High-average-power mid-infrared ZnGeP_2 optical parametric oscillator with a wavelength-dependent polarization rotator

Abstract

A wavelength-dependent polarization rotator is used to transform the orthogonal polarizations of the signal and idler of a near-degenerate type II KTP optical parametric oscillator (OPO) into a common polarization state. This common polarization allows a single ZnGeP2 OPO to fully utilize the 2 microm signal and idler KTP OPO outputs in a mid-IR conversion. The simple design of the wavelength-dependent polarization rotator yields a compact source that simultaneously generates four mid-JR wavelengths collinearly with a total mid-IR average power of 5.5 W at a >15 kHz pulse repetition rate.

Ho:YAG laser pumped walk-off compensated mid-infrared ZGP optical parametric oscillation

“…ZnGeP 2 (ZGP) crystals have been extensively used for mid-infrared generation through optical parametric oscillations (OPOs) [1][2][3][4][5][6][7][8][9][10][11]. The material has high nonlinear susceptibility (d 36~7 5 pm/V), however, a relatively low laser damage threshold (1-2 J/cm 2 at 2.05 μm) [12,13].…”

“…In order to avoid laser-induced damage in the OPO process, one solution is reducing the OPO pump requirement, which usually can be achieved by using a ZGP pair to compensate the spatial walk-off and double the OPO interaction length. Previously, walk-off compensation (WOC) has been widely used for lower threshold and high beam quality OPOs [5,10,11]. Conventional WOC can be achieved when two identical crystals are aligned with opposite walk-off directions.…”

“…In order to maintain the angular tunability for the OPO output wavelengths, the two single crystals need to be separately mounted on a precision setup that can synchronously tune the incident angles of the two crystals [5,10]. On the other hand, KTP stacks that are bonded by adhesive-free bond (AFB®) and other bonding technologies have been also used for WOC OPOs [11,14]. Such bonded crystal stacks can use more crystal pairs for WOC with negligible interface losses and phase shifts, and they don't require a complicated tuning setup for the OPO operations.…”

Adhesive-free bond quasi-noncritical phase-matched and quasi-phase-matched optical parametric oscillations

“…They are also important pump sources for midinfrared and terahertz (THz) radiations [3][4][5][6][7]. However, the spatial walk-off in critical phase-matched (CPM) OPO's can present a severe problem resulting in high threshold and distorted beam quality.…”

“…As a result, large-scale pump lasers with pulse energy at least above several millijoules are usually required [3,4]. In order to reduce the OPO threshold and increase its efficiency, a common method is using a walk-off compensated (WOC) KTP pair to replace the single KTP crystal, in which oppositely oriented KTP crystals can be arranged separately [5] or bonded together [6,8,9]. Although the output wavelengths of the later one cannot be tuned through the crystal polar angle (θ), it can have the following advantages [9]: 1.…”