Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1998-09-02
2001-06-26
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06252656
ABSTRACT:
BACKGROUND OF INVENTION
1. Technical Field
The present invention relates to a distributed sensing system; and more particularly, to a distributed seismic sensing system for sensing seismic disturbances and other phenomenon.
2. Description of the Prior Art
Seismic sensing systems are known in the art. For example, U.S. Pat. No. 5,623,455), issued to Norris.
One disadvantage of Norris's seismic data acquiring system is that the recording units RU and data acquisition units DAU use batteries which are expensive and time consuming to replace, and the recording units RU and data acquisition units DAU are also heavy devices that are not easily handled during deployment.
Moreover, the use of a fiber having fiber Bragg Gratings in combination with a piezoelectric transducer is shown and described in U.S. Pat. No. 5,675,674, issued to Weis, entitled “Optical Fiber Modulation and Demodulation System,” hereby incorporated by reference. See also U.S. patent application Ser. No. 08/933,544, filed Sep. 19, 1997, U.S. Pat. No. 5,986,749 hereby incorporated by reference, which shows and describes a fiber optic sensing system for an oil logging production system having a sensor element with an optical fiber and fiber Bragg Gratings, a piezoelectric transducer, a photo detector, and electroplates.
SUMMARY OF THE INVENTION
The present invention provides a seismic sensing system having a sensor, a transducer, an optical source and detection unit, an optical fiber and a measurement unit.
The sensor responds to a seismic disturbance, for providing a sensor signal containing information about the seismic disturbance. The sensor may be a geophone that detects vibrations passing though rocks, soil, etc., and provides an electrical sensor signal.
The transducer responds to the sensor signal, for providing a transducer force in the form of an expansion or contraction force that contains electromechanical information about the sensor signal. The transducer may be a piezoelectric, magnetostrictive or electrostrictive transducer that converts electrical energy into mechanical energy, and vice versa.
The optical source and detection unit provides an optical signal through the optical fiber. The optical signal may be a broadband or narrowband signal depending on whether a wavelength or time division multiplexing signal processing scheme is used.
The optical fiber responds to the transducer force, changes the phase or wavelength of the optical signal depending on the change in length of the optical fiber, for providing a transduced optical signal containing information about the transducer force. In effect, the optical fiber converts electromechanical energy from the transducer into optical energy by changing a fundamental characteristic and parameter such as the phase of the optical signal being transmitted or reflected through the optical fiber. The optical fiber is wrapped around the transducer and affixed thereon, for example by bonding. The optical fiber expands and contracts along with the transducer, which causes the change in the length of the optical fiber, which in turn causes the change in the phase of the optical signal. The optical fiber may also have one or more fiber Bragg Gratings therein which change the wavelength of the optical signal depending on the change in length of the fiber Bragg Grating. A fiber Bragg Grating pairs may also be arranged on a part of the optical fiber not bonded to the transducer when a cavity approach is used with the transducer arranged between the fiber Bragg Grating pair. In summary, the electrical voltage signal from the geophone causes the transducer to stretch or contract the optical fiber and change in the length of the fiber, which in turn causes a change of the phase or wavelength of the optical signal being transmitted or reflected through the optical fiber.
The measurement unit responds to the transduced optical signal, for providing a measurement unit signal containing information about the seismic disturbance. The measurement unit converts the transduced optical signal into the information about the seismic disturbance by detecting and processing the change in the phase or wavelength of the optical signal.
The seismic sensing system will typically have an array of geophones. Each geophone is connected to a respective piezoelectric, magnetostrictive or electrostrictive transducer, or the like. Each fiber Bragg Grating pair may have a respective wavelength &lgr;
1
, &lgr;
2
, . . . , &lgr;
n
, for providing a multiplexed fiber Bragg Grating optical signal containing information about changes in the optical fiber caused by the electromechanical force from the respective piezoelectric, magnetostrictive or electrostrictive transducer.
The present invention provides important advantages over the seismic sensing system of the aforementioned prior art patent to Norris. First, the transducer and optical fiber combination is a passive electrical element that is economical because it requires no batteries or time consuming and expensive battery replacement, and is also lightweight and easily handled during deployment of the seismic sensing system. Because of this, the seismic sensing system provides a much less cumbersome way for sensing and multiplexing seismic disturbances with a high channel count using optical fiber. Second, since voltage measurements are effectively made at the sensing locations, the seismic sensing system of the present invention does not suffer from the problem of signal attenuation. In operation, the electrical voltage signal is electro-optically converted into an optical phase or wavelength signal that is not adversely affected by amplitude attenuation as long as the amplitude of the light reaching the measurement unit is above some minimum value. Therefore, the voltage signal is effectively digitized at the sensor location without practically any quantization error. The optical detector system of the measurement unit and the property of the piezoelectric, magnetostrictive or electrostrictive transducer determine the accuracy of the voltage measurement.
The present invention, therefore, can be used in applications other than seismic sensing.
The invention will be fully understood when reference is made to the following detailed description taken in conjunction with an accompanying drawing.
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Kersey Alan D.
Maron Robert J.
Wu Jian-Qun
CiDRA Corporation
Font Frank G.
Nguyen Tu T.
Ware Fressola Van Der Sluys & Adolphson LLP
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