Effects of external reflector alignment in sensing applications of optical feedback in laser diodes

The alignment of the components in sensor systems based on external optical feedback is a critical factor due to the small size of laser aperture into which the returned light must be coupled. This paper presents experimental results and a theoretical explanation which show that the fringe frequency of a self-mixing interferometric sensor system can be doubled due to misalignment of the external reflector. The double frequency fringes are less sensitive to the alignment of the reflector than the normal fringes and offer a potential increase in the resolution of a sensing system of a factor of two.