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Speckle-based fibre-optic current sensor
Speckle-based sensor B.S. RAMPRASAD,
fibre-optic
current
T.S. RADHA BAI
A new fibre-optic sensor, based on the speckle for the measurement of current is The the advantages of simplicity and sensitivity, but requires a two-step measurement procedure. KEYWORDS:
fibre optics, speckle, current measurement
patterns can be made (which we will refer to as a specklegram). The method is similar to double exposure hologram recording. The key step underlying the present technique is to make the Young’s fringes, which one could obtain in the reconstruction mode from such a specklegram, serve as a sensitive indicator of the dimensional or displacement characteristics of the fibre and thus of the parameters of the source of the disturbance (for example, current or magnetic field).
Introduction An unconventional use of optical fibres, which has been gaining in stature over the past few years, is the development of tibre-optic sensors (FOS). Recent reviews’-5 have discussed the past, present and future of FOS and it appears that by now about 60 types have been developed in various laboratories around the world. Among the many efforts, the concerted attempts of the group at the Naval Research Laboratories in the USA in the development of FOS, are worthy of mention. To date, a host of parameters, such as temperature, pressure, rotation, acceleration, displacement, magnetic field and current, have been identified for sensing and measuring using intensity or phase modulation of the light propagating through an optical fibre. Depending upon whether the intensity or the phase aspect is used for sensing purposes, the techniques of FOS can essentially be divided into two categories, namely, non-interferometric and interferometric. Naturally the interferometric method is more sensitive, but problems in implementing this technique are more severe.
Experimental
Figure 1 shows a FOS configuration used by the authors for the measurement of magnetic field and current. A Pyrex glass fibre of 0.5 mm diameter drawn in the laboratory was fixed as a cantilever such that the free length was 6.3 cm. A HeNe laser (1 mW) beam was focused on the fixed end of the tibre through a microscope objective. A lens was used to collimate the output beam of the fibre and a nickel jacket mounted on the fibre at a suitable distance from the free end. A solenoid having a soft iron core was mounted on a xy stage with adjustments along the axis of the fibre and perpendicular to it. It was energized by a suitable
We have found that a technique involving the exploitation of speckle phenomena could be used for the development of a new type of FOS, and in this paper we report the results of the first generation of experiments on speckle based fibre-optic sensors (SBFOS) with particular emphasis on current measurement.
Principles
technique
OptIcal fi bre contilevnr
of the new technique
One can expect a speckle pattern to be generated at the output end of an appropriate optical fibre (for example, a multimode fibre) when coherent light is transmitted through it. However, if some method could be found to cause dimensional changes in the fibre (for example, through thermal expansion) or a displacement of a particular length of fibre through cantilever action, etc, then the original pattern will appear displaced. A record of the superimposition of these two speckle L
The authors are at the Central Instruments and Services Laboratory. Indian Institute of Science, Bangalore2. India. Received 26 May 1983. Revised 16 September 1983.
0030-3992/84/030156-D3/$03.00 156
Fig. 1 Schematic of speckle-based fibre-optic sensor. L = length of cantilever, d = gap between soft iron care and jacket
0
1984
Butterworth
B Co (Publishers)
Ltd
OPTICS AND LASER TECHNOLOGY.
JUNE
1984
Fig. 2
Specklegrams showing the Young’s fringes for different currents: a -
OPTICS AND
LASER TECHNOLOGY.
JUNE
1984
~OmA;b-2OmA;c-3OmA;d-4Om~e-5Om~f-SOmA
157
6-
Distance of jacket from the fixed end (cm)
To study the sensitivity, the nickel jacket was placed at different distances from the fixed end, keeping the current and the gap d constant. It was found that the free end of the fibre suffered maximum deflection when the nickel jacket was close to the fixed end, and the least when the nickel jacket was close to the free end. The reason for this is that, for a given current and gap, the displacement of the nickel jacket towards the core of the solenoid is the same; hence the larger deflection of the free end for a smaller loading distance L. The fibre behaved more like an optical lever than a loaded cantilever. The insert in Fig. 3 bears out this observation. As the loading distance increases the deflection decreases and consequently the fringe spacing increases. This means that a much lower current can be measured by having the jacket closer to the fixed end. The other parameters that can be operated upon are the gap d and the size of the speckle. The latter is possible by introducing an aperture in front of the lens. As the aperture is reduced the speckle size increases and one should be able to measure larger currents. However, experiments did not indicate a marked change in the fringe spacing By proper choice of loading distance and gap, different current ranges can be measured.
Conclusion
I 20
I 60
I 40 Current
(mA)
Fig. 3 Curve showing current against Young’s fringes spacing. Inset shows the variation of Young’s fringes spacing as a function of the nickel jacket position
dc power supply with provision for varying the current. Agfa 10E 75 photographic plates were used to record the speckle pattern. When the coil was energized the nickel jacket was attracted towards the solenoid. The photographic plate was exposed twice, once when the cantilever was static and the second time with the coil energized. The developed photographic plate, or specklegram, is a record of the two superimposed speckle patterns. When it is placed in an unexpanded laser beam Young’s interference fringes are seen on the screen. When the current is high the displacement of the cantilever is also high and the spacing between the fringes is small. As the current decreases the fringe spacing increases.
