New method for lens thickness measurement by the frequency-shifted confocal feedback

•A new method for lens thickness and air gap measurement is proposed.•The measurement method based on the frequency-shifted confocal feedback.•The measurement system has high sensitivity and belong to the heterodyne modulation.•The uncertainty is better than 0.0005 mm and the accuracy reaches micron range.

We describe a new method for lens thickness and air gap measurement based on the frequency-shifted confocal feedback. The light intensity fluctuation is eliminated by the heterodyne modulation and the detection sensitivity is improved prominently by the frequency-shifted feedback effect. The measurement results for different materials and kinds of lenses are presented in the paper, including K9 plain glasses, fused silica plain glass, and K9 biconvex lens. The uncertainty of the axial positioning is better than 0.0005 mm and the accuracy reaches micron range. It is promising to be applied in the multi-layer interface positioning and measurement area.