  We report a novel scheme for the generation of dense clusters of Raman
sidebands. The scheme uses a broadband-guiding hollow-core photonic crystal
fiber (HC-PCF) filled with a mixture of H2, D2, and Xe for efficient
interaction between the gas mixture and a green laser pump pulse (532 nm, 1 ns)
of only 5 uJ energy. This results in the generation from noise of more than 135
ro-vibrational Raman sidebands covering the visible spectral region with an
average spacing of only 2 THz. Such a spectrally dense and compact fiber-based
source is ideal for applications where closely spaced narrow-band laser lines
with high spectral power density are required, such as in spectroscopy and
sensing. When the HC-PCF is filled with a H2-D2 mixture the Raman comb spans
the spectral region from the deep UV (280 nm) to the near infrared (1000 nm).
