GB2407154A

GB2407154A - Acoustic emission sensor based on a fused tapered optical coupler with sharp taper angle

Info

Publication number: GB2407154A
Application number: GB0323946A
Authority: GB
Inventors: Rodney Alan Badcock; Thomas Butler; Rongsheng Chen; Gerard Franklin Fernando
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2003-10-13
Filing date: 2003-10-13
Publication date: 2005-04-20
Anticipated expiration: 2023-10-13
Also published as: GB2407154B8; WO2005040727A2; GB0323946D0; GB2407154B; GB2407154A8; WO2005040727A3

Abstract

An acoustic emission (AE) sensor comprising: a fused tapered optical coupler 7 receiving light from a light source 8 at its insertion end, and splitting this light to produce light signals at each of its exit end which may be connected to fibres 9,10. A detecting device which may include detectors D1, D2 detects light signals through these fibres and a comparator compares the signals to indicate the occurrence of an acoustic emission event during a steady state condition. The coupler 7 has a sharp taper angle, and a short interaction region to maximise the stress concentration following an AE event in the interaction region. The tapered region and coupling region may each be between 2 to 4 mm long. The sensor may be incorporated in a hydrophone, a microphone, a listening panel, a vibration monitor, a corrosion monitor or a temperature sensor.

Description

Improvements in and relating to Fibre Optic Sensors This invention relates

to apparatus and methods for detecting an acoustic event (AK) which may typically be within the range of DC (static) to 1.0 MHz through the use of fused-tapered optical fibre couplers.

Fused-tapered fibre optic couplers find many uses, such as in the telecommunications industry where they are used as multiplexers and de multiplexers and ideally distribute light among the branch fibres with little or no scattering loss or generation of noise due to ambient mechanical or thermal stresses. US 4,879,454 to Sperry Marine, Inc. teaches how such fused-tapered optical fibre couplers may be fabricated in an electric furnace, where electric heating coils are provided around adjacent fibres to be fused together.

Following heating of the coils, the fibres are then stretched and the light level from a light source directed down each of the fibres is monitored at the exit end of the fibres until the desired level of coupling is achieved, "hereafter heating and stretching of the fibres is stopped. This leaves, typically, a fused fibre optic coupler with a narrow taper angle and a fused interaction region of generally circular section of length, such as 25 mm, and 40 to 80 microns diameter so as to ensure minimum losses.

Such fibre optic couplers have also been used for acoustic emission detection and US 5,671,191 (Sperry Marine, Inc.) describes a variable coupler fibre optic sensor hydrophore where the coupler is encapsulated within a stress birefringent material such as an elastomer, over which is supported a rigid plate of high inertial mass which thereafter transfers incoming acoustic waves to the coupler, the mechanism being that the strain causes birefringence in the coating - , a- ma,, . _. _ material that interacts with the evanescent wave causing a coupling change which can thereafter be detected. The hydrophore acoustic sensor disclosed is stated as operating from sub- Hertz infrasound through frequencies "higher than KHz".

In EP 0431842 (Sperry Marine, Inc.) a fibre optic coupler based sensor is used for monitoring cardiovascular signals and is particularly useful for monitoring e.g. heartbeats or breathing of patients undergoing magnetic resonance imaging (MRI) where electrically based sensors would otherwise cause significant risk of injury due to the strong magnetic fields present during imaging. This sensor is stated as being sensitive up to about 10 KHz and the preferred embodiment of coupler is stated as being one made using the method described in US 4,879,454 discussed above.

However, as with the sensor described in US 5,671,191, that described in EP 0431842 operates over only a very low frequency range, whereas it is desirable for sensors to operate efficiently at much higher frequencies so as to be able to sense acoustic emission events resulting from e.g. breakage or fracture of structural components.

In our patent application PCT/GB01/00838 entitled "Acoustic Emission Detection Using a Fibre-Optic Mode-Coupling Element" (the disclosure of which is incorporated herein by reference), there is described a method and apparatus for sensing acoustic signals emitted from materials and structures by embedding or surface mounting a fibre optic mode-coupling element on or in a structure from which an acoustic emission signal is to be sensed, providing a narrow band-width light source and through the use of detectors for converting light output into electrical signals, subsequently comparing the output with a steady state to thereby detect an acoustic emission event, such as is generated when e.g. a fibre reinforced composite material fractures.

The present invention is derived from the realization that e.g. ultrasound sensors using fibre optic couplers can be made more sensitive and be operable over a relatively broad bandwidth by sacrificing light loss and maximising their sensitivity to stress during fabrication, as compared to fibre optic couplers traditionally used in e.g. the telecommunications industry, which are fabricated so as minimise sensitivity to stress and hence losses, as aforesaid.

