This book is made available in cooperation with Elsevier.
Quantum Photonics aims to serve as a comprehensive and systematic reference source for entrants to the field of quantum photonics, including updated topics on quantum photonics for researchers working in this field. The book reviews the fundamental knowledge of modern photonics related quantum technologies, key concepts of quantum photonic devices, and quantum photonics applications. The book is suitable for graduate students, researchers, and engineers who want to learn quantum photonics fundamentals.
The editors, who are leaders in this field, have formulated this book as an introduction to the cutting-edge research in quantum photonics. Researchers and students involved in the development of semiconductor optoelectronics and optical communication systems should also find this book helpful.
We study high-power high bit rate single-mode 1550 nm vertical-cavity surface-emitting lasers fabricated using wafer-fusion. The optical cavity was grown on an InP wafer, and the two AlGaAs/GaAs distributed Bragg reflectors were grown on GaAs wafers, all three by molecular-beam epitaxy. The active region is based on thin InGaAs/InAlGaAs quantum wells and a composite InAlGaAs tunnel junction. To confine current and optical radiation, we use a lateral-structured buried tunnel junction with ≈ 6 µm diameter and an etching depth of ≈ 20 nm. These VCSELs demonstrate up to 5 mW single-mode continuous-wave output power and a threshold current of ≈ 2 mA at 25 °C. Even at an ambient temperature of 85 °C, the maximum optical output power is larger than 1 mW. The lasers demonstrate a 34 Gbps non-return-to-zero data transfer rate and 42 Gbps (21 GBaud) using 4-level pulse amplitude modulation at 25 °C back-to-back conditions with ≈ 934 fJ/bit power consumption per bit, which is amongst the lowest values reported for this wavelength range and bit rate.
The essential performance parameters of present generations of Vertical Cavity Surface Emitting Lasers (VCSELs) like output power, f3dB cut-off frequency, are limited by extrinsic parameters. The most important among them is the shift of the gain maximum out of resonance with the DBR transmission due to increased heating of the active layer with increasing current, leading to a red shift of the emission. Another important limitation of f3dB is the unavoidable resistance by the p-type mirror and capacitance of the actual device generations. We present here a novel Multi-Hole Aperture (MuHA) VCSEL approach1 , based on variable aperture shapes and sizes, leading to increased output power for single/multi-mode emission, reduced series resistance, and larger f3dB of the devices. Holes in symmetric or asymmetric arrangements are etched from the top to the oxidizable layer(s). The aperture shape and size is realized by controlled oxidation of the oxidizable layer(s) through the holes. The holes are subsequently filled with gold, which effectively remove heat from the active layer. In MuHA VCSELs, the temperature of the active area for any given current is thus at least 50% lower than that of a comparable VCSELs processed using a “classical” design, resulting in larger rollover current, f3dB,… Combining MuHA to Multi-Aperture devices called Multi Aperture VCSELs (MAVs) is expected to lead to pseudo single mode emission with an output power of 8-10 mW across 50 μm Multi-Mode Fiber (MMF), enabling to cover much larger transmission distances than hitherto.
Vertical-Cavity Surface-Emitting Lasers (VCSELs) are widely used for optical interconnects, 3D sensing like face recognition, or automotive applications. Conventional VCSELs with top/bottom distributed Bragg reflectors (DBRs) for any wavelength range are costly and bulky, needing precise growth control. High-contrast subwavelength gratings (HCGs) show a near 100% reflectivity across a wide wavelength range, and have a typical thickness of a few hundred nanometers, much thinner than epitaxial DBRs. In addition, HCGs were reported to have the ability to tightly confine the field in the HCG-based vertical cavities, very promising for high-speed devices. Thus HCGs are ideal candidates for mirror replacements, at least at the top, to construct vertical cavities. Currently HCGs are often based on an oxide layer, being monolithically integrated, or air-suspended, and the fabrication of these HCGs is still challenging.
VCSELs at 940 nm have been attracting particular attention for short-wave wavelength division multiplexing and sensing. Here we report for the first time electrically injected 940-nm HCG-VCSELs using post-supported air-suspended HCGs. The HCG-VCSELs are fabricated without critical point drying, and the HCGs can be released with a 100% yield in water or isopropanol. Our first generation HCG-VCSELs achieve already a low threshold current of 0.65 mA, and a large side-mode suppression ratio of 43.6 dB at 25 ℃ under continuous-wave operation. Theoretically, these HCG-VCSELs have a smaller effective mode length of 1.38*(λ/n), than that of conventional VCSELs with λ/2 cavities. The relaxation resonance frequency will increase by 16%. A data rate of 100 Gbps for these HCG-VCSELs is expected for the on-off keying modulation format.
Our present design and fabrication methods of the HCG-VCSELs can be extended to other wavelength ranges.
We experimentally demonstrate for the first time electrically-injected Vertical-Cavity Surface-Emitting Lasers (VCSELs) with post-supported high-contrast gratings (HCGs) at 940 nm. The HCG-VCSELs have two posts to support the air-suspended HCGs, being realized by simple fabrication without critical point drying. The HCGs can be released with a 100% yield in water or isopropanol. The HCG-VCSEL with a 4 μm × 8 μm oxide aperture achieves a low threshold current of 0.65 mA and a large side-mode suppression ratio of 43.6 dB under continuous-wave operation at 25 degrees. Theoretically, the relaxation resonance frequency of the HCG-VCSEL will increase by 16% compared with that of the conventional VCSEL with a λ/2 cavity. The data rate of 100 Gbps in the on-off keying modulation format for the HCG-VCSEL is expected.
NIR VCSELs are proliferating from Datacom to Consumer Electronics and Automotive Applications. 3D object detection
and imaging at distances from 1 to 100m is in the focus. VCSEL arrays for Flash LiDARs are considered or already in use
for in-cabin/exterior-to-car detection, ADAS, gesture recognition, drones, robotics etc. In this work, we propose high peak
power NIR VCSEL arrays with large range of field of view (FOV). The effective refractive index (neff) of the core and
clad sections of the VCSEL structure based on industrial standard III-V processing is adapted to these applications.
Controlling the transverse lasing modes, beam divergences as large as 46° can be achieved. Using the symmetric radiation
profile and wide FOV, segmented VCSEL arrays offer a wide range of object detection applications, needing in addition
adapted driver ICs and angled diffusers.
Energy efficient 200+ Gbit/s single fiber data transmission systems can be realized by wavelength multiplexing the directly modulated Vertical-Cavity Surface-Emitting Lasers (VCSELs) presented here, emitting at the four wavelengths 850 nm, 880 nm, 910 nm, and 940 nm. Large energy efficiency defined by a heat to data ratio (HBR) of only 240 fJ/bit @ 50 Gb/s is observed. Tuning the cavity photon lifetime is demonstrated to lead to an increase of the data rate in concert with a reduction of the HDR. The large linearity of our L-I-characteristics will allow easily higher order modulation rates. Our results might impact ongoing discussions of new physical layer standards for IEEE 802.3cm and cd coarse wavelength multiplexing standards across OM5 multimode fiber enabling up to 400 Gbit/s error-free transmission.
High-contrast metastructures like one-dimensional high-contrast gratings (HCGs) are promising to improve the performance of conventional VCSELs, also presenting a basis for new applications. Different from the previous studies where HCGs are always modelled being of infinite size, we studied here the finite-size HCGs, which match the real situation. We observe finite-size HCGs behaving very differently from infinite-size HCGs. The reflectivity of a finitesize HCG strongly depends on the HCG size and the source size. At the same time, the simulation results show, that finite-size HCGs can shape the output beam, and a Gaussian-like reflected wave is typically achieved. Most important the normally incident light is partly redirected to the in-plane direction, showing unidirectional transmission. Monolithically integrated HCG-based optical sensors can be based on this novel effect. An integrable HCG reflector was fabricated with GaInP as the sacrificial layer targeting the application of HCG-VCSEL at 980 nm range. The measured reflectivity agrees well with the calculated reflectivity.
The optical feedback dynamics of two multimode InAs/GaAs quantum dot lasers emitting exclusively on sole ground or excited lasing states is investigated under the short delay configuration. Although the two lasers are made from the same active medium, their responses to the external perturbation are found not much alike. By varying the feedback parameters, various periodic and chaotic oscillatory states are unveiled. The ground state laser is found to be much more resistant to optical feedback, benefitting from its strong relaxation oscillation damping. In contrast, the excited state laser can easily be driven into very complex dynamics. While the ground state laser is of importance for the development of isolator-free transmitters, the excited one is essential for applications taking advantages of chaos such as chaos lidar, chaos radar, and random number generation.
Quantum dot nanostructures are one of the best practical examples of emerging nanotechnologies hence offering superior properties as compared to their quantum well counterparts. InAs/GaAs quantum dots allow producing energy- and cost-efficient devices with outstanding temperature stability, lowest threshold current, ultrafast gain dynamics, and low amplified spontaneous emission. This paper reports on the recent achievements in ultrafast and nonlinear dynamics properties of InAs/GaAs quantum dot lasers for radar systems, wireless communications and high-speed optical communications. Passive mode-locking is shown to exhibit a great potential for microwave, millimeter-wave and Terahertz signal generation with high repetition frequency tuning and jitter reduction. The optical feedback is also used to stabilize the pulse emission leading an integrated timing jitter as low as 90 fs without consuming additional power. Lastly, multimode optical feedback dynamics of InAs/GaAs QD lasers emitting on different lasing states is also studied. In particular, a chaos free operation is observed for the first time from the ground state lasing operation.
The progress of 1.3- and 1.5-μm waveband wafer-fused VCSELs is reported. The emission of single mode power of 6 - 8 mW at room temperature and up to 3 mW at 80°C were demonstrated. 10-Gb/s full wavelength-set VCSEL devices for CWDM systems with high yield and Telcordia-reliability were industrially manufactured. By increasing the compressive strain in the QWs and reducing the cavity photon life time the modulation bandwidth was increased to 11.5 GHz, and large-signal data transmission experiments show error-free operation and open eye diagrams from 25 to 35 Gb/s in both B2B and after 10-km, respectively.
