JPH11281940A - Optical modulator and reception equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the variance of the signal level of a photodetector which converts an optical signal to an electric signal regardless of the difference of transmission loss between two separated polarized light or the variance of an intensity ratio. SOLUTION: An optical modulator 1 is so constituted that one of two separated polarized light whose planes of polarization are perpendicular to each other is transmitted through a half-wave plate 24, a Farady rotator 25, and an optical attenuator 51 before the incidence of these separated polarized light on reflection optical modulators 21 and 22 and the optical attenuator 51 is continuously displaced perpendicularly to the optical path. This optical modulator 1 is mounted on a reception equipment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiving apparatus for receiving a signal radio wave such as a radio communication and a broadcasting via a receiving antenna, or measuring a radio wave intensity, a frequency and an electromagnetic noise in an EMC field. Also, the present invention relates to an optical modulator for converting an input electric signal into a light intensity signal.

[0002]

2. Description of the Related Art An optical modulator formed on the basis of an optical waveguide element using a material exhibiting an electro-optical effect as a substrate, such as lithium niobate, has a higher modulation efficiency than a previous bulk element.
It has features such as small size. This waveguide type optical modulator has also been used for measurement of spatial electric field strength and reception of radio waves such as broadcast waves.

FIG. 5 is a diagram showing a configuration of a conventional receiving apparatus. FIG. 6 is a diagram showing a configuration of a reflection type optical modulator including an optical waveguide element used in the receiving apparatus. FIG.
In the above, the reflection type optical modulator 43 is a reflection type branch interference type optical waveguide composed of a substrate 11 made of lithium niobate crystal, an input / output optical waveguide 12 by diffusion of titanium, a phase shift optical waveguide 13 and a reflector 14. And a modulation electrode 15 having a divided structure.

In FIG. 5, a broken line with an arrow indicates the direction in which light propagates, and a solid line with arrows on both ends indicates the polarization direction of the light (the same applies hereinafter). In the receiving apparatus shown in FIG. 5, two optical modulators 41 and 42 are used. The light separated into the respective polarization components by the polarization splitter / combiner 23 is incident on the optical modulators 41 and 42, while the AC voltage induced in the receiving antenna 33 by the electric field modulates the optical modulators 41 and 42. The electrodes are connected so as to be applied in the same phase. The light modulated and emitted by each of the optical modulators 41 and 42 is combined by the polarization separation / combination unit 23 and is incident on the photodetector 32 to be detected. In the case of this receiving apparatus, each optical modulator 41, 4
2, a single light source 31 is sufficient because the output light of the light source 31 is split into two polarization components and incident.

[0005]

However, in the conventional receiving apparatus shown in FIG. 5, the output light of the light source 31 is separated by the polarization separation / combination unit 23, and is output between two polarized lights having polarization planes perpendicular to each other. , The transmission loss is not always equal, or the transmission loss of the two optical modulators is not always equal. If the intensity differs between the two polarized lights or the intensity ratio fluctuates, the level of the signal converted from the optical signal to the electric signal by the photodetector 32 fluctuates. For example, when the optical signal fluctuates by 1 dB, the electric signal converted by the photodetector 32 fluctuates by 2 dB.

FIG. 7 is a diagram showing the configuration of another conventional receiving apparatus. By employing the Faraday rotator 25, the function equivalent to that of the receiving apparatus shown in FIG. 5 is achieved.

However, as in the receiver shown in FIG. 5, the transmission loss between the two polarized lights separated by the polarization splitter / combiner 23 is different in the receiver shown in FIG. If the transmission loss of the optical modulator is not equal, the level of the signal converted from the optical signal to the electric signal by the photodetector 32 fluctuates.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a photodetector for converting an optical signal into an electrical signal even if the transmission loss differs between the two separated polarizations or the transmission loss of the two optical modulators is not equal. It is an object of the present invention to provide an optical modulation device and a receiving device that suppress the fluctuation of the signal level of the optical modulator.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a light modulation device configured to emit incident light with varying intensity depending on the voltage of an applied electric signal. The light incident on the combiner is separated into two polarized lights whose polarization planes are perpendicular to each other, and one of the two polarized lights passes through a half-wave plate, a Faraday rotator and an optical attenuator, and is reflected light. The other polarized light enters the modulator and is transmitted through the half-wave plate and the Faraday rotator or the half-wave plate, the Faraday rotator and the optical attenuator, and is transmitted to the other reflection type optical modulator. The modulated light that is incident and emitted from each of the two reflection-type optical modulators is an optical modulation device that is combined and emitted by the polarization separation / combination device.

In the optical modulator according to the present invention, the half-wave plate and the Faraday rotator in the above configuration may be replaced by a Faraday rotator, or
The wave plate and the Faraday rotator need not be provided.

