Terahertz time-domain spectroscopy characterizes material properties by measuring complex responses in the frequency range from terahertz to tens of terahertz. In this frequency band, various resonance phenomena such as electron and phonon excitations in solid materials are usually observed.

In the laser phase-locked system, the most crucial part is the use of lock-in amplifier to achieve the test gas, reference gas two-way detection, or even to achieve the measured gas, reference gas and laser background three-way detection.

Photo Acoustic Spectroscopy (PAS) is an analytical instrument with a mechanism similar to that of a secondary quantum optical spectrometer.

Spectral measurement system built by a lock-in amplifier and a chopper, only measures the light intensity with a modulated frequency, so as to avoid the influence of other external noise, which can achieve the detection of low light with high precision.

The light from the beam splitter is converted into an electrical signal by the sensor. The sample beam and the reference beam are analyzed by two lock-in amplifiers with respective cut-off frequencies as reference frequencies, and the ratio of the two beams is calculated by the comparator.