In this work, the sensitivity to external optical feedback of two different InAs/GaAs QD Fabry-Perot (FP) lasers is investigated under long cavity regime. The first, which has a 1.5 mm-long cavity, emits on the GS while the second one, which is 1 mm long, radiates solely on the ES transition. The results indicate that for the same bias level, the ES laser presents a larger sensitivity to external feedback, the critical level being under 1% versus above 9% for the GS laser. In particular, the ES laser exhibits a route to chaos such that the first destabilization occurs for a lower feedback strength than for the GS laser.
Frequency conversion using highly non-degenerate four-wave mixing is reported in InAs/GaAs quantum-dot Fabry- Perot lasers. In order to compress the spontaneous emission noise, the laser is optically injection-locked. Under proper injection conditions, the beating between the injected light frequency and the cavity resonant frequency dominates the dynamic behavior and enhances a carrier modulation resonance at frequencies higher than the relaxation oscillation frequency. Conversion efficiencies as high as -12 dB associated to a large optical signal-to-noise ratio of 36 dB are reported. The conversion bandwidth is extended up to 2.1 THz for down-conversion (resp. 3.2 THz for up-conversion) with a quasi-symmetrical response between up- and down-converted signals.
We report on the dynamic properties of 1.31 μm InAs/GaAs and 1.55 μm InAs/InP quantum-dot Fabry-Perot lasers with the main focus on the increase of their large-signal modulation capabilities. A GaAs-based edge-emitter structure incorporating a standard p-doped active region with ten quantum-dot layers enables 15 Gbit/s data transmission at 70 °C upon direct modulation. The large number of layers and wide barriers cause significant carrier transport limitations. Since the carrier distribution across the stack is not uniform, a graded p-doping profile is implemented leading to an increased data rate of 20 Gbit/s, but at the expense of somewhat lower temperature stability. GaAs-based lasers operating exclusively from the first excited state demonstrate a further data rate increase to presently 25 Gbit/s, due to the larger degeneracy of the higher quantum-dot energy levels. 25 Gbit/s data transmission at 70 °C is also achieved with InAs/InP quantum-dot devices emitting in the C-band. Four- and eight-level pulse-amplitude modulation formats are utilized to increase the data rate at a given bandwidth compared to a standard on-off keying scheme. Data rates up to 35 Gbit/s are presented for both wavelength bands. Monolithically integrated two-section mode-locked lasers based on the graded pdoping structure provide low-jitter optical pulse trains and are utilized as optical sources for non-return-to-zero transmitters. 80 Gbit/s on-off keying and 80 GBd (160 Gbit/s) differential quadrature phase-shift keying data transmission based on optical time-division multiplexing are demonstrated using a packaged 40 GHz module.
Sergey Blokhin, Mikhail Bobrov, Nikolai Maleev, Alexander Kuzmenkov, Alexey Sakharov, Alexey Blokhin, Philip Moser, James Lott, Dieter Bimberg, Viktor Ustinov
Vertical-cavity surface-emitting lasers (VCSELs) based on the InGaAlAs-materials system on GaAs substrates are the key component for short-reach data and computer communications systems. Several different modulation schemes have been developed to realize high data bit rates based on various oxide-confined near-infrared VCSEL designs operated under direct current modulation. However, one open question to resolve is the optimal gain-to-cavity wavelength detuning to employ for temperature-stable high-speed performance. We investigate the static and dynamic characteristics of 850 nm high-speed oxide-confined VCSELs with different negative gain-to-cavity wavelength detunings. Our oxideconfined 850 nm VCSELs with a more common ~10 nm negative gain-to-cavity detuning demonstrate the conventional optical mode behavior with a classical single-resonance frequency response. With a larger (≥ 20 nm) negative detuning, our devices with large oxide-aperture size (>6 μm) show an anomalous start of lasing via higher order modes with a subsequent switching to lasing via the lowest order modes at higher currents. At intermediate currents, co-lasing via two types of transverse modes and a two-resonance modulation response is observed. The increase of operation temperature as well as the reduction in the oxide-aperture area resulted in classical lasing of index-guided VCSELs. The observed optical mode behavior can be attributed to the specific index guiding profile caused by the oxide-apertures, low internal optical losses, and the large gain-to-cavity detuning. Moreover, one can suggest that the complex shape of the modulation response results from the mode competition for the available gain during an interesting co-lasing operating regime.
Energy-efficient oxide-confined vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm, particularly well suited for optical interconnects operating at up to 85°C are presented. The modulation bandwidth f3dB of our VCSELs increases at low currents with increasing temperature up to 23 GHz at 85°C. The impact of cavity photon lifetime and oxide-aperture diameter on the energy efficiency, temperature stability, and static and dynamic properties of our VCSELs are analyzed. Error-free 40 Gb/s operation at 85°C with an energy-to-data ratio below 100 fJ/bit and a current density close to 10 kA/cm2 is predicted based on small signal modulation experiments.
Vertical-cavity surface-emitting lasers (VCSELs) are decisive cost-effective, energy-efficient, and reliable light sources for short-reach (up to ~300 m) optical interconnects in data centers and supercomputers. To viably replace copper interconnects and advance to on-chip integrated photonics, reliable VCSELs ideally must be able to operate highly energy efficient, but at large bit rates and without cooling up to 85 °C, with immunity to temperature variations. Our 980 nm VCSELs achieve such temperature-stable, energy-efficient, and high-speed operation coincidently. Record low 139 fJ/bit of dissipated heat for 35 Gbit/s error-free data transmission at 85 °C is reported. Careful design of both the VCSEL’s epitaxial structure and device geometry is of essence. Introducing a suitable gain-to-etalon wavelength offset simultaneously improves the temperature-stability, the maximum bit rate at high temperatures, and the energy efficiency. Tuning the photon lifetime additionally increases the bandwidth by changing the relation between damping and resonance relaxation frequency. Systematic temperature-dependent and oxide aperture-diameter-dependent measurements, including static L-I-V curves and emission spectra, small signal analysis, and data transmission experiments are reported. The modulation bandwidth, the parasitic cut-off frequency, the relaxation resonance frequency, lumped-circuit elements, and the K- and D-factors are derived, useful for energy-efficient optical interconnects based on 980 nm VCSELs.
In this paper we present the simulation results of an oxide-confined, InGaAs/GaAs based vertical-cavity surface-emitting laser with three different configurations of the oxide apertures. We analyze the impact of the number and position of oxide layers on the carrier distribution in the laser's active region, distribution of the optical modes, and modulation properties.
We describe the technique allowing for generation of low-noise wider frequency combs and pulses of shorter duration in quantum-dot mode-locked lasers. We compare experimentally noise stabilization techniques in semiconductor mode-locked lasers. We discuss the benefits of electrical modulation of the laser absorber voltage (hybrid mode-locking), combination of hybrid mode-locking with optical injection seeding from the narrow linewidth continues wave master source and optical injection seeding of two coherent sidebands separated by the laser repetition rate.
The use of Internet has increased and continues to increase exponentially, mostly driven by consumers. Thus bit rates in networks from access to WDM and finally the computer clusters and supercomputers increase as well rapidly. Their cost of energy reaches today 5-6 % of raw electricity production. For 2023 a cross over is predicted, if no new "green" technologies or "green" devices" will reduce energy consumption by about 15% per year. We present two distinct approaches for access and computer networks based on nanophotonic devices to reduce power consumption in the next decade.
Metal-cavity submonolayer (SML) quantum-dot (QD) microlasers are demonstrated at room
temperature under continuous-wave electrical injection for 2-μm-radius devices and pulsed
operation for 0.5-μm-radius devices. Compared to our previous quantum well devices, the superior
optical properties of SML QDs provide the possibility for further size reduction. Size-dependent
lasing characteristics are extracted from measurements to investigate the device physics for future
size reduction. An optical cavity model using the transfer matrix and the effective index method
including metal dispersion is developed and used for both the design and the experimental results
analysis. The laser uses an active region consisting of three groups of SML QDs, and each group
consists of 10 stacks of 0.5-monolayer InAs QD layers. The cylindrical microcavity is formed by
hybrid metal-distributed Bragg reflectors (DBRs) mirrors with an optimized SiNx passivation layer
on the sidewall to reduce the metal loss and to avoid the leakage current. The transverse optical
modes are solved using the Maxwell equations, and the resonance condition is determined by roundtrip
phase matching. Vertically-correlated QDs are modeled as quantum disks, and the wave
functions and eigenenergies in both conduction and valance bands are solved from Schrodinger
equation. Carrier-dependent material gain is calculated using Fermi’s golden rule and included in the
model. The lasing wavelengths, quality factors, and confinement factors for cavity modes are the
inputs for the rate-equation model, which predicts the light output power vs. current behavior and
has shown excellent agreement with experiments. Size-dependent physical quantities such as
leakage current and spontaneous emission coupling factor are extracted and investigated. Further
size reduction using only four pairs of DBRs is proposed.
Principles of energy-efficient high speed operation of oxide-confined VCSELs are presented. Trade-offs between oxideaperture diameter, current-density, and energy consumption per bit are demonstrated and discussed. Record energyefficient error-free data transmission up to 40 Gb/s, across up to 1000 m of multimode optical fiber and at up to 85 °C is reviewed.
Via experimental results supported by numerical modeling we report the energy-efficiency, bit rate, and modal properties of GaAs-based 980 nm vertical cavity surface emitting lasers (VCSELs). Using our newly established Principles for the design and operation of energy-efficient VCSELs as reported in the Invited paper by Moser et al. (SPIE 9001-02 ) [1] along with our high bit rate 980 nm VCSEL epitaxial designs that include a relatively large etalonto- quantum well gain-peak wavelength detuning of about 15 nm we demonstrate record error-free (bit error ratio below 10-12) data transmission performance of 38, 40, and 42 Gbit/s at 85, 75, and 25°C, respectively. At 38 Gbit/s in a back-toback test configuration from 45 to 85°C we demonstrate a record low and highly stable dissipated energy of only ~179 to 177 fJ per transmitted bit. We conclude that our 980 nm VCSELs are especially well suited for very-short-reach and ultra-short-reach optical interconnects where the data transmission distances are about 1 m or less, and about 10 mm or less, respectively.
