Research On The Damage Characteristics Of Bandpass Filters By Narrow-pulse Lasers Outside The Filter Band

In recent years,with the rapid development of small unmanned aerial vehicles（UAVs）,incidents of unauthorized flight and unauthorized filming in sensitive areas such as no-fly zones and military bases have become increasingly frequent.This poses a huge challenge to civil aviation safety and military secrecy protection.The commonly used UAV aerial photography television system mainly operates in the visible light band of 400-700 nm,while the laser radar mainly operates in the 905 nm band,and the strong laser system mainly operates at 1.06μm.Therefore,under the limitation of the filter,the laser cannot cause damage to the detector itself,but the filter in front of the detector can be damaged under the action of strong laser,ultimately causing the UAV imaging system to fail.Compared with the operating band of the imaging system,the strong laser is outside the operating band,so the damage caused by the strong laser to this type of optical window is called out-of-band damage.In view of the current development of UAV aerial photography technology and the urgent need to counter UAV technology,it is particularly important to carry out research on out-of-band damage of short-pulse lasers to optical windows.This paper focuses on the research of nanosecond pulse laser damage characteristics and laws on bandpass filters of400-700 nm and narrow-band filters of 905 nm from both theoretical and experimental aspects.Theoretical calculations were based on the Maxwell equation group and thermodynamics theory to establish a numerical calculation model for the out-of-band damage of narrow-pulse lasers to filters.The out-of-band damage characteristics of 1064 nm nanosecond pulse lasers on BP905 and BP420 bandpass filters under different incident angles and polarization states were analyzed.The change rules of the standing wave field,temperature field,and stress field of the two bandpass filters under different incident angles and polarization states were compared and analyzed,and the laser damage threshold of the bandpass filter under different incident angles was analyzed.In the experimental aspect,using a 1064 nm nanosecond pulse laser as the light source,an experimental system for the study of out-of-band damage characteristics of bandpass filters was established by narrow-pulse lasers.The laser damage threshold of the bandpass filter was measured using the 1-on-1 method.The effect of incident angle on the laser damage threshold and damage morphology of the bandpass filter was analyzed.Based on the above research,the following conclusions were drawn:1)The absorption coefficient of Ti₃O₅ film layer is much higher than that of Si O₂film layer,so the damage to the filter is mainly caused by the Ti₃O₅ film layer.2)The standing wave field also has a certain impact on laser damage.Due to the effect of optical path difference,the change of the incident angle of the laser causes a change in the standing wave field,which leads to a change in the thin film absorption rate.At the same time,the interface defects near the standing wave field peak absorb a large amount of laser energy,ultimately causing the temperature rise and damage of the film layer.3)Under the action of low-energy laser,the filter did not show melting damage,but exhibited film layer rupture caused by stress,so stress damage occurred before melting damage.The research work of this paper can provide relevant theoretical and technical support for anti-UAV technology.