Frequency comb spectroscopy with coherent and incoherent light

Optical frequency combs are a unique spectroscopic tool, providing an unparalleled combination of frequency accuracy, high-resolution and broad spectral coverage. The field of frequency comb spectroscopy has grown rapidly to encompass multiple spectroscopic scenarios, ranging from trace gas detection for atmospheric sensing to fundamental spectroscopy in chemistry, physics and biology. In this talk, I will highlight our recent development of frequency comb spectroscopy techniques that apply to both the active detection of coherent laser light, as well as passive detection of incoherent thermal light. In the case of coherent spectroscopy, we have developed extremely broad bandwidth mid-infrared frequency combs that span from 3 microns to nearly 25 microns. Read out of spectral information imprinted on the infrared frequency comb is enabled by electro-optic sampling, where a second comb at a wavelength of 1.5 micron, and a slightly different repetition rate, stroboscopically samples and upconverts the mid-infrared comb to the near-infrared. This approach has the benefit of shot-noise limited detection with robust 1.5 micron detectors that have significantly greater efficiency, and spectral and electrical bandwidth when compared to midinfrared detectors. In a second case, 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. Here we measure solar and atmospheric spectral features with 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. Additionally, we explore the direct heterodyne of incoherent thermal light with the frequency comb itself, thereby bringing 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.