This invention relates to an apparatus and method for monitoring a structure which employs a waveguide transmissive counter-propagating signal method and associated systems for locating events in fibre optic sensing systems.
The term xe2x80x9cstructurexe2x80x9d used in this specification and in the claims should be understood to mean machines, buildings, infra-structure such as pipe lines and the like to which the apparatus and method may be applied as well as waveguides themselves which act as a communication link for transmission of data from one place to another.
It should also be understood that the term xe2x80x9clightxe2x80x9d used in the specification and claims means both visible and non-visible parts of the electromagnetic radiation spectrum.
Optical devices are commonly used in industry and science and include laser cavities, waveguides, lenses, filters and other optical elements and their combinations. Such optical devices are used in a variety of instruments and installations.
Photonics technology has revolutionised the communications and sensor fields. This is mainly due to the rapid development of optical and opto-electronic devices. A wide variety of glass materials, material-dopants and waveguide structures are available and this invention relates to a waveguide transmissive counter-propagating signal method and associated systems for locating events in fibre optic sensing systems.
Presently, there is a very high demand for sensors and systems that provide real-time monitoring of the integrity or condition of structures such as machines, buildings and fibre optic communication links. Fibre optic sensors, in particular, are very promising for these applications because of their dielectric properties, their fine size, their ability to be remotely located and, in the case of intrinsic sensors, rapid response times. They also have particular advantages in hazardous environments. In addition, they have several clear advantages over existing conventional sensing techniques such as bulk optical measurements, potentiometric electrodes, resistive foil gauges and piezo-electric transducers.
Engineered structures are usually not monitored in real-time because of the difficulties in incorporating conventional sensors into the sensing environment and because of the limitations of the sensors. Furthermore, conventional sensors are generally point sensing devices, thus requiring a large number of sensors to cover a large area or long length of interest. The subsequent cost and complexity of such a system is most often impractical.
Fibre optic sensors overcome many of these difficulties by virtue of their inherent properties. In addition, optical sensors and optical processing systems are extremely fast and do not suffer from electro-magnetic interference (EMI), unlike their electronic counter-parts. The technology is gaining wide acceptance for monitoring applications and is expected to play a major role in the realisation of real-time structural integrity and machine condition monitoring systems, offering an advanced new generation of engineering sensors.
Fibre optic sensor technology has progressed at a rapid pace over the last decade. Many different sensing techniques have been developed to monitor specific parameters. Different configurations of fibre sensing devices have been developed for monitoring specific parameters, each differing by the principle of light modulation. Fibre optic sensors may be intrinsic or extrinsic, depending on whether the fibre is the sensing element or the information carrier, respectively. They are designated xe2x80x9cpointxe2x80x9d sensors when the sensing gauge length is localised to discrete regions. If the sensor is capable of sensing a measurand field continuously over its entire length, it is known as a xe2x80x9cdistributedxe2x80x9d sensor; xe2x80x9cquasi-distributedxe2x80x9d sensors utilise point sensors at various locations along the fibre length. Fibre optic sensors can be transmissive or can be used in a reflective configuration by mirroring the fibre end-face. So, fibre optic sensors are actually a class of sensing device. They are not limited to a single configuration and operation unlike many conventional sensors such as electrical strain gauges and piezoelectric transducers.
However, to-date most fibre optic sensor systems are based on point sensing devices, thus again requiring a large number of sensors to cover a large area or long length.
Very few distributed techniques have been developed and are commercially available. Of those that have been developed, fewer still have the capability to actually locate the region or position of the sensed parameter or disturbance along the fibre length; they simply detect, alert and sometimes quantify that an event has occurred.
Methods devised in the prior art for distributed sensing that are capable of locating the position of the sensed parameter include:
Most current techniques for monitoring fibre optic cable integrity are based on static or slowly varying measurements employing optical time domain reflectometry (OTDR) (ie., sharp bends, fibre fracture, fibre attenuation, connector losses, etc.). This method is essentially based on an optical radar technique, where a very narrow pulse of light launched into an optical fibre is back-scattered or back-reflected by anomalies in the fibre material or structure along its length (ie., fracture, localised compression, fault, etc.) and the measured time-of-flight determines the locations of the anomalies.
