2. BACKGROUND DISCUSSION Fiberoptic networks are susceptible to physical damage and disruption of service from a number of causes. Among the external causes, the infiltration of water in fiberoptic splice closures is particularly worrisome, particularly where freezing conditions are encountered.

Fiberoptic splice closures are expected to protect the equipment they house for many years in an outdoor environment. Water is susceptible to penetrate these closures, especially when they are installed underground. As a typical network operator may install hundreds or thousands of such boxes each year, it is not possible in practice to guarantee that none will leak over time.

Several problems can arise when water comes in contact with bare optical fibers or the optical fiber splices. A slow degradation process may cause some fibers to break. A more catastrophic failure happens when a group of fibers is torn by freezing water. When a break occurs, this may lead to a disruption of service if the affected part of the network has no built-in automatic restoration mechanism.

A typical fiberoptic splice closure may contain hundreds of fibers.

Repairing the broken fibers and restoring service may thus take days of work.

Even if there is no disruption of service, the impaired network remains more vulnerable until the broken part is repaired. Given the importance of maintaining the telecommunication networks in full operational condition all the time, the

ability to quickly identify leaking fiberoptic splice closures has thus been recognized for a long time in this industry as an important part of an effective maintenance program.

Many inventors have proposed ways to cope with the water infiltration problem with various degrees of success. Following is a brief description of the techniques they have proposed.

An effective way to avoid the problem is by pressurizing the cables to prevent the ingress of water in the outside cable plant. While this approach has been in use for a long time where copper telecommunication cables are installed, the characteristics of fiberoptic networks do not generally justify the use of this approach.

Many devices and solutions have been proposed and used to detect and signal the infiltration of water in fiberoptic splice closures. One of the simplest and most effective way to detect liquid water is by measuring the electrical conductivity between a pair of conductors. When a pair of conductors is immersed in water, the conductivity between them increases dramatically. A sensor unit can then signal the problem to a remote location through a dedicated pair of copper wires or by using the conductive sheath of the cable and the ground as a return path (as in U. S. Pat. No. 4,480, 251 to McNaughton or U. S. Pat. No. 5,077, 526 to Vokey et al.), or by radiating an alarm signal that can be detected by a receiver located in proximity (as in U. S. Pat. No.

5,903, 221 to Eslambolchi et al.). However, fiberoptic cables that include a pair of copper wire are more expensive than standard fiberoptic cables. When the conductive sheath of the cable is used instead, a stringent and costly maintenance program is needed to monitor the integrity of the cable sheath itself. With the growth in the deployment of optical fiber networks and the increase in the fiber count per cable, a demand has thus emerged for an all- optical solution that can use all-dielectric cables only.

When all-fiber devices are used, the simplest and preferred way to signal an alarm condition is through an increase in the optical transmission loss of the device. Most of the proposed all-fiber solutions use this scheme. The status of the sensors can then be monitored at designated locations (such as Central Offices) either manually or automatically through so called Remote Fiber Test Systems (RFTS). Optical Time-Domain Reflectometry (OTDR) is the preferred measurement method as it allows to identify the closure where the alarm comes from by measuring the distance along the link where the transmission loss increased. It may also be used to detect other problems that may affect the links.

Many all-fiber devices have been proposed to detect the presence of liquid water or humidity at a single point in the splice closure. One of these has been proposed by Lindow et al. in U. S. Pat. No. 6,099, 217 and 5,757, 988 and is commercialized under the trademark HydroSensor by Mark Products Inc.

Other devices have been proposed by Shen et al. in U. S. Pat. No. 5,430, 815, by Bonicel in U. S. Pat. No. 5,243, 670, by Johnson in U. S. Pat. No 5,966, 477, by Takaaki in Japan Pat. No. 2-128144 and in the references cited therein.

However, these devices are not suitable for use when the orientation of the splice closure is unknown because it is not possible in these instances to guarantee that the sensor is located at the lowest point in the closure, where water may accumulate first. On the other hand, relying on the detection of humidity alone is often not sufficient. Although a high level of humidity can be itself a cause for concern, the humidity sensors may not react when the temperature is low even though liquid water is present.

