# US 12,216,051 B2 · Dynamic phase-shift interferometer utilizing a synchronous optical frequency-shift > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 12,216,051 B2 - Document ID: US12216051B2 - Status: Granted - Kind: B2 - Application number: 17/581,534 - Filed: 2022-01-21 - Published: 2023-07-27 - Granted: 2025-02-04 - Earliest claimed: 2022-01-21 - Assignees: Onto Innovation Inc. - Applicants: Onto Innovation Inc. - Inventors: James Millerd - Web page: https://patentgrep.com/patent/US12216051B2 ## CPC - G01N 21/45: using interferometric methods; using Schlieren methods - G01N 21/21: Polarisation-affecting properties (G01N21/19 takes precedence) - G01N 2201/06113: Coherent sources; lasers - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) ## Abstract An optical metrology device characterizes a test object using a phase shift interferometer with synchronous time varying optical frequency shifts. A light source generates a beam having a time varying frequency, which is divided into two collinear, orthogonally polarized beams that differ by a first frequency shift. One or more optical cavities receive the beams and produce a pair of reference beams that differ from each other in frequency by the first frequency shift and a pair of test beams with a second frequency shift induced by the one or more optical cavities. The test beams differ from each other by the first frequency shift and differ from the reference beams by the second frequency shift. The first frequency shift has a pre-defined relationship with respect to the second frequency shift to generate interference between a reference beam and test beam that have frequency shift magnitudes with the pre-defined relationship. ## Claims ### Claim 1 An optical device for characterizing a test object, comprising: a tunable light source that is controlled to generate input light having a time varying frequency; a polarization frequency-shifting element that receives the input light and that is controlled to generate two beams that are orthogonally polarized and have respective frequencies that differ from each other by a first frequency shift; one or more optical cavities that receive the two beams, the one or more optical cavities comprising a reference path and a test path that are traversed by the two beams to produce a pair of reference beams and a pair of test beams, wherein an optical path difference between the test path and the reference path induces a second frequency shift on the pair of test beams that is a function of a static length of the optical path difference, the pair of test beams differ from the pair of reference beams in frequency by the second frequency shift, wherein the first frequency shift is controlled by the polarization frequency-shifting element to have a pre-defined relationship with respect to the second frequency shift; and a polarization phase sensor that receives the pair of reference beams combined with the pair of test beams and detects an interferogram produced by interference of a reference beam and test beam that are orthogonally polarized with respect to each other and that have frequency shift magnitudes with the pre-defined relationship. ### Claim 2 (depends on 1) The optical device of claim 1, wherein the one or more optical cavities is a Fizeau optical cavity, wherein the reference path and the test path are on a common optical axis. ### Claim 3 (depends on 1) The optical device of claim 1, wherein the tunable light source is a tunable laser. ### Claim 4 (depends on 1) The optical device of claim 1, wherein the tunable light source is a broadband light source and tunable narrowband filter. ### Claim 5 (depends on 1) The optical device of claim 1, wherein the pre-defined relationship is a match between the first frequency shift and the second frequency shift that produces an interference fringe contrast in the interferogram that is at least 5% over an integration time of the polarization phase sensor. ### Claim 6 (depends on 1) The optical device of claim 1, wherein the pair of reference beams comprises a first reference beam having the time varying frequency and a second reference beam having the time varying frequency and the first frequency shift, and the pair of test beams comprises a first test beam having the time varying frequency and the second frequency shift, and a second test beam having the time varying frequency and the first frequency shift and the second frequency shift, wherein the polarization phase sensor detects the interferogram produced by interference of the second reference beam and the first test beam. ### Claim 7 (depends on 1) The optical device of claim 1, wherein the tunable light source is controlled with a first drive signal and the polarization frequency-shifting element is controlled by a second drive signal to generate the first frequency shift with the pre-defined relationship with respect to the second frequency shift. ### Claim 8 (depends on 7) The optical device of claim 7, wherein the polarization frequency-shifting element comprises a moving minor that is controlled by the second drive signal to generate the first frequency shift by a Doppler shift. ### Claim 9 (depends on 7) The optical device of claim 7, wherein the polarization frequency-shifting element comprises at least one acousto-optic modulator that is controlled by the second drive signal to generate the first frequency shift. ### Claim 10 (depends on 1) The optical device of claim 1, wherein a reference surface in the one or more optical cavities is stationary with respect to a test surface during an acquisition time of the polarization phase