A homemade 532 nm pulse laser with 1 kHz repetition rate and 9.2 ns pulse duration, which is provided by an intra-cavity frequency-doubling electro-optical Q-switched Nd∶YAG laser, is used as the pump source for the optical parametric oscillator. To improve the peak power density of the 532 nm pump laser, a shrink-beam system is placed before the optical parametric oscillator cavity. Then, the 532 nm laser is used as the pump source of the singly resonant optical parametric oscillator. The optical parametric oscillator cavity is a 33 mm-long linear plane-concave resonator consisting of a plane mirror and a concave output coupler with a transmission of 35% at a signal wavelength of 972 nm.

The plane mirror has an antireflection coating at 532 nm and 1175 nm, and a high-reflection coating at 972 nm. A concave output coupler with a curvature radius of 2000 mm has an anti-reflection coating at 532 nm and 1175 nm and a local reflection coating at 972 nm. Thus, the optical parametric oscillator is singly resonant at 972 nm. Two type- Ⅰ LiB₃O₅ (LBO) crystals with a size of 4 mm×4 mm×12 mm and phase matching cut angles of θ=90° and φ=11.4° are used as the parametric crystals. The frequency-doubling unit is placed behind the optical parametric oscillator. A type- Ⅰ LBO crystal with a size of 4 mm×4 mm×12 mm and phase matching cut angles of θ=90° and φ=17.6° is used for the second harmonic generation from the 972 nm fundamental laser of the singly resonant optical parametric oscillator to the 486 nm blue laser.

Blue-green lasers have been widely used in ocean lidar systems owing to the optical transmission window of seawater. Researchers have found that 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. Comprehensively considering the lidar detection range and signal-to-noise ratio, a blue laser at approximately 488 nm has significant advantages for global ocean exploration. If the working wavelength of a laser detection system is located at 486.1 nm, which is also at the Fraunhofer dark line of the solar spectrum, the signal-to-noise ratio can be further improved, and the working hours can be extended. Generally, 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 pumped using a 355 nm laser. This study presents a novel method for obtaining a 486 nm blue laser using an optical parametric oscillator based on frequency-doubling technology. The optical parametric oscillator is pumped using a homemade 532 nm laser to avoid damage caused by an ultraviolet laser. We hope that this novel method will provide a reliable laser source for ocean laser detection systems.

a homemade 532 nm pump laser, a 972 nm singly resonant optical parametric oscillator, and a frequency-doubling unit from 972 nm to 486 nm.

Under a repetition of 1 kHz, when the pump energy is 3.87 mJ, a 972 nm output laser with a single pulse energy of 0.96 mJ is obtained in the optical parametric oscillator (Fig.5), and the optical to optical conversion efficiency of the optical parametric oscillator is 24.8%, which is close to the theoretical calculation value of 22.3%. The 972 nm pulse energy instability within 60 min is approximately 1.64% (Fig.6), and the pulse width is 7.5 ns with a smooth pulse temporal profile (Fig.7). The laser beam is slightly elliptical, approximately 1.7 mm in diameter, with beam quality factors of Mx2=1.30 and My2=1.22 in two directions (Fig.8). The 486 nm laser with single pulse energy of 49 μJ is obtained by frequency-doubling the 972 nm output laser with a corresponding frequency-doubling efficiency of 5.3%. The pulse duration of the blue laser is 6.9 ns, which is slightly narrower than that of the fundamental laser (Fig.10). The beam quality factors of the blue laser in the two directions are Mx2_{=1.26 and My2=1.15 (Fig.11).}

A 486 nm blue laser frequency doubled by a 972 nm singly resonant optical parametric oscillator pumped using a 532 nm laser is demonstrated. At a repetition rate of 1 kHz, the 972 nm signal laser energy of the singly resonant optical parametric oscillator reaches 0.96 mJ when the 532 nm pump laser energy is 3.87 mJ, with a corresponding conversion efficiency of 24.8%. The maximum energy of the frequency-doubled 486 nm laser is 49 μJ with a pulse width of 6.9 ns, and the corresponding frequency-doubling efficiency is 5.3%. The results show that high-repetition-rate blue laser pulses can be obtained using an optical parametric oscillator pumped by a 532 nm pulsed laser, which can avoid ultraviolet damage caused by the 355 nm laser. It can be used as a laser source for ocean laser LiDAR systems to achieve stable detection.