A strain isolated optical fibre Bragg grating sensor

Abstract: A strain isolated optical fibre Bragg grating sensor (1) has an optical fibre (2) provided at or adjacent one end (2a) with a Bragg grating (3). An open mouth cap (4) surrounds the fibre end (2a) and region provided with the Bragg grating (3) at a spacing from the fibre, fibre end and Bragg grating. The cap (4) is sealed to the fibre (2) at the open mouth at a side of the grating (3) remote from the fibre end (2a). The cap (4) is made from a low Young's modulus material and is operable to isolate the grating (3) from strain.

Description: This invention relates to a strain isolated optical fibre Bragg grating sensor suitable particularly, but not exclusively, for compensating non-strain isolated Bragg gratings for the effect of temperature. Fibre optics sensors may be used for measuring many parameters such as temperature, strain, pressure, displacement, electric field, refractive index, rotation, position and vibration. However because of sensitivity to these parameters, a fibre optic sensor can often suffer from cross sensitivity so that if used for measuring one parameter it may well also be effected by one or more of the other parameters. Although some of the cross sensitivity can be designed out of such sensors, it has proved particularly difficult to eliminate strain-temperature cross sensitivity. There is thus a need for an optical fibre Bragg grating sensor which is strain isolated and which therefore can be used for compensating for temperature effects on a similar non-strain isolated optical fibre Bragg grating sensor. An attempt to address the temperature compensation has been made in the article "Simultaneous Measurement of Temperature and Strain Using Two Fiber Bragg Gratings Embedded in a Glass Tube" by E. A. Song, published in Optical Fibre Technology: Materials, Devices and Systems volume 3, pages 194 to 196. Song discloses an optical fibre, a central portion of which is provided with a pair of closely-spaced Bragg gratings. A glass tube is bonded to either side of one of the Bragg gratings in an attempt to alleviate strain across that Bragg grating. According to one aspect of the present invention there is provided a strain isolated optical fibre Bragg grating sensor characterised by having an optical fibre provided at or adjacent one end thereof with a Bragg grating and an open mouth cap surrounding the fibre end and region provided with the Bragg grating at a spacing from the fibre, fibre end and Bragg grating, which cap is sealed to the optical fibre at the open mouth at a side of the Bragg grating remote from the fibre end, is made from a low Young's modulus material, and is operable to isolate the Bragg grating from strain. Preferably the cap is a thin walled capillary tube made of fused silica. Alternatively the cap is made of an epoxy resin. Conveniently the sensor contains a gaseous medium in the cap around the fibre, which medium has a low Young's modulus. Advantageously the gaseous medium is air. According to a further aspect of the present invention there is provided an optical fibre Bragg grating sensor system attachable to or incorporatable in an object, including one or more optical fibre Bragg grating sensors for sensing strain in or on the object and one or more strain isolated optical fibre Bragg grating sensors according to the present invention, for measuring temperature in or on the object. Preferably the system includes means for receiving a temperature output signal from the or each strain isolated optical fibre Bragg grating sensor, processing said temperature output signal and generating therefrom an output compensating for effect of temperature on the or each optical fibre Bragg grating sensor. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which; Figure 1 is a diagrammatic view of a strain isolated optical fibre Bragg grating sensor according to the present invention, Figure 2 is a graphical representation of amplitude versus wavelength showing how the wavelength peaks shift for constant strain with temperature for the sensor of Figure 1, Figure 3 shows the variation of wavelength peak positions with temperature for the sensor of Figure 1, and Figure 4 is a diagrammatic plan view of an optical fibre Bragg grating sensor system including at least one strain isolated optical fibre Bragg grating sensor according to the present invention, A strain isolated optical fibre Bragg grating sensor according to the present invention is shown schematically at 1 in Figure 1 of the accompanying drawings. The sensor 1 has an optical fibre 2 provided at or adjacent one end 2a thereof with a Bragg grating 3 which may be of equal spacing as shown or of variable spacing. An open mouth cap 4 surrounds the fibre end and region provided with the Bragg grating 3 at a spacing from the fibre, fibre end 2a and Bragg grating. The cap 4 is sealed to the optical fibre 2 at the open mouth at 5. The seal 5 lies at a side of the Bragg grating 3 remote from the fibre end 2a, is made from a low Young's modulus material and is operable to isolate the Bragg grating 3 from strain. The seal 5 may be a fusion splice or an epoxy resin. The cap 4 is preferably a piece of thin walled fused silica capillary tube which surrounds the fibre end 2a and grating 3 without coming into physical contact with the grating 3. Alternatively the cap 4 may be made from epoxy resin. A suitable optical fibre 2 is a standard germania doped-mode optical fibre, for example with a diameter of 125µm with the cap 4 being of 5mm length with an internal diameter of substantially 150µm and an external diameter of substantially 360µm. Conveniently the closed end of the cap 4 can be produced using a fusion splicer which can also be used for providing the seal 5 at the open mouth of the cap 4. The cap 4 effectively strain isolates the Bragg grating 3. The sensor of Figure 1 was attached to a bar of material which was strained using a three point bending test. Over a three point bend deflection of the bar over a range of 7.5 to 22.5 millimetres there was no strain reading from the sensor whereas a sensor not according to the invention showed a strain change over this deflection range of between 500 to 1500µε. Figure 2 of the accompanying drawings shows how the reflection wavelength peak shifts with temperature when the sensor of Figure 1 is heated. Figure 3 plots the reflection wavelength peak positions at specific temperatures for the sensor of Figure 1 when illuminated with a broadband superluminescent diode. The cap 4 sealed to the optical fibre 2 at 5 in the strain isolated fibre optic Bragg grating sensor of Figure 1 according to the present invention contains a gaseous medium around the fibre. This medium has a low Young's modulus which contributes to the strain isolating effect. Conveniently this gaseous medium is air. The strain isolated optical fibre Bragg grating sensor of the present invention is suitable for use in an optical fibre Bragg grating sensor system attachable to or incorporatable in an object on which strain is to be measured. Such a system is shown schematically in Figure 4 of the accompanying drawings. This system includes one or more non strain-isolated optical fibre Bragg grating sensors 6 and at least one strain isolated sensor 1 according to the invention. The temperature measured by the strain isolated sensor 1 is used to compensate for the temperature cross-sensitivity of the remaining non-isolated sensors 6 in the system. Means are provided for receiving a temperature output signal from the or each strain isolated sensor 1, processing the signal and generating therefrom an output compensating for effect of temperature on the sensors 6. The sensors 6 using the same optical fibre 2 and Bragg grating 3 as the strain isolated sensor 1 are compatible therewith and hence the temperature reading from the sensor 1 is comparable directly to that experienced by the sensor 6. Of course the sensor 1 and sensors 6 are placed in close proximity to one another.

