GB2281618A

GB2281618A - Method and apparatus for measuring a characteristic of an optical fibre

Info

Publication number: GB2281618A
Application number: GB9417580A
Authority: GB
Inventors: B Meggitt
Original assignee: Furukawa Research & Engineerin
Current assignee: Furukawa Research & Engineerin
Priority date: 1993-09-01
Filing date: 1994-09-01
Publication date: 1995-03-08
Anticipated expiration: 2014-09-01
Also published as: GB9417580D0; GB2281618B; GB9318104D0

Abstract

A method of measuring a characteristic of an optical fibre for use in a strain monitoring sensor includes: (i) producing a grating in a section of the fibre, the grating having a pitch length at least greater than the coherence length of a source; (ii) transmitting a signal along the fibre; (iii) receiving a signal reflected by the grating; (iv) passing the reflected signal through an optical processing interferometer, the difference between the path imbalance of the interferometer and the pitch of the grating being less than the coherence length of the source. The long-pitch reflective grating does not produce interference and is thus not spectrally selective: this allows up to 100 such gratings S1, S2.. to be provided along a fibre and multiplexed by a single source using OTDR techniques. The processing interferometer detects changes in phase of the reflected signals produced by strains in structures in which the fibre may be embedded. <IMAGE>

Description

METHOD AND APPARATUS FOR MEASURING A CHARACTERISTIC OF AN OPTICAL FIBRE The present invention relates to a method and apparatus for measuring a characteristic, such as for example strain, temperature and/or pressure, of an optical fibre. In particular it relates to an optical method and apparatus using a reflective grating.

The monitoring of structural integrity is an increasingly important engineering requirement in order to improve reliability and safety while reducing maintenance costs in a wide range of applications including, for example, strain monitoring of high temperature pipe-work and electrical machinery in a power generation plant, stress monitoring in bridges, dams and other civil engineering structures and static and dynamic strain loading in highly stressed aerospace structures.

Conventional strain monitoring techniques including foil and capacitive gauges are limited in their applicability, are not easily multiplexed in large sensor arrays and are suspectable to EMI interference.

Another known strain monitoring technique uses optical in-fibre strain sensors which, in contrast to foil and capacitive gauges, can operate in noisy electrical environments, can be operated in multi-sensor arrays and can be embedded on/into metal and composite structures since they are very compact in size and light in weight. Two types of optical fibre structures are particularly useful strain sensor applications; these are the in-fibre Fabry-Perot cavity and the spectrally selective fibre Bragg grating.

An experimental setup for making Bragg gratings in optical fibres is described in a paper entitled "Fibre Optic Bragg Grating Sensors" by WW Morey et al (SPIE vol 1169 "Fibre Optic and Laser Sensors" (vii) 9889 page 98).

Typically the grating is produced having a pitch length significantly less than the coherence length of the light source, with a high reflectivity of such a grating being necessarily from 10% to 90% in order to reflect back sufficient detectable energy.

Changes in a characteristic such as strain or temperature etc., of the fibre in which the grating is produced, produce a change in the pitch length of the grating. This in turn affects the frequency of the reflected radiation which, having been injected onto the fibre, is reflected back along the fibre by the grating.

A detection of this reflected frequency gives information about the change in the chosen characteristic(s), which in turn provides useful information in some of the applications listed above. Effectively, a Bragg grating operating in this way acts as a spectrally selective narrow band filter, reflecting back only radiation at an interfering wavelength of typically nanometres or less spectral bandwidth.

However the fibre Bragg grating operated as a reflective filter element described above has proved very limited in attempts to utilise the technique in multiple sensor arrays. The problem with conventional Bragg gratings is that, when operated in an array of similar spaced elements, they each compete for the same part of the source emission spectra.

This method, due to the high sensor finesse, limits the maximum number of multiplexed sensors to between about 5 and 10 at most. Alternatively, gratings of different pitch can be fabricated in parallel so that each grating operates over a separate and distinct portion of the available spectrum. Again, due to the bandwidth required by each grating, the maximum number of multiplexed sensors is limited to about 5 or 6.

