Optical parametric oscillation in a random polycrystalline medium

Abstract

We demonstrate an optical parametric oscillator (OPO) based on random phase matching in a polycrystalline χ (2) material, ZnSe. The subharmonic OPO utilized a 1.5-mm-long polished ZnSe ceramic sample placed at the Brewster's angle and was synchronously pumped by a Kerr-lens mode-locked Cr:ZnS laser with a central wavelength of 2.35 µm, a pulse duration of 62 fs, and a repetition frequency of 79 MHz. The OPO had a 90-mW pump threshold, and produced an ultrabroadband spectrum spanning 3-7.5 µm. The observed pump depletion was as high as 79%. The key to success in achieving the OPO action was choosing the average grain size of the ZnSe ceramic to be close to the coherence length (~ 100 µm) for our 3-wave interaction. This is the first OPO that uses random polycrystalline material with quadratic nonlinearity and the first OPO based on ZnSe. Very likely, random phase matching in ZnSe and similar random polycrystalline materials (ZnS, CdS, CdSe, GaP) represents a viable route for generating few-cycle pulses and multi-octave frequency combs, thanks to a very broadband nonlinear response.

Intra-pulse difference-frequency generation of mid-infrared (27–20 μm) by random quasi-phase-matching

“…External cooling of the nonlinear crystals can also be implemented to help reduce potential thermal-induced instabilities. In addition, the DFG signal has a resonant yield when the grain size of the polycrystalline material is close to the coherence length [20,21]. In the case for ZnSe, the grain size (50-70 μm) is still somewhat smaller than the coherence length (∼75 μm) when a 2 μm source is used.…”

“…In the case for ZnSe, the grain size (50-70 μm) is still somewhat smaller than the coherence length (∼75 μm) when a 2 μm source is used. This can be improved through annealing [21] to obtain larger and more suitable grain sizes. Considering the higher damage threshold, lower Δk for DFG, and the average grain size of off-the-shelf polycrystalline elements, pumping at 2 μm is the preferred route over 1 μm pumping for generating MIR through RQPM with ZnS and ZnSe crystals.…”

“…Random QPM (RQPM) [20] is an alternative approach which has recently been demonstrated in an optical parametric oscillator (OPO) [21]. The nonlinear medium is an aggregate of randomly oriented single-crystal grains that are tens of micrometers in size.…”

“…The needed high power has recently been achieved within the cavity of an OPO, which resulted in a broadband MIR continuum spanning from 3 to 7.5 μm [21]. Though successful, an OPO requires the alignment of two cavities at interferometric precision, which increases the complexity of the system.…”

Temporal Simultons in Optical Parametric Oscillators

“…Conversely, synchronously pumped degenerate OPOs have been used successfully to generate half-harmonic combs, but their pulse formation mechanisms are less understood. Key results include the demonstration of instantaneous octave-spanning spectra [18], few-cycle pulses [19], intrinsic phase and frequency locking [20] (which translates the coherence properties of the pump source onto the half-harmonic signal), and conversion efficiencies as high as 64% [21,22]. Recent work suggests a number of competing pulse formation mechanisms exist in such OPOs [22].…”