APS/Alan Stonebraker

Figure 1: 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 (PD$1$ and PD$2$) 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.