Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2001-06-04
2003-09-16
Kim, Robert H. (Department: 2882)
Optical waveguides
Optical waveguide sensor
C356S483000, C250S227140, C250S550000
Reexamination Certificate
active
06621947
ABSTRACT:
FIELD OF THE INVENTION
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 “structure” used in this specification and in the claims should be understood to mean machines, buildings, infrastructure 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 “light” used in the specification and claims means both visible and non-visible parts of the electromagnetic radiation spectrum.
ART BACKGROUND
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 electromagnetic 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 “point” 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 “distributed” sensor; “quasi-distributed” 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 lengt
Goode Jason R
Katsifolis Jim
Tapanes Edward E
Future Fibre Technologies Pty Ltd
Kim Robert H.
Michael & Best & Friedrich LLP
Suchecki Krystyna
LandOfFree
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