Attempts to overcome this limitation lead to complex and costly solutions such as the use of a number of parallel legs of fibre sensors. This solution is unsatisfactory since it is costly and difficult to implement.

Furthermore, when Bragg gratings are used in this way it is typical to use a spectrometer to measure the results produced. Since spectrometers have a relatively long integration time (i.e. a slow response time) this means that known systems cannot measure very rapid changes in whatever characteristic is being measured.

It has been known to use an interferometer instead of a spectrometer, and the interferometer is usually set up to measure the reflection at a given wavelength, and so requires a high degree of accuracy in the set up.

Typically the resolution is less than a few nanometres and therefore the system has a very low stability and is again limited in its scanning speed to the kilohertz region.

A further problem with known Bragg gratings is that, since the pitch of the grating has to be significantly less than the coherence length of the source, then the fabrication of the grating requires extremely high accuracy in order to get the required interference pattern. Typical accuracy required is in the order of a few nanometres.

The present invention aims to reduce some or all of the above problems by using an in-fibre non-spectrally selective reflective grating (an NSR grating).

Accordingly, in a first aspect, the present invention provides a method of measuring a characteristic of an optical fibre including: (i) producing a grating in a section of the fibre, the grating having a pitch length at least greater than the coherence length of a source to be used; (ii) transmitting a signal along the fibre using the source; (iii) receiving a signal reflected by the grating; (iv) passing the reflected signal through an optical processing interferometer (a "sensor" interferometer), the difference between the path imbalance of the interferometer and the pitch of the grating being less than the coherence length of the source.

In this way, by increasing the pitch of the grating from a typical 0.4 micrometers to possibly around 50 micrometers the present invention reduces the accuracy to which the grating needs to be fabricated, thus reducing the cost and complexity of using this method. In addition the use of an interferometer in this way requires much lower accuracy in setting up than that described in the prior art (or that required for use of a spectrometer), which again increases the practical applicability of these measuring techniques.

Furthermore the modified NSR gratings elements proposed here overcome the problems in multiplexing large numbers of elements since each sensor element, although of similar grating period, will reflect back a fraction of the total source spectral intensity (and not just the portion of which has the correct wavelength), therefore making much more efficient use of the available radiation. Thus a grating used according to the invention preferably does not act as a selective filter.

A prior art Bragg grating is usually described as phase matched i.e. it has a resonant effect. Each element of the Bragg grating reflects back a portion of the whole spectrum, but the elements then interfere so that only radiation of the Bragg wavelength is effectively reflected. In the case of an NSR grating according to the present invention there is little or no resonance, due to the increased pitch length, and so the portion of the whole spectrum is effectively reflected.

As an example a sensor of 1 cm length and having a 50 ym grating period, will reflect back approximately 10-3 of the incident power operating in the phase domain, while at the same time is able to utilise a high frequency heterodyne modulation technique that permits high response time operation.

Preferably a method according to the present invention includes the step of measuring a change in phase of the reflected signal. This allows for the production of a high precision strain monitoring system with strain resolution of around 5 microstrain and a large dynamic range, typically of +50 x 10+3 microstrain.

In prior art systems it is typically the wavelength of the reflected radiation which is measured. By measuring the phase change of the reflected signal, a high response time is obtainable and therefore quicker changes in the measured characteristic are detectable.

This sensor system may be suitable for both parallel point sensor configuration as well as a quasi-distributed multiplexed operation. This NSR grating system allows for multiplexing up to 100 sensor elements along a single fibre length making it ideally suitable for Smart Structure applications for embedding in or on composite or metal components including aircraft structures.

