WO1999049309A2

WO1999049309A2 - Improvements in and relating to inspection

Info

Publication number: WO1999049309A2
Application number: PCT/GB1999/000812
Authority: WO
Inventors: Qing Shan; Richard James Dewhurst
Original assignee: British Nuclear Fuels Plc
Priority date: 1998-03-26
Filing date: 1999-03-26
Publication date: 1999-09-30
Also published as: WO1999049309A3; AU3154999A

Abstract

The invention provides a non-destructive test (NDT) method and apparatus for remote detection of ultrasound, particularly from industrial rough surfaces. The technique enables the measurement of ultrasonic velocity, in-plane and out-of-plane ultrasonic displacements within Lamb wave frequencies. The technique is particularly applicable to chemical and nuclear industry process pipes and the like. The system provides high light gathering power, similicity in alignment, vibration tolerance, fibre compatibility and is suitable for use in hazardous environments, giving it particular relevance to NDT in the chemical and nuclear industry. The technique provides a method of investigating an item, the method comprising the steps of: generating ultrasound waves in the item; generating a first laser beam, forming a first probe beam and a first reference beam from the first laser beam, applying the first probe beam to a location on the item to generate a first return beam; applying the first reference beam and the first return beam to first detection means; generating a second laser beam, forming a second probe beam and a second reference beam from the second laser beam, applying the second probe beam to a location on the item to generate a second return beam; applying the second reference beam and second return beam to second detection means; analysing the output of the first and second detector means. The technique can also be implemented using a single probe beam to produce a first and second return beam and first and second reference beams.

Description

IMPROVEMENTS IN AND RELATING TO INSPECTION

This invention concerns improvements in and relating to inspection, particularly, but not exclusively to ultrasonic inspection of surfaces and thicknesses.

A great many situations make remote, non-contact inspection systems desirable. Such applications include the nuclear industry where such techniques would enable a wider range of areas to be inspected and also reduce the dose to the personnel carrying out those inspections. Inspections of surfaces for corrosion products, thicknesses of pipes to determine internal corrosion, the integrity of surface coatings and the integrity of welds are all applications where inspection is desirable.

Existing systems sometimes fail to attain the desired degree of performance for a variety of reasons. Additionally the type of determinations which can be made is limited.

According to a first aspect of the invention we provide a method of investigating an item, the method comprising the steps of :- generating ultrasound waves in the item; generating a first laser beam, forming a first probe beam and a first reference beam from the first laser beam, applying the first probe beam to a location on the item to generate a first return beam; applying the first reference beam and the first return beam to first detection means; generating a second laser beam, forming a second probe beam and a second reference beam from the second laser beam, applying the second probe beam to a location on the item to generate a second return beam; applying the second reference beam and second return beam to second detection means; analysing the output of the first and second detector means . The first probe beam and the second probe beam may be applied to separate first and second locations on the item.

The first return beam may be generated by the first probe beam. The second return beam may be generated by the second probe beam. The first return beam preferably arises from the incident location of the first probe beam on the item. The second return beam preferably arises from the incident location of the second probe beam on the item.

The paths of the first probe beam and the first return beam and/or the second probe beam and the second return beam are preferably similar or the same. The first and second probe beams and/or first and second return beams are preferably provided at the same angle to the surface. The first and/or second probe and/or first and/or second return beams may be perpendicular or non-perpendicular to the surface of the item. The first probe and first return beams are preferably offset from the second probe and second return beams .

Preferably the separation of the first and second probe beams and/or the separation of the first and second locations on the item is known. The separation may be determined by measuring the time of travel of a displacement through an item, of known phase velocity, between the first and second locations. The determining may form a stage of the method, for instance a calibration stage.

The passage of a displacement, preferably generated by the ultrasound, through the first location of the item preferably gives rise to a change in the first return beam. The passage of a displacement through the second location of the item preferably gives rise to a change in the second return beam.

An output in the first detector means may arise from a change in the first return beam. The change is preferably an optical phase change. An output from the second detector means may arise from a change in the second return beam. The change is preferably a phase change. Preferably the output from the first and/or second detector means comprises a voltage or an EMF, such as a voltage pulse or an EMF pulse.

