# US 2013/0239658 A1 · Photoacoustic sensor with mirror > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2013/0239658 A1 - Document ID: US20130239658A1 - Status: Published application - Kind: A1 - Application number: 13/419,827 - Filed: 2012-03-14 - Published: 2013-09-19 - Earliest claimed: 2012-03-14 - Assignees: Honeywell International Inc. - Inventors: Lisa Lust - Web page: https://patentgrep.com/patent/US20130239658A1 ## CPC - G01N 21/17: Systems in which incident light is modified in accordance with the properties of the material investigated (where the material investigated is optically excited causing a change in wavelength of the incident light G01N21/63) - G01N 2021/1704: {in gases} - 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 A photoacoustic sensing device includes a laser tuned to emit light to cause optical absorption by a gas to be detected, a resonant acoustic sensor positioned to receive pressure waves from the gas, wherein the laser is modulated to match a resonant frequency of the resonant acoustic sensor, and a first mirror positioned to receive light from the laser after the light has passed through the gas and to reflect the received light back through the gas to cause additional optical absorption. ## Claims ### Claim 1 A photoacoustic sensing device comprising: a laser tuned to emit light to cause optical absorption by a gas to be detected; a resonant sensor positioned to receive pressure waves from the gas, wherein the laser is modulated to match a resonant frequency of the resonant sensor; and a first mirror positioned to receive light from the laser after the light has passed through the gas and to reflect the received light back through the gas to cause additional optical absorption. ### Claim 2 (depends on 1) The device of claim 1 and further comprising a second mirror positioned relative to the first mirror to cause multiple further reflections of light through the gas between the first and second mirrors. ### Claim 3 (depends on 2) The device of claim 2 wherein the first and second mirror are positioned outside a range that would create a resonant cavity for the laser light. ### Claim 4 (depends on 2) The device of claim 2 wherein the second mirror is positioned between the laser and the first mirror and contains an opening to allow laser light to pass through the second mirror to reach the first minor. ### Claim 5 (depends on 4) The device of claim 4 wherein the laser is positioned to project light onto the first mirror at an angle from orthogonal to facilitate reflection back toward the second mirror at a suitable physical displacement from the opening. ### Claim 6 (depends on 5) The device of claim 5 wherein the first and second mirrors are substantially parallel to each other to reflect the laser light multiple times over a length of the mirrors. ### Claim 7 (depends on 6) The device of claim 6 wherein the resonant sensor comprises a tuning fork that is positioned between the two mirrors to receive the pressure waves and not receive the laser light. ### Claim 8 (depends on 2) The device of claim 2 wherein the resonant sensor is a microelectricalmechanical systems (MEMS) tuning fork. ### Claim 9 (depends on 1) The device of claim 1 wherein the laser light and gas are positioned proximate the tuning fork to optimize receipt of pressure waves from the optically heated gas. ### Claim 10 A method comprising: projecting a laser beam toward gas, wherein the laser beam is tuned to at least one gas absorption line of a gas to be detected; detecting pressure waves generated by optical absorption of the laser beam by the gas to be detected; and reflecting the laser beam back through the gas to increase an amplitude of the generated pressure waves. ### Claim 11 (depends on 10) The method of claim 10 and further comprising modulating the laser at a resonant frequency of a pressure wave detector. ### Claim 12 (depends on 11) The method of claim 11 wherein the pressure waves are detected by a tuning fork. ### Claim 13 (depends on 12) The method of claim 12 and further comprising converting the pressure waves to an electrical signal. ### Claim 14 (depends on 13) The method of claim 13 and further comprising processing the electrical signal to indicate whether or not the gas to be detected is present. ### Claim 15 A method comprising: positioning a pressure wave detector proximate a gas to be detected; aiming a laser toward the gas to be detected, wherein the laser is tuned to a optical absorption line corresponding to a desired gas; and positioning a first mirror to receive a laser beam that has gone through the gas to be detected such that it reflects the laser beam back through the gas to be detected. ### Claim 16 (depends on 15) The method of claim 15 and further comprising positioning a second mirror to reflect the laser beam reflected by the first mirror back through the gas to be detected. ### Claim 17 (depends on 16) The method of claim 16 and further comprising positioning the second mirror such that the laser beam is reflected back and forth between the first and second mirror through the gas to be detected several times. ### Claim 18 (depends on 17) The method of claim 17 wherein the number of reflections is at least fifty. ### Claim 19 (depends on 15) The method of claim 15 wherein the pressure detector comprises a piezoelectric tuning fork. ### Claim 20 (depends on 19) The method of claim 19 wherein the first and second mirrors form a cavity that directs pressure waves from the gas to the piezoelectric tuning fork.