Abstract
We demonstrate an optical parametric oscillator (OPO) based on random phase matching in polycrystalline ZnSe. The OPO was pumped by Cr:ZnS laser pulses (2.35 μm, 62 fs, 79 MHz), had a pump threshold of 90 mW, and produced an ultrabroadband spectrum spanning 3–7.5 μm.
Highlights
It has commonly been assumed that perfectly regular crystals are paramount for the operation of frequency converters based on quadratic nonlinearity χ(2), e.g. optical parametric oscillators (OPOs)
In our random phase matching (RPM) OPO approach with ZnSe, we took advantage of several important conditions: 1) For short pump pulses used in the experiment, the length of the nonlinear crystal does not need to be larger than ~1 mm – the temporal walk-off length, set by the group velocity dispersion difference between the pump and the OPO wavelength
The RPM ZnSe ceramic OPO in our experiment was synchronously pumped by a Kerr-lens mode-locked Cr:ZnS laser with a center wavelength of 2.35 μm, a 62-fs pulse duration, 650-mW average power, and a 79-MHz repetition rate
Summary
It has commonly been assumed that perfectly regular crystals are paramount for the operation of frequency converters based on quadratic nonlinearity χ(2), e.g. optical parametric oscillators (OPOs). As our recent results show, a disordered material consisting of randomly-oriented domains, with the nonlinear wave-coupling coefficient arbitrarily varying between its maximum (dNL) and minimum (-dNL) values, can perform well. Our approach is based on the phenomenon known as random phase matching (RPM) [1,2,3,4]. A broadband and flat response in RPM is the result of phase randomization due to arbitrary distribution of the crystalline domains, which eliminates destructive interference. The price to pay, is a slow growth of the output signal since the output intensity in RPM scales linearly with the sample length, as opposed to the quadratic dependence for both perfect phase and quasi-phase matching. In continuous-wave and nanosecond laser regimes, RPM is much less efficient than the conventional quasi-phase matching process [3]. Our finding is that RPM is very well suited for 3-wave processes when short (
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