A new record for energy-efficient oxide-confined 850 nm vertical-cavity surface-emitting lasers (VCSELs) particularly suited for optical interconnects is presented. Error-free performance at 25 Gb/s is achieved with only 56 fJ/bit of dissipated energy per quantum of information. The influence of the oxide-aperture diameter on the energy-efficiency of our VCSELs is determined by comparing the total and dissipated power versus the modulation bandwidth of devices with different aperture diameters. Trade-offs between various parameters such as threshold current, differential quantum efficiency, wall plug efficiency and differential resistance are investigated with respect to energy-efficiency. We show that our present single-mode VCSELs are more energy-efficient than our multimode ones.
The bandwidth-induced communication bottleneck due to the intrinsic limitations of metal interconnects is inhibiting the
performance and environmental friendliness of today´s supercomputers, data centers, and in fact all other modern
electrically interconnected and interoperable networks such as data farms and "cloud" fabrics. The same is true for
systems of optical interconnects (OIs), where even when the metal interconnects are replaced with OIs the systems
remain limited by bandwidth, physical size, and most critically the power consumption and lifecycle operating costs.
Vertical-cavity surface-emitting lasers (VCSELs) are ideally suited to solve this dilemma. Global communication
providers like Google Inc., Intel Inc., HP Inc., and IBM Inc. are now producing optical interconnects based on VCSELs.
The optimal bandwidth per link may be analyzed by by using Amdahl´s Law and depends on the architecture of the data
center and the performance of the servers within the data center. According to Google Inc., a bandwidth of 40 Gb/s has
to be accommodated in the future. IBM Inc. demands 80 Tbps interconnects between solitary server chips in 2020. We
recently realized ultrahigh bit rate VCSELs up to 49 Gb/s suited for such optical interconnects emitting at 980 nm. These
devices show error-free transmission at temperatures up to 155°C and operate beyond 200°C. Single channel data-rates
of 40 Gb/s were achieved up to 75°C. Record high energy efficiencies close to 50 fJ/bit were demonstrated for VCSELs
emitting at 850 nm. Our devices are fabricated using a full three-inch wafer process, and the apertures were formed by
in-situ controlled selective wet oxidation using stainless steel-based vacuum equipment of our own design. assembly,
and operation. All device data are measured, recorded, and evaluated by our proprietary fully automated wafer mapping
probe station. The bandwidth density of our present devices is expected to be scalable from about 100 Gbps/mm² to a
physical limit of roughly 15 Tbps/mm² based on the current 12.5 Gb/s VCSEL technology. Still more energy-efficient
and smaller volume laser diode devices dissipating less heat are mandatory for further up scaling of the bandwidth.
Novel metal-clad VCSELs enable a reduction of the device's footprint for potentially ultrashort range interconnects by 1
to 2 orders of magnitude compared to conventional VCSELs thus enabling a similar increase of device density and
bandwidth.
State-of-the-art vertical-cavity surface-emitting laser (VCSEL) based optical interconnects for application in high
performance computers and data centers are reviewed. Record energy-efficient data transmission is demonstrated with
850 nm single-mode VCSELs for multimode optical fiber lengths up to 1 km at bit rates up to 25 Gb/s. Total power
consumption of less than 100 fJ/bit is demonstrated for VCSELs for the first time. Extremely temperature stable 980-nm
VCSELs show lasing up to 200 °C. Error-free 44 Gb/s operation at room temperature and 38 Gb/s up to 85 °C is
achieved with these devices. We present record-high bit rates in a wide temperature range of more than 160 °C. Record
energy-efficient data-transmission beyond 30 Gb/s is achieved at 25 °C for this wavelength range. In view of the high
speed and advanced temperature stability we suggest long wavelength VCSELs for energy-efficient short and very short-distance
optical interconnects for future high performance computers.
Single mode (SM) 850 nm vertical-cavity surface-emitting lasers (VCSELs) are suitable for error-free (bit error ratio
<10-12) data transmission at 17-25 Gb/s at distances ~2-0.6 km over 50μm-core multimode fiber (MMF). Reduced
chromatic dispersion due to ultralow chirp of SM VCSELs under high speed modulation (up to 40 Gb/s) are responsible
for the dramatic length extension. Good coupling tolerances of the SM devices to the MMF manifest their applicability
for low cost optical interconnects. As the higher resonance frequency (up to 30 GHz) is reached at lower current
densities in small aperture (3 μm -diameter) devices the SM devices are also preferable due to reliability considerations.
Record energy-efficient oxide-confined 850-nm single mode and quasi-single mode vertical-cavity surface-emitting
lasers (VCSELs) for optical interconnects are presented. Error-free performance at 17 Gb/s is achieved with record-low
dissipated power of only 69 fJ/bit. The total energy consumption is only 93 fJ/bit. Transmission lengths up to 1 km of
multimode optical fiber were achieved. Our commercial quasi-single mode devices achieve error-free operation at
25 Gb/s across up to 303 m of multimode fiber. To date our VCSELs are the most energy-efficient directly modulated
light-sources at any wavelength for data transmission across all distances up to 1 km of multimode optical fiber.
The copper-induced communication bottleneck is inhibiting performance and environmental acceptance of
today's supercomputers. Vertical-cavity surface-emitting lasers (VCSELs) are ideally suited to solve this
dilemma. Indeed global players like Google, Intel, HP or IBM are now going for optical interconnects based on
VCSELs. The required bandwidth per link, however, is fixed by the architecture of the data center. According to
Google, a bandwidth of 40 Gb/s has to be accommodated. We recently realized ultra-high speed VCSELs suited
for optical interconnects in data centers with record-high performance. The 980-nm wavelength was chosen to be
able to realize densely-packed, bottom-emitting devices particularly advantageous for interconnects. These
devices show error-free transmission at temperatures up to 155°C. Serial data-rates of 40 Gb/s were achieved up
to 75° C. Peltier-cooled devices were modulated up to 50 Gb/s. These results were achieved from the sender side
by a VCSEL structure with important improvements and from the receiver side by a receiver module supplied by
u2t with some 30 GHz bandwidth. The novel VCSELs feature a new active region, a very short laser cavity, and a
drastically improved thermal resistance by the incorporation of a binary bottom mirror. As these devices might be
of industrial interest we had the epi-growth done by metal-organic chemical-vapor deposition at IQE Europe.
Consequently, the devices were fabricated using a three-inch wafer process, and the apertures were formed by
proprietary in-situ controlled selective wet oxidation. All device data were measured, mapped and evaluated by
our fully automated probe station. Furthermore, these devices enable record-efficient data-transmission beyond
30 Gb/s, which is crucial for green photonics.
Semiconductor optical amplifiers based on quantum dots show small-signal cross-gain modulation bandwidths exceeding
40 GHz. In large signal operation wavelength conversion at 80 Gb/s over 10 nm is presented. Two section mode-locked
lasers at 40 GHz yield ultra-low jitter of 200 fs in hybrid operation. Optical feedback presents an alternative way to
effectively reduce the jitter and opens up the possibility to extract a microwave signal, having the same properties as the
optical pulse comb, from the absorber section.
We report on the development of 25Gb/s 850nm VCSEL and PD components for efficient short-reach optical fiber
communication systems. VCSELs with the aperture size 6-7μm show the highest -3dB bandwidth (~20GHz) and Dfactor
(~8Ghz/mA1/2). K-factor is less than 0.25ns for VCSEL with 6 μm aperture. Eye diagrams are clearly open at 25C
up to 35Gb/s. The dark current of PDs remain below 1nA at T < 50°C and below 10nA when T < 90°C out to -10V. The
extracted PD capacity is linearly proportional to the detector area and less than 200fF even for 45μm PD diameter. Due
to elimination of contribution of diffusion process and quite small capacitance of the depletion region eye diagrams are
opened at 28Gb/s, even for the PDs with the largest active diameters. Using 35μm PD and 6μm VCSEL error-free
25Gb/s optical fiber communication links were tested over lengths of 203m and 103m at 25°C and 85°C, respectively.
Received optical power for the lowest BER is at both temperatures smaller than -4dBm. Obtained results indicate that
from the speed and power dissipation perspective developed high-speed CSELs and PDs are suitable for applications in
the next generation of short-reach multimode optical fiber interconnects.
The ever growing demand for more bandwidth in high-performance computing (HPC) applications leads to a continuous
replacement of traditional copper-based links by optical interconnects at ever shorter transmission distances. However,
this trend results in a more stringent performance requirements for laser light sources utilized in new generations of
optical interconnects in respect to single channel speed, packaging density, power consumption and temperature stability,
to make the technology competitive and commercially viable. Vertical cavity surface emitting lasers operating at
different wavelengths, e. g. 850 or 980 nm, represent one possible solution for the short distance high density
interconnects in HPC applications. Here we present ultra-high speed highly temperature stable 980 nm VCSELs
operating error-free at the record high bit rate of 44 Gbit/s at room temperature and 38 Gbit/s at 85 °C for future interand
intra-chip, and module-to-module optical links. Next we present high speed extremely energy efficient 850 nm
VCSELs with record low energy consumptions of only 83 fJ/bit while operating at 17 Gbit/s and of only 117 fJ/bit at 25
Gbit/s. Our VCSELs enable ecologically sound and economically practical HPC designs.
As a type-II heterostructure with exclusive hole confinement GaSb/(Al,Ga)As QDs are an ideal candidate for
a QD based memory device operating at room temperature. We investigated different Antimony-based QDs in
respect of localization energies and storage times with 8-band-k•p calculations as well as time-resolved capacitance
spectroscopy. In addition, we present a memory concept based on self-organized quantum dots (QDs) which could
fuse the advantages of today's main semiconductor memories DRAM and Flash. First results on the performance
of such a memory cell are shown and a closer look at Sb-based QDs as a storage unit is taken.
