An interferometric displacement sensor with useful properties has been built based on a laser with optical feedback from light that is backscattered by a moving object. Information about the object's motion is encoded in the phase of the backscattered light, which in turn influences the phase and the amplitude of the laser via injection-locking physics. We derive the properties of the amplitude and phase modulation of the laser from standard injection-locking relations augmented by a self-consistency condition. These predictions are then confirmed by experimental results. An off-the-shelf two-mode frequency-stabilized laser is used in two different interferometric configurations. First, the amplitude modulation of the laser is utilized for displacement measurements in a homodyne setup. Second, the phase modulation of the laser is used in a pseudoheterodyne interferometer. In both cases, the backscattered light from the object can be injected into the laser cavity without the help of any focusing optics. Thus the injection-locked sensor combines the advantages of readily available equipment and a straightforward optical setup without need for intricate alignment, and thereby meets two important conditions for industrial applications.
Digital phase demodulation is being developed as a flexible heterodyne demodulation technique for on-line dynamic displacement detection. The digitized modulated carrier and reference carrier signals from a heterodyne interferometer are demodulated for surface displacements using computer algorithms. Since the signal demodulation is performed in the computer, the heterodyne beat frequency or center frequency of the carrier signals can be easily optimized for the desired detection center frequency and the heterodyne beat frequency does not have to be stable. The resulting measured displacements are calibrated with respect to the wavelength of light used in the interferometer and remain accurate for displacements larger than the wavelength of light. No assumptions are made about the sideband frequency content or the amplitude of the surface displacements. The research presented here introduces digital phase demodulation concepts and shows initial experimental results from a heterodyne interferometer while monitoring vibrational and ultrasonic displacements. The initial results show that the heterodyne sensor system is capable of measuring displacements greater than three wavelengths of light and has a single-shot resolution better than 7 nm. Thus digital phase demodulation will allow a single instrument to monitor displacements produced by low-amplitude, high-frequency ultrasound and by high-amplitude, low-frequency vibrations.