Heterodyne detection of coherent Raman signals

摘要

A method and apparatus for heterodyne detection of coherent Raman signals from a Raman active sample exhibiting a Raman-induced Kerr effect. More specifically, a method and apparatus is disclosed for generating a local oscillator output and heterodyning this output with a portion of a probe laser output having its polarization shifted by 90 degrees due to a Raman-induced Kerr effect in a Raman active sample. A probe laser output and a pump laser output are directed into and intersect within the Raman active sample. When their frequency difference is equal to a Raman mode frequency, a non-linear optically induced birefringence in the sample shifts the probe polarization and produces a signal at the output of a polarization analyzer. This signal is heterodyned with a local oscillator output having substantially the same frequency. The local oscillator output is generated either by rotating the polarization analyzer, elliptically polarizing the probe laser output or reflecting a portion of the probe laser output directly into an optical detector. A combining of the local oscillator output and the polarization-shifted probe laser output creates a heterodyned cross-term signal which is linearly proportional to the Raman cross-section of the sample. In addition, an apparatus is disclosed for modulation of the pump laser output which in turn modulates the heterodyned cross-term signal and facilitates its subsequent detection and amplification.