JPH06289342A - Optical phase modulator - Google Patents

Abstract

PURPOSE:To provide the external optical phase modulator which has infinite controllability matching homodyne local oscillation optical phase control. CONSTITUTION:The optical phase modulator is equipped with a variable branch ratio type coupler 30 which branches input light into 1st output light and 2nd output light, a 1st phase modulating element which is arranged in the optical path of the 1st output light and has a >=180 deg. phase variation range, a 2nd phase modulating element which is arranged in the optical path of the 2nd output light and has a >=180 deg. phase variation range, a phase corrector 5 which is arranged in the optical path of the 1st or 2nd output light and corrects a phase difference generated by the variable branch ratio type coupler 30, and a 3dB coupler 7 which couples the 1st output light and 2nd output light inputted through the 1st phase modulating element, 2nd phase modulating element, or phase corrector 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external optical phase modulator having infinite controllability suitable for local oscillation phase control used in the homodyne system of coherent optical communication.

[0002]

2. Description of the Related Art Usually, one of two types of detection methods, heterodyne and homodyne, is adopted for coherent optical communication. Among them, in the detection method by heterodyne, the optical frequency is once set to the microwave intermediate frequency and then demodulated by a demodulator to obtain a baseband signal. Homodyne, on the other hand, obtains the baseband signal directly from the optical frequency.

On the receiving side, the signal light sent is mixed with the local oscillation light emitted from the laser for the local oscillation light source inside the receiver, and they are received by interfering with each other. On the other hand, in homodyne, at this time, it is necessary that the signal light and the local light have the same optical frequency and phase so that the baseband signal can be directly demodulated. Therefore, in homodyne, control is usually performed to match the optical frequency and phase of local light with the signal light. Up to now, a method of detecting a phase error signal from a received signal and feeding it back to an injection current of a semiconductor laser (LD) as a local light source has been usually used.

However, homodyne requires a very narrow laser line width. Therefore, a solid-state laser or an external cavity type semiconductor laser may be used instead of a normal semiconductor laser. In the solid-state laser, the method of changing the injection current as described above cannot be applied. Further, in the external cavity type semiconductor laser, it is possible to control by changing the injection current, but it is necessary to change the injection current largely for slight phase modulation. As a result, the generated power fluctuation may adversely affect the reception sensitivity, and the use of an external phase modulator allows more efficient control. In addition, Japanese Patent Application No. 04-
An external modulator may be required to perform polarization diversity with homodyne in the configuration described in “Optical receiver”.

Some external modulators capable of infinite phase modulation have been reported in the form of optical frequency modulators, optical SSB modulators, and the like. For example, in Noe, et al.'S report (Electronics Letters Vol.24 No.21 p1348) shown in FIG. 16, infinite phase modulation is realized by using an optical IC having a function of a half-wave plate. That is, the input light input from the optical fiber 101
It is input to the / 2 wavelength element 105 and further output to the optical fiber 101 via the polarization controller 103.

In this configuration, the polarization of the input light to the phase modulator must be a perfect circular polarization, but the perfect circular polarization is input to the optical waveguide of the optical IC because of the asymmetric structure of the waveguide. Difficult due to sex. If the input light is not a perfect circular polarization, the polarization of the output light will change depending on the phase modulation amount. In the coherent optical communication, the polarization variation of the local oscillation light causes a serious deterioration of the reception sensitivity, and thus the configuration using the 1/2 wavelength plate is not suitable.

In addition, Izutsu, et al.'S report (IEEE J.Qua
ntum Electron. Vol.QE-17 No.11 p2255)
SSB modulation is performed with the configuration shown in FIG. In this case,
The input light input to the Y-branch type 3 dB coupler 15 is split, and further split into four by the Y-branch type 3 dB coupler 15. At this time, the Y-branch type 3 dB coupler 1 on one side
5, a π / 2 phase shifter 21 is provided, and four branched waveguides are provided with phase modulators 109, respectively, of which two π phase shifters 107 are provided. . The input light transmitted through these four waveguides has two stages of 3
The dB coupler 7 combines them into one and outputs it.

When this structure is used as a phase modulator,
There is a loss of 6 dB after the phase modulation of Δφ, that is, 3 dB in the 3 dB coupler 7, and 3 dB in the subsequent coupling portion, that is, the 3 dB coupler 7, a total of essentially 6 dB.
If the loss inside the optical IC and the loss of the coupling system are included, the loss becomes very large near 10 dB.

[0009]

As described above, the conventional external infinite phase modulator does not have performance suitable for homodyne local oscillation phase control, such as fluctuation of output polarization and large loss. .

