A New Method to Extend the Measurement Range of a Displacement Measuring Interferometer by Measuring the Modulation Depth

Laser interferometers are widely used for sub-micrometer accuracy of measurements. However, conventional interferometers (homodyne and heterodyne interferometers) can be used to measure the displacement only. Therefore, the measurement signal must be continuous during the measurement process. This is difficult for a long measurement range with the presence of vibrations, misalignment of the optics, and tolerance of sliders. In this paper, a method of retrieving the absolute distance of the measured object by determining the modulation depth was proposed and demonstrated. This was the unique feature of the frequency-modulated interferometer. The modulation index is a function of the modulation excursion, modulation frequency, and the unbalanced length between the two arms of the interferometer. On the other hand, the modulation index can be determined through the Bessel function. Therefore, the absolute position of the target at special values of the modulation depth can be determined. These positions were utilized as markers on the scale to divide a large measuring range into small measuring ranges while ensuring the continuity of the system. Therefore, the proposed method can extend the measuring range of the interferometer even under the influence of vibrations or misalignment systems.