Fibre optic sensors are becoming increasingly popular due to their small size and their suitability for multiplexing together. As an example, in the field of hydrophones fibre-optic hydrophones are replacing piezo-electric hydrophones in an increasing number of applications. The use of optical fibres in hydrophones has many advantages-fibre is small and lightweight, immune to electro-magnetic interference (EMI), electrically passive, capable of being used over long distances and it can be easily multiplexed. WO-A-94/17366 for example, discloses a fibre-laser sensor that has shown considerable promise as a hydrophone.

Fibre optic coil-based hydrophones operate on the principle that pressure changes caused by an acoustic signal, such as a sound wave, are converted into a strain in a coil of optical fibre. This strain imposes a change in the phase of an optical signal passed through the coil, due to the physical change in length of the fibre and the stress optic effect. The phase change can be detected by beating the signal with a reference signal of a slightly different frequency which, when mixed, produces a beat frequency, or heterodyne carrier, equal to the difference in frequency of these two signals. The acoustic signal will therefore appear as a phase modulation on this carrier. It is known to form arrays of such optical hydrophones, which may be optically addressed using a variety of multiplexing techniques, e. g. time division multiplexing (TDM), wavelength division multiplexing (WDM), etc. Such hydrophone arrays are well known and will therefore not be described in detail herein. A more detailed explanation of the addressing of such arrays may be found in PCT Application PCT/GB00/01300, Publication Number WO 00/62021 assigned to"The Secretary of State for Defence (GB)".

Other optical fibre hydrophones comprise fibre-lasers. Fibre-lasers can take the form of a Distributed Bragg Reflector (DBR) or, more commonly, a Distributed

Feed Back device (DFB). The DBR fibre-laser geometry has two Bragg gratings (having identical reflection wavelengths) separated by a short section of fibre that is doped with a rare earth metal such as erbium. This structure forms a Fabry-Perot laser cavity which, when pumped by shorter wavelength radiation, lases at a very specific wavelength (determined by the grating pitch, cavity length and emission bandwidth of the dopant). The geometry of a DFB fibre-laser device is similar having two identical Bragg gratings separated by a quarter wavelength change in phase. A measurable change in output wavelength from such sensors is observed when an external stimulus acts upon that part of the fibre containing the laser. Fibre-lasers offer a number of attractive properties such as narrow linewidth, good wavelength selectability, stable and single polarisation operation and high signal to noise ratio. As well as being used as sensors fibre-lasers can be used as laser sources within fibre optic systems.

The extremely narrow linewidth of the fibre-laser is a result of the superposition of the narrow grating transfer function over the Fabry-Perot response of the active cavity. The combination of the grating profile, the narrow resonate pass band features of the Fabry-Perot cavity, and laser line narrowing, produces a relatively powerful and highly coherent single wavelength output. The resulting small diameter device is extremely sensitive to strain variations and, as such, is an ideal candidate for use in hydrophone design.

Fibre-laser devices although having many advantages are limited in the following aspects. The frequency response of a fibre-laser sensor is generally not flat over the frequency range of interest (lOHz-lOKHz). Typically, there exists a series of modal features related to the excitation of standing wave modes along the length of fibre exposed to an acoustic signal. As with standing waves on a string the resonant modes are influenced by the tension, resonant length and physical properties of the fibre. There can be a problem with providing a well-defined effective resonant length as the sensing element and fibre-optic down leads stimulated by the underwater acoustic signal are acoustically well matched.

Furthermore, the fibre-laser wavelength can be significantly altered by environment changes such as static pressure (depth) and temperature. A shift in the

wavelength of the device can cause problems with the separation of signals from a plurality of these devices, such signal separation often employing a wavelength division multiplexing technique.

Optical fibre sensors such as hydrophones are often used in harsh environments, for example in towed arrays. When deployed the array will see huge amounts of energy in the form of vibration transmitted to it down the tow cable and turbulence from the fact that it is travelling through the water. One problem associated with fibre optic sensors, for example fibre-laser hydrophones, is that they are extremely sensitive to vibrations that pass along coupling optical fibres. It has been found that a vibration acting upon the down lead fibre (over a metre either side of the sensing region) can be detected by a fibre-laser hydrophone. This is believed to be due to vibrations coupling into the fibre and travelling up and down the fibre as strain waves. As these strain waves cross the sensing region they are detected by the fibre optic sensor and appear as parasitic signals on the acoustic signal being measured.

