Kamalesh Dadi

Astrocombs are promising calibration sources for high-resolution astronomical spectrographs, offering stable, uniformly spaced calibration lines whose positions are traceable to an atomic frequency reference. Most spectrograph pixels fall in the gaps between astrocomb lines, remaining uninterrogated by the calibration light; however, it is becoming clear that detector inhomogeneities and intra-order variations in the spectrograph performance make it important to characterize the entire detector area to obtain the best radial-velocity precision. Here, this requirement is fulfilled by the demonstration of an astrocomb architecture in which lines can be swept in any chosen increment in frequency. Based on Fabry-Pérot filtering of a primary comb with a 1 GHz spacing, the astrocomb employs feed-forward locking to overlap a single-frequency laser onto a chosen primary comb line, a process that precisely selects which subset of primary comb modes will be transmitted by the Fabry-Pérot cavity. Operating from 650–1030 nm, and exhibiting continuous performance for >12 hours, the system demonstrates the long-term stability required by a practical astrocomb, while providing the fine sampling of the spectrograph instrument function likely to be necessary for achieving radial-velocity precisions supporting Earth-like exoplanet detection.

The precision, accuracy, and traceability demanded by contemporary exoplanet observations have motivated the astronomy community's adoption of astrocombs, a technology providing thousands of ultra-narrow, drift-free, regularly spaced optical frequencies on a selectable multi-GHz grid. To date, these systems have relied on high-power pump lasers and/or multiple stages of optical amplification to achieve the target performance. Here, we introduce a compact and simple astrocomb concept, in which a single laser diode pumps a 1 GHz Ti:sapphire oscillator, generating a 600 nm to 900 nm supercontinuum that is filtered to produce a 730 nm to 880 nm spectrum with a 15 GHz mode spacing, as a prerequisite for use as an astrocomb. Simple and low-cost astrocombs adopting this approach could be deployed in volume to enable a network of high-resolution spectrographs, needed to achieve sufficient observing time for comprehensive radial-velocity follow-ups of transiting exoplanet surveys.

Controlling cavity dispersion in a continuous-wave (cw) optical parametric oscillator operating in a doubly resonant oscillator (DRO) configuration, pumped by a single-mode laser source, has the potential to yield stable, broadband, multiaxial-mode output generation with flexible spectral coverage. This paper reports, for the first time to the best of our knowledge, the design considerations and theoretical analysis of the role of dispersion compensation on the cavity-mode structure and passive stability in a cw dispersion-compensated DRO (DCDRO) cavity. The study demonstrates that in a cw DCDRO, instabilities, including cluster hops and mode hops, can be eliminated, and spectral tunability can be controlled. A theoretical comparison with the conventional cw DRO (CDRO) emphasizes the unique properties of the analyzed cw DCDRO. The results also indicate that a cw DCDRO at degeneracy can be exploited for optical frequency comb (OFC) generation, offering increased flexibility in frequency comb spacing, practical powers, lower complexity, and lower cost.