According to a first aspect of the invention there is provided an AE sensor comprising a fused-tapered optical fibre coupler for receiving light from a light source at the insertion end of the coupler and, via an interaction region, splitting the light to produce light signals at each exit end of the coupler, detector means for detecting said light signals and comparator means for comparing said signals to thereby indicate the occurrence of an acoustic emission event during a steady state condition CHARACTERISED IN THAT the coupler has a sharp taper angle and a consequently short interaction region to maximise the stress concentration following an AE event in the interaction region, thereby increasing the sensitivity of the coupler and hence the sensor to stress.

Conveniently, the coupler used in the sensor of the invention has a tapered region of between 2 and 4 millimetres and coupling region between 2 and 4 millimetres long, although it has been found that coupling regions between 1 and 8 millimetres may still provide varying but high degrees of sensitivity to incoming acoustic events.

Conveniently, the diameter of the fibre in the interaction or coupling region is around 10 to 30 microns although diameters between the ranges of 5 to 40 microns have also been tested with varying degrees of success.

According to a second aspect of the invention there is provided a method of sensing an AE event using a coupler as described in the first aspect of the invention including the steps of embedding the coupler on or in a material in which an AE event is to be sensed, providing a light source at the insertion end of the coupler and monitoring changes in light output from the coupler to thereby indicate the occurrence of an AE event. Such method may conveniently be used for e.g. embedding the coupler in a composite structural material, such as the wing of an aircraft, to thereby continuously monitor AE events. These may conveniently be monitored cumulatively such that upon, say, the accumulation of a given number of AE events at or above a given threshold a warning signal may be provided indicating potential failure of the structural component.

The invention will now be described, by way of example only, with reference to the accompanying Examples and Figures in which: Figure 1 is a schematic view of an experimental set-up of a sensor using a fusedtapered optical fibre coupler of the invention and a piezoelectric sensor as a reference submerged in a water bath, Figure 2 show response curves to a 155kHz continuous acoustic way of excitation, Figure 3 shows corresponding response curves following an AE event, Figure 4 is a schematic view of an experimental set-up of a sensor using a fusedtapered optical fibre coupler of the invention and a piezoelectric sensor as a reference arranged on a composite plate, Figure 5 show response curves to a 155kHz continuous acoustic way of excitation, Figure 6 shows corresponding response curves following an AE event.

Since the fused tapered optical fibre coupler used in accordance with the sensor of the invention requires characteristics not normally found in conventional couplers having relatively long tapered regions so as to minimise light loss, bespoke couplers were made using Joinwit Coupler workstations of the type shown at www.ioinwit.com/english/iw2101.htm, although other kinds of coupler workstations may, of course, be used. Since the object was to fabricate a coupler having a sharp taper angle relative to conventional couplers, a relatively short interaction region and a relatively narrow waist the thermal and physical characteristics of the flame used to fuse the optical fibres together needed to be tightly controlled. It was found that using a hydrogen flame with an effective size of about 3-5mm using a torch of diameter 8mm and a gas flow rate of 140-150SCCM gave satisfactory results. The pulling and heat duration was typically of 3-5 minutes, the draw distance typically 11-16mm and a draw rate of typically 2-4mm per minute. This enabled fabrication of couplers having a length of taper of typically just 2-4mm, a waist or interactive region of typically just 1 5mm in length, a total length of typically 10-15mm and a waist (diameter) of typically 0.005-0.01 mm. During fabrication the first and subsequent cross over points were monitored, these being the points where light launched down one fibre at the input end starts to couple between the two fibres until such time as the light intensity in both are equal, and at a point when the optical coupling ratio between the two output fibres is equal or nearly equal, fabrication was stopped.

Example 1

In accordance with the above method of fabrication, a fused tapered optical fibre coupler was made using a pair of single mode fibres with a cut-off wavelength of 600nm resulting in a splitting ratio of 50%. The coupler was then fixed at each end by UV cured epoxy resin into a silica groove of dimensions 25mm x 2mm x 2mm.

The experimental set up is shown in Figure 1 in which, within a water filled bath 1, rests at the bottom thereof an AE source 2 in the form of a piezoelectric transducer (model R15) driven by a signal generator 3. A piezoelectric sensor 4 (model R1 5S) is used as a reference and is connected via an oscilloscope 5 to a computer 6 for processing data from the oscilloscope 5 via a GPIB interface (not shown).

In accordance with this embodiment of the invention, a fused-tapered optical fibre coupler 7 acts as the sensing element of the AE sensor and is placed proximate the piezoelectric sensor 4 so as to be equi-distant from the AE source emanating from the piezoelectric transducer 2 by about 50mm. The AE sensor also includes a light source in the form of a 633nm 10 mW He He laser 8 for launching light via a laser-to fibre coupler (not shown) into a first fibre 9 and, via coupler 7, sharing or coupling this light with a second fibre 10, the light outlet ends of fibres 9, 10 being optically coupled to first and second light detectors 1 1, 12. These convert the light signals into electric signals which are subsequently amplified by amplifier 13 which signals are then displayed and analysed respectively by the oscilloscope 5 and computer 6.

