PM<sub>2.5 </sub>and PM<sub>10</sub> are very important indexes reflecting the air quality in atmospheric environment monitoring. Based on Mie angular scattering theory, a system measuring particulates diameter and concentration of PM<sub>2.5 </sub>and PM<sub>10</sub> was designed, which used optical fiber as transmission medium. Theoretical deduction and Matlab program simulation were presented to illustrate the basic principle of Mie angular scattering. The influences of laser wavelength, detection angle and other parameters on the measurement accuracy were discussed in detail. It had been found that the measurement results had multi-valued problem when the measurement angle range was too small. However, this problem was eliminated obviously when the angle range is greater than 25 degrees.
The intrinsic phase noise of distributed feedback (DFB) fiber laser greatly reduces the signal to noise ratio (SNR) of unbalance interferometric fiber sensor system, which has a negative influence on the demodulation of tiny signal. In order to suppress the phase noise of DFB fiber laser (DFB-FL), a self-injecion locking DFB-FL was presented. Result of experiment demonstrated that the phase noise can be suppressed by the self-injection locking structure and the effect of suppression was improved with the increase of the length of locking ring. When the length of locking ring is two meters, the phase noise above 500Hz decreases by 8 dB / √Hz demodulated by an one meter optical path difference asymmetric Michelson Interference, and the mode is not hoping in eight hours. Contrast with the unlocked DFB-FL, the pump efficiency of self-injection locking DFB-FL is increased by 30%.
The magnitude of light intensity on the photo-to-electric detector fluctuates all the time in an optic fiber sensing system, because of the influence of various factors in the fiber optic sensing system and from the external environment. As a result of the excessive intensity, the electric signal will be overload after the amplifier circuit with constant enlargement factor, and when the light intensity becames too small, it will reduce the signal-to-noise ratio of the electric signal. Therefore, it is necessary to introduce an automatic gain control (AGC) module into the system, which can insure the electric signal in a reasonable magnitude. In order to solve the problem of optic intensity fluctuating in the optical fiber sensing system with PGC modulation and demodulation, in this paper, firstly, it is analyzed that the impact of different magnitudes of interferential intensity to the PGC demodulation in theory. Secondly, a reasonable control method is put forward and an AGC module based on the AD602 chip is designed and produced. Finally, it is proved that the optic fiber sensor system with an AGC module has strong ability to resist fluctuation of light intensity within 40dB.