Abstract: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 transformlimited 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.

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 10 6 . narrow linewidth | silicon photonics | phase noise | coherent optical communications A lmost from the inception of the semiconductor distributed feedback (DFB) laser, there has been a continuous effort to improve its coherence. The methods used to this end include long cavities (1), longitudinal mode engineering via multiple phaseshifts (2, 3), optimization of the active medium [e.g., strained quantum well (QW)] (4), and wavelength detuning (5, 6). Progress has been hindered by the inevitable penalty paid for the coherencelimiting optical absorption, the result of spatially colocalizing both photons and electrons in a highly absorbing active medium.The finite coherence of laser light is of fundamentally quantum-mechanical origin, the result of spontaneously generated photons entering the lasing mode from the active region of the laser medium. Under the effect of many independent spontaneous emission events, the laser field phasor performs a random walk in the complex plane, which results in a phase excursion given by (7)where N th is the number of excited carriers at threshold, W sp is the spontaneous emission rate ðs −1 Þ into the lasing mode, n is the average number of coherent photons in the lasing mode, α is the linewidth enhancement factor due to coupling of amplitude and phase fluctuations, and τ is the symbol duration (s). The numerator and denominator of Eq. 1 conceptually represent spontaneous photon generation and photon storage, respectively. Increasing the quality factor, Q, of the laser cavity provides a double benefit to phase noise by reducing the number of excited carriers needed to reach threshold, thus decreasing spontaneous photon generation while increasing photon storage.The quality factor of conventional I...

We demonstrate optical manipulation of nanoliter aqueous droplets containing surfactant or lipid molecules and immersed in an organic liquid using near infrared light. The resulting emulsion droplets are manipulated using both the thermocapillary effect and convective fluid motion. Droplet pair-interactions induced in the emulsion upon optical initiation and control provide direct observations of the coalescence steps in intricate detail. Droplet-droplet adhesion (bilayer formation) is observed under several conditions. Selective bilayer rupture is also realized using the same infrared laser. The technique provides a novel approach to study thin film drainage and interface stability in emulsion dynamics. The formation of stable lipid bilayers at the adhesion interface between interacting water droplets can provide an optical platform to build droplet-based lipid bilayer assays. The technique also has relevance for understanding and improving microfluidics applications by devising Petri dish based droplet assays requiring no substrate fabrication.

Abstract: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. ©2012 Optical Society of America

We propose and demonstrate a novel approach to increase the effective bandwidth of a frequencymodulated 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.