Classifications

• • 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
  • G01K11/3206—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 at discrete locations in the fibre, e.g. using Bragg scattering
• • 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
  • G01D5/268—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 using optical fibres
• • 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
  • G01J5/06—Arrangements for eliminating effects of disturbing radiation; Arrangements for compensating changes in sensitivity
  • G01J5/061—Arrangements for eliminating effects of disturbing radiation; Arrangements for compensating changes in sensitivity by controlling the temperature of the apparatus or parts thereof, e.g. using cooling means or thermostats
• • 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
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4201—Packages, e.g. shape, construction, internal or external details
  • G02B6/4246—Bidirectionally operating package structures
• • 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
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
  • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
  • G02B6/02195—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for tuning the grating
  • G02B6/02204—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for tuning the grating using thermal effects, e.g. heating or cooling of a temperature sensitive mounting body

Definitions

• This invention relates to a method of and apparatus for use in measuring the magnitude of a physical quantity that has the potential to influence the reflectance of an optical filter.
• the invention has particular application to the measurement of temperature in inaccessible locations and is hereinafter described in such context. However, it will be understood that the invention does have broader application, to the measurement of any condition that has the potential to induce a change in the reflectance of an optical filter, for example by inducing stress or strain within the optical filter.
• BACKGROUND OF THE INVENTION Various systems have been developed or proposed which incorporate optical fibre sensors and which utilise the spectral response of optical filters to determine the magnitude of physical quantities.
• United States Patents numbered 4,806,012 and 4,996,419 disclose systems which employ filters in the form of in- fibre Bragg gratings and which are suitable for use in measurement of temperature and strain in high voltage, corrosive or constricted environments.
• most of the prior art arrangements are inherently expensive in that, for example, they require the use of elaborate light source systems and optical spectrum analysers.
• Other prior art arrangements exhibit unsatisfactory long term reliability.
• the present invention has been developed from an initial requirement of the electrical power industry for a relatively low cost apparatus that is suitable for use in measuring the temperature in electrical equipment, including in the core of high voltage transformers. Whilst it might be appropriate to use expensive measuring apparatus, such as those incorporating optical spectrum analysers, in conjunction with expensive equipment in power generating stations, the same type of measuring apparatus cannot justifiably be used in relation to relatively inexpensive equipment, such as transformers, where temperature monitoring may be desired for the purpose of determining the effect of load current on the lifetime of the equipment.
• the present invention seeks to meet the need for an apparatus that may be employed for effecting measurement of the types above discussed and which has cost compatibility with equipment in relation to which the apparatus is to be used.
• the present invention may be defined broadly in terms of an apparatus for measuring the magnitude of a physical quantity that has the potential to influence the reflectance of an optical filter.
• the apparatus comprises an optical fibre, at least one optical filter located in or coupled to the optical fibre, a light source arranged to launch light into the optical fibre, and control means for controlling the light source in a manner to vary the centre wavelength of the spectrum of light emitted by the source over a range which embraces a substantial portion at least of the expected spectral response of the or each filter.
• the apparatus includes means for detecting light reflected back through the optical fibre by the or each filter, for detecting the occurrence of peak reflection and for determining a measure of the physical quantity that exists at the or each filter by reference a condition that is controlled by the control means when light is emitted that gives rise to the peak reflection.
• the invention may be defined as a method of measuring the magnitude of a physical quantity that has the potential to influence the reflectance of an optical filter.
• the method comprises the steps of launching light into an optical fibre that incorporates or is coupled to at least one optical filter, controlling the light source in a manner to vary the centre wavelength of the spectrum of light emitted by the source over a range which embraces a substantial portion at least of the expected spectral response of the or each filter, detecting light reflected back through the optical fibre by the or each filter, detecting for the occurrence of peak reflection and determining a measure of the physical quantity that exists at the or each filter by reference to a condition that is controlled by the control means when light is emitted that gives rise to the peak reflection.
• the invention as above defined does not require a determination to be made of the complete reflection characteristics of the filter or, if more than one filter is employed, of each of the filters. Nor does the invention require a measure to be made of the actual centre wavelength at which peak reflection occurs. Rather, detection is made for the occurrence of peak reflection and a determination is made as to the magnitude of the physical quantity (that determines the spectral position of the peak reflection) by reference to the control condition exercised over the light source.
