Paul M. Belden

Generation of Kerr combs centered at 4.5 μm in crystalline microresonators pumped with quantum-cascade lasers

We report on the generation of mid-infrared Kerr frequency combs in high-finesse CaF2 and MgF2 whispering-gallery-mode resonators pumped with continuous-wave room-temperature quantum cascade lasers. The combs were centered at 4.5 μm, the longest wavelength to date. A frequency comb wider than one half of an octave was demonstrated when approximately 20 mW of pump power was coupled to an MgF2 resonator characterized with quality factor exceeding 10(8).

Multiwatt continuous-wave and Q-switched Er:YAG lasers at 1645 nm: performance issues

The cw and Q-switched performance of Er:YAG lasers operating at 1645 nm were measured. Guided by previous work in the literature, we sought to improve efficiency at low pulse repetition frequencies by decreasing the doping level from 0.5 to 0.25 at.% to reduce upconversion losses. Only a small improvement was obtained with this first-time-tested lower-doped material. Measurements of the fluorescence due to upconversion directly indicated that loss due to this process could not account for the observed power loss at low pulse repetition frequencies. Enhanced green emission during Q-switched operation, resulting from two-photon absorption of 1645 nm intracavity laser light, is reported for what we believe to be the first time. Measurements indicated that the output loss from this process is negligible.

Narrowband Er:YAG nonplanar ring oscillator at 1645 nm

We report 1645 nm narrowband operation of a monolithic Er:YAG nonplanar ring oscillator resonantly pumped at 1532 nm. Unidirectional cw power up to 0.5 W was obtained with a measured linewidth of 21 kHz.

Transient reflectivity as a probe of ultrafast carrier dynamics in semiconductors: A revised model for low-temperature grown GaAs

We revisit pump-probe transient reflectivity (PPTR) as a probe of ultrafast carrier dynamics in photoconductive materials, using low-temperature grown GaAs (LT-GaAs) as an exemplar. The carrier dynamics in a series of annealed LT-GaAs wafers were measured by PPTR. The wafer growth and anneal conditions were tailored to produce a material system with a sub-picosecond carrier lifetime. The PPTR signals from these wafers are bipolar with time constants on the order of 100 fs and 1 ps, consistent with previous literature reports on LT-GaAs. We examined the utility of numerical simulations of the pump-probe transients described in [V. Ortiz et al., J. Appl. Phys. 102, 043515 (2007)] to model our experimental results. We discovered a discrepancy between the models predictions and experiment with respect to the scaling of the PPTR response with injected carrier density, and show that this discrepancy is rooted in how the model accounts for the index of refraction change due to band filling (BF) and band gap renormalization (BGR). We demonstrate that any model that includes a non-negligible BGR effect is inconsistent with our experimental observations of LT-GaAs. We present a revised model to simulate PPTR signals that account for BF and incorporate optical absorption from mid-gap states. This model can reproduce the experimental results on LT-GaAs and enables comparative assessments of alternate trapping and recombination hypotheses. For LT-GaAs, we compared point defects and nanoparticles as sites for Shockley-Read-Hall recombination, with the result that nanoparticle trapping and recombination centers most accurately reproduce the PPTR probe of carrier dynamics in LT-GaAs.

Development of pulsed fiber lasers for long-range remote sensing

Abstract. Pulsed fiber lasers bear great promise as optical transmitters for remote sensors, having tight size, power consumption, and ruggedness constraints. We discuss recently introduced laser architectures and components supporting power scaling of efficient fiber-based laser sources for long-range operation consistent with imaging and/or spectroscopic sensing from high-altitude and space-based platforms.

Nanosecond-Pulsed, mJ-Level Single-Mode Fiber Master Oscillator Power Amplifier with Polarization Maintaining Tapered Gain Fiber

We have demonstrated mJ pulse energy with nanosecond pulses, narrow linewidth, and single-mode beam quality operation in a master oscillator power amplifier (MOPA) fiber laser system using an Yb-doped polarization maintaining tapered amplifier.

Daytime Lidar Characterization of Subvisual Cirrus Layers

A mobile Rayleigh lidar facility was used at two sites in the western United States to detect and characterize subvisual cirrus layers by analyzing the separated polarization components of the signal. The polarized scatter was range gated and used for optical attenuation measurements of visual cirrus, and the depolarized signal was used for sensitive detection of subvisual cirrus. For temporal windows of 1 minute the optical density sensitivity was 0.02, slightly larger than the optical density of subvisual cirrus layers. The optical densities of the subvisual layers were calculated using a backscatter to extinction ratio calculated from visual cirrus layers or by temporal averaging of uniform layers. Using a combination of temporal averaging and a backscatter to extinction ratio improved the optical density sensitivity by seven orders of magnitude.