WO2011114096A2 - Multiple pathlength gas cell - Google Patents

Multiple pathlength gas cell Download PDF

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Publication number
WO2011114096A2
WO2011114096A2 PCT/GB2011/000359 GB2011000359W WO2011114096A2 WO 2011114096 A2 WO2011114096 A2 WO 2011114096A2 GB 2011000359 W GB2011000359 W GB 2011000359W WO 2011114096 A2 WO2011114096 A2 WO 2011114096A2
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WO
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Prior art keywords
spectrometer
laser
paths
pathlength
optical
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PCT/GB2011/000359
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French (fr)
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WO2011114096A3 (en
Inventor
Ruth Lindley
Lain Howieson
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Cascade Technologies Limited
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Publication of WO2011114096A2 publication Critical patent/WO2011114096A2/en
Publication of WO2011114096A3 publication Critical patent/WO2011114096A3/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
    • G01N21/03—Cuvette constructions
    • G01N21/031—Multipass arrangements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G01N2021/396—Type of laser source
    • G01N2021/399—Diode laser
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/06—Illumination; Optics
    • G01N2201/066—Modifiable path; multiple paths in one sample
    • G01N2201/0662—Comparing measurements on two or more paths in one sample
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/06—Illumination; Optics
    • G01N2201/066—Modifiable path; multiple paths in one sample
    • G01N2201/0668—Multiple paths; optimisable path length
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/06—Illumination; Optics
    • G01N2201/069—Supply of sources
    • G01N2201/0696—Pulsed
    • G01N2201/0697—Pulsed lasers

