Patent Application
20090290147
The present invention is directed generally to optical fiber sensors, and more particularly to dynamic polarization based optical fiber sensors for detecting dynamic events acting on optical fibers.
The present invention involves a proposed solution to address some of the shortcomings and complexity experienced with fiber sensing techniques applied to respond to dynamic events acting on an optical fiber. Dynamic sensing is used to track and measure events with some frequency or time-resolved component—typically above 20 Hz-30 Hz, such as vibration, acoustic, rotation rate, pressure, temperature, magnetic field, or other physical parameter that alters light propagation in an optical fiber. These changes are tracked over time and processed to provide a measurement of some parameter acting on a length of fiber assembled in a sensor transducer. Typically this measurement is performed using phase sensitive optical interferometers which, although highly sensitive, are difficult to construct and involve complex and expensive signal detection and processing equipment and software. This limits the cost effectiveness of the interferometric approach to address a number of applications beyond ones that can justify a high cost per sensing point.
Solutions to the above-mentioned problems include dynamic fiber optic sensors which include intensity-modulated sensors that measure power changes in a fiber under stress such as in vibration monitoring, or light scatter intensity in a medium such as in a gas flow meter. Such sensors also include some highly specialized spectrally or wavelength-modulated sensors using extrinsic Fabry-Perot devices of fiber Bragg gratings configured to respond to dynamic events. These sensors have been demonstrated in laboratory and some low volume niche applications; however the bulk of commercially successful dynamic sensors are interferometric based which leverage the sensitivity achievable with the technology in a number of relatively high performance applications.
These sensors are constructed among a number of classical interferometer configurations such as Fabry-Perot, Mach Zehnder, and Michelson typically used in commercial acoustic, flow, and seismic sensing; and Sagnac in inertial and magnetic field sensing. There are also some emerging interferometric-based intrusion detection systems used in asset and facility security systems that use a range of configurations.
Interferometric sensors measure slight dynamic fiber path-length changes that result in phase change of light propagating down the sensing fiber. These changes are detected as an intensity signature of frequency peaks or fringes that are processed electronically and interpreted as path length changes over time. This is then correlated to the magnitude of the measurand over time. In some cases, multiple interferometric sensors are arranged in an array of sensors to track speed of an event as in the case of acoustic wave velocity in seismic sensing, or velocity of pressure disturbances in flow meters. To resolve these measurements requires complex optical interrogation equipment, including expensive modulation and receiver modules, and relatively complex processing electronics and software. The high cost of this interrogation is compounded in multi-point sensing such as acoustic systems (seismic) in which sensor interrogation equipment becomes unwieldy and prohibitively expensive in all but the most critical applications. In addition the construction of the transducer and sensing fiber packaging becomes quite demanding in the precision of fiber lengths and fiber mounting or coil winding which becomes a significant cost component of the system.
There is an ongoing need for a simpler and more inexpensive approach to accurately detecting dynamic events occurring along an optical fiber.
In accordance with the present invention, an optical fiber sensor system to detect dynamic events includes a single mode optical fiber which serves as the sensing element. The fiber single mode propagation is due to the small size of the fiber core in which by design only a limited number of wavelengths will transmit above the specified fiber operating wavelength. Single mode fiber however supports two subsequent polarization modes or eigenmodes, which in a perfectly circularly symmetric fiber are degenerate with identical propagation velocity. In practical application however, slight fiber imperfections and external perturbations acting on the fiber will break the degeneracy, creating a difference in propagation velocity between the polarization modes, so that the fiber becomes birefringent. The polarization state of light launched into the fiber will transform slightly because of slight intrinsic waveguide imperfections, a result of the fiber manufacturing process. This polarization state will be further transformed due to external perturbations or stresses acting on the fiber that couple power between the polarization modes. Besides the inevitable mechanical bending encountered when installing or packaging the fiber, most external stresses are dynamic due to changing environments from a range of thermal, mechanical, vibrational, acoustic, and magnetic effects of which fiber polarization and birefringence can be quite sensitive. Detecting these dynamics events according to this invention is accomplished by configuring a linear polarizing component in communication with the sensing optical fiber. The linear polarizing component includes a polarization sensing fiber to be disposed adjacent to—preferably collinear with—the optical fiber. A light source communicates with the linear polarizing component for generating a light signal along the optical fiber. A reflector is disposed along the optical fiber for reflecting the light signal along the optical fiber. An optical detector communicates with the linear polarizing component. A signal processor communicates with the optical detector and is configured for determining from the reflected light signal dynamic events along the optical fiber.