Fibre Optic Distributed Temperature Sensor (DTS) systems have been developed for continuous temperature measurements along the entire length of an optical fibre, and any surface or structure which the fibre is attached to. In the majority of distributed temperature sensing, the ratio of the intensity of the Stokes and anti-Stokes return signals are measured in an optical time domain reflectometry (OTDR) configuration. The end result is a true measurement of the temperature profile along the entire length of the sensor.
Various OTDR back-scattering techniques for strain and pressure measurements have also been investigated, although no commercial technology is yet available.
Physical placement of Sagnac interferometer loops at specific locations or geometric configurations have also been used for distributed fibre disturbance detection and location. In a Sagnac interferometer, light is launched into opposite ends of a sensing fibre loop such that two beams circulate through the loop in opposite directions and then recombine to produce a phase interference pattern on a single photodetector. No use of the time of travel or time delay between the counter-propagating signals is used in these methods.
The most common methods for locating events are based on techniques using the back-scattering or back-reflection of extremely narrow pulses of laser light, combined with some other form of sensing mechanism to extract further information about the actual sensed parameter (ie., temperature, strain, pressure, etc.). However, while modern advances in photonics devices have allowed very precise and accurate systems to be developed and commercialised, they are often very complex and expensive. The main reasons for the complexity and high cost of these units is largely in the requirement for very high accuracy and high speed components needed in order to generate extremely narrow pulses of laser light, detect optical signals of extremely low power (often this involves photon-counting and significant averaging of the signals), and provide extremely accurate timing for the time-of-flight measurements of the light pulses.
Owing to the requirement of measuring and averaging the time-of-flight of very narrow, low power pulses, these techniques are often limited to monitoring static or very slowly varying parameters. In addition, to-date most systems based on this principle monitor only temperature. However, they may offer one significant advantage over most other techniques, including that of the present application; namely, the ability to provide the profile of the sensed parameter along the entire length of the fibre.
Nevertheless, it would be a significant advance to be able to also obtain real-time, quasi-static and dynamic information about any form of disturbance to the optical fibre, particularly transient events which are too quickly occurring to detect with OTDR techniques. This can be achieved by combining a distributed sensing technique incapable of locating the events with a compatible technique which is capable of locating the events. This would have the further advantage of monitoring any structure or material near the fibre or to which the fibre is attached. Such a capability should enable truly distributed sensing applications such as structural integrity monitoring, pipeline leak detection, ground monitoring, machine condition monitoring and intrusion detection of high security areas.
The object of the present invention is to provide a waveguide transmissive counter-propagating signal method and associated systems for locating events in waveguide sensing systems.
The present invention provides an apparatus for monitoring a structure and for locating the position of an event including;
a light source;
a waveguide for receiving light from the light source so that the light is caused to propagate in both directions along the waveguide to thereby provide counter-propagating optical signals in the waveguide, the waveguide being capable of having the counter-propagating optical signals or some characteristic of the signals modified or effected by an external parameter caused by or indicative of the event to provide modified counter-propagating optical signals; and
detector means for detecting the modified counter-propagating optical signals effected by the parameter and for determining the time delay or difference between the modified counter-propagating optical signals in order to determine the location of the event.
The present invention relies on the measurement of the time delay or difference between transmissive counter-propagating optical signals affected by the same event. In this novel arrangement, optical signals, preferably continuous-wave (CW) optical signals, are launched, preferably from a single light source, into the waveguide and simultaneously detected by a detector such as two separate photodetectors. Pulsing of the optical signal is not necessary, although it may be employed in some arrangements. Any sensed parameter which acts to alter the counter-propagating signals will effect both signals in the same manner, but because the effected counter-propagating signals must each continue travelling the remainder of the waveguide length to their respective photodetectors there is a resultant time delay or time difference between the detected signals. The time delay is directly proportional to the location of the sensed event along the waveguide length. Therefore, if the time delay or difference is detected and measured, the location of the event can be determined. At the same time, if a compatible sensing mechanism is being employed the sensed event can be quantified and/or identified (ie., strain, vibration, acoustic emission, temperature transient, etc.). In addition, non-sensitive fibre optic delay lines may be connected to the waveguide at either or both ends in order to add additional delay between the transmissive counter-propagating signals and to provide insensitive lead fibres. This may assist engineering the technique into a practical working system.
The invention also has the advantage of operating on virtually any existing type of transmissive distributed fibre optic sensor, enabling dynamic and transient events to be detected, quantified and located anywhere along the length of the optical fibre which forms the waveguide. Furthermore, it operates in a transmissive configuration, thus, delivering substantially the entire optical signal and power back to the detector and not requiring signal averaging, and it determines the location of events via the time delay measurement between counter-propagating optical signals effected by the same disturbance.
