Intracavity wavelength modulation of an optical parametric oscillator for coherent Raman microscopy.

We present a novel intracavity frequency modulation scheme in a tunable, picosecond optical parametric oscillator (OPO). The OPO signal wavelength can be modulated with a depth of more than 10 nm at a rate of 38 MHz (one half its repetition rate). We discuss the design and construction of the light source and its application to the recently-developed frequency modulation coherent anti-Stokes Raman scattering (FM-CARS) and stimulated Raman scattering (SRS) techniques. The new light source allows for real time subtraction of the interfering background signal in coherent Raman imaging, yielding images with purely chemical contrast.