As the density of transistors in CMOS integrated circuits continues to roughly double each two years the processor
computational power also roughly doubles. Since the number of input/output (I/O) devices can not increase without
bound I/O speed must analogously approximately double each two years. In the Infiniband EDR standard (2011) a single
channel bit rate of 26 Gb/s is foreseen. The maximum reliable and efficient copper link length shrinks at bit rates above
10 Gb/s to a few meters at best. At higher bit rates the length of a given multimode fiber link must also shrink, due to
both modal and wavelength dispersions. Although the modal dispersion in modern multimode OM3 and OM4 fibers that
are optimized for 850 nm vertical-cavity surface-emitting lasers (VCSELs) is reduced, the wavelength dispersion
remains a serious issue for standard multimode VCSELs. An ultimate solution to overcome this problem is to apply
single-mode VCSELs to extend and ultimately maximize the link length. In this paper we demonstrate recent results for
single-mode VCSELs with very high relaxation resonance frequencies. Quantum well 850 nm VCSELs with record high
30 GHz resonance frequencies are demonstrated. Additionally single-mode data transmission at 35 Gb/s over multimode
fiber is demonstrated. For comparison we also present specific device modeling parameters and performance
characteristics of 850 nm single-mode quantum dot (QD) VCSELs. Despite a significant spectral broadening of the QD
photoluminescence and gain due to QD size dispersion we obtain relaxation resonance frequencies as high as 17 GHz.
The progressive penetration of optical communication links into traditional copper interconnect markets greatly expands
the applications of vertical cavity surface emitting lasers (VCSELs) for the next-generation of board-to-board, moduleto-
module, chip-to-chip, and on-chip optical interconnects. Stability of the VCSEL parameters at high temperatures is
indispensable for such applications, since these lasers typically reside directly on or near integrated circuit chips. Here
we present 980 nm oxide-confined VCSELs operating error-free at bit rates up to 25 Gbit/s at temperatures as high as 85
°C without adjustment of the drive current and peak-to-peak modulation voltage. The driver design is therefore
simplified and the power consumption of the driver electronics is lowered, reducing the production and operational costs.
Small and large signal modulation experiments at various temperatures from 20 up to 85 °C for lasers with different
oxide aperture diameters are presented in order to analyze the physical processes controlling the performance of the
VCSELs. Temperature insensitive maximum -3 dB bandwidths of around 13-15 GHz for VCSELs with aperture
diameters of 10 μm and corresponding parasitic cut-off frequencies exceeding 22 GHz are observed. Presented results
demonstrate the suitability of our VCSELs for practical high speed and high temperature stable short-reach optical links.
Quantum well (QW) VCSELs have a tendency to switch their polarization from one linearly polarized (LP)
mode to the orthogonal one when changing the operation conditions. As polarization properties of VCSELs are
governed by anisotropies, namely stress-induced birefringence and dichroism, the inherent anisotropy of quantum
dots (QDs) is expected to influence the polarization properties of QD VCSELs. In this paper we summarize our
experimental results on polarization properties of QD VCSELs with the main focus on polarization switching
phenomena. Close to threshold the laser emits linearly polarized light which changes to elliptically polarized
(EP) at some current. The main axes of these states are not aligned and the angle between them increases
with current. As the current is still increased polarization switching accompanied by polarization mode hopping
occurs. Distinctive feature of the observed switching is that the two EP states between which switching occurs are
nonorthogonal. The angle between their major exes is 40 deg. Polarization mode hopping has been characterized
in terms of the dwell time and the current-dependence of this factor examined. Apparently, the dwell time
decreases when the pump current is increased which differs from what has been published for QW VCSELs. The
average dwell time is 20 ns. Similarly to QW VCSELs the distribution of the dwell time is exponential. The
statistics is the same for the two EP states and such symmetric switching is maintained in the whole range of
currents at which the light is elliptically polarized. Large-signal modulation experiments show that the frequency
at which polarization switching disappears is about 100 MHz. This indicates that the switching is of thermal
origin.
KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Data communications, Oxides, Reliability, Eye, Data transmission, Picosecond phenomena, Photodetectors, Signal to noise ratio
Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and
storage area network (LAN/SAN) optical fiber data communication systems and all other short-reach high-speed data
transfer applications. The intrinsic limitations of copper-based electrical links at data rates exceeding 10 Gbit/s leads to a
progressive movement wherein optical communication links replace traditional short-reach (300 m or shorter) copper
interconnects. The wavelength of 850 nm is the standard for LAN/SAN applications as well as for several other evolving
short-reach application areas including Fibre Channel, InfiniBand, Universal Serial Bus (optical USB), and active optical
cables. Here we present our recent results on 850 nm oxide-confined VCSELs operating at data bit rates up to 40 Gbit/s
at low current densities of ~10 kA/cm2 ensuring device reliability and long-term stability based on conventional industry
certification specifications. The relaxation resonance frequencies, damping factors, and parasitic cut-off frequencies are
determined for VCSELs with oxide-confined apertures of various diameters. At the highest optical modulation rates the
VCSELs' high speed operation is limited by parasitic cut-off frequencies of 24-28 GHz. We believe that by further
reducing device parasitics we will produce current modulated VCSELs with optical modulation bandwidths larger than
30 GHz and data bit rates beyond 40 Gbit/s.
Hybrid mode-locking in monolithic quantum dot lasers is studied experimentally and theoretically. A strong asymmetry of the locking range with respect to the passive mode locking frequency is observed. The width of this range increases linearly with the modulation amplitude for all operating parameters. Maximum locking range found is 30 MHz. The results of a numerical analysis performed using a set of five delay-differential equations taking into account carrier exchange between quantum dots and wetting layer are in agreement with experiments and indicate that a spectral filtering element could improve locking characteristics. Asymptotic analysis of the dependence of the locking range on the laser parameters is performed with the help of a more simple laser model consisting of three delay differential equations.
Based on frequency resolved optical gating, a pulse shape and phase characterization of a monolithic-two-section,
quantum-dot mode-locked laser (QD-MLL) at 1.3 μm, at a repetition rate of 40 GHz, is presented. The dynamics of the
absorber and the gain section are investigated in detail. Increasing the gain current leads to an increase of mostly linear
chirp inducing significant pulse broadening. The absorber dynamics, namely the sweep out time of the carriers of the
QDs, is enhanced at larger reverse biases. The quantum confined stark effect (QCSE) however reduces the absorber
efficiency. Thus the shortest pulse width occurs for medium voltages. Pulses generated by hybrid mode locking are
compared to passive mode-locked ones. Only a slight suppression of the trailing part of the pulses is found. Simulations
as well as experiments demonstrate that the linear part of the chirp can be easily compensated leading to pulse
compression. A pulse width of 700 fs is achieved almost independent of operating conditions. Temperature changes of
8°C leads to pulse broadening of a few hundred femtoseconds. Pulse combs up to 160 Gbit/s are generated using optical
time division multiplexing (OTDM). Eye diagrams and autocorrelation measurements prove the suitability of our
approach.
We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond
the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design
for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal
conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a
minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic
high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in
the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in
a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential
resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is
capable of error-free transmission (BER<10-12) under direct modulation at a record-high bit-rate of 32 Gb/s over 50 m of
OM3 fiber at room temperature, and at 25 Gb/s over 100 m of OM3 fiber at 85 °C. We also demonstrate transmission at
40 Gb/s over 200 m of OM3+ fiber at room temperature using a subcarrier multiplexing scheme with a spectrally
efficient 16 QAM modulation format. All transmission results were obtained with the VCSEL biased at current densities
between 11-14 kA/cm2, which is close to the 10 kA/cm2 industry benchmark for reliability. Finally, we show that by a
further reduction of the oxide capacitance and by reducing the photon lifetime using a shallow surface etch, a record
bandwidth of 23 GHz for 850 nm VCSELs can be reached.
KEYWORDS: Waveguides, Semiconductor lasers, Photonic crystals, Active optics, Cladding, Near field, Near field optics, High power lasers, Crystals, Phase matching
The concepts, features, modeling and practical realizations of high power high brightness semiconductor diode lasers
having ultrathick and ultrabroad waveguides and emitting in the single vertical single lateral mode are analyzed.
Ultrathick vertical waveguide can be realized as a photonic band crystal with an embedded filter of high order modes. In
a second approach a tilted wave laser enables leakage of the optical wave from the active waveguide to the substrate and
additional feedback from the back substrate side. Both designs provide high power and low divergence in the fast and the
slow axis, and hence an increased brightness. Lateral photonic crystal enables coherent coupling of individual lasers and
the mode expansion over an ultrabroad lateral waveguide. Experimental results are presented. Obtained results
demonstrate a possibility for further expansion of the concept and using the single mode diodes having an ultrabroad
waveguide to construct single mode laser bars and stacks.
Just as the density of transistors on a silicon chip about doubles with each new generation, processor bandwidth also
about doubles. Consequently the speed of input-output (I/O) devices must grow and today we find processor I/O speed
approaching or slightly surpassing 10 Gb/s (G) per channel for 100G Ethernet server applications. Similarly Storage
Area Networks are supported by Fibre Channel FC16G transceivers operating at the newly standardized serial signaling
rate of 14 Gbaud. Further upgrades will require within only a few years links at 25, 28 and 40 Gbaud, speeds that are
barely feasible with copper cabling, even for very short reach distances. Thus the role of optical interconnects will
increase dramatically as the data transfer rates increase. Furthermore an increased bandwidth demand necessitates an
equal or greater demand for low cost and highly power efficient micro-laser and -detector components along with their
associated driver and transimpedance amplifier (TIA) integrated circuits (ICs). We summarize our recent achievements
in vertical cavity surface emitting lasers (VCSELs) and PIN photodetectors suitable for very short reach multimode fiber
links that enable bit rates up to and beyond 40 Gb/s. We address achievements in current modulated VCSELs,
electrooptically modulated VCSELs, top illuminated PIN photodiodes, TIA and driver ICs, and packaging solutions.
Efficient generation of polarized single or entangled photons is a crucial requirement for the implementation
of quantum key distribution (QKD) systems. Self-organized semiconductor quantum dots (QDs) are capable of
emitting one polarized photon or an entangled photon pair at a time using appropriate electrical current injection.
We realized highly efficient single photon sources (SPS) based on well established semiconductor technology: In
a pin structure a single electron and a single hole are funneled into a single InAs quantum dot using a submicron
AlOx current aperture. Efficient radiative recombination leads to emission of single polarized photons with an
all-time record purity of the spectrum. Non-classicality of the emitted light without using additional spectral
filtering is demonstrated. Out-coupling efficiency and emission rate are increased by embedding the SPS into a
micro-cavity of Q = 140. The design of the micro-cavity is based on detailed modeling to optimize its performance.