Fiberoptic cables that are sensitive to the ingress of moisture have also been proposed by Sawano et al. in U. S. Pat No. 4,812, 014 and by Michie et al. in U. S. Pat. No. 5,744, 794. However, such cables are susceptible to react to humidity and the level of the water-induced optical attenuation depends on the length of the cable section that is affected. It is often preferable to have a

sensor that produces only a clear"yes"or"no"signal to indicate its status.

Moreover, the maximum one-way optical transmission loss of a device should preferably be limited to the minimum value that is needed to produce a clear signature (for example, 1 dB) in order to allow to continue to monitor the rest of the link. It may also be desirable to be able to use the monitored link for temporary or continuous data transmission service. In this case, the link should suffer only a small increase in transmission loss at the service wavelength in order to remain operational after a sensor has triggered.

A device that uses a water-activated battery as the water sensor element and an optical fiber intensity modulator has been proposed by Vokey et al. in U. S. Pat. No. 5,349, 182 and 5,262, 639. However, it requires proprietary equipment at the remote location to detect and identify the alarm signal that it generates.

SUMMARY OF THE INVENTION A fiberoptic water sensor that is compatible with OTDR or other standard fiber loss measurement techniques is desirable. This water sensor can be used in situations where it is necessary to detect the presence of water in a chamber without using electrical wires to communicate an alarm signal.

It is thus the purpose of the present invention to provide an all-fiber water detector that can be used with all-dielectric cables, that is sensitive to liquid water, that can detect the presence of water in any part of the chamber where it is installed, that preferably produces a binary output signal, that is low cost and that is compatible with the use of OTDRs and other standard fiberoptic loss measurement equipment.

To accomplish the foregoing and other objects, features and advantages of the present invention, there is provided a fiberoptic water sensor that includes a water-activated battery element adapted to be disposed in a fiberoptic splice

closure. The water activated battery element is positioned so as to accept water that enters the closure. At least one storage capacitor is coupled to the water- activated battery element and is adapted to be charged from a potential established at the water-activated battery element when subjected to water. A control circuit that is responsive to the charging of the storage capacitor generates an electrical actuation signal. An electromechanical actuator is responsive to this actuation signal. An optical fiber is provided, with the level of optical transmission loss of the fiber to be measured locally or remotely by ensuring that the optical fiber of the fiberoptic water sensor reaches the measuring instruments through an external fiberoptic link. The electromechanical actuator, in response to the electrical actuation signal, causes a bending of a segment of the optical fiber from an initial position to a bent position so as to change the optical transmission loss of the fiber and thus signal the presence of water in the splice closure.

In accordance with further aspects of the present invention, the water- activated battery element may be of elongated configuration such as in cable or ribbon form and typically is of a length between 1 and 2 meters. The cathode of the water-activated battery may comprise a hydrogen electrode and the anode may comprise magnesium or a magnesium alloy. The support for the hydrogen electrode may comprise a copper wire or a plurality of such wires. A dielectric spacer element may be disposed between the cathode and anode and may comprise a braided polyester sleeving. Furthermore, there may be provided a jacket about the cathode and anode to hold them together and to provide electrical insulation from the metal parts of the enclosure which they may contact.

In accordance with one embodiment of the invention described herein, the storage capacitor may comprise either a single capacitor or a capacitor bank. The control circuit is preferably based on an hex Schmitt-trigger inverters integrated circuit of the CMOS HC family. The integrated circuit is preferably

powered from the storage capacitor through an RC filter. The control circuit may comprise a pulse generator, a voltage multiplier coupled from the pulse generator, and a power-switching transistor (preferably, a Metal Oxide Semiconductor Field Effect Transistor (MOSFET)) coupled from the voltage multiplier. The pulse generator may comprise a plurality of inverters (preferably, Schmitt-trigger inverters). There may also be provided a further RC circuit associated with the first inverter for forming a free running oscillator of the pulse generator. The following inverter output is preferably directed to a load resistor such that the outgoing pulse cannot reach the input threshold voltage of the next inverter and propagate beyond that point if the supply voltage has not reach a minimum value. The voltage multiplier may comprise a pair of series connected Schmitt-trigger inverters. A capacitive load may be provided at the output of the first inverter of the voltage multiplier to introduce a delay in the propagation of the pulse to the second inverter. Coupling to the power switching transistor is preferably provided through both a Schottky barrier diode and a capacitor that are connected respectively to the input and the output of the first and second inverters of the voltage multiplier.

