  When illuminated by a white light source, the discrete resonances of a
Fabry-Perot interferometer (FP) provide a broad bandwidth, comb-like spectrum
useful for frequency calibration. We report on the design, construction and
laboratory characterization of two planar, passively stabilized, low finesse
(~40) FPs spanning 380 nm to 930 nm and 780 nm to 1300 nm, with nominal free
spectral ranges of 20 GHz and 30 GHz respectively. These instruments are
intended to calibrate astronomical spectrographs in radial velocity searches
for extrasolar planets. By tracking the frequency drift of three
widely-separated resonances in each FP we measure fractional frequency drift
rates as low as 1 x 10^(-10) / day. However we find that the fractional drift
rate varies across the three sample wavelengths, such that the drift of two
given resonance modes disagrees with the ratio of their mode numbers. We
explore possible causes of this behavior, as well as quantify the temperature
and optical power sensitivity of the FPs. Our results demonstrate the
advancement of Fabry-Perot interferometers as robust and frequency-stable
calibrators for astronomical and other broad bandwidth spectroscopy
applications, but also highlight the need for chromatic characterization of
these systems.