The resulting resonant single-QD diode generates single polarized photons at a repetition rate of 1 GHz exhibiting
a second order correlation function of g(2)(0) = 0.
Eventually, QDs grown on (111) oriented substrate are proposed as source of entangled photon pairs. Intrinsic
symmetry-lowering effects leading to the splitting of the exciton bright states are shown to be absent for this
substrate orientation. As a result the XX → X → 0 recombination cascade of a QD can be used for the generation
of entangled photons without further tuning of the finestructure splitting via QD size and/or shape. We present
first micro-photoluminescence studies on QDs grown on (111) GaAs, demonstrating a fine structure splitting less
than the spectral resolution of our set-up.
We have designed, fabricated and measured the performance of two types of edge emitting lasers with unconventional
waveguides and lateral arrays thereof. Both designs provide high power and low divergence in the fast and the slow axis,
and hence an increased brightness. The devices are extremely promising for new laser systems required for many
scientific and commercial applications. In the first approach we use a broad photonic crystal waveguide with an
embedded higher order mode filter, allowing us to expand the ground mode across the entire waveguide. A very narrow
vertical far field of ~ 7° is resulting. 980 nm single mode lasers show in continuous wave operation more than 2 W,
ηwp ~ 60%, M2 ~ 1.5, beam parameter product of 0.47 mm×mrad and a brightness ~ 1×108 Wsr-1cm-2 respectively. First
results on coherent coupling of several lasers are presented. In the second approach we use leaky designs with feedback.
The mode leaks from a conventional waveguide into a transparent substrate and reflects back, such that only one mode at
a selected wavelength is enhanced and builds up, others are suppressed by interference. 1060 nm range devices
demonstrate an extremely narrow vertical far field divergence of less than 1°.
Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and
storage area network (LAN/SAN) optical fiber data communication systems and short-reach computer interconnects. The
continuing rapid increase of serial transmission data rates driven by multi-core microprocessor's bandwidth upgrades
cannot be sustained via conventional copper-based links as bit rates move beyond 10 Gbit/s and distances greater than 1
m. The intrinsic limitation of copper at high single-channel data rates facilitates the need to transition to optical fiberbased
links at ever shorter distances. For LAN/SAN applications the 850 nm wavelength is standard. This same
wavelength is also the standard for several other evolving short-reach application areas including Fibre Channel, CEI,
USB, InfiniBand, and HDMI optical link systems. Herein we present our recent results on 850 nm oxide-confined
VCSELs operating at data bit rates up to 40 Gbit/s. The low operational current density in the range of ~10 kA/cm2
ensures viable device reliability and long-term stability based on well-known industry certification specifications. Key
VCSEL device parameters including the relaxation resonance frequency, damping, and parasitic cut-off frequency are
determined for VCSELs with oxide-confined apertures of various diameters. We find that a parasitic cut-off frequency of
24-28 GHz limits the VCSEL's high speed operation at the highest optical modulation rates. We believe that with some
effort the device parasitics can be further reduced such that current modulated VCSELs can be realized with larger than
30 GHz optical modulation bandwidth and reliable and practical operation beyond 40 Gbit/s.
Efficient generation of polarized single photons or entangled photon pairs is a crucial requirement for the implementation
of quantum key distribution (QKD) systems [1] [2]. In this context, self-organized semiconductor quantum dots (QDs)
[3] [4] play a decisive role as they are capable of emitting only one polarized photon at a time using appropriate
electrical current injection [5] [6]. Therefore, a single QD embedded in a LED can be used as a single photon source. By
tuning the electronic structure it is possible to use QD as source for entangled photons. Resonant cavity-induced
enhancement of spontaneous emission and out-coupling efficiency can improve external quantum efficiency of quantum
dot based single photon sources dramatically. In order to optimise the device geometry detailed numerical device
modelling must be performed. The modelling of the electromagnetic field were done using eigenmode-techniques. The
essential design parameters, such as cavity length, aperture diameter, position and thickness were systematically varied.
We designed and fabricated optimized resonant cavity light emitting diodes combined with a submicron oxide current
aperture, to pump individual InGaAs/GaAs QDs electrically. These devices demonstrates more than ten times increased
single photon rate in comparison to the simple LED design. Pulsed correlation measurements demonstrated true single
photon emission with g2(0) = 0 at a rate of 1 GHz.
We report on the modeling, epitaxial growth, fabrication, and characterization of 830-845 nm vertical cavity surface
emitting lasers (VCSELs) that employ InAs-GaAs quantum dot (QD) gain elements. The GaAs-based VCSELs are
essentially conventional in design, grown by solid-source molecular beam epitaxy, and include top and bottom gradedheterointerface
AlGaAs distributed Bragg reflectors, a single selectively-oxidized AlAs waveguiding/current funneling
aperture layer, and a quasi-antiwaveguiding microcavity. The active region consists of three sheets of InAs-GaAs
submonolayer insertions separated by AlGaAs matrix layers. Compared to QWs the InAs-GaAs insertions are expected
to offer higher exciton-dominated modal gain and improved carrier capture and retention, thus resulting in superior
temperature stability and resilience to degradation caused by operating at the larger switching currents commonly
employed to increase the data rates of modern optical communication systems. We investigate the robustness and
temperature performance of our QD VCSEL design by fabricating prototype devices in a high-frequency ground-sourceground
contact pad configuration suitable for on-wafer probing. Arrays of VCSELs are produced with precise variations
in top mesa diameter from 24 to 36 μm and oxide aperture diameter from 1 to 12 μm resulting in VCSELs that operate in
full single-mode, single-mode to multi-mode, and full multi-mode regimes. The single-mode QD VCSELs have room
temperature threshold currents below 0.5 mA and peak output powers near 1 mW, whereas the corresponding values for
full multi-mode devices range from about 0.5 to 1.5 mA and 2.5 to 5 mW. At 20°C we observe optical transmission at 20
Gb/s through 150 m of OM3 fiber with a bit error ratio better than 10-12, thus demonstrating the great potential of our QD
VCSELs for applications in next-generation short-distance optical data communications and interconnect systems.
Mode-locked lasers (MLLs) and semiconductor optical amplifiers (SOAs) based on quantum dot (QD) gain material will
impact the development of next generation networks like the 100Gb/s Ethernet. Hybrid mode-locked lasers consisting of
a monolithic two section device presently already generate picosecond pulse trains at 40 GHz with an extremely low
jitter in the range of 200 fs under optimum operating conditions. A detailed chirp analysis which is prerequisite for
optical time division multiplexing applications is presented. QD SOAs are showing superior performance for linear
amplification as well as nonlinear signal processing. Wavelength conversion via cross-gain modulation is shown to have
a small signal bandwidth beyond 40 GHz under high bias current injection. This makes QD SOAs much superior to
conventional SOAs.
Presently VCSELs covering a significant spectral range (840-1300 nm) can be produced based on quantum dot (QD)
active elements. Herein we report progress on selected QD based vertical-cavity surface-emitting lasers (VCSELs)
suitable for high-speed operation. An open eye diagram at 20 Gb/s with error-free transmission (a bit-error-rate < 10-15)
is achieved at 850 nm. The 850 nm QD VCSELs also achieve error-free 20 Gb/s single mode transmission operation through multimode fiber without the use of optical isolation. Our 980 nm-range QD VCSELs achieve error free transmission at 25 Gb/s at up to 150°C. These 980 nm devices operate in a temperature range of 25-85°C without current or modulation voltage adjustment. We anticipate that the primary application areas of QD VCSELs are those that require
degradation-robust operation under extremely high current densities. Temperature stability at ultrahigh current densities,
a forte of QDs, is needed for ultrahigh-speed (> 40 Gb/s) current-modulated VCSELs for a new generation of local and storage area networks. Finally we discuss aspects of QD vertical extended-cavity surface emitting lasers with ultra high power density per emitting surface for high power (material processing) and frequency conversion (display) applications.
980 nm VCSELs based on sub-monolayer growth show for 20 Gbit/s large signal modulation clearly open eyes without
adjustment of the driving conditions between 25 and 120 °C. To access the limiting mechanism for the modulation
bandwidth, a temperature dependent small signal analysis is carried out on these devices. Single mode devices are
limited by damping, whereas multimode devices are limited by thermal effects, preventing higher photon densities in the
cavity.
We discuss wavelength stabilized all-epitaxial Tilted Cavity Lasers (TCLs). Optical cavity of a TCL favors propagation of only one tilted optical mode ensuring wavelength-selective operation. The possibility of full control of the thermal shift of the lasing wavelength d λ/dT in TCL
including positive, zero or negative shift, is proved theoretically. Broad-area
(100 μm) 970-nm-range devices have been fabricated showing a high temperature stability of the lasing wavelength
(0.13 nm/K), a high power operation (> 7 W in pulsed mode and > 1.5 W in continuous wave (cw) mode), and a narrow
vertical far-field beam divergence (FWHM ~ 20°). Single transverse mode edge-emitting 4 μm-wide-ridge TCLs
demonstrated high-power spatial and spectral single mode cw operation with a longitudinal side mode suppression ratio
(SMSR) up to 41.3 dB at 93 mW output power. Such a result is similar to the best values achieved for DFB lasers in the
same spectral range, while no etching and overgrowth is used in present case.
Low transparency current density and improved temperature stability with a large characteristic temperature
T0 > 650 K up to 80 °C are demonstrated for 1.3 μm MBE grown InGaAs quantum dot (QD) edge emitting
lasers. Digital modulation with an open eye pattern up to 12 Gb/s at room temperature and bit error rate below
10-12 for 10 Gb/s modulation was realized for this wavelength. Semiconductor optical amplifiers based on
InGaAs QD gain media achieved a chip gain of 26 dB. A conventionally doped semiconductor DBR QD-VCSEL
containing 17 p-modulation doped QD layers demonstrated a cw output power of 1.8 mW and a
differential efficiency of 20 % at 20 °C. The maximum -3dB modulation bandwidth at 25 °C was 3 GHz. First
MOCVD-grown QD-VCSELs with selectively oxidized DBRs and 9 QD-layers were realized, emitting at 1.1
μm. A cw multimode output power of 1.5 mW, 6 mW in pulsed operation, and an cw external efficiency of 45 %
were achieved at 20 °C. The minimum threshold current of a device with 2 μm aperture was 85 μA.