Claims: A strain isolated optical fibre Bragg grating sensor having an optical fibre (2) provided at or adjacent one end (2a) thereof with a Bragg grating (3), and an open mouth cap (4) surrounding the fibre end (2a) and region provided with the Bragg grating (3) at a spacing from the fibre, fibre end and Bragg grating, which cap (4) is sealed to the optical fibre (2) at the open mouth at a side of the Bragg grating remote from the fibre end (2a), is made from a low Young's modulus material, and is operable to isolate the Bragg grating (3) from strain. A sensor according to claim 1, wherein the cap (4) is a thin walled capillary tube made of fused silica. A sensor according to claim 1, wherein the cap (4) is made of an epoxy resin. A sensor according to any one of claims 1 to 3, containing a gaseous medium in the cap (4) around the fibre (2), which medium has a low Young's modulus. A sensor according to claim 4, wherein the gaseous medium is air. An optical fibre Bragg grating sensor system attachable to or incorporatable in an object, including one or more optical fibre Bragg grating sensors (6) for sensing strain in or on the object and one or more strain isolated optical fibre Bragg grating sensors (1) according to any one of claims 1 to 5, for measuring temperature in or on the object. A sensor system according to claim 6, including means for receiving a temperature output signal from the or each strain isolated optical fibre Bragg grating sensor (1), processing said temperature output signal and generating therefrom an output compensating for effect of temperature on the or each optical fibre Bragg grating sensor (6).

Inventors: FOOTE PETER D, HARAN FRANCIS M, FOOTE, PETER D., HARAN, FRANCIS M., Foote, Peter D. British Aerospace PLC, Haran, Francis M., 20 Craigendoran Avenue
Applicants: BAE SYSTEMS PLC
Application year: 1998
Main classification: G01D5
IPC codes: G01D5, G01B11, G01K11, G02B6, G01L1