Definitions

  • the present invention relates to an optical cell for detecting samples, for example fluids such as gases or liquids.
  • an optical spectrometer for example a gas or liquid optical sensor or detector that has a single measurement cell that supports two or more optical paths.
  • the paths may be perpendicular to each other.
  • the paths may be non- perpendicular.
  • the paths may be parallel to each other.
  • the paths may have the same or different lengths.
  • One pathlength may have a length selected from: at least ten times the other pathlength; at least twenty times the other pathlength; at least thirty times the other pathlength; at least forty times the other path length; at least fifty times the other path length; at least sixty times the other path length; at least seventy times the other path length.
  • Each path may be defined by two or more mirrors.
  • the mirrors may be astigmatic.
  • the mirrors may be spherical.
  • the mirrors may be flat.
  • the two or more paths may not include any mirrors.
  • a detector may be provided for detecting light from each path or one detector can detect light from both paths.
  • the sensor or detector preferably includes a laser.
  • the laser may be a chirped laser.
  • the wavelength variation provided by the wavelength chirp itself is used to provide a wavelength scan. This means that the sampling rate can be very high and a full spectral analysis can be done very quickly.
  • the chirp rate may be selected so that there is a time delay between spots on the ends of the paths of the cell sufficient to substantially prevent light interference from occurring, wherein the spots define locations at which the injected chirp is reflected from the cell walls.
  • the chirped laser may be a semiconductor laser, for example a semiconductor diode laser.
  • the chirped light is generated by applying a one or a series of substantially step function electrical pulses to the semiconductor diode laser to cause the laser to output one or more pulses, each having a continuous wavelength chirp, for injecting into the optical cell.
  • the laser may be a quantum cascade laser.
  • Each applied pulse has a duration that is greater than 150ns, in particular greater than 200ns.
  • Each applied pulse may have a duration that is in the range of 150 to 300ns, for example 200 to 300ns. This can provide a tuning range of about 60GHz.
  • Each detected pulse may have a duration that is greater than 150ns, in particular greater than 200ns.
  • Each detected pulse may have a duration that is in the range of 150 to 300ns, for example 200 to 300ns.
  • One or more beam splitters may be provided for dividing light from the source into a plurality of beams. Each beam is injected into a different optical path.
  • Figure 1 shows a multiple pathlength gas sensing cell
  • Figure 2 shows typical spot patterns associated with each path
  • Figure 3 shows a detailed design of the dual pathlength cell body
  • Figure 4 shows temporal displacement associated with long and short paths.
  • Figure 1 shows an elongate closed gas sensing cell 10 that has two paths 12 and 14 of different pathlength perpendicular to each other.
  • Light from a single source 16 is split at beam splitter 18 to form two beams that enter the cell independently via a first input 17 and a second input 19.
  • the length of the cell 10 is used to support a long pathlength 12, for example 40m, using two astigmatic mirrors 20a, b. Perpendicular to this and at the centre of the length of the cell is the shorter pathlength 14, for example 0.8m, that uses two spherical mirrors 22a, b. Both pathlengths 12 and 14 are configurable, and typically can vary between 2 and 100 m for the long path, and 0.1 and 5 m for the short path. Typical spot patterns for each mirror 20a, 20b, 22a and 22b are shown in Figure 2.
  • the mirror 20a allows a propotion of light that has passed along the long pathlength to leave the cell. This is detected by detector 24.
  • the mirror 22a allows a propotion of light that has passed along the short pathlength to leave the cell. This is detected by detector 26.
  • the input light from the source 16 is detected by a third detector 28 and may be used as a reference.
  • the source 16 may be a chirped semiconductor laser, for example a quantum cascade laser.
  • the chirped light is generated by applying a one or a series of substantially step function electrical pulses to the semiconductor diode laser to cause the laser to output one or more pulses, each having a continuous wavelength chirp, for injecting into the optical cell.
  • Each applied pulse typically has a duration that is greater than 150ns, in particular greater than 200ns.
  • Detected pulses typically have a duration greater than 150ns, in particular greater than 200ns.
  • Figure 3 shows the external cell body 30. This is constructed from aluminium, and includes heaters, purge ports for the astigmatic mirrors, inlet and exhaust lines for allowing a sample to be introduced into and removed from the cell, and ports for temperature and pressure sensors.
  • the cell is coated with PFA to reduce gaseous adsorption to the surfaces, and can be heated to 200C. Aluminium is used for the mirror substrate, and the cell body material is matched to this to maintain alignment over a 100C temperature range.
  • the volume of the cell is 320 ml and the lid can be fully removed to allow access to the mirrors for cleaning and alignment.
  • the cell has BaF 2 windows suitable for use over a wavelength range of 0.14 to 14 microns. All seals are made with high temperature o-rings.
  • the use of multiple pathlengths set perpendicular to each other within one cell affords significant advantages such as improved dynamic range and detection limit; simultaneous measurement; improved response time; reduced optical train and coherent noise removal.
  • the recovered signals can be used to detect or identify gases in the sample area. Typically, this involves comparing the detected signal with one or more fingerprints for known materials.
  • optical pathlengths for example one of 40m and the other of 0.8 m, affords up to a 50 times increase in dynamic range compared to using a single pathlength system. This improvement in dynamic range is achieved without any degradation in the gas detection limit associated with the longer optical pathlength.
  • further optical pathlengths could be supported within this cell to further improve the sensors dynamic range or indeed provide better overlap in the detection ranges associated with each optical pathlength.
  • the improvement in dynamic range and detection limit could also be afforded by using multiple gas absorption cells each with different pathlengths.
  • This approach presents a problem in that each cell would exhibit a measurement lag with respect to one another, precluding simultaneous measurement.
  • the use of multiple cells would also significantly increase the gas reside/response time of the sensor making it unsuitable for modern process applications.
  • the single cell approach, and in particular a design whereby the optical pathlengths are perpendicular to each other and centred on each axis, overcomes both these problems and affords a significantly more compact and stable optical train.
  • the signal from each optical path can be recorded by two detectors simultaneously. By dividing the signal of the long path detector with the short path detector it is possible to remove common noise sources associated with the cell. This would include cell induced optical fringing, pressure fluctuation and intensity variation noise.
  • both pulses are temporally displaced with respect to each other such that they can be detected by a single detector element. This is shown in Figure 4.
  • the delay associated with the transit time for the light pulse to propagate would be 330nS, while for 1m pathlength the delay associated with the transit time for the light pulse to propagate would be 3nS, giving a temporal displacement between the long and short pathlength light pulses of ⁇ 327nS. This would enable pulse durations no greater than 327ns and typically 150 - 300nS to be recorded on a single detector element without overlap or interference.
  • the delay associated with the transit time for the light pulse to propagate would be 165nS, while for 2m pathlength the delay associated with the transit time for the light pulse to propagate would be 6nS, giving a temporal displacement between the long and short pathlength light pulses of ⁇ 159nS. This would enable pulse durations no greater than 159ns and typically 100 - 150nS to be recorded on a single detector element without overlap or interference.
  • the temporally displaced signals associated with each path and detected by a single detector can be captured on a single channel digitizer.
  • the long path and short path signals can then be divided to remove common noise sources to further improve the performance of the spectrometer.
  • the ability to use a single detector element by utilizing the temporal displacement associated with each pathlength affords significant advantages. These include: a simplified optical train, associated reductions in complexity and cost related to the use of a single detector and single channel digitizer, and absolute uniformity in detector responsivity and bandwidth associated with the use of a single detector for each path. The latter is particularly important to enhancing the performance of the spectrometer by ensuring both paths provide an equivalent response across the increased dynamic range of the spectrometer.