We have studied the modulation properties of a vertical cavity surface-emitting laser (VCSEL) coupled to an
electrooptical modulator. It is shown that, if the modulator is placed in a resonant cavity, the modulation of the light
output power is governed predominantly by electrooptic, or electrorefraction effect rather than by electroabsorption. A
novel concept of electrooptically modulated (EOM) VCSEL based on the stopband edge-tunable distributed Bragg
reflector (DBR) is proposed which allows overcoming the limitations of the first-generation EOM VCSEL based on
resonantly coupled cavities. A new class of electrooptic (EO) media is proposed based on type-II heterostructures, in
which the exciton oscillator strength increases from a zero or a small value at zero bias to a large value at an applied
bias. A EOM VCSEL based on a stopband-edge tunable DBR including a type-II EO medium is to show the most
temperature-robust operation. Modeling of a high-frequency response of a VCSEL light output against large signal
modulation of the mirror transmittance has demonstrated the feasibility to reach 40 Gb/s operation at low bit error rate.
EOM VCSEL showing 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths is
realized.
N. Yu. Gordeev, M. Maximov, Y. Shernyakov, I. Novikov, L. Ya. Karachinsky, V. Shchukin, T. Kettler, K. Posilovic, N. Ledentsov, D. Bimberg, R. Duboc, A. Sharon, D. Arbiv, U. Ben-Ami
Direct laser diodes can typically provide only a limited single mode power, while ultrahigh-brightness is required for
many of the market-relevant applications. Thus, multistage power conversion schemes are applied, when the laser diodes
are used just as a pumping source. In this paper we review the recent advances in ultra-large output aperture edge-emitting
lasers based on the photonic band crystal (PBC) concept. The concept allows near- and far-field engineering
robust to temperature and strain gradients and growth nonuniformities. High-order modes are selectively filtered and the
effective optical confinement of the fundamental mode can be dramatically enhanced. At first, we show that robust ultra-narrow
vertical beam divergence (<5 deg. FWHM) can be achieved simultaneously with ultrahigh differential efficiency
(80-85%) and significant single mode power for several wavelengths of the key regions. A maximum single mode power
of 1.4 W is achieved for 980 nm lasers. At second we extend the PBC concept towards the 2D photonic crystal. A
significant field extension in the vertical direction allows a robust fabrication of the field-coupled lateral multistripe PBC
arrays with a total multistripe width of 0.2 mm. We also demonstrate that the concept of high-order modes filtering
works well also in the lateral direction. Finally, we address possible options for 3D managing of light towards
wavelength stabilized laser operation by processing of the multistripe arrays along their lengths. The concept opens a
way for 3D photonic crystal edge emitting lasers potentially allowing scalable single mode power increase to arbitrary
high levels.
Recent results on GaAs-based high-speed mode-locked quantum dot (QD) lasers and optical amplifiers with an operation
wavelength centered at 1290 nm are reviewed and their complex dependence on device and operating parameters is
discussed on the basis of experimental data obtained with integrated fiber-based QD device modules.
Hybrid and passive mode-locking of QD lasers with repetition frequencies between 5 and 80 GHz, sub-ps pulse widths,
ultra-low timing jitter down to 190 fs, high output peak power beyond 1 W and suppression of Q-switching are reported,
showing the large potential of this class of devices for O-band optical fiber applications.
Results on cw and dynamical characterization of quantum dot semiconductor optical amplifiers are presented. QD
amplifiers exhibit a close-to-ideal noise figure of 4 dB and demonstrate multi-wavelength amplification of three CWDM
wavelengths simultaneously. Modelling of QD polarization dependence shows that it should be possible to achieve
polarization insensitive SOAs using vertically coupled QD stacks. Amplification of ultra-fast 80 GHz optical combs and
bit-error-free data signal amplification at 40 Gb/s with QD SOAs show the potential for their application in future 100
Gb Ethernet networks.
N. Ledentsov, F. Hopfer, A. Mutig, V. Shchukin, A. V. Savel'ev, G. Fiol, M. Kuntz, V. A. Haisler, T. Warming, E. Stock, S. S. Mikhrin, A. R. Kovsh, C. Bornholdt, H. Eisele, M. Dähne, N. D. Zakharov, P. Werner, D. Bimberg
Advanced types of QD media allow an ultrahigh modal gain, avoid temperature depletion and gain saturation effects,
when used in high-speed quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). An anti-guiding VCSEL
design reduces gain depletion and radiative leakage, caused by parasitic whispering gallery VCSEL modes. Temperature
robustness up to 100°C for 0.96 - 1.25 &mgr;m range devices is realized in the continuous wave (cw) regime. An open eye
20 Gb/s operation with bit error rates better than 10-12 has been achieved in a temperature range 25-85°C without current
adjustment. A different approach for ultrahigh-speed operation is based on a combination of the VCSEL section,
operating in the CW mode with an additional section of the device, which is electrooptically modulated under a reverse
bias. The tuning of a resonance wavelength of the second section, caused by the electrooptic effect, affects the
transmission of the system. The second cavity mode, resonant to the VCSEL mode, or the stopband edge of the second
Bragg reflector can be used for intensity modulation. The approach enables ultrahigh speed signal modulation. 60GHz
electrical and ~35GHz optical (limited by the photodetector response) bandwidths are realized.
KEYWORDS: Photonic crystals, Laser crystals, Waveguides, Semiconductor lasers, Refractive index, Crystals, Reflectivity, High power lasers, Gallium arsenide, Near field optics
High concentration of optical power in a narrow exit angle is extremely important for numerous applications of laser diodes, for example, for low-cost fiber pumping and coupling, material processing, direct frequency conversion, etc. Lasers based on the longitudinal photonic band crystal (PBC) concept allow a robust and controllable extension of the fundamental mode over a thick multi-layer waveguide region to achieve a very large vertical optical mode spot size and, consequently, a very narrow vertical beam divergence. Many undesirable effects like beam filamentation, lateral multimode operation and catastrophic optical mirror damage (COMD) are strongly reduced. 650 nm GaInP/GaAlInP PBC lasers show narrow far field pattern (FWHM~7°) stable up to the highest output powers. Differential efficiency up to 85% is demonstrated. Total single mode output power as high as 150 mW is achieved in 4 μm-wide stripes in continuous wave operation, being limited by COMD due to not passivated facets. The lateral far field FWHM is 4 degrees. 840 nm GaAs/GaAlAs PBC lasers show a vertical beam divergence of 8° (FWHM) and a high differential efficiency up to 95% (L=500 μm). A total single mode CW power approaches 500 mW for 1 mm-long 4 μm-wide stripes devices at ~500 mA current, being COMD-limited. The lateral far field FWHM is 5 degrees. Another realization of a longitudinal PBC laser allows lasing in a single high-order vertical mode, a so-called tilted mode, which provides wavelength selectivity and substantially extends the possibility to control the thermal shift of the lasing wavelength. In a multilayer laser structure, where the refractive index of each layer increases upon temperature, it is possible to reach both a red shift of the lasing wavelength for some realizations of the structures, and a blue shift for some others. Most important, the absolute thermal stabilization of the lasing wavelength of a semiconductor laser can be realized.
F. Hopfer, A, Mutig, G. Fiol, M. Kuntz, S. Mikhrin, I. Krestnikov, D. Livshits, A. Kovsh, C. Bornholdt, V. Shchukin, N. Ledentsov, V. Gaysler, N. Zakharov, P. Werner, D. Bimberg
980 nm vertical-cavity surface-emitting laser based on sub-monolayer growth of quantum dots show at 25 and 85°C for 20 Gb/s without current adjustment clearly open eyes and error free operation with bit error rates better than 10-12. For these multimode lasers the small signal modulation bandwidth decreases only from 15 GHz at 25°C to 13 GHz at 85°C. Single mode devices demonstrate at 20°C a small signal modulation bandwidth of 16.6 GHz with 0.8 mW optical output power and a record high modulation current efficiency factor of 19 GHz/mA1/2.
We report on a miniature solid state emitter structure, which allows electrical pumping of only one single InAs quantum
dot (QD) grown in the Stranski-Krastanow mode. The emitter is based on a single layer of low density (~108 cm-2) QDs
grown by Molecular Beam Epitaxy and a submicron AlOX current aperture defined by selective oxidation of high
aluminium content AlGaAs layers. The device demonstrates strongly monochromatic polarized emission of the single
QD exciton at subnanoampere current pumping. No other emission is observed across a spectral range of 500 nm, proving that indeed just one single QD is contributing. Correlation measurements of the emitted photons show a clear
antibunching behavior.
Nanotechnology is a driver for novel opto-electronic devices and systems. Nanosemiconductors like quantum dots allow controlled variation of fundamental electronic and optical properties by changing the size and shape of the nanostructures. This applies directly to self-organized quantum dots which find a versatile use in many kinds of photonic devices.
Wavelength tunability, decreased laser threshold, scalability of gain by stacking quantum dot layers, low linewidth enhancement factor and temperature stability are consequences of three-dimensional carrier confinement in semiconductor quantum dots. Directly modulated lasers using quantum dots offer further advantages like strongly damped relaxation oscillations yielding low patterning effects in digital data transmission. Quantum dot mode-locked lasers feature a broad gain spectrum leading to ultra-short pulses with sub-ps width and a low alpha factor for low-chirp. Thereby, optical comb generators for the future 100G Ethernet are feasible. Semiconductor optical amplifiers based on quantum dots show advantages as compared to classical ones: broad bandwidth due to the inhomogeneous quantum dot size distribution, ultrafast gain recovery for high-speed amplification and small patterning in optical data transmission. We present our most recent results on temperature stable 10 Gb/s, 23°-70°C direct modulation of lasers, ultrafast 80 GHz and short 710 fs optical pulse combs with mode-locked lasers and semiconductor optical amplifiers showing ultrafast amplification of these optical combs as well as error-free 40 Gb/s data modulation, all based on a quantum dot gain medium.
