  We discuss the ongoing development of single-mode fiber Fabry-Perot (FFP)
Interferometers as precise astro-photonic calibration sources for high
precision radial velocity (RV) spectrographs. FFPs are simple, inexpensive,
monolithic units that can yield a stable and repeatable output spectrum. An FFP
is a unique alternative to a traditional etalon, as the interferometric cavity
is made of single-mode fiber rather than an air-gap spacer. This design allows
for excellent collimation, high spectral finesse, rigid mechanical stability,
insensitivity to vibrations, and no need for vacuum operation. The device we
have tested is a commercially available product from Micron Optics. Our
development path is targeted towards a calibration source for the
Habitable-Zone Planet Finder (HPF), a near-infrared spectrograph designed to
detect terrestrial-mass planets around low-mass stars, but this reference could
also be used in many existing and planned fiber-fed spectrographs as we
illustrate using the Apache Point Observatory Galactic Evolution Experiment
(APOGEE) instrument. With precise temperature control of the fiber etalon, we
achieve a thermal stability of 100 $\mu$K and associated velocity uncertainty
of 22 cm s$^{-1}$. We achieve a precision of $\approx$2 m s$^{-1}$ in a single
APOGEE fiber over 12 hours using this new photonic reference after removal of
systematic correlations. This high precision (close to the expected
photon-limited floor) is a testament to both the excellent intrinsic wavelength
stability of the fiber interferometer and the stability of the APOGEE
instrument design. Overall instrument velocity precision is 80 cm s$^{-1}$ over
12 hours when averaged over all 300 APOGEE fibers and after removal of known
trends and pressure correlations, implying the fiber etalon is intrinsically
stable to significantly higher precision.
