Abstract
State-of-the-art Doppler experiments require wavelength calibration with precision at the cm/s level. A low-finesse Fabry-Perot interferometer (FPI) can provide a wavelength comb with a very large bandwidth as required for astronomical experiments, but unavoidable spectral drifts are difficult to control. Instead of actively controlling the FPI cavity, we propose to passively stabilize the interferometer and track the time-dependent cavity length drift externally. A dual-finesse cavity allows drift tracking during observation. The drift of the cavity length is monitored in the high-finesse range relative to an external standard: a single narrow transmission peak is locked to an external cavity diode laser and compared to an atomic frequency. Following standard locking schemes, tracking at sub-mm/s precision can be achieved. This is several orders of magnitude better than currently planned high-precision Doppler experiments. It allows freedom for relaxed designs rendering this approach particularly interesting for upcoming Doppler experiments. We also show that the large number of interference modes used in an FPI allows us to unambiguously identify the interference mode of each FPI transmission peak defining its absolute wavelength solution. The accuracy reached in each resonance with the laser concept is then defined by the cavity length that is determined from the one locked peak and by the group velocity dispersion. The latter can vary by several 100m/s over the relevant frequency range and severely limits the accuracy of individual peak locations. A potential way to determine the absolute peak positions is to externally measure the frequency of each individual peak with a laser frequency comb (LFC). Thus, the concept of laser-locked FPIs may be useful for applying the absolute accuracy of an LFC to astronomical spectrographs without the need for an LFC at the observatory.
Highlights
High-precision radial velocity (RV) measurements have become a well established technique for detecting and characterizing planets around other stars, and they are in the focus for precision experiments like determing fundamental constants, measuring the cosmic microwave background temperature, and directly observating the expansion of the Universe (Maiolino et al 2013)
We show in the following that for this exercise, it is not necessary to determine the position of two transmission peaks through ultra-high precision measurements, but that the accuracy provided with standard calibration techniques in the astronomical spectrograph is sufficient
A Fabry-Pérot interferometer operated with a white light lamp can provide a large number of well-defined and uniform transmission peaks that are suitable for frequency calibration in astronomical spectrographs
Summary
High-precision radial velocity (RV) measurements have become a well established technique for detecting and characterizing planets around other stars, and they are in the focus for precision experiments like determing fundamental constants, measuring the cosmic microwave background temperature, and directly observating the expansion of the Universe (Maiolino et al 2013). The light leakage from the partly attenuated modes gets amplified in the final stage of the second harmonic generation, shifting the line center observed by the spectrograph (Schmidt et al 2008) Such technical difficulties are likely to be overcome during the years, but the question when LFC technology becomes a turn-key and affordable solution for astronomical observatories is still open. Our strategy eliminates the need for comparing the FPI signal to gas emission lines on the CCD and provides absolute calibration during observation Such a system has all advantages of the optical frequency comb but can be built entirely from simpler (and cheaper) off-the-shelf technology.
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