We report on lasers and light emitting diodes based on the longitudinal photonic bandgap crystal (PBC) concept. The PBC design allows achieving a robust and controllable extension of the fundamental mode over a thick multi-layer waveguide region to obtain a very large vertical optical mode spot size and a very narrow vertical beam divergence. An efficient suppression of high order modes can be realized either by the optical confinement factor selection of the fundamental mode, which is localized at the "optical defect" region and has a higher overlap with the gain region. All the other modes spread across the thicker PBC waveguide. In another approach leakage loss selection can be used to suppress excited modes in case of absorbing substrate or the substrate with a higher-refractive index. In this paper we concentrate on growth and performance of high power single mode visible (650 nm) GaInP/AlGaInP PBC lasers, giving a comprehensive example. The devices show narrow far field pattern (full width at half maximum of vertical beam divergence of about 7°), which is stable up to the highest output powers. Differential efficiency up to 85% is demonstrated. Total continuous wave single mode output power as high as 120 mW is achieved in 4 micrometer-wide stripes. Infrared (980 nm) InGaAs/AlGaAs PBC lasers with a beam divergence down to 4.2 degrees and a high temperature stability of the threshold current are also demonstrated.
We have performed a systematic study of structural and optical properties of Quantum dot (QDs) lasers based on InAs/InGaAs quantum dots grown on GaAs substrates emitting in the 1.3 - 1.5 μm range. 1.3 μm range QD lasers are grown using GaAs as matrix material. It is shown that the lasers, grown with large number of QD stacks are metamorphic, with plastic relaxation occurring through the formation of misfit dislocations. Thus, 1.3 μm QD lasers with large number of stacks grown without strain compensation are metamorphic. Another type of defects is related to local dislocated clusters, which are the most dangerous. When proper optimization of the growth conditions is carried out, including a selective thermal etching off of statistically formed dislocated clusters through the defect-reduction technique (DRT), no significant impact of misfit dislocations on the degradation robustness is observed. In uncoated devices a high cw single mode power of ~700 mW is realized limited by thermal roll-over, which is not affected by 500 h ageing at room temperature. At elevated temperatures the main degradation mechanism revealed is catastrophic optical mirror damage (COMD). When the facet are passivated, the devices show the extrapolated operation lifetime in excess of 106 h at 40°C at ~100 mW cw single mode output power. Longer wavelength (1.4 - 1.5 μm) devices are grown on metamorphic (In,Ga,Al)As layers deposited on GaAs substrates. In this case, the plastic relaxation occurs through formation of both misfit and threading dislocations. The latter kill the device performance. Using DRT in this case enables blocking of threading dislocation with growth of QDs in defect-free upper layers. DRT is realized by selective capping of the defect-free areas and high-temperature etching of nano-holes at the non-capped regions near the dislocation. The procedure results in etching of holes and is followed by fast lateral overgrowth with merger of the growth fronts. If the defect does not propagate into the upper layer when the hole is capped, the upper layers become defect-free. Lasers based on this approach exhibited emission wavelength in the 1.4 -1.5 μm range with a differential quantum efficiency of about ~50%. The narrow-stripe lasers operate in a single transverse mode and withstand continuous current density above 20 kA cm-2 without degradation. A maximum continuous-wave output power of 220 mW limited by thermal roll-over is obtained. No beam filamentation was observed up to the highest pumping levels. Narrow stripe devices with as-cleaved facets are tested for 60°C (800 h) and 70°C (200 h) on-chip temperature. No noticeable degradation has been observed at 50 mW cw single mode output power. This shows the possibility of degradation-robust devices on foreign substrates. The technology opens a way for integration of various III-V materials and may target degradation-free lasers on silicon for further convergence of computing and communications.
Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.
Extensive mode-locking investigations are performed in InGaAs/InAs/GaAs quantum dot (QD) lasers. Monolithic mode-locked lasers are fabricated using QD material systems grown by MOCVD and MBE techniques and emitting at 1.1μm and 1.3μm, respectively. The mode-locking performance is evaluated using a variety of laser designs, with various ridge waveguide geometries, cavity and absorber lengths. Passive and hybrid mode-locking are studied and compared in 3.9mm long devices emitting at 1.1μm and operating at a repetition rate of 10GHz. Using 2.1mm long devices emitting at 1.3μm, 18GHz passive mode locking with 10ps Fourier transform limited pulses is demonstrated. This confirms the potential of quantum dot laser for low chirp, short optical pulse generation. Preliminary investigation of the timing jitter of QD passively mode-locked lasers and the behaviour of the QD absorber are also presented. Finally, we report 36GHz passive mode-locking with 6ps optical pulse obtained using 1.1mm long QD lasers emitting at 1.3μm.
We analyse the sensitivity of quantum dot semiconductor lasers to optical feedback. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and strongly damped relaxation oscillations.
The influence of local fields on the excitonic Rabi oscillations
in isolated, arbitrary shaped quantum dot (QD) has been theoretically investigated. Hamiltonian of the system "QD+electromagnetic field" has been obtained. Both QD interaction with classical electromagnetic field and ultrashort optical pulse has been considered. As a result, the bifurcation and anharmonism in the Rabi oscillations in a QD exposed to the monochromatic field have been predicted. The dependence of Rabi oscillations period on the QD depolarization parameter, which characterize local field has been obtained. It has been shown, that for the Gaussian pulse the final state of inversion as a function of peak pulse strength demonstrates step-like transitions.
The molecular beam epitaxy of self-assembled quantum dots (QDs) has reached a level such that the principal advantages of QD lasers can now be fully realized. We overview the most important recent results achieved to date including excellent device performance of 1.3 μm broad area and ridge waveguide lasers (Jth<150A/cm2, Ith=1.4 mA, differential efficiency above 70%, CW 300 mW single lateral mode operation), suppression of non-linearity of QD lasers, which results to improved beam quality, reduced wavelength chirp and sensitivity to optical feedback. Effect of suppression of side wall recombination in QD lasers is also described. These effects give a possibility to further improve and simplify processing and fabrication of laser modules targeting their cost reduction. Recent realization of 2 mW single mode CW operation of QD VCSEL with all-semiconductor DBR is also presented. Long-wavelength QD lasers are promising candidate for mode-locking lasers for optical computer application. Very recently 1.7-ps-wide pulses at repetition rate of 20 GHz were obtained on mode-locked QD lasers with clear indication of possible shortening of pulse width upon processing optimization. First step of unification of laser technology for telecom range with QD-lasers grown on GaAs has been done. Lasing at 1.5 μm is achieved with threshold current density of 0.8 kA/cm2 and pulsed output power 7W.
The dephasing time in semiconductor quantum dots and quantum-dot molecules is measured using a sensitive four-wave mixing heterodyne technique. We find a dephasing time of several hundred picoseconds at low temperature in the ground-state transition of strongly-confined InGaAs quantum dots, approaching the radiative-lifetime limit. Between 7 K and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. On a series of InAs/GaAs quantum-dot molecules having different interdot barrier thicknesses a systematic dependence of the dephasing dynamics on the barrier thickness is observed. The results show how the quantum mechanical coupling of the electronic wavefunctions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.
We analyse the sensitivity of quantum dot semiconductor lasers to optical. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and of strongly damped relaxation oscillations.
Interaction between strongly localized charge carriers in zero-dimensional systems like quantum dots (QD) depends sensitively on the geometrical roperties of the dots. The recently observed monolayer splitting with eight well resolved peaks (in low excitation photoluminescence (PL)) together with eight-band kp theory as the appropriate tool for modeling electronic and optical properties offers direct spectroscopic access to details of the QD morphology. By this achievement it became possible to link single-dot spectra obtained by cathodoluminescence measurements via the exciton transition energy to structural properties of the probed QD. In view of theory this situation constitutes an ideal starting point to study few-particle interactions for realistic InAs QDs as a function of their structural properties. This is done using the configuration interaction method. The wavefunctions are obtained from eight-band kp calculations of single-particle states including explicitly piezoelectric effects in the confinement potential.
Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids.
We have demonstrated for QD-lasers a record low transparency current density of 6A/cm2 per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz.
GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm2.
Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M2, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers.
Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained.
Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 µm, probably up to 1.55 μm.
In this work we present a detailed study of picosecond optical pulse generation using high-repetition rate mode-locked quantum dot lasers. MOCVD-grown quantum dot lasers emitting at 1.1μm and MBE-grown quantum dot lasers emitting at 1.3μm are investigated. Passive mode-locking at 10GHz, 18GHz and 36GHz with pulse widths in the 6-12ps range are reported. Hybrid mode-locking is demonstrated at 10GHz, showing a significant improvement in the RF spectral characteristics when compared with passive mode-locking. A timing jitter of 600fs (2.5MHz to 50MHz) is measured in the 18GHz passively mode-locked laser. Autocorrelation techniques are used to characterise the high repetition rate mode-locked lasers as well as the time-bandwidth product of the optical pulses. Fourier-transform
limited pulses are obtained from passively mode-locked QD lasers.
1.3 μm GaAs-based quantum dot (QD) lasers demonstrate parameters improved over InP-based devices. They exhibit lower threshold current densities and losses, higher differential efficiencies and improved temerature stability. Highspeed operation is demonstrated. Reduced linewidth enhancement factor advantageous for low-chirp operation makes it possible to suppress dramatically filamentation effects destroying lateral far-field pattern. GaAs-based QD 1.3 μm VCSEL with 8 μm oxide aperture wavelength emits up to 1.2 mW CW multimode.