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Physics & Mathematics (AREA)
  • General Health & Medical Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
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  • Pathology (AREA)
  • Optics & Photonics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Optical Measuring Cells (AREA)

Abstract

An optical spectrometer, for example a gas or liquid optical sensor or detector that has a single measurement cell that supports two or more optical paths.

Description

Multiple Pathlength Gas Cell
Field of the Invention
The present invention relates to an optical cell for detecting samples, for example fluids such as gases or liquids.
Background
Incoming legislation governing emissions monitoring has become ever more stringent. This has put increasing demands on metrology for monitoring environmental emissions. In particular, there has been a drive towards gas detection systems that can provide improved accuracy of measurement and lower limits of gas detection, combined with a large dynamic range. Mature gas detection technologies including NDIR and chemi-luminescence have been unable to meet this demand. The use of coherent laser light with its well known propagation characteristics can be exploited in gas detection to enhance a gas sensor dynamic range. By passing the light through an optical beam splitter, two beams of light from a single source can be produced. If these two beams are subsequently passed through further optical beam splitters, then any number of beams of light can be generated from a single source. In reality, the number of beams is limited by the decreasing light intensity that results from splitting, but in this respect QC lasers and in particular chirped QC lasers are ideally suited due the intrinsic high power afforded by these devices when used in chirped mode. Absorption spectroscopy, following the Beer Lambert law is well established. The relationship between the distance passed through a gas (optical pathlength) and strength of absorption signal can be used to tailor both the dynamic range and gas detection limit of a gas spectrometer. In reality, a compromise is typically required when choosing optical pathlength to ensure both an acceptable upper measurement range and lower detection limit is achieved. This compromise can be a significant factor in limiting the use of absorption spectroscopy in many applications.
Summary of the Invention
According to one aspect of the present invention, there is provided an optical spectrometer, for example a gas or liquid optical sensor or detector that has a single measurement cell that supports two or more optical paths. By providing two or more paths, one can be optimised for the upper gas measurement range and the other optimised for the lower detection limit without requiring any significant compromise between the two. The paths may be perpendicular to each other. The paths may be non- perpendicular. The paths may be parallel to each other.
The paths may have the same or different lengths. One pathlength may have a length selected from: at least ten times the other pathlength; at least twenty times the other pathlength; at least thirty times the other pathlength; at least forty times the other path length; at least fifty times the other path length; at least sixty times the other path length; at least seventy times the other path length.
Each path may be defined by two or more mirrors. The mirrors may be astigmatic. The mirrors may be spherical. The mirrors may be flat.
The two or more paths may not include any mirrors.
A detector may be provided for detecting light from each path or one detector can detect light from both paths.
The sensor or detector preferably includes a laser. The laser may be a chirped laser. In this case, the wavelength variation provided by the wavelength chirp itself is used to provide a wavelength scan. This means that the sampling rate can be very high and a full spectral analysis can be done very quickly.
The chirp rate may be selected so that there is a time delay between spots on the ends of the paths of the cell sufficient to substantially prevent light interference from occurring, wherein the spots define locations at which the injected chirp is reflected from the cell walls.
The chirped laser may be a semiconductor laser, for example a semiconductor diode laser. The chirped light is generated by applying a one or a series of substantially step function electrical pulses to the semiconductor diode laser to cause the laser to output one or more pulses, each having a continuous wavelength chirp, for injecting into the optical cell. The laser may be a quantum cascade laser. Each applied pulse has a duration that is greater than 150ns, in particular greater than 200ns. Each applied pulse may have a duration that is in the range of 150 to 300ns, for example 200 to 300ns. This can provide a tuning range of about 60GHz. Each detected pulse may have a duration that is greater than 150ns, in particular greater than 200ns. Each detected pulse may have a duration that is in the range of 150 to 300ns, for example 200 to 300ns.
One or more beam splitters may be provided for dividing light from the source into a plurality of beams. Each beam is injected into a different optical path.
Brief Description of the Drawings
Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:
Figure 1 shows a multiple pathlength gas sensing cell;
Figure 2 shows typical spot patterns associated with each path;
Figure 3 shows a detailed design of the dual pathlength cell body, and Figure 4 shows temporal displacement associated with long and short paths. Detailed Description of the Drawings
Figure 1 shows an elongate closed gas sensing cell 10 that has two paths 12 and 14 of different pathlength perpendicular to each other. Light from a single source 16 is split at beam splitter 18 to form two beams that enter the cell independently via a first input 17 and a second input 19.
The length of the cell 10 is used to support a long pathlength 12, for example 40m, using two astigmatic mirrors 20a, b. Perpendicular to this and at the centre of the length of the cell is the shorter pathlength 14, for example 0.8m, that uses two spherical mirrors 22a, b. Both pathlengths 12 and 14 are configurable, and typically can vary between 2 and 100 m for the long path, and 0.1 and 5 m for the short path. Typical spot patterns for each mirror 20a, 20b, 22a and 22b are shown in Figure 2.
