In an optical delay line, translating the stage changes the optical path length. When the translation stage moves by L meters, the introduced optical path length difference is 2L meters. This is because the beam reflects from two mirrors, and each reflection contributes a path-length change, giving a total of 2L.
Since the propagation speed of light in air is approximately the speed of light c, the time delay Δt is:
Δt = 2L / c
where:
L is the translation-stage displacement (meters).
c is the speed of light, approximately 3 × 108 m/s.
Δt is the time delay; the displayed unit is automatically selected as fs, ps, or ns based on magnitude.