Arseny Vasilyev

Precise control of broadband frequency chirps using optoelectronic feedback

We demonstrate the generation of wideband frequency sweeps using a semiconductor laser in an optoelectronic feedback loop. The rate and shape of the optical frequency sweep is locked to and determined by the frequency of a reference electronic signal, leading to an agile, high coherence swept-frequency source for laser ranging and 3-D imaging applications. Using a reference signal of constant frequency, a transform-limited linear sweep of 100 GHz in 1 ms is achieved, and real-time ranging with a spatial resolution of 1.5 mm is demonstrated. Further, arbitrary frequency sweeps can be achieved by tuning the frequency of the input electronic signal. Broadband quadratic and exponential optical frequency sweeps are demonstrated using this technique.

High-coherence semiconductor lasers based on integral high-Q resonators in hybrid Si/III-V platforms.

Significance The data rate of modern optical fiber communication channels is increasingly constrained by the noise inherent in its principal light source: the semiconductor laser (SCL). Here, we examine the phase noise of SCLs due to the spontaneous recombination of excited carriers radiating into the lasing mode as mandated by quantum mechanics. By incorporating a very high-Q optical resonator as an integral part of a hybrid Si/III-V laser cavity, we can remove most of the modal energy from the optically lossy III-V active region, thereby reducing the spontaneous emission rate while increasing the number of phase-stabilizing stored photons. Our fabricated SCLs boast more than a 10× linewidth improvement compared with commercial SCLs, with the possibility of a further major coherence increase. The semiconductor laser (SCL) is the principal light source powering the worldwide optical fiber network. The ever-increasing demand for data is causing the network to migrate to phase-coherent modulation formats, which place strict requirements on the temporal coherence of the light source that no longer can be met by current SCLs. This failure can be traced directly to the canonical laser design, in which photons are both generated and stored in the same, optically lossy, III-V material. This leads to an excessive and large amount of noisy spontaneous emission commingling with the laser mode, thereby degrading its coherence. High losses also decrease the amount of stored optical energy in the laser cavity, magnifying the effect of each individual spontaneous emission event on the phase of the laser field. Here, we propose a new design paradigm for the SCL. The keys to this paradigm are the deliberate removal of stored optical energy from the lossy III-V material by concentrating it in a passive, low-loss material and the incorporation of a very high-Q resonator as an integral (i.e., not externally coupled) part of the laser cavity. We demonstrate an SCL with a spectral linewidth of 18 kHz in the telecom band around 1.55 μm, achieved using a single-mode silicon resonator with Q of 106.

Suppression of stimulated Brillouin scattering in optical fibers using a linearly chirped diode laser

The output of high power fiber amplifiers is typically limited by stimulated Brillouin scattering (SBS). An analysis of SBS with a chirped pump laser indicates that a chirp of 2.5 × 10(15) Hz/s could raise, by an order of magnitude, the SBS threshold of a 20-m fiber. A diode laser with a constant output power and a linear chirp of 5 × 10(15) Hz/s has been previously demonstrated. In a low-power proof-of-concept experiment, the threshold for SBS in a 6-km fiber is increased by a factor of 100 with a chirp of 5 × 10(14) Hz/s. A linear chirp will enable straightforward coherent combination of multiple fiber amplifiers, with electronic compensation of path length differences on the order of 0.2 m.

Multiple source frequency-modulated continuous-wave optical reflectometry: theory and experiment

We propose and demonstrate a novel approach to increase the effective bandwidth of a frequency-modulated continuous-wave (FMCW) ranging system. This is achieved by algorithmically stitching together the swept spectra of separate laser sources. The result is an improvement in the range resolution proportional to the increase in the swept-frequency range. An analysis of this system as well as the outline of the stitching algorithm are presented. Using three distinct swept-frequency optical waveforms, we experimentally demonstrate a threefold improvement in the range resolution of a three-sweep approach over the conventional FMCW method.

Phase-locking and coherent power combining of broadband linearly chirped optical waves.

