# US 8,772,706 B2 · Multiple wavelength configuration for an optical encoder readhead including dual optical path region with an optical path length difference > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,772,706 B2 - Document ID: US8772706B2 - Status: Granted - Kind: B2 - Application number: 13/452,782 - Filed: 2012-04-20 - Published: 2013-10-24 - Granted: 2014-07-08 - Earliest claimed: 2012-04-20 - Assignees: Mitutoyo Corporation - Applicants: Joseph D. Tobiason - Inventors: Joseph D. Tobiason - Web page: https://patentgrep.com/patent/US8772706B2 ## CPC - G01D 5/38: by diffraction gratings - G01D 5/268: {using optical fibres (G01D5/28 - G01D5/38 take precedence)} - G01D: MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR ## Abstract A readhead and scale arrangement comprises: a readhead comprising a light source configured to output diverging source light comprising a plurality of individually detected wavelengths and at least a first optical signal receiver channel configured to provide a first set of position signals; and a scale track extending along the measuring axis direction on a scale member comprising a scale grating, the scale track configured to diffract the first diverging source light to provide first and second diffracted scale light portions of each of the individually detected wavelengths to the first optical signal receiver channel which provide interference fringes. The readhead comprises an optical path difference element, arranged such that respective wavelengths undergo different phase shifts. Such an encoder is robust to scale track contamination and other signal degradation. ## Claims ### Claim 1 A readhead and scale arrangement usable to provide an indication of a position between two members that move relative to one another along a measuring axis direction, the readhead and scale arrangement comprising: a readhead comprising a light source configured to output diverging source light comprising a plurality of individually detected wavelengths and at least a first optical signal receiver channel configured to provide a first set of position signals; and a scale track comprising a scale grating extending along the measuring axis direction on a scale member, the scale track configured to diffract the diverging source light to provide first and second diffracted scale light portions of each of the individually detected wavelengths to the first optical signal receiver channel, wherein: the readhead is configured such that the first and second diffracted scale light portions of each individually detected wavelength interfere to provide interference fringes proximate to an input of the first optical receiver channel; the readhead comprises an optical path difference element comprising a first optical path region and a second optical path region wherein the first and second optical path regions have an optical path length difference, and the optical path difference element is arranged such that the first diffracted scale light portion of each of the individually detected wavelengths is derived from source light that passes through the first optical path region and emerges with a first wavelength-dependent phase shift, and the second diffracted scale light portion of each of the individually detected wavelengths is derived from source light that passes through the second optical path region and emerges with a second wavelength-dependent phase shift, such that for each of the individually detected wavelengths, the interference fringes arising from the first and second scale light portions have a different position-independent spatial phase shift that depends on that wavelength and the optical path length difference of the optical path difference element; and the input of the first optical signal receiver channel comprises a first spatial filter having a first spatial phase and configured to filter the interference fringes of the individually detected wavelengths such that each member of the first set of position signals comprises a position-dependent periodic signal of one of the individually detected wavelengths, the position-dependent periodic signal having a position-independent phase shift corresponding to the position-independent spatial phase shift of the interference fringes corresponding to that wavelength and the spatial phase of the first spatial filter. ### Claim 2 (depends on 1) The readhead and scale arrangement of claim 1, wherein the first optical path region and the second optical path region comprise different thicknesses of the same material. ### Claim 3 (depends on 1) The readhead and scale arrangement of claim 1, wherein the first optical path region comprises a material having a different index of refraction than a material of the second optical path region. ### Claim 4 (depends on 3) The readhead and scale arrangement of claim 3, wherein a material of the second optical path region comprises air. ### Claim 5 (depends on 1) The readhead and scale arrangement of claim 1, wherein a material of the optical path difference element extends to cover the input of the first optical receiver channel. ### Claim 6 (depends on 5) The readhead and scale arrangement of claim 5, wherein the first spatial filter comprises a periodic phase mask formed on the material that extends to cover the input of the first optical receiver channel. ### Claim 7 (depends on 1) The readhead and scale arrangement of claim 1, wherein the first optical path region transmits approximately half of the diverging source light, and the second optical path region transmits approximately half of the diverging source light. ### Claim 8 (depends on 1) The readhead and scale arrangement of claim 1, wherein the diverging source light comprises at least three detected wavelengths. ### Claim 9 (depends on 1) The readhead and scale arrangement of claim 1, further comprising a signal detector which includes a wavelength filtering element that separates each wavelength in the first set of position signals in order to provide position signals corresponding to each wavelength. ### Claim 10 (depends on 1) The readhead and scale arrangement of claim 1, wherein the scale grating is a phase grating. ### Claim 11 (depends on 1) The readhead and scale arrangement of claim 1, wherein the first optical receiver channel comprises a fiber optic receiver channel. ### Claim 12 (depends on 1) The readhead and scale arrangement of claim 1, wherein the first optical receiver channel comprises a photodetector. ### Claim 13 (depends on 12) The readhead and scale arrangement of claim 12, wherein the photodetector comprises a photodetector array that is configured as the periodic spatial filter. ### Claim 14 (depends on 1) The readhead and scale arrangement of claim 1, wherein: the readhead comprises a second optical receiver channel configured to provide a second set of position signals; the scale track diffracts the diverging source light to provide third and fourth diffracted scale light portions of each of the individually detected wavelengths to the second optical signal receiver channel, wherein: the readhead is configured such that the third and fourth diffracted scale light portions of each individually detected wavelength interfere to provide interference fringes proximate to an input of the second optical receiver channel; the third diffracted scale light portion of each of the individually detected wavelengths is derived from source light that passes through the first optical path region and emerges with the first wavelength-dependent phase shift and the fourth diffracted scale light portion of each of the individually detected wavelengths is derived from source light that passes through the second optical path region and emerges with the second wavelength-dependent phase shift such that for each of the individually detected wavelengths, the interference fringes arising from the third and fourth scale light portions have a different position-independent spatial phase shift that depends on that wavelength and the optical path length difference of the optical path difference element; and the input of the second optical signal receiver channel comprises a second spatial filter that is phase shifted relative to the first spatial filter and is configured to filter the interference fringes of the individually detected wavelengths such that each member of the second set of position signals comprises a position-dependent periodic signal of one of the individually detected wavelengths, the position-dependent periodic signal having a position-independent phase shift corresponding to the position-independent spatial phase shift of the interference fringes corresponding to that wavelength and the spatial phase of the second spatial filter. ### Claim 15 (depends on 1) The readhead and scale arrangement of claim 1, wherein the first spatial filter is formed on material which comprises a thickness of at least 250 microns, and the sum of the thickness of the material of the first spatial filter and a maximum thickness of the optical path difference element is at most 1600 microns.