GB2257505A - Rapid-response calorimetric gauge using fibre optic interferometer - Google Patents
Rapid-response calorimetric gauge using fibre optic interferometer Download PDFInfo
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- GB2257505A GB2257505A GB9114715A GB9114715A GB2257505A GB 2257505 A GB2257505 A GB 2257505A GB 9114715 A GB9114715 A GB 9114715A GB 9114715 A GB9114715 A GB 9114715A GB 2257505 A GB2257505 A GB 2257505A
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- 238000002310 reflectometry Methods 0.000 claims abstract description 10
- 239000013307 optical fiber Substances 0.000 claims abstract description 7
- 230000008542 thermal sensitivity Effects 0.000 claims abstract description 6
- 239000011248 coating agent Substances 0.000 claims description 9
- 238000012545 processing Methods 0.000 claims description 5
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- 238000012360 testing method Methods 0.000 description 9
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- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 3
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- 239000005350 fused silica glass Substances 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
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- 239000000567 combustion gas Substances 0.000 description 1
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
- G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K17/00—Measuring quantity of heat
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N25/00—Investigating or analyzing materials by the use of thermal means
- G01N25/20—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity
- G01N25/48—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on solution, sorption, or a chemical reaction not involving combustion or catalytic oxidation
- G01N25/4806—Details not adapted to a particular type of sample
- G01N25/4813—Details not adapted to a particular type of sample concerning the measuring means
- G01N25/482—Details not adapted to a particular type of sample concerning the measuring means concerning the temperature responsive elements
-
- 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
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
- G01J2009/0226—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
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
- G01J2009/0257—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods multiple, e.g. Fabry Perot interferometer
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- Chemical & Material Sciences (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Life Sciences & Earth Sciences (AREA)
- Combustion & Propulsion (AREA)
- Health & Medical Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- Engineering & Computer Science (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Measuring Temperature Or Quantity Of Heat (AREA)
Abstract
A high speed sensor for measuring heat flux directly uses an optical fibre Fabry-Perot interferometer 40 which is mounted with one of its end faces 42 flush with the surface of a specimen eg. a turbine blade, whose heat transfer characteristics are to be investigated. The sensor 40 is illuminated by a laser source 10 via a directional coupler 20 and an addressing fibre 30 of identical optical characteristics to the sensor 40. The addressing fibre 30 and the sensor 40 have low reflectivity coatings on their end faces such that some of the incident light is reflected to an interference detector 50. Here the optical phase change in the sensor reflection is detected when the sensor 40 is exposed to heat flux thereby changing its refractive index. A signal processor (not shown) relates the optical phase change to the mass density, specific heat capacity and thermal sensitivity of the sensor 40 to give a direct reading of the heat flux. A Faraday isolator 15 reduces reflections, and a second detector 60 monitors and regulates laser power output. <IMAGE>
Description
RAPID-RESPONSE CALORIMETRIC GAUGE
The present invention relates to calorimetric gauges and in particular to a rapid-response calorimetric gauge based on a miniature optical fibre Fabry-Perot interferometer.
Such devices are of especial interest to the determination of heat transfer characteristics between the hot combustion gases and turbine blades in gas turbine engines. The standard technique for determining the heat transfer characteristics of a particular blade design is to construct a ceramic model of the blade and to assess its performance in a transient wind tunnel. Here it is exposed to short duration pulses of hot or cold gases, typical run times in such installations extending from about 10-1000 milliseconds. The surface of the test blade is equipped with an array of rapid response temperature sensors which map the changes in temperature across the test blade during its exposure to the gas pulse.
To date, the best temperature sensors available for this task are platinum thin film resistance thermometers, which can be built to have a thermal resolution as low as 25 mK and a measurement bandwidth up to 100 kHz. They work reliably in an environment where pressure changes of up to 10 Bar are common and where the gas temperature variation can be as high as 200"C.
However, thin film resistance thermometers of this kind suffer from a number of drawbacks, not least of which is their fragility.
They are also quite difficult to build and are relatively large which limits their spatial resolution. Moreover, their electrical outputs are small and this can cause additional difficulties in the wind tunnel environment where high levels of electromagnetic interference may be experienced.
A further limitation of thin film resistance thermometers is that they are incapable of measuring heat flux directly. They measure only surface temperature which is related to heat flux by a semi-infinite, one-dimensional Laplace equation. Although it is possible to derive a heat flux reading from the temperature output by signal processing, such a procedure is subject to excessive noise which is exacerbated by the electromagnetic interference experienced in the test environment.
An alternative method of measuring heat flux directly is to use a compound sensor in which one sensor element is placed on the surface of the specimen whilst the other sensor element is embedded within it.
Unfortunately such compound sensors have poor response times.
