We report the first demonstration of frequency conversion in an orientation patterned GaAs (OPGaAs) crystal with a fan-out grating configuration. Room temperature frequency doubling of a continuous-wave carbon dioxide laser tuned from 9.26 to 10.65 micrometers was obtained using the large-aperture OPGaAs crystal (>3 mm thick, >7.5 mm wide and 40 mm long) with fan-out grating periods ranging between 212.5 μm and 220 μm.
High efficiency second harmonic generation of a pulsed TEA CO2 laser operating at 9.569 μm was demonstrated in
a quasi-phase-matched GaAs structure, 1.48 mm thick, 39.7 mm long and 8.3 mm wide, and having a grating period
of 219.6 μm. The structure was grown by hydride vapor phase epitaxy and was dual-band anti-reflection coated on
both entrance and exit surfaces. Energy of 1.2 mJ was obtained at 4.78 μm from single pass conversion with incident
energy of 2.56 mJ.
μA 760 μm thick GaAs crystal was grown using HVPE. Transmission spectrum of this sample showed minimal
absorption for light having photon energy below the bandgap energy, indicating the absence of the EL2 defects
commonly found in Bridgman grown samples. Irradiance dependent absorption measured at 1.535 μm using
100 ns duration laser pulses showed increased nonlinear absorption in the HVPE grown GaAs compared to
Bridgman grown samples. The dominant nonlinear absorption process in both samples was absorption due to
free carriers generated by two-photon absorption. The HVPE grown sample showed higher nonlinear absorption
due to longer carrier lifetimes.
Using recently published results of intrinsic and free carrier nonlinear absorption coefficients in InP, nonlinear
refraction was investigated at 1.064 μm using ns duration lasers to characterize refraction from generated free
carriers. A phase retrieval algorithm was implemented to determine the amplitude and phase profiles of the incident
beam. Accurate spatial and temporal profiles of the incident field were used to model nonlinear propagation
through and beyond the sample. With the sample held fixed at focus and the incident energy increased, images
of the transmitted beam a fixed distance away were recorded as a function of irradiance. Excellent agreement
was observed between recorded beam images and those generated from the numerical model.
A new nonlinear optical crystal, CdSiP2, has recently been developed and a bulk crystal cut for phase-matching of second harmonic generation from 4.8 to 2.4 μm has been tested. Numerical and theoretical results are presented. Accounting for Fresnel losses, internal conversion efficiency was over 55% at a peak incident irradiance of 56 MW/cm2.
Irradiance scan measurements of the nonlinear absorption and refraction coefficients of CdTe were performed
at room temperature and at 77 K and compared to the values obtained in previous experiments using a
different technique.
Integral expressions for the pump and generated fields are presented here for the case of second harmonic
generation of a focused Gaussian pump beam incident on a nonlinear crystal . The birefringent walk-off of the
generated beam and the effect of pump depletion are included in the theory.
Recently there has been increasing interest in high quality ceramic laser gain materials, particularly for high-energy lasers, due to the successful application of high-volume advanced ceramics consolidation techniques to transparent oxide gain materials. In this paper, a brief comparison of manufacturing techniques is presented, including an overview of the co-precipitation process and the solid-state reaction process. Merits and risks of each will be presented from a processing viewpoint. Ceramic Nd:YAG in particular shows promise for high power laser design. The program reported here is also compiling a definitive database to compare ceramic and single crystal Nd:YAG materials. Uniform doping levels of up to 9 at% Nd3+ have been reported by Konoshima Chemical Co. in ceramic Nd:YAG, and studied by the US Army Research Laboratory and the US Air Force Research Laboratory. All ceramic Nd:YAG materials studied to date have exhibited similar, if not identical, spectroscopic parameters to those measured for single crystal samples. Thermal properties, laser damage thresholds and refractive indices for a range of temperatures and wavelengths are reported. Diode-pumped free running laser experiment results with highly concentrated (up to 8 at% Nd3+) ceramics and their comparison with our modeling results are presented. High pulse repetition frequency actively (AO) Q-switched laser experiments are in progress. While there are still challenges in the manufacturing of ceramic laser gain materials, and the benefits of the application of ceramic technology to laser material are yet to be fully realized, ceramic Nd:YAG shows promise and could provide new options to the laser design engineer.
The ceramic form of yttrium aluminum garnet (YAG) was studied to determine its suitability for high power lasers. The high Nd3+ doping, the large material size, and the variable doping level with position in the sample, all achievable in ceramics as opposed to single crystal, may lead to higher power solid state lasers than those currently available. We have compared the optical properties of ceramic YAG doped with 0-9 at% Nd3+ to single-crystal, 1 at% Nd:YAG material. Measurements included scattering, thermo-optic behavior, absorption, fluorescence, and laser damage.
Measurements of absorption and emission features showed a small but approximately linear increase in line width with increasing Nd3+ concentration. Nd3+ fluorescence lifetime was rapidly concentration quenched with the 240-μs lifetime for the 1 at% material decreasing to 30 μs for the 9 at% ceramic material. Bulk and surface laser damage thresholds were measured for undoped and 1% Nd-doped ceramic YAG samples using ns-duration laser pulses at 1.064 μm. Both bulk and surface damage threshold values were found to be at least as high as that of single crystals. Measurements of the refractive index and thermo-optic coefficients showed no difference between the single crystal and ceramic materials at 1% Nd3+ doping levels. The scattering in the ceramic material was less than half that of the single crystal.
These results suggest that for most optical characteristics, the ceramic material is equal to and in some cases superior to the single crystal material.
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