OPTICAL INTERFEROMETER INCORPORATING BEAM SHEAR MEASUREMENT
Publication Date: 2016-Aug-16
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Optical interferometers are instruments that can be used to make very precise measurements of a positional displacement of a target object. In what is known as a homodyne interferometer, a laser beam of a given wavelength is split into two parts by an optical splitter, with one part constituting a reference beam having a first polarization and another part constituting a measurement beam having a second polarization. Positional displacement information of the target object with respect to the optical splitter can be obtained be generating an interference pattern between the reference beam and the measurement beam as a result of the two beams being coherent.
Fig. 1 A traditional homodyne interferometer
Fig. 1 above shows a traditional homodyne interferometer that incorporates such an architecture. As shown, the optical splitter receives the laser beam from the laser and splits the laser beam into a measurement beam having a vertical polarization and a reference beam having a horizontal polarization. The reference beam is directed towards a reference object that is stationary, while the measurement beam is directed towards a target object that can be a movable object. The traditional homodyne interferometer is configured to detect and measure an amount of positional displacement of the target object with respect to the optical splitter. More particularly, the traditional homodyne interferometer is configured to detect an extent of movement of the target object either towards the optical splitter or away from the optical splitter.
The movement of the target object towards the optical splitter or away from the optical splitter adds a Doppler component (ωd) into the measurement beam that is reflected by the target object back towards the optical splitter. The reflected measurement beam is combined with the reference beam that is also reflected back towards the optical splitter by the reference object, and the optical beam containing the reflected measurement beam and the reflected measurement beam is routed to a detector by using one or more mirrors.
Understandably, as a result of combining the reflected reference beam with the reflected measurement beam, the optical beam provided to the detector contains both a horizontal polarization component and a vertical polarization component. The detector generates an interferometric pattern using the combined optical beam, and then uses the interferometric pattern to measure an extent of movement, as well as a direction of movement, of the target object with respect to the optical splitter.
While the traditional homodyne interferometer shown in Fig. 1 does prove useful for obtaining displacement information of the target object along an x-axis (indicated in Fig. 1 by the bi-directional arrow) it fails to provide displacement information in other directions, such as for example,...