In accordance with still further aspects of the present invention, the electromechanical actuator may comprise a solenoid. There may also be included an actuation shaft supported for activation from the solenoid and including an arm engageable with the segment of optical fiber. A biasing spring may be associated with the actuation shaft for biasing the arm toward the bent position of the segment of optical fiber. A disk may be provided supported from the actuation shaft and engageable by the electromechanical actuator. The electromechanical actuator may comprise a solenoid having a plunger, and the disk may be provided with a slot for receiving the tip of the plunger and preferably limiting the displacement of the plunger toward the disk in the engaged position. A spring is engageable with the plunger so as to bias the plunger toward the disk.

In accordance with still a further feature of the present invention, the segment of optical fiber may be formed in an arcuate segment. Two spaced points of the segment may be fixed to the structure chassis, preferably near the actuation shaft. The aforementioned has the optical fiber segment disposed in a predetermined plane and extending in an arc. The bent position rotates the segment arc out of the plane, preferably around the actuation shaft. The segment may be rotated on the order of one half to three-quarters of a turn.

In accordance with another feature of the present invention, part of the actuation shaft extends outside the housing of the device and is terminated with a socket to allow the device to be rearmed manually with an appropriate tool after it has triggered. The extension also includes a position mark to allow to visually determine the status of the device without having to measure the insertion loss through the optical fiber.

Also, in accordance with the present invention there is provided a method of detecting water in an enclosure that houses optical fibers. This method provides a signaling of a detection of water at a remote site. The method comprises the steps of providing a water-activated battery element in the enclosure for establishing therefrom an electrical actuation signal in response to detection of a minimum amount of water in the enclosure. Next is the step of providing an optical fiber, with the optical transmission loss of the fiber being measured at the remote site. Next, is using the electrical actuation signal to trigger a mechanical actuation moment. The last step is using the mechanical actuation moment to bend a segment of the optical fiber from an initial position to a bent position so as to change the optical transmission loss that is measured at the remote site.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description as taken in conjunction with the accompanying drawings, in which: FIG. 1 is a simple perspective view of the system of the invention including a central office and a fiberoptic water sensor as used in a fiberoptic splice closure; FIG. 2 is a plan view of the sensor in its unactivated (or armed) state; FIG. 3 is a plan view like that shown in FIG. 2 but in the activated (or triggered) state; FIG. 4 is a cross-sectional view illustrating mechanical operation in the unactivated state; FIG. 5 is a cross-sectional view similar to FIG. 4 but in the activated state; FIG. 6 is a block diagram illustrating the control circuit of the present invention as well as the water-activated battery element; FIG. 7 is a more detailed circuit diagram of the control circuit, also illustrating the water-activated battery element and the electromechanical actuator; and . FIG. 8 is a simple perspective view of the housing of the fiberoptic water sensor and the water sensor element, the optical fiber pigtail and the status- reset extension; FIG. 9 is a cross-sectional view of the water sensor element; FIG. 10 is a plan view of an alternative embodiment of the sensor in which a watch mechanism is used to trigger the sensor; FIG. 11 is a plan view of an alternative embodiment of the sensor in which a blade is used to break the fiber; and FIG. 12 is a detail of the blade and the fiber of the sensor of FIG. 11 in the armed position.

DETAILED DESCRIPTION OF THE INVENTION Reference is now made to FIG. 1 of the present drawings for an illustration of the fiberoptic water sensor 9 of the present invention as disposed within a splice closure 35. The drawing of FIG. 1 also illustrates the splice trays 29 and the fiberoptic cables 33 and 34. Cables 33 and 34 are considered as coupling from a central office 30 whereat a remote measurement of the transmission state of the fiber can be made either manually or automatically through a Remote Fiber Test System (RFTS), such as depicted as box 32 in FIG. 1. Typically, a plurality of splice closures in cascade are found along the optical fiber path to the Central Office. Typically, communications equipment 31 is provided at the Central Office.

