JPH11242195A - Optical modulation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a sensitivity from being lowered by connecting modulation electrodes of at least two optical modulators among optical modulators in series each other so that the same electric signals are impressed on them with the same phases to use single mode optical fibers without depending on the polarized plane of the outgoing light of a light siurce. SOLUTION: The AC voltage induced in a receiving antenna 33 passes through an impedance mudulating circuit 35 to be impressed on two optical modulators 41, 42 connected in series. The modulation electrodes of the two modulators 41, 42 are connected in series so that the same electric signals are impressed on them with the same phases. In this case, since coaxial cables of 50 Ωs are used in this device, the impedance of a closed circuit consisting of the modulation electrodes and the impedance modulating circuit becomes double and the voltage drip to be impressed on the modulation electrodes becomes half and the sensitivity is enhanced. Then, the light made incident on respective optical modulators 41, 42 is modulated by impression voltages impressed on the modulation electrodes and is emitted to be made incident on a photodetector 32.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator for converting an input electric signal into a light intensity signal, and more particularly to a receiver for receiving a signal radio wave via a receiving antenna, and a radio wave intensity and frequency in the EMC field. The present invention relates to an optical modulation device for converting an input electric signal into a light intensity signal, which is used for an electric field sensor for measuring electromagnetic noise and the like.

[0002]

2. Description of the Related Art FIG. 3 is a diagram showing the configuration of a conventionally known optical modulator comprising an optical waveguide element. An optical modulator 43 configured based on an optical waveguide element using a material exhibiting an electro-optical effect as a substrate, such as lithium niobate, has features such as higher modulation efficiency and smaller size as compared with the previous bulk element. Waveguide-type optical modulators have also been used for measurement of spatial electric field strength, reception of radio waves of broadcast waves, and the like. The optical modulator 43 has a substrate 11 made of lithium niobate crystal, and is composed of a reflective branch interference optical waveguide formed thereon and a modulation electrode 15 having a divided structure. The reflection-type branch interference type optical waveguide has an input / output optical waveguide 12 and a phase shift optical waveguide 13 formed by diffusion of titanium on a substrate 11 made of lithium niobate crystal, and a reflection section 20 formed at an end. .

FIG. 4 is a diagram showing a configuration of a conventional electric field sensor and a receiving device.

[0004] In FIG. 3 and FIG.
The AC voltage induced in 3 is guided to the modulation electrode 15 and applied to the phase shift optical waveguide 13. The output light of the light source 31 passes through the polarization-maintaining optical fiber 26, the optical circulator 37, and the polarization-maintaining optical fiber 28, and passes through the optical modulator 4 sequentially.
The light enters the third input / output optical waveguide 12 and is divided into two phase-shifted optical waveguides 13, reflected by the reflection unit 20, and emitted again through the input / output optical waveguide 12.
Opposite voltages are applied to the two phase-shifted optical waveguides 13. The intensity of the light emitted to the polarization maintaining optical fiber 28 changes according to the applied voltage. Therefore, by detecting the change in the intensity of the outgoing light with the photodetector 32, the intensity and frequency of the radio wave induced by the receiving antenna 33, signals contained in the radio wave, and the like can be detected.

[0005]

In the receiving apparatus shown in FIG. 4, since the optical modulator 43 operates only for a fixed polarization component, the polarization maintaining light is applied to the optical fiber through which the light incident on the optical modulator 43 passes. The use of fibers 26, 28 is inevitable. When a normal single mode optical fiber is used, the polarization state changes due to distortion, temperature change, etc.
Since the power of light modulated by the optical modulator fluctuates, it is not practical.

However, polarization maintaining optical fibers are more expensive than ordinary single mode optical fibers. further,
When the polarization maintaining optical fiber is used, only a certain polarization component of the output light of the light source 31 is used.

The present invention solves these problems, that is, using a single-mode optical fiber substantially entirely without depending on the polarization plane of the light emitted from the light source,
It is an object of the present invention to provide an optical modulation device in which sensitivity does not decrease.

[0008]

SUMMARY OF THE INVENTION The present invention comprises a plurality of optical modulators so that incident light exits with varying intensity depending on the voltage of an applied electric signal. In such a light modulator, the modulation electrodes of at least two of the light modulators are light modulators connected in series with each other so that the same electric signal is applied in the same phase. .

