MXPA00007196A - Fiber optic sensor system and method - Google Patents

Abstract

A system and method are provided for converting an electrical signal to an optical signal for a fiber optic system. The electrical signal produced by a sensor (10) based upon a parameter being measured is connected across a material (12, 34, 40) that changes dimension responsive to an applied electrical signal. An optical fiber (14, 30, 38) is coupled to the material (12, 34, 40) where dimension changes of the material (12, 34, 40) produce strain in the optical fiber (14, 30, 38). This strain is operable to affect light traveling through the optical fiber (14, 30, 38) to produce an optical signal for a fiber optic system. The sensor (10) can be a geophone or a hydrophone, and the material (12, 34, 40) that changes dimension can be a piezoelectric ceramic cylinder, a PVDF film, or other piezo-polymer material.

Description

OPTIC FIBER SENSOR SYSTEM AND METHOD TECHNICAL FIELD OF THE INVENTION The present invention relates generally to the field of sensor arrangements and, more particularly, to a fiber optic sensor system and method.

BACKGROUND OF THE INVENTION Electrical geophones can be used to measure the speed of a vibration by moving a coil of copper wire through a magnetic field based on vibration. This movement induces a voltage across the coil proportional to the movement that can be used to determine the speed of the vibration. Similarly, a piezoelectric polyvinylidene fluoride (PVDF) ceramic or hydrophone sensor can create an electrical signal output that is proportional to the detected sound pressure. Traditionally, said electrical type sensors have required electronic components for signal conditioning and preamplification near the sensor elements in order to be able to transmit the output signals to the sensor arrangement recording and processing equipment. These additional electronic components can add significant complexity and costs to the set of outdoor sensor regulation panels.

