Thermal-light heterodyne spectroscopy with frequency comb calibration

Abstract

Precision laser spectroscopy is key to many developments in atomic and molecular physics and the advancement of related technologies such as atomic clocks and sensors. However, in important spectroscopic scenarios, such as astronomy and remote sensing, the light is of thermal origin, and interferometric or diffractive spectrometers typically replace laser spectroscopy. In this work, we employ laser-based heterodyne radiometry to measure incoherent light sources in the near-infrared and introduce techniques for absolute frequency calibration with a laser frequency comb. Measuring the solar continuum, we obtain a signal-to-noise ratio that matches the fundamental quantum-limited prediction given by the thermal photon distribution and our system’s efficiency, bandwidth, and averaging time. With resolving power R ∼ 10 6 , we determine the center frequency of an iron line in the solar spectrum to sub-MHz absolute frequency uncertainty in under 10 min, a fractional precision 1/4000 the linewidth. Additionally, we propose concepts that take advantage of refractive beam shaping to decrease the effects of pointing instabilities by 100 × , and of frequency comb multiplexing to increase data acquisition rates and spectral bandwidths by comparable factors. Taken together, our work brings the power of telecommunications photonics and the precision of frequency comb metrology to laser heterodyne radiometry, with implications for solar and astronomical spectroscopy, remote sensing, and precise Doppler velocimetry.

2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments

“…An LFC referenced to an atomic clock and the SI second has become an essential wavelength calibration tool. As the LO in a heterodyne radiometer [413], they could provide higher resolution Doppler maps of the sun than those produced by NASA's Solar Dynamics Observatory [414]. But in the astronomy community, LFCs have found their greatest application in ground-based Precision Radial Velocity (PRV) measurements, and when specialized for this role, are dubbed 'astrocombs' [28,29,415].…”

Design of an all hollow fiber‐coupled middle infrared laser heterodyne radiometer (LHR)

“…2,3 Compared with current FTS, LHR offers merits: high sensitivity, high spectral/spatial resolution, and cost-effective compact instrument size. [2][3][4][5][6][7][8] In the near-infrared (IR), LHR has developed rapidly in recent years. 9 Wilson et al, [10][11][12] Rodin et al, 13,14 and Wang et al 15 developed LHRs applying distributed feedback (DFB) lasers as local oscillators (LOs) for measuring CO 2 and CH 4 in the atmospheric column.…”

“…Ground‐based passive laser heterodyne radiometer (LHR), similar to the Fourier‐transform spectrometer (FTS), 1 uses the Sun as a signal source to carry out long‐term, long‐distance, and remote sensing of atmospheric gases 2,3 . Compared with current FTS, LHR offers merits: high sensitivity, high spectral/spatial resolution, and cost‐effective compact instrument size 2–8 …”