Preferably, the proposed technique employs a whitelight low coherence method that utilises a broadband source such as an LED or fluorescent fibre laser. The processing interferometer may be formed from the first (frequency shifted) and zero-order (unshifted) diffractive beams from a Bragg cell modulator. By making the spacing of the in-fibre Bragg gratings at a period greater than the coherence length of the broadband source (e.g. 50 ym) no interference effects are observed in the backreflected light.

However, when the processing interferometer is placed in series with the fibre sensor and given a similar path imbalance, then the output signal from the system will be a high frequency carrier signal at the Bragg cell frequency (e.g. 40Mz - 1GHz), the phase of which is modulated by the change in pitch of the Bragg grating induced by the strain field of interest.

Therefore, the system has a high response time capability.

The source may be pulsed, and by pulsing the source, either directly or indirectly, optical time domain reflectometry (OTDR) techniques coupled with frequency down conversion methods (to produce intermediate frequencies) can be used to multiplex the sensor array.

Provided that the sensor elements are spaced at a separation greater than the equivalent pulse width (e.g.

about a few meters), this will give a strain slew rate of > 108 strain/sec.

In-fibre Bragg gratings may be produced by the sideillumination of the fibre by an interference pattern at W wavelengths of around 260nm (e.g. using an Excimer laser or frequency doubled Argon laser) in high Ge concentration silica fibres. Conventional prior art gratings have a pitch spacing of optical wavelengths and operate as narrow-band reflective filters operating about a specific wavelength. They change their reflected wavelength with change in grating pitch by stress or thermal effects and require monitoring by a high resolution monochromator which have slow response times.

The long-pitch gratings proposed here have a grating pitch greater than the source coherence length so that they reflect in a broad-band fashion. By use of the second processing interferometer formed by the active Bragg cell modulation unit that has a similar path imbalance as the sensor unit it may be possible to track the optical phase changes in the sensor unit to < lmradian resolution. Use of such a long-pitch grating reduces the degree of stability required in its production and greatly reduces the stability required in the source emitted wavelength.

Since strain sensors in general are temperature sensitive device that respond to both "real" and "apparent" strain effects in may be desirable to remove the apparent strain component in the present system (e.g.

around 5 yt/ C). To do this a second "dummy" sensor element may be used that is placed in the same vicinity as the strain sensor but is designed to be isolated from the strain field of interest and therefore only responds to the temperature effects. Subtraction of the two measurements will eliminate the apparent strain component.

In the NSR grating system according to the present invention it is possible to provide the "dummy" sensor by supplying a second parallel fibre sensor arm from the output port of the directional coupler at the source output. By producing the sensors at the same location along the second sensor fibre, the "dummy" sensors are isolated from the strain field by enclosing the sensor length in a slightly larger diameter (loose fitting) silica tube that is attached to the surface of interest along side the active strain sensor element. One sensor therefore senses strain and temperature effects and the other senses only temperature effects, allowing the apparent strain to be eliminated.

In a second aspect, the present invention provides an apparatus for measuring a characteristic of an optical fibre including: means for producing a grating whose pitch length is greater than the coherence length of the source to be used; means for transmitting a signal along the fibre from the source and receiving a signal reflected from the grating; and an optical processing interferometer into which the reflected signal may be passed, the difference between the path imbalance of the interferometer and the grating pitch being preferably in near balance and, in any event, less than the coherence length of the source.

The path length of the interferometer may be modulated in a periodic fashion to create a carrier, for example by a frequency shifter, moving mirror, Bragg cell or other method. The use of a Bragg cell may be best for high speed operation.

Preferably means are included for measuring a change in phase of the reflected signal. This allows for the production of a high precision strain monitoring system with strain resolution of around 5 microstrain and a large dynamic range, typically of +50 x 1owt3 microstrain.

The present invention may be used with either single or multimode fibres and is particularly useful when applied to SMART structures, such as these used in altering the shape of an aircraft wing due to stress.

Ambiguity may arise in the measurements when there is a phase change of greater than one fringe produced in the grating. To avoid this, two closely spaced sources of similar wavelength (typically between 5 and 10 nanometres in separation) may be used and the measurements analysed.