The mirror 20a allows a propotion of light that has passed along the long pathlength to leave the cell. This is detected by detector 24. The mirror 22a allows a propotion of light that has passed along the short pathlength to leave the cell. This is detected by detector 26. The input light from the source 16 is detected by a third detector 28 and may be used as a reference. The source 16 may be a chirped semiconductor laser, for example a quantum cascade laser. The chirped light is generated by applying a one or a series of substantially step function electrical pulses to the semiconductor diode laser to cause the laser to output one or more pulses, each having a continuous wavelength chirp, for injecting into the optical cell. Each applied pulse typically has a duration that is greater than 150ns, in particular greater than 200ns. Detected pulses typically have a duration greater than 150ns, in particular greater than 200ns. Figure 3 shows the external cell body 30. This is constructed from aluminium, and includes heaters, purge ports for the astigmatic mirrors, inlet and exhaust lines for allowing a sample to be introduced into and removed from the cell, and ports for temperature and pressure sensors. The cell is coated with PFA to reduce gaseous adsorption to the surfaces, and can be heated to 200C. Aluminium is used for the mirror substrate, and the cell body material is matched to this to maintain alignment over a 100C temperature range. The volume of the cell is 320 ml and the lid can be fully removed to allow access to the mirrors for cleaning and alignment. The cell has BaF2 windows suitable for use over a wavelength range of 0.14 to 14 microns. All seals are made with high temperature o-rings.
The use of multiple pathlengths set perpendicular to each other within one cell affords significant advantages such as improved dynamic range and detection limit; simultaneous measurement; improved response time; reduced optical train and coherent noise removal. The recovered signals can be used to detect or identify gases in the sample area. Typically, this involves comparing the detected signal with one or more fingerprints for known materials.
The use of two optical path lengths, for example one of 40m and the other of 0.8 m, affords up to a 50 times increase in dynamic range compared to using a single pathlength system. This improvement in dynamic range is achieved without any degradation in the gas detection limit associated with the longer optical pathlength. In addition, further optical pathlengths could be supported within this cell to further improve the sensors dynamic range or indeed provide better overlap in the detection ranges associated with each optical pathlength.
The improvement in dynamic range and detection limit could also be afforded by using multiple gas absorption cells each with different pathlengths. This approach presents a problem in that each cell would exhibit a measurement lag with respect to one another, precluding simultaneous measurement. The use of multiple cells would also significantly increase the gas reside/response time of the sensor making it unsuitable for modern process applications. The single cell approach, and in particular a design whereby the optical pathlengths are perpendicular to each other and centred on each axis, overcomes both these problems and affords a significantly more compact and stable optical train.
The signal from each optical path can be recorded by two detectors simultaneously. By dividing the signal of the long path detector with the short path detector it is possible to remove common noise sources associated with the cell. This would include cell induced optical fringing, pressure fluctuation and intensity variation noise.
These sources are typically the predominant noise source associated with all optical based spectrometers and their removal will typically reduce the lower detection limit of the gas analyzer by a further order of magnitude. The ability to record both paths simultaneously and divide out common noise sources is a significant advantage over conventional measurement methodologies.
By careful selection of the two optical pathlengths and pulse duration, whereby the time delay associated with the transit time for the light pulse to propagate through the long path is sufficiently different to the time delay associated with the transit time for the light pulse to propagate through the short path, both pulses are temporally displaced with respect to each other such that they can be detected by a single detector element. This is shown in Figure 4.
As an example, for a 100m pathlength, the delay associated with the transit time for the light pulse to propagate would be 330nS, while for 1m pathlength the delay associated with the transit time for the light pulse to propagate would be 3nS, giving a temporal displacement between the long and short pathlength light pulses of ~327nS. This would enable pulse durations no greater than 327ns and typically 150 - 300nS to be recorded on a single detector element without overlap or interference.
As a further example, for a 50m pathlength, the delay associated with the transit time for the light pulse to propagate would be 165nS, while for 2m pathlength the delay associated with the transit time for the light pulse to propagate would be 6nS, giving a temporal displacement between the long and short pathlength light pulses of ~159nS. This would enable pulse durations no greater than 159ns and typically 100 - 150nS to be recorded on a single detector element without overlap or interference.
The temporally displaced signals associated with each path and detected by a single detector can be captured on a single channel digitizer. The long path and short path signals can then be divided to remove common noise sources to further improve the performance of the spectrometer.
The ability to use a single detector element by utilizing the temporal displacement associated with each pathlength affords significant advantages. These include: a simplified optical train, associated reductions in complexity and cost related to the use of a single detector and single channel digitizer, and absolute uniformity in detector responsivity and bandwidth associated with the use of a single detector for each path. The latter is particularly important to enhancing the performance of the spectrometer by ensuring both paths provide an equivalent response across the increased dynamic range of the spectrometer.
A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

