Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
1997-02-18
2001-03-20
Kim, Robert H. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S035500, C356S329000, C356S340000, C250S227130, C250S227180, C250S227230, C385S012000, C385S014000
Reexamination Certificate
active
06204920
ABSTRACT:
TECHNICAL FIELD
The invention generally relates to sensing systems and, in particular, an optical fiber sensor system having multiple light responsive sensors for sensing various parameters and for use in communication systems.
BACKGROUND ART
Fiber optic intracore Bragg gratings have been used in prior sensing systems as reliable, localized sensors of strain, temperature and other physical parameters. The wavelength encoding of strain and temperature in these Bragg grating sensors make them suited for monitoring structures to track strain and temperature over periods of time. Such sensors are not affected by electrical interference as compared to electrical based sensors. However, the prior systems which have employed such sensors have been very expensive and inflexible in their ability to detect different parameters. In addition, such prior systems were high in volume and weight (e.g., large and heavy) which made them unacceptable in mobile applications.
An example of such a prior art two channel system employing Bragg grating sensors is illustrated in FIG.
1
. Each channel includes a sensor array SA
1
, SA
2
which is embedded in or attached to the structure to be monitored, such as within a concrete deck support girder for a bridge. Each sensor of each array reflects light of a particular bandwidth which is different from the bandwidth of the other sensors in the array. For example, a first sensor of array SA
1
may be detecting strain and reflecting light within a bandwidth having a wavelength of 1295-1297 nanometers (nm) whereas a second sensor of sensor array SA
2
may be detecting temperature and reflecting light within a bandwidth having a wavelength of 1329-1331 nanometers (nm), the specific wavelength of reflection being related to the strain or temperature of the sensor with changes tracking generally linearly. Consequently, the sensors of each array of each channel must be illuminated by light from a different, separate broad band light source BBLS
1
which emits light corresponding to the sensor's reflective operating bandwidth.
Optical fibers transmit the light from the source via a 2×1 coupler to the sensor array and transmit the reflected light from the sensors of the array via the coupler to a tunable filter TF
1
, TF
2
driven by a waveform generator WG which is scanned to detect a narrow band of reflected light. A peak detector PD
1
, PD
2
detects the peak of any reflected light within the narrow band of the filter for each channel. Each peak detector generates a digital pulse representative of the peak of the reflected light. The digital pulses generated during scanning are used as a trigger for the A/D converter and thus are converted to a value which is proportional to a particular wavelength. Knowing the tunable filter's response, the converted values can be identified as an exact wavelength. Using a model of the sensor's relationship of wavelength to a particular parameter, a value based on this parameter can be made.
Such systems have been found to be very expensive, primarily because of the cost associated with the various light sources needed to illuminate each sensor array with light of the appropriate bandwidth and multiple tunable filters to locate the sensors. In addition, the need for various or multiple light sources tends to add the significant weight and volume of such prior art systems which limit the applicability of the system for use in mobile applications or other environments in which a light weight or compact monitoring system is needed or desired.
There is a need for an improved optical fiber sensor system which avoids these prior art deficiencies and would be useful in mobile systems such as systems which monitor the structural integrity of aircraft and systems by monitoring parameters indicating safe conditions such as structural monitors for bridges. In addition, there is a need for an improved optical fiber sensor system which could be used in telecommunications.
DISCLOSURE OF INVENTION
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an optical fiber sensor system which uses only one light source in combination with multiple arrays of sensors. The invention accomplishes this by employing a tunable light source which is multiplexed to address the sensors of the multiple arrays. The tunable light source of the invention reduces system weight and size so that systems according to the invention meet the need for an optical fiber sensor system which has a reduced weight and a miniaturized package of reduced cost. This invention is also directed to meeting the need for such a system which employs a single light source so that its cost is reduced without a loss in flexibility or accuracy.
In particular, the inventors have taken advantage of the advent of brighter low cost tunable light sources and have addressed the cost problem at the source (i.e., in the section of the layout prior to the light reaching the sensors) rather than at the detector (i.e., in the section after the light leaves the sensors on its way to the detectors), as most prior art has done. This invention provides an optical fiber sensor system which uses one tunable source of light to all the sensors. The single tunable source allows the elimination of many tunable filters located before the photodetectors and peak detectors as in the prior art. This results in a size and weight savings as well as an increase in reliability.
The implementation of a single, tunable light source now permits optical systems to be used in mobile environments, such as aircraft, while maintaining flightworthy system specifications. As a result, single light source distributed systems according to the invention prove useful in monitoring various parameters as well as in communications.
Even though the system of the invention employs a single tunable light source, the invention provides light to a plurality of sensors operating within different bandwidths. This overcomes the limitations of the prior art systems and provides a flexibility not previously recognized in the prior art. Consequently, the invention becomes particularly useful in high performance, low cost Bragg grating sensor systems.
Another advantage of the optical fiber sensor system of the invention is that it allows sensor systems to employ Bragg grating sensors in combination with a digital signal processor. Because the processor according to the invention need only process a reduced number of data points per trace, the processor quickly determines peaks so that the system is higher in speed.
The low cost and low weight advantages of the optical fiber sensor system of the invention provide a system that is useful in monitoring structural integrity over time. In particular, systems of the invention provide substantially continuous monitoring of vibration, strain, wear and other parameters. Furthermore, such parameters can be monitored in fixed structures such as bridges as well as mobile structures such as aircraft.
The adaptability of the invention also provides an optical fiber sensor system for use in telecommunications such as substation to substation communication. The flexibility of the invention also has advantages in that it provides a system which can meet a rising market need for more sensors per system while being useful in both commercial and military markets. In addition, the invention provides an optical fiber sensor system which uses simple strain gauges and provides full health monitoring of structural integrity of stationary as well as mobile systems.
In one form, the invention comprises a sensing system having a first sensor, a tunable light source, a detecting circuit and a processor. The first sensor has a first operating bandwidth and modifies light provided to it at a wavelength representative of or indicative of a first sensed parameter. The tunable light source provides light which illuminates the sensor. This provided light has a wavelength which varies over the first bandwidth. The detecting circuit provides a signal representative
Belk John H.
Ellerbrock Philip J.
Johnson Bartley C.
Kim Robert H.
Lee Andrew H.
McDonnell Douglas Corporation
Senniger Powers Leavitt & Roedel
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