[0009]

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

FIG. 1 is a diagram showing an equivalent circuit of an optical modulation device according to the present invention. FIG. 2 is a configuration diagram of a receiving device using the optical modulation device of the present invention. In FIG. 2, the light modulation device 1 is inside a frame separated by a wavy line.

1 and 2, two optical modulators 41 and 42 are used. These optical modulators are shown in FIG.
Is the same as that shown in FIG. Hereinafter, the description of the optical modulator is based on FIG.

The output light of the light source 31 is
As a result, the polarization plane is divided into two linearly polarized light beams that are perpendicular to each other, and are incident on the two optical modulators 41 and 42, respectively. Here, single mode optical fibers 44 and 46 are used from the light source 31 to the polarization splitter / combiner 23. Then, the polarization separation / combination unit 23 and the two optical modulators 4
The polarization maintaining optical fibers 26 and 27 are connected between 1 and 42. In practice, the distance between the polarization separation / combination unit 23 and the two optical modulators 41 and 42 is extremely shorter than the distance between the light source 31 and the polarization separation / combination unit 23. That is,
Substantially throughout, single mode optical fibers are used. The optical circulator 37 and the photodetector 32 are connected by a single mode optical fiber 45.

The AC voltage induced in the receiving antenna 33 is applied to modulation electrodes of two optical modulators 41 and 42 connected in series via an impedance conversion circuit 35. The light incident on each of the light modulators 41 and 42 is modulated and emitted by the applied voltage applied to the modulation electrode, and is combined again by the polarization separation / combination unit 23, and is transmitted to the photodetector 32 through the optical circulator 37. Incident.

The optical modulator 1 of this configuration has a so-called polarization compensation function that the sum is constant regardless of the optical power ratio of the incident light of the two optical modulators 41 and 42. There are features.

Next, in FIG. 1, equivalent circuits 51 and 52 of two optical modulators having the same characteristics are shown in a frame separated by a broken line, and these are connected in series.
Lm, Cm, and Rm are the inductance, capacitance, and resistance components of the modulation electrodes of the two optical modulators, respectively. R54 is the resistance component of the coaxial cable,
V53 is an induced voltage component of the receiving antenna, and L55 and C56 constitute an impedance conversion circuit. The impedance conversion circuit and the modulation electrode constitute a series resonance circuit.

When a single optical modulator is connected, 5
When a 0Ω coaxial cable is used, the impedance of a closed circuit formed by the modulation electrode of the optical modulator and the impedance conversion circuit is usually adjusted to be about 5Ω. The modulation electrodes 15 (FIG. 3) of the two optical modulators 41 and 42 having the same characteristics are connected in series so that the same signal is applied in the same phase. Since a 50Ω coaxial cable is used, the impedance of the closed circuit formed by the modulation electrode of the optical modulator and the impedance conversion circuit is doubled, and the closed circuit formed by the modulation electrode of the optical modulator and the impedance conversion circuit is doubled. The impedance of the circuit will be about 10Ω. Therefore, the drop of the voltage applied to the modulation electrode is halved, and the sensitivity is improved by 3 dB.

[0017]

As described above, according to the present invention, regardless of the optical power ratio of the incident light to the two optical modulators, the sum is constant, that is, while maintaining the so-called polarization compensation. In addition, an optical modulator in which the impedance of a closed circuit formed by the modulation electrode of the optical modulator and the impedance conversion circuit does not decrease can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an equivalent circuit of a light modulation device according to the present invention.

FIG. 2 is a configuration diagram of a receiving device using the optical modulation device of the present invention.

FIG. 3 is a configuration diagram of a conventional optical modulator including an optical waveguide element.

FIG. 4 is a diagram showing a configuration of a conventional electric field sensor and a receiving device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical modulator 11 Substrate 12 Incoming / outgoing optical waveguide 13 Phase shift optical waveguide 15 Modulation electrode 17 Terminal 20 Reflector 23 Polarization separation / synthesizer 26, 27, 28 Polarization maintaining optical fiber 31 Light source 32 Photodetector 33 Receiving antenna 35 Impedance Conversion circuit 37 Optical circulator 41, 42, 43 Optical modulator 44, 45, 46 Single mode optical fiber 51, 52 Equivalent circuit of optical modulator 53 V 54 R 55 L 56 C

Claims (1)

[Claims] 1. An optical modulation device including a plurality of optical modulators so that incident light changes its intensity depending on the voltage of an applied electric signal and emits the light. An optical modulator, wherein the modulation electrodes of at least two of the optical modulators are connected in series so that the same electric signal is applied in the same phase.