[OPTICAL REVIEW Vol. 6, No. 4 (1999) 273-283]
Wavelength-Tunable Laser-Diode Interferometer
Department of Electronics, University of Industrial Technology, Sagamihara, 4-1-1, Hashimotodai, Sagamihara, 229-1196 Japan
(Received April 19, 1999; Accepted May 10, 1999)
Laser diodes (LDs) have been applied to a phase-measuring interferometer through the wavelength tunability of LDs by controlling their currents. Laser-diode interferometers based on a heterodyne technique are reviewed. A two-wavelength laser-diode interferometer is demonstrated with current control of dual LDs in opposite directions. A synthetic wavelength makes it possible to extend the range of interferometric measurements. The wavelength is controlled by the laser injection current and is stepwise or rampwise changed to introduce a time-varying phase difference between the two beams of an interferometer with unbalanced optical path lengths. The optical output is demodulated with a phase-extraction algorithm. Systematic phase errors caused by the LD-power variation and by the difference between the beat frequency and ramp frequency are analyzed. A feedback interferometer with electronics is used to eliminate the phase error by locking the interferometer on a preset phase. Typical experimental results are shown.
Key words : laser diode, wavelength tunability, phase measurement, phase-shifting method, heterodyne interferometer, feedback control, two-wavelength interferometer, systematic phase errors