Stimulated Raman spectral microscope using synchronized Er- and Yb-fiber lasers

Abstract

We report our development of a stimulated Raman scattering (SRS) spectral microscope employing synchronized fiber-laser (FL)-based pulse sources, which generate both pump and Stokes pulses. The pump pulses are obtained through the second-harmonic generation of Er-FL pulses. The high-frequency noise in the photocurrent of the pump pulses is successfully suppressed to the shot-noise limit by the collinear balanced detection (CBD) technique using a compact fiber delay line. Fast wavelength-tunable Stokes pulses are generated through the spectral filtering of broadband Yb-FL pulses and post amplification. To achieve a wide wavelength tunability, we introduce a double-pass spectral filter and confirm that the wavelength tunability can be extended to ∼280 cm −1 , while the wavelength tunability with a single-pass spectral filter is ∼190 cm −1 . The spectral filter is also used to compensate for the group delay dispersion (GDD) of Stokes pulses, which is induced by fiber amplifiers. We show that the negative dispersion can be introduced to the pulses simply by shifting the position of the diffraction grating in the spectral filter. Although the wavelength tunability is slightly decreased to 230 cm −1 after the compensation, it is sufficient for covering the CH-stretching vibration region. The SRS imaging and microspectroscopy of polymer beads and HeLa cells are successfully demonstrated.