The analysis may include determining the phase velocity through the item, most preferably between the first and second locations. The calculation of the phase velocity may be based on the time delay between signal from the first detector means and a signal from the second detecting means, in combination with the known separation of the first and second locations.

The first detector means and second detector means may be provided in parallel between an electrical potential, for instance between an applied potential an earth.

The method of investigation may be used to monitor out- of-plane displacements in the item. The method may be used to investigate Rayleigh waves and/or symmetrical Lamb waves.

According to a second aspect of the invention we provide a method of investigating an item, the method comprising the steps of : - generating ultrasound waves in the item; generating a first laser beam, forming a first probe beam and a first and a second reference beam from the first laser beam, applying the first probe beam to a location on the item; obtaining a first return beam and a second return beam from the location; applying the first reference beam and the first return beam to first detection means; applying the second reference beam and the second return beam to second detection means; analysing the output of the first and second detector means .

The first return beam and second return beam may be generated by a first probe beam. The first and second return beams preferably arise from the incident location of the first probe beam on the item.

The paths of the first return beam and second return beam are preferably different. The path of the first probe beam is preferably different to the path of the first return beam and/or the second return beam. The first and second return beams are preferably provided at the same angle to the surface. The first probe beam is preferably provided at a different angle to the surface than the first and/or second return beam. Preferably the first and/or second return beams are non-perpendicular to the surface of the item. The first probe beam is preferably perpendicular to the surface. The first and/or second return beams and/or first probe beam are preferably offset from one another.

In another form a second probe beam may also be provided, the second probe beam potentially forming the second reference beam and/or second return beam.

The first probe beam and the second probe beam may be applied to the same location on the item.

The first return beam may be generated by a first probe beam. The second return beam may be generated by the second probe beam. The first return beam preferably rises from the incident location of the first probe beam on the item. The second return beam preferably arises from the incident location of the second probe beam on the item.

Paths of the first probe beam and the first return beam and/or the second probe beam and the second return beam are preferably similar or the same. The first and second probe beams and/or first and second return beams are preferably provided at the same angle to the surface. The first and/or second probe beams and first and/or second return beams are preferably non-perpendicular to the surface of the item. The first probe and first return beams are preferably offset from the second probe and second return beam. The passage of a displacement, preferably generated by the ultrasound, through the incident location on the item preferably gives rise to a change in the first return beam. The passage of a displacement through the incident location on the item preferably gives rise to a change in the second return beam.

The change in the first and/or second return beam may be a Doppler frequency shift. Preferably the change in the first return beam is a frequency increase and the change in the second return beam is a frequency decrease or vice versa.

An output from the first detector means may arise from a change in the first return beam. The change is preferably an optical phase change. An output from the second detector means may arise from a change in the second return beam. The change is preferably a phase change. Preferably the output from the first and/or second detector means comprises a voltage or an EMF, such as a voltage pulse or an EMF pulse.

The analysis of signals from the detectors may include signal frequency analysis, signal amplitude analysis, item thickness determination, coating thickness determination, item integrity investigations, and item composition.

The first detector means and second detector means may each be provided between a separate electrical potential and a lower potential, such as earth. Preferably the electrical potentials are of opposing polarity. Most preferably the electrical potentials are of equivalent level, but opposing polarity.

The method may be used to investigate in-plane displacements of an item. The method may be used to investigate a symmetric Lamb waves.

Further details, options, possibilities and potential features and steps for the first and second aspects of the invention include the following.

The ultrasound may be generated in the item by a laser beam. Preferably a separate laser source to the probe beam laser source is used. The ultrasound generating laser beam may be pulsed. A pulse duration of nanoseconds is preferred.

This ultrasound may be generated by thermal expansion, for instance arising from localised heating by the laser pulse on the item, or the ultrasound may be generated by ablation caused by the laser. Alternatively the ultrasound may be generated in the item by a conventional transducer system, for instance using the piezoelectric effect.

A separate location for ultrasound generation may be provided.