The limitations of the electrical sensor systems and improvements offered by a fiber optic system have been well documented. In addition, the concept of using an optical fiber in detection applications is not new. The Naval Research Laboratory of E.U.A. (NRL) has been a leader in this area, and the NRL and others have described a series of optical systems. For example, the patent of E.U.A. No. 4,648,083, issued to Gialorenzi, describes a typical optical fiber system. In this system, an optical phase equivalent to the acoustic pressure in a hydrophone was measured. In addition, Hofler, Garret and Brown of the Naval Postgraduate School have described fiber optic vibration sensors. Common fiber optic sensors consist of coils of fiber wrapped around mandrels (see U.S. Patent No. 4,525,818, issued to Cielo, et al.) Or on bending discs (see U.S. Patent No. 4,959,539, issued to Hofler, et al.). Then the coils are attached to optical couplers to create an interferometer. In these conventional optical sensor systems, the physical phenomenon that is being measured is directly converted into a differential optical phase acting on the interferometer. In other words, the acoustic pressures or vibrations force the arms of the interferometer creating an optical phase shift in the interferometer. Some arrangements require extended channel group lengths to achieve the required signal at noise ratio. In the case of an impeller arrangement, a series of elements and hydrophone (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 the US patent. No. 5,668,779, issued to Dandridge, et al. and the patent of E.U.A. No. 5,317,544, issued to Maas, et al. These extended interferometers are relatively complicated to manufacture and isolate only certain parts of the interferometer is difficult. Another fiber optic sensor approach consists of fiber Bragg grid-based sensors. Bragg fiber gratings can be used in different ways to measure a given phenomenon. One method is to use the grid as a reflector, creating a Fabry-Perot interferometer. In this case, a similar change is measured in the light phase. In a second method, the grid itself is the sensor, and the tension on the grid changes the period of the grid which changes the wavelength of light reflected from the grid. This change in wavelength is proportional to the voltage on the grid. With either type of fiber optic sensor, the sensor arrangements can be significantly improved by fiber optic telemetry. However, the sensors have become more , complicated and, in many cases, conventional fiber optic systems have produced sensors with lower performance and / or higher cost.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, there is disclosed a method and system for converting an electrical signal, such as the output of an electrical sensor or a total group of electrical sensors, to an optical signal for a fiber optic system that provides advantages over the conventional sensor systems. According to one aspect of the present invention, the electrical signal produced by a sensor based on a parameter being measured is connected through a material that changes the sensitive dimension to an applied electrical signal. An optical fiber is coupled to the material where changes in the dimension of the material produce tension in the optical fiber. This voltage 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 may be a geophone sensor that produces an electrical signal proportional to the movement of the geophone sensor. In another embodiment, the sensor may be a hydrophone sensor that produces an electrical signal proportional to the acoustic pressure inherent in the hydrophone sensor. Also, the material that changes the sensitive dimension to an applied electrical signal may 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 detection and transmission system can be used to replace the signal conditioning and preamp components that are used in conventional electrical sensor arrangement systems. This can be achieved by converting the electrical output signals of the sensors into optical phase signal information. Another technical advantage of the present invention is that the disadvantages of the above systems can be overcome by providing a telemetry system that combines the high performance and low cost of the electrical sensors with the advantages offered by a passive optical telemetry system. The passive nature can eliminate many faults created in the electronic components of active signal conditioning or other optical configuration that requires electrical power in water. Additional technical advantages of the present invention will be apparent from the drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention and advantages thereof can be gained by reference to the following description taken in conjunction with the accompanying drawings, in which reference numerals indicate similar characteristics, and wherein: Figure 1 is a diagram of a mode of a system using a Mach-Zehnder interferometer with an arm wound around a PZT to convert an electrical signal into a differential interferometric phase; Figure 2 is a diagram of one embodiment of a system using both arms of a Mach-Zehnder interferometer around separate PZTs connected with opposite polarities in a symmetrical arrangement to convert electrical signals into differential interferometric phase; Figure 3 is a diagram of one embodiment of a system using fiber Bragg gratings in a Fabry-Perot interferometer wound around a PZT to convert an electrical signal into a differential interferometric phase; Figure 4 is a diagram of one embodiment of a system using a single fiber Bragg grid wrapped around a PZT to convert an electrical signal into diverse optical wavelength information; Figure 5 is a diagram of one embodiment of a system using a Mach-Zehnder interferometer with an arm attached to a PVDF film to convert an electrical signal into a differential interferometric phase; Figures 6A and 6B are diagrams of modalities of total groups of electrical sensors converted into an interferometric phase; and Figure 7 is a diagram of a configuration configuration of electrical sensors converted to interferometric phase.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 is a diagram of one embodiment of a system using a Mach-Zehnder interferometer with an arm wound around a PZT to convert an electrical signal, such as from an electrical sensor or a total group of sensors, into an interferometric phase differential. As shown, a sensor 10 creates a voltage output related to the parameter it is measuring, such as speed or sound pressure. The output voltage is then placed through a material 12 that changes the dimension (e.g., shrinks and expands) sensitive to the applied output voltage. An optical fiber 14 is wound around the material 12, and the optical fiber 14 is subjected to stress by the change in dimension of the material 12. The system also includes a reference optical fiber 16. To make an interferometer, optical couplers 18 can be joining by fusion, indicated at 20, to the detection optical fiber 14 and reference optical fiber 16, as shown. In one implementation, the sensor 10 in Figure 1 may be a conventional geophone sensor. A geophone sensor can be used to measure the speed of a vibration by moving a coil of copper wire through a magnetic field. This movement induces a voltage through the coil provided to the movement. In this implementation, the voltage output of the sensor 10 can be directly connected to a piezoelectric ceramic cylinder 12. Typically, a geophone used in seismic exploration applications produces a voltage output in the order of one volt from peak to peak (although this can vary by changing the number of turns of copper wire or the magnetic field). The application of this voltage