Raman lasers for trace gas detection (Conference Presentation)

Abstract

The correct identification of the third-order nonlinear optical effect of stimulated Raman scattering (SRS) led in the last years to a versatile method to generate new laser wavelengths resulting from a photon-phonon-interaction. There is the possibility to down- (Stokes) or up-shifting (anti-Stokes) of the pump laser frequency. The size of the frequency shift depends on the Raman-active material and the excitability of their SRS-promoting vibration-modes. Prominent Raman crystals include BaNO3 and other nitrates, KGW and other tungstates, YVO4 and other vanadates as well as diamond. Recently, we observed SRS in the laser crystal LuAlO3 with one SRS-active phonon mode and the natural crystal Spodumene (α-LiAlSi2O6), which has three corresponding SRS-active vibration modes. Selective amplification of one particular spectral line generated through SRS is possible by placing the Raman crystal into a frequency-selective optical resonator, whose optical feedback is selective for only one Stokes- or anti-Stokes component. Raman lasers can be used in many applications, e.g. differential absorption LIDAR systems (DIAL, Light Detection and Ranging) to detect trace gases like carbon dioxide (CO2), ozone (O3) or water vapor (H2O). Various pumping schemes and resonator designs have been investigated focusing on good conversion efficiency, high spatial beam quality and high pulse energy of the output beam. The DIAL technique requires laser sources with high average output power combined with an excellent beam quality (M2 < 2). One possible solution can be found in an effect called beam-cleanup, which takes place by using Raman lasers and amplifiers.