This is a major drawback in operation, particularly when used in towed arrays, where the coupling optical fibres are subject to large external forces with vibrations being transmitted along the fibre.

A first aspect of the present invention provides a vibration protection structure for fibre optic sensors or sources comprising: first and second end members and a support coupling said first and second end members together, said end members each being operable to be secured to an optical fibre in optical communication with a fibre optic sensor and/or source located between said end members such that in operation said vibration protection structure at least partially isolates the sensor and/or source from vibrations passing along the optical fibre.

Fibre optic components and in particular sources such as fibre-laser sources or fibre-laser sensors that are optically coupled to optical fibres are often very sensitive to vibrations passing along the coupling optical fibre. This causes distortions in the signal produced or detected. A vibration protection structure that can be secured to the optical fibre of a fibre optic source or sensor to provide at least partial isolation from

vibrations travelling along a coupling optical fibre can alleviate these problems and enhance the efficiency of the device by reducing noise from extraneous signals. The vibration protection structure can be secured directly to the optical fibre of the optical fibre component or it may be secured to optical fibre (s) that are coupled to the component. A further problem associated in particular with fibre optic sensors comprising fibre-lasers is that the effective resonant length can be ill-defined and can vary with external conditions. The use of a vibration protection structure that is secured to the optical fibre helps provide a well-defined length for the optical fibre sensor thereby providing some control over the resonant frequency of the fibre. This is important as the length can be defined to avoid fibre resonant frequencies occurring in frequency ranges of interest. Alternatively it might be advantageous to set the resonant frequency of the fibre to that of a known, or signature, frequency emitted by an object (for example a ships propeller) thereby enhancing the sensitivity of the sensor for a specific application.

Preferably, said vibration protection structure comprises a substantially rigid cage. A rigid cage like structure provides vibration isolation without adding too much to the weight of the overall device. The rigidity of the structure helps to hold the optical fibre sensor at a set length.

Although the cage may consist of, for example, a housing possibly having slits therein, in preferred embodiments the supports comprise a plurality of elongate members, preferably three, said first and second end members being connected together by said plurality of elongate members. This is a simple and easy to construct arrangement for the vibration protection structure.

Although the vibration protection structure can be made of a variety of materials it is preferably made of metal. Metal is rigid and robust and can survive in many harsh environments.

Preferably, said vibration protection structure has an adjustable length such that said first end member can be moved with respect to said second end member.

Providing the vibration protection structure with an adjustable length enables it to be altered depending on the circumstances. Altering the length of the structure (with the fibre firmly attached to respective end members) alters the tension under which the fibre optic source or sensor is held. With a fibre-laser sensor this has two effects it alters the resonant frequency of the fibre and it affects the wavelength of the fibre-laser as it alters the pitch of the Bragg gratings. It can thereby be used to tune particular sensors to make them sensitive to certain acoustic frequency ranges, and to compensate for any drift in the operating wavelength of the fibre-laser sensor that may occur due to changes in the Bragg grating spacing as a result of external temperature and pressure changes. It has also been found that the sensitivity of many optical fibre sensors can be enhanced by altering the fibre tension.

Advantageously, said vibration protection structure further comprises an actuator operable to cause said first end member to move with respect to said second end member.

In some embodiments, said actuator is electrically controlled while in others it is manually controlled. Electrical control of the actuator enables the length to be adjusted in-situ and enables an operator to tune a sensor while in operation and to adjust this tuning in response to external factors. Manual control of the actuator has the advantage of being simple and requiring no electrical connections. Often the sensors are used in harsh environments, such as underwater, where any electrical connections would be under severe strain.

Although the actuator could be achieved in a variety of ways, in preferred embodiments, said actuator is associated with said first end member and said actuator is arranged to cause the first end member to move relative to said support to effect relative movement between first and second end members. Providing the actuator in association with an end member is a simple way of providing the relative movement.

Preferably, said actuator incorporates said first end member, an element attached to said support and a mechanism for moving said first end member relative to said element.

In some embodiments, said first end member comprises a bolt, said element comprises a disk secured to said support and said actuator further comprises a nut, said nut being mounted on said bolt and being in juxtaposition with said disk but not attached thereto, said bolt being slideably mounted on said support such that on rotation of said nut on said bolt said bolt moves along said support. In other embodiments it is the disk that is secured to the fibre and the bolt that is secured to the support. In any case the bolt, nut and disk arrangement is a mechanically simple and robust way of effecting the relative movement. Furthermore, this arrangement allows the length and/or the tension on the optical fibre to be altered without imparting any twisting force to the fibre.

