ZA200508065B - A fibre optic sensor for measurement of refractive index - Google Patents
A fibre optic sensor for measurement of refractive index Download PDFInfo
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- ZA200508065B ZA200508065B ZA200508065A ZA200508065A ZA200508065B ZA 200508065 B ZA200508065 B ZA 200508065B ZA 200508065 A ZA200508065 A ZA 200508065A ZA 200508065 A ZA200508065 A ZA 200508065A ZA 200508065 B ZA200508065 B ZA 200508065B
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- wave
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- guide
- core
- cladding
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- 239000000835 fiber Substances 0.000 title claims description 21
- 238000005259 measurement Methods 0.000 title claims description 13
- 238000005253 cladding Methods 0.000 claims description 34
- 239000000126 substance Substances 0.000 claims description 16
- 230000003287 optical effect Effects 0.000 claims description 15
- 238000001228 spectrum Methods 0.000 claims description 8
- 230000010363 phase shift Effects 0.000 claims description 6
- 239000000463 material Substances 0.000 claims description 4
- 230000001902 propagating effect Effects 0.000 claims description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 8
- 239000013307 optical fiber Substances 0.000 description 7
- PEDCQBHIVMGVHV-UHFFFAOYSA-N Glycerine Chemical compound OCC(O)CO PEDCQBHIVMGVHV-UHFFFAOYSA-N 0.000 description 6
- 239000000203 mixture Substances 0.000 description 5
- 230000008878 coupling Effects 0.000 description 4
- 238000010168 coupling process Methods 0.000 description 4
- 238000005859 coupling reaction Methods 0.000 description 4
- 238000002474 experimental method Methods 0.000 description 4
- 235000011187 glycerol Nutrition 0.000 description 3
- 238000007654 immersion Methods 0.000 description 3
- 230000003993 interaction Effects 0.000 description 3
- 239000007788 liquid Substances 0.000 description 3
- 239000000523 sample Substances 0.000 description 3
- 230000035945 sensitivity Effects 0.000 description 3
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
- 238000005530 etching Methods 0.000 description 2
- 238000001727 in vivo Methods 0.000 description 2
- 238000012795 verification Methods 0.000 description 2
- 206010014405 Electrocution Diseases 0.000 description 1
- 239000012080 ambient air Substances 0.000 description 1
- 239000012491 analyte Substances 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 238000009529 body temperature measurement Methods 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000010790 dilution Methods 0.000 description 1
- 239000012895 dilution Substances 0.000 description 1
- 235000019441 ethanol Nutrition 0.000 description 1
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- 238000012886 linear function Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
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
- G01N21/43—Refractivity; Phase-affecting properties, e.g. optical path length by measuring critical angle
- G01N21/431—Dip refractometers, e.g. using optical fibres
-
- 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
- G01N21/41—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
- 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
- G01N2021/458—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods using interferential sensor, e.g. sensor fibre, possibly on optical waveguide
Description
A FIBRE OPTIC SENSOR FOR MEASUREMENT OF REFRACTIVE INDEX
This invention relates to a fibre optic sensor for measurement of refractive index.
The refractive index of a material carries information regarding its chemical composition. For example, dilution of ethyl alcohol with water will change its refractive index slightly. For liquids that do not mix, the boundary between the constituents will usually be distinguishable by a step in the refractive index.
Optical fibre sensors are ideally suited for application in the chemical and medical field because of the absence of electrical signals in the sensor head and the fibre that carries the signals between the sensor and the processing electronics. This eliminates the danger of explosions in chemical applications or electrocution of of other damage to the patient in medical applications. } Usually, the guided mode that propagates in the core of an optical fibre is confined to the immediate vicinity of the core. Because the cladding layer surrounds the core, it shields the guided mode from interaction with any chemical substance that may be in contact with the fibre. Therefore, optical 95 fibre chemical sensing requires a means to enable interaction between the guided mode and the chemical environment. This may be done by removing some part of the cladding, for example by etching 1}, or by coupling a part of the guided mode into a cladding mode.
