Speaker
Description
We describe the laser frequency comb that is now installed and operating as the primary wavelength calibrator for the Habitable Zone Planet Finder (HPF) spectrograph at the 10 m Hobby-Eberly Telescope. The laser frequency comb, with 30 GHz mode spacing, is built around a combination of electro-optic and integrated-photonic technologies to address the challenges of bandwidth, mode spacing, and robustness. The central tooth of the comb is the 1064 nm continuous wave (CW) light from a semiconductor laser that feeds waveguide electro-optic modulators driven by a 30 GHz microwave source. This results in a comb of approximately 100 teeth spaced exactly by the microwave drive frequency. The CW laser, microwave source, and all other frequencies in the system are referenced to a GPS-disciplined clock that provides absolute traceability to the SI second with fractional uncertainty below 1e-12 at 1 night of averaging. The initial comb is amplified, spectrally broadened and temporally compressed to a pulse width of 70 fs, and then focused into a 25 mm long nonlinear silicon nitride waveguide. The waveguide provides the combination of tight confinement and engineered dispersion for nonlinear spectral expansion of the 30 GHz comb across 700-1600 nm with only 525 mW of incident average power (18 pJ of pulse energy). This low power reduces the thermal loading and aides long-term operability. Static and programmable amplitude filters are used to flatten the spectral envelope across the HPF band of 800-1300 nm. The laser frequency comb has been running autonomously and continuously since May 2018, and we have used it to achieve on-sky stellar RV’s with precision near 1.53 m/s. We have further shown that the intrinsic stability of the HPF and comb support RV precision below 10 cm/s. This same electro-optic frequency comb architecture can be employed for coverage from <600 nm to >2500 nm.
In addition to the 30 GHz LFC, we have built and installed an evacuated and temperature-controlled plane-parallel Fabry-Perot etalon as a supplementary calibrator for the HPF. Extensive pre-deployment testing allowed us to simultaneously track multiple Fabry-Perot resonances between 800 and 1300 nm. While showing slow linear drift and excellent frequency stability of the etalon modes below 1e-9 per day, these laboratory tests revealed chromatic variation in the linear drift not supported by a simple description of the etalon. This study has continued post-deployment, where interleaved comb and etalon exposures at the HPF provide the ability to cross-calibrate and track the drift of each individual mode of the etalon relative to the absolute reference provided by the comb. Such measurements across the full HPF bandpass with precision <1m/s have further confirmed unexpected chromatic structure in the etalon drift. Updated results and our interpretation of the etalon’s chromatic drift properties will be presented.
