HIGH-ACCURACY MID-IR LASEER-BASED GAS SENSOR

• US 20170248517A1
• Filed: 05/15/2017
• Published: 08/31/2017
• Est. Priority Date: 09/09/2011
• Status: Active Grant

• Current Assignee: Thermo Fisher Scientific GmbH (Thermo Fisher Scientific Incorporated)
• Original Assignee: Thermo Fisher Scientific GmbH (Thermo Fisher Scientific Incorporated)
• Inventors: SCHERER, James J., PAUL, Joshua B., JOST, Hans-Juerg
• Granted Patent: US 10,107,751 B2
• Time in Patent Office: Days
• CPC Class Codes: G01N 21/031 Multipass arrangements
• CPC Class Codes: G01N 21/0332 with temperature control control of temperature G05D23/00; cryostats F17C3/08
• CPC Class Codes: G01N 21/05 Flow-through cuvettes G01N21/09 takes precedence; handling fluid samples G01N1/10
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: G01N 21/61 Non-dispersive gas analysers G01N21/3504 takes precedence
• CPC Class Codes: G01N 33/0037 for NOx
• CPC Class Codes: G01N 33/004 for CO, CO2
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Abstract

A gas sensor system is provided, comprising: a gas cell operable so as to receive a sample gas; a vacuum system fluidically coupled to the gas cell operable to maintain the sample gas within the gas cell at a sub-ambient pressure; a pressure sensor operable to sense a pressure of the sample gas; a thermally insulated enclosure having the gas cell therein; a heat source or heat exchanger operable to influence an interior temperature of the thermally insulated enclosure; a light source within the thermally insulated enclosure operable to provide a mid-infrared (mid-IR) light into and through the gas cell; a photodetector within the thermally insulated enclosure operable to receive the attenuated mid-IR; and a control system electronically coupled to the vacuum system and to the pressure sensor operable to maintain the sample gas within the gas cell at the predetermined pressure to within one torr (1 Torr).

Claims

1. A gas sensor system for detecting and measuring the concentration of a gaseous molecular species within an environment having an ambient temperature and an ambient pressure, the system comprising:

a gas cell operable so as to receive the sample gas from the environment;

a vacuum system fluidically coupled to the gas cell operable to maintain the sample gas within the gas cell at a predetermined pressure less than the ambient pressure;

a pressure sensor operable to sense a pressure of the sample gas within the gas cell;

a thermally insulated enclosure having the gas cell disposed therein;

a heat source or heat exchanger operable to influence a temperature of the interior of the thermally insulated enclosure;

a light source within the thermally insulated enclosure operable to provide a mid-infrared (mid-IR) light into the gas cell so as to be transmitted through the sample gas therein, wherein a wavelength of the mid-IR light coincides with a rotationally resolved absorption line of the gaseous molecular species;

a photodetector within the thermally insulated enclosure operable to receive the mid-IR light transmitted through the sample gas in the gas cell; and

a control system electronically coupled to the vacuum system and to the pressure sensor operable to maintain the sample gas within the gas cell at the predetermined pressure to within one torr (1 Torr).

2. A gas sensor system as recited in claim 1, further comprising:

a temperature sensor operable to sense a temperature of the sample gas within the gas cell; and

a control system electronically coupled to the heat source or heat exchanger and to the temperature sensor operable to maintain the sample gas within the gas cell at a predetermined temperature to within one degree Kelvin (1°

K).

3. A gas sensor system as recited in claim 2, wherein the predetermined temperature is greater than the ambient temperature.

4. A gas sensor system as recited in claim 2, wherein the predetermined temperature is within a range between about 30°

C. to about 40°

C.

5. A gas sensor system as recited in claim 2, wherein the heat source or heat exchanger comprises a thermoelectric element disposed within an aperture of the thermally insulated enclosure that is operable to either transfer heat into or out of the thermally insulated enclosure.

6. A gas sensor system as recited in claim 5, further comprising a first and a second heat sink and fan assembly in thermal contact with the thermoelectric element and disposed outside of and within the thermally insulated enclosure, respectively.

7. A gas sensor system as recited in claim 1, further comprising:

a laser module within the thermally insulated enclosure, the light source being disposed within the laser module;

an optical module within the thermally insulated enclosure, the photodetector being disposed within the optical module, the optical module being optically coupled to the gas cell; and

an optical fiber coupled to the light source module and to the optical module, wherein the optical fiber is operable to direct the mid-IR light from the light source into the optical module.

8. A gas sensor system as recited in claim 1, further comprising:

a laser module within the thermally insulated enclosure comprising;

a first and a second laser operable to provide, respectively, a first near-infrared (near-IR) light having a first wavelength and a second near-IR light having a second wavelength; and

a wavelength division multiplexer (WDM) optically coupled to the first laser and to the second laser and operable to receive the first and second near-IR lights therefrom;

an optical fiber having a first end and a second end, the first end optically coupled to the WDM and operable to receive the first and second near-IR lights therefrom; and

an optical module within the thermally insulated enclosure and having the photodetector disposed therein comprising;

an optically non-linear crystal optically coupled to the second end of the optical fiber and operable to receive the first and second near-IR lights therefrom and to generate the mid-IR light by difference frequency generation; and

an optical filter optically coupled to the non-linear crystal and operable to transmit the mid-IR light while blocking transmission of the first and second near-IR lights,

wherein the optical filter is optically coupled to the gas cell such that, in operation, the mid-IR light is transmitted through the optical filter to the gas cell.

9. A gas sensor system as recited in claim 8, wherein the gas cell is fluidically coupled to the optical module such that, in operation, the sample gas flows from the gas cell into the optical module.

