WO2004088290A1

WO2004088290A1 - A fibre optic sensor for measurement of refractive index

Info

Publication number: WO2004088290A1
Application number: PCT/IB2004/050375
Authority: WO
Inventors: Pieter Lodewikus Swart
Original assignee: Rand Afrikaans University
Priority date: 2003-04-02
Filing date: 2004-03-31
Publication date: 2004-10-14
Also published as: ZA200508065B

Abstract

This invention relates to a fibre optic sensor for measurement of refractive index. 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, 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 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.

Description

A FIBRE OPTIC SENSOR FOR MEASUREMENT OF REFRACTIVE INDEX

FIELD OF THE INVENTION

This invention relates to a fibre optic sensor for measurement of refractive index.

BACKGROUND TO THE INVENTION

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 or 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 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 lilted fibre Bragg grating [2] or a long-period grating ("LPG") can facilitate the coupling from a core mode to a cladding mode without etching of the fibre [3, 4]. As the cladding mode propagates by total internal 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 is 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. OBJECT OF THE INVENTION

It is an object of this invention to provide a fibre optic sensor for the measurement of refractive index.

SUMMARY OF INVENTION

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 waveguide. 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.

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 waveguide, 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 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.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 a 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 1 a;

Figure 1c shows details of return or reflected optical paths in a sensor element of the sensor of figure 1 a; Figure 2a shows a graph of the experimental verification of the sensor of figure 1 a, 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 1 a;

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.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

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. n this embodiment the reflector is a mirror as is known in the art. The waveguide 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 wave-guide between the LPG and the mirror 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 Pi 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 1 b, 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 1 b, upon reflection f om 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 a 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 P₃. 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.

EXPERIMENTAL VERIFICATION OF THE INVENTION

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. It is 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.

REFERENCES

[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", Journal 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

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 at a 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 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.

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. A sensor as claimed in any one of claims .3 to 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.

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 to13 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 external 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 property/ies 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 claims 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.