High-resolution high-speed tunable grating filter for stimulated Raman spectral imaging
2012
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 -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.
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