Laser-diode interferometry

Abstract

Publisher Summary This chapter presents a phase-measuring technique in an unbalanced interferometer using direct frequency modulation of laser diodes (LD) sources by changing currents. The wavelength is stepwise, sinusoidally changed to introduce a time-varying phase difference between the two beams of the interferometer. The operation of the LD and the phase-extracted method from measured interferograms are demonstrated. In particular, a two-wavelength LD interferometer has been elaborately constructed to extend the range of interferometric phase measurement by using a long wavelength synthesized by two wavelengths. Features of the recent development in LD interferometers are described on a feedback interferometer to calibrate the phase shifts and to lock the interferometer on a phase-shift condition by controlling the bias and modulation currents of the LD. The changes in laser-diode power violate the assumption of constant intensity in the phase-extraction algorithm. The system can eliminate the phase shift error between the target phase shift and the real phase shift. An electrically normalizing technique with a photodiode or a phase-extraction algorithm insensitive to LD-power changes is applied to alleviate this problem. Laser-diode interferometry is applied to holographic interferometry and phase-conjugate interferometry with LD wavelength tuning, keeping interferometric capabilities.