Optical waveguides – Optical waveguide sensor – Including physical deformation or movement of waveguide
Reexamination Certificate
2001-07-26
2003-12-23
Kim, Robert H. (Department: 2871)
Optical waveguides
Optical waveguide sensor
Including physical deformation or movement of waveguide
C385S012000, C385S037000, C356S035500
Reexamination Certificate
active
06668105
ABSTRACT:
BACKGROUND OF THE INVENTION
A number of strain sensor technologies exist that apply to a wide variety of commercial, military, and industrial markets. Resistance strain gages have been the most widely used in the past, and are the most readily available technology at this time. A variety of configurations are available, including those with limited temperature compensation and endurance to harsh environments. However, the inherent disadvantages of resistance strain gages, including installation costs, complexity, weight, long-term measurement drift, susceptibility to electromagnetic noise, and dangers from electrical power requirements, have limited their application in certain fields.
More recently, a new variety of strain sensors have been developed based on fiber optic technology, such as extrinsic Fabry-Perots, in-line fiber etalons, intrinsic Fabry-Perots and Bragg gratings. All of these optical sensors share a common property in which imposed strains on the respective sensor portion of optical fibers alter the transmitted electromagnetic spectrum in a manner that can be detected and measured by optical interrogation instrumentation. This optical sensor technology has overcome many of the difficulties inherent in resistance strain gages and electrical transmission networks. Among the many advantages are:
Accuracy: Optical sensors are self-referencing, with virtually unlimited resolution. This means that measurements are absolute, providing long-term stability that does not require frequent sensor calibration or bridge balancing as required with resistance gage technology. In addition, because these sensors are optical in nature, they do not emit electromagnetic noise and are themselves not susceptible to electromagnetic interference from other electrical systems.
Safety: Optical fiber sensors cannot generate sparks or other forms of heat that might cause unwanted fires or explosions in the presence of fuels or other combustible sources.
Reliability: The mechanical strength of optical fibers has been found to approach three times that of the strongest carbon steel. Hermetic coated fibers are immune to corrosion and do not induce corrosion on metals. In addition, environmental tests conducted to date suggest that temperature and humidity have limited influence of the optical characteristics of Bragg gratings.
Weight and Space Savings: Compared with copper wires used in conventional sensing systems, optical fibers are four times lighter per unit volume and provide greater than ten thousand times more signal bandwidth in a smaller cross section.
Within the past two decades, a number of manufacturers have attempted to exploit this fiber optic sensor technology, with limited results. The costs associated with the electronic and optical systems required to interrogate the fiber optic sensors were prohibitively high for most applications. Application of fiber optic technology to the growing telecommunications industry has greatly mitigated this problem. The drive to multiplex as many communications channels within a single data path as possible has revolutionized the telecommunications industry, and further developments are expected to increase multiplexing capability by orders of magnitude in the future. This will be accompanied by constant improvements in the performance and reliability of optical fiber components, while at the same time, reducing costs. This multiplexing capability will for the first time allow for a high density of strain sensors to be implemented in a single optical network, thus permitting a large distributed sensing capability as required by such emerging areas as the structural health monitoring field.
A key characteristic that has limited the application of fiber optic strain sensors is their inherent sensitivity to temperature variations. This sensitivity makes measurements due to thermal variations indistinguishable from mechanical strain measurements. The temperature sensitivity of fiber optic sensors will continue to limit their application to most fields, unless additional means are implemented to compensate for temperature variations. Some applications of fiber optic strain sensors have solved this temperature dependence problem, such as by the use of Fabry-Perot sensors to decouple the temperature dependent characteristic of optical fibers. However, in most cases, this is achieved at the sacrifice of multiplexing capability.
The present invention provides for temperature compensation and multiplexing capability in a single robust, hermetic sensor package. The present fiber optic sensor “flatpack” houses and protects two fiber optic sensors in a single pre-packaged unit, which compensates for temperature. The present flatpack simplifies the installation of sensors by field personnel by exploiting the adhesives and installation techniques used with well-established resistance strain gage technology.
The present fiber optic sensor flatpack is applicable to a diverse range of applications in defense, civil infrastructure, and industrial applications. For example, in the defense area, the technology is applicable to service life extension programs of ships and submarines. In the civil infrastructure area, the technology is applicable to monitoring the condition of bridges, dams, and highways. In the industrial area, the technology is applicable to various uses in production facilities, transportation, construction, aerospace or wherever structural integrity must be monitored.
The present fiber optic sensor flatpack relies on a number of technologies, as described below.
For example, fiber Bragg gratings (FBG) recently were developed as narrow band optical filters for the telecommunications industry, enabling the transmission of a large number of telephone calls on the same optical fiber link. This is made possible with wavelength division multiplexing (WDM) using FBGs or equivalent spectral filtering technology. This WDM capability can be combined with conventional time and spatial division multiplexing schemes, based on optical switching technology, to enable literally hundreds of sensors to be multiplexed and decoded using the same hardware used to decode just a few sensors.
FBGs are fabricated by exposing a germanium doped or boron co-doped optical fiber to a periodic intensity distribution, as shown in FIG.
1
. These fibers are photosensitive, in that the refractive indices thereof change when exposed to ultraviolet (UV) light. Because of this photosensitivity, the impinging sinusoidal intensity distribution results in a sinusoidal refractive index distribution in the fiber core. The combined effect of the periodic index distribution is the reflection of light at a very specific wavelength known as the “Bragg wavelength.” This wavelength is predictable in terms of the mean refractive index n and the pitch of the periodicity &Lgr; given by &lgr;
B
=2n&Lgr;.
Sensors are made from these FBGs by exploiting the characteristic that the grating pitch and refractive index are both functionally dependent on strain. Therefore, strain on and temperature of the grating causes the Bragg wavelength to shift left or right. The wavelength-encoded nature of FBGs offers the greatest potential for multiplexing these sensors in the wavelength domain along a single length of optical fiber.
Multiplexing is accomplished by producing an optical fiber with a sequence of spatially separated FBGs, each having a different pitch &Lgr;
k
, where k=1, 2, 3, . . . , N. The resulting Bragg wavelengths associated with each pitch therefore are given by &lgr;
Bk
=2n&Lgr;
k
, where k=1, 2, 3, . . . , N. Because the unstrained Bragg wavelength of each FBG is different, the information from each sensor is individually determined by examining the wavelength spectrum. For example, where a strain field is uniquely encoded as a perturbation to Bragg wavelength &lgr;
B2
, at 1558 nanometers, the Bragg wavelengths associated with the other two gratings &lgr;
B1
and &lgr;
B3
remain unchanged.
Because both the grating pitch A and refractive index n change with te
Chen Peter C.
Chen Shiping
Caley Michael H
Dykema Gossett PLLC
Kim Robert H.
Systems Planning & Analysis, Inc.
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