  Advances in high-resolution laser spectroscopy have enabled many scientific
breakthroughs in physics, chemistry, biology and astronomy. Optical frequency
combs have 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 frequency of the laser because of
fluctuations in the scan speed. Here we demonstrate a simple spectroscopy
scheme with a frequency modulated diode laser that references the diode laser
on-the-fly to a fiber cavity with sub-15 Hz frequency precision over an 11-THz
range at a measurement speed of 1 THz/s. This is an improvement of more than
two 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 several
applications, including dispersion measurement of an ultra-high-Q
microresonator and spectroscopy of an HF gas cell, which can be used for
absolute frequency referencing of the tunable diode laser. The simplicity,
robustness and low costs of this spectroscopy scheme could prove extremely
valuable for out-of-the-lab applications like LIDAR, gas spectroscopy and
environmental monitoring.
