Significance Due to their narrow spectral range, single-frequency lasers have the advantage of long coherence length due; thus, such lasers are widely used in coherent detection. Recently, the demand for atmospheric aerosol detection, wind field information, atmospheric gas concentration distribution, and coherent imaging have become urgent. A pulsed single-frequency laser source is significant for coherent laser detection. Single-frequency laser pulses with high output pulse energy are required for long-distance coherent detection.

To avoid the scattering of a single-frequency-pulsed laser in the atmosphere from damaging the human eye, the light source of a lidar system must consider the safety of laser irradiation. According to the International Electrotechnical Commission IEC60825 international application standard and laser safety classification method, 1.4-2.6 μm laser irradiation is less harmful to human eyes than 1.06 μm laser irradiation under the same laser pulse energy. Thus, the 1.4-2.6 μm band is called eye safety band. The eye-safe laser is represented by the 1.6-μm band output generated with an erbium-doped gain medium and the 2-μm band output generated with a holmium-doped gain medium. In recent years, with the development of 1.4-1.5 μm and 1.9-μm laser diodes and resonant pumping technology, Er³⁺ doped single-frequency lasers ~1.6 μm and the Ho³⁺ doped single-frequency lasers ~2 μm have been greatly promoted. The 1.6-μm band locates in the communication band and the atmospheric window. The corresponding devices, such as detectors are more mature and efficient. Therefore, the 1.6 μm band is more suitable for long-distance lidar. One output line of Er∶YAG is ~1.65 μm, and there are characteristic absorption peaks of CH₄ gas; therefore, a single-frequency laser at 1.645 μm can be used for differential absorption detection of methane. Tm³⁺ and Ho³⁺ lasers ~2 μm are also located in the atmospheric window and have higher atmospheric transmittance than 1.6 μm lasers. Additionally, Ho∶YLF single-frequency lasers at a wavelength of 2.05 μm can be used for differential absorption detection of CO₂.

Aiming at the application requirements of lidar for single-frequency lasers, this article reviews the research progress of continuous and pulsed all-solid-state single-frequency lasers in the 1.6- and 2-μm bands.

Progress Typically, continuous-wave (CW) operation of a single-frequency laser is realized by inserting a longitudinal mode selection device in a standing wave resonator or a one-direction ring cavity while pulsed single-frequency lasers are usually obtained via a CW narrow linewidth seed laser injected into the driven laser to achieve amplification and single-frequency-pulsed laser output. In the latter case, power is increased through the main oscillation power amplification (MOPA). In this paper, considered the application requirements of single-frequency lasers in lidar systems, the technical developments of CW and pulsed all-solid-state single-frequency lasers are reviewed, and the output characteristics of single-frequency lasers in the 1.6- and 2-μm bands are compared and analyzed. The technical characteristics of different injection locking methods are discussed, and combined with the application requirements of lidar systems, the future development of eye-safe all-solid-state single-frequency lasers is considered.

Conclusions and Prospects Given the requirements of coherent wind measurement lidar and differential absorption lidar, eye-safe single-frequency lasers have improved rapidly in recent years. CW single-frequency laser technology, pulsed laser technology, resonant pump technology, seed-injection locking technology, and MOPA amplification technology have made significant progress. However, eye-safe single-frequency all-solid-state lasers need to be further studied in terms of energy enhancement of the seed-injection regenerative amplifier, MOPA amplification, pulse width control, new type gain media, and laser structure optimization to further improve the characteristics of all-solid-state single-frequency lasers. Further studies are expected to improve the performance of long-range coherent laser wind measurement and differential absorption lidar, for example in terms of detection length and accuracy.