METHOD FOR MEASURING TIME DIFFERENCE OF LIGHT PROPAGATION

PURPOSE:To make the accurate measurement of refractive indices possible by using coherent light, measuring the fluctuation of the light power inputting to a photo- detector, determining a peak value of visibility, and measuring the time difference of light propagation from the difference in optical path lengths. CONSTITUTION:The light out from a light source 21 is bisected by a beam splitter 22, one of which is turned back by a fixed mirror 23, and the other is turned back by a mirror 24 placed in a variable optical delay path 25. If these two beams of light are so adjusted as to superpose on each other in a photodetector 26, the optical path lengths of the two beams of light cause level fluctuation owing to the interference of the two beams within coherence length. In this case, if a light source of short coherence length is used as the light source, the light passing the optical path 2a and the light passing the optical path 2b are matched on the photodetecting surface 26 with high accuracy in time, and refractive indices are measured accurately.