Examples of non-locating distributed fibre optic sensing techniques which the present invention could be compatible with, without imposing any limitations, include:
Modalmetric interferometers
Sagnac interferometers
Michelson interferometers
Long-length Fabry-Perot interferometers
Mach-Zehnder interferometers
Two-mode interferometers
Preferably the waveguide is a silica waveguide.
Preferably the light from the light source is launched simultaneously into opposite ends of the waveguide.
Preferably the light source is a single light source. However, in other embodiments two CW or synchronised light sources could be used for launching light simultaneously into opposite ends of the waveguide.
Preferably the waveguide is one or more optical fibres which forms an event sensitive optical fibre.
Preferably further silica waveguides are connected to the sensing waveguide at either or both ends in order to add additional delay between the transmissive counter-propagating signals and to provide insensitive lead waveguides.
Preferably the detector means comprises:
first and second photodetectors for simultaneously receiving the radiation from the counter-propagating signals in the silica waveguide; and
processing means for receiving signals from the first and second photodetectors for determining the time delay or difference between the signals effected from the same disturbance and therefore determining the location of the sensed event.
Preferably a waveguide coupler or set of couplers is arranged between the light source and photodetectors and the silica waveguide so that the light can be simultaneously transmitted from the light source to both ends of the silica waveguide and the detector means also being connected to the coupler or couplers so that the counter-propagating transmissive radiation can be directed via the coupler or couplers from the silica waveguide to the detector means.
The invention also provides a method for monitoring a structure to locate the position of an event, including the steps of;
launching light into a waveguide so that the light is caused to propagate in both directions along the waveguide to thereby provide counter-propagating optical signals in the waveguide, the waveguide being capable of having the counter-propagating optical signals or some characteristic of the signals modified or effected by an external parameter caused by the event, to provide counter-propagating optical signals; and
detecting the modified counter-propagating optical signals effected by the parameter and for determining the time delay or difference between the modified signals in order to determine the location of the event.
Preferably the light is launched into both ends of the waveguide to provide the counter-propagating signals.
Preferably the light is launched into both ends of the waveguide from a single light source.
The preferred embodiment provides a waveguide transmissive counter-propagating signal method and associated systems for locating events in optical waveguides, which may include:
providing an optical fibre (single or multi moded) formed from a waveguide material designed to simultaneously transmit counter-propagating optical signals;
providing a sensor configuration (single or multi moded), with any appropriate waveguide length, any suitable geometry and compatible with the waveguide transmissive counter-propagating signal method and associated systems for locating events in optical waveguides, designed to optimise the sensor sensitivity and detection capabilities;
providing a lead optical fibre (single or multi moded) formed from a waveguide material which acts as an insensitive light guide between the sensing fibre and the sensing and locating system optics and optoelectronics interface;
providing a lead optical fibre (single or multi moded) formed from a waveguide material which acts as an insensitive light guide between the sensing fibre and the excitation source;
fusion splicing, or otherwise connecting, the sensor waveguide and the lead optical fibres so that cores of the waveguides are aligned and remain fixed at the splice;
delivering the counter-propagating signals from the waveguide sensor, via the lead optical fibres, to an appropriate optical and electronic arrangement such that the time delay or difference between the signals may be measured and utilised to determine the location of the sensed event; and
registering any changes in the waveguide sensor optical signals that may be utilised with a compatible sensing technique, such that the sensed parameter may be quantified and/or identified.
The preferred embodiment may also be said to reside in a method for producing a waveguide transmissive counter-propagating signal method and associated systems for locating events in optical waveguides, including, but not limited to, the steps of:
Preparing an optical fibre (single or multi moded) formed from a waveguide material designed to simultaneously transmit counter-propagating optical signals.
Preparing a sensor configuration (single or multi moded), with any appropriate waveguide length, any suitable geometry and compatible with the waveguide transmissive counter-propagating signal method and associated systems for locating events in optical waveguides, designed to optimise the sensor sensitivity and detection capabilities.
