WO2005103781A1 - Dispositif optique d'etablissement de moyenne du bruit modal - Google Patents

Dispositif optique d'etablissement de moyenne du bruit modal Download PDF

Info

Publication number
WO2005103781A1
WO2005103781A1 PCT/US2005/002853 US2005002853W WO2005103781A1 WO 2005103781 A1 WO2005103781 A1 WO 2005103781A1 US 2005002853 W US2005002853 W US 2005002853W WO 2005103781 A1 WO2005103781 A1 WO 2005103781A1
Authority
WO
WIPO (PCT)
Prior art keywords
optical fiber
multimode optical
multimode
fiber
temperature
Prior art date
Application number
PCT/US2005/002853
Other languages
English (en)
Inventor
Bernard Patrick Masterson
Eric C. Huelson
Ian S. Smith
Original Assignee
Zolo Technologies, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from PCT/US2004/010048 external-priority patent/WO2004090496A2/fr
Application filed by Zolo Technologies, Inc. filed Critical Zolo Technologies, Inc.
Priority to EP05712335A priority Critical patent/EP1730563A4/fr
Priority to CA2561711A priority patent/CA2561711C/fr
Priority to KR1020067020404A priority patent/KR101212439B1/ko
Priority to CN200580010448.0A priority patent/CN1938621B/zh
Priority to AU2005236385A priority patent/AU2005236385A1/en
Priority to US10/599,233 priority patent/US7787728B2/en
Priority to JP2007506152A priority patent/JP2007534983A/ja
Publication of WO2005103781A1 publication Critical patent/WO2005103781A1/fr

Links

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
    • G01N21/17—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
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • 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
    • G01J3/30—Measuring the intensity of spectral lines directly on the spectrum itself
    • G01J3/36—Investigating two or more bands of a spectrum by separate detectors
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/07—Controlling or regulating
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N5/00—Systems for controlling combustion
    • F23N5/02—Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium
    • F23N5/08—Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium using light-sensitive elements
    • F23N5/082—Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium using light-sensitive elements using electronic means
    • 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
    • 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
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0218—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using optical fibers
    • 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
    • G01J3/027—Control of working procedures of a spectrometer; Failure detection; Bandwidth calculation
    • 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
    • G01J3/0297—Constructional arrangements for removing other types of optical noise or for performing calibration
    • 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
    • G01J3/108—Arrangements of light sources specially adapted for spectrometry or colorimetry for measurement in the infrared range
    • 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
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/0014—Radiation pyrometry, e.g. infrared or optical thermometry for sensing the radiation from gases, flames
    • 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
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/0014—Radiation pyrometry, e.g. infrared or optical thermometry for sensing the radiation from gases, flames
    • G01J5/0018—Flames, plasma or welding
    • 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
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/58—Radiation pyrometry, e.g. infrared or optical thermometry using absorption; using extinction effect
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
    • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
    • G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
    • 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
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/359—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using near infrared light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35338—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using other arrangements than interferometer arrangements
    • G01D5/35341—Sensor working in transmission
    • G01D5/35345—Sensor working in transmission using Amplitude variations to detect the measured quantity
    • 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
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0818—Waveguides
    • G01J5/0821—Optical fibres
    • 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
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/14—Mode converters