Quantum dot (QD) is one of the most perspective candidates to be used as an active region of temperature-insensitive 1.3-micron GaAs based lasers for optical networks. However, the limited optical gain achievable in QD ground state hindered their practical use. In this work we have demonstrated that using of high number of QDs stacks grown under proper conditions by MBE is an effective way to considerably increase the optical gain of QD lasers. Ridge waveguide laser diodes with width of 2.7 μm and 4.5 μm based on various numbers of QD layers (N=2, 5, 10) were fabricated and studied in this work. Ultra-low threshold current of 1.43 mA was achieved for 2-stack QD. Regime of simultaneous lasing at ground- and excited-states was discovered. This effect was accounted for the finite time of carriers capture to the ground-state in QD. Multi-stack QD structures enabled to maintain continuous work ground-state lasing up to the current density of 10 kA = 100xJth. Enhanced optical gain allowed us to unite very high differential efficiency (>75%) with low threshold current (<100 A/cm2) and characteristic temperature (T0>100K). For example, laser diode of 1-mm cavity length has shown single mode output power of 100mW at operating current of 195 mA and at high operation power demonstrated insensibility to the changes of temperature. The combination of parameters achieved is quite competitive to all technologies currently used for 1.3-micron lasers including traditional InP-based lasers and makes QD gain medium very promising for VCSEL and telecom laser applications.
Facet overheating is considered a potential source for device degradation of diode lasers. We test two different concepts for the reduction of facet temperatures of high-power diode lasers by measuring the facet temperatures by means of Raman spectroscopy. For conventional high-power broad area lasers we demonstrate the reduction of the facet overheating by the introduction of current blocking layers by a factor of 3-4. For another set of devices among them quantum well and quantum-dot lasers with almost the same device design we find a reduction of the overheating by 40 to 60 percent for the dot devices. Thus we qualify two very different but promising technological approaches for increasing device reliability.
We present temperature-dependent measurements of the dephasing time in the ground-state transition of strongly-confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. At low temperature we measure a dephasing time of several hundred picoseconds. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a sharp zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. We also explore the dephasing time beyond the one exciton occupation, by electrically injecting carriers. Electrical injection into the barrier region results in a dominantly pure dephasing of the excitonic ground-state transition. Once the injected carriers have filled the electronic ground state, additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.
Recent results on molecular-beam epitaxy growth of the quantum dot InGaAs/GaAs heterostructures for long-wavelength lasers on GaAs substrates are presented. As a result of optimization of the growth procedure for active region and emitter layers low-threshold current density (45 - 80 A/cm2) long-wavelength (1.27 - 1.3 μm) laser diodes may be fabricated with high reproducibility.
KEYWORDS: Monte Carlo methods, Quantum dots, Thermodynamics, Chemical species, Diffusion, Semiconductors, Process engineering, Nanostructures, Physics, Nanotechnology
We study the heteroepitaxial growth of self-assembled quantum dots in strained semiconductors in the Stranski-Krastanov growth mode using kinetic Monte Carlo simulations. Optimization of growth parameters such as temperature, deposition rate, coverage, and growth interruption time is discussed. In particular, we investigate the crossover between kinetically controlled and thermodynamically limited growth, and thereby resolve the seemingly contradictory temperature dependence of the average dot size.
For memory structures based on optically induced charge in self-organized quantum dots, the concept of wavelength-domain multiplexing in the quantum dot ensemble is an essential prerequisite. The electric properties of quantum dots in various material systems as studied by time-resolved capacitance spectroscopy are summarized, and candidates suitable for future memory applications are discussed. By combining optical excitation and capacitance spectroscopy, direct evidence is obtained for energy-selective hole charge generation and storage in InAs/GaAs quantum dots. A clear dependence of the activation energy of the emitted holes on the energy of the excitation is observed.
We have produced GaAs-based quantum-dot edge-emitting lasers operating at 1.16 μm with record-low transparency current, high output power, and high internal quantum efficiencies. We have also realized GaAs-based quantum-dot lasers emitting at 1.3 μm, both high-power edge emitters and low-power surface emitting VCSELs. We investigated the ultrafast dynamics of quantum-dot semiconductor optical amplifiers. The dephasing time at room temperature of the ground-state transition in semiconductor quantum dots is around 250 fs in an unbiased amplifier, decreasing to below 50 fs when the amplifier is biased to positive net gain. We have further measured gain recovery times in quantum dot amplifiers that are significantly lower than in bulk and quantum-well semiconductor optical amplifiers. This is promising for future demonstration of quantum dot devices with high modulation bandwidth.
The development of 1.3 micron VCSELs is currently considered to give a strong impulse for a wide use of ultra-fast local area networks. In the present work we discuss MBE growth and characteristics of InAs/GaAs quantum dot (QD) lasers, we also give characteristics of 1.3 micron QD VCSELs grown on GaAs and compare them with those of 1.3 micron InGaAsN/GaAs QW VCSELs. Overgrowing the InAs quantum dot array with thin InGaAs layer allows us to achieve 1.3 micron emission. Long stripe lasers showed low threshold current density (<100 A/cm2), high differential efficiency (>50%), and low internal loss (1-2 cm-1). Maximum continuous wave (CW) output power for wide stripe lasers was as high as 2.7 W and 110 mW for single mode devices. Uncoated broad area lasers showed no visible degradation of characteristics during 450 hours (60C, ambient environment). 1.3 micron InGaAsN/GaAs QW VCSELs are characterized by higher optical loss and lower differential efficiency than QD VCSELs. Due to high gain in the active region QW VCSELS demonstrate high output power (1 mW). QW VCSELs show extremely low internal round-trip optical loss (<0.05%), low threshold currents (<2 mA), high differential efficiency (40%) and output power (600 microW).
GaAs and InGaAs (311) surfaces may be spontaneously corrugated with a height and a period controlled by the film composition, strain and polarity of the substrate. Using image-processed high-resolution transmission electron microscopy we found that both GaAs - AlAs interfaces in short-period superlattices (SPSL) grown on (311)A GaAs substrates are corrugated with a height of 1 nm and a lateral periodicity of 3.2 nm. The same lateral periodicity is also revealed for SPSL grown on (311)B surfaces, but the corrugation height and the degree of order are strongly reduced in this case. A strong optical anisotropy (up to 60%) is found in photoluminescence (PL) spectra for SPSLs grown on (311)A surface and not for (311)B-grown SPSLs. We observed a strong mixing between (Gamma) and X states in the conduction band for the SPSLs grown on (311)A surface which allowed realization of bright PL at room temperature at 650 nm. (311)B and (100)GaAs-AlAs SPSLs grown side by side demonstrated only weak long-wavelength PL due to disorder- induced states. (311)A SPSLs demonstrate also a slow carrier relaxation with characteristic LO-phonon scattering times in excess of 10 ps. Corrugated SLs are particularly advantageous for polarization stabilized surface emitting lasers, bright-red AlGaAs lasers and far infrared emitters and detectors.
Inter-sublevel transitions in InGaAs/AlGaAs quantum dots (QDs) in the mid-infrared (MIR) wavelength range are investigated by means of absorption and optically and electrically pumped emission spectroscopy. Charging dependent energy shifts of inter-sublevel transitions observed in calorimetric absorption spectra are attributed to few-particle effects in the QDs. MIR emission from near-infrared QD lasers is observed in the MIR lasing mode below threshold, which is confirmed by a theoretical modelling of such a bipolar lasing device. In contrast, spontaneous MIR emission is recorded for optically pumped Qds.
Continuous wave room-temperature output power of approximately 3 W for edge-emitters and of about 1 mW for vertical-cavity surface-emitting lasers is realized for GaAs-based devices using InAs quantum dots (QDs) operating at 1.3 micrometers . Long operation lifetimes are manifested. The breakthrough became possible due to development of self- organized growth and defect-reduction techniques in QD technology. We show that the basic parameters of QD lasers outperform the parameters of the devices fabricated using competing GaAs-based `quantum well' technologies.
A simple way to generate wavelength tunable ((Delta) (lambda) > 50 nm) semiconductor laser pulses with a width of a few hundred femtoseconds and a timing jitter well below 1 ps is self-seeding of a gain-switched Fabry-Perot laser diode with subsequent chirp compensation and soliton compression. The low timing jitter of the single mode laser pulses allows self-seeding to be used e.g. in high temporal resolution electro-optic sampling systems. Additionally, by controlling the electrical phase delay between two self seeded laser diodes femtosecond pulses with electrically adjustable time delay can be generated.
Theoretical study of threshold characteristics of a quantum dot (QD) laser in the presence of excited-state transitions is given. The effect of microscopic parameters (degeneracy factor and overlap integral for a transition) on the gain is discussed. An analytical equation for the gain spectrum is derived in an explicit form. Transformation of the gain spectrum with the injection current is analyzed. The threshold current density is calculated as a function of the total losses. The conditions for a smooth or step-like change in the lasing wavelength with the losses are formulated. Threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in GaAs matrix are simulated. A small overlap integral for transitions in such QDs (and hence large spontaneous radiative lifetime) is shown to be a main possible reason for a low value of the maximum single-layer modal gain of the respective structure which is deficient to attain lasing at moderately short (several hundreds of micrometers) cavity lengths.
Nanoscale coherent insertions of narrow gap material in a single-crystalline matrix, or Quantum Dot (QD) provide a possibility to extend the basic principles of heterostructure lasers. The idea to use heterostructures with dimensionality lower than two in semiconductor lasers appeared a quarter of a century ago, simultaneously with the proposal of a quantum well laser. However, fabrication of quantum wire- and, particularly, QD heterostructure (QDHS) lasers appeared to be much more difficult. The breakthrough occurred when techniques for self-organized growth of QDs allowed fabrication of dense arrays of uniform in shape and size coherent islands free from undesirable defects. Recently, some key parameters of QD lasers were significantly improved as compared to those for QW devices. High-power operation, record low threshold current densities, strongly reduced chirp and extension of the wavelength range on GaAs substrates up to 1.3 micrometer range were demonstrated. It also became clear that unique properties of QDs may give rise to a new generation of semiconductor lasers, such as far and middle infrared light emitters based on interlevel electron transitions in QDs or single quantum dot vertical-cavity surface-emitting lasers.
We experimentally investigate the modal properties of vertical cavity surface emitting lasers with vertically coupled quantum dot active regions. Etched air-post structures with aluminum-gallium-oxide apertures and aluminum-oxide distributed Bragg reflectors are electrically-pumped below the lasing threshold. The wavelengths of the resonant cavity modes are revealed by room temperature electroluminescence measurements. In concert with our earlier theoretical predictions, we find that the resonant cavity modes blueshift as the radius of the oxide aperture decreases.
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