High-resolution high-speed tunable grating filter for stimulated Raman spectral imaging

Abstract

Stimulated Raman scattering (SRS) microscopy can visualize molecular vibration with high sensitivity and high contrast, allowing label-free imaging of biological samples. In order to specify molecules, it is important to obtain Raman spectrum at each pixel. High-speed wavelength scanning would allow such spectral imaging. Here, we demonstrate a tunable optical filter for spectral imaging with SRS microscopy. In the filter, an incident beam is reflected by a galvanometer scanner, and then imaged onto a Littrow grating by 4-f relay lenses. The diffracted beam is reflected back to the galvanometer scanner, and then launched into a fiber collimator. The transmission wavelength of this filter can be tuned because the Littrow angle is dependent on the angle of the galvanometer scanner. This configuration allows high spectral resolution of ~0.3 nm and high-speed wavelength scanning with a response time of a few milliseconds. Furthermore, the optical path length is kept constant when the wavelength is scanned. This property is important because SRS microscopy uses two-color laser pulses, which should coincide in time. In the experiment, broadband pulses from a 38-MHz ytterbium fiber laser is filtered out by the optical filter, and then amplified by Yb-doped fiber amplifiers. The wavelength of the amplified pulses is tunable over ~24 cm^-1 and the spectral width of the pulses is < 3.3 cm^-1. These pulses are synchronized with a 76-MHz train of 5-ps pulses generated by a Ti:sapphire laser. By using these two-color pulses, SRS spectral imaging of polymer beads is successfully accomplished.