The present invention has been made in view of the above problems, and an object thereof is to provide an external optical phase modulator having infinite controllability suitable for homodyne local oscillation phase control.

[0011]

In order to achieve the above object, the first invention of the present application relates to a variable branching ratio coupler having first and second outputs and a coupler disposed in each optical path.
A phase modulator having a phase variable range of 80 degrees or more, and a phase corrector arranged in either one of the two optical paths for correcting the phase difference caused by the variable branching ratio coupler,
The gist is to have a 3 dB coupler that couples the two outputs.

The second invention of the present application is the first and second inventions.
A variable branching ratio coupler having an output, a phase modulation element having a phase variable range of 180 degrees or more arranged in one optical path, and a phase modulation having a phase variable range of 360 degrees or more arranged in the other optical path. 3 to connect the element and the two outputs
The gist is to have a dB coupler.

The third invention of the present application is the first and second inventions.
Mach-Zehnder interferometer type branching ratio variable coupler having an output of 1 and an optical path arranged at either of two output optical paths.
It is a gist to have a phase modulation element having a phase variable range of 80 degrees or more and a directional coupler type 3 dB coupler for coupling two outputs.

Further, a fourth invention of the present application is that a Mach-Zehnder interferometer type variable branching ratio coupler having first and second outputs and a phase of 180 degrees or more arranged in either of the two output optical paths. Phase modulator with variable range and 2
The gist is to have a π / 2 phase shifter arranged in one of the optical paths of two outputs and a Y-branch type 3 dB coupler for coupling the two outputs.

In the above items, the constituent elements are preferably integrated on the non-linear optical material substrate in the form of an optical waveguide and operate by the electro-optic effect. Further, the non-linear optical material substrate is made of niobate. It is a lithium substrate. Alternatively, the constituent elements are integrated on a semiconductor substrate.

[0016]

According to the first invention of the present application, a phase modulation element having a phase variable range of 180 degrees or more is arranged in each of the optical paths of the first and second outputs of the variable branching ratio type coupler. A phase corrector is arranged on either side to correct the phase difference caused by the variable branching ratio coupler, and the two outputs are further combined by a single-stage 3 dB coupler.

In the second invention of the present application, a phase modulation element having a phase variable range of 180 degrees or more is arranged in one optical path of the first and second outputs of the variable branching ratio coupler, and in the other optical path. A phase modulation element having a phase variable range of 360 degrees or more is arranged, and further two outputs are coupled by a single stage 3 dB coupler.

According to a third aspect of the present invention, a phase modulation element having a phase variable range of 180 degrees or more is arranged in the optical path of either the first output or the second output of the Mach-Zehnder interferometer type variable branching ratio coupler. Further, the two outputs are combined by a single-stage directional coupler type 3 dB coupler.

According to a fourth aspect of the present invention, a phase modulator having a phase variable range of 180 degrees or more is arranged in the optical path of either the first output or the second output of the Mach-Zehnder interferometer type variable branching ratio coupler. And π in either of the two optical paths
A / 2 phase shifter is arranged, and two outputs are combined by a single stage Y-branch type 3 dB coupler.

[0020]

First, referring to the block diagram shown in FIG.
The operation principle of the present invention will be described. The principle of the present invention is based on the formula: cos (ωt + φ) = cosωtcosφ−sinωtsinφ (1). Here, ω = 2πf, f: frequency of light, and φ is a phase to be modulated. First, the incident light cosωt
Of the optical power of the cos ²
φ: sin ² φ (amplitude ratio | cosφ |: | sinφ |)
Branch to. At this time, the phase of the light of the second output with respect to the phase of the light of the first output is determined by the branching ratio variable coupler 1 to be used.
Depends on the type. Therefore, the electric field of the first output light can be written as | cosφ | cosωt, and the electric field of the second output light can be written as | sinφ | cos (ωt + θ). θ is the difference between the phase of the light of the first output and the phase of the light of the second output.

Here, the phase switch 3 of each optical path is modulated so that the absolute value symbol of the electric field amplitude is removed. That is, taking the first output as an example, if cosφ> 0, no modulation is performed, and if cosφ <0, 180 degree phase modulation is performed. This 180 degree phase modulation causes cosωt
Becomes cos (ωt ± π) = − cosωt, and if written including the amplitude, − | cosφ | cosωt = cosφco
sωt. The same operation is performed on the second output.

Further, in order to correct θ, the phase corrector 5 is added to either one. Since θ is the relative phase difference between the first output and the second output, the phase corrector 5 may be inserted in either side. Here, the phase corrector 5 is inserted in the optical path of the second output.

