Reduction in thickness error of optical coatings by dividing thick layers and monitoring with multiple witness glasses

•A monitoring strategy based on using several witness glasses is presented.•Some thick layers are divided into two layers.•The thickness error is reduced.•Spectral performance close to theoretical design is achieved.

We present a monitoring strategy based on using two pieces of witness glass, which are brought to the measuring position in a specially chosen sequence, each witness glass is monitored by one single wavelength. To reduce the thickness error, some thick layers are divided into two layers and monitored by different witness glasses. Theoretical analysis and experimental results have demonstrated that the proposed monitoring strategy can achieve spectral performance close to the theoretical design.