Although the fiberoptic water sensor of the present invention is used, in the preferred embodiment, to detect the presence of water in a splice closure having a splice tray, the fiberoptic water sensor of the present invention could be used to detect the presence of water in any chamber in applications where it is preferable not to use electrical wires to communicate a detection signal. Such applications could be situations in which the distance from the monitoring unit is too great, environments where the electrical interferences are common, locations where corrosion is common, applications is which there is a risk of explosion, etc.

During the installation of the fiberoptic water sensor in a fiberoptic splice closure, the two optical fiber leads from the sensor are spliced to the fibers of the cable plant that are designated for this purpose. The sensor reacts to the presence of water by increasing the transmission loss over its fiber. This change can, in turn, be detected at a remote location.

The invention is considered as being composed of the following general components: Water sensor element, Storage capacitor, Control circuit, Electro- mechanical actuator, Mechanism, Optical Fiber, and Housing.

Water sensor element The water sensor element 1 is a water-activated battery. When water reaches the water sensor element or part of it, it generates an electric current that charges the storage capacitor C1 (FIG. 7) and energizes the control circuit.

Capillary attraction ensures that a sufficient portion of the electrodes is wet even when the water touches the sensor element at a single point. As depicted in FIG. 8 and 9 that show close-up views of the water sensor element and its components, the active portion of the water-activated battery preferably starts at about 30 cm from the housing 26 of the fiberoptic water sensor.

The water sensor element has an elongated shape (like a cable or a ribbon), being typically between 1 to 2 meters long. It could be longer or shorter if needed. It is flexible so that it can be wound inside a fiberoptic splice closure or any similar chamber in order to be able to detect the accumulation of water in any part thereof. Refer to FIG. 1 that illustrates the water sensor element 1 wound about the splice tray 29.

The cathode 2 of the sensor element is a hydrogen electrode. The support for this electrode could be made of any electrically conductive material having a low hydrogen overpotential and suitable mechanical and corrosion resistance characteristics. This includes, but is not limited to, copper, graphite and noble metals such as platinum, gold and palladium and numerous alloys.

The preferred material for the cathode is copper, as it is inexpensive and readily available in many shapes and sizes with appropriate mechanical and corrosion resistance characteristics.

The anode 3 is preferably made of a magnesium alloy, such as AZ31B.

The selected alloy shall offer good corrosion resistance, be relatively ductile and also retain much of the high electronegativity of magnesium. Unalloyed magnesium can also be used. No other common base metal at the anode can

produce a single cell voltage higher than magnesium. Zinc, for instance, produces a single cell open circuit voltage that is only about half that of magnesium, requiring that the water sensor element be composed of at least two such cells in order to be able to energize the sensor electronics.

For both the anode and the cathode, the specified electrode material could also be plated, cladded or coated over a suitable base substrate to constitute an acceptable electrode that falls within the scope of the present invention.

With a magnesium anode, a copper cathode and water from common sources (rainwater, groundwater, melted snow from urban areas, tap water, <BR> <BR> seawater, heavy water from nuclear power plants, etc. , which may contain in addition various contaminants such as hydrocarbons) as the electrolyte, the open circuit voltage of the water-activated cell is typically around 1.5 volt, which is sufficient to energize the control circuit.

When a portion of the water sensor element is wet, the current it produces can be relatively small. It is possible to increase the current generating capacity of the water sensor element by providing soluble ionic salts that can dissolve when water is present. Other cathodic half-cell reactions could be contemplated in such a case. However, if such salts are included near the electrodes, they could make the device exceedingly sensitive to humidity as they could absorb a substantial amount of moisture. Furthermore, they could become a source of corrosion affecting the water sensor itself or parts of the closure or its content. It is therefore deemed preferable to avoid the use of such salts altogether.

A spacer dielectric element 4 is used to prevent the electrodes from contacting each other while allowing water to flow freely around the electrodes.