sensor to produce the static length of the optical path difference between the test path and the reference path. ### Claim 11 (depends on 1) The optical device of claim 1, wherein the interferogram is produced by interference of the reference beam having the first frequency shift and the test beam having the second frequency shift. ### Claim 12 A method of characterizing a test object with an optical device, comprising: generating input light having a time varying frequency with a tunable light source; generating from the input light two beams that are orthogonally polarized and have respective frequencies that differ from each other by a first frequency shift with a polarization frequency-shifting element that receives the input light; receiving the two beams with one or more optical cavities, the one or more optical cavities comprising a reference path and a test path that are traversed by the two beams to produce a pair of reference beams and a pair of test beams, wherein an optical path difference between the test path and the reference path induces a second frequency shift on the pair of test beams that is a function of a static length of the optical path difference, wherein the first frequency shift is controlled by the polarization frequency-shifting element to have a pre-defined relationship with respect to the second frequency shift; and receiving with a polarization phase sensor the pair of reference beams combined with the pair of test beams and detecting an interferogram produced by interference of a reference beam and test beam that are orthogonally polarized with respect to each other and that have frequency shift magnitudes with the pre-defined relationship. ### Claim 13 (depends on 12) The method of claim 12, wherein the one or more optical cavities is a Fizeau optical cavity, wherein the reference path and the test path are on a common optical axis. ### Claim 14 (depends on 12) The method of claim 12, wherein the tunable light source is one of a tunable laser or a broadband light source with a tunable narrowband filter. ### Claim 15 (depends on 12) The method of claim 12, wherein the pre-defined relationship is a match between the first frequency shift and the second frequency shift that produces an interference fringe contrast in the interferogram that is at least 5% over an integration time of the polarization phase sensor. ### Claim 16 (depends on 12) The method of claim 12, wherein the pair of reference beams comprises a first reference beam having the time varying frequency and a second reference beam having the time varying frequency and the first frequency shift, and the pair of test beams comprises a first test beam having the time varying frequency and the second frequency shift, and a second test beam having the time varying frequency and the first frequency shift and the second frequency shift, wherein detecting the interferogram comprises detecting the interferogram produced by interference of the second reference beam and the first test beam with the polarization phase sensor. ### Claim 17 (depends on 12) The method of claim 12, further comprising controlling the tunable light source with a first drive signal and controlling the polarization frequency-shifting element with a second drive signal to generate the first frequency shift with the pre-defined relationship with respect to the second frequency shift. ### Claim 18 (depends on 17) The method of claim 17, wherein the polarization frequency-shifting element comprises a moving minor that is controlled by the second drive signal to generate the first frequency shift by a Doppler shift. ### Claim 19 (depends on 17) The method of claim 17, wherein the polarization frequency-shifting element comprises at least one acousto-optic modulator that is controlled by the second drive signal to generate the first frequency shift. ### Claim 20 (depends on 12) The method of claim 12, wherein a reference surface in the one or more optical cavities is stationary with respect to a test surface during an acquisition time of the polarization phase sensor to produce the static length of the optical path difference between the test path and the reference path. ### Claim 21 (depends on 12) The method of claim 12, wherein detecting the interferogram comprises detecting the interferogram produced by the reference beam having the first frequency shift and the test beam having the second frequency shift. ### Claim 22 An optical device for characterizing a test object, comprising: means for generating input light having a time varying frequency; means for generating from the input light two beams that are orthogonally polarized and have respective frequencies that differ from each other by a first frequency shift; means for receiving the two beams with one or more optical cavities, the one or more optical cavities comprising a reference path and a test path that are traversed by the two beams to produce a pair of reference beams and a pair of test beams, wherein an optical path difference between the test path and the reference path induces a second frequency shift on the pair of test beams that is a function of a static length of the optical path difference, wherein the first frequency shift is controlled to have a pre-defined relationship with respect to the second frequency shift; and means for receiving the pair of reference beams combined with the pair of test beams and detecting an interferogram produced by interference of a reference beam and test beam that are orthogonally polarized with respect to each other and that have frequency shift magnitudes with the pre-defined relationship.