Tracking type laser interferometer for objects with rotational degrees of freedom

摘要

A laser interferometer and a method for operating a laser interferometer perform a differential position measurement by laser interferometry of two elements while offering a rotational degree of freedom to one of the elements using a reflecting sphere as a mirror for the laser beam. The laser interferometer and method do not require the object to be aligned with the rotation axis, but instead can track the object in off-centered geometries. This is achieved by employing the pointing of the reflected beam from the sphere as a feedback signal to realign the interferometer which then has a constant beam pointing to the center of the sphere in all cases. The laser interferometer and method keep the direction of the measurement constant. The laser interferometer and method are suitable for homodyne and heterodyne types of laser interferometer technology.

主权项

1. A laser interferometer, comprising:
a) a coherent light source emitting a source laser beam; b) interferometer optics forming a laser interferometer module configured to split the source laser beam into two beam parts propagating with orthogonal polarization; c) a flat first reference surface and a quarter-wave retarding wave-plate, one of the two beam parts being reflected by said first flat reference surface and passing said quarter-wave retarding wave-plate twice while propagating back to said interferometer module, to rotate the polarization of the one beam part by 90 degrees and then transmit the one beam part by said interferometer optics; d) imaging optics, a reflecting sphere or reflecting cylinder forming a second reference surface and another quarter-wave retarding wave plate, the other of the two beam parts being imaged to said reflecting sphere or reflecting cylinder by said imaging optics and during propagation passing said other quarter-wave retarding wave plate twice to rotate the polarization of the other beam part by 90 degrees and then reflect the beam by said interferometer optics yielding two output beams of the laser interferometer having different polarization making the beam parts distinguishable; e) said interferometer optics recombining the two beam parts into a combined output beam having distinguishable polarization; f) a non-polarizing beam splitter splitting the combined output beam into two beam components to be measured for analysis; g) a polarizer receiving the combined output beam and being aligned to transmit the two beam components approximately equally with the beam components interfering; h) an interference detector detecting an interference pattern and emitting a signal corresponding to a differential measurement of said first and second reference surfaces in a longitudinal direction of the source laser beam configured to achieve down to a sub-nm interferometric resolution; i) another polarizer and a 1D position sensitive detector or a 2D position sensitive detector, said other polarizer receiving the two beam components and being aligned to transmit only the other beam part reflected by said spherical surface or reflecting cylinder and perform a position measurement of the reflected other beam part by said 1D position sensitive detector or said 2D position sensitive detector.