Definitions

  • the present invention is directed toward combustion monitoring, and more particularly toward an apparatus and method for averaging mode noise associated with a multimode optical fiber.
  • Laser-based optical molecular species sensors have recently been implemented to address the concerns associated with extraction measurement techniques.
  • Laser-based measurement techniques can be implemented in situ and offer the further advantage of high-speed feedback suitable for dynamic process control.
  • a particularly promising technique for measuring combustion gas composition, temperature and other combustion parameters is tunable diode laser absorption spectroscopy (TDLAS).
  • TDLAS is well suited for the control and monitoring of coal-fired combustion processes.
  • TDLAS is equally well suited for the monitoring of other combustion processes.
  • the spectroscopy techniques described herein are useful for monitoring and controlling jet aircraft engine combustion parameters.
  • TDLAS is typically implemented with diode lasers operating in the near-infrared and mid-infrared spectral regions.
  • Suitable lasers have been extensively developed for use in the telecommunications industry and are, therefore, readily available for TDLAS applications.
  • Various techniques of TDLAS which are more or less suitable for the sensing and control of combustion processes have been developed. Commonly known techniques are wavelength modulation spectroscopy, frequency modulation spectroscopy and direct absorption spectroscopy. Each of these techniques is based upon a predetermined relationship between the quantity and nature of laser light received by a detector after the light has been transmitted through a combustion process chamber and absorbed in specific spectral bands which are characteristic of the gases present in the process or combustion chamber. The absorption spectrum received by the detector is used to determine the quantity of the gas species under analysis plus associated combustion parameters such as temperature.
  • Typical coal-fired power plants have combustion chamber dimensions of 10-20 meters on a side.
  • the plants are fired by pulverized coal, which results in a combustion process which both obscures the transmission of laser light because of the high dust load and which is extremely luminous.
  • the environment is also highly turbulent.
  • the overall transmission rate of light through the process chamber will fluctuate dramatically over time as a result of broadband absorption, scattering by particles or beam steering owing to refractive-index fluctuations.
  • There is also intense thermal background radiation from the burning coal particles which can interfere with detector signals.
  • the environment outside of the power plant boiler also makes the implementation of a TDLAS sensing or control system problematic. For example, any electronics, optics or other sensitive spectroscopy components must be positioned away from intense heat, or adequately shielded and cooled. Even though the implementation of a TDLAS system is extremely difficult under these conditions, TDLAS is particularly well suited to monitor and control a coal combustion process.
  • optical fiber coupling is particularly advantageous for the implementation of a TDLAS system.
  • one or more probe beams which may consist of multiplexed light of various relevant wavelengths are delivered to a pitch-side (transmit) optical apparatus and projected into the combustion chamber.
  • the probe beam is received in a catch-side (receive) optical apparatus after transversing the combustion chamber.
  • a multimode optical fiber in the catch-side optical train.
  • the use of a multimode fiber necessarily results in mode noise, which is a change in the signal level of detected light that results from non-uniform time and wavelength varying light distribution in the core of the multimode fiber used to collect and transport light.
  • mode noise is a change in the signal level of detected light that results from non-uniform time and wavelength varying light distribution in the core of the multimode fiber used to collect and transport light.
  • "Catch" -side mode noise can obscure absorption features which must be observed for effective TDLAS.
  • mode noise is not limited to or caused by TDLAS implementations which feature a catch-side multimode optical fiber.
  • mode noise will inevitably occur in any multimode optical fiber of substantial length which is transmitting light.
  • Mode noise is inevitable in a multimode fiber because the greater cross sectional diameter of a multimode fiber as compared to a single-mode fiber allows transmitted light to propagate along numerous light paths or modes. Some paths or modes are longer or shorter than others.
  • constructive and destructive interference necessarily occur resulting in non-uniform time and wavelength varying light distribution in the core of the multimode fiber which gives rise to a typical mode noise speckle pattern.
  • mode noise occurs in computing, telecommunication, or other scientific applications utilizing substantial lengths of multimode fiber. Whether or not mode noise interferes with the efficiency of a given optical system depends upon the requirements of the particular system.
  • the present invention is directed toward overcoming one or more of the problems discussed above.
  • the present invention is an optical mode noise averaging device including a multimode optical fiber and means for averaging a modal-noise-induced signal level variation of light propagating within the multimode optical fiber.
  • the device may average modal noise induced signal level variations by cyclically varying an index of refraction of the multimode optical fiber over a select period of time, scrambling a light distribution within the multimode optical fiber, or both.
  • the index of refraction of the multimode optical fiber may be cyclically varied by cyclically varying the temperature of the multimode optical fiber.
  • the index of refraction may be varied or the light distribution within the multimode optical fiber may be scrambled by cyclically and physically manipulating the multimode optical fiber.
  • the temperature of the multimode optical fiber may be varied through the action of a thermal element placed in thermal communication with the multimode optical fiber.
  • Suitable devices for use as a thermal element include, but are not limited to, a thermoelectric module, a resistive heater, an infrared heater, a chemical heater, a conventional refrigeration device, a chemical cooler, a source of fluid cooled below ambient temperature, or a source of fluid heated above ambient temperature.
  • the optical device may include a temperature sensor such as a thermocouple in thermal contact with the multimode optical fiber and a controller receiving input from the temperature sensor and controlling the thermal element.
  • a temperature sensor such as a thermocouple in thermal contact with the multimode optical fiber
  • a controller receiving input from the temperature sensor and controlling the thermal element.
  • the manipulation may include twisting, stretching, or shaking the multimode optical fiber.
  • a piezo stretcher may be used to accomplish the cyclical stretching of the multimode optical fiber.
  • a motor may be used to cyclically twist a portion of the multimode optical fiber in alternate clockwise and counterclockwise directions with respect to the longitudinal axis of the fiber and relative to a fixed portion of the fiber.
  • the present invention is further directed to a method of optical mode noise averaging in a multimode optical fiber including coupling light to the input of a multimode optical fiber, cyclically varying an index of refraction of the multimode optical fiber, and receiving the averaged light at the output of the multimode optical fiber.
  • the method of mode noise averaging may include varying the index of refraction by one of cyclically varying the temperature of the multimode optical fiber and cyclically manipulating the multimode optical fiber.
  • the temperature of the multimode optical fiber may be cyclically varied by providing a thermal component in thermal communication with the multimode optical fiber.
  • the multimode optical fiber may be cyclically manipulated by twisting, stretching, or shaking the multimode optical fiber.
  • FIG. 1 is a schematic diagram of a TDLAS sensing apparatus
  • FIG. 2 is a schematic diagram of a TDLAS sensing apparatus featuring remotely located components optically coupled to components near a combustion chamber
  • FIG. 3 is a schematic diagram of an optical mode noise averaging device consistent with the present invention
  • FIG. 4 is an exploded view of a temperature-based phase shifting apparatus with a source of fluid heated above or cooled below ambient temperature as a thermal element
  • FIG. 5 is an exploded view of a temperature-based phase shifting apparatus utilizing a series of thermoelectric devices as the thermal element
  • FIG. 6 is an alternative exploded view of the temperature-based phase shifting apparatus of FIG.
  • FIG. 7 is a schematic diagram of an optical mode noise averaging device using a motor for mechanical manipulation of a multimode optical fiber
  • FIG. 8 is a schematic diagram of an optical mode noise averaging device using a piezo stretcher for mechanical manipulation of a multimode optical fiber
  • FIG. 9 is a schematic diagram of a pitch-side mode noise reduction device.