A tilted fibre Bragg grating [2] or a long-period grating (“LPG”) can facilitate the coupling from a core mode ta a cladding mode withaut etching of the fibre [3, 4]. As the cladding mode propagates by total intemal reflection at the boundary between the cladding and the external environment, the chemical properties of the environment will influence the propagation In a refractometer, the interaction ie mainly due to the refractive index of the chemical environment. In another embodiment of the optical fibre refractometer, two multimode fibres are fused together to form a hemispherical sensing head or transducer [5]. A light source illuminates the sensing head through one of the fibres, and the other fibre returns the remaining light to a photo-detector. The refractive index of the chemical surrounding the sensing transducer determines the coupling efficiency into the second fibre.
State-of-the-art fibre optic refractive index sensors as described in the previous paragraph, are transmission type devices. Therefore, they require two fibres: one for delivering the excitation to the measuring head or the transducer, and one to carry the signal to the detector and processing "electronics. This could be a serious limitation, especially for in vivo applications of the sensors.
It is an object of this invention to provide a fibre optic sensor for the measurement of refractive index.
In accordance with this invention there is provided a fibre optic sensor for measurement of refractive index comprising a sensor element consisting of a wave guide having a grating along part of its length and terminating in a reflector at a reflector end thereof; the guide being arranged to guide an incident core wave, from an input end thereof, in its core, through the grating where part of the wave is divided out into a cladding of the wave guide to from an incident cladding wave which, together with the remainder of the incident core wave, propagates to the reflector which reflects the incident core and cladding waves to form reflected core and cladding waves propagating back to the grating where part of the reflected cladding wave is divided back into the core of the wave guide to Interfere with the reflected core wave to form an interference wave.
There is provided for the sensor to include a routing device such as an optical coupler or an optical circulator connected to the input end of the wave-guide.
The sensor further includes a wave source connected to the routing device for guiding the incident wave from the wave source to the input end of the wave- guide.
A further feature of the invention provides for a measuring means such as a spectrometer or a spectrum analyser to be connected to the routing device and for the routing device to guide the interference wave to the spectrometer. s
A still further feature of the invention provides for the measurement means to measure the power spectrum of the reflected interference wave as a function of wavelength or the phase shift of the interference wave.
Further features of the invention provide for the reflected interference wave to be converted to an electrical signal by the measurement means; for the electrical signal to be amplified; for the phase of the electrical signal to be measured; alternatively, for the interference fringes of the electrical signal to be measured.
A still further feature of the invention provides for the wave-guide to be surrounded by a medium with properties that may change because of an external perturbation such as, for example, a change in concentration.
There is provided for the wave-guide to have an outer buffer layer extending from its input end to the grating.
The changing properties affect the phase or the amplitude of the core and cladding propagating waves in a different way or to a different degree. Proper operation may require removal of some part of a buffer layer of the wave- guide, and/or treatment of the core or the cladding of the wave-guide.
The cladding of the wave-guide shields the core from the ambient chemical environment, but not from the temperature of the environment. Proper choice 5 of materials or feedback may negate the influence of temperature on the wave-guide in cases were temperature is not measured.
These and other features of the invention are described in more details below.
A preferred embodiment of the invention is described below, by way of example only, and with reference to the accompanying drawings in which:
Figure 1a shows a schematic representation of the system configuration of a fibre optic sensor; ’
Figure 1b shows details of forward or incident optical paths in a sensor element of the sensor of figure 1a;
Figure 1c shows details of retum or reflected optical paths in a sensor element of the sensor of figure 1a;
Figure 2a shows a graph of the experimental verification of the . sensor of figure 1a, representing two immersion depths of the sensor element in water;
Figure 2b shows the Fourier spectrum of interference versus wavelength signal of the sensor of figure 1a;
Figure 3 shows a graph of the phase change of the signals used in the sensor of figure 1a as a function of depth of immersion of the sensor element in water;
Figure 4 shows a graph of the phase change of the signals used in the sensor of figure 1a as a function of the concentration of glycerine in water; and
Figure 5 shows a graph of the phase change of the signals used in the sensor of figure 1a as a function of temperature.
