Electric field sensor

Abstract: An electric field sensor comprises an electric field sensor head (13) for varying an intensity of a propagating light beam from a light source (19) in response to an electric field intensity of an input signal received by a reception antenna (11). A photo-electric converter (21) converts the propagating light beam into an output signal. A correcting device (27) compares the output signal with a reference signal to produce and supply a control signal to the photo-electric converter.

Description: Background of the Invention: This invention relates to an electric field sensor for measuring an electric field intensity of a signal which is an electromagnetic wave propagating through a space. In the manner which will later be described more in detail, a conventional electric field sensor comprises a reception antenna, an electric field sensor head, an incident optical fiber, an outgoing optical fiber, a light source, a photo-electric converter, and a setting device. The reception antenna is responsive to an input signal. The electric field sensor head is for varying a light intensity of a propagating light beam propagating therethrough in response to an electric field intensity of the input signal received by the reception antenna. The incident and the outgoing optical fibers are connected to the electric field sensor head. The light source is connected to one end of the incident optical fiber and is for emitting a light beam to the incident optical fiber as the propagating light beam. The photo-electric converter is supplied with the propagating light beam delivered from the electric field sensor head through the outgoing optical fiber. The photo-electric converter is for converting the propagating beam into an electric signal as an output signal. The setting device is for setting the output signal by a first reference signal which has a first reference characteristic. However, the output signal of the photo-electric converter is set by a second reference signal having a second reference characteristic when a measuring temperature changes. The second reference characteristic is different from the first reference characteristic. As a result, the conventional electric field sensor is incapable of reliably measuring the electric field intensity of the input signal. Motohisa Kanda & Keith D. Masterton, Optically Sensed EM-Field Probes for Pulsed Fields, 8078 Proceedings of the IEEE, 80(1992) January, No.1, pages 209-215 shows an electric field sensor comprising an antenna, an electro-optic modulator varying the light intensity according to the field, a laser as light source, a detector, and optical fibers connecting the modulator with the laser and the detector, respectively. US 5,210,407 shows an electric field sensor comprising an antenna, an optical IC with a light modulator varying the light intensity according to the field, incident and outgoing optical fibers, a light generator, a light detector and optical fibers connecting the optical IC with the light generator and the light detector, respectively. The light modulator comprises optical waveguides on the optical IC and modulating electrodes receiving a signal from the antenna. In the GB 2 052 053 A, various embodiments for measuring fields using an optical modulator are described. Light is emitted from a LED, led through a fiber and a modulator, reflected, led through the modulator and a fiber to a dectector and converted into an electrical signal. The lowpass-filtered output signal of the detector is compared with a reference signal, and the comparison result is used to control the LED to compensate for instabilities of the LED, the fibers and the detector. Summary of the Invention : It is therefore an object of this invention to provide an electric field sensor which is capable of reliably measuring the electric field intensity of the input signal. Other objects of this invention will become clear as the description proceeds. The object is attained by an electric field sensor according to claim 1. Further developments of the invention are specified in the dependent claims, respectively. Brief Description of the Drawing: Fig. 1 is a block diagram of a conventional electric field sensor; Fig. 2 is a graph for use in describing operation of the conventional electric field sensor illustrated in Fig. 1; Fig. 3 is a block diagram of the electric field sensor according to an embodiment of this invention; Fig. 4 is a schematic front view of an electric field sensor head of the electric field sensor illustrated in Fig. 3; and Fig. 5 is a block diagram of a correcting device of the electric field sensor illustrated in Fig. 3. Description of the Preferred Embodiment: Referring to Fig. 1, a conventional electric field sensor will be described for a better understanding of this invention. The conventional electric field sensor comprises a reception antenna 11, an electric field sensor head 13, an incident optical fiber 15, an outgoing optical fiber 17, a light source 19, a photo-electric converter 21, and a setting device 23. The reception antenna 11 is responsive to an input signal. The electric field sensor head 13 is for varying a light intensity of a propagating light beam propagating therethrough in response to an electric field intensity of the input signal received by the reception antenna 11. The incident and the outgoing optical fibers 15 and 17 are connected to the electric field sensor head 13. The light source 19 is connected to one end of the incident optical fiber 15 and is for emitting a light beam to the incident optical fiber 15 as the propagating light beam. The photo-electric converter 21 is supplied with the propagating light beam delivered from the electric field sensor head 13 through the outgoing optical fiber 17. The photo-electric converter 21 is for converting the propagating beam into an electric signal as an output signal 25. The setting device 23 is for setting the output signal 25 by a first reference signal which has a first reference characteristic. For example, the first reference characteristic is represented by a first reference characteristic curve A. However, for example, the output signal 25 of the photo-electric converter 21 is set by a second reference signal having a second reference characteristic which is represented by a second reference characteristic curve B when a measuring temperature changes. The second reference characteristic is different from the first reference characteristic. As a result, the conventional electric field sensor is incapable of reliably measuring the electric field intensity of the input signal. Referring to Figs. 3, 4, and 5, the description will proceed to an electric field sensor according to an embodiment of this invention. Similar parts are designated by like reference numerals. In Fig. 3, the electric field sensor comprises the reception antenna 11, the electric field sensor head 13, the incident optical fiber 15, the outgoing optical fiber 17, the light source 19, the photo-electric converter 21, and a correcting device 27. The correcting device 27 is connected to the photo-electric converter 21. In Fig. 4, the electric field sensor head 13 comprises a substrate 29, an incident optical waveguide 31, two phase-shift optical waveguides 33, an outgoing optical waveguide 35, and modulation electrodes 37. The incident optical waveguide 31 is formed on the substrate 29 to be connected to the incident optical fiber 15. The two phase-shift optical waveguides 33 is formed on the substrate 29 to be branched from the incident optical waveguide 31 and have a variable refractive index which varies in response to the electric field intensity. The outgoing optical waveguide 35 is formed on the substrate 29 to be connected at one end to the outgoing optical fiber 17 and at the other end to the junction of the two phase-shift optical waveguides 33. The modulation electrodes 37 are formed in the vicinity of the two phase-shift optical waveguides 33. Each of the modulation electrodes 37 is connected to the reception antenna 11 through a pad 39 formed on the substrate 29. An incident light beam is incident from the incident optical fiber 15 and passes through the incident optical waveguide 31 to be branched to the two phase-shift optical waveguides 33. When the reception antenna 11 receives the input signal, electric voltages are induced and applied to the modulation electrodes 37 to produce, in the two phase-shift optical waveguides 33, electric field components having directions opposite to each other in a depth direction. As a consequence, variation in refractive index is produced in the two phase-shift optical waveguides 33 due to an electrooptical effect so that a phase difference is produced between light beams propagating through the two phase-shift optical waveguides 33. The phase difference has a level dependent upon the electric field intensity applied to the modulation electrodes 37. The light beams are joined and combined in the outgoing optical waveguide 35 as an outgoing light beam having a light intensity which varies due to an interference therebetween. In other words, the outgoing light beam emitted to the outgoing optical fiber 17 has an intensity which varies in response to the electric field intensity applied to the modulation electrodes 37. Turning back to Fig. 3, the correcting device 27 is supplied with the output signal 25 from the photo-electric converter 21. The correcting device 27 compares the output signal 25 with a reference signal to produce and supply a control signal 41 to the photo-electric converter 21. In Fig. 5, the correcting device 27 comprises a reference signal producing device 43, a comparing device 45, and a controller 47. The comparing device 45 is connected to the photo-electric converter 21. The comparing device 45 is supplied with the output signal 25 from the photo-electric converter 21. The output signal 25 has an output level when the electric field sensor head 13 is supplied with a reference electric field intensity of the input signal. The reference signal producing device 43 produces the reference signal 49 which has a reference level. The comparing device 45 is connected to the reference signal producing device 43 and is supplied with the reference signal. The comparing device 45 compares the output level of the output signal 25 with the reference level of the reference signal 49 to produce a deflection signal 51 representing a deflection between the output level and the reference level. The controller 47 is connected to the comparing device 45 and the photo-electric converter 21. The controller 47 is supplied with the deflection signal from the comparing device 45 to produce and supply the control signal 41 to the photo-electric converter 21. The controller 47 controls the photo-electric converter 21 so that the output signal 25 is set by the reference signal even when a measuring temperature changes.

