ABSTRACT
We report here the first-ever, to the best of our knowledge, observation of an inconsistency in the fringe disappearance that occurs in self-mixing interferometers. The disappearance of fringes has been observed in vibration and absolute distance sensing schemes under moderate/strong feedback regimes, and it has a major impact on the design of self-mixing sensors. The number of missing fringes that mostly depends on the feedback strength is also linked to the establishment of the initial stable solution, and, as a consequence, the first modulation period will result in more fringes than expected in the case of an already permanent modulation. We demonstrate that this phenomenon is entirely predicted by the well-admitted dynamic rate equation model of the laser under optical feedback followed by the perfect agreement with experimental results.
ABSTRACT
It is established in the optical feedback interferometry (OFI) theory that the shape of the interferometric fringe has an impact on the detector's response to very small displacement measurements. In this paper, we validate-for the first time, to the best of our knowledge, based on experimental results-this statement by comparing experiments to an established model implementation. Through these experiments, we show that the amplitude of the signals induced by sub-λ/2 optical path variations is linearly dependent on the slope of the underlying fringe. Thus, careful control of the phase allows us to maximize the detection amplitude of very small displacements by positioning the phase where the fringe slope is the steepest. These results are directly applicable to established OFI applications that measure sub-λ/2 optical path variations, such as OFI vibrometers or acoustic imaging though the acousto-optic effect.
