Simulation analysis and preparation of a high optical density laser protection filter

High optical density (OD) filters have been widely used in space observation, optical detection, and laser protection. However, the lack of high OD value filters is restricting their application. This paper reports the formulation of a three-dimensional mesh model that can help predict the effects of pinhole defects, thickness error, and uniformity on the transmittance and OD value of optical filters. A laser protection filter (LPF) with a high OD value was prepared on fused silica using a microwave plasma-assisted pulsed DC reactive sputtering technique. The transmittance and OD value of the LPF were measured. Comparing the designed, measured, and simulated results, we found that the thickness error and uniformity of the layers mainly affected the passband transmittance of the LPF and had little effect on the OD value of the blocking band. In contrast, the pinhole defects were the main factor that decreased the OD value of the blocking band. The average OD values of the prepared LPF in the blocking bands of 527-532 and 755-833 nm were 8.832 and 10.191, respectively. By comparing the transmittance and OD value of the simulated and measured results, we found that the LPF has ${\pm 1}\% $±1% uniformity error and 0.5% pinhole ratio. Suggestions for preparing high OD optical filters are provided, and further improvements are summarized.