Results and discussion The current was raised from 10 mA to 60 mA for a fixed position of the nickel jacket and the gap (6) between the core and the jacket. Several specklegrams were recorded and some typical Young’s fringes obtained from them are shown in Fig. 2. It can be seen that the fringe spacing decreases as the current is increased. A plot of current against fringe spacing is given in Fig. 3.
158
A novel method of using fibre-optic speckle for the measurement of current has been described. While the technique has the advantages of simplicity and sensitivity, the disadvantage of requiring a two-step measurement procedure is a problem at the moment. The use of real-time methods such as electronic speckle pattern interferometry (ESPI) may remove this disadvantage, but cost-effectiveness of the FOS developed on the basis of this simple principle may be lost. We have observed that the same technique could be used for the measurement of magnetic fields as well. Detailed investigations using regular communicationgrade fibres are being carried out in our laboratory and a paper will appear elsewhere.
Acknowledgement The authors are grateful to Prof S.V. Pappu for his kind encouragement and interest in this work
References Giallorenzi, T.G., et al. ‘Optical fiber sensor technology’. IEEE J Quant Ekcrron 18, (4). (1982) 626-665 Hermer, A.L. ‘Principles of optical fiber sensors and instrumentation’ Meas. Control. 15. 4. (1982) 143-15 I James, KA., Quick, W.H., Strahan, V.H. ‘Fiber optics: the way to true digital sensors’ Control Ennn (1979) 30-33 Baumbic< R.J., Alexander, J. ‘Fiber optics sense process variables’. Control Engg, (1980) 75-77 Tebo, A.R ‘Sensing with optical fibers: an emerging technology’. Elecrro-oprical Systems Design (I 982) 39-45
OPTICS AND LASER TECHNOLOGY. JUNE 1984
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Reprints Reprints of all articles in this journal are available in quantities of 100 or more.
Reprints
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-
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For full details of prices and availability of reprints, please write to The Reprint Department Butterworth Scientific Limited PO Box 63 Westbury House Bury Street Guildford Surrey GU2 5BH England
OPTICS AND LASER TECHNOLOGY.
JUNE 1984
159
fibre-optic
current
T.S. RADHA BAI
A new fibre-optic sensor, based on the speckle for the measurement of current is The the advantages of simplicity and sensitivity, but requires a two-step measurement procedure. KEYWORDS:
fibre optics, speckle, current measurement
patterns can be made (which we will refer to as a specklegram). The method is similar to double exposure hologram recording. The key step underlying the present technique is to make the Young’s fringes, which one could obtain in the reconstruction mode from such a specklegram, serve as a sensitive indicator of the dimensional or displacement characteristics of the fibre and thus of the parameters of the source of the disturbance (for example, current or magnetic field).
Introduction An unconventional use of optical fibres, which has been gaining in stature over the past few years, is the development of tibre-optic sensors (FOS). Recent reviews’-5 have discussed the past, present and future of FOS and it appears that by now about 60 types have been developed in various laboratories around the world. Among the many efforts, the concerted attempts of the group at the Naval Research Laboratories in the USA in the development of FOS, are worthy of mention. To date, a host of parameters, such as temperature, pressure, rotation, acceleration, displacement, magnetic field and current, have been identified for sensing and measuring using intensity or phase modulation of the light propagating through an optical fibre. Depending upon whether the intensity or the phase aspect is used for sensing purposes, the techniques of FOS can essentially be divided into two categories, namely, non-interferometric and interferometric. Naturally the interferometric method is more sensitive, but problems in implementing this technique are more severe.
Experimental
Figure 1 shows a FOS configuration used by the authors for the measurement of magnetic field and current. A Pyrex glass fibre of 0.5 mm diameter drawn in the laboratory was fixed as a cantilever such that the free length was 6.3 cm. A HeNe laser (1 mW) beam was focused on the fixed end of the tibre through a microscope objective. A lens was used to collimate the output beam of the fibre and a nickel jacket mounted on the fibre at a suitable distance from the free end. A solenoid having a soft iron core was mounted on a xy stage with adjustments along the axis of the fibre and perpendicular to it. It was energized by a suitable
We have found that a technique involving the exploitation of speckle phenomena could be used for the development of a new type of FOS, and in this paper we report the results of the first generation of experiments on speckle based fibre-optic sensors (SBFOS) with particular emphasis on current measurement.
Principles
technique
OptIcal fi bre contilevnr
of the new technique
One can expect a speckle pattern to be generated at the output end of an appropriate optical fibre (for example, a multimode fibre) when coherent light is transmitted through it. However, if some method could be found to cause dimensional changes in the fibre (for example, through thermal expansion) or a displacement of a particular length of fibre through cantilever action, etc, then the original pattern will appear displaced. A record of the superimposition of these two speckle L
The authors are at the Central Instruments and Services Laboratory. Indian Institute of Science, Bangalore2. India. Received 26 May 1983. Revised 16 September 1983.