Further, in the optical modulator according to the present invention, the optical attenuator is configured to be continuously displaced perpendicularly to the transmitted light so that the intensity of the transmitted light continuously changes. Features.

In addition, according to the present invention, a receiving antenna equipped with these optical modulators for inducing an AC voltage by an electromagnetic wave, the optical modulator to which an AC voltage is applied, and emitting light to the optical modulator. A light source, a light detector that converts light emitted from the light modulation device into an electric signal, and a reception device that includes a light source, a light modulation device, and an optical fiber optical transmission line between the light modulation device and the light detector. is there.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a receiving apparatus according to an embodiment of the present invention. In FIG. 1, the inside of the frame partitioned by the broken line is the light modulation device. The light modulation device 1 includes a polarization splitter / combiner 23, a half-wave plate 24, a Faraday rotator 2
5, optical attenuator 51, two reflection-type optical modulators 21, 22,
And polarization-maintaining optical fibers 26, 27, 28, 29, and 30 as optical transmission paths therebetween. The half-wave plate 24 and the Faraday rotator 25 are bonded together.

FIG. 2 is a diagram showing a configuration of the reflection type optical modulator 21 used in the optical modulation device. In the following, the reflection type optical modulator 22 will also be described with reference to FIG. The reflection type optical modulator 21 includes a substrate 11 made of a lithium niobate crystal, and a reflection type branch interference optical waveguide composed of an input / output optical waveguide 12 by diffusion of titanium, a phase shift optical waveguide 13 and a reflection portion 14 on the substrate 11, Modulation electrode pair 1
5 and 16 are formed. Modulation electrodes 15, 1
6 is divided into a plurality of electrodes in the light traveling direction and is capacitively coupled.

In FIG. 1, the light incident on the polarization splitter / combiner 23 is split into two polarizations whose polarization planes are perpendicular to each other, and the polarization maintaining optical fibers 26 and 27 are used as transmission lines, respectively, and the Faraday rotator 25 is used. Through. One polarized light propagates through the polarization-maintaining optical fiber 29, passes through the optical attenuator 51 provided on the optical path, passes through the polarization-maintaining optical fiber 30, and adjusts the polarization direction. , One of the reflection type optical modulators 22. The optical attenuator 51 is a flat plate made of glass and having a wedge-shaped cross section as shown in FIG. The other polarized light transmitted through the Faraday rotator 25 is converted into a polarization maintaining optical fiber 28.
Then, the polarization direction is adjusted, and the light enters the reflection type optical modulator 21.

On the other hand, the modulation electrodes of the two optical modulators 21 and 22 are connected in series so that the same signal is applied in the same phase. Each reflection type optical modulator 21,
The light incident on 22 is modulated by the voltage applied to the modulation electrodes 15 and 16 and emitted. Light modulator 21,
The light emitted from the light source 22 passes through the same optical path as that of the incident light, and is synthesized by the polarization separation / combination device 23, and the light source 31
Is emitted from a port different from the port on which the output light is incident, and is incident on the photodetector 32.

As described above, the polarization separation / combination unit 23
If the transmission loss between the two polarized lights separated by the optical modulator 32 is not equal, or the transmission loss of the two optical modulators is not equal, the level of the signal converted from the optical signal to the electric signal by the photodetector 32 is changed. fluctuate. Therefore, in the receiving apparatus having the configuration shown in FIG. 1, the optical attenuator 51 is continuously displaced perpendicularly to the optical path so that the intensity of light transmitted through the optical attenuator 51 is continuously changed. However, the transmission loss of the two polarized lights can be made uniform. As a result, the electric signal level of the photodetector 32 can be stabilized.

FIG. 3 is a diagram showing a configuration of a receiving apparatus according to another embodiment of the present invention. In FIG. 3, the inside of the frame partitioned by the broken line is the light modulation device. In the receiving apparatus according to the present embodiment, a Faraday rotator 25 is used instead of the half-wave plate 24 and the Faraday rotator 25 of the receiving apparatus shown in FIG. Under this configuration, the optical attenuator 51 is continuously displaced perpendicularly to the optical path so that the intensity of light passing through the optical attenuator 51 changes continuously, as in the receiving apparatus of FIG. To adjust the transmission loss of the two polarized lights. As a result, the photodetector 32
The electric signal level of the signal can be stabilized. With this configuration, the present receiving apparatus has functions equivalent to those of the receiving apparatus shown in FIG.

