180 Basic Instrumentation for Optical Measurement Detector E 0 E i L 1 L 2 Mirror Scan Figure 2.5.1 Basic configurations of a wavelength meter based on a Michelson interferometer. An optical wavelength meter can be built from a simple Michelson interfer- ometer by scanning the optical length of one of the two interferometer arms. Based on the Michelson interferometer configuration shown in Figure 2.5.1, assuming that the input optical field is E i , the power-splitting ratio of the cou- pler is a, and the optical lengths are L 1 and L 2 for the two arms, then the optical field reaches the receiving photodetector is p ffiffiffiffiffiffiffiffiffiffiffi p ffiffiffi E 0 ¼ 1 À a a E i ½ exp jð2bL 1 þ ' 1 Þ þ exp jð2bL 2 þ ' 2 Þ e jot p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ' þ ' 2 ' À ' 2 jot cos bDL þ 1 e ¼ 2 að1 À aÞ E i exp j bðL 1 þ L 2 Þ þ 1 2 2 Mirror