In a project with the Canadian Space Agency (CSA), we have developed prototypes of 1.55 μm frequencystabilized lasers for space applications. These lasers can be used as metrology sources for internal calibration of spectrometers such as the Cross-track Infrared Sounder (CrIS). Our prototypes include a 1552 nm DFB laser frequency-locked to H13CN using external phase modulation. The prototypes feature high quality characteristics such as CW output power of 8 mW and a narrow linewidth of 1.5 MHz. The frequency of the laser is known to a few ppm. The frequency stability levels at 10-10 between 30 and 10 000 s. The relative intensity noise (RIN) falls from -100 to -140 dBc/Hz between 1 Hz and 10 kHz, and levels at -140 dBc/Hz between 10 kHz and 1 MHz. Further improvement to reduce the linewidth to a few kHz can be provided using an all-fiber interferometer and correction of the laser injection current accordingly.
Frequency noise reduction of semiconductor lasers using electrical feedback from an optical frequency
discriminator is an efficient and simple approach to realize narrow linewidth lasers. These lasers are of great
interest for applications such as LIDAR, RF photonics and interferometric sensing. In this paper, we review
the technological choices made by TeraXion for the realization of its Narrow Linewidth Laser modules. The
method enables to decrease the linewidth of DFB lasers from several hundreds of kHz to a few kHz. We
present the work in progress to integrate such system into a miniature package and to incorporate advanced
functionalities such as multi-laser phase locking.
In many sensing systems, a highly coherent laser source is necessary to perform sensitive interferometric or coherent
measurements. At TeraXion, we have built a compact laser system that provides a stable laser frequency with a very
narrow linewidth using a 60 mW DFB semiconductor laser. The linewidth reduction system uses a frequency
discriminator to measure the laser frequency noise and provides an electrical feedback to reduce this noise over a given
bandwidth. Experimental work shows that the phase noise of the DFB semiconductor laser can be reduced by more than
4 orders of magnitude from 10 Hz to 100 kHz. We analyzed the effect of the particular frequency noise spectrum of
such a laser on its degree of coherence, its linewidth and the resulting interferometric noise. The laser linewidth
computed from the power spectral density of frequency noise of the laser is reduced from 570 kHz down to an
equivalent of 1.8 kHz when the output signal is observed for 30 ms, and from 370 kHz to 18 Hz for 1 ms. Similarly, the
coherence length is increased from 145 m up to 45 km for fringes observed over 30 ms. Each result is compared with
those obtained with a fiber laser.
We present the architecture and the characterization results of a Master Laser prototype that can be used to distribute a phase-coherent millimeter wave reference within the Atacama Large Millimeter Array. This source is obtained by frequency-locking a 1556-nm narrow linewidth DFB fiber laser to a two-photon transition in rubidium 85 at 778 nm after second harmonic generation in a non-linear waveguide crystal. The prototype yielded an absolute wavelength of 1556.210 843 nm, a stability of 2x10-12 at tau = 1 s, a linewidth of 2 kHz over 1 ms, a coherence of 40% at 50 km over 1 ms, and a RIN below -145 dBc/Hz for f>10 MHz. Using this laser, the transmission of an 18.6 GHz reference over 10 km of fiber was achieved with a residual phase fluctuations lower than 0.22 degrees RMS (33 fs RMS) over 10 s.
Quasi phase-matched second-harmonic generation in the co-propagating geometry
is demonstrated in an asymmetric quantum well waveguide. Modulation
of the nonlinear susceptibility along the waveguide was achieved using a patterned
quantum well intermixing process. Photoluminescence measurements ofthe quanturn
well band edges indicated that a grating of alternating regions of intermixed
and as-grown asymmetric quantum wells was produced for periods between 2 jtm
and 12 jim . The variation of the second harmonic intensity generated by guided
incident light between A=1480 nm and 1600 nm was measured. The resulting
second-harmonic spectra show sharp quasi-phase matching resonances for grating
periods near 3 pm , demonstrating that a periodic modulation of the quantum well
nonlinear susceptibility was produced.
Broadband discrete and continuous tuning of erbium-doped fiber lasers is demonstrated in this paper. First, an exact
multiple of 100 GHz frequency step-tunable L band erbium-doped fiber ring laser is presented. Precise multiple of
100 GHz step-tunability is obtain by proper angle positioning of a 100 GHz Fabry-Perot micro etalon in the laser
cavity. Secondly, continuous tuning over 90 nm is shown in a laser using gain-band switching between C and L band
EDFAs. Lastly, a tunable laser design that uses a single gain medium to obtain 100 nm of tuning in the C and L
bands is demonstrated.
Distributed feedback semiconductor lasers used in optical telecommunication are submitted to an optical injection: various phenomena are experimentally observed, including frequency and progressive phase locking, frequency generation, chaotic behavior, bistability and sensitive detection. The theoretical interpretation is based on the laser transfer function in the frequency domain which is calculated for this kind of laser.
A single-frequency fiber laser for frequency referencing in DWDM communication systems is presented. The frequency selective element is a sampled fiber Bragg grating and the laser can be step-tuned to exact multiples of 100 GHz over 16 lines between 192.0 and 193.5 THz.
This is a progress report on the realization of a compact and transportable frequency standard at 1556 nm based on a two-photon transition in rubidium at 778 nm. These hyperfine transitions present great metrological interest. They have a narrow theoretical linewidth of 150 kHz when observed with a 1556 nm laser, and their absolute frequency is known with an uncertainty of 5.2 X 10-12. In this experiment, we use a high power 1556 nm DFB laser and reduce its linewidth to the 10 kHz level using optical feedback from a confocal cavity. We generate its second harmonic in a periodically poled LiNbO3 crystal and use this signal to injection-lock a Fabry-Perot laser emitting 42 mW at 778 nm. The slave laser is used to observe the Doppler-free two- photon transitions: two counter-propagating beams excite rubidium atoms which emit a blue fluorescence on resonance. This 420 nm light is detected on the side of the Rb cell with a photomultiplier. Such an optical frequency standard at 1556 nm, standing in the multiwavelength telecommunications systems window, becomes an attractive source for absolute frequency calibration of WDM components, optical spectrum analyzers and wavemeters.
