  Raman scattering spectroscopy is widely used as an analytical technique in
various fields, but its measurement process tends to be slow due to the low
scattering cross-section. In the last decade, various broadband coherent Raman
scattering spectroscopy techniques have been developed to address this
limitation, achieving a measurement rate of about 100 kSpectra/s. Here, we
present a significantly increased measurement rate of 50 MSpectra/s, which is
500 times higher than the previous state-of-the-art, by developing time-stretch
coherent Raman scattering spectroscopy. Our newly-developed system, based on a
mode-locked Yb fiber laser, enables highly-efficient broadband excitation of
molecular vibrations via impulsive stimulated Raman scattering with an
ultrashort femtosecond pulse and sensitive time-stretch detection with a
picosecond probe pulse at a high repetition rate of the laser. As a
proof-of-concept demonstration, we measure broadband coherent Stokes Raman
scattering spectra of organic compounds covering the molecular fingerprint
region from 200 to 1,200 cm-1. This high-speed broadband vibrational
spectroscopy technique holds promise for unprecedented measurements of
sub-microsecond dynamics of irreversible phenomena and extremely
high-throughput measurements.