through a piezoelectric ceramic cylinder (PZT) will induce, for example, a change of approximately 5 nm / volt of the average diameter of a PZT having a diameter of 2.54 cm and a wall of 0.127 cm thickness . This change can be translated into a length change in the optical fiber 14 of 4.75p nm per revolution. The number of turns may vary to adjust the optical scale factor of the system. The relative phase change in an interferometer is given by the equation:? F = 2p-n- (? L /?), Where n is the refractive index for the fiber y? It is the wavelength of light. According to the present invention, a fiber optic interferometer can be constructed with an arm wound around a piezoelectric cylinder 12 as shown in Figure 1. In operation, the output of the electrical sensor 10 (e.g., a geophone) it is connected through the piezoelectric cylinder 12. The voltage output of the electrical sensor 10 causes the piezoelectric cylinder 12 to expand and contract, thereby causing the detection optical fiber 14 of the interferometer to expand and contract. This induces a phase change in the interferometer proportional to the parameter that is being measured by the electrical sensor 12. The optical signal can then be multiplexed with optical signals from other sensors in an arrangement according to conventional methods. Figure 2 is a diagram of one embodiment of a system using both arms of a Mach-Zehnder interferometer around separate PZTs connected with opposite polarities in a symmetrical arrangement to convert electrical signals into differential interferometric phase. In contrast to the system of Figure 1, a second material 22 that changes the sensitive dimension to an applied output voltage is used and is connected to the output voltage of the sensor 10 at an opposite polarity such as the material 12 so that the material 22 is reflected in the material 12. The material 22 can be, for example, a piezoelectric ceramic cylinder (PZT) as mentioned above. By winding the reference fiber 16 in the material 22 and connecting the materials 12 and 22 with opposite polarities, the fiber 16 is caused to contract when the fiber 14 expands and vice versa. With this structure, the system of Figure 2 provides a symmetric configuration for converting the electrical output of the sensor 10 into optical signals that can be used to increase the scale factor by a factor of two. Figures 3 and 4 demonstrate configurations for an alternate approach that includes the addition of fiber Bragg gratings to the concept of signal conversion. Figure 3 is a diagram of one embodiment of a system using fiber Bragg gratings in a Fabry-Perot interferometer wound around a PZT to convert the electrical sensor output to differential interferometric phase according to the present invention. As shown, a Fabry-Perot interferometer is created between two reflective fiber Bragg gratings 26 and 28. The light traveling down the optical fiber 30 is partially reflected by the first grid 26. The light continues downwards from the optical fiber 32 which is wound around the cylinder PZT (or other electro-sensitive material) 12 to the next grid 28 where the light is reflected back. A voltage across PZT 12 produced by a sensor 10 (not shown in Figure 3) induces a phase change between the signals reflected back from the grids 26 and 28. Consequently, the system of Figure 3 uses grids. 26 and 28 as reflectors, creating a Fabry-Perot interferometer. In general, a fiber containing grids for the Fabry-Perot interferometer can be wound around a PZT cylinder or other electro-sensitive material so that the two grids are positioned on each side of the cylinder. The optical fiber between the grids is sensitive when an electrical sensor output is subjected to stress which causes a change in phase of the light measured between the reflected signals of the two grids. Figure 4 is a diagram of one embodiment of a system using a single fiber Bragg grid wound around a PZT to convert the electrical sensor output into various optical wavelength information according to the present invention. As shown, an optical fiber 38 with an integral fiber Bragg grid 36 is attached to a material 15 that changes the sensitive dimension to a voltage output through the material 40 (eg, PZT) of a sensor 10 (not it is shown in figure 4). The voltage output of the sensor 10 applied through the material 40 induces a voltage in the grid 36. This voltage causes the grid period 36 to change, and in turn causes the wavelength of the reflected light back from the grid. 36 change. This change in wavelength is to provide the voltage output of the sensor 10. Therefore, in operation, the grid 36 is the sensor for converting an optical signal from the original output voltage of the sensor 10. Although the above modes Using a piezoelectric cylinder (PZT) to induce tension in the optical fiber, this PZT could be replaced by a PVDF film or other material with change of dimension or other electro-sensitive characteristics. Figure 5 is a diagram of one embodiment of a system using a Mach-Zehnder interferometer with an arm attached to a PVDF film to convert an electrical sensor output to a differential interferometric phase according to the present invention. As shown, the PZT cylinder 12 of Figure 1 is replaced with a PVDF film 46 (or other material). An optical fiber 44 is connected to the material 46, and the electrical output of a sensor 48 is applied through the material 46. The electrical output of the sensor 48 induces a sensible change in the material 46 which in turn induces tension in the optical fiber 44. In addition to being extended as shown in Figure 5, the material 46 could be wrapped around a mandrel or placed in other configurations as appropriate for the desired application. Likewise, other types of interferometers could also be used, such as the Michelson interferometer, in addition to those described above. Figures 6A and 6B are diagrams of modalities of total groups of electrical sensors. As shown in Figure 6A, a sensor 50 may include a group of electrical sensors or sensor elements 52 that are connected in parallel. Similarly, as shown in Figure 6B, the sensor 50 may include a group of electrical sensors or sensor elements 52 that are connected in series. In the case of an impeller, a group of sixteen sensing elements 52 are typically connected together over an arrangement length of 12.5 meters to form an electrical signal output. This is also common with other types of sensors, although the group is often referred to as a single element. Figure 7 is a diagram of an electrical sensor arrangement configuration. The layout configuration has a wet end portion 54 (where the arrangement is placed in water) and a dry end portion 56. The dry end portion 56 may contain optoelectronic components for processing optical signals. The layout configuration of Figure 7 includes four sensor subgroups 58 each of which contains four sensors 60. Sensors 60 may be single sensors or groups of sensors (e.g., as shown in Figures 6A and 6B). The sensors 60 are connected to optical output signals 62 and provide optical input signals 64. In the illustrated arrangement, each sensor 60 in a group 58 is connected to the same optical output signal 62. Also in this arrangement, each sensor 60 in a group 58 it is connected to a different line to provide input optical signals 64. This arrangement is essentially a frequency division multiplexed telemetry, for example like the one shown and described in the US patent No. 4,648,083, issued to Gialorenzi. However, other optical telemetries could be used to drive an N-channel arrangement. Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made thereto without departing from the spirit and scope of the invention. the invention as defined by the appended claims.