The item may be a discrete item or a part of a larger item. The item may be in-situ. The item may be a surface of a vessel, pipe or conduit. The item may be of metal, non- metal, have a metal coating or have a non-metal coating.

The first and second probe beams and first and second reference beams may be provided by a single laser source. The first probe beam and first and second reference beams may be provided by a single laser source.

The respective probe beam(s) and reference beam(s) may be formed by splitting a single beam. The probe beam(s) and reference beam(s) may be formed by introducing the beam to a splitter. Preferably the beams are introduced to a polarised beam splitter. The reference beam may pass through the splitter and the probe beam may be reflected from the splitter, or vice versa. The probe beam leaving the splitter may enter optical directing means.

The first probe/first reference forming beam and second probe/second reference forming beam may be formed by applying a beam to a beam splitter, preferably a 50%-50% splitter. The first probe/reference forming beam may pass through the splitter and the second probe/reference beam may be reflected from the splitter, or vice versa.

The first probe beam and the first and second reference forming beam may be formed by applying a beam to a beam splitter, preferably a 50%-50% splitter. The first probe bea may pass through the splitter and the first/second reference forming beam may be reflected from the splitter, or vice versa. The first reference beam and the second reference beam may by formed by applying the beam to a beam splitter.

Optical directing means may be provided to convey the probe beam(s) towards the location on the item. The optical directing means may include one or more optical fibres and/or fibre couplings and/or focussing means, such as lenses.

Optical directing means may be provided to receive and/or convey the return beam(s) towards the detector means. The optical directing means may include one or more optical fibres and/or fibre couplings and/or collecting means, such as lenses.

Preferably separate detector means are provided for the first return/reference beam and for the second return/reference beam.

The first return beam and first reference beam are preferably directed to the same location on first detecting means. The second return beam and second reference beam are preferably directed to the same location on second detecting means. Preferably the return beam and reference beam form an interference pattern on the detector means. Preferably the interference pattern is stable in the absence of charge in the return beam.

The detector means are preferably photo-refractive crystals. The detector means may exhibit a non-steady state photo-induced EMF effect. The detector may be a gallium; arsenic crystal, most preferably doped with chromium, but other combinations suitable for the present invention exist.

The detector means preferably is capable of forming photo-generated carriers. Preferably the photo-generated carriers diffuse away from areas of intense radiation in the interference pattern. Preferably the interference pattern causes a charge pattern to form. The charge pattern is preferably stationary in the absence of charge in the return beam.

A change in the return beam preferably results in a change in the detector.

The change may be a modulation of the beam, preferably a frequency modulation.

Preferably the change in the detector means is the generation of an emf. The change in the return beam may cause a change in the interference pattern. A change in the interference pattern may result in movement of the photo- generated charge carriers with the detector means, most preferably as they move away from areas of intense radiation in the new interference pattern. Preferably the change in the return beam is momentary. Preferably the original interference pattern returns on removal of the change to the return beam.

The method may include two modes, one for detecting in- plane displacement and one for detecting out-of-plane displacements. Means for switching between the two modes may be provided. Switching may be provided by transferring the first and second detector means from a first configuration to a second configuration. Preferably the first and second detector means are provided in parallel with a potential across them in first configuration and are provided with separate opposing potentials across them in the second configuration. Preferably in the first configuration out-of- plane displacements are detected. Preferably in the second configuration in-plane displacements are detected. Preferably in the first configuration in-plane displacements are not detected. Preferably in the second configuration out-of-plane displacements are not detected. A means for switching between the first and second configurations may be mechanical or electronic. The method may include two modes of probe delivery. In the first mode first and second probe means may be delivered to a single location. The second mode may provide delivery of the first and second probes to different locations.

According to a third aspect of the invention we provide apparatus for investigating an item, the apparatus comprising a laser beam source, means for forming a first probe beam and a first reference beam from the laser beam source and means for applying the first probe beam to a location on the item to generate a first return beam and means for applying the first reference beam to first detection means and means for applying the first return beam to the first detection means, and further comprising a means for generating a second probe beam and a second reference beam from a laser source, means for applying the second probe beam to a location on an item to generate a second return beam, means for applying the second reference beam to second detection means and means for applying the second return beam to the second detection means and means for investigating the output of the first and second detector means.

