LASERS WITH OPTICAL FEEDBACK AS DISPLACEMENT SENSORS

An interferometric displacement sensor with useful properties has been built based on a laser with optical feedback from light that is backs cattered by a moving object. Information about the object's motion is encoded in the phase of the backscattered light, which in turn influen ces the phase and the amplitude of the laser via injection-locking phy sics. We derive the properties of the amplitude and phase modulation o f the laser from standard injection-locking relations augmented by a s elf-consistency condition. These predictions are then confirmed by exp erimental results. An off-the-shelf two-mode frequency-stabilized lase r is used in two different interferometric configurations. First, the amplitude modulation of the laser is utilized for displacement measure ments in a homodyne setup. Second, the phase modulation of the laser i s used in a pseudoheterodyne interferometer. In both cases, the backsc attered light from the object can be injected into the laser cavity wi thout the help of any focusing optics. Thus the injection-locked senso r combines the advantages of readily available equipment and a straigh tforward optical setup without need for intricate alignment, and there by meets two important conditions for industrial applications.