Heterodyne interferometry with a frequency-modulated laser diode

Abstract

A digital phase measuring interferometer with a frequency-modulated laser diode using the integratedbucket technique is described. The injection current is continuously changed to introduce a time- varying phase difference between the two beams of an unbalanced Twyman-Green interferometer. The intensity of the interference patterns is integrated with a CCD array sensor for intervals of one-quarter period of the fringe. Using the intensity data a microcomputer calculates the phase to be detected. Some experimental results with the interferometer are presented; the rms repeatability obtained was lambda/80.

Sinusoidal phase-modulating laser diode interferometer capable of accelerated operations on four integrating buckets

“…[3][4][5] When variable current i m (t) is injected into a LD, the intensity distribution of the interference pattern in a general two-beam interferometer can be expressed by…”

“…[1][2][3][4][5][6] Originally introduced in Ref. 1, the technique was used in our sinusoidal phasemodulating ͑SPM͒ 2 and SPM laser diode ͑LD͒ 3 interferometers.…”

Coaxial phase-shifting laser diode interferometer based on pulse modulation

“…Several types of interferometers that employ LDs have been proposed and used in practical applications. Although the LD wavelength is generally modulated directly using a stepwise [2,3], triangular [4,5], or sinusoidal signal [6,7], this direct modulation degrades the measurement accuracy because the intensity of the interference signal varies with the injection current. Figure 1 shows an example of the conventional direct modulation that uses a triangular signal as an injection current.…”

Method to measure frequency change of tunable laser based on Jamin shearing interferometer

“…When the wavelength variation is large enough, for example some nanometers, wavemeters can be used for measurement. When the wavelength variation is relatively small (<0.1nm), the resolution of wavemeters is not adequate for the frequency change measurement, and therefore other instruments such as Fabry-Perot interferometer 6, 7 , Self-heterodyne interferometry [8][9][10][11] , and Michelson interferometer 12,13 are applied. In measurement using a Fabry-Perot etalon with known cavity space 6 , the transmitted power reach its maximum when the laser is tuned to the resonant condition of the etalon.…”

“…Michelson interferometer is simple with uncomplicated experimental setup 13 . However the accuracy is not high because the phase difference is obtained using estimation of interference orders.…”