Claims: An electric field sensor comprising: a reception antenna (11) for receiving an input signal; an electric field sensor head (13) for varying an intensity of a propagating light beam propagating therethrough in response to an electric field intensity of said input signal; an incident optical fiber (15) and an outgoing optical fiber (17) both of which are connected to said electric field sensor head (13); a light source (19) connected to one end of said incident optical fiber (15) for emitting a light beam to said incident optical fiber (15) as said propagating light beam; a photo-electric converter (21) supplied with said propagating light beam delivered from said electric field sensor head (13) through said outgoing optical fiber (17) for converting said propagating beam into an electric signal as an output signal (25); and characterised in that the sensor further comprises : a correcting device (27) supplied with said output signal (25) from said photo-electric converter (21) for comparing said output signal (25) with a reference signal (49) generated within the correcting device to produce and supply a control signal (41) to said photo-electric converter (21). An electric field sensor as claimed in claim 1, said correcting device comprising: a reference signal producing device (43) for producing the reference signal (49) having a reference level; a comparing device (45) connected to said photo-electric converter (21) and said reference signal producing device (43) for comparing an output level of said output signal (25) when said electric field sensor head (13) is supplied with a reference electric field intensity of said input signal with said reference level to produce a deflection signal (51); and a controller (47) connected to said comparing device (45) and said photo-electric converter (21) for receiving said deflection signal (51) from said comparing device (45) to produce and supply said control signal (41) to said photo-electric converter (21). An electric field sensor as claimed in claim 1 or 2, said electric field sensor head comprising a substrate (29), an incident optical waveguide (31) formed on said substrate to be connected to said incident optical fiber (15), two phase-shift optical waveguides (33) formed on said substrate (29) to be branched from said incident optical waveguide (31) and having a variable refractive index which varies in response to said electric field intensity, an outgoing optical waveguide (35) formed on said substrate (29) to be connected at one end to said outgoing optical fiber (17) and at the other end of the junction of said two phase-shift optical waveguides (33), and at least one modulation electrode (37) formed in the vicinity of at least one of said two phase-shift optical waveguides (33).

Inventors: MURAMATSU RYOJI, MURAMATSU, RYOJI, MURAMATSU, RYOJI, C/O TOKIN CORPORATION
Applicants: NEC TOKIN CORP, NEC TOKIN CORPORATION
Application year: 1995
Main classification: G01R29
IPC codes: G01R29