The apparatus may also include means for generating ultrasound waves in the item.

According to a fourth aspect of the invention we provide apparatus for investigating an item, the apparatus comprising a laser beam source, means for forming a first probe beam, and first reference beam and a second reference beam from the laser source, means for applying the first probe beam to a location on the item, means for obtaining a first return beam and a second return beam from the location, means for applying the first reference beam to first detection means, means for applying the first return beam to the first detection means, means for applying the second reference beam to second detection means, means for applying the second return beam to the second detection means and means for investigating the output of the first and second detector means.

The apparatus may also provide means for generating ultrasound waves in the item.

The third and fourth aspects of the invention include the structure, features, possibilities, options and steps described elsewhere in this document including the first and second aspects of the invention and suitable means for implementing the method steps thereof.

Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which : -

Figure la illustrates a first mode of ultrasound generation;

Figure lb illustrates a second mode of ultrasound generation;

Figure 2 illustrates a first embodiment of an instrument according to the invention;

Figure 3 illustrates a second embodiment of an instrument according to the invention;

Figure 4 illustrates a third embodiment of an instrument according to the invention; and

Figure 5 illustrates a still further embodiment of the invention.

The technique of the present invention is based around the generation of ultrasound within a sample under test. This is conveniently achieved using a laser, although a variety of other techniques, including piezoelectric generation or acoustic emission, may be used. The monitoring of the ultrasound effects resulting and the interpretation of the results can give useful information about the sample. The technique is directed towards examining a variety of situations, including weld examination and the determination of corrosion. The technique is particularly useful for analysis of surfaces or materials within hostile environments were contact access is not possible or is undesirable.

Two techniques for generating ultrasound within the sample under investigation using lasers are employed, principally depending on the type of investigation being pursued. These techniques are illustrated schematically in Figure la, thermal, and Figure lb, ablation. In each case short, i.e. nanosecond, laser pulses are employed.

In thermal generation of ultrasound, thermoelastic, the laser beam 1 is used as a practically instantaneous source of heat at a discrete point location 3 on the surface 5 by suitable focussing. The surface is not damaged by the laser due to the low power and short duration of application of the pulse. However, the pulse does generate an expanding acoustic front 7 within the sample. The shape of the front is dependant on the location size, the size of the sample, the sample material composition, the pulse width and wavelength. As the thermal expansion 3 is generated within the thermoelastic regime for the material only a small normal compression component, relative to the surface of the item, in the ultrasound occurs. The ultrasound energy mostly propagates in the form of side lobes at angles determined by the conditions of the investigation.

Thermal ultrasound generation is particularly suited to the generation of so called Lamb waves within a sample. Lamb waves provide a potential investigative route for evaluating material types, welds and the integrity of portions of a sample .

In ablation generation of ultrasound a higher rate of thermal input is provided with the result that a small portion of the surface 5 is turned into a vapour or plasma and is expelled from the surface. The equal and opposite reaction 11 for this plume generates, from Newton's 2nd Law, an ultrasound field with a very large normal component, relative to the laser beam.

Ablation generation of ultrasound is particularly suited to the generation of compressional waves within a sample. Such waves are of use, for instance, in measuring the thickness of the sample, which may be a pipe wall or vessel wall.

The gathering of the ultrasound produced, and in particular the variations arising in the ultrasound from the features of the sample under investigation, in a viable manner is a complex and problematical task. A variety of options have been considered and evaluated as being inadequate for a number of reasons. Some of the proposals suffered problems with the frequency detection limits, decreasing sensitivity and particularly with the optical complexity and ease with which such systems could be damaged in the uses envisaged. Other approaches also proved too complicated for practical systems and/or too expensive in terms of the equipment which might realistically be damaged or contaminated in use. Other problems arise from variations in surface reflectivity and surface roughness.

The technique employed by the present invention is based around the use of photo induced emf ' s in photo refractive crystals, with varying configurations used to maximise the information obtained from the particular investigations .