Recently, interest has grown in the use of fibre optic techniques for measuring rapid changes in temperature, amongst which those techniques based on interferometry have proved to be the most sensitive. The temperature sensitivity of such fibre optic devices is due to the temperature dependence of both the fibre length and its refractive index. For most fused quartz fibres, the temperature dependence of the refractive index dominates by a factor of about 20.
Fibre optic temperature sensors have the added advantage that they have an all-dielectric construction which means that they are immune to the effects of electromagnetic interference such as might be encountered in the transient wind tunnel environment.
In the majority of reports which refer the use of fibre optic interferometers for measuring rapid temperature changes, a Mach
Zehnder interferometer has been proposed, although use of a Michelson interferometer was described by Corke, Kersey, Jackson and Jones in a paper entitled "All Fibre Michelson Thermometer" (Electron. Letts., vol 19, pp.471-473, 1983). In these devices, heat flux enters the sensing element through the sides of the fibre so the speed of response is limited by the rate at which heat diffuses through the fibre cladding. Nevertheless, the use of thick metallic coatings in which a temperature rise is effected by ohmic heating has enabled bandwidths of tens of kHz to be achieved.
It has now been discovered that an optical fibre Fabry-Perot interferometer can be configured to measure heat flux directly with the fibre axis normal to the incident heat flux and the fibre end face flush with the test surface. Satisfactory operation of such a device is achieved provided that the length of the fibre which forms the interferometer cavity is longer - preferably at least four times longer - than the depth of penetration of the heat pulse which it is desired to measure. This ability to measure heat flux directly is based on the assumption that the system behaves as a one dimensional model in which the heat flux propagates in the direction of the fibre axis.Measurement of the heat flux is then accomplished by relating the optical phase change which occurs in response to a change in temperature of the interferometer to the mass density, specific heat capacity and thermal sensitivity of the sensor element in accordance with the following equation: pc dX
Equation (1) k dt where q is the heat flux experienced by the sensor element;
p is the mass density;
c is the specific heat capacity;
k is the thermal sensitivity, and
dX
- is the time derivative of the optical phase change.
dt
Thus signal processing to derive the heat flux q from the measured optical phase change is quite simple.
It is therefore an object of this invention to provide a rapidresponse calorimetric gauge which overcomes many of the drawbacks associated with known temperature mapping devices such as the requirement for complex signal processing and susceptibility to noise.
The invention is a rapid-response calorimetric gauge comprising: a Fabry-Perot cavity sensor element comprising a length of single mode optical fibre having its ends coated with a low reflectivity coating; an addressing fibre spliced to and aligned with one end of the sensor element, said addressing fibre having the same optical characteristics as the sensor element and having a low reflectivity coating at its spliced end; a laser source for illuminating the Fabry-Perot cavity via a directional coupler and the addressing fibre; an interference detector for measuring the variation with time of the intensity of light reflected from the splice and from the end of the sensor element remote from the splice so as to determine the optical phase change which occurs in the reflected light from the sensor element when said sensor element is exposed to heat flux, and pre-calibrated signal processing means which relates the optical phase change output of the detector to the mass density, specific heat capacity and the thermal sensitivity of the sensor element to give a direct reading of the heat flux experienced by the sensor element.
The directional coupler is a solid state fibre optic device which facilitates extraction of the light from the fibre system for analysis. It operates in a similar fashion to a conventional beam splitter, so that the composite reflection from the splice and from the distal end of the sensor element is divided into two beams, one of which is directed to the interference detector and the other of which is directed back towards the laser source.
In order to keep the frequency noise floor as low as possible, it is preferable to minimise feedback to the laser source, for example by incorporating a Faraday isolator in the launch optics. In addition, the tip of the addressing fibre into which the laser light is launched may be cut at a small angle up to 10 from square to prevent formation of a cavity between the fibre end and the launch objective.
Numerous measures may be adopted throughout the optical system to minimise unwanted reflections. Such measures help to suppress the effects of parasitic interferometers which would otherwise complicate the changing intensity pattern seen by the interference detector. An example of such a measure is the interposition of a microscope cover slip between the tip of the addressing fibre and the arm of the directional coupler which delivers light to it from the laser source.
The cover slip, which is butted against the end of the addressing fibre and secured in place with an index matching adhesive, serves to prevent light travelling back along the arm of the directional coupler leading from the launch optics from being reflected into the fibre system. If such reflections were to occur, they could form a parasitic Fabry-Perot cavity with the ends of either of the two output arms of the directional coupler.
