Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
1998-01-23
2001-11-06
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
C356S340000
Reexamination Certificate
active
06314056
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to the field of sensor arrays and, more particularly, to a fiber optic sensor system and method.
BACKGROUND OF THE INVENTION
Electrical geophones can be used to measure the velocity of a vibration by moving a coil of copper wire through a magnetic field based upon the vibration. This movement induces a voltage across the coil proportional to the movement which can be used to determine the velocity of the vibration. Analogously, a piezoelectrical ceramic or polyvinylideneflouride (PVDF) hydrophone sensor can create an electrical signal output that is proportional to sensed acoustic pressure. Traditionally, such sensors of the electrical type have required signal conditioning and preamplifying electronics near the sensing elements to be able to transmit the output signals to sensor array recording and processing equipment. These additional electronics can add significant complexity and cost to the outboard sensor suite.
The limitations of electrical sensor systems and improvements offered by a fiber optic system have been well documented. Further, the concept of using an optical fiber in sensing applications is not new. The U.S. Naval Research Laboratory (NRL) has been a leader in this area, and the NRL and others have disclosed a number of optical systems. For example, U.S. Pat. No. 4,648,083, issued to Gialorenzi, describes a typical fiber optic system. In this system, an optical phase equivalent to acoustic pressure in a hydrophone was measured. In addition, fiber optic vibration sensors have been disclosed by Hofler, Garrett and Brown of the Naval Post Graduate school. Common fiber optic sensors consist of coils of fiber wrapped around mandrels (see U.S. Pat. No. 4,525,818, issued to Cielo, et al.) or onto flexing disks (see U.S. Pat. No. 4,959,539, issued to Hofler, et al.). The coils are then attached to optical couplers to create an interferometer. In these conventional optical sensor systems, the physical phenomenon being measured is directly converted into a differential optical phase by acting on the interferometer. In other words, the acoustic pressures or vibrations stress the arms of the interferometer creating an optical phase shift in the interferometer. Some arrays require extended channel group lengths in order to achieve the required signal to noise ratio. In the case of a towed streamer array, a number of hydrophone elements (16 is common) are electrically connected together to create an output over an extended length. Optical versions of the extended group length have been described for example in U.S. Pat. No. 5,668,779, issued to Dandridge, et al. and U.S. Pat. No. 5,317,544, issued to Maas, et al. These extended interferometers are relatively complicated to fabricate and isolating only certain parts of the interferometer is difficult.
Another fiber optic sensor approach consists of fiber Bragg grating based sensors. The fiber Bragg gratings can be used in different manners to measure a given phenomenon. One method is to use the grating as a reflector, creating a Fabry-Perot interferometer. In this case, a similar change in phase of the light is measured. In a second method, the grating itself is the sensor, and strain on the grating changes the period of the grating which changes the wavelength of light reflected from the grating. This change in wavelength is proportional to the strain on the grating.
With either type of fiber optic sensor, sensor arrays can be significantly improved by the fiber optic telemetry. However, along the way, the sensors have become more complicated, and, in many cases, conventional fiber optic systems have yielded sensors with lower performance and/or higher cost.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and system for converting an electrical signal, such as the output of an electrical sensor or a summed group of electrical sensors, to an optical signal for a fiber optic system are disclosed that provide advantages over conventional sensor systems.
According to one aspect of the present invention, the electrical signal produced by a sensor based upon a parameter being measured is connected across a material that changes dimension responsive to an applied electrical signal. An optical fiber is coupled to the material where dimension changes of the material produce strain in the optical fiber. This strain is operable to affect light traveling through the optical fiber to produce an optical signal for a fiber optic system. In one embodiment, the sensor can be a geophone sensor that produces an electrical signal proportional to motion of the geophone sensor. In another embodiment, the sensor can be a hydrophone sensor that produces an electrical signal proportional to acoustic pressure incident on the hydrophone sensor. Also, the material that changes dimension responsive to an applied electrical signal can be, for example, a piezoelectric ceramic cylinder or a PVDF film or other piezo-polymer material.
A technical advantage of the present invention is that an electrical signal produced by a sensor can be converted to an optical signal for use in a fiber optic system.
Another technical advantage is that a laser controlled optical transmission and detection system can be used to replace signal conditioning and preamplifying components in use in conventional electrical sensor array systems. This can be accomplished by the conversion of electrical output signals from the sensors into optical phase signal information.
A further technical advantage of the present invention is that disadvantages of prior systems can be overcome by providing a telemetry system that combines the high performance and low cost of electrical sensors with the advantages offered by a passive optical telemetry system. The passive nature can eliminate many failures created in the active signal conditioning electronics or other optical configuration requiring electrical power in water.
Additional technical advantages of the present invention should be apparent from the drawings, description and claims.
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D.A. Jackson, R. Priest, A. Dandridge, and
Bunn J. Brett
Bunn James S.
Jaaskelainen Mikko
Maas Steven J.
Arnold & Associates
Lobo Ian J.
Petroleum Geo-Services
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