10. A gas sensor system as recited in claim 8, further comprising:

a temperature sensor operable to sense a temperature of the sample gas within the gas cell; and

a control system electronically coupled to the heat source or heat exchanger and to the temperature sensor operable to maintain the sample gas within the gas cell at a predetermined temperature to within one degree Kelvin (1°

K).

11. A gas sensor system as recited in claim 8, wherein the first and second lasers respectively comprise first and second diode lasers, further comprising:

a laser diode current driver operable to repeatedly modulate a drive current supplied to one of the first and second diode lasers such that a wavelength of the mid-IR light repeatedly traverses across a wavelength range of the rotationally resolved optical absorption line;

a digitizer electronically coupled to an output of the photodetector such that an output of the photodetector is digitized at each of a plurality of discrete time points during each modulation of the drive current so as to generate a plurality of digital detector signal values corresponding to a direct absorption spectrum; and

a digital memory storage device operable to store the plurality of digital detector signal values.

12. A gas sensor system as recited in claim 1, further comprising:

a laser module within the thermally insulated enclosure comprising;

a first laser operable to provide a first near-IR light having a first wavelength; and

a second laser operable to provide a second near-IR light having a second wavelength;

a first optical fiber having a first end and a second end, the first end optically coupled to the first laser and operable to receive the first near-IR light therefrom;

a second optical fiber having a first end and a second end, the first end optically coupled to the second laser and operable to receive the second near-IR light therefrom;

an optical module within the thermally insulated enclosure and having the photodetector disposed therein comprising;

a beam combiner or multiplexer optically coupled to the second end of the first optical fiber and to the second end of the second optical fiber, and operable to receive first and second near-IR lights from the first and second optical fibers, respectively;

an optically non-linear crystal optically coupled to beam combiner or multiplexer and operable to receive the first and second near-IR lights therefrom and to generate the mid-IR light by difference frequency generation; and

an optical filter optically coupled to the non-linear crystal and transmitting the mid-IR light and blocking transmission of the first and second near-IR lights,

wherein the optical filter is optically coupled to the gas cell.

13. A gas sensor system as recited in claim 12, wherein the gas cell is fluidically coupled to the optical module such that, in operation, the sample gas flows from the gas cell into the optical module.

14. A gas sensor system as recited in claim 12, further comprising:

a temperature sensor operable to sense a temperature of the sample gas within the gas cell; and

a control system electronically coupled to the heat source or heat exchanger and to the temperature sensor operable to maintain the sample gas within the gas cell at a predetermined temperature to within one degree Kelvin (1°

K).

15. A gas sensor system as recited in claim 12, wherein the first and second lasers respectively comprise first and second diode lasers, further comprising:

a laser diode current driver operable to repeatedly modulate a drive current supplied to one of the first and second diode lasers such that a wavelength of the mid-IR light repeatedly traverses across a wavelength range of the rotationally resolved optical absorption line;

a digitizer electronically coupled to an output of the photodetector such that an output of the photodetector is digitized at each of a plurality of discrete time points during each modulation of the drive current so as to generate a plurality of digital detector signal values corresponding to a direct absorption spectrum; and

a digital memory storage device operable to store the plurality of digital detector signal values.

16. A gas sensor system as recited in claim 1, wherein the light source comprises:

a first and a second diode laser operable to provide, respectively, a first near-infrared (near-IR) light having a first wavelength and a second near-IR light having a second wavelength; and

an optically non-linear crystal optically coupled to both the first and the second diode laser so as to receive both the first and the second near-IR lights and to generate the mid-IR light by difference frequency generation; and

an optical filter optically coupled to the non-linear crystal and operable to transmit the mid-IR light while blocking transmission of the first and second near-IR lights,

wherein the optical filter is optically coupled to the gas cell such that, in operation, the mid-IR light is transmitted through the optical filter to the gas cell.

17. A gas sensor system as recited in claim 16, further comprising:

a laser diode current driver operable to repeatedly modulate a drive current supplied to one of the first and second diode lasers such that a wavelength of the mid-IR light repeatedly traverses across a wavelength range of the rotationally resolved optical absorption line;

a digitizer electronically coupled to an output of the photodetector such that an output of the photodetector is digitized at each of a plurality of discrete time points during each modulation of the drive current so as to generate a plurality of digital detector signal values corresponding to a direct absorption spectrum; and

a digital memory storage device operable to store the plurality of digital detector signal values.

18. A gas sensor system as recited in claim 1, wherein the wavelength of the mid-IR light is within the range of approximately 3.0 microns to 4.8 microns.

19. A gas sensor system as recited in claim 18, wherein the gaseous molecular species is methane (CH₄) and a center wavelength of the rotationally resolved absorption line is within the range of approximately 3.2 microns to 3.4 microns.

20. A gas sensor system as recited in claim 18, wherein the gaseous molecular species is chosen from the group consisting of carbon monoxide (CO), nitrous oxide (N₂O) and carbon dioxide (CO₂) and a center wavelength of the rotationally resolved absorption line is within the range of approximately 4.2 microns to 4.7 microns.

21. A gas sensor system as recited in claim 18, wherein the gaseous molecular species is carbon monoxide (CO) and a center wavelength of the rotationally resolved absorption line is within the range of approximately 4.57 microns to 4.65 microns.

22. A gas sensor system as recited in claim 18, wherein the gaseous molecular species is nitrous oxide (N₂O) and a center wavelength of the rotationally resolved absorption line is within the range of approximately 4.510 microns to 4.555 microns.