In the embodiment illustrated in Figure 2 a sample 100 is investigated using a first probe laser beam 102 and a second probe laser beam 104 generated by laser source 106. The emitted beam 108 is split by 50*-to 50% beam splitter 110 into beams 112a and 112b. These beams are reflected by mirrors 114a and 116a and 116b respectively into polarised beam splitters 118a and 118b to give the probe beams 102 and 104 respectively and, via mirrors 120a and 120b, reference beams 122 and 124 respectively. The probe beams 102, 104 are focussed and otherwise conveyed to the sample 100 by optical systems 123a and 123b. The optical systems 123a, 123b are equivalent to one another and may be formed of optical fibres with appropriate connectors so as to convey the beam to the desired location for application to the sample 100. The optical systems also include a return path for the probe beams 102, 104 to convey the return beams back.

The return beam 126 of probe beam 102 and the reference beam 122 are played onto first photo-refractive crystal 130. The return beam 128 of probe beam 104 and the reference beam 124 are played onto second photo-refractive crystal 132.

For each photo-refractive crystal 130, 132, the probe beam and reference beam are interfered on the crystal's surface so as to form a secular/periodic intensity pattern. The photo-generated carriers formed within the crystal diffuse away from the regions of intense optical radiation occurring in the pattern. In the absence of any change in the optical signal from the probe beam these carriers effectively become trapped in the interference pattern and form a stationary charge pattern themselves. This in turn gives a corresponding stationary space charge field.

If a deviation in the sample 100 occurs or is encountered at the incident location of the probe beam this results in a variation in the probe beam and hence in the interference pattern on the crystal surface. The change in the intense radiation locations results in a change in the charge carrier position as they re-adjust. Depending on the manner of this re-adjustment, a detectable signal can arise in the systems electronics.

Where the variation occur as a result of environmental vibration, for instance of the sample or instrument, the rate of variation is usually such that the space charge grating tracks the fringe motion and no emf arises. The system is, therefore, immune to signals generated by many background influences in the environment of use as the frequency of such vibrations is too low to give detectable events. Vibration of the sample, instrument and their surroundings are thus accounted for.

Where non-steady state semi-conductor type detectors are employed, such as GaAs.Cr crystals, the dynamic spatial wavefront compensation normally covers the range upto 1 to 10kHz; the vast majority of background vibrations.

Where on the other hand the variation in the beam occurs as a result of an ultrasound pulse arriving at the incident location of the probe beam modulation of the return beam causes a change in the intensity pattern which is faster than the material tracking time and the space charge grating cannot track the motion. The result is an induced emf across the photo-refractive material which can be detected.

Detectors of the GaAs : Cr type, as discussed above, offer a sensitivity of 3.9xlO^~5A(W/Hz) with an active area of the detector of size 3.7mm x 6.2mm.

Detectors employed in the invention have been evaluated in this research to be particularly beneficial for this type of investigation. The system gives combined optical compensation and detection stages through a single detector and without the need for an optical readout beam of the wavefront information. Additionally the reflectivity of the surface involved and its roughness do not interfere with the signal information obtained.

Ultrasound can be generated and analysed for a variety of purposes: Lamb waves can be used to investigate pipe work in non-destructive tests due to the relatively low cut off frequency response; equally the high frequency response of the system makes it suitable for investigating Rayleigh waves which can be used to investigate defects or surface coatings, for instance. The system illustrated in Figure 2 is principally intended for the monitoring of out-of-plane displacements relative to the plane of the sample.

In the sample 100, if ultrasound is generated which passes the location of the probe beam 102 and the ultrasound has a significant out of plane displacement, the travel time of this pulse as it propagates to the second probe beam 104 can be measured.

The generation of the displacement at the probe beam 102 gives rise to the modulation of the return beam 126 for that probe beam 102 and hence to a change in the interference pattern for crystal 130 and hence an emf in the form of a pulse.

The displacement travels across the sample to the incident location of the second return beam 128 where it also causes modulation of the second probe beam 104 and hence a change in the interference pattern and the generation of an emf in crystal 132.

