Inverted signal-frequency detection and velocity measurement by self-coupling laser sensor array using modulation frequency difference

In this paper, we report the theory of a self-coupling laser sensor array system for improving the signal-frequency inversion problem and the results of velocity measurements with this system. A self-coupling laser sensor is an interferometer that uses optical beats produced by the interference between the light returned from the laser target and the light in the active layer of the laser diode. Using wavelength modulation, this system can simultaneously measure multiple metrological quantities, such as the absolute distance to a target and velocity of a target. However, in a self-coupling laser sensor using wavelength modulation, the signal frequency is inverted and becomes negative if the Doppler shift of the returned light owing to the movement of the target is larger than the signal frequency when the target is stopped. In this case, it is impossible to detect the positive or negative value of the signal directly, resulting in a large measurement error. This has been regarded as a problem that limits the measurement dynamic range of the modulated self-coupling laser sensors. In this study, we propose a system to accurately detect the signal-frequency inversion and improve the measurement dynamic range. The proposed system detects the positive or negative value of the signal frequency from the relationship between the velocity and signal frequency obtained by irradiating multiple beams with different modulation frequencies and then recalculates the accurate measurement value. The measurement results reveal that this system can accurately measure the moving velocity of a target, even when the signal frequency is inverted.