Yuk Shan Cheng

Typical degenerate femtosecond optical parametric oscillators adopt an astigmatism-compensating resonator employing few-100 µm, Brewster-cut periodically-poled crystals to minimize intracavity dispersion and maximize the parametric gain bandwidth, but these crystals are fragile and hard to manufacture, especially when multiple gratings are required. Here, we show how the opposite approach—using a mm-thick crystal and considerable intracavity material—can still achieve few-cycle pulses and multi-100-nm spectral bandwidths. Using plane mirrors to eliminate astigmatism, our design balances the normal dispersion of the gain crystal with anomalous dispersion from thick intracavity lenses. By confining poled regions to less than the physical crystal length, we achieve a broad parametric gain bandwidth while allowing the crystal length to become an independent parameter for engineering the intracavity dispersion. We experimentally demonstrate the concept in an MgO:PPLN optical parametric oscillator, synchronously pumped by a 500 MHz Ti:sapphire laser, resulting in 42 fs pulses at 1564 nm with a -20 dB bandwidth of 300 nm and an average output power of 112 mW. The plane-mirror design is compact, robust, stable, easily aligned, and readily adapted to a wide range of repetition rates. Degenerate optical parametric oscillators based on this approach offer a route to simple generation of phase-coherent light when pumped by a femtosecond frequency comb.

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.

The independent manipulation of each line in a laser frequency comb is a powerful resource for emerging applications in optical waveform synthesis, spectroscopy, quantum technology, microwave photonics, astrophotonics, and metrology. Previous demonstrations of line-by-line modulation reported the control of hundreds of comb modes, with bandwidth:resolution ratios of a few thousand. Here, we present a spectral shaper concept achieving precise amplitude control of 10,000 comb modes spanning 580–950 nm (200 THz), with a bandwidth:resolution ratio exceeding 20,000. We demonstrate its application to the dynamic flattening and free spectral range control of an astrocomb, together with fully arbitrary line-by-line grayscale modulation. Our approach is directly extendable to dual amplitude and phase shaping as an enabler for high-repetition-rate optical waveform synthesis and precision spectral shaping across unprecedented bandwidths.