Fast spectro-imaging

Nature 502, 355–359 (2013)

Researchers in Germany and France have developed a coherent anti-Stokes Raman spectroscopy system that can perform spectroscopic imaging over a broad spectral bandwidth within a few microseconds. The approach taken by Takuro Ideguchi and co-workers is to use two laser frequency combs with repetition frequencies of f + δf and f to interrogate a sample. A train of pulses in the first frequency comb periodically excites the molecular vibration, which is then probed by the pulses of the second frequency comb with a linearly increasing time delay. The resulting filtered anti-Stokes radiation forms a temporal interferogram, which is recorded by a single silicon photodiode. The thus obtained spectra span Raman shifts from 200 cm⁻¹ to 1,400 cm⁻¹. A resolution of 4 cm⁻¹ was achieved using δf = 100 Hz and 5 Hz and measurement times of 14.8 μs and 295.5 μs, respectively. The researchers say that the use of a single photodetector ensures that the spectra are recorded consistently. However, they admit that the present experiment is limited by the low duty cycle time (the ratio between the time it takes to measure an interferogram and the time before the next interferogram is measured). The scientists say that the approach will enable new applications in nonlinear microscopy and nonlinear spectroscopy.