Tunable FSRS Measurements with Reduced Background Signals: Using a Low-Finesse Etalon Filter to Generate Picosecond Pump Pulses in the 460-650 nm Range

Generating wavelength-tunable picosecond laser pulses from an ultrafast laser source is essential for femtosecond stimulated Raman scattering (FSRS) measurements. Etalon filters produce narrowband (picosecond) pulses with an asymmetric temporal profile that is ideal for stimulated resonance Raman excitation. However, etalon filters with high finesse are typically only useful at the laser fundamental and harmonic frequencies due to very low throughput and limited tunability. Here, we show that a single etalon filter with low finesse (15 cm–1 bandwidth, 172 cm–1 free spectral range) provides an efficient and tunable option for generating Raman pump pulses over a wide range of wavelengths when used in combination with an optical parametric amplifier and a second harmonic generation (SHG) crystal that has an appropriate phase-matching bandwidth. Tuning the SHG wavelength to match individual transmission lines of the etalon filter gives asymmetric picosecond pump pulses over a range of 460-650 nm. The SHG crystal length determines the temporal rise time of the pulse, which is an important property for reducing background and increasing Raman signals compared with symmetric pulses having the same total energy. This approach provides a relatively simple and efficient method to generate optimally shaped picosecond pump pulses for resonance-enhanced FSRS measurements across the visible region of the spectrum.