  Optical frequency combs have the potential to improve the precision of the
radial velocity measurement of celestial bodies, leading to breakthroughs in
such fields as exoplanet exploration. For these purposes, the comb must have a
broad spectral coverage in the visible wavelength region, a wide mode spacing
that can be resolved with a high dispersion spectrograph, and sufficient
robustness to operate for long periods even in remote locations. We have
realized a comb system with a 30 GHz mode spacing, 62 % available wavelength
coverage in the visible region, and 40 dB spectral contrast by combining a
robust erbium-doped-fiber-based femtosecond laser, mode filtering with newly
designed optical cavities, and broadband-visible-range comb generation using a
chirped periodically-poled LiNbO3 ridge waveguide. The system durability and
reliability are also promising because of the stable spectrum, which is due to
the use of almost all polarization-maintaining fiber optics, moderate optical
power, and good frequency repeatability obtained with a wavelength-stabilized
laser.