In an especially preferred form of the invention, the spare output arm of the directional coupler (i.e. the output arm which is not connected to the addressing fibre) is routed to a second detector through a length of multi-mode fibre spliced onto it with the aid of index matching adhesive. Multi-mode fibre is used here because it helps to minimise any weak reflections which might otherwise occur at the interface between the fibre and the second detector. Such reflections could form a parasitic Michelson interferometer with the tip of the addressing fibre. The second detector is used to monitor the power output of the laser source. Where appropriate, reactive control may be used to minimise fluctuations in the laser power to maintain it within acceptable limits.
The invention will now be described by way of example with reference to the drawings, in which:
Figure 1 is a schematic diagram of the basic optical system;
Figure 2 is a schematic diagram of a preferred optical system, and
Figure 3 is a sectional view of a sensor element mounted in a test
specimen.
Referring now to Figure 1, there is shown a coherent light source 10 such as a pulsed Nd/YAG laser diode, a directional coupler 20, a single mode addressing fibre 30 and a sensor element 40 mounted such that one of its end faces lies normal to the surface experiencing heat flux.
The directional coupler 20 is solid state fibre optic device equivalent in function to a conventional beam splitter. Its purpose in the present apparatus is therefore to facilitate access to the light in the fibre system so that at least a portion of it can be diverted to a detector 50 where its optical characteristics are determined. Directional coupler 20 has a pair of output arms 23 and 24, the first of which is in communication with the addressing fibre 30 and the second of which leads to an absorbing fibre end 25.
Preferably, this fibre end is treated with a black coating such as graphite to assist in minimising unwanted reflections.
The sensor element 40 is illuminated by the laser source via the directional coupler 20 and the addressing fibre 30. It is formed from a Fabry-Perot interferometer cavity comprising a length of single mode optical fibre having identical optical characteristics to those of the addressing fibre 30 to which it is reflectively spliced at 35. Both the proximal end 41 and distal end 42 of the sensor element are given a low reflectivity coating of vapour deposited aluminium, with the splice end 36 of the addressing fibre being similarly treated.
Typically, reflectivities of 5 to 25% are used, but optimum performance is achieved with a value of reflectivity around 10%.
As a result of these reflective coatings at the fibre ends, a fraction of the light passing through addressing fibre 30 is reflected at the splice 35, and serves as a reference beam. The remainder of the light passes to the distal end 42 of the sensor element, where a fraction of the light is again reflected to form a signal beam.
The signal and reference beams are both guided back to the directional coupler 20 and a fraction of this reflected light (typically 50%) is directed to the detector 50. Here, interference between the signal and reference beams is detected, the intensity of such interference following the relationship: I < a 1 + Vcos 0 where 0 is the temperature dependent phase retardance associated with propagation of the light through the sensor element 40, and
V is the visibility of the interference (to).
The heat flux q experienced by the sensor element 40 is directly related to the time derivative of the optical phase change by the material properties of mass density (p) and specific heat capacity (c) and by the thermal sensitivity (k) of the fibre core according to equation (1) above.
The optical phase change output of the detector 50 is fed to a pre-calibrated signal processor (not shown) which converts the optical phase change reading to a direct reading of heat flux.
Figure 2 shows a preferred form of the optical system described above and uses like reference numerals to refer to components which are common to both systems. The preferred system of Figure 2 incorporates additional measures to minimise unwanted reflections within the fibre system and to improve the power control of the laser source.
Laser source 10 illuminates a sensor element 40 as before via a directional coupler 20 and an arbitrary length of addressing fibre 30 of identical optical characteristics to the sensor element. This addressing fibre is reflectively spliced to the sensor element 40 at splice 35 and is coated at its splice end 36 with a low reflectivity coating. The proximal and distal ends 41, 42 of the sensor element are similarly coated.
Operation of the preferred optical system is as previously described, except that a Faraday isolator 15 is interposed between the laser source 10 and the directional coupler 20 to assist in preventing light reflected through the addressing fibre from being back-reflected to the fibre system by the launch optics.
In addition, the second output arm 24 of the directional coupler is now connected to a second detector 60 via a length of multi-mode fibre 26. The second detector 60 is used to monitor the power output of the laser source 10 and may drive a reactive control device (not shown) which minimises fluctuations in the laser power and maintains it within acceptable limits. Multi-mode fibre is used in the connection 26 between the directional coupler and the second detector to minimise any weak residual reflections at the fibre/detector interface which might otherwise form a parasitic Michelson interferometer with the end of the addressing fibre.
These additional measures help to minimise environmentally-induced noise within the system.
