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High Sensitivity Detection Based on Tunable Laser

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a laser spectrum gas detection technology developed in recent years. Compared with common electrochemical and ion-conducting ceramic technologies, it has strong selectivity, high sensitivity, With the advantages of fast response, on-line measurement, and strong anti-background spectral interference ability, it is suitable for long-term on-line detection of gases in complex environments. Oxygen (O2) is an important gas in the human living environment. The detection of oxygen concentration is widely used in various fields of production and life and is of great significance.

According to Memes Consulting, recently, the team of Professor Ma Yufei from Harbin Institute of Technology used TDLAS technology to achieve high-sensitivity measurement of oxygen concentration in the air. This study calculated the oxygen concentration in the air based on the direct measurement method, and used the wavelength modulation method to further improve the sensitivity of the system, and finally obtained the detection limit at the ppm level, which can meet the application requirements in many fields. Relevant research results have been published in the journal "Chinese Optics".

TDLAS technology is mainly divided into direct absorption spectroscopy (Direct Absorption, DA) and wavelength modulation spectroscopy (Wavelength Modulation Spectroscopy, WMS). Direct absorption spectroscopy is often used for inversion of gas concentration by measuring the spectral absorbance signal. Its working principle is to control the output wavelength range by changing the working temperature and current of the laser, so as to obtain an absorption line with a complete absorption peak. On this basis, the gas concentration can be directly obtained by calculating the integral value of the absorbance curve in the frequency domain, combined with the gas concentration calculation formula.

In this study, in order to improve the signal strength of the TDLAS detection system, the researchers optimized the selection of the oxygen absorption line. According to the principle of spectral sensing spectral line selection: the selected laser spectral line in the experiment should have strong absorption intensity; there is only oxygen absorption line near the spectral line, and there is no other gas interference; the spectral line should be available in the market. Within the output band of the laser light source. The laser used in the experiment is a semiconductor laser with a maximum output power of 18 mW, which can stably output a single-mode Gaussian beam with a wavelength of about 760 nm.

Using direct absorption spectroscopy, the researchers measured the oxygen concentration in the experimental environment with a pressure of 99.5 kPa and a temperature of 291 K to be 20.56%, and the minimum detection limit was 5.53 × 10⁻³. Since semiconductor lasers have the advantages of narrow line width and fast frequency modulation, wavelength modulation can be performed by injecting current. In the wavelength modulation method, the researchers optimized the laser wavelength modulation depth to obtain a complete second harmonic waveform for calibration of oxygen concentration. Through calculation, the signal-to-noise ratio of the TDLAS sensing system is 380.74, and the minimum detection limit of oxygen concentration is 540 × 10⁻⁶. It is verified that the sensing system proposed in this study has good oxygen detection ability and can be widely used in oxygen concentration detection in various fields.

In conclusion, this study achieved a highly sensitive measurement of oxygen concentration in air using the TDLAS technique. By optimizing the selection of the oxygen absorption line, the oxygen absorbance curve was obtained by direct measurement, the oxygen concentration in the air was calculated, and the modulation depth was optimized by the wavelength modulation method, finally an excellent detection limit of ppm level was obtained. The sensing system will play an important role in many fields.

We can customized tunable laser source for gas detection.

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