Frequency shifting of pulsed narrow-band laser light in a multipass Raman cell

A multipass cell is described which allows efficient stimulated
Raman frequency shifting for low pump laser intensities and low gas
pressures. The latter is important for Raman shifting of
narrow-band Fourier-transform limited light pulses (Δv=75 MHz). It
is shown that frequency broadening of the Raman shifted light can
be largely avoided in the Dicke narrowing regime at low pressures.
For 75 MHz pump pulses and an H2 density of 2.5 amagat we found a
negligible broadening to 90 MHz of the stimulated Stokes light.
This is far below the value of 250 MHz expected from spontaneous
emission. The narrow-band Stokes pulses achieved in CO2 enabled us
to measure the pressure shift coefficient (-0.71×10-2 cm-1/amagat)
of this gas. It is demonstrated, for the example of benzene, that
our technique provides a very practical light source for high
resolution molecular spectroscopy.