Optical feedback signals involving the misalignment information

The optical feedback characteristics of the laser intensities at both emission directions are studied in a single-mode HeNe laser under low optical feedback strength. The sign inversion phenomenon can be observed when the photodetector at the main emission direction only receives the main light spot. There is no sign inversion phenomenon but intensity branch phenomena or fringe frequency-doubled phenomenon when the photodetector at the main emission direction receives both the light spots. When the optical feedback signal has intensity branch, it involves the misalignment information of the external cavity which can be used to detect the angular pitching of the reflector. The fringe frequency-doubled phenomenon can be used to double the resolution of an optical feedback system.