In the preferred embodiment, this element is a braided polyester sleeving that covers one of the electrodes. Another sleeving of a similar type is used as a

jacket 5 to provide electrical insulation from the metal parts of the enclosure which the electrodes may contact and to hold them together.

It should be clear that a wide range of arrangements and materials can be considered to make, hold, and protect the electrodes of the water sensor element. For instance, the jacket and the spacer element could be distinct elements as in the preferred embodiment or they could be combined into a single entity that provides the same functionality. The spacer and/or the jacket can also be made of a capillary material or structure to promote wetting of a longer section. The electrodes could be of a multiplicity of braided or stranded wires, as is commonly found in the electrical industry. In any such case, the result would still be a water-activated battery that could be suitable for the present application. All such arrangements therefore fall within the scope of the present invention. In the same way, any multicell design that constitutes an elongated water-activated battery having the required characteristics also falls within the scope of the present invention.

Storage capacitor The electric energy from the water sensor element 1 is accumulated in the storage capacitor C1 in order to drive the electromechanical actuator S1.

Depending on the characteristics of the electromechanical sub-system, the storage capacity could be typically between 5,000 and 50, 000/iF. The storage capacitor could be of a type that can be fully discharged in a short time, typically within a few milliseconds. Its equivalent series resistance (ESR) should preferably be less than 0.1 Q. Aluminum electrolytic capacitors are the preferred type for this application. The storage capacitor C1 can be provided by a single element, as is illustrated in FIG. 7 or it can be provided by a capacitor bank such as the bank CB illustrated in FIG. 6 and comprise two or possibly more capacitors all connected in parallel. The storage capacitor preferably used has a

capacity of 22, 000 F. The preferred dimensions for capacitor C1 are 0. 2" in diameter and 0. 4" in height. As will be readily understood, a smaller storage capacitor would be preferred, if available.

Control circuit The function of the control circuit is to initiate the discharge of the storage capacitor C1 through the electromechanical actuator S1 when capacitor C1 is charged to a sufficiently high voltage. The control circuit is composed of a pulse generator 6, a voltage multiplier 7 and a power-switching element Q1.

In the preferred embodiment, the control circuit is constructed around a set of six Schmitt-trigger inverters (U1a, U1b, U1c, U1d, U1e, U1f) of the Complementary Metal Oxide Semiconductor (CMOS) HC family. These inverters are packed in a single integrated circuit U1 of type 74HC14. When used at a relatively slow switching speed as in the present application, these logic gates can operate at a supply voltage that is less than 1 volt, which is much below their recommended supply voltage range, that is between 2 and 6 volts. Capacitor C2 and resistor R2 are used to filter the supply voltage at U1 to ensure that it remains fully energized while the discharge of the main capacitor C1 proceeds.

The role of the pulse generator 6 is to produce a pulse when the voltage at the storage capacitor C1 has reached a sufficiently high value that we shall designate thereafter as the threshold voltage of the control circuit. As the capacity of the storage capacitor C1 is relatively high and the current generation capacity of the water sensor element 1 is relatively low, charging C1 may take as long as 15 minutes or more in some cases, once water has contacted the sensor element.

The pulse generator includes a positive feedback loop through resistor R4. Because of the asymmetry of the inverter gates, the output voltage of the

pulse generator 6 always starts in the low voltage state when the supply voltage rises slowly from zero.

An oscillator built around circuit U1a, capacitor C3 and resistor R3 produces a low frequency square wave signal that is buffered through circuit U1b and fed to circuit U1c through capacitor C4 and load resistor R4. Because the output characteristic of the Schmitt-trigger inverters, in particular that of U1b, is a non-linear function of its supply voltage, when the supply voltage is lower than the threshold voltage of the control circuit, the input voltage at U1c cannot reach its own positive-going threshold voltage. The pulses from the oscillator are then blocked at that point and the output of the pulse generator 6 remains low.

However, when the supply voltage at capacitor C1 reaches the threshold voltage of the control circuit, circuit U1c is triggered by the oscillator output. The positive feedback loop through resistor R4 then latches the output of the pulse generator at a high value for the remainder of the half-cycle of the oscillator, which should be long enough to complete the triggering sequence of the device.