The AE sensor of the invention operates differential phase modulation caused by acoustic waves driven by the piezoelectric transducer 2 onto the coupling waist of the coupler 7 between the fundamental (even-LPO,) and the second (odd-LP,' ) modes in the coupling waist. Without acoustic perturbation for single port excitation E, (O) = l and E2 (0) = O the corresponding normalized two output peak powers of the fibre coupler are given by Pi = IE;I2(i = 1,2): P=cos2 (1) P2=sin2 (2) where,BO' and,BO2 are the propagation constants of theLPO, mode and theLp, mode is the accumulated phase difference between LPo' mode and LP'' mode and can be expressed as [{[,llo'(z)-,8(z)]/2}dz, 1 is the length of the coupling waist. When an acoustic field acts on a 50:50 coupling ratio fibre coupler (COS2 = sin2 = 1/2) the powers from two outputs can be written as P' = (1/ 2) {1 - sin[2(t)] } (3) P2 = (1/2){1+sin[2646(t)]} (4) whereA() is the extra phase difference caused by the acoustic perturbation between theLPO, mode andLP,'mode and can be written as A() = 1[fo (Z) - {, (z)] (z't)dz (5) wheree(z,) is the strain experienced by the coupling waist due to the acoustic wave.

The relevant power change caused by the acoustic wave is AP = P2 - PI = sin[264(t)] (6) In order to enlarge this effect the coupling waist is expected to be very thin in diameter, typically 10 microns. When the coupling waist in the fused tapered fibre coupler is much thinner than the un-tapered section the strain caused by acoustic field in the coupling waist is found to be much larger than the other parts of the coupler. The coupler with thinner coupling waist will therefore

be more sensitive to the acoustic field.

In this Example, the coupling waist of the coupler 7 was about 3mm long and about 10 microns thick, the length of each tapered region was about 3 mm, the taper angle therefore being relatively large as compared to those in conventional fused-tapered optical fibre couplers.

When the piezoelectric transducer 2 was driven continuously at its resonant frequency of 155kHz, strong signals were sensed. This can be seen from Figure 2 which shows the oscilloscope waveforms from the piezoelectric sensing apparatus, represented by the top trace, and the fibre optic sensing apparatus, represented by the bottom trace. The peakpeak signal changes were 1.625V for the piezoelectric sensor and 1.219V for the fibre-optic coupler based sensor respectively in the 155kHz acoustic field. Based on a given 10 V DC output from the AE sensor the coupling ratio change caused by the acoustic effects at 155kHz can be expected to be up to 10%. Comparing the amplitude of the signal with that obtained from the piezoelectric sensor 4 and its respective calibration certificate it can be determined that the maximum acoustic pressure change where the sensor 4 and coupler 7 were situated is about 2.9 mbar. The sensitivity of the respective sensor elements based on the certification of the piezoelectric sensor 4 is expected to be 0.74 V/mbar with less than 1 microbar acoustic noise floor for the piezoelectric sensor 4 and 0.56 V/mbar with 18 microbar acoustic noise floor for the optical flbre sensor at 155 kHz. As will be evident, the acoustic response of the fibre-optic coupler 7 is different from that of the amplitude modulator described in the paper by S.G. Farwell et al., "Low- loss all-fibre amplitude modulator at 1.55 m", Electron. Leff., 1996, 32, pp. 577-578 in which the frequency of the output from the modulator is twice the acoustic drive frequency.

Example 2

Using the same experimental set-up, a pulse generator was used to drive the piezoelectric excitation transducer 2 and a plot of the respective waveforms is shown in Figure 3 where Figure 3(a) relates to the signal level obtained from a piezoelectric sensor and Figure 3(b) relates to a sensor signal from a fibre-optic coupler 7 where the AE detection was simulated.

Example 3

In Figure 4 is shown an experimental set up in which the AE event was the breaking of a pencil at a point P on a fibre composite plate 14. Again, a piezoelectric sensor (Model R15) 4 with a 155 kHz resonant frequency was used as a reference and all common components to those shown with reference to Figure 1 are numbered accordingly.

The piezoelectric sensor 4 and the fused-tapered fibre coupler 7 were mounted on the surface of the plate 14. The AE event was generated by breaking the pencil at a point P about 10 mm from the sensor 4 and coupler 7.

The pencil break emitted a wide-band AE signal which was transmitted through the plate and picked up by the two sensing elements, namely the piezoelectric sensor 4 and the coupler 7. With the AE event being transmitted through the composite plate 14, it will be understood that such could represent a structural component e.g. of an aircraft. Hence, in this experimental set up, unlike that shown in Example 1 where the acoustic field acts directly on the interaction region or waist of the coupler via the medium of water, the primary transmission of the AE signal is through the composite plate 14.