• the or each filter effectively is illuminated by scanning the light source through a spectrum that covers the expected spectral response of the filter(s) . When the spectrum of the light source overlaps with the spectrum of the filter, or with that of each of the filters in turn, light is reflected back along the optical fibre to a detector.
• the detector provides an output signal which rises as the light source spectrum shifts to match that of the or each filter and which peaks when maximum overlap is achieved.
• this approach permits the employment of a relatively inexpensive, conventional type of laser diode as the light source and provides for detection by use of a simple photodiode.
• the optical fibre When employed in the measurement of temperature, the optical fibre will be positioned to locate the optical filter at a point of interest or to locate each of the optical filters at respective points at which measurements are to be taken.
• any change in the temperature of the filter will cause a corresponding shift in the spectral response of the filter and, knowing how the centre wavelength of the spectral response relates to temperature in a given filter, a prevailing temperature may be determined by reference to a control condition that exists when peak reflection of the filter is detected.
• the light source may be controlled by a variable temperature heat source to emit a light spectrum whose centre wavelength varies
• condition of the filter may be correlated with the temperature of the heat source, when the maximum reflection occurs, to determine the prevailing temperature of the filter.
• the or each filter preferably comprises an in-fibre Bragg grating, and where more than one filter is required to be located within a single length of the optical fibre each grating will be formed to provide a different spectral response in order that discrimination might be made between sensing points.
• the filter may be formed as a multilayer- dielectric-stack interference filter having alternating high and low refractive index layers vapour-deposited on a polished endface of the fibre.
• Figure 2 shows an enlarged view of a sensor portion of the apparatus
• Figure 3 shows a diagrammatic representation of optical components of the apparatus
• FIG. 4 shows a diagrammatic representation of electrical control/processing components of the apparatus
• Figure 5 shows a graph of a photodetector response and the temperature appropriate to control scanning of the light source
• Figure 6 shows a graph of the light source temperature against temperature of a typical in-fibre grating.
• the apparatus comprises a length of optical fibre 10 which connects an opto-electronic portion 11 of the apparatus to a location 12 where temperature is to be measured.
• a display screen 13 is coupled to the portion 11 of the apparatus for providing a visual display of recorded temperature.
• the end region 14 of the optical fibre 10 that is positioned in the temperature measuring location 12 incorporates a sensor 15 which is illustrated in Figure 2 and which comprises an in-fibre Bragg grating 16.
• the end region 14 of the optical fibre is exposed to a window, for example, in the form of a glass tube 15.
• the optical fibre is contained within and protected by a plastics material sleeve 18, and the full length of optical fibre is contained within a heat resistant sleeve 19 composed of polytetrafluoroethylene.
• the sleeve 19 or a plurality of such sleeves may be located permanently within a piece of equipment, such as an electrical power transformer, and the or each of the sleeves may be positioned to extend from a gland to a zone at which temperature measurements may be required.
• the optical fibre 10 will be inserted into the sleeve to an extent such that the end region 14 locates in the zone at which the temperature measurement is to be made.
• the optical fibre 10 may comprise any photosensitive single mode fibre having the usual core and cladding, and the Bragg grating is formed by interferometrically side- writing the grating into the fibre using UV radiation.
• the technique employed in this process is well understood and does not form a part of the present invention. However, it has been found that by apodising the grating spectrum, ripples from side band reflections is minimised.
• the grating 16 is formed to provide a predetermined spectral response and one whose centre wavelength shifts with changes in temperature to which the end region 14 of the fibre is exposed.
• the spectral response of the Bragg grating will also vary with changes in strain induced in the grating and it is for this reason that the end region 14 of the optical fibre is located within the glass tubing, to isolate the grating from any strain-inducing conditions.
• the optical fibre will be chosen to exhibit strain-dependent reflectance variation.
• the optical fibre may be selected as one which does not exhibit a temperature-dependent reflectance variation.
• compensating optics may be employed to negate the effect of any temperature variations.
• the optical fibre 10 is coupled to an optical system 20 which is shown in Figure 3 and which forms a part of the opto-electronic portion 11 of the apparatus.
• the optical system 20 includes a multi-longitudinal-mode, gain-guided semiconductor laser diode 21 which is mounted within a metal block 22, such as one formed from aluminium or copper.
• the metal block has minimal thermal mass and, thus, responds quickly throughout its volume to any change in temperature.
• the metal block 22 which mounts the laser diode 21 is mounted to a Peltier effect thermoelectric temperature regulator 23.
• the temperature regulator 23 is driven by a square wave current, having equal high and low periods of approximately 30 seconds, and this causes a substantially triangular-wave temperature cycling of the laser diode 21 at the periodic rate of the current supply to the temperature regulator 23.
• the gain-guided laser diode 21 is selected to provide a substantially linear temperature-wavelength response, the temperature regulator 23 is employed to control the laser diode 21 in a manner to vary the centre wavelength of the spectrum of light emitted by the laser diode to cover the full width of the spectral response of the grating 16.