Claims (39)

NOVELTY OF THE INVENTION CLAIMS

1. - A system for converting an electrical signal to an optical signal for an optical fiber system, comprising: a sensor that produces an electrical signal based on a parameter that is being measured; a material that changes the sensitive dimension to an applied electrical signal, the electrical signal produced by the sensor connected through the material; and an optical fiber coupled to the material, where changes in the dimension of the material produce tension in the optical fiber; the operable voltage to affect the light that travels through the optical fiber to produce an optical signal for a fiber optic system.

2. The system according to claim 1, further characterized in that the sensor is a geophone sensor that produces a voltage output proportional to the movement of the geophone sensor.

3. The system according to claim 1, further characterized in that the sensor is a hydrophone sensor that produces a voltage output proportional to the acoustic pressure inherent to the hydrophone sensor.

4. The system according to claim 1, further characterized in that the sensor comprises a group of detection components connected together to produce the electrical signal.

5. - The system according to claim 1, further characterized in that the material is a piezoelectric ceramic cylinder.

6. The system according to claim 1, further characterized in that the material is a PVDF film.

7. The system according to claim 1, further characterized in that the material is a piezo-polymer material.

8. The system according to claim 1, further characterized in that the optical fiber is part of an optical interferometer and the voltage produced in the optical fiber creates a phase change in the interferometer.

9. The system according to claim 8, further comprising: a second material that changes the sensitive dimension to an applied voltage, the voltage output of the sensor connected through the second material in an opposite polarity; and a second optical fiber coupled to the second material, where the changes of dimension of the second material produce tension in the second optical fiber; The voltage output of the sensor thus produces opposite dimensional changes in the optical fibers.

10. The system according to claim 8, further characterized in that the interferometer is a Mach-Zehnder interferometer.

11. - The system according to claim 8, further characterized in that the interferometer is a Michelson interferometer.

12. The system according to claim 8, further characterized in that the interferometer is a Fabry-Perot interferometer.

13. The system according to claim 8, further characterized in that the interferometer is a Fabry-Perot interferometer constructed using Bragg gratings as reflectors.

14. The system according to claim 1, further characterized in that the optical fiber has an integral Bragg grid with the Bragg grid coupled to the material, and further characterized in that the voltage produced in the optical fiber causes a period of Bragg grid change which changes the wavelength of the light reflected by the Bragg grid.

15. The system according to claim 1, further characterized in that the optical signal is multiplexed with other optical signals of a sensor arrangement using frequency division multiplexing techniques.