An embodiment of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a long pitch reflective grating according to the present invention, showing back reflected amplitudes; Figure 2 is a schematic diagram of one form of a processing interferometer section of the present invention; Figure 3 is a graph of spectral intensity against spectral wavelength showing spectral energy reflected from a long pitch reflective in-fibre grating according to the present invention; and Figure 4 is a schematic diagram of a quasidistributed reflecting fibre grating sensor network according to the present invention.

The principle of operation (illustrated in Fig.1) of the current application is the production of a reflective in-fibre sensor grating (Fig. 1) of a pitch (2), 1s, in a fibre (4). The pitch (2) is of optical path length greater than the coherence length, Lc, (i.e. 2 n. ls La) of the broad spectral bandwidth source used. This means that interference effects are not observed in the back reflected radiation, the amplitude of which can be shown to have the form;

The corresponding intensity function is then given from the standard form; Ir = Ar.Ar where Ar is the complex conjugate of Ar and where a2 iS the reflection coefficient of an individual grating element due to the modified refractive index at that point, N is the number of elements and n is the fibre core refractive index .10 is the incident intensity.

The returned radiation from the grating may be passed through a second interferometer of path imbalance, ip, similar to that of the grating optical path length (lpsnls~Lc/2)l as is known in "white-light" interferometry, and interference effects are then established in the output waveform such that changes in the pitch of the reflective fibre grating are observed as a change in phase of the output fringe pattern of the second processing interferometer.

In addition, a modulation can be introduced in the processing interferometer to produce a carrier signal in the output waveform about which the phase changes can be conveniently measured. The carrier can be formed, for example, by either modulating the optical path length of the processing interferometer e.g. oscillating one mirror in a Michelson, Mach Zhender or low finesse Fabry-Perot interferometer, or by use of a Bragg frequency modulator in one arm of the interferometer as indicated in Fig. 2.

Fig. 2 shows introducing a Path Imbalance, lp, between the two output beams (a first order diffracted beam, 6, and a zero order beam, 8) to give an overall output beam, 10. The optical high frequency carrier is operated at frequency fB using a Bragg cell.

For a processing interferometer with arm imbalance, and and a transfer function, for example, of the form Ir' =1/2 (1+cos (4't1p/Xo), the output interferogramme can be described by the function:

where the expontial terms represent the visibility of the interferometric fringes produced, Co is the source entral wavelength, a the spectral width of the source (in wave number) at the 1/e intensity points, and nl is the graphic index (n' = n - Xdn/dX) The reflective fibre grating as described is responsive to external stimuli since, for example, its output phase change Af is sensitive to longitudinal strain and temperature effects according to the relationship: Power - y. [1-(n-l)y] Iow where 8n/8T and 815/84 are the temperature coefficient of refractive index and strain coefficient of the grating pitch respectively. As an example, for a longitudinal fibre strain of 10,000 yt a phase change of around 6 radians is produced giving a stain resolution of < 2yt.

As an example of the returned back-reflected intensity of such an in-fibre reflective grating, a 200 element grating of pitch 25ym with a source of coherence length Lea 50 tim (X,=780nm) and a fibre launch power of 5mW and a grating reflection coefficient of t2, approximately 10-5, will give a detected signal of approximately 2 W. Therefore, it is seen that for an array of such fibre reflective gratings, in the order of > 100 sensors can be multiplexed by a single source using OTDR techniques, as indicated in Fig. 4. The power reflected back by each sensor is given by approximately: Power . y. [1- (n-l)yl Iol where Y is the reflectivity of the grating elements (typically 5x10-3), n is the sensor position in the array and Io is the launch optical power into the fibre. For 100 sensors the smallest return signal (i.e. 100th sensor) may be about 0.25 tiW, which is adequate for conventional processing techniques.