  • a preferred embodiment of the present invention is an optical mode noise averaging apparatus.
  • the optical mode noise averaging apparatus is described in detail in Section E below.
  • the optical mode noise averaging apparatus is particularly well suited to, but not limited to, averaging the mode noise which is inherent in a catch-side (or receive side) multimode optical fiber associated with a fiber-coupled tunable diode laser absorption spectroscopy (TDLAS) sensing apparatus.
  • TDLAS fiber-coupled tunable diode laser absorption spectroscopy
  • a fiber-coupled TDLAS sensing apparatus is described below.
  • the preferred embodiments of the present invention are suitable for averaging optical mode noise in any optical system where mode noise is present.
  • the optical mode noise averaging apparatus may be implemented in any computing, telecommunications, scientific research, or other system which features a significant length of multimode optical fiber transmitting light.
  • the averaging apparatus will be useful in any optical system where the efficiency of the system can be enhanced by averaging the optical mode noise inherent in light propagating within a multimode optical fiber.
  • FIG. 1 An embodiment of a sensing apparatus 10 suitable for the sensing, monitoring and control of a combustion process is shown in FIG. 1.
  • the sensing apparatus 10 performs tunable diode laser abso ⁇ tion spectroscopy (TDLAS) using laser light from a series of tunable diode lasers 12 lasing at select frequencies in the near-infrared or mid-infrared spectrum.
  • TDLAS tunable diode laser abso ⁇ tion spectroscopy
  • the output of each tunable diode laser 12 is coupled to an individual optical fiber which may be a single-mode optical fiber 14 and routed to a multiplexer 16.
  • Coupled As used herein, “coupled”, “optically coupled” or “in optical communication with” is defined as a functional relationship between counte ⁇ arts where light can pass from a first component to a second component either through or not through intermediate components or free space.
  • the laser light of some or all of the frequencies generated is multiplexed to form a multiplexed probe beam having multiple select frequencies.
  • the multiplexed probe beam is coupled to a pitch-side (or transmit side) optical fiber 18 and transmitted to a pitch optic 20 or collimator operatively associated with a process chamber which, in FIG. 1, is shown as a combustion chamber 22.
  • the pitch optic 20 is oriented to project the multiplexed probe beam through the combustion chamber 22.
  • a catch optic 24 Across the combustion chamber 22 in optical communication with the pitch optic 20 is a catch optic 24.
  • the catch optic 24' is preferably substantially opposite the pitch optic 20 and is operatively associated with the combustion chamber 22.
  • the catch optic 24 is positioned and oriented to receive the multiplexed probe beam projected through the combustion chamber 22.
  • the catch optic 24 is optically coupled to a catch-side optical fiber 26 which transmits the portion of the multiplexed probe beam which is received by the catch optic 24 to a demultiplexer 28.
  • each wavelength of demultiplexed laser light is coupled to an output optical fiber 30.
  • Each output optical fiber 30 in turn is optically coupled to a detector 32, which typically is a photodetector sensitive to one of the select frequencies of laser light generated and multiplexed to form the probe beam.
  • the detectors 32 generate an electrical signal based upon the nature and quantity of light transmitted to the detector 32 at the detector frequency.
  • the electrical signal from each detector 32 is typically digitized and analyzed in data processing system 34.
  • the digitized and analyzed data can be used to sense physical parameters within the process chamber including but not limited to the concentrations of various gas species and the combustion temperature within the combustion chamber 22.
  • the data processing system 34 can further be used to send signals through a feedback loop 36 to combustion control apparatus 38 and thereby actively control select process parameters.
  • the process parameters controlled can include fuel (e.g., pulverized coal) feed rates; oxygen feed rates and catalyst or chemical agent addition rates.
  • FIG. 2 schematically depicts the overall component placement of a fiber-coupled, multiplexed sensing system 40.
  • the sensing system 40 generally consists of a system rack 42, a breakout box 44, a transmitter head 46 having pitch optics 48, a receiver head having catch optics 52 and connecting optical fibers.
  • the system rack 42 is preferably located in a remote control room situated a distance, for example one kilometer, from the combustion chamber 54. The control room typically will have a moderate controlled environment.
  • the system rack 42 contains the lasers 56, detectors 58, wavelength multiplexers 60 and wavelength demultiplexers 62.
  • the system rack 42 also houses the system electronics and control software (not shown in FIG. 2).
  • the system rack 42 may optionally house an alignment light source 64.
  • the optical fibers connecting the system rack 42 with the breakout box 44 are typically standard single-mode telecom optical fiber.
  • This type of fiber is inexpensive, readily available, low-loss and allows the laser light to be directed to various off-the-shelf telecom components to manipulate the light, such as optical switches, splitters, and wavelength division multiplexers. Without optical fiber coupling, the laser light would have to be directed through free space all the way to the combustion chamber 54, which would be very difficult to implement or, alternatively, sensitive electronic and optical components would have to be situated in close proximity to the combustion chamber 54.
  • the breakout box 44 is a ruggedized enclosure located close to the boiler.
  • the breakout box 44 contains optical switches, splitters and couplers (collectively 66) which may be used as discussed below to direct the optical signals to multiple transmitter-receiver head pairs.
  • a third group of system components as shown in FIG. 2 are the transmitter and receiver heads 46, 50.
  • the optics and electronics in the transmitter and receiver heads 46, 50 must convert the light in the fiber 68 into a collimated beam, direct the beam accurately through the combustion chamber 54, capture the beam on the far side of the combustion chamber 54 and couple the beam into the fiber 70.
  • the choice of optics to accomplish this is determined by the transmission distance, the turbulence of the combustion zone, its effect on the transmitted beam's quality, and the core size of the fiber 70. Selection of the core size is a matter of design choice dictated by the application. A larger core will capture more of the laser light but also much more of the background light.
  • Fiber coupling on the catch (receiver) side has several advantages. In particular, only light in the same location as the laser light and traveling in the same direction is focused into the fiber 70. This drastically reduces the amount of background light that is sensed. Light may be captured into one of several receiver fibers and an optical switch or other optical routing device can select light from one fiber for routing to the detectors 58. Only one catch optic is shown in FIG. 2.
  • both the pitch and catch optics 48, 52 are custom-designed and aberration-corrected for wavelengths from 660 nm to 1650 nm so that multiple laser signals can be efficiently transmitted and received at the same time.
  • Tunable diode laser abso ⁇ tion spectroscopy is performed using techniques known to those skilled in the art of laser spectroscopy. Generally, TDLAS is performed by the transmission of laser light through a target environment, followed by the detection of the abso ⁇ tion of the laser light at specific wavelengths, due to target gases such as carbon monoxide or oxygen. Spectral analysis of the detected light allows identification of the type and quantity of gas along the laser path.
  • laser light provides the high brightness necessary to receive detectible transmission in the presence of severe attenuation (typically greater than 99.9% loss of light) that may be seen in some of these environments.
  • severe attenuation typically greater than 99.9% loss of light
  • the laser light may be brought in to the target environment through armored optical fiber.
  • Effective sensing of temperature or multiple combustion process component gasses requires the performance of TDLAS with multiple widely spaced frequencies of laser light.
  • the frequencies selected must match the abso ⁇ tion lines of the transitions being monitored. For example, it is useful to monitor NO 2 at a wavelength of 670 nm to approximate emission NO concentrations. It is also quite useful to monitor oxygen, water vapor (temperature), and carbon monoxide in a coal-fired utility boiler.
  • Suitable abso ⁇ tion lines, and thus suitable lasing frequencies can be selected based upon an assumption that the laser probe path length through a combustion chamber is equal to 10 meters and that the mole fraction of each species is CO (1%), O 2 (4%), CO 2 (10%), and H 2 O (10%).
  • the process temperature can be assumed to be 1800 K which is slightly higher than what is typically observed in a coal-fired plant, but the cushion serves as a safety factor in the calculations.
  • three water abso ⁇ tion lines can be selected for TDLAS that meet the following criteria:
