Schematic of the setup for ultrasensitive detection of gas species by measuring their absorption spectra with a laser frequency comb source. An Er:fiber femtosecond laser is locked to a high-finesse optical cavity containing the gas sample. An electro-optic modulator (EOM) is used to phase modulate the comb light at 14 megahertz, and the cavity reflected light is dispersed by a reflection grating and imaged on two photo detectors (PD1 and PD2) in order to create error signals at two different wavelengths. The feedback is sent to a laser current controller and to a PZT (piezoelectric transducer) inside the laser cavity. The cavity transmitted light is coupled through a polarization-maintaining fiber into a fast-scanning Fourier transform spectrometer. The two outputs of the interferometer (beams 1 and 2) are incident on two photodiodes of the autobalancing photodetector. The beam of a continuous-wave 780-nanometer external cavity diode laser (ECDL), used for frequency calibration, is propagating parallel to the frequency comb beam and incident on a separate detector (not shown). P = polarizer, PBS = polarizing beam splitter cube, λ/4 = quarter-wave plate, λ/2 = half-wave plate.