We present a new architecture for frequency-modulation stimulated Raman Scattering (FM-SRS) that allows broadband background-free acquisitions. The system is based on a femtosecond laser oscillator (Stokes beam), used to pump a narrowband picosecond optical parametric oscillator (pump beam). Using an electro-optic modulator and polarizing beam splitter, we separate the broadband Stokes beam into two intensity-modulated Stokes beams that are spectrally filtered using a high-speed, narrowband acousto-optic tunable filter (AOTF) and a narrowband etalon. The two Stokes sub-beams and the pump beam are then recombined. By detecting the signal as the difference between in-Raman-resonance and off-Raman-resonance wavenumbers, we achieve background-free SRS measurements. Our scheme significantly simplifies and gives flexibility in selecting the pair of wavenumbers for FM-SRS, and allows background-free acquisitions from the fingerprint to the CH-stretch region of the Raman spectrum.
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