Preparing a waveguide sensor and optical fibre lead by cleaving or polishing their ends so as to establish a flat, smooth surface. After taking necessary precautions to remove any contaminants from the cleaved or polished waveguide sensor and fibre lead end-faces, the waveguide sensor and fibre lead are placed end-to-end in a fusion splicing apparatus and fused together using the appropriate or desired fusion are times and currents. The fusion splicing procedure may be repeated a number of times if necessary. The core and overall diameters of the waveguide sensor and fibre lead are not limited and translation stages or V-grooves may be used on the fusion splicing apparatus to centrally align the cores of the waveguide sensor and fibre lead before the fusion splicing procedure. Different combinations of waveguide sensor and fibre lead may require a different or unique set of fusion splicing parameters.
Cleaving or polishing the waveguide sensor at any location after the fusion splice so as to establish a flat, smooth surface. The position of the cleave or polished surface establishes the localised length or sensing region of the sensor. After taking necessary precautions to remove any contaminants from the cleaved or polished waveguide sensor end-face, it is fusion spliced at a desired location to a second fibre lead.
Preparing or connectorising the free ends of the fibre leads in any manner which facilitates attaching, connecting, splicing or coupling the fibre leads to the appropriate combination and arrangement of light source, couplers, photodetectors and signal processing electronics which achieves the transmissive counter-propagating signal method for locating events in optical waveguides.
Preferably the manufactured sensor and/or the exposed fusion spliced region(s) may be protected by encapsulating or coating the desired region in a suitable device or material (ie., heatshrink fusion splice protector, acrylate, enamel, epoxy, polyimide, etc.).
In a preferred embodiment the sensor waveguide is a multimode fibre and the lead fibres are singlemode fibres.
In other embodiments a plurality of multimode fibres and singlemode fibres are fusion spliced in end-to-end relationship to form several sensitive and insensitive regions along the entire fibre assembly.
In other embodiments a plurality of singlemode fibres are fusion spliced to respective multimode fibres and the plurality of singlemode fibres are connected to a coupler which in turn is connected to a further singlemode fibre to form a multiplexed sensor arrangement.
In an alternate arrangement, the sensor fibre may be replaced by two or more suitably configured optical fibres (single or multi moded) and additional couplers may be utilised to connect the plurality of sensor fibres to the fibre optic leads. In this arrangement, a further number of couplers and photodetectors may be required at the instrumentation to facilitate the increased number of sensing and lead fibres.
In a preferred embodiment of the above alternate arrangement the sensing part is formed by two suitably configured singlemode fibres and the insensitive leads are singlemode fibres. The two sensing fibres are connected to a lead fibre by the use of a singlemode coupler at either or both ends.
The present invention is effective on any optical waveguiding distributed sensing technique that may be arranged in a transmissive configuration. In a preferred embodiment, but without limitation, the distributed sensing technique is based on a modalmetric technique utilising the fusion splicing of insensitive singlemode fibre to sensitive multimode fibre. In yet another preferred embodiment, but without limitation, the distributed sensing technique is based on a Mach-Zehnder or Michelson interferometer utilising two singlemode fibres as the sensitive region.
Preferably the waveguide comprises at least one optical fibre and/or at least one optical fibre device. In some embodiments of the invention the waveguide may merely comprise an optical fibre without any additional elements. However, the optical fibre can include passive or active elements along its length. Furthermore, the optical fibre can include sensing elements along its length and those sensing elements can comprise devices which will respond to a change in the desired parameter in the environment of application and influence the properties and characteristics of the electromagnetic radiation propagating in the waveguide to thereby provide an indication of the change in the parameter.
Preferably any suitable CW or pulsed single-frequency or multiple wavelength source or plurality of sources may be employed. In a preferred embodiment, without limitation, a CW or pulsed coherent laser diode is utilised to supply the optical signal. In an alternate arrangement, multiple light sources, of the same or varying wavelengths, may be used to generate the counter-propagating signals. The preferred embodiments of the present invention offer the potential to utilise all-fibre, low-cost optical devices in conjunction with laser diodes, light emitting diodes, photodetectors, couplers, isolators, circulators and filters. Any suitable light source, coupler and photodetector arrangement may be used with the sensor and locating systems. In a preferred embodiment, the required optical properties of the light source are such that light may be launched into and propagated in the singlemode waveguide. For localisation, the light propagated in a singlemode fibre must remain singlemoded during the entire period of travel in the singlemode fibre. Once the light is launched into the multimode fibre from the singlemode fibre, several modes may be excited and the multimoded fibre will be sensitive to various parameters. Once the light is launched back into the singlemode fibre from the multimode fibre, only a single mode is supported and travels to the optical components of the system. Lead-in/lead-out fibre desensitisation and sensor localisation is achieved in this manner. In practical applications, the singlemode fibre should be made sufficiently long to attenuate all cladding modes in order to improve the signal-to-noise ratio. This preferred embodiment applies for both directions of travel of the transmissive counter-propagating optical signals.