We study the spectral profile of a semiconductor laser and the Allan variance according to its frequency noise spectral density, particularly in the case of filtered white noise.
Novel step-tunable single-frequency fiber lasers in ring or linear configurations are presented. The frequency discriminating elements are periodic filters, either sampled fiber Bragg gratings or Fabry-Perot micro-etalons. The lasers can be step-tuned to exact multiples of 100 GHz and they find applications in frequency referencing for DWDM communication systems.
This paper presents a summary of some of the research activities conducted at the Centre d'optique, photonique et laser from Universite Laval that find applications in the field of optical communications. The subjects considered are: absolute frequency control, compensation of dispersion and nonlinear effects within dense wavelength division multiplexed systems, writing of in-fiber Bragg gratings, optical fast frequency-hopped code division multiple access, gain dynamics in Erbium doped fiber amplifiers, short pulsed emission at high repetition rates, dual wavelength operation of semiconductor lasers, applications of Fourier-transform spectrometers and holographic interconnections.
We have undertaken a research directed to the realization of frequency-stabilized lasers for multifrequency optical communications in the 375 THz, 229 THz, and 193 THz (0.8, 1.3, and 1.55 micrometers ) bands. In this paper, we present an overview of our latest results in the 1.55 micrometers band. We compare the performance of optical frequency references based on lasers frequency-locked to acetylene molecules and rubidium atoms. The absolute vales and the frequency stability improvements are discussed. We also present techniques to transfer those performance to multiple frequencies for multifrequency communication systems. We study the use of an absolutely calibrated multimode Fabry-Perot optical resonator with transmission peals set at exact multiples of 100 GHz. We also study the use of a calibrated wavemeter based on a sum-frequency surface emitting multilayered nonlinear crystal to allow the precise tuning at any frequency in the vicinity of an absolute optical frequency standard.
Optical heterodyne has been proposed to generate the microwave signals to be used in phased array antenna systems. Optical injection-locking of secondary lasers to distinct FM-sidebands of a current-modulated laser diode has been used to improve the spectral purity of the microwave signal generated by heterodyne. A very narrow linewidth microwave signal superimposed over a Lorentzian shaped noise floor has been obtained. The spectral purity of the produced microwave signal has been characterized by three distinct means: power spectrum, phase fluctuations, and time-domain frequency stability (Allan variance). The power spectrum shows a linewidth smaller than 25 mHz (FWHM) and the Allan standard deviation gives a level 1 X 10-11 for an averaging time of 1 sec.
We describe a simple technique for aligning optical fibers prior to fusion splicing. The technique relies on the fact that well-cleaved fiber ends have extremely sharp edges. By making the narrow pencil of light emerging from one fiber scan laterally over the entrance face of a second fiber, and by monitoring the light diffracted past its sharp edges, we can locate precisely the geometric center of the output fiber. With this technique, we have aligned fiber cores with a mean lateral offset of 0.81 μm, the major part of this offset caused by the eccentricity of the core relative to the cladding's circular perimeter.
Experimental results on the direct injection-locking of a microwave oscillator by an intensity modulated optical signal are presented. Starting with a free-running oscillator, RF spectrum measurements are done as we vary the modulation frequency of the optical signal impinging on the oscillator. The locking range (as much as 5 MHz) is measured for different modulating signal power levels on the laser diode at the fundamental frequency and at the 2nd, 3rd and 5th subharmonics. Frequency stability measurements are conducted in both time domain (Allan variance) and frequency domain (power spectral density). It shows that the phase noise and the long term stability of the microwave oscillator are greatly improved under locked condition. The Allan standard deviation of the microwave oscillator at (tau) equals 0.1 s improves from 10-6 to 10-10 reaching the precision limit of the measurement setup.
We describe a simple method to produce a clock transition with purely optical means through modulated pumping. We observe the field-independent ground state resonance of 87Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation). We detect the 6.835 GHz resonance with a modulation frequency of 1.139 GHz. A high contrast resonance peak is observed and a condition for zero light shift is found. With a beam radius of approximately 3 mm and a buffer gas (N2) pressure of 680 Pa (5.1 Torr), a minimal linewidth of 290 Hz is observed. A theoretical study of the excitation of hyperfine coherences by a modulated laser beam is given.
Recently we demonstrated a technique to achieve the absolute calibration of a Fabry-Perot resonator used as a multi-frequency discriminator with evenly distributed reference values. These references are used to frequency-lock an ensemble of laser sources to precisely known values. We present in this paper a summary of the procedure that we established and discuss the frequency setting capability and the accuracy of such an optical frequency generator.
We report the progress of our work on the observation and the characterization of 87Rb vapor resonances in order to frequency-lock a 1529 nm DFB semiconductor laser. We present energy levels diagrams corresponding to the 5P3/2 yields 4D5/2 and 4D3/2 transitions of rubidium atoms, respectively at 1529.4 and 1529.3 nm (196.02 and 196.04 THz). Basic emission characteristics of our DFB laser are also given. We then show the absorption profiles of a probe laser emitting around 1529 nm while the 87Rb vapor is optically pumped at 780.2 nm (D2 line at 384.2 THz) using a 30 mW AlGaAs laser. We finally present, for this experiment, the dependence of the probe laser absorption resonance depth and linewidth on the pump power.
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