Preferably, said disk is spring mounted against said nut.

A further aspect of the present invention provides, an optical fibre sensor comprising: at least one fibre optic sensor comprising an optical fibre having a sensing portion; a vibration protection structure comprising a first and a second end member coupled together by a support, said first and second end members being secured to an optical fibre in optical communication with said fibre-laser sensing portion such that said sensing portion of said fibre optic sensor lies between said end members; wherein said vibration protection structure is operable to at least partially isolate said sensing portion of said fibre optic sensor from vibrations passing along said optically communicating optical fibre.

Preferably, said end members are secured directly to said optical fibre at first and second attachment points, any protective coating of said optical fibre being removed at said attachment points prior to said end members being secured.

Attachment in this way enables the optical fibre to be held securely and improves the vibrational isolation of the device.

In preferred embodiments said vibration protection structure is glued to said optical fibre. This provides a secure and simple way of attaching the structure.

Advantageously, said sensing portion of said fibre optic sensor is coated in a compliant material, preferably polyurethane. This coating increases the sensitivity of the fibre optic sensor. Furthermore, the arrangement of the vibration structure with its end members being connected by longitudinal members means that it is simple to apply the polyurethane once the vibration protection structure is in place. Although the polyurethane coating increases the sensitivity of the device it does have a drawback in that it causes a shift in the lasing wavelength of the device. The use of a tuneable vibration structure in conjunction with the coating in certain embodiments enables the coated sensor to be tuned to the desired wavelength, such that the shift caused by coating the device can be compensated for.

In preferred embodiments said polyurethane coating directly contacts the optical fibre of said fibre optic sensor, any protective coating covering said sensing portion of said fibre optic sensor being removed prior to application. of said polyurethane coating. The polyurethane increases the effectiveness of the hydrophone by transmitting any received vibrations through its body thus giving a sideways spread to the vibrations. Protective coatings are generally not in hard contact with the optical fibre core, i. e. there can be movement between the fibre and the coating, this can impede the reception of these signals.

Although the optical fibre sensor can be a variety of things, preferably it comprises a fibre-laser sensor comprising a DBR or a DFB device. These devices are particularly sensitive to vibrations passing along the optical fibre and as such benefit from a vibration protection structure. Furthermore, they can be tuned by adjusting the tension of the optical fibre. It is believed that a length change in such a device (achieved by means of altering the tension of the fibre) results in a shift in the grating pitch and thus, a change in the lasing wavelength. Additionally, the sensitivity may be enhanced by altering the tension of the fibre.

A further aspect of the present invention comprises an optical fibre sensing assembly comprising a plurality of optical fibre sensors according to a second aspect of the present invention, said plurality of optical fibre sensors being optically coupled together by optical fibre.

Although the plurality of optical fibre sensors can be coupled in a variety of ways, they are preferably optically coupled in series with each other.

Advantageously, said assembly is formed from a single optical fibre. This reduces optical losses due to optical couplings. It also reduces the cost and diameter of the assembly.

Preferably, said optical fibre sensing assembly further comprises a plurality of heat shrink sleeving, said heat shrink sleeving coating said optical fibre between said vibration protection structures. Heat shrink sleeving is simple to apply to the optical fibres and provides a cheap yet effective protection to the coupling optical fibre between the vibration protection structures.

Preferably, said end members each comprise a recess in an outer surface, said recesses being operable to receive said heat shrink sleeving. This provides greater protection for the device with the heat shrink sleeving being held securely in place and with it being less likely that gaps open between the vibration protection structure and the heat shrink sleeving with changes in length of the various components due to external conditions such as temperature.

Although the plurality of optical fibre sensors can all be set to lase at approximately the same wavelength in some embodiments at least one of said plurality of optical fibre sensors are set to lase at different wavelengths to said other plurality of optical fibre sensors. When using a plurality of sensors together it has been found convenient to separate the signals by using wavelength multiplexing. Thus, using

lasers that operate at different wavelengths provides a simple way of separating the signals.

A still further aspect of the present invention provides a method of manufacturing an optical sensor comprising the following steps: securing a vibration protection structure according to a first aspect of the present invention, to an optical fibre in optical communication with an optical fibre having a fibre-laser sensing portion; coating said sensing portion with a compliant sensitivity enhancing coating; and during said coating process, monitoring a wavelength generated by said fibre-laser and adjusting a length of said vibration protection structure in response to said monitored wavelength.

