We report a simple ad hoc method for designing an aperiodic grating structure to quasi-phase match two arbitrary second-order nonlinear processes simultaneously within the same electric-field-poled crystal. This method also allows the relative strength of the two processes to be adjusted freely, thereby enabling maximization of the overall conversion efficiency. We also report an experiment that is based on an aperiodically poled lithium niobate crystal that was designed by use of our method. In this crystal, parametric oscillation and second-harmonic generation are simultaneously phase matched for upconversion of a femtosecond Ti:sapphire laser to 570 nm. This self-doubling optical parametric oscillator provides an experimental verification of our design method.
In this paper, we model the performance of a device with a simple architecture for high-power mid-wave infrared beam generation at a wavelength of 3.8 microns. The device is a seeded idler efficiency-enhanced optical parametric generator (IEE-OPG) based on an aperiodically poled MgO-doped LiNbO3 (APMgLN) grating pumped by a high-repetition rate nanosecond-pulsed 1064-nm laser and seeded by a low-power 1478-nm distributed feedback diode laser. In the IEE-OPG, two optical parametric amplification (OPA) processes, OPA-1 and OPA-2, are simultaneously phase matched in a single APMgLN grating. The signal at 1478 nm is amplified and the idler at 3800 nm is generated as a result of OPA-1, the signal acts as the pump for OPA-2 and the conversion efficiency of the idler is enhanced as a result of OPA-2. Also, a difference-frequency beam at 2418 nm is generated.We characterized the device performance using a realistic model that takes the diffraction of the beams into account. We designed multiple aperiodic gratings with varying relative strengths of OPA-1 and OPA-2. For various crystal lengths, optimum relative strengths of the two processes and input pump power levels for achieving the maximum mid-wave infrared conversion efficiency and output power are determined.Efficiency-enhanced mid-wave infrared beam generating optical parametric oscillators (OPOs) based on APMgLN gratings were reported before. However, no attempt was made for the optimization of the relative strengths of the simultaneously phase-matched processes in these devices. Our model calculations show that it is possible to reach and exceed the mid-wave infrared conversion efficiencies of these OPOs by correctly choosing the design parameters of the seeded OPGs based on relatively long APMgLN gratings.
In this paper, we design aperiodic gratings based on orientation-patterned gallium arsenide (OP-GaAs) for converting 2.1 μm pump laser radiation into long-wave infrared (8-12 μm) in an idler-efficiency-enhanced scheme. These single OP-GaAs gratings placed in an optical parametric oscillator (OPO) or an optical parametric generator (OPG) can simultaneously phase match two optical parametric amplification (OPA) processes, OPA 1 and OPA 2. We use two design methods that allow simultaneous phase matching of two arbitrary χ(2) processes and also free adjustment of their relative strength. The first aperiodic grating design method (Method 1) relies on generating a grating structure that has domain walls located at the zeros of the summation of two cosine functions, each of which has a spatial frequency that equals one of the phase-mismatch terms of the two processes. Some of the domain walls are discarded considering the minimum domain length that is achievable in the production process. In this paper, we propose a second design method (Method 2) that relies on discretizing the crystal length with sample lengths that are much smaller than the minimum domain length and testing each sample's contribution in such a way that the sign of the nonlinearity maximizes the magnitude sum of the real and imaginary parts of the Fourier transform of the grating function at the relevant phase mismatches. Method 2 produces a similar performance as Method 1 in terms of the maximization of the height of either Fourier peak located at the relevant phase mismatch while allowing an adjustable relative height for the two peaks. To our knowledge, this is the first time that aperiodic OP-GaAs gratings have been proposed for efficient long-wave infrared beam generation based on simultaneous phase matching.
Abstract:We report a nanosecond sum-frequency generating optical parametric oscillator based on a single KTiOAsO 4 crystal that is simultaneously phase matched for optical parametric generation and sum-frequency generation. Pumped at a wavelength of 1064 nm by a Q-switched Nd:YAG laser, this device produces 10.4-ns-long 8.3 mJ red pulses at a wavelength of 627 nm with 21% energy conversion efficiency. This device provides a simple and efficient method for converting high energy Nd:YAG lasers to a red wavelength.
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