Heterodyne spectroscopy using comb and tunable laser with low-speed detector
摘要
Heterodyne spectroscopy using comb and continuous-wave (CW) gives a highly sensitive and accurate way to measure the absorption spectrum with resolved comb lines. Here, to avoid using a high-speed optical detector and ADC card, low-bandwidth heterodyne spectroscopy is demonstrated by using a tunable CW laser and a probing comb laser, realizing easy data acquisition, real-time storage, and processing. The absolute frequency accuracy of the CW laser is guaranteed by locking the laser to one of the optical frequency comb teeth. And the multi-heterodyne beats within a bandwidth of 350 MHz are acquired at each sampling frequency step. The beat signal between comb teeth is suppressed by introducing a dispersion compensation fiber, avoiding its confusion between the multi-heterodyne signal. Two working modes, the wideband searching mode and the precise analyzing mode are integrated for fast searching the gas absorption spectrum over a wideband range and analyzing the absorption feature within a determined span. In experiment, the H13CN absorption spectrum is analyzed with spectrum resolution equal to the comb tooth spacing of 100 MHz. The maximum relative deviation of the spectral transmittance is approximately 3%, with a time resolution of 400 µs per sampling step and an acquisition time of about 2 ms for a single absorption line.