Noncritical, Narrowband, Intracavity Pumped CdSe OPO

We report on a noncritically phase-matched cascade CdSe optical parametric oscillator with an output energy of 0.54 mJ at 100 Hz, temperature tuning within 9274–9314 nm, and narrow spectrum achieved by a Volume Bragg Grating.

CdSe was among the first nonlinear optical crystals developed for the mid-IR part of the spectrum, subsequently applied mostly for frequency down-conversion, up to 20–24 µm [1]. Its moderate nonlinear coefficient, pm/V, is offset by the relatively long samples available (>40 mm) and one of the lowest residual losses reported ( at 10.6 uµm according to Cleveland Crystals). Operating long crystals is related to limitations originating from the spatial walk-off effect: Fortunately, the hexagonal (6mm), optically positive CdSe, with nonzero effective nonlinearity only for type-II oeo interaction, is phase-matchable at . This is essential at high repetition rates when the single pulse energies and peak intensities are relatively low, and tighter focusing is indispensable [2]. Temperature tuning is limited in and the pump wavelength shall exceed ~ 1.5 µm to avoid two-photon-absorption. Tandem optical parametric oscillators (OPOs) offer more flexibility for this compared to laser pumped OPOs because the noncritical tuning in the mid-IR can be extended by varying the pump wavelength for the CdSe-based second stage. The development of periodically-poled ferroelectric materials such as (PPKTP) opened new opportunities for the first oxide-crystal-based stage of such cascade OPOs, which is noncritically phase-matched, too. However, having in mind the large CdSe length, its narrow pump spectral acceptance bandwidth presents another challenge as type-0 (eee) phase-matching (PM) is used in PPKTP. To circumvent this effect, which is stronger near degeneracy, we employ here a Volume Bragg Grating (VBG) for spectral narrowing the first stage signal wave which is used as a pump for CdSe. In addition, to utilize the higher intracavity pump intensity we place CdSe in a coupled cavity. Intracavity pumping of CdSe has been demonstrated before only in a tunable Cr:ZnSe laser [3] but under critical PM. Here we report on an all-noncritical, cascade CdSe OPO with narrowband output and temperature tuning. Fig. 1.

Schematic of the intracavity pumped, cascade CdSe-OPO.