Figure 5 shows the respective waveforms of the detected signals, (a) representing the signal level from the piezoelectric sensor 4 and (b) representing the signal detected by the coupler 7. Figure 6 shows the corresponding frequency responses to the pencil break in the form of a Fast Fourier Transform frequency spectrum where, again, (a) represents the piezoelectric sensor 4 and (b) represents the sensing coupler 7. As will be evident, the strongest frequency response from the piezoelectric sensor 4 was around 155 kHz, its resonant frequency. As for the sensing coupler 7, the response shows that the AE signal was successfully detected, there being several strong response peaks, which may be natural resonant frequencies of the coupler 7. As can also be seen, the frequency response of the sensing coupler 7 may be up to 300 kHz and is even wider than that of the piezoelectric sensor 4 and at least an order of magnitude higher than the prior art devices referred to earlier.

From the foregoing it will be seen that the invention provides a highly sensitive broadband apparatus and method for sensing AE events by abandoning conventional design practice for fused-tapered fibre optic couplers in order to provide an effective alternative to electrically based sensors and/or prior art low frequency sensors using fibre optic couplers of conventional manufacture and geometry. In particular, due to the wide bandwidth of operation, relatively flat broadband response, high sensitivity, low directionality and DC response, the sensor may find application in many areas. As shown in Example 1, the sensor can operate when immersed in liquid with a very broad, sensitive, frequency response range, making it suitable for use as a hydrophore, which could be used to create a towed sonar array. Similarly, due to the responsivity down to DC levels the device may provide a suitable broadband microphone and may, for example, be suitable for embedment within materials so as to be used to construct a "listening" panel for use in buildings. Vibration monitoring is also a possibility for applications within machine and structural engineering environments. Due to the relatively small size, and lack of metallic structure, the fibre optic sensor may be used to monitor corrosion that cannot be met by conventional sensors known in the prior art. The very good DC and high frequency response to stress also makes it possible to use the sensor for stress and strain monitoring applications, for example the sensor could be used to monitor axial and lateral strain and stress, pressure and torsional twist. Due to the geometrical structure of the sensor which is aimed at maximising the 1 concentration of stress, and the residual stresses in the structure of the coupler, it is possible to use the DC or low frequency response of the sensor to form a strain-isolated temperature sensor. However, as aforesaid, many other applications may be found for this invention and it will be understood that these examples are not intended to be limiting, other variations and uses being envisaged without departing from the spirit or scope of the invention.

Claims

- -Be. .-e a. : . . . . . Claims 1. An AE sensor

comprising a fused-tapered optical fibre coupler for receiving light from a light source at the insertion end of the coupler and, via an interaction region, splitting the light to produce light signals at each exit end of the coupler, detector means for detecting said light signals and comparator means for comparing said signals to thereby indicate the occurrence of an acoustic emission event during a steady state condition CHARACTERISED IN THAT the coupler has a sharp taper angle and a consequently short interaction region to maximise the stress concentration following an AE event in the interaction region, thereby increasing the sensitivity of the coupler and hence the sensor to stress.
2. An AE sensor according to claim 1 further CHARACTERISED IN THAT the coupler has a tapered region of between 2 and 4 millimetres and a coupling region between 2 and 4 millimetres long.
3. An AE sensor according to claim 1 further CHARACTERISED IN THAT the coupling region is between 1 and 8 millimetres long.
4. An AE sensor according to any preceding claim further CHARACTERISED IN THAT the diameter of the fibre in the coupling region is around 10 to 30 microns. l

Or;'r' _ _. a, ,

-
5. An AE sensor according to any one of claims 1 to 3 further CHARACTERISED IN THAT the coupling region is around 5 to 40 microns.
6. A hydrophore incorporating an AE sensor according to any preceding claim.
7. A microphone incorporating an AE sensor according to any one of claims 1 to5.
8. A listening panel incorporating a microphone according to claim 7.
9. A vibration monitor for monitoring machine and structural environments incorporating an AE sensor according to any of claims 1 to 5.
10. A corrosion monitor incorporating an AE sensor according to any one of claims 1 to 5.
A stress and strain monitor incorporating an AE sensor according to any one of claims 1 to 5.
12. A temperature sensor incorporating an AE sensor according to any one of claims 1 to 5.

. . .
13. A method of sensing an AE event using a coupler as claimed in any of claims 1 to 5 including the steps of embedding the coupler on or in a material in which an AE event is to be sensed, providing a light source at the insertion end of the coupler and monitoring changes in light output from the coupler to thereby indicate the occurrence of an AE event.