Mismatching of gratings to achieve phase shift in an optical position detector

An optical position detector (100, 200) for installation in an optical servo system including a reference grating (108, 208) having a first fundamental spatial frequency, comprising:

• (a) a light source (110, 210) to provide light beams;
• (b) a first light detector (112, 212) spaced apart from said light source (110, 210) to detect non-interfering light beams from the light source;
• (c) a second light detector (114, 214) spaced apart from said light source (110, 210) and spaced apart from said first light detector (114, 214) to detect non-interfering light beams from the light source;
• and
• (d) a coherent detector grating (118, 218) to diffract the light beams, and overlying said first light detector (112, 212) and said second light detector (114, 214), wherein said coherent detector grating (118, 218) has a second fundamental spatial frequency which does not match the first fundamental spatial frequency of the reference grating.

An optical position detector system (100-102, 200-202), comprising:

• (a) a reference grating (108, 208) having a first fundamental spatial frequency;
• (b) a light source (110, 210) positioned to shed light on said reference grating;
• (c) a first light detector (112, 212) spaced apart from said light source (110, 210) and positioned to detect non-interfering light beams diffracted by said reference grating (108, 208);
• (d) a second light detector (114, 214) spaced apart from said light source (110, 210), spaced apart from said first light detector (114, 214) and positioned to detect non-interfering light beams diffracted by said reference grating (108, 208);
• and
• (e) a coherent detector grating (118, 218) overlying said first light detector (112, 212) and said second light detector (114, 214), wherein said detector grating (118, 218) has a second fundamental spatial frequency which does not match the first fundamental spatial frequency of said reference grating (108, 208).

A method of fabricating an optical position detector system (100, 102), comprising the steps of:

• (a) placing a light source (110), a plurality of detectors (112, 114) and a coherent diffraction grating (118) having a first fundamental spatial frequency and comprising a source filter and a detector filter on the same surface, on a first body (116);
• (b) placing a reference grating (108) on a reflective second body (104), the reference grating having a second fundamental spatial frequency that is mismatched with the first fundamental spatial frequency;
• and
• (c) spatially offsetting one (112) of the plurality of detectors on the first body (116) relative to at least one other (114) of the plurality of detectors so that at least two of the plurality of detectors are configured to detect non-interfering light beams provided by the light source and reflected at different locations on the reference grating (108).

A method of fabricating a transmissive optical position detector system (200, 202), comprising the steps of:

• (a) placing a light source (210) and a source grating (217) on a first body (216);
• (b) placing a plurality of detectors (212, 214) and a coherent detector filter (218) having a first fundamental spatial frequency on a second body (216′);
• (c) placing a reference grating (208) having a second fundamental spatial frequency on a third, transmissive body (204), wherein the first and second fundamental spatial frequencies are mismatched;
• and
• (d) spatially offsetting one (212) of the plurality of detectors relative to at least one other (214) of said plurality of detectors so that at least two of the plurality of detectors are configured to detect non-interfering light beams provided by the light source and reflected at different locations on the reference grating (208).

A method of fabricating a reflective optical position detector system (100, 102) including a coherent diffraction grating (118) and a reference grating (108) having a first fundamental spatial frequency, comprising the steps of:

• (a) fabricating the coherent diffraction grating (118) so as to filter spatial frequencies of the reference grating (108);
• and
• (b) selecting a second fundamental spatial frequency for the coherent diffraction grating (118), the second fundamental spatial frequency being mismatched with the first fundamental spatial frequency for providing signals having maximized amplitude and a preselected phase shift for a pair of detectors;
• wherein
• selecting a second fundamental spatial frequency includes employing a fundamental spatial frequency mismatch calculated according to the formula:
• F=(N−phi/360)/X,
• where N is the number of matched filter grating periods between the detectors (112, 114), X is the spacing between the detectors (112, 114), and phi is the preselected phase shift.

A method of fabricating a reflective optical position detector system (100, 102), comprising the steps of:

• (a) placing a light source (110), a plurality of detectors (112, 114) and a coherent diffraction grating (118) on a first body, the plurality of detectors being configured to detect non-interfering light beams from the light source, the coherent diffraction grating having a first fundamental spatial frequency and comprising a source filter and a detector filter on the same surface;
• (b) placing a reference grating (108) having a second fundamental spatial frequency on a reflective second body (104, 106);
• and
• (c) purposely mismatching the first fundamental spatial frequency to the second fundamental spatial frequency.

A method of fabricating a transmissive optical position detector system (200, 202), comprising the steps of:

• (a) placing a light source (210) and a source grating (217) on a first body (216);
• (b) placing a plurality of detectors (212, 214) and a coherent detector filter (218) on a second body, the plurality of detectors being configured to detect non-interfering light beams from the light source, the coherent detector filter having a first fundamental spatial frequency;
• (c) placing a reference grating (208) having a second fundamental spatial frequency on a third, transmissive body (204);
• and
• (d) purposely mismatching the first fundamental spatial frequency to the second fundamental spatial frequency.

An optical position detector comprising:

• a light source;
• a first and a second light detector spaced apart from each other to detect non-interfering light beams from the light source;
• and
• a coherent diffraction grating having a first fundamental spatial frequency and overlying the first and the second light detector, wherein the optical position detector is configured to be positioned in a servo system including a reference grating having a second fundamental spatial frequency that has a mismatch with the first fundamental spatial frequency, and wherein the mismatch provides a phase shift between the first light detector and the second light detector.