  • the preferred (de)multiplexing grating efficiency calculates to be only 40% in this region, however, suitable laser power should be available for reasonable measurement efficiency.
  • the use of fiber coupling on both the pitch and catch-sides of a TDLAS sensing apparatus requires critical alignment of the pitch and catch optics. Active alignment is preferably accomplished with a select alignment wavelength.
  • One possible alignment wavelength is 660 nm because high power (45 mW) diodes are available at this frequency and 660 nm would be near the peak of 14th order grating operation.
  • Other alignment wavelengths may be determined to be equally or more suitable.
  • a reasonable set of wavelengths selected for multiplexing to a probe beam for TDLAS as embodied in the present invention are as shown in Table 2. It should be noted that this wavelength set is for one embodiment of a TDLAS sensing apparatus suitable for the sensing and control of a coal-fired power plant. Other wavelength sets can be equally suitable. Table 2
  • TDLAS Using Multiplexed Beam 40
  • a particular advantage of TDLAS with a wavelength-multiplexed probe beam is increased accuracy of temperature measurements.
  • the temperature of the monitored gas In order to make accurate concentration measurements with TDLAS, the temperature of the monitored gas must be known.
  • the strength of a molecular abso ⁇ tion is a function of temperature.
  • the temperature To convert the amplitude of an abso ⁇ tion feature to concentration, the temperature must be known.
  • Certain previous attempts to measure the concentration of combustion species such as CO suffer from insufficiently accurate temperature measurements leading to errors in quantification. This is particularly true for diode-laser-based ammonia slip monitors that have traditionally not inco ⁇ orated temperature measurement at all.
  • temperature may be determined by measuring the ratio of the intensity of two or more molecular water lines.
  • the ratio of the integrated intensity of two lines is a function of temperature only (assuming constant total system pressure).
  • two lines provide an accurate temperature.
  • two lines do not suffice to determine the temperature distribution.
  • two lines can only determine a "path- averaged" temperature.
  • measuring the integrated amplitude of more than two lines allows temperature non-uniformity to be probed.
  • Mode noise is defined herein as a change in the signal level of detected light that results from non-uniform time and wavelength varying light distribution in the core of a fiber used to collect and transport the light to and from the process chamber being measured.
  • P Po + ⁇ ⁇ c ⁇ E 1 E J cos[(27 ⁇ 'o ⁇ n 1 jZ/c + ⁇ y (T, ⁇ ))] (1)
  • P 0 wavelength independent average power
  • E amplitude of light
  • n the ith transverse mode c y overlap integral between the ith and jth transverse mode
  • ⁇ ny refractive index difference between the ith and jth modes
  • ⁇ jj phase shift between ith and jth modes due to temperature and stress
  • the total index change, ⁇ n is ⁇ 1%, but most modes spend the bulk of transmission time close to the fiber core center, and therefore, ⁇ n; j ⁇ 0.0005, in general.
  • the commonly available optical fiber GIF50 supports approximately 135 modes, which is sufficiently coarse to produce prominent mode noise during a wavelength scan given reasonably achievable beam clipping levels.
  • the intensity at a point z along the fiber is:
  • single-mode fiber could be used in the catch-side optical train of a system as described above, mode noise would not be an issue.
  • multimode fiber must typically be used in the catch-side optical train of a fiber-coupled TDLAS system for two reasons.
  • the measurement volume a combustion chamber with a measurement path in excess of 10 meters
  • the initially single-mode (Gaussian spatial distribution) beam is significantly degraded in quality.
  • the coupling efficiency of this severely distorted beam into single-mode fiber would be very poor. This is an unacceptable situation since the beam is attenuated by 3 - 4 orders of magnitude when passing through the measurement volume primarily due to scattering and obscuration by soot and fly ash.
  • the core of a multimode fiber presents at least 25 times the target cross sectional area of a single-mode fiber.
  • the effects of beam steering can be significantly reduced.
  • the coupling efficiency into multimode fiber is independent of the spatial mode of the light, the poor beam quality obtained after passing through the fireball is not an issue.
  • mode dependent losses occurring in the multimode fiber train are a significant design challenge.
  • the light distribution emanating from the core of a multimode fiber exhibits a random speckle pattern, i.e., a random pattern of light and dark areas caused by constructive and destructive interference between different modes of the fiber. If the speckle pattern was totally invariant as a function of time and wavelength, it would not present a problem. However, slow variations in the speckle pattern particularly as a function of wavelength can cause mode noise if the beam is clipped anywhere in the multimode catch-side optical train as described above. This clipping is impossible to avoid; it can only be reduced. Therefore, additional measures to reduce mode noise must be implemented to improve the detection sensitivity of the system.
  • mode noise may be reduced by:
  • the catch optics of a fiber-coupled TDLAS sensing system are designed and implemented to inco ⁇ orate all of the above in order to reduce modal noise.
  • the optics are designed such that any beam clipping should occur at a low level given near perfect alignment of the system. Efforts should be made to keep the length of multimode fiber to a minimum; however, for some applications z must be long in order to have the control electronics in an environmentally controlled area.
  • the value of ⁇ njj may be reduced by using premium low-dispersion multimode fiber.
  • excellent results may be obtained by averaging the modes by cyclically varying the index of refraction or mechanical manipulation of a catch-side multimode fiber and extracting data from the averaged light signal collected.
  • the speckle pattern exhibited in a multimode fiber varies as a function of time and wavelength and also as a function of the mechanical position of the fiber. Both transmission time and wavelength are affected by the index of refraction of the fiber. Flexing the fiber and manipulating it in specific ways can cause the speckle pattern to change. If these mechanical manipulations or a cyclic variation of the index of refraction are performed continuously, over a period of time, the spatial distribution of light emanating from the fiber averages to a relatively uniform pattern.
  • the index of refraction of a fiber may be changed by stretching or twisting the fiber or varying the temperature of the fiber. Varying the temperature of the fiber causes a change in the refractive index difference between the ith and jth transverse modes, Any. This change in the index of refraction of the fiber shifts the modal noise by the function cos(27iTo ⁇ nijz)/c given in equation 2. '
  • the cyclical phase shifting or scrambling of modal noise to produce a time averaged measurement may be accomplished with an optical device 300.
  • the optical device 300 will include a multimode optical fiber 302 having an input 304 and an output 306.
  • Light may be coupled to the input 304 of the multimode optical fiber 302 and will generally propagate through the system in the direction of the arrows shown in FIG. 3 in association with the input 304 and the output 306.
  • the optical device 300 will also include an averaging component 308 operatively associated with the multimode fiber 302.
  • the averaging component 308 may include apparatus for cyclically varying an index of refraction of the multimode optical fiber 302 over a select period of time.
  • the averaging component 308 may include an apparatus for scrambling a light distribution within the multimode optical fiber 302. Variation of an index of refraction or scrambling a light distribution may be accomplished by the averaging component 308 through cyclically varying the temperature of the multimode optical fiber 302, cyclically manipulating the multimode optical fiber 302, or both.
  • the averaging component 308 may twist, stretch, or shake the multimode optical fiber 302.
  • various thermal elements or thermal components may be provided in thermal communication with the multimode optical fiber. Any apparatus which will affect the temperature of the multimode optical fiber 302 can be included in the averaging component 308.
  • Representative devices which can be used to affect the temperature of the multimode optical fiber 302 include a thermoelectric module, a resistive heater, an infrared heater, a chemical heater, a conventional refrigeration device utilizing compressed fluids and heat exchangers, a chemical cooler, a source of fluid cooled below ambient temperature, and a source of fluid heated above ambient temperature. Some of these devices are discussed in detail below.
  • a sensor 310 may also be placed in thermal communication with the multimode optical fiber 302.
  • the sensor 310 may provide information to a controller 312 which in turn may control the averaging component 308 through a control line 314.
