Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-11-18
2001-12-11
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227120
Reexamination Certificate
active
06329648
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fiber optic sensor systems and more particularly to an optical fiber communication system where a relatively long wave length of light can be used in the detection of a small physical displacement , for example, for measurements of a parameter which effects a small full scale displacement therefore requiring resolution of micro-displacements for accurate measurement.
BACKGROUND OF THE INVENTION
Fiber optic sensor systems generally rely on varying the intensity of a light beam or upon measurements based on the wavelength of light. The first approach is analog in nature and, therefore, limited for high performance applications. The second approach is capable of greater performance but still has limitations.
An example of the limitations of the second approach is based upon light interference. The wavelength of a typical coherent light source is 0.83 micrometers or 33 micro-inches. An optical resonant cavity designed for this wavelength will display interference maximums and minimums at one half wavelength intervals, or 16.5 micro-inches. If strictly digital performance is desired, then, these interferences must be counted to measure a displacement. A high performance sensor, however, can easily require one part in one million resolution so that the required displacement for this type of measurement would be 16.5 microinches×one million=16.5 inches. Since this is generally not practical for parameters such as high pressure, a compromise must be msde. For example, a displacement of 50 interferences would be 0.0008 inches, which would give a resolution of 2%, based on digital techniques. Any additional required resolution for this maximum displacement would then have to be obtained from analog interpolation of phase information to further resolve the displacement between interferences.
Other disadvantages of these wavelength of light approaches are the requirement for single mode fibers and potentially sophisticated, expensive readout interface equipment. Single mode fibers are required to preserve the coherency of the light source in the optical system. These fibers have to have an extremely small diameter core (on the order of 0.0004 inches) and therefore are difficult to splice, connect and handle. The disadvantages of complex, expensive readout equipment speak for themselves.
SUMMARY OF THE INVENTION
This invention is intended to address the difficulties of high resolution by being based on a relatively low frequency light modulation (such as 5 or 10 Mhz) instead of the wavelength of light. This circumvents the need for single mode fibers and coherent light sources, but does not preclude their use. It is also an object of this invention to provide the ability to function with small sensor displacements using this long wavelength modulation as well as directly provide a high resolution digital output.
In a first embodiment of the present invention, a light source is located at one location and a sensor is located at a remote second location.
The concept is to transmit light, modulated at a relatively low frequency, from the light source to the sensor along a fiber optic line element arrangement which can be thousands of feet in length. A phase reference signal and a phase signal from the sensor are then returned back to operate a phase comparator at the location of the light source and to control the modulation frequency of the transmitted light as a function of the input displacement parameter of the sensor. The input displacement developed within the sensor is derived from an input parameter, such as pressure or temperature. The arrangement further nulls out the effects of the length of the optic fiber line element arrangement and permits measurements of micro-inch displacements of the input displacement parameter of the sensor.
In the first embodiment, light is transmitted along a first optic fiber line element to a sensor unit. At the sensor unit, a second optic fiber line element is utilized to return the transmitted light to a first photo detector located at the light source location. The light is applied to the sensor unit and reflected light from the sensor unit returns to a second photo detector at the light source.
The sensor unit includes an optical delay line which creates a phase shift of the light passing through it. When the measurement parameter changes, the distance to the reflector surface changes with respect to the optic fiber ends and this reflected light to the optic fiber ends produces a phase change of the light wave which is sensed at the second photo detector located with the light source. This change is very small for this configuration but this arrangement is shown for illustration. The method of obtaining large changes will be covered below.
The two photo detectors output to a phase comparator which senses the phase shift and drives a voltage controlled oscillator to adjust the frequency modulating the light source. This frequency is adjusted until a 180 degree phase difference exists between the inputs of the phase comparator. This frequency, then, will be determined by the combined path length of the sensor delay line and reflector, If all other delays were to cancel or be calibrated out Measuring the frequency can be used to determines the parameter measurement.
In a modification of the first embodiment, the sensor unit can obtain a reference reflector and at the other location, a reference light source. A frequency select switch is utilized to switch the measurement reflection and the reference selection between the two optic fiber line element. At the sensor unit, a second optic fiber line element is used to return the phase reference light to a first photo-detector located at the light source location. The transmitted light is also applied at the sensor unit to an optical delay line module and then to a reflector. Reflected light passes again through the optical delay line and returns to a second photo-detector at the light source location.
In a modification of the first embodiment, the sensing unit can contain a reference reflector and, at the other location, a second light source. A frequency select switch is utilized to switch the measurement and reference reflections between the two optic fiber line elements. Adding the two resultant frequencies will null out the effect of the line length.
In a second embodiment, a single optical fiber line element is utilized together with a WDM communication protocol. By using two different optical frequencies, the single optical fiber line element can process both a reference phase signal and a measurement phase signal from a sensor unit with fewer components than the first embodiment.
This invention is particularly useful in oilfield completions where the length of the line element can reach 15,000 feet or more.
REFERENCES:
patent: 6246482 (2001-06-01), Kinrot et al.
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