# PMT Fluorescence Signal Denoising Processing Based on Wavelet Transform and BP Neural Network
## 2 Detection System Section  
### Optical Path Section  
Firstly, based on the principle of photoluminescence, sulfur dioxide molecules are irradiated by ultraviolet light with a wavelength of 200 nm-220 nm to produce excited sulfur dioxide molecules, which emit fluorescence with a wavelength of 240 nm-420 nm during their return to the ground state. Within a certain concentration range, the fluorescence intensity is directly proportional to the concentration of sulfur dioxide in the sample air. The optical path and assembly diagram of the detection equipment are shown in Figure 1.  
The excitation light source selected is the L12745-01 pulse xenon lamp [20] from Hamamatsu Photonics Co., Ltd. in Iwata City, Japan. It has a rated power of 20 W and can provide spectra in the 185 nm-2500 nm wavelength range. The duration of a single excitation signal is about 5 microseconds.  
In order to obtain the required wavelength range of ultraviolet light for the experiment and reduce the interference of impurity light, four reflective bandpass filters are placed on the excitation light path, with a wavelength range of 209 nm-219 nm [21], the incident angle is 45\({}^{\circ}\). Reflective filters have a stronger retention effect on the required spectral intensity than transmissive filters, which can increase the proportion of spectral intensity required and reduce interference from impurity light. Afterwards, through the combination of multiple positive lens, the light entering the reaction chamber is concentrated directly below the PMT, which can obtain the strongest fluorescence signal. In addition, a conical structure needs to be designed at the rear end of the chamber, with the inner side sprayed with absorbing material. The purpose of doing so is to absorb the light entering the chamber as much as possible, avoid diffuse reflection in the chamber, and be received by PMT again, which will reduce the proportion of fluorescence signal.  
The photoelectric converter selected is the R11558 photomultiplier tube from Hamamatsu Photonics Co., Ltd. in Japan, equipped with a magnetic shielding shell, with a response band of 300 nm-650 nm. The PMT power supply module adopts the original C12597-01 high-voltage base. Due to the introduction of power supply, it will bring more electromagnetic crosstalk, which will add impurities to the output of PMT. Therefore, resistor programming has been chosen for high-voltage base power supply. The high-voltage base provides a reference power line that can provide gain voltage to the PMT through resistive voltage division, thereby avoiding the introduction of more electromagnetic crosstalk. A bandpass filter with a wavelength range of 280 nm-400 nm is placed directly below the PMT, which can effectively detect the fluorescence emitted by sulfur dioxide molecules during their return to the ground state and reduce the interference of some impurity light. Of course, it is also necessary to place a combination of plano-convex lenses to integrate the optical path, ensuring that the generated fluorescence signal is focused on the effective detection part of PMT, in order to obtain the strongest fluorescence signal.