Connor Fredrick’s research while affiliated with University of Colorado Boulder and other places

The search for Earth-like exoplanets with the Doppler radial velocity (RV) technique is an extremely challenging and multifaceted precision spectroscopy problem. Currently, one of the limiting instrumental factors in reaching the required long-term 10⁻¹⁰ level of radial velocity precision is the defect-driven subpixel quantum efficiency (QE) variations in the large-format detector arrays used by precision echelle spectrographs. Tunable frequency comb calibration sources that can fully map the point spread function (PSF) across a spectrograph’s entire bandwidth are necessary for quantifying and correcting these detector artifacts. In this work, we demonstrate a combination of laser frequency and mode spacing control that allows full and deterministic tunability of a 30 GHz electro-optic comb together with its filter cavity. After supercontinuum generation, this gives access to any optical frequency across 700–1300 nm. Our specific implementation is intended for the comb deployed at the Habitable-Zone Planet Finder (HPF) spectrograph and its near-infrared Hawaii-2RG array, but the techniques apply to all laser frequency combs (LFCs) used for precision astronomical spectrograph calibration and other applications that require broadband tuning.

Disk-integrated observations of the Sun provide a unique vantage point to explore stellar activity and its effect on measured radial velocities. Here, we report a new approach for disk-integrated solar spectroscopy and evaluate its capabilities for solar radial velocity measurements. Our approach is based on a near-infrared laser heterodyne radiometer (LHR) combined with an optical frequency comb calibration, and we show that this combination enables precision, disk-integrated solar spectroscopy with high spectral resolution (~800,000), high signal-to-noise ratio (~2,600), and absolute frequency accuracy. We use the comb-calibrated LHR to record spectra of the solar Fe I 1565 nm transition over a six-week period. We show that our measurements reach sub-meter-per-second radial velocity precision over a single day, and we use daily measurements of the absolute line center to assess the long-term stability of the comb-calibrated LHR approach. We use this long-duration dataset to quantify the principal uncertainty sources that impact the measured radial velocities, and we discuss future modifications that can further improve this approach in studies of stellar variability and its impact on radial velocity measurements.

The search for earth-like exoplanets with the Doppler radial velocity technique is an extremely challenging and multifaceted precision spectroscopy problem. Currently, one of the limiting instrumental factors in reaching the required long-term $10^{-10}$ level of radial velocity precision is the defect-driven sub-pixel quantum efficiency variations in the large-format detector arrays used by precision echelle spectrographs. Tunable frequency comb calibration sources that can fully map the point spread function across a spectrograph's entire bandwidth are necessary for quantifying and correcting these detector artifacts. In this work, we demonstrate a combination of laser frequency and mode spacing control that allows full and deterministic tunability of a 30 GHz electro-optic comb together with its filter cavity. After supercontinuum generation, this gives access to any optical frequency across 700 - 1300 nm. Our specific implementation is intended for the comb deployed at the Habitable Zone Planet Finder spectrograph and its near-infrared Hawaii-2RG array, but the techniques apply to all laser frequency combs used for precision astronomical spectrograph calibration and other applications that require broadband tuning.

The detection of light of thermal origin is the principal means by which humanity has learned about our world and the cosmos. In optical astronomy, in particular, direct detection of thermal photons and the resolution of their spectra have enabled discoveries of the broadest scope and impact. Such measurements, however, do not capture the phase of the thermal fields--a parameter that has proven crucial to transformative techniques in radio astronomy such as synthetic aperture imaging. Over the last 25 years, tremendous progress has occurred in laser science, notably in the phase-sensitive, broad bandwidth, high resolution, and traceable spectroscopy enabled by the optical frequency comb. In this work, we directly connect the fields of frequency comb laser spectroscopy and passive optical sensing as applied to astronomy, remote sensing, and atmospheric science. We provide fundamental sensitivity analysis of dual-comb correlation spectroscopy (DCCS), whereby broadband thermal light is measured via interferometry with two optical frequency combs. We define and experimentally verify the sensitivity scaling of DCCS at black body temperatures relevant for astrophysical observations. Moreover, we provide comparison with direct detection techniques and more conventional laser heterodyne radiometry. Our work provides the foundation for future exploration of comb-based broadband synthetic aperture hyperspectral imaging across the infrared and optical spectrum.

The introduction of nonlinear nanophotonic devices to the field of optical frequency comb metrology has enabled new opportunities for low-power and chip-integrated clocks, high-precision frequency synthesis and broad-bandwidth spectroscopy. However, most of these advances remain constrained to the near-infrared region of the spectrum, which has restricted the integration of frequency combs with numerous quantum and atomic systems in the ultraviolet and visible ranges. Here we overcome this shortcoming with the introduction of multisegment nanophotonic thin-film lithium niobate waveguides that combine engineered dispersion and chirped quasi-phase matching for efficient supercontinuum generation via the combination of χ⁽²⁾ and χ⁽³⁾ nonlinearities. With only 90 pJ of pulse energy at 1,550 nm, we achieve gap-free frequency comb coverage spanning 330–2,400 nm. The conversion efficiency from the near-infrared pump to the ultraviolet–visible region of 350–550 nm is 17%, and our modelling of optimized poling structures predicts an even higher efficiency. Harmonic generation via the χ⁽²⁾ nonlinearity in the same waveguide directly yields the carrier-envelope offset frequency and a means to verify the comb coherence at wavelengths as short as 350 nm. Our results provide an integrated photonics approach to create visible and ultraviolet frequency combs that will impact precision spectroscopy, quantum information processing and optical clock applications in this important spectral window.

We lock a laser to a solar iron line thereby adapting laboratory spectroscopy techniques to thermal light that has traveled 150 million km. This permits frequency tracking of solar dynamics with Doppler precision approaching 1 ppb.

We quantify the complicated chromatic drift of ~ 5,000 modes across 500 nm of bandwidth of Fabry-Pérot etalons used for radial velocity measurements at the~10 ⁻ ¹⁰ level. We trace the behavior to the dielectric mirrors.

We describe the design and long-term performance of the 30 GHz optical frequency comb used as the primary calibration source of the Habitable Zone Planet Finder (HPF) spectrograph, a system used for one of the most demanding spectroscopic applications in astronomy, the detection of planets beyond our solar system through Doppler spectroscopy.

With optical heterodyne, we measure the spectrum of thermal light using two near-infrared combs with 5-GHz repetition rates. Spectral multiplexing across dense wavelength-division multiplexing channels enables high signal-to-noise ratio.