On-the-fly precision spectroscopy with a dual-modulated tunable diode laser and Hz-level referencing to a cavity

Advances in laser spectroscopy have enabled many scientific breakthroughs in physics,chemistry,biology,and astronomy.Optical frequency combs pushed measurement limits with ultrahigh-frequency accuracy and fast-measurement speed,while tunable-diode-laser spectroscopy is used in scenarios that require high power and continuous spectral coverage.Despite these advantages of tunable-diode-laser spectroscopy,it is challenging to precisely determine the instantaneous laser frequency because of fluctuations in the scan speed.Here,we demonstrate a simple spectroscopy scheme with a frequencymodulated diode laser that references the laser on-the-fly to a fiber cavity.The fiber cavity's free spectral range is on-the-fly calibrated with sub-10-Hz frequency precision.We achieve a relative precision of the laser frequency of 2×10^(-8)for an 11-THz frequency range at a measurement speed of 1 THz∕s.This is an improvement of more than 2 orders of magnitude compared to existing diode-laser-spectroscopy methods.Our scheme provides precise frequency calibration markers,while simultaneously tracking the instantaneous scan speed of the laser.We demonstrate the versatility of our method through various applications,including dispersion measurement of a fiber,ultrahigh-Q microresonators,and spectroscopy of a hydrogen fluoride gas cell.The simplicity,robustness,and low cost of this spectroscopy scheme are valuable for out-of-the-lab applications like lidar and environmental monitoring.