The emf pulse in the crystal 130 and the emf pulse in the crystal 132 will be spaced by a time period dependant on the ultrasonic wave phase velocity and the physical separation of the two probe beams 102, 104. The two crystals 130, 132 are connected in parallel between potential source 150 and ground 152 so as to present two pulses in the electronic output signal.

The separation of the two probe beams 102, 104, can be determined accurately in calibration tests using a sample with a known ultrasonic velocity. Hence in unknown samples the monitoring gives the ultrasonic wave phase velocity. Determining the phase velocity and/or attenuation gives a variety of information which can be used to for instance, to determine propagation, material properties and the like.

The system described above is particularly suited to monitoring out of plane displacements, such as Rayleigh waves or asymmetric Lamb waves. In many situations, and particularly with asymmetric Lamb waves, the majority of the displacement occurs as in- plane vibrations. The monitoring of such displacements has been particularly problematical. The embodiment of the invention presented in Figure 3 successfully monitors such vibrations .

In this embodiment a sample 200 is investigated using a first probe laser beam 202 and a second probe laser beam 204 generated by laser source 206. The emitted beam 208 is split by 50%-to 50% beam splitter 210 into beams 212a and 212b. These beams are reflected by mirrors 214a and 216a and 216b respectively into polarised beam splitters 218a and 218b to give the probe beams 202 and 204 respectively and, via mirrors 220a and 220b, reference beams 222 and 224 respectively.

The probe beams 202, 204 are focussed and otherwise conveyed to the sample 200 by optical systems 223a and 223b. The two probe beam 202, 204 in this embodiment, however, are targeted at the same location 225 on the sample 200 by mirrors 227a, 227b.

Once again the first return beam 226 of first probe beam 202 and first reference beam 222 are played onto first photo-refractive crystal 230. The second return beam 228 of second probe beam 204 and second reference beam 224 are played onto second photo-refractive crystal 232. An interference pattern is thus generated on each crystal.

Displacements in the sample, for instance vector δ, have an in plane component δVy as well as an out of plane component δVx. The component δVy will introduce a positive Doppler frequency shift to the return beam 226 of probe beam 202 and a negative Doppler frequency shift to the return beam 228 of probe beam 204, or vice versa. Each of the frequency shifts causes a change in the interference pattern for its respective crystal and hence an emf. In the configuration shown the two crystals are provided such that the emf ' s add to one another to double the signal obtained.

Out-of-plane displacements in this configuration also give modulation of the probe beams, but the emf ' s are cancelled on the load resistor 250. Detection of out of plane components is therefore avoided.

In the embodiment of Figure 4 a similar structure is used to the embodiment of Figure 3, equivalent components are given equivalent numbers, increased by 100.

In this case, however, the crystals are configured such that the emf ' s arising from the respective Doppler shifts caused by the in-plane displacements cancel one another out. The out of plane generated emf's, however, are added upon the load resistor 350 thus allowing their measurement.

In all cases it is necessary to deliver the probe pulse (s) to the location of interest and to recover at least part of the optical output to form a return beam.

The embodiment of Figure 5 illustrates the delivery of the probe beam 402 to the sample 400 via multimode optical fibres 440 and focussing optics 442. The output from the location 444 under investigation is collected on one side, return beam 445, by collection optics 446 and conveyed by further multimode optical fibres 448 onto crystal 430. The collection system on the other side, return beam 449, of the location 444 is similarly provided with collection optics 450 and optical fibres 452 to deliver the output to crystal 432.

The use of optical fibres for the delivery and extraction system for the light is beneficial for a number of reasons including avoiding the need for direct line of sight with the location 444, (relatively tortuous routes can be used facilitating access) ; increased flexibility in the route through which the light is conveyed by avoiding the use of fixed optics; and increased separation of location and instrument, (reducing dose exposure and contamination potential for the instrument) .

The instrument illustrated in Figure 5 enables both in- plane and out-of-plane displacements to be monitored through the switching system 470. A mechanical switch is illustrated, but electronic switching could equally well be used.