We propose, analyze and demonstrate the optoelectronic phase-locking of optical waves whose frequencies are chirped continuously and rapidly with time. The optical waves are derived from a common optoelectronic swept-frequency laser based on a semiconductor laser in a negative feedback loop, with a precisely linear frequency chirp of 400 GHz in 2 ms. In contrast to monochromatic waves, a differential delay between two linearly chirped optical waves results in a mutual frequency difference, and an acoustooptic frequency shifter is therefore used to phase-lock the two waves. We demonstrate and characterize homodyne and heterodyne optical phase-locked loops with rapidly chirped waves, and show the ability to precisely control the phase of the chirped optical waveform using a digital electronic oscillator. A loop bandwidth of ~ 60 kHz, and a residual phase error variance of < 0.01 rad(2) between the chirped waves is obtained. Further, we demonstrate the simultaneous phase-locking of two optical paths to a common master waveform, and the ability to electronically control the resultant two-element optical phased array. The results of this work enable coherent power combining of high-power fiber amplifiers-where a rapidly chirping seed laser reduces stimulated Brillouin scattering-and electronic beam steering of chirped optical waves.

Stimulated Brillouin Scattering Suppression With a Chirped Laser Seed: Comparison of Dynamical Model to Experimental Data

A numerical model is developed to simulate stimulated Brillouin scattering (SBS) in high power single-mode fiber amplifiers. The time dependent model incorporates both laser and Stokes wave amplification and initiates the Brillouin scattering from thermal phonons. A frequency chirped laser is used as the seed to suppress SBS. Experiments with Yb-doped fiber amplifiers show good agreement with the modeling. Using experimentally determined parameters, the model is used to predict chirp requirements for multi-kilowatt amplifiers with tens of meters of delivery fiber. A comparison is made between a chirped seed source and random phase modulation for SBS suppression.

Using a linearly chirped seed suppresses SBS in high-power fiber amplifiers, allows coherent combination, and enables long delivery fibers

When seeding a high power fiber amplifier with a frequency-chirped seed, the backward Brillouin scattering can be kept at the spontaneous level because the coherent laser/Stokes interaction is interrupted. Operating a conventional vertical cavity surface-emitting diode laser in an optoelectronic feedback loop can yield a linear frequency chirp of ~1016 Hz/s at a constant output power. The simple and deterministic variation of phase with time preserves temporal coherence, in the sense that it is straightforward to coherently combine multiple amplifiers despite a large length mismatch. The seed bandwidth as seen by the counter-propagating SBS is large, and also increases linearly with fiber length, resulting in a nearly-length-independent SBS threshold. Experimental results at the 600W level will be presented. The impact of a chirped seed on multimode instability is also addressed theoretically.

Phase noise reduction of a semiconductor laser in a composite optical phase-locked loop

The bandwidth and residual phase noise of optical phase- locked loops (OPLLs) using semiconductor lasers are typically con- strained by the nonuniform frequency modulation response of the laser, limiting their usefulness in a number of applications. It is shown in this work that additional feedback control using an optical phase modula- tor improves the coherence between the master and slave lasers in the OPLL by achieving bandwidths determined only by the propagation de- lay in the loop. A phase noise reduction by more than a factor of two is demonstrated in a proof-of-concept experiment using a commercial distributed feedback semiconductor laser. C 2010 Society of Photo-Optical Instru-

Terahertz chirp generation using frequency stitched VCSELs for increased LIDAR resolution

We stitch the frequency chirps of two vertical-cavity surface-emitting lasers in a frequency-modulated imaging experiment at 1550nm. The effective frequency excursion is 1 THz, corresponding to a free-space axial resolution of 150 micrometers.

Sideband locking of a single-section semiconductor distributed-feedback laser in an optical phase-lock loop

The bandwidth and performance of optical phase-lock loops (OPLLs) using single-section semiconductor lasers (SCLs) are severely limited by the nonuniform frequency modulation response of the lasers. It is demonstrated that this restriction is eliminated by the sideband locking of a single-section distributed-feedback SCL to a master laser in a heterodyne OPLL, thus enabling a delay-limited loop bandwidth. The lineshape of the phase-locked SCL output is characterized using a delayed self-heterodyne measurement.