With reference to
The system 10 directly measures any perturbation imparted onto the structure of the optical fiber 12 which causes a modulation of the birefringence of the waveguide or creates an exchange of the light energy from one orthogonal propagating mode to the other (cross coupling). These perturbations can be the result of, for example, pressure disturbances, vibration, temperature, or acoustic waves.
The optical system 10 can be mathematically modeled using Jones calculus matrices as follows:
Where eox,y represent the output light vectors and einx,y represent the input light vectors. The function g(t) is the signal modulating the birefringence of the sensing waveguide. The objective of the signal processing system is to reproduce the function g(t) electronically with very high spectral fidelity so that application specific analysis can be completed. The architecture shown in
With reference to
An optical detector 122 communicates with the polarizer/analyzer circuit 108 for sensing the reflected light signal. A signal processor 124 communicates with the optical detector 122 for processing information extracted from the reflected light signal. The system 100 is configured to allow multiple sections of the same optical fiber to function as stand alone sensors providing an array type feature.
The fiber Bragg grating (FBG) reflectors 112, 114, 116 are configured to reflect the same wavelength slot of the source light. In this case it is necessary to process the signals in the time domain which can be performed in conjunction with pulsing of the light source 110. Using a pulsed system with the timing characteristics shown in
With reference to
The pulse width, and duty cycle of the light source 110 is chosen to coincide with the length of the sensor to enable the deconvolution of each sensor cell. In an alternative configuration a wavelength division multiplexing (WDM) system can be employed to also allow the analysis of each sensing cell independently. This requires using FBGs with different wavelengths, but alleviates the length restriction of the sensor as well as avoidance of any pulsing electronics in the source and signal processor. A WDM demultiplexer is preferably incorporated into a receiver unit so that each section as defined by wavelength of the corresponding FBG is individually processed. A WDM configuration is shown by way of example in
Turning to
A WDM demultiplexer 322 includes an input 324 coupled to the polarizer/analyzer circuit 308, and includes three outputs 326, 328, 330 each coupled to a corresponding one of three optical detectors 332, 334, 336. A signal processor 338 communicates with the optical detectors 332, 334, 336 via respective outputs 340, 342, 344 of the optical detectors for processing information extracted from the reflected light signal.
Slow drifts of the polarization state of the optical signal are very commonplace in standard (Non PM) fibers and are difficult to detect. However, the detection of AC type signals and especially the comparison of these signals from separate portions of the optical fiber over a very short period avoids the need for any absolute calibration. Any slow drift component is essentially the same to all of the sensors and can be eliminated easily using any common-mode rejection algorithm.
The use of a polarization based optical sensor in accordance with the present invention can be used to directly measure very minute perturbations applied to a sensing fiber section. Typically this measurement has been performed using phase sensitive optical interferometers. This method requires complicated processing and pulsing electronics as well as ultra precise location of sensing fiber lengths. Both of these issues limit the cost effectiveness of the interferometric approach from both a hardware/software complexity and manufacturing/test perspective. The polarization architectures presented in the present application are relatively simple to manufacture and require low cost signal processing electronics. In addition the light source required for the polarization sensor can be a broad band low coherence source as compared to the more complex laser sources needed for the interferometric architectures. The sensitivity of the polarization based optical sensor can be enhanced by the use of special fiber waveguide designs such as operation at or near second mode cutoff wavelength (high V value) and low-birefringence twisted or spun fiber, and fiber coatings that impart sensitivity or improved coupling to the measurand such as high modulus (preferably about Shore D 70 or higher) polymers for acoustic sensing.
Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present invention.