With reference to figure 1, a fibre optic sensor is shown which includes a wave source in the form of a super-luminescent diode, a routing device in the form of an optical circular, a measuring means in the form of a spectrometer and a sensor element. The sensor element consists of a wave-guide in the form of a length of optical fibre terminating in a reflector at a reflector end thereof. in this embodiment the reflector is a mirror as is known in the art. The wave- guide includes a long period grating (LPG) along part of its length.
The circulator forms a hub connected to the sensor element, super- luminescent diode and spectrometer for routing an incident wave to the input end of the wave guide for propagation of the incident wave in the core of the wave guide and for routing an interference wave received from the input end of the wave guide to the spectrometer.
The section of the waveguide between the LPG and the mimor forms a transducer or measurement section. A buffer layer covers the rest of the wave-guide.
In use, an incident wave propagates from the super-luminescent light emitting diode (a broadband optical source) to port Py of the optical circulator. It travels through the circulator to port P2 and propagates in the core of the wave-guide to the LPG. The LPG has an appropriate centre wavelength and an appropriate coupling value. As shown in figure 1b, part of the incident wave continues to travel along the core towards the mirror located on the reflector end of the fibre. The other part of the wave couples into the cladding of the wave-guide and travels along the cladding as a cladding mode(s), also towards the mirror located on the reflector end of the fibre.
As shown in figure 1b, upon reflection from the mirror, the incident waves in the core and in the cladding retrace their paths towards the mode coupler
(LPG). At the LPG, part of the reflected core wave couples from the core into the cladding and is lost as indicated by dashed lines. Similarly, part of the cladding wave couples back into the core, while the rest of the cladding mode continues to propagate in the cladding where it is lost (dashed lines). The two reflected waves traveling in the core towards port P, of the optical circulator, interfere with each other. Thus, the reflected core wave and the part of the reflected cladding wave that coupled back into the core at the LPG, forms a reflected interference wave. These interfering waves propagate through the circulator and exit at port Pa. An optical spectrum analyser or any other suitable instrument determines the power spectrum of the interference wave as a function of wavelength.
Figure 2(a) shows measured interference patterns for the wavelength range 1480 nm to 1600 nm for a particular embodiment of the invention in which, for the first dashed-line curve, the outer end (mirror) of the sensor is submerged 45mm into water, and for the second solid-line curve, the sensor element was left in the ambient air in the laboratory.
The shape (amplitude and phase) of the interference spectrum depends on the difference in the optical path lengths (the product of physical length and effective refractive index) of the two optical paths between the LPG and the mirror (one in the core and the other in the cladding). An external perturbation may act on these path lengths simultaneously (this could be changing temperature), or it may be allowed to act on only one of the paths (such as a chemical substance that comes into contact with the cladding alone).
By the proper choice of the materials constituting the fibre core, the fibre cladding, and by a proper choice of the period and the refractive index modulation of the LPG, the sensor may be optimised for temperature measurements or for measurement of chemical composition of the surrounding substance. Adjustment of the length of the optical fibre between the LPG and the mirror enables one to tailor the sensitivity.
In the first experiment to demonstrate the principle, the transducer section of the optical fibre of the sensor was immersed to different depths in water. The buffer layer of the fibre was removed before the experiment to allow contact between the cladding and the analyte (water in this instance). Figure 3 shows the measured phase shift as a function of the depth of immersion. The measured phase shift is a linear function of the depth.
In the second experiment, the transducer section was immersed to a fixed depth into varying concentrations of glycerine in water. The concentration of the glycerine varied between 0% and 80%. Figure 4 shows the measured phase shift of the interferometer signal as a function of the concentration. itis possible to use the phase shift to determine the concentration of an unknown sample of the mixture.