The phase corrector 5 sets the second output to sin φcos ωt. When this is mixed by the 3 dB coupler 7, the phase of the light output in the diagonal direction is shifted by 90 degrees in the 3 dB coupler 7, so the 3 dB coupler 7 shown in FIG.
On the upper output branch above 4

[Outer 1] Light is obtained.

In order to follow φ → infinity by this method, the branching ratio of the coupler on the input side is sin ² φ, cos ²
By using φ and the periodicity as shown in FIG. 15, the range of the arrow in the drawing may be folded and used. Also,
Since the 180-degree phase modulation given after branching only switches between 180 degrees, there is no problem with infinite controllability. Further, as can be seen from FIG. 15, this switch switches when the optical power passing through the respective optical paths is 0, so that there is no unnecessary fluctuation in the output optical power.

When this method is used, a bulk element may be used, but it is expected that a waveguide structure is often adopted in consideration of modulation efficiency, optical path length error, and the like. In this case, the incident polarization is almost limited to linear polarization. (Any more complicated structure can be used to support arbitrary incident polarized waves.) Unlike the circular polarized waves described in the conventional example, stable linear polarized waves can be obtained easily by using a polarizer. Is. Also, with this method, the essential loss due to modulation is 3 dB.

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical phase modulator according to the present invention.

The optical phase modulator shown in FIG. 1 is an example integrated on a lithium niobate substrate. In this example,
Directional coupler type variable branching ratio coupler 30, 180 degree phase switch 3 arranged in both optical paths, phase correctors 5, 3 dB coupler 7, voltage conversion circuit 11, and modulation phase signal φ Controller 9 for controlling
Consists of. Further, although it is possible to configure them with a bulk element, it is difficult for the bulk element to match the optical path length difference from branching to mixing, so it is desirable that they are integrated in some form.

The input variable branch ratio coupler 30 is a directional coupler type. The operation of the directional coupler type with respect to the applied voltage V ₂ is as shown in FIG. To obtain the change of the sine wave is the input signal voltages V ₁ proportional to the phase difference to be modulated, an input signal voltages V ₁ voltage conversion circuit 5 with the input-output characteristics such as pre FIG 2 (b) It is necessary to convert it to the applied voltage V ₂ . Further, in the directional coupler type as shown in FIG. 1, the variable range of the branching ratio from 0: 1 to 1: 0 is not always obtained, and therefore, in order to guarantee that, FIG.
Alternatively, an inverted Δβ type as shown in may be used.

In FIG. 1, the output 3 dB coupler 7
However, if it is not actually 3 dB, the output amplitude fluctuates, so it is advisable to attach electrodes as shown in FIG. 3 so that the branching ratio can be adjusted. Alternatively, as shown in FIG. 5, a Y-branch type may be used for the output 3 dB coupler 7, which is relatively easy to obtain a characteristic close to 3 dB. In the Y branch, there is no 90 degree phase shift as in the directional coupler type 3 dB coupler, so the operating range of the phase corrector 5 is 90 degree shifted from that of the directional coupler type 3 dB coupler 7.

As for the phase corrector 5, as shown in FIG. 6, the phase corrector 5 may be omitted and the modulation voltage of the 180 ° phase switch 3 may be changed.

The variable branching ratio type coupler 1 provided on the input side has a slightly complicated structure without depending on the directional coupler type, but a Mach-Zehnder type coupler as shown in FIG. 7 may be used. . In the Mach-Zehnder type, the branching ratio changes sinusoidally with respect to the applied voltage, so that an input characteristic conversion circuit, unlike the case of the directional coupler type, is not necessary. Further, if a high applied voltage is allowed, the turning point may be not only π / 2 shown in FIG. 8 but also an integral multiple of π / 2.

That is, since the two outputs of the Mach-Zehnder variable branching ratio variable coupler 19 are cosφ: sinφ with no absolute value sign, depending on how the folding point is taken, as shown in FIG. Need only be provided on one side. For example, the turning point is shown in FIG.
If it is set like the area A shown in FIG. 8, it is sufficient to switch only on the sin φ side, and if it is set like the area B shown in FIG. 8, only the cos φ side is required.

Further, since there is no phase difference between the two outputs in the Mach-Zehnder type, when the directional coupler type is used for the output 3 dB coupler, the phase corrector 5 can be omitted (FIG. 9). When the output coupler is the Y-branch type 15, the π / 2 phase shifter 21 is required in either optical path (FIG. 10).

