Detailed investigation of anomalous modal behavior in fabricated bottom-emitting intra-cavity contacted 960 nm range
vertical cavity surface emitting lasers (VCSELs) have been performed. At low currents the broad-aperture VCSELs show
multi-mode operation at 945 nm via whispering gallery-like modes. Subsequent increase of pump current results in rapid
increase of fundamental mode intensity and switching to a pure single transverse mode lasing regime at 960 nm with the
higher slope efficiency. As a result record single transverse mode output power of 15 mW with a side-mode-suppressionratio
(SMSR) above 30 dB was achieved. The observed phenomena cannot be explained by oxide-index guiding or
changes in current pumping. 2D heat transport simulations show a strong temperature gradient inside the microcavity
due to an effective lateral heat-sinking. This creates an effective waveguide and results in lower optical losses for the
fundamental mode. At fixed pump current in pulsed regime (pulse width < 400 ns) high-order modes dominate, however
the subsequent increase of pulse width leads to a rapid rise of optical power for the fundamental mode and SMSR
increasing. Thus the self-heating phenomena play a crucial role in observed VCSEL unusual modal behavior.
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.
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.
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.
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.
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.
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