A vibration protection structure that has an adjustable length can be used when manufacturing a fibre-laser sensor and in particular when coating it with a compliant coating that enhances the pressure sensitivity of the fibre laser. During the coating process the coating may cure and harden and thereby exert anisotropic forces on the fibre-laser, these forces can affect the lasing wavelength. By means of a feed back loop it is possible to vary the length of the vibration protection structure in response to changes in the measured wavelength of the device thus at least reducing changes in the operating wavelength of the fibre-laser sensor during the coating/curing process.

Although the sensitivity enhancing coating can be made of a number of different compounds that act to increase the sensing portions sensitivity external pressure changes, in preferred embodiments said sensitivity enhancing coating comprises polyurethane.

Polyurethane has been found to be particularly effective at increasing fibre- laser sensitivity. It is also a robust compound and can be used in harsh environments.

In preferred embodiments said length of said vibration protection structure is adjusted in order to keep said wavelength within predetermined limits.

A yet further aspect of the present invention provides a method of protecting a fibre optic sensor and/or source from vibrations comprising: securing a structure to a first portion of an optical fibre in optical communication with a fibre optic sensor and/or source; securing said structure to a second portion of an optical fibre in optical communication with said fibre optic sensor and/or source; wherein said first and second portions are located at either side of said fibre optic sensor and/or source such that said first and second portions are held at a predetermined distance from each other and thereby at least partially isolate said fibre optic sensor and/or source from vibrations passing along optical fibre in optical communication with said fibre optic sensor and/or source.

A further aspect of the present invention provides an optical fibre-laser having a vibration protection structure; wherein said vibration protection structure comprises a first and a second end member coupled together by a support, said first and second end members each being secured to an optical fibre in optical communication with said optical fibre-laser located between said end members, such that said optical fibre-laser is at least partially isolated from vibrations passing along optically communicating optical fibre.

The vibration protection structure of the present invention is particuarly effective for fibre-lasers. Preferred embodiments of the structure having a variable length can also be used to tune a fibre-laser to a required wavelength.

Particular embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, of which: Figure 1A shows a tuneable vibration protection structure according to an embodiment of the present invention; Figure 1B shows a tuneable vibration protection structure according to an embodiment of the present invention attached to a fibre-laser sensor;

Figure 2 shows an optical fibre comprising an optical fibre sensor; Figure 3 schematically shows a plurality of optical fibre sensors arranged in series and being mounted within tuneable vibration protection structures; Figure 4 schematically shows two fibre-laser sensors held at different points by a vibration protection structure (not shown); and Figure 5 shows a fibre-laser held within an adjustable length vibration protection structure.

The vibration protection structure shown in Figures 1A and B consists of a cage like structure 5 having an end member 10, three elongate members or locating rods 20 a disk 30, nut 35 and bolt 37 which acts as the other end member of the vibration protection structure.

Figure 2 shows an optical fibre 50 having a sensing portion 40 which forms the fibre optic sensor or in this particular embodiment the hydrophone. Although the sensor and coupling fibre may be formed from a single piece of fibre, in the embodiment shown a plurality of lengths of fibre are used, and these are spliced together. Protective tubing 52 is mounted around the spliced connections to provide them with some degree of mechanical protection.

The vibration protection structure of Figures 1A and 1B is adhesively secured in use by a rigid glass-metal adhesive to a portion of an optical fibre 50 such as that shown in Figure 2 from which the protective coating has been removed. In this embodiment the bolt 37 is also secured to the optical fibre 50 by being glued to a portion of it from which the protective coating has been removed. By firmly bonding the cage to the optical fibre, strain waves passing through optically coupling optical fibre are coupled into the supporting cage structure rather than into the sensing region of the optical fibre. The sensing portion of the optical fibre 40 (see Figure 2), lies within the cage structure. Although the Figure is described as showing a fibre-laser

sensor it could equally well show a fibre-laser that is used as a laser source in an optical fibre system.

