Research Of Key Technologies Of Stabizing Ld In Gas Detection System

In design of tunable diode laser absorption spectroscopy (TDLAS) based gas concentration detection system, laser diodes (LD) are considered as the core components of the system. The performance of LD makes a direct impact on the accuracy and reliability of the system. Therefore, it is important to keep the working condition of LD stable. The main factors affecting the working condition of LD are the injection current and temperature, so in gas detection system the key to keep the working condition of LD stable is the stable control of injection current and temperature.The working principles of LD first are analyzed and basic structure and characteristics of vertical-cavity surface-emitting laser (VCSEL), which is selected as the laser in experiments, are described in detail. Methane is selected as the target gas of the system, and the basic principles of gas spectral absorption characteristics is introduced. The wavelength modulation based harmonic detection scheme of methane gas concentration detection system is analyzed in detail, the requirements of LD working condition in the detection system and the output wavelength are made clear. Secondly, according to the requirements of LD output wavelength in the detection scheme, the output wavelength modulation scheme by the variation of injection current is designed to ensure the stability of injection current, which has significant impact on the working conditions of LD. Through the analysis of selection of key components, the LD drive circuit is realized to ensure the stability of injection current. However, the premise to realize the output wavelength modulation by injection current is the stability of temperature, which is another important factor affecting the output wavelength of LD.In order to ensure the temperature of LD to meet the requirements of detection scheme, in combination with typical temperature control theory, the heat conduction structure of LD and factors affecting the temperature of LD are both analyzed. Combining the characteristics of heat conduction, the control scheme including the outer environmental temperature control and inner thermoelectric refrigerator(TEC). The temperature control circuit of LD is completed in this paper.At the end of the paper, the overall tests and experiments show that, the LD achieves stable working conditions, and the designed driving circuit and temperature control circuit can fulfill the requirements of detection system. Combining the characteristics of detection schemes, an evaluation method of LD working conditions based on the amplitude of harmonic signal is proposed. The proposed evaluation method is simple and helpful to the setting and debugging of LD working conditions.