Interferometric miniature grating encoder readhead using fiber optic receiver channels

A method for operating a device for measuring the relative displacement between two members, the device comprising:

• a scale having a scale grating pattern formed along a measuring axis direction;
• and
• a fiber optic readhead arrangement positionable relative to the scale grating pattern to provide an operable interference illumination field arising from light diffracted by the scale grating pattern, wherein the interference illumination field comprises respective light and dark interference fringe zones that extend along a direction approximately perpendicular to the scale grating pattern, the fiber optic readhead arrangement comprising:
• a transparent mask substrate;
• and
• a plurality of fiber-optic receiver channels, each respective fiber-optic receiver channel comprising:
• a respective receiver channel spatial phase mask portion having a respective spatial phase and having its light-blocking elements arranged at a pitch that is operable for spatially filtering the light of the operable interference illumination field, the respective receiver channel spatial phase mask portion generally located at a nominal receiver plane that is operable for spatially filtering the light of the operable interference illumination field;
• and
• at least one respective receiver channel optical fiber having an input end that receives a respective receiver channel optical signal light;
• wherein:
• each respective receiver channel spatial phase mask portion is fabricated on a surface of the transparent mask substrate with its light-blocking elements positioned along the measuring axis direction with respect to the light-blocking elements of the other receiver channel spatial phase mask portions in a manner that establishes desired relationships between the respective spatial phases of the respective receiver channel spatial phase mask portions;
• the respective receiver channel optical signal light received by the at least one respective receiver channel optical fiber comprises light arising from the scale grating pattern and collected through the respective receiver channel spatial phase mask portion over a respective collected light area having a collected light area dimension along the measuring axis direction that is at least three full periods of the respective receiver channel spatial phase mask portion;
• and
• at least each collected light area and each input end are positioned entirely within a cylindrical volume having an axis perpendicular to the nominal receiver plane and having a cylinder radius that is at most 5 millimeters;
• the method comprising:
• operably positioning the fiber optic readhead arrangement relative to the scale grating pattern;
• receiving the light of the operable interference illumination field at the nominal receiver plane with at least first and second respective channels of the plurality of fiber-optic receiver channels and spatially filtering respective portions of the light of the operable interference illumination field to provide at least first and second respective receiver channel optical signals having at least first and second respective signal phases;
• and
• outputting the at least first and second respective receiver channel optical signals along respective optical fibers to provide relative displacement measurement information in the form of a plurality of respective optical output signals, the respective optical output signals arising from spatially filtered scale light without the use of an electronic photodetector element.