A laser-lock concept to reach cm s^-1-precision in Doppler experiments with Fabry-Pérot wavelength calibrators

Abstract

State-of-the-art Doppler experiments require wavelength calibration with precision at the cm s −1 level. A low-finesse Fabry-Pérot 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 using the 87 Rb D 2 atomic line. A dual-finesse cavity allows drift tracking during observation. In the low-finesse spectral range, the cavity provides a comb transmission spectrum tailored to the astronomical spectrograph. 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 from a Doppler-free transition. Following standard locking schemes, tracking at sub-mm s −1 precision can be achieved. This is several orders of magnitude better than currently planned high-precision Doppler experiments, and it allows freedom for relaxed designs including the use of a single-finesse interferometer under certain conditions. All components for the proposed setup are readily available, rendering this approach particularly interesting for upcoming Doppler experiments. We also show that the large number of interference modes used in an astronomical 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 100 m s −1 over the relevant frequency range and severely limits the accuracy of individual peak locations, although their interference modes are known. 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.

State of the Field: Extreme Precision Radial Velocities

“…However, the systems need to be referenced to LFCs, lamps or atomic features (Bauer et al 2015;McCracken et al 2014;Reiners et al 2014;Schwab et al 2015;Halverson et al 2014). Precise vacuum and temperature controls are generally required to stabilize the devices and it is important to track and correct for drifts in the dispersion.…”

Radial velocity information content of M dwarf spectra in the near-infrared

“…The japanese IRD will use a laser frequency comb approach exclusively (Kotani et al 2014). The development of precise wavelength calibrators is an active line of research, and significant improvement has been made over the past few years (e.g., Wildi et al 2012;Reiners et al 2014).…”

Frequency stability characterization of a broadband fiber Fabry-Pérot interferometer

“…7(a)-(g), we simulate the case in which the FFP cavity length is locked at one wavelength, e.g., to an optical frequency reference as in [23,24,26], with the expectation that frequency stabilizing one mode would stabilize all FFP modes. The precision with which the out-of-loop modes are stabilizedparticularly those far from the stabilizing frequency -remains an outstanding question in the field.…”

“…A passive Fabry-Pérot (FP) etalon has also been suggested as an astronomical wavelength calibration source for high precision Doppler measurements [21][22][23]. When illuminated by a broadband light source, the transmission of an etalon consists of a broad array of comb-like spectral features.…”