FIG. 4 is a diagram showing a configuration of a receiving apparatus according to still another embodiment of the present invention. In FIG. 4, the light modulation device is shown in a frame partitioned by a broken line. The output light of the light source 31 is separated into two polarized lights whose polarization planes are perpendicular to each other by a polarization splitter / combiner 23, and one of the polarized lights is converted into light while being incident on each of the reflection type optical modulators 21 and 22. The light passes through the attenuator 51. Under this configuration, the optical attenuator 51 is continuously displaced perpendicularly to the optical path, as in the receiving device of FIG. As a result, the electric signal level of the photodetector 32 can be stabilized.

Incidentally, an optical attenuator is incorporated in the two optical paths,
A method may be used in which two polarization transmission losses are mutually adjusted and aligned. Also, the polarization separation / combination unit 2 in the light modulation device
3 and the optical modulator 22, the half-wave plate, the Faraday rotator and the optical attenuator, or the Faraday rotator and the optical attenuator are the same even if the order of their arrangement on the optical path is changed. Functions and effects can be obtained.

[0022]

As described above, according to the optical modulator of the present invention, the transmission loss between two separated polarized lights is not equal, or the transmission loss between two optical modulators is not equal. In addition, fluctuations in the signal level of a photodetector that converts an optical signal into an electric signal can be suppressed, and stable reception can be achieved by a receiver equipped with this optical modulator.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a receiving device according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a reflection type optical modulator.

FIG. 3 is a diagram showing a configuration of a receiving device according to another embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of a receiving device according to still another embodiment of the present invention.

FIG. 5 is a diagram showing a configuration of a conventional receiving apparatus.

FIG. 6 is a diagram showing a configuration of a reflection type optical modulator including an optical waveguide element.

FIG. 7 is a diagram showing a configuration of another conventional receiving apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Optical modulator 11 Substrate 12 Incoming / outgoing optical waveguide 13 Phase shift optical waveguide 14 Reflector 15, 16 Modulation electrode 17, 18 Terminal 21, 22 Reflection type optical modulator 23 Polarization separation / combiner 24 1/2 wavelength plate 25 Faraday rotation Numerals 26, 27, 28, 29, 30 Polarization maintaining optical fiber 31 Light source 32 Photodetector 33, 34 Receiving antenna 35, 36 Resonant circuit 37 Optical circulator 41, 42, 43 Reflection type optical modulator 44, 45 Single mode light Fiber 51 Optical attenuator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/14 10/04 10/06 10/152 10/142

Claims (5)

[Claims] In an optical modulator configured to change the intensity of incident light depending on the voltage of an applied electric signal and emit the light, the light incident on the polarization splitter / combiner is controlled by the light modulator. The polarization plane is separated into two polarizations perpendicular to each other, and one of the two polarizations passes through the Faraday rotator and the optical attenuator and is incident on the reflection type optical modulator, and the other polarization is The modulated light that has passed through the Faraday rotator or the Faraday rotator and the optical attenuator, is incident on another reflection type optical modulator, and is emitted from each of the two reflection type optical modulators is the polarization separation / combination. An optical modulator characterized in that the light is combined and emitted by a light modulator. 2. An optical modulator configured so that incident light varies in intensity depending on the voltage of an applied electric signal and is emitted, wherein the light incident on the polarization splitter / combiner is The polarization plane is separated into two polarizations perpendicular to each other, and one of the two polarizations passes through a half-wave plate, a Faraday rotator and an optical attenuator, and is incident on a reflection type optical modulator. The other polarized light is transmitted through a half-wave plate and a Faraday rotator or a half-wave plate, a Faraday rotator and an optical attenuator, and is incident on another reflection type optical modulator. An optical modulation device, wherein modulated light emitted from each of the reflection type optical modulators is combined and emitted by the polarization separation / combination device. 3. An optical modulator configured so that incident light changes its intensity depending on the voltage of an applied electric signal and emits the light. The polarization plane is separated into two polarizations perpendicular to each other, one of the two polarizations is transmitted through the optical attenuator and is incident on the reflection type optical modulator, and the other polarization is directly or After passing through the optical attenuator, it is incident on another reflection type optical modulator,
An optical modulator, wherein modulated light emitted from each of the two reflection-type optical modulators is combined and emitted by the polarization splitter / combiner. 4. The optical attenuator according to claim 1, wherein the intensity of the transmitted light is continuously changed by continuously displacing the transmitted light perpendicularly. The light modulation device according to claim 1 or 2. 5. A receiving antenna on which the optical modulation device according to claim 1 is mounted and which induces an AC voltage by an electromagnetic wave; and the optical modulation device to which the AC voltage is applied; A light source for emitting light to the light modulator, a light detector for converting the light emitted from the light modulator to an electric signal, and a light source and the light modulator, between the light modulator and the light detector. And an optical fiber transmission line.