The threshold voltage of the control circuit is set through resistor R4 at a value around 1.2 volt, which is low enough to be reached under all circumstances when the water sensor element 1 or part of it is wet with any form of water that is susceptible of being encountered.

In order to drive the power-switching element Q1, the outcoming pulse from the pulse generator is amplified through the voltage multiplication circuit 7.

In the preferred embodiment, a nominal doubling of the pulse amplitude is obtained by first charging capacitor C6 through the low leakage current Schottky barrier diode D1. After a delay that is set forth by the discharge of capacitor C5 through circuit U1e to allow the charging of C6 in the way that is described above, the output of circuit U1f goes high and adds to the voltage at capacitor C6, raising the control voltage of the power-switching element Q1 above 2 volts.

R5 is used to prevent that a charge coming from leak currents or fluctuations in the circuit accumulate at the gate of the transistor Q1. This would lead Q1 to partially conduct and would therefore uncharge C1 without the current being great enough to trigger the solenoid S1. C4 is used to reduce the power consumption of the circuit by limiting the output current of U1 b.

The power-switching transistor Q1 of the control circuit is a power MOSFET transistor. The transistor is preferably of n-channel type and is characterized by a low gate threshold voltage, preferably less than 1 volt, and a low on-resistance, preferably less than 0.1 Q. A suitable transistor for this application is part number NDC631 N manufactured by Fairchild Semiconductors. In the control circuit described above, the charge-discharge cycle repeats indefinitely, as long as the water sensor element can supply enough current to the device.

Electromechanical actuator The electromechanical actuator S1 converts the high current electric pulse generated by the control circuit into a mechanical displacement that can be used directly or indirectly to bend or break the optical fiber 15. In the preferred embodiment, the electromechanical actuator S1 is a solenoid of the plunger type. A wire spring 13 is anchored to the printed circuit board 11 at one end and is inserted in the plunger 12 through a hole near its tip at the other end.

The wire spring 13 is bent in such a way as to bias the plunger 12 toward the disc 20.

The weight of the plunger 12 should be minimized in order to reduce the sensitivity of the device to mechanical shocks. Increasing the return force of the spring 13 further reduces the sensitivity of the device to mechanical shocks. The return force of the spring 13 must however be less than the pull force exerted by

the solenoid S1 when it is triggered. In order to maximize the pull force of the solenoid S1, its coil 8 is built to a typical resistance of a fraction of an ohm.

In another embodiment, the electromechanical subsystem described in the sections above (the solenoid, the control circuit and the storage capacitor) is replaced with a micropower step motor with the appropriate control circuit and demultiplication gears. Such a motor produces the displacement required to bend the fiber in order to achieve the required level of optical transmission loss.

A low-cost analog quartz wristwatch mechanism can be used advantageously for that purpose, since it includes all the drive electronics as well as the demultiplication gears. It is only necessary to add a capacitor of a few microfarads (the equivalent of C1) at the power input to regulate the voltage from the high impedance water-activated battery element.

Some commercially available watch mechanisms start to run at 1 volt and draw as low as 1 microampere. They are compatible with the water- activated battery, even though its open-circuit voltage can drop below 1.3 V after a few hours of continuous operation as a result of the polarization of the electrodes. Depending on the level of demultiplication, the transition between the high and the low transmission states of the fiber may take less than one hour up to a few hours.

Mechanism A mechanism is depicted in FIGS. 2 to 5 to transfer or amplify the movement produced by the electromechanical actuator S1 in order to bend the optical fiber 15. The actuation shaft 14 may have a diameter of 1/16". It may be supported on each side by the chassis 17 with low friction bearings 17B and 17C so that it is free to rotate along its axis but is restrained from translation in any direction, especially in a direction perpendicular to the axis. The optical fiber 15 is positioned relative to the actuation shaft 14 in such a way that a short

segment 16 makes an arc immediately under the actuation shaft 14 (see FIG.

2). This segment 16 is glued to the chassis at two points 17A close to the shaft.

The solenoid S1 is also fixed to the chassis 17, perpendicular to the actuation shaft 14. Preferably, points 17A are bent portions of the chassis.