  • Temperature-based mode shifting is a particularly efficient method of dealing with modal noise because varying the temperature of the fiber changes in the index of refraction of all traverse modes and temperature changes can occur over a significant length of the fiber.
  • index of refraction of the fiber By changing the index of refraction of the fiber, one guarantees that all transverse modes will shift, and no modes can remain 'frozen' with the signal.
  • any type of heating/cooling system can be placed in thermal communication with a multimode fiber to cycle the temperature of the fiber.
  • Electric resistive heaters conventional refrigeration devices, heated or cooled fluids, Peltier or other thermoelectric devices, infrared devices, or chemical devices could all be used to affect the temperature of a fiber.
  • FIG. 4 An exploded view is illustrated in FIG. 4.
  • An embodiment of a fluid device 400 which uses vortex air tubes 402A, 402B to alternately blow hot and cold air across and around the fiber is shown in FIG. 4.
  • Air delivered from a compressed air source (not shown in FIG. 4) is alternately delivered to one of two vortex tubes 402A, 402B.
  • the vortex tubes 402 A, 402B are coupled in fluid communication with the interior of a chamber 404. As shown in FIG.
  • the chamber 404 may be formed of an enclosure 406 having sides 408A, 408B, a top plate 410, a front 412, and a rear access port 414.
  • the entire enclosure 406 may be suitable for rack mounting in a typical data processing equipment rack.
  • a rack mountable enclosure 406 is particularly convenient, any enclosure shape, type, or style which is suitable for receiving a spool 416 holding a length of multimode optical fiber 418 may be used to implement the fluid device 400.
  • a device without an enclosure 406 could be used.
  • the vortex tubes 402A, 402B are in fluid communication with the interior of the enclosure through access port 414, and are thus in fluid and thermal communication with the multimode optical fiber 418 wound onto the spool 416. Accordingly, the multimode optical fiber 418 may be cyclically heated and/or cooled by the application of air heated or cooled above or below ambient temperature by the vortex tubes 402 A, 402B.
  • Suitable vortex tubes 402A, 402B are readily available.
  • EXAIR ® 3230 vortex tubes are available from EXAIR ® Co ⁇ oration. These or similar vortex tubes operating at 30 ft 3 /minute throughput can provide air heated to +60°C or cooled to -20°C depending on the orientation of the tube. In addition, it is relatively easy to cycle between heated and cooled air when using vortex tubes. It is important to note, however, that any apparatus or method of cyclically supplying a heated or cooled fluid in thermal communication with the multimode optical fiber 418 is suitable for the implementation of an embodiment of the fluid device 400.
  • the heating and cooling fluid may be air as discussed above, however, water, heating/cooling oils, compressed gases, or other fluids may be used to heat or cool the multimode fiber.
  • one of the vortex tubes 402A, 402B may deliver heated air until the fiber reaches a temperature about 10°C greater than the inlet temperature.
  • the temperature of the fiber may be determined by a thermocouple 420 or other temperature sensor embedded in contact with the fiber.
  • a temperature control unit (not shown in FIG. 4) may receive input from the thermocouple 420 and trigger a solenoid switch so that air is sent to the other vortex tube 402A, 402B for cooling.
  • the use of temperature controller can be eliminated and replaced with a timed relay to cyclically switch the vortex tubes 402A, 402B between heating and cooling. Cycling between temperatures is preferably continuous during operation of the device.
  • thermoelectric modal phase shifting device 500 includes a spool 502 holding a select length of multimode optical fiber 504.
  • One or more thermoelectric heating/cooling modules 506 are placed in thermal communication with the multimode optical fiber 504.
  • multiple thermoelectric heating/cooling modules 506 are radially disposed around the inside of the spool 502. Thermal communication is facilitated by the use of thermoelectric grease between the outer surface of a thermoelectric module 506 and the coil of multimode optical fiber 504.
  • the spool 502 is constructed with an open rim to facilitate contact between the thermoelectric modules 506 and the multimode optical fiber 504 and to facilitate winding the multimode optical fiber 504.
  • thermoelectric modules 506 One arrangement for positioning of the thermoelectric modules 506 in contact with the multimode fiber 504 and relative to the spool 502 is illustrated in the exploded view of FIG. 6.
  • One or more heat sinks 508 may also be operatively disposed in thermal communication with the thermoelectric modules 506.
  • the heat sinks 508 are fabricated from a highly thermally conductive material such as aluminum or copper and have fins or other apparatus designed to increase the surface area of each heat sink 508.
  • a fan 510 may be operatively positioned to force or draw air across the heat sinks 508, facilitating the rapid extraction of heat from the thermoelectric modules 506 and thus the rapid heating or cooling of the multimode fiber 504. As shown in FIG. 5 and FIG.
  • a framework 512 having a top ring 514 and a bottom ring 516 may be employed to hold the components of the thermoelectric device 500 in a proper orientation with respect to each other without hindering the flow of air over and around the heat sinks 508.
  • openings 518 are formed in the framework 512 to assure free airflow.
  • thermoelectric modules 506 which are based upon the Peltier principle. Direct current is provided to a thermoelectric module 506 through leads 520. With a thermoelectric module 506 operating under the Peltier principle, opposite surfaces of the thermoelectric module 506 heat or cool depending on the direction of the DC current provided. Thus, these modules provide certain advantages because it is relatively easy to selectively heat or cool the multimode optical fiber 504 by selectively switching the polarity of the direct current provided to the leads 520. It is important to note, however, that other types of devices may be implemented to heat and/or cool the multimode optical fiber 504. For example, direct resistive heaters, conventional refrigeration apparatus, infrared heating devices, and/or chemical reaction based heaters and/or coolers could be used to vary the temperature of the multimode fiber 504.
  • power may be delivered from a suitable power supply to the thermoelectric modules 506 mounted in a cylindrical configuration as shown in FIG. 5 and FIG. 6.
  • Current switching electronics may be used to cyclically reverse the polarity of the DC power delivered to each thermoelectric module 506.
  • Modal phase shifting occurs in the length of multimode fiber 504, preferably Premium GIF50 multimode fiber, that is in thermal communication with the thermoelectric modules 506. 55m to 100m has been found to be a suitable length of multimode fiber to accomplish modal phase shifting and averaging. Other lengths may be suitable as well. If 100m of wrapped fiber is used approximately 50% of the fiber is in direct contact with the thermoelectric coolers.
  • thermocouples 522 or other temperature measuring devices may be mounted between the thermoelectric modules 506 and the multimode optical fibers 504 and used to monitor the temperature of the multimode optical fiber 504 at all times.
  • a temperature control unit (not shown in FIG. 5 and FIG. 6) may receive the thermocouple- determined temperature and cycle the current direction based on the temperature readings. Temperate ranges of about 35°C to 50°C are achievable with readily available thermoelectric modules.
  • thermoelectric modules 506 heat may be dissipated on the opposite side of the thermoelectric modules 506 by means of heat sinks 508 mounted in thermal communication with the thermoelectric modules 506. Forced air may be fed or drawn through the heat sink fins to aid in the dissipation of heat equivalent to the input power. Efficient heat dissipation may be accomplished by forcing air through openings 518 at the bottom of the fins so that air flows through the system and out the top of the unit. Other configurations which allow sufficient airflow are suitable as well.
  • the thermoelectric device 500 could be bathed in cooling fluid or otherwise cooled.
  • the fan is in constant operation while the device is running. Any suitable fan or fluid source may be utilized to dissipate heat, however, a 300CFM fan has been found effective to remove heat from the system as shown in FIG. 5 and FIG. 6.
  • Controlling electronics may be associated with heating and cooling of the thermoelectric modules 506.
  • Feedback controlling electronics may sense the temperature of the fiber based upon input from the thermocouple 518, a thermometer, or other temperature sensor.
  • the controller may switch the current direction of power delivered to a thermoelectric module 506 and adjust power levels for the heating and cooling cycles (heating is typically more efficient so less power may be needed).
  • the controller may control the maximum and minimum temperatures delivered to the fiber-optics, and shut off the drive circuitry in the event of overheating.
  • thermoelectric device 500 Thermoelectric Module Phase Shift System Tests