Claims
1. An optical spectrometer, for example a gas or liquid optical sensor or detector that has a single measurement cell that supports two or more optical paths.
2. A spectrometer as claimed in claim 1 wherein the paths are perpendicular to each other.
3. A spectrometer as claimed in claim 1 wherein the paths are non- perpendicular, for example parallel to each other.
4. A spectrometer as claimed in any of the preceding claims wherein the paths are the same or different lengths.
5. A spectrometer as claimed in any of the preceding claims wherein each path is defined by two or more mirrors.
6. A spectrometer as claimed in claim 5 wherein the mirrors are selected from: astigmatic; spherical; flat.
7. A spectrometer as claimed in any of the preceding claims wherein a detector is provided for detecting light from each path or one detector for detecting light from both paths.
8. A spectrometer as claimed in any of the preceding claims including a laser, for example a chirped laser.
9. A spectrometer as claimed in claim 8 comprising means for applying one or a series of substantially step function electrical pulses to a semiconductor diode laser to cause the laser to output one or more pulses, each having a continuous wavelength chirp, for injecting into the optical cell.
10. An arrangement as claimed in claim 9 wherein each applied pulse has a duration greater than 150ns, in particular greater than 200ns.
11. An arrangement as claimed in claim 9 or claim 10 wherein each applied pulse has a duration in the range of 150 to 300ns, preferably 200 to 300ns.
12. An arrangement as claimed in any of claims 9 to 11 wherein the detected pulse duration is greater than 150ns, in particular greater than 200ns.
13. An arrangement as claimed in any of claims 9 to 12 wherein the detected pulse duration is in the range of 150 to 300ns, preferably 200 to 300ns.
14. A spectrometer as claimed in any of claims 8 to 13 wherein the chirped laser is a semiconductor laser, for example a semiconductor diode laser, and in particular a quantum cascade laser.
15. A spectrometer as claimed in any of claims 8 to 13 wherein the laser is pulsed and a single detector is provided, and wherein the pulse duration and/or pathlengths are selected to avoid overlap or interference of signals from the two paths at the detector.
16. A spectrometer as claimed in any of claims claim 8 to 15 wherein the chirp rate is selected so that there is a time delay between spots on the ends of the paths of the cell sufficient to substantially prevent light interference from occurring, wherein the spots define locations at which the injected chirp is reflected from the cell walls.
17. A spectrometer as claimed in any of the preceding claims wherein one or more beam splitters is provided for dividing light from the source into a plurality of beams. 8. A spectrometer as claimed in claim 17 wherein each beam is injected into a different optical path. 19. A spectrometer as claimed in any of the preceding claims wherein one pathlength has a length selected from: at least ten times the other pathlength; at least twenty times the other pathlength; at least thirty times the other pathlength; at least forty times the other path length; at least fifty times the other path length; at least sixty times the other path length; at least seventy times the other path length.
PCT/GB2011/000359 2010-03-16 2011-03-15 Multiple pathlength gas cell WO2011114096A2 (en)

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US10823671B2 (en) 2015-10-17 2020-11-03 General Electric Company Gas detector and method of detection
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