A helical spring 19 along the shaft 14 provides the torque needed to turn the actuation shaft 14 and bend the fiber segment 16. When the device is armed, the rotation of the actuation shaft 14 is blocked by the plunger 12 as it is engaged in the slot 21 on the disk 20 that is fixed to the actuation shaft 14. The slot 21 is deep enough to reliably block the rotation of the actuation shaft 14 when the plunger 12 is engaged. A stroke slightly longer than the depth of the slot 21 is needed to disengage the plunger 12 and allow the actuation shaft 14 to rotate. The moment of inertia of the actuation shaft 14 and disc 20 assembly and the friction on the bearings 17B and 17C are minimized so that the rotation can be completed before the solenoid S1 is deactivated and the plunger 12 returns back on the disc 20 under the force of the spring 13.

When the actuation shaft 14 is freed, an arm 22 attached to the actuation shaft 14 winds the section 16 of the optical fiber about the actuation shaft 14, as depicted in FIG. 3. The amplitude of the rotation is limited by arm 22 as it impinges on stop 17D. Stop 17D can be made from a bent portion of the chassis 17. The final position of the actuation shaft 14 determines the level of transmission loss in the optical fiber 15 when the device is triggered. In a typical sensor, the fiber may need to be winded one half to three quarters of a turn around the shaft to induce the required transmission loss.

In a preferred embodiment, an extremity of the actuation shaft 14 extends outside the housing and is terminated by a hex socket flat head 25.

This allows to rearm the device after it has triggered and to check the status of the device by looking at the position of a reference mark embossed on the head 25 relative to a reference mark on the metal tube 23. This feature is particularly useful to test the device.

A number of alternate mechanisms can be conceived to bend the optical fiber. For example, the solenoid could be of the clapper-type. Compression or extension helical springs could be used to push the plunger or the clapper on the disc. The plunger could engage in the disc through its face instead of through its edge. The energy required to bend the optical fiber 15 as in the preferred embodiment is typically less than 1 millijoule, which is many times less than the energy stored in the capacitor C1 when charged at the threshold voltage. A more direct action mechanism that does not require the energy stored in the spring 19 to bend the fiber 16 is thus conceivable with little or no change in the drive electronics. A microbending of the fiber could also be used instead of the macrobending that is obtained when the fiber is wound about a shaft as in the preferred embodiment. All such variations remain within the scope of the present invention.

As shown in FIG 10, when a watch mechanism 45 is used instead of the solenoid to bend the fiber 15, the minute or the hour axis of the watch mechanism can be directly coupled to the actuation shaft 14. The time-set feature 46 of the watch mechanism can be used to rearm the device. The preferred method to check the status of the sensor is by looking through a window 47 to a drum 48 with a color-coded surface attached to the actuation shaft. The torsion spring 19 on the actuation shaft is preferably kept to assist the mechanism in bending the fiber. In this case, the motor works more like a controlled break. This, in effect, reduces the threshold voltage at which the motor can rotate the actuation shaft and bend the fiber.

For the watch mechanism 45, the preferred dimensions are 0. 75" in diameter and 0. 12" thick.

Optical fiber The optical fiber 15 of the present invention is sensitive to bending. It is also desirable that it has the same mode field diameter as that of the fiber that is deployed in the optical fiber network in which the device is installed.

Singlemode silica fiber is presently universally used for high data rate transmission in long-haul and metropolitan networks. This type of fiber is generally compatible with the present invention. In the preferred embodiment, the optical fiber 15 is standard singlemode fiber (SMF) of a standard diameter of 125/im with a plastic jacket of 250, um.

As the macrobending loss of a singlemode optical fiber increases sharply with the wavelength of the transmitted light, it is preferable to use the invention at a long wavelength of the transmitted light, such as 1550 of 1625 nm for standard telecommunication fiber. At a shorter wavelength or with a fiber having a lower sensitivity to bending, such as a dispersion-shifted fiber (DSF) or nonzero dispersion fiber (NDF), the fiber 16 needs to be farther bended to achieve the same level of optical transmission loss. This can be done by adjusting the position of the stop 17D.