In the third experiment, the sensor was placed in a temperature-controlled laboratory oven and the temperature set point was changed between 16 °C and 90 °C. Figure 5 depicts the measured phase change as a function of temperature. For this particular embodiment of the sensor, the temperature sensitivity of the phase change is approximately 11.3 degrees/°C.
The embodiment of the invention described above thus provides a compact refractometer with only one optical fibre probe to measure the refractive index and other related properties (such as chemical concentration and composition) of various types of solids, liquids and gases; to determine the level of a substance, for example the level of a liquid in a container; to measure temperature; to probe a patient in vivo with minimum invasiveness; and to increase the sensitivity by employing optical interference.
The sensor may monitor one or more of these measurands simultaneously.
[1] K. Schroeder, W. Ecke, R. Mueller, R. Willsch and A. Andreev, "A fibre
Bragg grating refractometer’, Measurement Science and Technology, vol. 12, 2001, pp. 757-764.
[2] G. Laffont and P. Ferdinand, "Tilted short-period fibre-Bragg-grating- induced coupling to cladding modes for accurate refractometry’,
Measurement Science and Technology, vol. 12, 2001, pp. 765-770.
[3] H.J. Patrick, A.D. Kersey and F. Bucholtz, "Analysis of the response of long-period fiber gratings to external index of refraction®, Jounal of Lightwave
Technology, vol. 16(9), 1998, pp. 1606-1612.
[4] K.S. Chiang, Y. Liu, M.N. Ng and X. Dong, "Analysis of etched long-period fibre grating and its response to external refractive index’, Electronics Letters, vol. 36 (11), 2000, pp 966-967.
[5] V. Svirid, S. Khotiaintsev and P.L. Swart, "Novel optical fiber refractometric transducer employing hemispherical detection element’, Optical Engineering, vol. 41(4), 2002, pp. 779-787.
Claims (22)
1. A fibre optic sensor for measurement of refractive index comprising a sensor element consisting of a wave guide having a grating along part of its length and terminating in a reflector ata reflector end thereof; the guide being arranged to guide an incident core wave, received from an input end thereof, in its core, through the grating where part of the wave is divided out into a cladding of the wave guide to from an incident cladding wave which, together with the remainder of the incident core wave, propagates to the refiector which reflects the incident core and cladding waves to form reflected core and cladding waves propagating back to the grating where part of the reflected cladding wave is divided back into the core of the wave guide to interfere with the reflected core wave to form an interference wave.
2. A sensor as claimed in claim 1 in which the sensor includes a routing device.
3. A sensor as claimed in any one of the preceding claims in which the sensor includes a measuring means.
4. A sensor as claimed in any one of the preceding claims in which the sensor includes a wave source.
5. A sensor as claimed in claim 2 in which the routing device is an optical coupler connected to the input end of the wave-guide.
6. A sensor as claimed in claim 2 in which the routing device is an optical circulator connected to the input end of the wave-guide.
7. A sensor as claimed in any one of claims 4 to 6 in which the wave source is a diode.
8. A sensor as claimed in any one of claims 4 to 7 in which the wave source is connected to the routing device so that an incident wave from the wave source Is guided to the input end of the wave-guide through the routing device.
9. Asensorasclaimed inany one of claims 3.0.8 in which the measuring. . «=~ means is a spectrometer or a spectrum analyser.
10. A sensor as claimed in any one of claims 3 to 9 in which the measuring means is connected to the routing device so that the interference wave will be guided from the input end of the wave-guide to the measuring means.
11. A sensor as claimed in any one of claims 3 to 10 in which the measurement means is arranged to measure the power spectrum of the reflected interference wave as a function of wave length
12. A sensor as claimed in any one of claims 3 to 10 in which the measurement means is arranged to measure the phase shift of the interference wave. 3
13. A sensor as claimed in any one of the preceding claims in which the reflected interference wave is converted to an electrical signal by a, or by the, measurement means.
