In recent years, 0.9 μm pulsed lasers have shown great potential in various fields such as laser medicine, precision machining, remote sensing, and communications. The second harmonic of 0.9 μm pulsed lasers falls within the seawater transmission window (450?490 nm), making them particularly suitable for applications in ocean lidar. Notably, the Fraunhofer H-β absorption line in the solar radiation spectrum has a central wavelength of 486.1 nm, which allows lasers operating at the H-β line wavelength to effectively improve the signal-to-noise ratio and enhance the detection accuracy. Currently, the main methods for generating high-power 0.9 μm pulsed lasers include quasi-three-level transitions in Nd-doped lasers, second harmonic generation in Tm-doped lasers, and optical parametric oscillators (OPOs). In this paper, a 972 nm nanosecond pulsed laser based on a four-mirror ring-cavity KTiOAsO? (KTA) crystal OPO is proposed. This laser addresses the issue of insufficient single-pulse energy in existing 972 nm pulsed lasers and is expected to improve the detection accuracy of spaceborne ocean lidar.

The laser system consists of four parts (Fig.1): seed laser, small slab double-pass pre-amplifiers, multi-stage large slab single-pass main-amplifiers, external cavity frequency doubling, and single resonant OPO. The seed laser is a passively Q-switched NPRO Nd∶YAG end-pumped laser with a repetition frequency of 100 Hz. Solid-state amplifiers increase the seed laser energy. Both the pre-amplifiers and main-amplifiers are designed with laser diode side-pumped Nd∶YAG slab crystals with a doping atomic fraction of 1%. High-power 808 nm laser diode arrays (LDAs) with a pump pulse width of 150 μs are used for pumping. A type-I phase matching LiB₃O₅ (LBO) crystal with a size of 10 mm×10 mm×15 mm, cutting angles of θ=90° and φ=11.3°, and an effective non-linear coefficient of d_eff=0.832 pm/V is used for the second harmonic generation from the 1064 nm laser to the 532 nm laser. The front and back surfaces have anti-reflection (AR) coatings for 1064 nm and 532 nm. The OPO uses a four-mirror ring cavity design with a cavity length of 290 mm, in which two KTA crystals with size of 8 mm×8 mm×10 mm are placed in a walk-off-compensated placement. The KTA crystals are cut at angles of θ=90° and φ=27.2°, with AR coatings for 532, 972, and 1176 nm on the two end surfaces of the crystals.

At a repetition rate of 100 Hz, the NPRO laser produces 80 μJ pulses at a wavelength of 1064 nm. After multi-stage solid-state laser amplifications, the single-pulse energy of the 1064 nm pulsed laser increases to 224 mJ, with a near-field spot beam size of 6.01 mm×6.40 mm exhibiting a Gaussian distribution (Fig. 2). The LBO crystal is used for frequency doubling to produce a 532 nm pulsed laser with a single-pulse energy of 128 mJ (Fig. 3) and a pulse width of 6.2 ns (Fig. 4). After the beam reduction, the spot size is 1.93 mm×2.34 mm (Fig. 5) with a central wavelength of 532.2 nm (Fig. 6). The 532 nm pulsed laser is used as the pump source for the OPO. To prevent damage to the KTA crystal, the single-pulse energy of the 532 nm pump laser is limited to 40 mJ. The threshold pump energy of the OPO is 15 mJ. When the pump energy reaches 40 mJ (2.67 times of the threshold), the maximum single-pulse energy of the 972 nm pulsed laser reaches 10.2 mJ (Fig. 7), with a pump-to-signal conversion efficiency of 25.5%. The corresponding pulse width is 5.9 ns (Fig. 8), the central wavelength is 972.3 nm, the spectral linewidth is 0.19 nm (Fig. 9), and the near-field spot size is 2.89 mm×2.81 mm (Fig. 10).

A passively Q-switched Nd∶YAG NPRO laser with a repetition frequency of 100 Hz generates a 1064 nm pulsed seed source with a single-pulse energy of 80 μJ, which is amplified to 224 mJ by multi-stage solid-state laser amplifiers. A 532 nm second-harmonic pulse with a single-pulse energy of 128 mJ is generated through frequency-doubling with an LBO crystal. Additionally, a 40 mJ 532 nm pulsed laser is used to pump the four-mirror ring cavity OPO containing two KTA crystals, resulting in a 10.2 mJ 972 nm pulsed laser with a pulse width of 5.9 ns, a peak power of 1.73 MW, a central wavelength of 972.3 nm, a linewidth of 0.19 nm, and a pump-to-signal conversion efficiency of 25.5%. The experiment demonstrates that using the 532 nm pulsed laser generated by extra-cavity frequency doubling as the pump source for the OPO is an effective method for obtaining a 972 nm pulsed laser with high single-pulse energy and high repetition frequency. This results provide a reference for high-energy 486 nm pulsed laser output through frequency doubling.