Femtosecond Laser Micro-woven PVDF Sensor And Its Application In Photoacoustic Detection

Optical components damage has always limited the load capacity of high-power laser systems,and once damage occurs,the smooth operation of the laser system is jeopardized,so it is particularly important to propose a nondestructive,real-time online,and convenient laser damage detection method.Photoacoustic detection method can provide real-time online detection compared to traditional damage detection methods,but the detection sensitivity is insufficient,and the preparation of pressure sensors by means of photolithography and etching is complicated,affecting the detection precision.In this thesis,circular truncated cone microarray of Polyvinylidene Fluoride(PVDF)thin film pressure sensor is fabricated by combining femtosecond laser processing and hot embossing,and photoacoustic detection of fused silica laser damage is carried out based on the fabricated microstructure sensor.This method is characterized by low cost,simple process and high sensitivity,and can be applied to the online monitoring of optical component damage in large high-power laser devices to improve the reliability of laser devices.This thesis has the following main research contents:1.Based on the flexoelectric effect and COMSOL finite element analysis,a physical model of the circular truncated cone microarray of PVDF thin-film sensor is established,and the effects of parameters such as the diameter and period of the circular truncated cone microarray on the strain gradient of the PVDF thin-film sensor are analyzed.The results show that the strain gradient of the PVDF thin-film sensor is larger when the height of the circular truncated cone is 5 μm,the diameter of the circular truncated cone is between 30μm-200 μm,and the period of the circular truncated cone microarray is between 50 μm-200μm,the smaller the diameter of the circular truncated cone and the period of the circular truncated cone microarray.The strain gradient of the PVDF thin-film sensor decreases with increasing the height of the circular truncated cone between 1 μm and 5 μm for a circular truncated cone diameter of 30 μm and a circular truncated cone microarray period of 50 μm..2.A circular truncated cone microarray PVDF pressure sensor is successfully prepared by combining femtosecond laser processing and hot embossing.Firstly,a femtosecond laser with a laser wavelength of 1030 nm,a pulse width of 300 fs and a repetition frequency of 100 kHz is used to process perforated structures of different diameters and periods on the surface of PET(polyester)films with a thickness of 50 μm,and the cone angle error of the perforated PET film samples is calculated to be less than 0.2° by measuring the diameters of the upper and lower surfaces of the perforations.Then the structure is transferred using hot embossing technology and the circular truncated cone microarray PVDF thin film sensor is successfully prepared.In the force-electric coupling test,the smaller the diameter and period of the circular truncated cone microarray,the higher the sensitivity of the PVDF sensor.The sensitivity of the sensor with a period of 50 μm and a 30 μm diameter circular truncated cone microarray is 8.98 mV/kPa,which is 10.9 times higher than that of the unstructured PVDF thin film sensor.3.Based on the prepared microstructured PVDF thin-film sensor,a laser irradiated fused silica glass damage photoacoustic detection study is carried out.The PVDF sensor IS laminated to the rear surface and side of the fused silica,and the effects of the irradiation point position and the irradiation laser fluence on the photoacoustic signal are investigated.The experimental results show that the sensor can detect micron-level damage pits under the laser wavelength of 1064 nm,pulse width of 10 ns and single-pulse energy laser of 0.75 J.With the increase of the distance between the irradiation point and the sensor,the diffusion of the shock wave makes the photoacoustic signal amplitude decrease continuously.In this thesis,a method combining femtosecond laser processing and hot embossing is proposed for the preparation of microstructured PVDF film pressure sensors,and laser irradiation fused silica damage detection research is carried out,which not only provides a new preparation means for the preparation of pressure sensors,but also provides a new technical idea for laser damage photoacoustic detection.