0030-3992/84/030156-D3/$03.00 156
Fig. 1 Schematic of speckle-based fibre-optic sensor. L = length of cantilever, d = gap between soft iron care and jacket
0
1984
Butterworth
B Co (Publishers)
Ltd
OPTICS AND LASER TECHNOLOGY.
JUNE
1984
Fig. 2
Specklegrams showing the Young’s fringes for different currents: a -
OPTICS AND
LASER TECHNOLOGY.
JUNE
1984
~OmA;b-2OmA;c-3OmA;d-4Om~e-5Om~f-SOmA
157
6-
Distance of jacket from the fixed end (cm)
To study the sensitivity, the nickel jacket was placed at different distances from the fixed end, keeping the current and the gap d constant. It was found that the free end of the fibre suffered maximum deflection when the nickel jacket was close to the fixed end, and the least when the nickel jacket was close to the free end. The reason for this is that, for a given current and gap, the displacement of the nickel jacket towards the core of the solenoid is the same; hence the larger deflection of the free end for a smaller loading distance L. The fibre behaved more like an optical lever than a loaded cantilever. The insert in Fig. 3 bears out this observation. As the loading distance increases the deflection decreases and consequently the fringe spacing increases. This means that a much lower current can be measured by having the jacket closer to the fixed end. The other parameters that can be operated upon are the gap d and the size of the speckle. The latter is possible by introducing an aperture in front of the lens. As the aperture is reduced the speckle size increases and one should be able to measure larger currents. However, experiments did not indicate a marked change in the fringe spacing By proper choice of loading distance and gap, different current ranges can be measured.
Conclusion
I 20
I 60
I 40 Current
(mA)
Fig. 3 Curve showing current against Young’s fringes spacing. Inset shows the variation of Young’s fringes spacing as a function of the nickel jacket position
dc power supply with provision for varying the current. Agfa 10E 75 photographic plates were used to record the speckle pattern. When the coil was energized the nickel jacket was attracted towards the solenoid. The photographic plate was exposed twice, once when the cantilever was static and the second time with the coil energized. The developed photographic plate, or specklegram, is a record of the two superimposed speckle patterns. When it is placed in an unexpanded laser beam Young’s interference fringes are seen on the screen. When the current is high the displacement of the cantilever is also high and the spacing between the fringes is small. As the current decreases the fringe spacing increases.
Results and discussion The current was raised from 10 mA to 60 mA for a fixed position of the nickel jacket and the gap (6) between the core and the jacket. Several specklegrams were recorded and some typical Young’s fringes obtained from them are shown in Fig. 2. It can be seen that the fringe spacing decreases as the current is increased. A plot of current against fringe spacing is given in Fig. 3.
158
A novel method of using fibre-optic speckle for the measurement of current has been described. While the technique has the advantages of simplicity and sensitivity, the disadvantage of requiring a two-step measurement procedure is a problem at the moment. The use of real-time methods such as electronic speckle pattern interferometry (ESPI) may remove this disadvantage, but cost-effectiveness of the FOS developed on the basis of this simple principle may be lost. We have observed that the same technique could be used for the measurement of magnetic fields as well. Detailed investigations using regular communicationgrade fibres are being carried out in our laboratory and a paper will appear elsewhere.
Acknowledgement The authors are grateful to Prof S.V. Pappu for his kind encouragement and interest in this work
References Giallorenzi, T.G., et al. ‘Optical fiber sensor technology’. IEEE J Quant Ekcrron 18, (4). (1982) 626-665 Hermer, A.L. ‘Principles of optical fiber sensors and instrumentation’ Meas. Control. 15. 4. (1982) 143-15 I James, KA., Quick, W.H., Strahan, V.H. ‘Fiber optics: the way to true digital sensors’ Control Ennn (1979) 30-33 Baumbic< R.J., Alexander, J. ‘Fiber optics sense process variables’. Control Engg, (1980) 75-77 Tebo, A.R ‘Sensing with optical fibers: an emerging technology’. Elecrro-oprical Systems Design (I 982) 39-45
OPTICS AND LASER TECHNOLOGY. JUNE 1984
I would like further details on exhibiting at lnspex ‘85 Name Position Company Address
Tel :
Reprints Reprints of all articles in this journal are available in quantities of 100 or more.
Reprints
are essential
-
0 for the company that wants to distribute impartial comment on its activities to potential customers and clients 0 for the company that wants to up-date its technical staff on new techniques and new technologies
For full details of prices and availability of reprints, please write to The Reprint Department Butterworth Scientific Limited PO Box 63 Westbury House Bury Street Guildford Surrey GU2 5BH England
OPTICS AND LASER TECHNOLOGY.
JUNE 1984
159