2-μM pulsed holmium laser for a future CO₂/ H₂o space lidar mission

In the context of greenhouse gases monitoring such as carbon dioxide (CO₂), a powerful emitter in the near infrared (1.5-2 μm) is needed to get a useful precision on concentration measurement (< 1 %) with reasonably space and time resolution. Energy pulses larger than several millijoules at a pulse repetition frequency (PRF) of several hundred of Hertz are usually required. Such requirements call for a solid-state laser configuration at least for a part of it as demanded pulse energy is well beyond current pulsed fiber laser potential performances. These DIAL emitters also call for a specific multiple wavelength emission around the chosen atmospheric gas absorption line, single mode operation, high spectral purity and stability, high pulse energy stability, good beam quality, linear pulse polarization and good overall wall plug efficiency, especially for space Integrated Path Differential Absorption (IPDA) lidar measurements. In this paper we report on the development and the demonstration of a two-wavelength single-frequency Ho:YLF oscillator that was developed in the framework of a ESA contract. This laser is especially suitable for atmospheric carbon dioxide (CO₂) measurement using the R30 CO₂ absorption line at 2050.967 nm. The oscillator consists in a fiber-coupled and freespace solid-state hybrid system and can be used in high-energy middle-rate or moderate-energy high-rate configurations depending the detection scheme of the lidar. The pulse energy and frequency stabilities are specially documented in twofrequency single-mode operations in the context of CO₂ space borne IPDA measurements.