Coherent anti-Stokes Raman scattering spectroscopy using an optical parametric oscillator

Abstract

A -barium borate optical parametric oscillator (OPO) pumped by the third harmonic of a pulsed Nd:YAG laser was developed and its potentials and limits to carry out coherent anti-Stokes Raman scattering spectroscopy are studied. Spectral properties of the OPO operated in a noncollinear phase-matching configuration are detailed, demonstrating that the OPO bandwidth can be controlled and considerably enlarged from the classical value of 1 nm up to 40 nm. The broadband signal wave emitted in this case fulfils the requirements of the tunable source needed for coherent anti-Stokes Raman scattering experiments in liquids. For gas phase studies, where the Raman linewidths are narrower than the distance separating the OPO cavity modes, scanning CARS is preferable. Injection seeding of the OPO with an external-cavity tunable diode laser reduces the signal wave linewidth to less than 500 MHz. Using this seeded OPO and its continuous tunability over 100 GHz, coherent anti-Stokes Raman scattering spectroscopy experiments are performed and molecular hydrogen Q(5) line profiles at different pressures and temperatures are measured.