Optimization of Fiber Amplifier Design for Stimulated Raman Scattering Microscopy

Spectral focusing is a rapid, reliable, and simple method for acquiring hyperspectral coherent Raman images using chirped femtosecond lasers. Previous work has demonstrated the use of a parabolic fiber amplifier for Stokes pulse amplification, which increases the bandwidth of Stokes pulses in stimulated Raman scattering (SRS) microscopy with spectral focusing, thereby expanding the spectral range. However, determining the optimal parameters of the fiber amplifier design — such as fiber lengths, input pump power, and peak power of the input Stokes pulse being amplified — has not been explored in depth for this application. In this study, we performed numerical simulations to find the ideal fiber amplifier parameters to address this limitation. We constructed a Stokes pulse fiber amplifier (SPFA) using fiber section lengths that produced the optimal combination of high-power spectral density, spectral bandwidth, chirp rate, and chirp linearity for the amplified Stokes pulse, and verified the results empirically. We demonstrate the high-quality performance of the SRS microscope with the integrated SPFA for imaging biological samples with a spectral range increased to 400 cm^-1 from the typical 200 cm-1 without such amplification. This work provides valuable insights and detailed analysis for those looking to build similar systems, offering important information that has not been thoroughly addressed in previous literature.