The disk 30 is spring mounted against a nut 35 which is threaded onto a bolt 37. Turning the nut 35 as indicated by arrow A causes the bolt to move as indicated by arrow B and this increases or decreases the tension under which the optical fibre 50 (see Figure 2) of the fibre optic sensor 40 is held. Altering the tension of the optical fibre of the fibre optic sensor in this way produces two effects. It alters the output wavelength of the laser and also the resonant properties of the length of the sensing fibre and thus the frequencies to which it is sensitive. Furthermore, holding the optical fibre under tension tends to increase the sensitivity of many fibre-laser devices. This is thought to be because the Bragg gratings of such devices may have been written onto the optical fibre when it was held under tension. Although in this embodiment it is the bolt that is secured to the optical fibre and the disk that is secured to the cage, in other embodiments the disk could be secured to the fibre with the bolt being secured to the cage.

The ability to alter the tension (i. e. tune) a fibre-laser device offers significant advantages in terms of device performance and in terms of device integration in a multiplexed array of sensors. This ability to tune the sensor is a way of compensating for any drift in the lasing wavelength of the device due to coating and operation over a wide range of temperatures and pressures. The sensitivity of the device to a particular frequency range can similarly be adjusted in this manner.

Although the vibration protection structure of Figures 1A and 1B has a manually operated actuator it is envisaged in some embodiments to have electrically actuatable tensioners. This would enable the sensor to be tuned in-situ and any wavelength drift due to changes in temperature and pressure to be compensated for.

Figure 4 shows two examples of fibre-laser sensors 40 and 42 showing the attachment position of a vibration protection cage. In the two examples the cage has different lengths li, and 12. This illustrates the possibility of altering the length of a

cage prior to it being attached to the fibre. In this way its length can be set to a particular value that is appropriate for the resonant frequencies of the fibre for a particular sensing application. Once glued to the fibre the length of the cage can again be altered in the direction B. In this case, as well as affecting the length of the fibre, the tension under which the fibre is held is also affected. Thus, although the resonant frequency of the fibre can be adjusted once the cage is attached such an adjustment also affects the wavelength of the laser and this may limit the possible adjustments for a particular embodiment. Thus, the possibility of altering the length of the cage before attachment to the fibre can be useful in some situations.

In use the hydrophone portion of the optical fibre is mounted within the tuneable cage of Figure 1, with the cage being glued to portions of the optical fibre at either side of the sensing portion. Figure 3 schematically shows a plurality of hydrophones 40 in series on an optical fibre 50, encased in tuneable cages 5. The fibre protruding from either end of the cage is sheathed in a small diameter tubing 55 which itself is bonded to the ends of the cage. In preferred embodiments this tubing is a heat shrink tubing such that it can be simply bonded to the fibre. The end members of the cage comprise recesses for receiving this tubing in some embodiments. This arrangement helps protect the optical fibre from transverse shear forces and also helps isolate it from external vibrations.

Polyurethane 60 coats the hydrophone within the cage and improves its acoustic sensitivity.

One advantage of the cage-like structure of the vibration protection structure is that the coating of polyurethane 60 can be simply applied to the sensor 40 once the cage is attached to the optical fibre. This coating increases the sensitivity of the sensor, but would clearly be difficult to apply prior to mounting the vibration protection structure as this must be glued securely to the optical fibre. The polyurethane is applied by placing the cage in a mould and injection moulding the polyurethane so that it fills all cavities surrounding the laser. The cage is designed to maintain good longitudinal stiffness while allowing the fibre to be coated to an

optimum diameter, which in this embodiment is 5mm. Any protective coating surrounding the sensing portion of the optical fibre is removed prior to coating with polyurethane in order to improve the sensitivity of the device.

Figure 5 shows a fibre-laser sensor to which a coating of polyurethane is to be applied. The device is set-up such that the wavelength of the fibre-laser can be monitored by a wavelength sensor 70 and this signal is fed to a controller 80 which adjusts the length of the vibration protection structure 12. The vibration protection structure 12 has an electrically operated actuator 82 for adjusting its length, thus the controller 80 sends signals to the actuator in response to changes in the fibre-laser wavelength detected. The coating of polyurethane takes some time to complete as the polyurethane needs to dry. As it dries it exerts changing forces on the fibre-laser sensor that affect the pitch of the Bragg gratings and thus the wavelength of this device. By altering the cage length (and hence fibre tension) during the drying process the wavelength of the fibre-laser can be held within predetermined limits. This not only enables a sensor of a desired wavelength to be produced but it also helps reduce the number of fibre sensors which no longer function following the coating process.

Although a particular embodiment of the invention has been described herein, it will be apparent that the invention is not limited thereto and that many modifications and additions may be made within the scope of the invention. For example, various combinations of the features of the independent claims could be made with the features of the dependent claims without departing from the scope of the present invention.