  • Tests have been performed with the thermoelectric device 500 described above. Tests were performed by pitching four wavelength bands through an abso ⁇ tion free, nitrogen purged chamber. Without an abso ⁇ tion species in the pathlength, the laser should exhibit a linear wavelength response after being divided by a reference signal. The deviation in linearity of the slope is caused predominantly by modal noise.
  • the start and end of each wavelength cycle may be neglected due to time delays cased by travel times of the pitch and receive heads. These time delays cause significant variation between the tap and observed signal at the start and end of each cycle.
  • thermoelectric device 500 using single sided and double sided thermoelectric modules 506, multiple averaging times, and different fiber lengths.
  • modal phase shifting and averaging produced reduced modal noise signal deviations.
  • the reduction of noise was dependent on the length of fiber used in the experiment as given in equation (3); long fibers have more high frequency modal noise deviations than do short fibers. Because of this, the relative resolution enhancement qf modal phase shifting is witnessed more in long fibers. Results are presented below for the various configurations:
  • mode noise may be averaged and smoothed by cyclically varying the index of refraction or by mechanical manipulation of a multimode fiber and extracting data from the average light signal collected.
  • the temperature-based phase shifting apparatuses discussed above accomplish modal phase shifting by varying the index of refraction of a multimode fiber through cyclical temperature variations.
  • mechanical manipulation of a multimode fiber may also be employed to vary the index of refraction.
  • mechanical manipulation may result in averaging and smoothing of a mode-noise affected signal through light's inability to completely follow a particular mode within a waveguide as the fiber is manipulated.
  • the averaging and smoothing of a mode noise induced speckle pattern within a length of multimode fiber may be accomplished by a combination of phase shifting and mechanical scrambling.
  • Some specific modes of mechanical fiber manipulation are more effective at averaging mode noise than others.
  • twisting the fiber about its longitudinal (z) axis relative to some other point on the fiber causes the speckle pattern to change.
  • the dominant change obtained is a rotation of the speckle pattern around the z-axis.
  • the pattern does not rotate as far around the axis as the fiber is mechanically rotated.
  • a secondary effect is that the actual light distribution is somewhat altered by the rotation.
  • the rotation of the speckle pattern is not predominantly due to stress induced refractive index changes in the fiber, although this may explain small changes in the speckle intensity pattern. Rather, the rotation is due to the light's inability to completely follow the waveguide as it is manipulated in a torsional motion.
  • a mechanical mode noise averaging apparatus (mechanical device) 700 is shown schematically in FIG. 7.
  • the mechanical device 700 uses a hollow shaft motor 702 through which a multimode fiber 704 is placed and fastened.
  • a remote section of fiber 706 is held fast relative to the shaft position of the motor 702, and the motor is repetitively swept through a twisting motion which is preferably +360 degrees and then -360 degrees of motion.
  • the frequency of this motion preferably is greater than or equal to 10 Hz to enable effective averaging of the transmitted signal, and significantly reduce the effect of catch-side mode noise.
  • the twisting of a multimode fiber along its longitudinal axis has been determined to be effective at scrambling mode noise, other mechanical manipulations such as shaking, stretching, or bending may be employed as well.
  • Multimode fiber can be stretched with a piezo stretcher. Piezo devices are generally used to introduce a modulated time delay in a single-mode fiber. Multimode fiber is not used in piezo stretching devices because time delays in multimode fiber are not controllable because of the multiple paths or modes that the light can travel. However, the piezo stretching devices, although not practical for producing a time delay, may be used to introduce modal phase shifts.
  • a piezo device 800 works by winding several meters of multimode optical fiber 802 around half cylinders 804, and then oscillating the half cylinders 804 at a prescribed oscillation frequency and distance. As the distance between the half cylinders 804 expands and contracts, stresses in the fiber 802 harmonically oscillate. This oscillation causes the index of refraction of fiber 802 to fluctuate.
  • the effectiveness of the modal shift is function of both the change in the length of the fiber (z) and the change in the index of refraction ( ⁇ n,j) in the fiber (equation (1)).
  • Modal phase shifting can be accomplished by one of two techniques using a piezo device 800.
  • a piezo device 800 is implemented with enough fiber 802 and configured to introduce enough stress to produce a great degree of modal variation so that a uniform signal level could be achieved by averaging the many modes.
  • a piezo device 800 could potentially be operated to oscillate in a manner that produces a mode shift of a harmonic of 180° at minimum and maximum stretching distances.
  • modal noise could be reduced, not by time averaging many mode shifts, but by optimizing the stretching characteristics so that a phase shift of 180° is achieved.
  • modal noise could then be averaged in as little as one cycle, allowing for rapid data acquisition with modal noise elimination.
  • the pitch-side optical train of a fiber-coupled TDLAS sensing apparatus also presents a significant design challenge due to the necessity of producing a single-mode beam for all wavelengths to be transmitted through the measurement region. If single-mode fiber could be used throughout the pitch-side optical train, mode noise would not be an issue. However, fiber only operates as a single-mode waveguide over a limited wavelength window. Beyond the short wavelength cutoff for a particular fiber, light can be transported through the fiber in several higher order spatial modes. These higher order modes will interfere to produce a speckle pattern when the light exits the fiber. The speckle pattern is time and wavelength varying. Even a small amount of beam clipping then gives rise to noise in the measurement.
  • This problem can be acute in the fiber-coupled, wavelength multiplexed TDLAS sensing and control device described above due to the need to multiplex wavelengths as long as 1.67 microns with wavelengths as short as 760 nm or 670 nm.
  • Photonic crystal fiber may in the future provide a solution to this dilemma, but photonic crystal fiber technology is currently in its infancy.
  • the problem of multiplexing and pitching light in a single- mode beam from 670nm or 760nm to 1670nm may be minimized by utilizing a very short transmission section of multimode fiber 120 that does not allow the higher order spatial modes for a wavelength shorter than the single-mode cutoff to develop.
  • L the length of multimode fiber
  • mode noise will be minimized.
  • the 760 nm light is coupled to a short section of single-mode fiber with a cutoff wavelength of 1280 nm (e.g., Corning SMF 28), the 760 nm light remains single-mode for at least a few meters.
  • a solution to pitch- side mode noise is to couple the 760 nm light into a fiber which is single-mode for wavelengths longer than 1280 nm but could be multimode for 760 nm, with only a short distance to traverse before it is collimated to be transmitted through the measurement zone.
  • FIG. 9 and FIG. 2 A schematic diagram of such a system is shown in FIG. 9 and FIG. 2.
  • multiple diode laser sources 902 lasing at widely spaced lasing frequencies are coupled to discrete single-mode optical fibers 904A - 904n.
  • the diode lasers lasing at wavelengths between 1349nm and 1670nm are multiplexed with a multiplexer 906.
  • the output of multiplexer 906 is coupled to a pitch-side fiber-optic 908 having suitable dimensions for transmitting light with wavelengths ranging from 1349nm - 1670nm, both without substantial transmission losses and without the introduction of mode noise.
  • a suitable fiber-optic for these wavelengths is Corning SMF28.
  • the 760nm input if multiplexed and coupled to an SMF28 optical fiber would, after transmission over a relatively short distance, become multimodal. Accordingly, the output of the 760nm laser is coupled to a fiber which is single-mode for wavelengths less than 1280nm such as SMF750.
  • the laser light transmitted in the input fiber 904n and the multiplexed laser light transmitted in the pitch-side optical fiber 908 can be coupled nearby the pitch optic 910.
  • the coupler 912 and pitch optic 910 are preferably optically connected by a short length of transmission optical fiber 914 with the transmission optical fiber 914 being selected to transmit all of the coupled and multiplexed wavelengths without significant loss.
  • a suitable transmission optical fiber for the system depicted in FIG. 9 would be Corning SMF28.
  • the 760nm laser light coupled to the transmission optical fiber 914 will not exhibit significant multimodal behavior.
  • the transmission optical fiber must be kept to a length of 3 meters or less to avoid the introduction of significant multimodal noise.
  • coupler 134 receives input from both a 760 nm diode laser and a multiplexed beam from diode lasers having substantially longer wavelengths.

