Derryck T. Reid

Directly diode-pumped Ti:sapphire oscillators can provide a compact source of GHz-rate femtosecond pulses, however their pulse energies are limited by the power, beam quality and wavelength of the available pump diodes. Here, we investigate power and energy scaling of a 785 nm, 1-GHz Kerr-lens-modelocked Ti:sapphire oscillator by using chirped-pulse amplification in a tapered semiconductor optical amplifier. Seed pulses with durations of 114 fs and energies of ≤ 20 pJ were amplified to energies of 650 pJ, corresponding to 650 mW average output power. After amplification, the pulses were recompressed to durations of 347 fs, limited by gain narrowing and nonlinear chirp in the semiconductor optical amplifier, the second of which was identified using a second-harmonic generation frequency-resolved optical gating measurement. Our results show the viability of semiconductor optical amplification of GHz-rate pulses close to the Ti:sapphire gain peak, but also highlight the practical trade-offs between pulse duration and pulse energy, which are relevant to applications like multiphoton microscopy and nonlinear frequency conversion.

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.

Dual-comb distance metrology combines sub-µm precision with absolute distance measurement over meter-scale non-ambiguity ranges. The most common interferometric implementation requires sophisticated approaches to achieve continuous streaming due to the need for digitization at near-gigasample/s rates. Here, we exploit the naturally low data burden of two-photon dual-comb LiDAR to demonstrate continuously-streamed distance metrology at up to 20 kHz, reaching a precision of nearly 1 µm in an averaging time of 10 ms. Using free-running 500 MHz Er,Yb:glass femtosecond lasers, we achieve sampling rates sufficient to demonstrate the capture of a four-minute audio track from the displacement of a loudspeaker-mounted mirror. The ability to perform high-speed yet low-data-burden distance metrology using unstabilized lasers presents exciting opportunities for diverse applications in industrial machine-tool control and calibration.

Quasi-phase-matching in χ ( 2 ) media underpins many nonlinear conversion techniques, yet conventional sinc-squared gain models assume monochromatic pumps and wavelength-independent efficiency, limiting their accuracy for broadband sources. We introduce spectral-crystal efficiency mapping (SCEM), an analytical framework that combines input spectra with the nonlinear properties of a crystal to evaluate conversion potential. SCEM incorporates pump spectral profiles, higher-order interactions, and cascaded processes, providing an upper bound on efficiency that complements more computationally demanding numerical models. The resulting maps agree with experiment and simulation, while offering experimentalists an intuitive tool for assessing nonlinear gain media and guiding optimization of broadband frequency-conversion systems.

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.

Broadband frequency combs with mode spacings of ≥ 1 GHz provide a valuable resource for optical frequency metrology and astrophotonics. Significant average powers are often needed to reach the pulse energies required for supercontinuum generation at 1 GHz repetition rates, putting this beyond the reach of most simple ultrafast lasers. Here, by using dispersion-engineered Si ₃ N ₄ waveguides, we report octave-spanning comb generation from 539 to 1078 nm (−20 dB bandwidth) pumped with a three-element 1 GHz Ti:sapphire laser powered by a single laser diode. Laser repetition-rate stability of 790 mHz is achieved over a 1-hour duration, and carrier-envelope-offset control and stabilization to a single-frequency cw laser is presented. The system offers a simple route to a coherent, broadband supercontinuum spanning the visible to the near-infrared, with potential as an enabling technology for optical frequency metrology, quantum-timekeeping and astrophysical spectrograph calibration.

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.

High-resolution cross-dispersion spectrographs are widely used in spectroscopy, but the two-dimensional format of the spectrum requires sophisticated calibration, conventionally performed by illuminating the instrument with a broadband hollow-cathode lamp and cross-referencing the result to an emission-line atlas. Here, we introduce a new technique to completely calibrate a high-resolution echelle spectrograph using only a laser frequency comb. Selected individual comb lines are removed from a broadband 20 GHz laser frequency comb—revealing their exact location in the spectrograph echellogram—and wavelength-tagged with sub-fm accuracy. In a complementary procedure, the comb is reduced to contain one line per echellogram order, enabling the spectrograph’s free spectral range to be visualized and the exact concatenation between orders to be determined. In this way, the complete calibration of the high-resolution spectrograph is achieved using only a laser frequency comb, directly providing GPS-referenced accuracy without the need to bootstrap the calibration by indexing the comb modes using a broadband atomic-line source.

We demonstrate mid-infrared time-domain optical coherence tomography (OCT) with an orientation-patterned GaP optical parametric oscillator. Instantaneous broadband mid-infrared spectra provide reduced scattering for OCT applications including cultural heritage, quality assurance, and security. B-scan calibrations performed across the wavelength tuning range show depth resolutions of 67 µm at 5.1 µm and 88 µm at 10.5 µm. Volumetric imaging inside a plastic bank card is demonstrated at 5.1 µm, with a 1 Hz A-scan rate that indicates the potential of stable broadband OPO sources to contribute to mid-infrared OCT.