Laser diode pumped high-energy single-frequency Er:YAG laser with hundreds of nanoseconds pulse duration

Coherent light detection and ranging (Lidar) is an effective tool for remote sensing detection of atmospheric information. For example, coherent wind measurement Lidar can detect three-dimensional wind field information, and differential absorption Lidar can be used for remote sensing of the greenhouse gas. In recent years, 1645 nm high-energy single-frequency pulsed lasers have been widely used in coherent Lidar detection systems. The wavelength at 1645 nm belongs to the eye-safe range, locates in the atmospheric window with high atmospheric transmittance, and covers the absorption peak of methane, which could be used for differential absorption detection of methane gas concentration. At the same time, this wavelength locates in the communication band, so the corresponding devices such as detectors are relatively mature. This makes lasers at wavelength of 1645 nm are widely used as light sources for Lidars.

For long range detection coherent Lidar, a high-energy single-frequency laser with long pulse duration (narrow linewidth) is required. The higher single pulse energy, the longer detection distance could be observed, and pulse duration of hundreds of nanoseconds ensures high coherent detection accuracy and high ranging accuracy. In previous research, limited single-pulse energy and complicated laser system make it difficult to satisfy the requirements of long distance high-precision coherent Lidar. Therefore, the development of a 1645 nm high-energy single frequency laser with hundreds of nanoseconds pulse duration is important for the development of an eye-safe coherent Lidars.

The 1645 nm high-energy single frequency laser with hundreds of nanoseconds pulse duration is introduced by Shanghua Li and Dr. Qing Wang from a research group from the Beijing Institute of Technology in Chinese Optics Letters, Vol. 18, Issue 3, 2020 (Shanghua Li, Qing Wang, Rui Song, Fangfang Hou, Mingwei Gao, Chunqing Gao. Laser diode pumped high-energy single-frequency Er:YAG laser with hundreds of nanoseconds pulse duration[J]. Chinese Optics Letters, 2020, 18(3): 031401). Via a ring cavity configuration with double Er:YAG ceremics pumped by two 1.47 μm laser diodes (LDs), up to 28.6 mJ single-frequency laser pulses with hundreds of nanoseconds pulse duration at 1645 nm under a pulse repetition frequency of 200 Hz are obtained. Pulse energy of 28.6 mJ is the highest single-frequency pulse energy around 1.6μm based on Erbium doped gain mediums, while the pulse duration maintains hundreds of nanoseconds.

A highly stable LD pumped Er:YAG non-planar ring oscillator (NPRO) is used as the seed laser, and it is injected into the slave ring cavity to achieve single-frequency pulsed laser. By comparing effects on pulse energy storage and crystal thermal effect with different pump wavelengths, a resonant pumping scheme with two Er:YAG ceramic gain media end pumped by two wavelengths of 1.47 μm LDs is designed. The pulse energy and beam quality have been significantly improved compared with previous work.

The group leader Professor Gao Chunqing of Beijing Institute of Technology indicates that this kind of eye-safe high-energy single-frequency Er:YAG laser with hundreds of nanoseconds pulse duration is an effective laser source for coherent Lidars. The method has a simple scheme without extra travelling wave amplification stage, and the output is stable and reliable. The future research of the group will focus on the application of single-frequency lasers in coherent wind measurement Lidar and differential absorption Lidar systems.

Characteristics of single-frequency pulse energy and pulse width under different pulse repetition frequency.