Study On Spectrum Distortion Correction And Digital Conditioning For Tunable Diode Laser Absorption Spectroscopy System

With the globalizing of world economy, problem of energy and environment is becoming more and more serious. Energy saving and pollution control has become the consensus of the world. Emission Monitoring is the premise of polluting gas control, and tunable diode laser absorption spectrum (TDLAS) is one of the most representative technologies. In a TDLAS system, wavelength modulation will also induce a light amplitude modulation, and which leads to distortion of the second harmonic spectrum as increase the difficulty of digital conditioning. To address this problem, the effect of the remnant light amplitude modulation on the harmonic detection and digital conditioning were studied. The method of synchronous light modulation suppression is proposed, and a experimental system based on MEMS VOA is set up, and corresponding compensation experiments on a TDLAS CO2 gas analyzer were carried out .Firstly, in this paper, the principles of TDLAS technology and the linear function of gas absorption spectrum are introduced. The theory of the second harmonic detection based on wavelength modulation technique is derived. The realization of wavelength modulation is put forward, the source of noise and the control method of the system is analyzed.Secondly, the effect of the remnant light amplitude modulation of TDLAS gas concentration detection system is analyzed. According to the remnant light amplitude modulation, the synchronous compensation system based on tunable attenuator is put forward. The gas cell with collimation and convergent optical system is analyzed and designed by the Gauss beam approximation method. To extract weak signal, phase locking amplification technique is introduced, and the digital phase locking amplification circuit based on FPGA is studied. The design and simulation of the main modules are accomplished.Thirdly, according to the compensation method, a TDLAS experimental system is set up, which includes optical system, dSPACE AutoBox signal simulation box, the control circuit laser diode of wavelength modulation and the driving circuit of the attenuator. Laser characteristics testing experiments and remnant light amplitude modulation compensation experiments without absorption gas were carried out; the parameters of the attenuator driving model was then determined, good compensation results of the remnant light amplitude modulation were obtained. Thereafter, the compensation experiment with CO2 absorption gas is carried out. By comparing the spectrum of the before and after compensation, the effect of our method compensation is demonstrated. The distortion second harmonic spectral signal is reduced to 20% of its original value and the first order light amplitude, which is the noise of the TDLAS system, was reduced to 0.1% of its original value correspondingly.The remnant light amplitude modulation compensation method using tunable synchronous light attenuator greatly improved the TDLAS's signal processing, and has potential application value in a real on site TDLAS system.