Referring now to Figure 3, a fibre optic Fabry-Perot sensor element 140 is shown embedded in a test specimen 170 with the fibre axis normal to the test surface. The sensor element communicates with the optical source and detector assembly (not shown) via an arbitrary length of addressing fibre 130 of the same type as that from which the sensor element 140 is made. This communication is facilitated by means of a reflective splice 135 which is formed in the following manner:
In order to minimise angular misalignments and to ensure that lateral misalignment is negligible in relation to the diameter (typically 4-5 pm) of the fibre core, the sensor element 140 and the splice end 136 of the addressing fibre 130 are supported in an adhesive-filled precision capillary alignment tube 137 made from fused quartz or ceramic material. The addressing fibre 130 is first cleaved and then inserted into the tube 137. The fibre that is to form the sensor element 140 is then cleaved, given a low reflectivity aluminium coating by vapour deposition on what will become its proximal face and inserted into the other end of the tube 137. When the fibres have been correctly butted in the centre of the alignment tube 137, the adhesive is cured using a UV lamp or by heating as appropriate. With the adhesive set, the alignment tube 137 is cut to produce the desired sensor length, typically between 2 and 4 mm for its application to transient wind tunnel trials on experimental blade profiles. This cleaved face is then polished prior to depositing an aluminium coating as before to form the distal end of the sensor element. The sensor assembly is then ready for mounting flush with the surface of the test specimen.
The geometry of the sensor is such that the fibre end face acts as the sensing area, the fibre axis being normal to the surface of the test specimen at which the heat flux measurement is made. The sensitive region of the fibre is its core which is typically 4-5 pm in diameter, but the spatial resolution of the invention is defined by the overall diameter of the sensor element which includes the fibre cladding and the alignment tube. The finished sensor ready for mounting therefore has a diameter of around 100 pm, which is still very much smaller than the thin film thermometers used previously.
Claims (4)
1. A rapid-response calorimetric gauge comprising: a Fabry-Perot cavity sensor element comprising a length of single mode optical fibre having its ends coated with a low reflectivity coating; an addressing fibre spliced to and aligned with one end of the sensor element, said addressing fibre having the same optical characteristics as the sensor element and having a low reflectivity coating at its spliced end; a laser source for illuminating the Fabry-Perot cavity via a directional coupler and the addressing fibre; an interference detector for measuring the variation with time of the intensity of light reflected from the splice and from the end of the sensor element remote from the splice so as to determine the optical phase change which occurs in the reflected light from the sensor element when said sensor element is exposed to heat flux, and pre-calibrated signal processing means which relates the optical phase change output of the detector to the mass density, specific heat capacity and the thermal sensitivity of the sensor element to give a direct reading of the heat flux experienced by the sensor element.
2. A rapid-response calorimetric gauge as claimed in claim 1 in which a Faraday isolator is interposed between the laser source and the directional coupler to minimise feedback to the laser source of light reflected from the splice and from the end of the sensor element remote from the splice.
3. A rapid-response calorimetric gauge as claimed in claim 1 or claim 2 in which a second detector is coupled to the directional coupler through a length of multi-mode optical fibre to monitor the power output of the laser source, said second detector having reactive control means to minimise fluctuations in the laser power and maintain it within predetermined limits.
4. A rapid-response calorimetric gauge substantially as described with reference to Figures 1 and 3 or with reference to Figures 2 and 3 of the drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9114715A GB2257505A (en) | 1991-07-08 | 1991-07-08 | Rapid-response calorimetric gauge using fibre optic interferometer |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9114715A GB2257505A (en) | 1991-07-08 | 1991-07-08 | Rapid-response calorimetric gauge using fibre optic interferometer |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9114715D0 GB9114715D0 (en) | 1991-08-28 |
| GB2257505A true GB2257505A (en) | 1993-01-13 |
Family
ID=10698004
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9114715A Withdrawn GB2257505A (en) | 1991-07-08 | 1991-07-08 | Rapid-response calorimetric gauge using fibre optic interferometer |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2257505A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8253945B2 (en) | 2004-04-08 | 2012-08-28 | The Science And Technology Facilities Council | Optical sensor |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2096784A (en) * | 1981-04-09 | 1982-10-20 | Standard Telephones Cables Ltd | Optical fibre temperature sensors |
| EP0260894A1 (en) * | 1986-09-12 | 1988-03-23 | Cogent Limited | Optical fibre measuring system |
-
1991
- 1991-07-08 GB GB9114715A patent/GB2257505A/en not_active Withdrawn
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2096784A (en) * | 1981-04-09 | 1982-10-20 | Standard Telephones Cables Ltd | Optical fibre temperature sensors |
| EP0260894A1 (en) * | 1986-09-12 | 1988-03-23 | Cogent Limited | Optical fibre measuring system |
| GB2200986A (en) * | 1986-09-12 | 1988-08-17 | Cogent Ltd | Optical fibre measuring system |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US8253945B2 (en) | 2004-04-08 | 2012-08-28 | The Science And Technology Facilities Council | Optical sensor |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9114715D0 (en) | 1991-08-28 |
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