Interferometric encoder systems

An encoder interferometry system comprising:

• an encoder head, the encoder head being configured to direct light to an encoder scale to produce a plurality of twice-diffracted measurement beams,
• wherein the encoder head comprises
• a single optical element arranged to receive an input beam from a light source, wherein the optical element is configured to direct a first portion of the input beam along a measurement path to define a measurement beam,
• wherein the optical element is further configured and arranged to receive a first once-diffracted measurement beam and a second once-diffracted measurement beam from the encoder scale and to redirect the first once-diffracted measurement beam and the second once-diffracted measurement beam toward the encoder scale to produce a first twice-diffracted measurement beam and a second twice-diffracted measurement beam such that the first twice-diffracted measurement beam and the second twice-diffracted measurement beam propagate along non-parallel beam paths having an angular separation α through the optical element following the second diffraction at the encoder scale, and the non-parallel beam paths of the first and second twice-diffracted beams at least partially overlap;
• a first detector arranged to receive the first twice-diffracted measurement beam and a first reference beam;
• a second detector arranged to receive the second twice-diffracted measurement beam and a second reference beam;
• and
• an electronic processor configured to receive an interference signal from each of the first and second detectors, each interference signal comprising a phase related to the relative displacement between the encoder scale and the encoder head, and to determine information about a degree of freedom of the encoder head or the encoder scale based on the phase for each interference signal.

A method comprising:

• directing, from an encoder head, a portion of an input beam to an encoder scale, wherein the portion of the input beam is diffracted by the encoder scale into a first once-diffracted measurement beam and a second once-diffracted measurement beam;
• receiving at the encoder head the first once-diffracted measurement beam and the second once-diffracted measurement beam following diffraction from the encoder scale;
• redirecting, from the encoder head, the first once-diffracted measurement beam and the second once-diffracted measurement beam back toward the encoder scale to produce a first twice-diffracted measurement beam and a second twice-diffracted measurement beam, such that the first twice-diffracted measurement beam and the second twice-diffracted measurement beam propagate along non-parallel beam paths having an angular separation α through the encoder head, and the non-parallel beam paths of the two twice-diffracted beams at least partially overlap;
• receiving, at a first detector, a first output beam comprising the first twice-diffracted measurement beam and a first reference beam to produce a first interference signal;
• receiving, at a second detector, a second output beam comprising the second twice-diffracted measurement beam and a second reference beam to produce a second interference signal, wherein each interference signal comprises a phase related to the relative displacement between the encoder scale and the encoder head;
• receiving, at an electronic processor, the first and second interference signals;
• and
• determining, by the electronic processor, information about a degree of freedom of the encoder scale or the encoder head based on the phase for each interference signal.

An encoder interferometry system comprising:

• an encoder head comprising:
• a plurality of optical elements arranged and configured to
• direct a first measurement beam and a second measurement beam toward an encoder scale to produce a first once-diffracted measurement beam and a second once-diffracted measurement beam,
• receive the first once-diffracted measurement beam and the second once-diffracted measurement beam from the encoder scale,
• redirect the first once-diffracted measurement beam and the second once-diffracted measurement beam toward the encoder scale to produce a first twice-diffracted measurement beam and a second twice-diffracted measurement beam such that the first twice-diffracted measurement beam and the second twice-diffracted measurement beam propagate along non-parallel beam paths having an angular separation α following the second diffraction at the encoder scale, wherein the non-parallel beam paths at least partially overlap, and
• combine the first twice-diffracted measurement beam and the second twice-diffracted measurement beam with a third beam and a fourth beam, respectively, wherein the third beam and the fourth beam have the same angular separation α such that the first twice-diffracted measurement beam and the third beam propagate along a first co-linear path and so that the second twice-diffracted measurement beam and the fourth beam propagate along a second co-linear path;
• a first detector arranged to receive the first twice-diffracted measurement beam and the third beam;
• a second detector arranged to receive the second twice-diffracted measurement beam and the fourth beam;
• and
• an electronic processor configured to receive an interference signal from each of the first and second detectors, each interference signal comprising a phase related to the relative displacement between the encoder scale and the encoder head, and to determine information about a degree of freedom of the encoder head or the encoder scale based on the phase for each interference signal.