In the position shown out-of-plane displacements, such as the arrival of a displacement at location 444, are monitored and in plane displacements are cancelled out in the detectors. By switching to the other position the out-of- plane displacements can be cancelled and the in-plane displacements can be monitored instead. Due to the different apparent displacements of the surface of the item 400 relative to the direction of reflection, the return beams 445, 449, are Doppler shifted in opposing manners.

For the various in-plane monitoring techniques, describe above and shown in Figures 3,4 and 5, other monitoring mechanisms can be employed to obtain the ultrasonic phase velocities.

Claims

CLAIMS :

1. A method of investigating an item, the method comprising the steps of : - generating ultrasound waves in the item; generating a first laser beam, forming a first probe beam and a first reference beam from the first laser beam, applying the first probe beam to a location on the item to generate a first return beam; applying the first reference beam and the first return beam to first detection means; generating a second laser beam, forming a second probe beam and a second reference beam from the second laser beam, applying the second probe beam to a location on the item to generate a second return beam; applying the second reference beam and second return beam to second detection means; analysing the output of the first and second detector means .

2. A method according to claim 1 in which the first return beam is generated by the first probe beam, the second return beam is generated by the second probe beam, the first return beam arises from the incident location of the first probe beam on the item and the second return beam arises from the incident location of the second probe beam on the item.

3. A method according to claim 1 or claim 2 in which the first probe beam and the second probe beam are applied to separate first and second locations on the item.

4. A method according to any preceding claim in which the separation of the first and second probe beams and/or the separation of the first and second locations on the item is known .

5. A method according to claim 4 in which the passage of a displacement through the first location of the item gives rise to a change in the first return beam and the passage of the displacement through the second location of the item gives rise to a change in the second return beam.

6. A method according to claim 5 in which an output in the first detector means arises from a change in the first return beam, the change being an optical phase change and an output from the second detector means arises from a change in the second return beam, the change being a phase change.

7. A method according to claim 5 or claim 6 in which the analysis includes determining the phase velocity through the item between the first and second locations, the determination of the phase velocity being based on the time delay between a signal from the first detector means and a signal from the second detecting means, in combination with the known separation of the first and second locations.

8. A method according to any preceding claim in which the method is to monitor out-of-plane displacements in the item, such as Rayleigh waves and/or symmetrical Lamb waves.

9. A method of investigating an item, the method comprising the steps of :- generating ultrasound waves in the item; generating a first laser beam, forming a first probe beam and a first and a second reference beam from the first laser beam, applying the first probe beam to a location on the item; obtaining a first return beam and a second return beam from the location; applying the first reference beam and the first return beam to first detection means; applying the second reference beam and the second return beam to second detection means; analysing the output of the first and second detector means .

10. A method according to claim 9 in which the first return beam and second return beam are generated by a first probe beam.

11. A method according to claim 9 or claim 10 in which the first probe beam is provided at a different angle to the surface than the first and second return beam.

12. A method according to claim 10 or claim 11 in which the passage of a displacement through the incident location on the item gives rise to a change in the first return beam and the passage of a displacement through the incident location on the item gives rise to a change in the second return beam.

13. A method according to any of claims 10 to 12 in which the change in the first and/or second return beam is a Doppler frequency shift.

14. A method according to claim 13 in which the change in the first return beam is a frequency increase and the change in the second return beam is a frequency decrease or vice versa.

15. A method according to any of claims 10 to 14 in which the method is used to investigate in-plane displacements of an item, for instance asymmetric Lamb waves.

16. A method according to any preceding claim in which the respective probe beam(s) and reference beam(s) are formed by splitting a single beam.

17. A method according to any preceding claim in which separate detector means are provided for the first return/reference beam and for the second return/reference beam.

18. A method according to any preceding claim in which the first return beam and first reference beam are directed to the same location on first detecting means and the second return beam and second reference beam are directed to the same location on second detecting means.

19. A method according to claim 17 or claim 18 in which the return beam and reference beam form an interference pattern on the detector means .

20. A method according to any preceding claim in which the detector means are photo-refractive crystals.