# US 6,721,079 B2 · Optical path length tuning methods in etalons > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 6,721,079 B2 - Document ID: US6721079B2 - Status: Granted - Kind: B2 - Application number: 10/215,054 - Filed: 2002-08-08 - Published: 2003-02-20 - Granted: 2004-04-13 - Earliest claimed: 2001-08-09 - Assignees: Accumux Technologies, Inc. - Inventors: Pochi Albert Yeh; Scott Patrick Campbell - Web page: https://patentgrep.com/patent/US6721079B2 ## CPC - G02B 1/06: made of fluids in transparent cells - G02B 5/284: {of etalon type comprising a resonant cavity other than a thin solid film, e.g. gas, air, solid plates (etalons for fibre optic multiplexing G02B6/29358; etalons for spectral measurement G01J3/26)} - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS ## Abstract Fine-tuning of the optical path length in etalon cavities is achieved by slight variations of the cavity's index of refraction. Such index of refraction variations are accomplished by varying the relative gas mixture in gas-gap etalons or UV-exposing doped glass layer(s) in solid etalons or gas-gap+glass etalons. ## Claims ### Claim 1 A method for tuning an optical path length of an etalon with a cavity of length L and comprised of a plurality of gases having at least two refractive indices, the cavity having a net refractive index across the length L which is a function of the refractive indices of the plurality of gases, the optical path length defined as the length L multiplied by the net refractive index, the method comprising: adjusting a relative volume of the plurality of gases to fine-tune the net refractive index. ### Claim 2 (depends on 1) The method as recited in claim 1, further comprising: in-situ monitoring the optical path length of the etalon while adjusting the relative volume of the plurality of gases; and ceasing adjustment of the relative volumes of the plurality of gases when the desired optical path length is achieved. ### Claim 3 A method for tuning an optical path length of an etalon with a cavity of length L and comprised of one or more solid media, one or more of the solid media having a refractive index that is adjustable when exposed to radiation, the cavity having a net refractive index across the length L which is a function of the refractive indices of the solid media, the optical path length defined as the length L multiplied by the net refractive index, the method comprising: irradiating one or more of the solid media having a refractive index that is adjustable when exposed to radiation to fine-tune the net refractive index. ### Claim 4 (depends on 3) The method as recited in claim 3, further comprising: in-situ monitoring the optical path length of the etalon while irradiating the solid media; and ceasing irradiation of the solid media when the desired optical path length is achieved. ### Claim 5 (depends on 3) The method as recited in claim 3, further comprising: adjusting a relative volume of the one or more of the solid media having a refractive index that is adjustable when exposed to radiation to fine-tune the net refractive index. ### Claim 6 (depends on 3) The method as recited in claim 3, further comprising: irradiating one or more of the solid media having a refractive index that is adjustable when exposed to radiation in a spatially varying pattern so that the refractive index of the irradiated solid media varies; and tuning the optical path length by transverse displacement of a light beam across the irradiated solid media. ### Claim 7 (depends on 6) The method as recited in claim 6, further comprising: selecting the spatially varying pattern to match an optical path length error profile. ### Claim 8 A method for tuning a reflectivity and an effective length of a mirror stack of an etalon comprised of one or more solid media, one or more of the solid media of the mirror stack having a refractive index that is adjustable when exposed to radiation, the reflectivity and the effective length of the mirror stack being a function of the refractive indices of the solid media, the method comprising: irradiating one or more of the solid media having a refractive index that is adjustable when exposed to radiation to fine-tune the reflectivity and the effective length of the mirror stack. ### Claim 9 (depends on 8) The method as recited in claim 8, further comprising: in-situ monitoring the reflectivity and the effective length of the mirror stack of the etalon while irradiating the solid media; and ceasing irradiation of the solid media when the desired reflectivity and effective length of the mirror stack is achieved. ### Claim 10 An etalon comprising: two parallel mirrors separated by a distance L; a cavity formed between the two mirrors; and a plurality of gases having at least two refractive indices within the cavity; wherein the cavity has a net refractive index across the distance L which is a function of the refractive indices of the plurality of gases, the optical path length is defined as the distance L multiplied by the net refractive index; and wherein the cavity contains a relative volume of each of the plurality of gases to produce a desired optical path length. ### Claim 11 An etalon comprising: two parallel mirrors separated by a distance L; a cavity formed between the two mirrors; and one or more solid media having a refractive index that is adjustable when exposed to radiation within the cavity; wherein the cavity has a net refractive index across the distance L which is a function of the refractive indices of the solid media, the optical path length is defined as the distance L multiplied by the net refractive index; and wherein the cavity contains one or more radiated solid media having a refractive index that is adjustable when exposed to radiation to produce a desired optical path length. ### Claim 12 (depends on 11) The etalon as recited in claim 11, further comprising: wherein the cavity contains a relative volume of the one or more of the solid media having a refractive index that is adjustable when exposed to radiation to produce a desired optical path length. ### Claim 13 (depends on 11) The etalon as recited in claim 11, wherein the cavity contains one or more solid media having a refractive index that is adjustable when exposed to radiation irradiated in a spatially varying pattern so that the refractive index of the irradiated solid media varies across its surface. ### Claim 14 (depends on 13) The etalon as recited in claim 13, wherein the spatially varying pattern is selected to match an optical path length error profile. ### Claim 15 An etalon comprising: two parallel mirrors, at least one of the mirrors formed from a mirror stack comprised of one or more solid media, one or more of the solid media having a refractive index that is adjustable when exposed to radiation, the mirror stack having a reflectivity and effective length which is a function of the refractive indicies of the solid media; and a cavity formed between the two mirrors; wherein the mirror stack contains one or more irradiated solid media having a refractive index that is adjustable when exposed to radiation to produce a desired reflectivity and effective length for the mirror stack.