It should be noted that, since the grating is not acting as an interference filter, it reflects back a fraction (14) of the total source spectral profile (12) as shown in Fig. 3 since it is not spectrally selective.

Therefore, unlike an array of similar spectral filter type Bragg gratings, each sensor does not complete for the same part of the spectrum.

Additionally, it does not require tuning of the grating pitch to the source wavelength and therefore the long pitch grating will operate with a wide range of source wavelengths. This is a major advantage when considering the variability of semiconductor source wavelengths.

Further, since the technique uses a second interferometer as the processing unit, it can operate at high modulation speeds when using, for example, a Bragg cell frequency shifter (Fig.2). This can be compared with the fibre Bragg grating technique that requires a monochromator to demodulate the returned spectral signal with limited response time, typically < lkHz.

Claims

CLAIMS:

1. A method of measuring a characteristic of an optical fibre including: (i) producing a grating in a section of the fibre, the grating having a pitch length at least greater than the coherence length of a source to be used; (ii) transmitting a signal from the source along the fibre; (iii) receiving a signal reflected by the grating; (iv) passing the reflected signal through a first optical processing interferometer, the difference between the path imbalance of the interferometer and the pitch of the grating being less than the coherence length of the source.

2. A method according to claim 1, including the step of measuring a change in phase of the reflected signal.

3. A method according to claim 2, wherein a second processing interferometer, having a similar path imbalance to the first interferometer, is used to measure any optical phase change in the first interferometer.

4. A method according to any one of the above claims, wherein said grating is a non-spectrally selective reflective grating.

5. A method according to any one of the above claims, wherein the in-fibre Bragg grating is produced by the side-illumination of the fibre by an interference pattern at W wavelength.

6. A method according to any one of the above claims, wherein the pitch of the grating is around 40-60 micrometers.

7. A method according to any one of the above claims, wherein in Step (i) a plurality of gratings are produced.

8. A method according to any one of the above claims, wherein a "dummy" sensor element is used which is located in the same vicinity as the first grating but is isolated from strain effects on the fibre.

9. A method according to claim 5, wherein the dummy sensor includes a second parallel fibre sensor arm supplied from an output port of a directional coupler at the source output, a "dummy" grating being produced at substantially the same location along the second sensor fibre as the grating produced in the first fibre.

10. A method according to claim 9, wherein the dummy sensor is enclosed in a loose fitting tube that is attached to the surface of interest alongside the fibre in which the first grating is produced.

11. A method according to any one of the above claims, in which the source is pulsed in order to facilitate the use of optical time domain reflectometry techniques to multiplex an array of sensors.

12. A method of measuring a characteristic of an optical fibre substantially as any one herein described with reference to the accompanying drawings.

13. An apparatus for measuring a characteristic of an optical fibre including: means for producing a grating whose pitch length is greater than the coherence length of the source; means for transmitting a signal along the fibre and receiving a signal reflected from the grating; and an optical processing interferometer having its optical path modulated into which the reflected signal may be passed, the difference between the path imbalance of the interferometer and the grating pitch being less than the coherence length of the source.

14. Apparatus according to claim 13 wherein the difference between the path imbalance of the interferometer and the grating pitch is in near balance.

15. Apparatus according to any of claims 12 to 14, wherein the source is a broadband source.

16. Apparatus according to any one of claims 12 to 15, wherein the processing interferometer is formed from the first (frequency shifted) and zero-order (unshifted) diffractive beams from a Bragg cell modulator and is coupled with a frequency down conversion to allow phase changes to be conveniently measured.

17. Apparatus according to any one of claims 12 to 16, including means for measuring a change in phase of the reflected signal.

18. Apparatus according to any one of claims 12 to 17 including two closely spaced sources of similar wavelength.

19. Apparatus according to claim 18, wherein the difference in wavelength between the two sources is between 5 and 10 nanometres.

20. Apparatus for measuring a characteristic of an optical fibre substantially as any one embodiment herein described with reference to the accompanying drawings.