Utilisation of properties and characteristics of the electromagnetic radiation propagating in the waveguide sensor enables monitoring to take place in a non-destructive manner. Thus, the sensor is not necessarily damaged, fractured or destroyed in order to monitor and locate the desired parameter.
In the method, according to the preferred embodiment of the invention, electromagnetic radiation is launched into an optical waveguide (single or multi moded), such as an optical fibre, from a light source, such as a pigtailed laser diode, fibre laser or light emitting diode, and propagates along the optical waveguide. The optical waveguide is connected (temporarily or permanently) to one arm of an optical waveguide light splitter or coupler and when the electromagnetic radiation reaches the light splitter the electromagnetic radiation can branch out into the two output waveguide arms of the light splitter. Each of the output arms of this light splitter are fusion spliced to other couplers, thus the optical radiation from the laser source is simultaneously launched into each of the other two couplers. These two couplers form the launch and detection ports for the dual-ended, counter-propagating method described above. The optical signal is simultaneously launched to the output waveguide arms of the couplers. Only one output arm is used in each coupler, the other is fractured or otherwise terminated to avoid back-reflections. The output arms of the couplers are either connected (temporarily or permanently) directly to the waveguide sensing element or to a lead optical waveguide which is connected (temporarily or permanently) to the waveguide sensing element. Any one of the output waveguide arms of the light splitter may be used to deliver the electromagnetic radiation to the sensor waveguide via an optical waveguide lead. Likewise, a plurality of output waveguide arms may be used to deliver the electromagnetic radiation to a number of individual or multiplexed waveguide sensors. Each of the counter-propagating signals transmitted into the waveguide sensor propagates along the entire length of the waveguide until they reach the opposite ends and are launched back into the latter couplers in the opposite direction to the initial launch signals. The signals are each split in the reverse direction through the latter couplers. Part of the signals travel back towards the first coupler and laser, and the remainder of the signals travel along the unused arms of the latter couplers, which are terminated at photodetectors. The optical signals are simultaneously monitored by the two photodetectors. Appropriate electronics, signal processing schemes and algorithms process the signals from each detector and provide the location of the sensed event by determination of the time delay or difference between the signals effected by the same disturbance. The insensitive fibre optic leads may be very long to provide an additional time delay between the optical signals. This may assist engineering the technique into a practical working system.
In the method, according to an alternate preferred embodiment of the invention, the sensing section is formed by two or more suitably configured fibres (single or multi moded) and the insensitive leads are singlemode fibres. The plurality of sensing fibres are connected to the lead fibres by the use of additional singlemode couplers at either or both ends of the sensing fibres.
Preferably the instrument optical and electronic arrangements will utilise noise minimisation techniques.
Preferably, all the optical and electrical components will be located in a single instrument control box, with individual optical fibre input ports.
Electro-optic devices, acousto-optic devices, magneto-optic devices and/or integrated optical devices may also be utilised in the system.
The invention provides an apparatus for monitoring an optic fibre communication link into which data signals are launched and from which the data signals are received, and for locating the position of a disturbance to the link including;
a light source for launching light into the link so that the light is caused to propagate in both directions along the link to thereby provide counter-propagating optical signals in the link, the link being capable of having the counter-propagating optical signals or some characteristic of the signals modified or effected by the disturbance to provide modified counter-propagating optical signals; and
detector means for detecting the modified counter-propagating optical signals and for determining the time delay or difference between the modified counter-propagating optical signals in order to determine the location of the disturbance.
The invention still further provides a method for monitoring an optical fibre communication link into which data signals are launched and from which the data signals are received, to locate the position of a disturbance to the link, including the steps of;
launching light into the link so that the light is caused to propagate in both directions along the link to thereby provide counter-propagating optical signals in the link, the link being capable of having the counter-propagating optical signals or some characteristic of the signals modified or effected by the disturbance to provide counter-propagating optical signals; and
detecting the modified counter-propagating optical signals effected by the disturbance and for determining the time delay or difference between the modified signals in order to determine the location of the disturbance.