Cascaded Orientation Patterned Gallium Arsenide Optical Parametric Oscillator For Improved Longwave Infrared Conversion Efficiency

Optical parametric oscillators (OPOs) utilizing quasi-phase matched materials offer an appealing alternative to direct laser sources. Quasi-phase matched materials provide a useful alternative to traditional birefringent nonlinear optical materials and through material engineering, higher nonlinear coefficients can now be accessed. Orientation patterned gallium arsenide (OPGaAs) is an ideal material because of its broad IR transmission and large nonlinear coefficient. In contrast to ferroelectric materials, such as lithium niobate, where the pattern is fabricated through electric poling, zincblende materials, like OPGaAs, are grown epitaxially with the designed pattern. Generating longwave output from a much shorter pump wavelength, however, is relatively inefficiency due to the large quantum defect when compared to similar devices operating in the 3-5 μm regime. One method to increase pump to idler conversion efficiency is to recycle the undesired and higher energy signal photons into additional idler photons via a second nonlinear stage. An external amplifier stage can be utilized, where the signal and idler from the OPO are sent to a second nonlinear crystal in which the idler is amplified at the expense of the signal. Alternatively, the second crystal can be placed within the original OPO cavity where the signal from the first-stage acts as the pump for the second crystal and the resonant intensity of the signal is higher. Pumping the second crystal within the OPO should lead to higher conversion efficiency into the longwave idler. The grating period needed for the second crystal to use the signal from the first crystal to produce additional idler has the fortuitous advantage that it will not phase match to the original pump wavelength, avoiding unwanted nonlinear interactions. Therefore, a simple linear cavity can be utilized where the pump from the first-stage will simply propagate through the second crystal without undesired results. Without this feature, the pump would need to be coupled out of the cavity before it enters the second crystal. Initial numerical simulations using a custom model, implemented in MATLAB® for the proposed linear, two-stage, cascaded, OPGaAs nanosecond OPO suggest a significant improvement in conversion efficiency over a single-stage device can be obtained. The numerical model includes diffraction, crystal loss, phase mismatch, pump depletion, and back conversion, it assumes monochromatic waves and neglects group velocity dispersion. For a singly resonant oscillator (SRO) pumped by a 2.052 μm Tm:Ho,YLF laser with 45 ns pulse width, the addition of the second crystal in the cavity increases idler generation by a factor of two and exceeds the quantum defect limit. Experimentally, the cascaded OPGaAs OPO demonstrated a ~3% slope efficiency. Limited output may be the result of improper phase matching, given that two distinct idlers wavelengths were observed. Tuning the OPGaAs crystals to generate identical idlers should improve efficiency. The linewidth of the signal serving to pump the second-stage likely reduced efficiency as well. To our knowledge, this is the first cascaded OPO using OPGaAs, and the first cascaded OPO operating in the longwave infrared where the same longwave idler was generated in the both crystals.