Landscapes

  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geochemistry & Mineralogy (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Mechanical Light Control Or Optical Switches (AREA)
  • Control Of Combustion (AREA)

Abstract

L'invention concerne un dispositif optique d'établissement d'une moyenne du bruit modal (300) comprenant une fibre optique multimode (302) ainsi qu'un moyen (308) permettant d'établir la moyenne d'une variation du niveau de signal provoquée par un bruit modal de la lumière se propageant dans la fibre optique multimode (302). Ce dispositif peut établir la moyenne de variations de niveau de signal provoquées par le bruit modal par variation cyclique de l'indice de réfraction de la fibre optique multimode (302) sur une durée sélectionnée, par embrouillement de la répartition de la lumière dans la fibre optique multimode (302), ou les deux. L'indice de réfraction de la fibre optique multimode peut être modifié cycliquement par variation cyclique de la température de cette fibre optique multimode (302). Alternativement, l'indice de réfraction peut être varié ou la répartition de la lumière dans la fibre optique multimode peut être embrouillée, par manipulation cyclique de la fibre optique multimode (302).
PCT/US2005/002853 2004-03-31 2005-02-02 Dispositif optique d'etablissement de moyenne du bruit modal WO2005103781A1 (fr)

Priority Applications (7)

Application Number Priority Date Filing Date Title
EP05712335A EP1730563A4 (fr) 2004-03-31 2005-02-02 Dispositif optique d'etablissement de moyenne du bruit modal
CA2561711A CA2561711C (fr) 2004-03-31 2005-02-02 Dispositif optique d'etablissement de moyenne du bruit modal
KR1020067020404A KR101212439B1 (ko) 2004-03-31 2005-02-02 광학 모드 노이즈 평균화 장치
CN200580010448.0A CN1938621B (zh) 2004-03-31 2005-02-02 光模式噪声平均装置
AU2005236385A AU2005236385A1 (en) 2004-03-31 2005-02-02 Optical mode noise averaging device
US10/599,233 US7787728B2 (en) 2004-03-31 2005-02-02 Optical mode noise averaging device
JP2007506152A JP2007534983A (ja) 2004-03-31 2005-02-02 光学的モードノイズ平均化デバイス

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
PCT/US2004/010048 WO2004090496A2 (fr) 2003-03-31 2004-03-31 Procede et appareil pour surveiller et reguler une combustion
USPCT/US2004/010048 2004-03-31

Publications (1)

Publication Number Publication Date
WO2005103781A1 true WO2005103781A1 (fr) 2005-11-03

Family

ID=35197120

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US2005/002853 WO2005103781A1 (fr) 2004-03-31 2005-02-02 Dispositif optique d'etablissement de moyenne du bruit modal

Country Status (7)

Country Link
EP (1) EP1730563A4 (fr)
JP (1) JP2007534983A (fr)
KR (1) KR101212439B1 (fr)
CN (1) CN1938621B (fr)
AU (1) AU2005236385A1 (fr)
CA (1) CA2561711C (fr)
WO (1) WO2005103781A1 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008065336A1 (fr) * 2006-11-29 2008-06-05 Cascade Technologies Limited Perturbateur de fibre multimode
KR20120058512A (ko) * 2009-08-10 2012-06-07 졸로 테크놀러지스, 아이엔씨. 다중모드 전송 광섬유를 이용한 광신호 노이즈 저감
US9366621B2 (en) 2012-04-19 2016-06-14 Zolo Technologies, Inc. In-furnace retro-reflectors with steerable tunable diode laser absorption spectrometer
CN107543613A (zh) * 2017-08-23 2018-01-05 西安科技大学 一种井下红外测温精度影响因素测试装置及测试方法

Families Citing this family (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP5181689B2 (ja) * 2008-01-18 2013-04-10 横河電機株式会社 近赤外光及び赤外光の分光分析装置
JP5606720B2 (ja) * 2009-03-04 2014-10-15 オリンパス株式会社 光検出装置
US8456634B2 (en) * 2009-06-15 2013-06-04 General Electric Company Optical interrogation sensors for combustion control
JP5455528B2 (ja) 2009-09-29 2014-03-26 三菱重工業株式会社 燃焼制御装置
US9746375B2 (en) * 2014-04-08 2017-08-29 Yokogawa Electric Corporation Systems, methods, and apparatus for optical noise management in optical spectroscopy
CN111175987A (zh) * 2018-11-09 2020-05-19 青岛海信激光显示股份有限公司 激光消散斑装置、激光消散斑方法及激光投影设备
CN111089661B (zh) * 2019-12-04 2020-11-03 北京航空航天大学 一种基于激光吸收光谱的温度快速提取方法

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4895421A (en) * 1987-02-20 1990-01-23 The Board Of Trustees Of The Leland Stanford Junior University Dynamic couplers using two-mode optical waveguides
US4915468A (en) * 1987-02-20 1990-04-10 The Board Of Trustees Of The Leland Stanford Junior University Apparatus using two-mode optical waveguide with non-circular core
US5396506A (en) * 1993-12-09 1995-03-07 United Technologies Corporation Coupled multiple output fiber laser
WO2004090496A2 (fr) 2003-03-31 2004-10-21 Zolo Technologies, Inc. Procede et appareil pour surveiller et reguler une combustion
JP2004354671A (ja) 2003-05-29 2004-12-16 Nikon Corp スペックルパターン分散装置及びレーザ光照射装置

Family Cites Families (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS63133035A (ja) * 1986-11-26 1988-06-04 Anritsu Corp 光伝送特性試験装置
DK0435825T3 (da) * 1989-12-27 1996-02-12 Ciba Geigy Ag Apparat til homogenisering af den inhomogene lysfordeling i et laserstrålelysbundt
US5291013A (en) * 1991-12-06 1994-03-01 Alamed Corporation Fiber optical monitor for detecting normal breathing and heartbeat motion based on changes in speckle patterns
CN1163665A (zh) * 1994-12-23 1997-10-29 西门子公司 利用法拉第效应、带有对强度变化和温度影响进行补偿的测量磁场的方法和装置
JPH10301153A (ja) * 1997-04-23 1998-11-13 Sony Corp 光源装置とこれを用いた光学測定装置および露光装置
JP4038631B2 (ja) * 1998-08-28 2008-01-30 株式会社堀場製作所 半導体レーザ分光法を用いた温度・濃度・化学種の高速計測方法および計測システム
US6351587B1 (en) * 1999-11-12 2002-02-26 Lucent Technologies Inc. Multi-fiber digital delay line
US20020158202A1 (en) * 2001-01-08 2002-10-31 Webber Michael E. Laser-based sensor for measuring combustion parameters
JP2003084324A (ja) * 2001-09-12 2003-03-19 Nippon Telegr & Teleph Corp <Ntt> マルチモード導波路
JP2003156698A (ja) * 2001-11-22 2003-05-30 Toshiba Corp レーザ光源装置
KR100417000B1 (ko) * 2001-12-03 2004-02-05 삼성전자주식회사 저 편광 모드 분산을 위한 장치

