LBO crystal for Singly-Resonant Optical Parametric Oscillator Pumped by 532 nm Lasers

Abstract

Blue - green lasers have been widely used in ocean lidar systems owing to the optical transmission window of seawater. The green lasers in the 520 - 580 nm region penetrate deeper into coastal seawater, whereas blue lasers in the 420 - 510 nm region are more suitable for deep clean seawater.

There are two methods to obtain a blue laser with a wavelength of more than 480 nm: one is based on a frequency - quadrupling Tm - doped fiber laser, and the other is based on an optical parametric oscillator (OPO) pumped using a 355 nm laser.

Traditional optical parametric oscillators can be divided into singly - resonant Optical Parametric Oscillator (SROPO) and dual-resonant optical parametric oscillator (DROPO) according to the number of resonant waves.In DROPO, the signal light and idle light in the resonant cavity oscillate back and forth in the cavity at the same time to obtain gain amplification and finally achieve output. In SROPO, the resonant cavity only responds to one light wave frequency of the signal light or idle light, and the light wave of the other frequency is completely transmitted. Only one parametric light oscillates in the cavity and is coupled out.

Extracavity Frequency System

532 nm Laser System Frequency Doubling Unit

A Q-switched Nd:YAG laser with a repetition rate of 1 kHz, when fitted with a type I phase-matched LBO crystal inside its resonant cavity, yields pulsed 532 nm laser output. The LBO crystal has cutting angles of θ = 90° and φ = 11°, with dimensions of 4 × 4 × 12 mm.

In the subsequent optical path of the 532 nm laser, a half-wave plate is inserted to adjust the polarization state of the 532 nm laser pulses. A beam expander system with a focal length ratio of 5:2 is used to compress the diameter of the 532 nm laser beam, enhancing the power density of the pump light entering the OPO crystal. The plane mirror M1 is coated with high-transmittance films at 532 nm and high-reflectance coating at 972 nm. It forms a plano-concave cavity with the curved mirror M2, which has a curvature radius of 2000 mm and is coated with high-transmittance films at 532 nm and partially reflective films at 972 nm (with a reflection rate R=65%). Both cavity mirrors are high-transmittance for the 1175 nm idler wavelength to ensure single-resonant output of the OPO signal light at 972 nm.

The beam splitter BS1 is coated with high-reflectance films at 532 nm and high-transmittance films at 972 nm. The 972 nm signal light pulses output from the OPO via mirror M2 are incident on the LBO frequency-doubling crystal (LBO3) after passing through BS1, resulting in the generation of 486 nm blue light laser pulses. The dimensions of the LBO crystal used for frequency doubling are 4 mm × 4 mm × 12 mm, with a cut angle of θ=90° and φ=17.6°. It has an effective nonlinear coefficient of 0.82 pm/V and a walk-off angle of 10.4 mrad.