Compact beam re-tracing optics to eliminate beam walk-off in an interferometer

#Interferometers commonly use polarization encoding to distinguish reference beams from measurement beams. In a plane-mirror interferometer 100 illustrated in FIG. 1, for example, an input beam IN contains two linearly polarized components having orthogonal linear polarizations. A polarizing beam splitter 110 in interferometer 100 separates the two components creating a reference beam and a measurement beam.

#In FIG. 1, polarizing beam splitter 110 reflects the component corresponding to the reference beam. The reference beam thus travels down a path R1 through a quarter-wave plate 120 to a reference mirror 130. Reference mirror 130 has a fixed mounting relative to polarizing beam splitter 110 and is aligned perpendicular to path R1 so that the reference beam reflects from a reference mirror 130 and travels back through quarter-wave plate 120 along path R1. Passing twice through quarter-wave plate 120 effectively rotates the polarization of the reference beam by 90° so that the reference beam returning on path R1 passes through polarizing beam splitter 110 and enters a cube corner reflector 140 along a path R2.

#Cube corner reflector 140 reflects the reference beam onto an offset path R3, and the reference beam traverses polarizing beam splitter 110 directly to a collinear path R4. The reference beam then continues along a path R4 through quarter-wave plate 120 before again reflecting from reference mirror 130 and returning through quarter-wave plate 120 along path R4. The second pair of trips through quarter-wave plate 120 changes the polarization of the reference beam, so that polarizing beam splitter 110 reflects the reference beam from path R4 onto an output path ROUT.

#FIG. 1 illustrates a conventional interferometer having a misaligned measurement mirror that causes beam walk-off.

#FIG. 2 shows an embodiment of the invention that eliminates walk-off using a reflector that returns measurement and reference beams to retrace their respective paths.

#FIGS. 3A and 3B show interferometer optics for embodiments of the invention having alternative systems for separating input and output beams.

#Use of the same reference symbols in different figures indicates similar or identical items.

#In accordance with an aspect of the invention, an interferometer has a passive optical system that returns measurement and reference beams for additional passes through interferometer optics to eliminate beam walk-off in the final output beam. In one embodiment, the returned beams exactly retrace their respective paths to exit as collinear beams that coincide with the input beam. For this embodiment, a plane mirror perpendicular to the output path of a conventional interferometer can return reference and measurement beams for the retracing. Alternatively, the returning reference and measurement beams can be parallel to but offset from the original output paths. An optical element such as an isosceles prism can return the beams with the desired direction and offset.

#In a multi-axis interferometer, splitting that separates beams for the separate measurement axes can occur after a combined beam makes a first pass through the interferometer optics and after a reflection from a return reflector. The combined beams are “combined” in that the beams have not yet been split for measurements along different axes. As a result, a multi-axis interferometer in accordance with the invention that eliminates beam walk-off only requires one more beam path than does a conventional multi-axis interferometer that is subject to beam walk-off. Further, the multi-axis interferometer with no beam walk-off can use a single return reflector for the combined beam rather than separate return reflectors for each measurement axis. A multi-axis interferometer in accordance with the invention can be made compact.