A Twyman-Green phase-measuring feedback interferometer is constructed to calibrate a phase shift which affects the measurement accuracy given by a frequency tuning of a laser diode source. PHASE-SHIFTING LASER-DIODE FEEDBACK INTERFEROMETER Laser diodes (LDs) have been proved to be useful light sources in optical interferornetry for a single-mode operation and a frequency tunability. In contrast to using a phase shifter such as a piezoelectric transducer a phase-measuring LD interferometry can be realized by changing the un?alanced optical path length of the interferorneter with the frequency-modulated (FM) LD. The principle of the phase-measuring technique into practical LD interferometer together with the LD operation and the digital fringe data processing is shown in Fig. 1. The problem with the measurement is that the deviations of phase shift from its nominal value due to mode instability of LD violate the assumption of known phase shift in the phaseextracti9n algorithm causing less accuracy. To avoid this difficulty a feedback interfe rometer with TTL electronics is made to stabilize the phase shift using the frequency tuning of LD. The selection of LDs suitable for the interferometric experiment may be followed by the requirement of the singlemode behavior yielding the long coherence length. The light source such as an AlGaAs channeled-substrate planar-type LD operated at 780-nm wavelength is available for the interferometric use. The wavelength is controlled by the current such that the temperature must be kept