14. A sensor as claimed in claim 13 in which the electrical signal is amplified.
15. A sensor as claimed in any of claims 12 to13 in which the phase of the electrical signal to be measured.
16. A sensor as claimed in any of claims 12 t013 in which the interference fringes of the electrical signal are measured.
17. A sensor as claimed in any one of the preceding claims in which at least part of the wave-guide is surrounded by a medium with properties that may change because of an external perturbation.
18. A sensor as claimed in claim 17 in which the extemal perturbation is a change in concentration.
19. A sensor as claimed in any one of the preceding claims in which the wave-guide has an outer buffer layer extending from its input end to the grating.
20. A sensor as claimed in claim 17 in which a change of the medium propertyfies affects the phase or the amplitude of the core and/or cladding propagating incident and/or reflected waves.
21. A sensor as claimed in any one of the preceding ctaims in which the cladding of the wave-guide shields the core from the ambient chemical environment, but not from the temperature of the environment.
22. A sensor as claimed in any one of the preceding claims in which the wave-guide material or feedback negates the influence of temperature on the wave-guide.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| ZA200508065A ZA200508065B (en) | 2003-04-02 | 2005-10-06 | A fibre optic sensor for measurement of refractive index |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| ZA200302582 | 2003-04-02 | ||
| ZA200508065A ZA200508065B (en) | 2003-04-02 | 2005-10-06 | A fibre optic sensor for measurement of refractive index |
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| Publication Number | Publication Date |
|---|---|
| ZA200508065B true ZA200508065B (en) | 2007-01-31 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| ZA200508065A ZA200508065B (en) | 2003-04-02 | 2005-10-06 | A fibre optic sensor for measurement of refractive index |
Country Status (2)
| Country | Link |
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| WO (1) | WO2004088290A1 (en) |
| ZA (1) | ZA200508065B (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN100561198C (en) * | 2005-07-01 | 2009-11-18 | 曾祥楷 | Fibre-optical microstructure Michelson interfere type surface plasma resonance chemistry and biology sensor and system |
| CN100451609C (en) * | 2005-07-01 | 2009-01-14 | 重庆工学院 | Interference evanescent wave chemical and biological sensor and system with fibre-optical Michelson |
| CN100557416C (en) * | 2006-12-26 | 2009-11-04 | 重庆工学院 | Minisize multiplexing interfering biomolecule effect sensing method and probe |
| CN102364313B (en) * | 2011-10-15 | 2013-05-01 | 浙江师范大学 | High-temperature sensing method based on optical fiber micro Michelson interference on spherical end face |
| CN103630975B (en) * | 2013-03-08 | 2015-12-23 | 北方工业大学 | Centre wavelength and the adjustable light pectination interferometer of channel spacing |
| CN104535534B (en) * | 2014-12-15 | 2017-08-04 | 哈尔滨工程大学 | A kind of optical fiber precast rod refractivity profile measurement apparatus and measuring method based on the absolute light path comparison method of white light interference |
| CN105866071B (en) * | 2016-06-02 | 2019-04-26 | 吉林大学 | A kind of fiber optic interferometric method surveys the device of refractive index |
| CN108181271A (en) * | 2017-12-22 | 2018-06-19 | 北京信息科技大学 | For fibre-optical microstructure sensor of temperature and liquid refractivity sensing measurement and preparation method thereof |
| CN111257283B (en) * | 2020-02-25 | 2021-09-03 | 厦门大学 | Refractive index sensing measurement device and method |
| CN114324240B (en) * | 2021-12-31 | 2024-04-09 | 清华大学 | Refractive index measuring device and refractive index measuring method |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6111999A (en) * | 1998-09-24 | 2000-08-29 | Lucent Technologies Inc. | Fiber device having variable refractive index region proximal the core |
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2004
- 2004-03-31 WO PCT/IB2004/050375 patent/WO2004088290A1/en active Application Filing
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| WO2004088290A1 (en) | 2004-10-14 |
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