Inside the housing 26, the path followed by the optical fiber 15 is such that the bending radius of the fiber is always greater than about 2 cm in the normal (armed) state illustrated by 16A. The optical transmission loss of the fiber is then negligible in all cases and the probability of a fatigue related break occurring over time is also negligible.

Reference is also made to FIGS. 2 and 3 for an illustration of the fiber segment 16, identified in its normal position at 16A in FIG. 2 and in an arc fixed at end points. Also illustrated is the arm at a position relating to the normal position of the fiber. Also indicated in FIG. 3 is the arm along with an illustration of the optical fiber in an alarm or bent condition. This is illustrated at 16B in FIG.

3.

In FIG. 8, the optical fiber 15 and the optical fiber pigtail or cable 27 are shown more in detail. In a preferred embodiment, the optical fiber pigtail comprises two loose tubes 40 and 41 surrounding input and output optical fibers 38 and 39.

Housing A housing 26 is required to protect the electronic circuit and the mechanism from the environment. In particular, the device has to remain operational when it is completely immerged in water as well as when it is located in a warm and humid environment for a prolonged period.

In the preferred embodiment, the device is built in such a way that all interface elements (the water sensor element 1, the optical fiber pigtail 27 and the position modifier 25) are located on the same face of the housing. The housing is of plastic molded around the device. A low viscosity lubricating fluid seals the bearing for the extension of the actuation shaft 14 through the chassis. The position modifier 25 which terminates the extension of the actuation shaft 14 outside the housing 26 is protected from the environment by a metal tube 23 that also extends from the housing 26 and a removable vinyl dust cap 24. The position modifier 25 is shown more in detail in FIG. 8.

In the preferred embodiment, both leads of the optical fiber 15 are protected by 900 im loose tube that run through the optical fiber cable 27. The optical fiber 15, the loose tubes 40 and 41 and the cable 27 are firmly anchored to the housing 26. Preferably, a strain relief 37 is used to prevent extreme bending of the cable 27 at the housing 26 which could result in breakage of the fibers.

In still another preferred embodiment, the electromechanical actuator is used to break the fiber in response to the actuation signal. When a fiber is broken, its optical transmission loss can increase considerably, up to the point

where light is completely blocked, and a strong optical reflection usually appears. Moreover, these changes happen with both singlemode and multimode fibers of any type and they can be observed remotely in the same way that has been described previously. However, because of the large transmission loss, it is generally not possible to continue to monitor the other sensors that may be present on the same fiberoptic link after one sensor has triggered or to use the fiber at the same time for data transmission purposes.

Furthermore, because the detection of water results in the fiber being broken and because the broken fiber cannot be replaced, this device can only be used once and its operation cannot be tested by the user.

The same electronic circuit, including the water sensor element and the solenoid, is used in this embodiment. However, the mechanical subsystem differs significantly. As shown in FIG. 11 and in FIG. 12, a sharp blade 50 made of a hard material, preferably tungsten carbide, is secured at the inside end of the plunger 52. The plunger is biased towards the outside of the solenoid with a spring 13. Preferably, the fiber 15 has its jacket removed on a short section 54 and is secured in a position such that the blade 50 can hit the bare section 54.

When the solenoid S1 is activated with an electric pulse from the control circuit, the plunger 52 accelerates towards the fiber 15 until the blade 50 impinges on the bare section 54. The device is designed so that the fiber normally breaks on the first impact. However, the cycle described above repeats itself automatically as long as the water-activated battery can power the control circuit above its threshold voltage. This increases the likelihood that the fiber will break and thus the reliability of the device.

To increase the effectiveness of the impact, the portion of the fiber that receives the impact is slightly bent in the direction of the plunger in order that the bent-induced stress in the fiber assists in the propagation of the fracture through the diameter the fiber. There is also provided a solid seat 55 under and in contact with the bare section 54 of the fiber that receives the impact in order

to provide the stiffness that is necessary for an efficient energy transfer from the plunger 50 to the fracture zone.

Having now described a limited number of embodiments of the present invention, it is apparent that numerous other embodiments and modifications thereof are contemplated as following within the scope of the present invention as defined by the appended claims.