• mode-hopping of the laser diode should be suppressed in the interest of obtaining a linear temperature-wavelength response over a reasonable range, and this may be achieved by driving the laser with a stable injection current just above that required to establish the lasing threshold. This has the effect of distributing the optical power among many longitudinal modes and allows for smooth transfer of power between modes as the temperature is cycled.
• the light emitted from the laser diode is directed through an optical coupler such as a frustrated total internal reflection (FTIR) beam*splitter cube 24 as illustrated in the drawings.
• FTIR frustrated total internal reflection
• One-half of the emitted light is reflected away from the various components of the optical system and the remaining 50% of the emitted light is directed through a focusing lens 25.
• the light- receiving end of the optical fibre 10 is positioned at the focal point of the lens 25 and, thus, light emitted by the laser diode 21 is launched into the optical fibre.
• Transmitted light which has a wavelength remote from the centre wavelength of the grating passes through and beyond the grating.
• light which is reflected by the grating 16 at the end region 14 of the optical fibre 10 is reflected in the reverse direction, back along the optical fibre and through the lens 25 and beam splitter 24.
• 50% of the reflected light is deflected through 90° to impinge on a photodetector (i.e., a photodiode) 26.
• the reflected light is focused to a waist adjacent the photodetector, and a pinhole aperture 27 is located immediately in front of the photodetector for the purpose of masking any stray background light.
• the grating spectrum is effectively interrogated by scanning (i.e., varying) the centre wavelength of the spectrum of the light emitted by the laser diode 21 to cover a substantial portion at least of the rejection bandwidth of the grating, and the reflected light is detected by the photodetector 26 for the purpose of determining when peak reflectance occurs and, thus, when a centre wavelength is emitted which corresponds with the Bragg wavelength of the grating at a given temperature.
• a processor 28 is employed to track the reflection of the grating and to identify peak reflectance during each scan cycle.
• the processor 28 is illustrated in Figure 4 and is connected to the optical system 20 by way of an electrical interface 29.
• a measure of the wavelength of the peak reflection is derived by monitoring the temperature of the laser diode 21 using a thermistor 30 which is positioned adjacent and coupled thermally to the copper block 22 that mounts the laser diode 21.
• the processor 28 receives signals representative of the instantaneous temperature of the laser diode 21 and the output from the photodetector 26, the former being derived from the thermistor 30 and the latter representing the instantaneous temperature of the grating 16.
• the processor 28 simultaneously reads the two signals through an analogue-to-digital converter, the signals are analysed and processed, and a measured temperature output is displayed.
• two external electrical circuits are employed in addition to the processor 28.
• One of these circuits comprises a laser current control circuit 31 which provides stable injection current for the laser 21 and the other circuit, which is controlled by an output from the processor 28, is a regulator drive circuit 32 which provides the necessary current to drive the temperature regulator 23.
• a graph of a typical laser diode operating temperature signal and photodetector signal received in respect of a heating portion of a temperature scan cycle is shown in Figure 5.
• the processor 28 is employed to acquire and build an array for a predetermined temperature range, for example an array of 2,000 points for a measured temperature range of 200°C.
• the processor 28 is employed to locate the index of the maximum element value of the photodetector signal array and it then finds the corresponding element value of the laser diode temperature at the same array index. This value is used in the final scaling of the measured temperature.
• the slope indicates a gain of approximately 30 and, in the processor, when the zero- point offset is subtracted from the measured laser diode temperature at the peak of the photodetector signal, the resultant laser diode temperature is multiplied by the gain factor.
• This product then represents the scaled measured temperature of the grating at the time when the peak reflectance occurs during a given scan cycle.
• two substantially identical gratings may be employed.
• One grating is exposed to monitored ambient temperature and, thus, provides a reflectance peak at a centre wavelength which is shifted relative to that which is exhibited by the filter which is exposed to the "unknown" temperature.
• a measure of the "unknown" temperature is then obtained by determining the difference between the laser operating temperatures at which the respective peaks occur and by then multiplying the resultant value by the gain factor which, as previously mentioned, is derived as the slope of the curve shown in Figure 6.
• the forward voltage of the laser diode 21 may be employed in lieu of the thermistor voltage as a measure of the junction temperature. The instantaneous forward voltage is determined by the junction temperature when current is flowing to create laser emission and, thus, the voltage provides a measure of the junction temperature.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Measuring Temperature Or Quantity Of Heat (AREA)
• Optical Transform (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)

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Citations (8)

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• 1995
  • 1995-05-15 AU AUPN2971A patent/AUPN297195A0/en not_active Abandoned
• 1996
  • 1996-05-14 WO PCT/AU1996/000293 patent/WO1996036276A1/en not_active Application Discontinuation
  • 1996-05-14 EP EP96911876A patent/EP0871397A1/en not_active Withdrawn
  • 1996-05-14 JP JP8534392A patent/JPH11509925A/ja active Pending

Patent Citations (8)

Non-Patent Citations (1)

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