TDLAS technology is essentially a spectral absorption technology. The concentration of gas can be obtained by analyzing the selective absorption of laser by gas. It differs from traditional infrared absorption technology in that the spectral width of semiconductor lasers is much smaller than that of gas absorption lines.

Principle:

Therefore, TDLAS technology is a high resolution spectral absorption technology. In the Lambert-Beer formula, the intensity attenuation of semiconductor lasers passing through the measured gas can be expressed by Lambert-Beer law. IV, 0 and IV represent the intensity of the absorption spectrum of the gas at the incident frequency V and after the gas passing through pressure P, concentration X and path L, respectively; S (T) represents the intensity of the absorption spectrum of the gas; and the linear function g (v-v0) represents the intensity of the absorption spectrum of the gas. The shape of the absorption line was characterized. Usually, the absorption of gas is small, and the absorption of gas can be approximated by formula (4-2). These relationships indicate that the higher the gas concentration, the greater the attenuation of light. Therefore, the concentration of gas can be measured by measuring the attenuation of gas to laser.

The absorption of gas molecules is always related to the energy level transition from low energy state to high energy state. Line strength S (T) reflects the net effect of the intensity between stimulated absorption, stimulated radiation and spontaneous radiation during the transition process. It is the most basic property of the absorption spectrum line, which is determined by the transition probability between energy levels and the number of molecules in the upper and lower levels. The distribution of molecules between different energy levels is affected by temperature, so the line strength of spectral lines is also related to temperature. If the reference line strength S (T0) is known, the line strength at other temperatures can be calculated by the following formula: Q (T) is the molecular partition function; h is the Planck constant; C is the speed of light; K is the Boltzmann constant; En is the lower energy level. Line strength S (T0) of absorption spectra of various gases can be consulted in relevant spectral databases.