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4895421A (en) * 1987-02-20 1990-01-23 The Board Of Trustees Of The Leland Stanford Junior University Dynamic couplers using two-mode optical waveguides
US4915468A (en) * 1987-02-20 1990-04-10 The Board Of Trustees Of The Leland Stanford Junior University Apparatus using two-mode optical waveguide with non-circular core
US5396506A (en) * 1993-12-09 1995-03-07 United Technologies Corporation Coupled multiple output fiber laser
WO2004090496A2 (fr) 2003-03-31 2004-10-21 Zolo Technologies, Inc. Procede et appareil pour surveiller et reguler une combustion
JP2004354671A (ja) 2003-05-29 2004-12-16 Nikon Corp スペックルパターン分散装置及びレーザ光照射装置

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of EP1730563A4 *

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008065336A1 (fr) * 2006-11-29 2008-06-05 Cascade Technologies Limited Perturbateur de fibre multimode
JP2010511167A (ja) * 2006-11-29 2010-04-08 カスケイド テクノロジーズ リミテッド マルチモードファイバ摂動体
RU2468400C2 (ru) * 2006-11-29 2012-11-27 Кэскейд Текнолоджиз Лимитед Устройство возмущения многомодового оптического волокна
US8411273B2 (en) 2006-11-29 2013-04-02 Cascade Technologies Limited Multi mode fibre perturber
KR20120058512A (ko) * 2009-08-10 2012-06-07 졸로 테크놀러지스, 아이엔씨. 다중모드 전송 광섬유를 이용한 광신호 노이즈 저감
EP2464846A1 (fr) * 2009-08-10 2012-06-20 Zolo Technologies, Inc. Atténuation du bruit de signal optique à l'aide d'une fibre d'émission multimodale
US8786857B2 (en) 2009-08-10 2014-07-22 Zolo Technologies, Inc. Mitigation of optical signal noise using a multimode transmit fiber
EP2464846A4 (fr) * 2009-08-10 2015-01-21 Zolo Technologies Inc Atténuation du bruit de signal optique à l'aide d'une fibre d'émission multimodale
KR101716917B1 (ko) * 2009-08-10 2017-03-15 졸로 테크놀러지스, 아이엔씨. 다중모드 전송 광섬유를 이용한 광신호 노이즈 저감
US9366621B2 (en) 2012-04-19 2016-06-14 Zolo Technologies, Inc. In-furnace retro-reflectors with steerable tunable diode laser absorption spectrometer
CN107543613A (zh) * 2017-08-23 2018-01-05 西安科技大学 一种井下红外测温精度影响因素测试装置及测试方法
CN107543613B (zh) * 2017-08-23 2024-02-02 西安科技大学 一种井下红外测温精度影响因素测试装置及测试方法

Also Published As

Publication number Publication date
EP1730563A4 (fr) 2011-11-16
JP2007534983A (ja) 2007-11-29
CN1938621B (zh) 2015-04-01
EP1730563A1 (fr) 2006-12-13
KR101212439B1 (ko) 2012-12-14
CA2561711C (fr) 2012-12-18
KR20060135850A (ko) 2006-12-29
CA2561711A1 (fr) 2005-11-03
AU2005236385A1 (en) 2005-11-03
CN1938621A (zh) 2007-03-28

Similar Documents

Publication Publication Date Title
US7787728B2 (en) Optical mode noise averaging device
CA2561711C (fr) Dispositif optique d&#39;etablissement de moyenne du bruit modal
US7248755B2 (en) Method and apparatus for the monitoring and control of combustion
Spearrin et al. Simultaneous sensing of temperature, CO, and CO 2 in a scramjet combustor using quantum cascade laser absorption spectroscopy
KR101716917B1 (ko) 다중모드 전송 광섬유를 이용한 광신호 노이즈 저감
CN107727638B (zh) 一种带谐振腔增强的激光拉曼光谱气体分析仪
US7939805B2 (en) Method and apparatus for infrared spectrometry
CA2748793A1 (fr) Procede et appareil pour controler des proprietes de combustion dans un interieur d&#39;une chaudiere
JP2011145680A (ja) 光学的モードノイズ平均化デバイス
Yu et al. Quantum cascade laser spectroscopy-based high-sensitive temperature measurement technology
Rabe et al. Mode filter for LED-based absorption spectroscopy
Seymour Spectroscopic Measurements of Path-Averaged Species Correlations in Turbulent Flare Plumes
Culshaw Fiber optic systems for remote laser spectroscopy

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): AE AG AL AM AT AU AZ BA BB BG BR BW BY BZ CA CH CN CO CR CU CZ DE DK DM DZ EC EE EG ES FI GB GD GE GH GM HR HU ID IL IN IS JP KE KG KP KR KZ LC LK LR LS LT LU LV MA MD MG MK MN MW MX MZ NA NI NO NZ OM PG PH PL PT RO RU SC SD SE SG SK SL SM SY TJ TM TN TR TT TZ UA UG US UZ VC VN YU ZA ZM ZW

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): BW GH GM KE LS MW MZ NA SD SL SZ TZ UG ZM ZW AM AZ BY KG KZ MD RU TJ TM AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HU IE IS IT LT LU MC NL PL PT RO SE SI SK TR BF BJ CF CG CI CM GA GN GQ GW ML MR NE SN TD TG

121 Ep: the epo has been informed by wipo that ep was designated in this application
WWE Wipo information: entry into national phase

Ref document number: 2007506152

Country of ref document: JP

REEP Request for entry into the european phase

Ref document number: 2005712335

Country of ref document: EP

WWE Wipo information: entry into national phase

Ref document number: 2561711

Country of ref document: CA

Ref document number: 1020067020404

Country of ref document: KR

Ref document number: 200580010448.0

Country of ref document: CN

Ref document number: 2005236385

Country of ref document: AU

Ref document number: 2005712335

Country of ref document: EP

NENP Non-entry into the national phase

Ref country code: DE

WWW Wipo information: withdrawn in national office

Ref document number: DE

WWE Wipo information: entry into national phase

Ref document number: 2866/KOLNP/2006

Country of ref document: IN

ENP Entry into the national phase

Ref document number: 2005236385

Country of ref document: AU

Date of ref document: 20050202

Kind code of ref document: A

WWP Wipo information: published in national office

Ref document number: 2005236385

Country of ref document: AU

WWE Wipo information: entry into national phase

Ref document number: 10599233

Country of ref document: US

WWP Wipo information: published in national office

Ref document number: 2005712335

Country of ref document: EP

WWP Wipo information: published in national office

Ref document number: 1020067020404

Country of ref document: KR

WWP Wipo information: published in national office

Ref document number: 10599233

Country of ref document: US