16. The system according to claim 1, further characterized in that the optical signal is multiplexed with other optical signals of a sensor arrangement using wavelength division multiplexing techniques.

17. - The system according to claim 1, further characterized in that the optical signal is multiplexed with other optical signals of a sensor arrangement using time division multiplexing techniques.

18. The system according to claim 1, further characterized in that the optical signal is multiplexed with other optical signals from a sensor arrangement using a hybrid of frequency division multiplexing, wavelength division multiplexing and time division multiplexing techniques.

19. A system for converting an electrical signal to an optical signal for a fiber optic system, comprising: a sensor that produces an electrical signal based on a parameter that is being measured; a material that changes the sensitive dimension to an applied electrical signal, the electrical signal produced by the sensor connected through the material; and an interferometer characterized in that changes in the dimension of the material cause a mirror to move and cause a change in the intensity of the reflected optical signal.

20. The system according to claim 19, further characterized in that changes in the dimension of the material cause the mirror to move and cause a change in the phase of the reflected optical signal.

21. A method for converting an electrical signal to an optical signal for an optical fiber system, comprising: connecting an electrical signal produced by a sensor based on a parameter that is being measured through a material that changes the dimension sensitive to an applied electrical signal; coupling an optical fiber to the material where the material dimension changes produce tension in the optical fiber; and producing an optical signal for a fiber optic system from the effect of the voltage on the optical fiber in the light traveling through the optical fiber.

22. The method according to claim 21, further characterized in that the sensor is a geophone sensor that produces an electrical signal proportional to the movement of the geophone sensor.

23. The method according to claim 21, further characterized in that the sensor is a hydrophone sensor that produces an electrical signal proportional to the acoustic pressure inherent to the hydrophone sensor.

24. The method according to claim 21, further characterized in that the sensor comprises a group of detection components connected together to produce the electrical signal.

25. The method according to claim 21, further characterized in that the material is a piezoelectric ceramic cylinder.

26. The method according to claim 21, further characterized in that the material is a PVDF film.

27. The method according to claim 21, further characterized in that the material is a piezo-polymer material.

28. - The method according to claim 21, further characterized in that the optical fiber is part of an optical interferometer and the voltage produced in the optical fiber creates a phase change in the interferometer.

29. The method according to claim 28, further comprising: connecting the output voltage of the sensor in an opposite polarity through a second material that changes the sensitive dimension to an applied voltage; and coupling a second optical fiber to the second material where the changes in dimension of the second material produce tension in the second optical fiber; The voltage output of the sensor thus produces opposite dimensional changes in the optical fibers.

30. The method according to claim 28, further characterized in that the interferometer is a Mach-Zehnder interferometer.

31. The method according to claim 28, further characterized in that the interferometer is a Michelson interferometer.

32. The method according to claim 28, further characterized in that the interferometer is a Fabry-Perot interferometer.

33. The method according to claim 32, further characterized in that the interferometer is a Fabry-Perot interferometer constructed using Bragg gratings as reflectors.

34. - The method according to claim 21, further characterized in that the optical fiber has an integral Bragg grid with the Bragg grid coupled to the material, and further characterized in that the voltage produced in the optical fiber causes a period of the grid of Bragg change which changes the wavelength of the light reflected by the Bragg grid. The method according to claim 21, further comprising multiplexing the optical signal with other optical signals from a sensor arrangement using frequency division multiplexing techniques. 36. The method according to claim 21, further comprising multiplexing the optical signal with other optical signals from a sensor array using wavelength division multiplexing techniques. 37. The method according to claim 21, further comprising multiplexing the optical signal with other optical signals of a sensor arrangement using time division multiplexing techniques. 38.- A method for converting an electrical signal to an optical signal for a fiber optic system, comprising: connecting an electrical signal produced by a sensor based on a parameter that is being measured through a material that changes the dimension sensitive to an applied electrical signal; and coupling an interferometer to the material where changes in the dimension of the material cause a mirror to move and cause a change in the intensity of the reflected optical signal. 39.- The method according to claim 38, further characterized in that changes in the dimension of the material cause the mirror to move and cause a change in the intensity of the reflected optical signal.