Ocean carbon sequestration accounts for one-third of total plant carbon sequestration of the world, and particulate organic carbon (POC) is a major form of ocean-based carbon. Accurate detection of POC content can help China achieve the goal of carbon peak and neutrality. Light detection and ranging (LiDAR) active detection is the only known technology that can directly penetrate ocean bodies to realize the detection of marine euphotic zones. When the characteristic parameters of the phase, frequency, amplitude, and polarization of the optical signal are analyzed and the characteristics of the detected target are inverted, the vertical distribution of ocean POC over a long range can be obtained. A 532 nm frequency-doubled blue-green wavelength is typically used in ocean LiDAR systems, as lower attenuation is found at 532 nm in coastal waters. Additional research shows that if the laser wavelength of the LiDAR operates near the Fraunhofer dark lines of the solar spectrum, such as 518.36, 486.13, and 434.05 nm, the interference of solar background noise on the laser ocean detection system can be effectively reduced. It can also improve the signal-to-noise ratio and extend the working period of the LiDAR system. This study proposes a high-repetition-frequency three-wavelength laser system that provides a new technical route for the development of a LiDAR light source for POC hyperspectral detection in oceans.

A multi-wavelength laser system for POC hyperspectral detection LiDAR with a high repetition rate is developed using a frequency-stabilized seed laser (Fig.1) combined with fiber-bulk hybrid amplification and nonlinear frequency conversion technologies (Fig.2). A 1064-nm distributed feedback (DFB) semiconductor laser with a linewidth of ~1 MHz is used as the single-frequency continuous-wave (CW) seeder, and an iodine molecular absorption pool is designed to control the frequency stability. The seeder laser is chopped into nanosecond pulse trains with a repetition rate of 5 kHz using an acousto-optic modulator following continuous fiber amplification; it is then amplified by a double-clad fiber amplifier. The output is collimated and coupled into a solid-state amplification system for further pulse energy scaling. The solid-state amplification system comprises a two-stage Nd∶YVO₄ crystal dual-pass preamplifier and two-stage Nd∶YVO₄ crystal main amplifier. The crystals at each level are end-pumped using a fiber-coupled laser diode (LD). The amplified laser is incident on the nonlinear frequency-conversion module after turning mirror. Type-I LiB₃O₅ (LBO) crystals with a size of 4 mm×4 mm×20 mm and phase matching cut angles of θ=90° and φ=11° are used as second-harmonic-generation crystals. Type-II LBO crystals with a size of 3 mm×3 mm×10 mm and phase matching cut angles of θ=42.7° and φ=90° are used as sum-frequency-generation crystals. Finally, two type-I BBO crystals with a size of 5 mm×5 mm×20 mm and phase matching cut angles of θ=29.6°and φ=90° are used as parametric crystals to produce a 486-nm blue laser.

With a laser wavemeter, the center wavelength of the DFB semiconductor laser is measured to be 1064.49061 nm. After being chopped by an acousto-optic modulator (AOM) and amplified by a fiber pulse preamplifier, the single-pulse energy is approximately 1 μJ. The pulse width is approximately 25.4 ns and the repetition frequency is 5 kHz. When the pump energy of the entire system is approximately 29.3 mJ, the single-pulse energy of the amplified output laser is approximately 6.8 mJ, and the total extraction efficiency reaches 23.2%. At the maximum output energy, the measured amplified laser pulse width is approximately 17 ns (Fig.3). The measured diameter of the near-field spot is approximately 1.2 mm and 1.9 mm in the x and y directions, respectively, which correspond to divergence angles of 2.2 mrad and 2.9 mrad, respectively (Fig.4). The beam quality factors are Mx2=1.03 and My2=1.15. Following frequency doubling by the first LBO crystal, a green laser with a single-pulse energy of 3 mJ is generated. The center wavelength of the green laser is measured to be 532.2448 nm with a spectral linewidth of less than 400 fm, which is less than the resolution limit of the wavemeter. The frequency jitter of the green laser is less than 20 MHz within 30 min (Fig.7). With the type-II phase-matching LBO, a 355-nm ultraviolet pulse laser output with a single-pulse energy of 2.6 mJ is obtained. The remaining 532-nm green laser pulse energy is approximately 0.53 mJ. The ultraviolet pulse laser pumped optical parametric oscillator (OPO) crystal finally achieves a blue laser output with a single-pulse energy greater than 0.7 mJ, corresponding to a conversion efficiency of 26.9%. The laser center wavelength is ~486.1 nm at a linewidth of ~0.16 nm (Fig.8). Table 1 presents the parameters of the successfully developed high-repetition-rate three-wavelength laser.

A multi-wavelength, high-repetition-rate laser based on a fiber-bulk hybrid cascaded amplifier is experimentally investigated as a laser source for POC detection LiDAR. An iodine molecular absorption pool is used to control the frequency stability of the DFB laser. An AOM is used to chop the CW output of the DFB laser into nanosecond pulse trains with a repetition rate of 5 kHz. Fundamental frequency laser output at 1064 nm with a single-pulse energy of approximately 6.8 mJ and pulse width of approximately 17 ns is obtained through fiber solid-state hybrid amplification. Following third-harmonic generation and OPO using nonlinear crystals, a three-wavelength laser beam output is obtained. The corresponding single-pulse energies are 2.24 mJ@1064 nm, 0.53 mJ@532 nm, and 0.7 mJ@486 nm. The center wavelength of the green laser is measured to be 532.2448 nm with a spectral linewidth of less than 400 fm. The results of this study provide a new technical route for the high-spectral-resolution detection of POC using LiDAR systems in seawater, with the advantages of a high repetition rate and narrow linewidth.