In the case of the Mach-Zehnder type, the internal 3
If the branching ratio of the dB coupler 17 is not actually 3 dB, the branching ratio of 0 to 1 cannot be obtained. Therefore, in order to ensure this, as shown in FIG. You may be able to adjust.

So far, the materials integrated on the lithium niobate substrate have been described, but other materials may be used as long as they have an electro-optical effect and can be formed into a waveguide.

It is known that the refractive index of a semiconductor is changed by the injection of carriers, but it is also possible to manufacture a semiconductor having the structure as described above.

Since a relatively large change in the refractive index of a semiconductor can be easily obtained, the variable branching ratio coupler may be made of a Y branching type variable branching ratio coupler 25 as shown in FIG.

If a substance having a very large electro-optical constant is used, the symmetry between the two optical waveguides 13 is not broken even in the directional coupler type variable branching ratio coupler 29 as shown in FIG.
The structure can change the bond length. This Figure 1
With the configuration shown in FIG. 3, the branch output ratio is cosφ: sinφ as in the case of the Mach-Zehnder type, and the phase difference between the two outputs is always π / 2.
By making the dB coupler 7 a Y-branch type, a simple structure as shown in FIG. 13 can be obtained. Of course, the output 3 dB coupler 7 can be of a directional coupler type, in which case a π / 2 phase shifter is required for phase compensation.

The arrangement of the electrodes in the drawings of the above embodiments is based on the assumption that a Z-cut lithium niobate substrate and a substrate having a large electro-optic constant in the same direction as that of the Z-cut lithium niobate substrate are used. Even with a substrate having a large electro-optic constant, the electrode arrangement may be appropriately changed and used.

[0040]

As described above, by using the constitution of the present invention,
It is possible to obtain a phase modulator having an infinite phase tracking property, a small loss, no output polarization fluctuation, and performance suitable for homodyne local oscillation phase control.

[Brief description of drawings]

FIG. 1 is an embodiment of the present invention, which is an example in which a directional coupler type is used for a coupler having a variable branching ratio, which is integrated on a lithium niobate substrate.

FIG. 2A is a diagram showing an operation with respect to an applied voltage of the variable branch ratio coupler, FIG. 2B is an input / output characteristic of the voltage conversion circuit, and FIG. 2C is an operation of the variable branch ratio coupler with respect to an input voltage. It is a characteristic.

FIG. 3 is a part of an embodiment of the present invention, in which the branching ratio is variable,
This is an example using a B coupler.

FIG. 4 is a part of an embodiment of the present invention, which is an example in which an inversion Δβ type branching ratio variable coupler is used.

FIG. 5 is a part of an embodiment of the present invention, which is an example using a Y-branch type 3 dB coupler.

FIG. 6 is an example of a part of an embodiment of the present invention in which a 180-degree phase switch and a phase corrector are integrated.

FIG. 7 is an example of using a Mach-Zehnder variable branching ratio coupler as a part of the embodiments of the present invention, and FIG. 7A shows an input 3 dB coupler of a Mach-Zehnder variable branching ratio coupler having a Y-branch type. (B) is an example in which the input 3 dB coupler of the Mach-Zehnder variable branching ratio coupler is a directional coupler type.

FIG. 8 is a diagram showing operating characteristics of a Mach-Zehnder variable branch ratio coupler.

FIG. 9 is a block diagram when a Mach-Zehnder type is used for a variable branching ratio coupler according to one embodiment of the present invention.

FIG. 10 is a block diagram when a Mach-Zehnder type is used for the variable branching ratio coupler in one of the embodiments of the present invention.

FIG. 11 is a part of an embodiment of the present invention in which a Mach-Zehnder variable branch ratio coupler is used, and a 3 dB coupler inside the Mach-Zehnder is a variable branch ratio type, and (a) is a Mach-Zehnder. The input 3 dB coupler of the variable type branching ratio coupler is an example of a Y-branch type, and (b) is an example of the input 3 dB coupler of the Mach-Zehnder type variable branching ratio coupler of a directional coupler type.

FIG. 12 is one of the embodiments of the present invention, which is an example in which a Y-branch type is used as a coupler having a variable branching ratio, which is integrated on a semiconductor substrate.

FIG. 13 is one of the embodiments of the present invention, which is an example in which a directional coupler is used for a coupler having a variable branching ratio, which is integrated on a nonlinear optical material substrate.

FIG. 14 is a block diagram showing the principle of the present invention.

FIG. 15 is a diagram for explaining the operation of the present invention.

FIG. 16 is a diagram showing an example of a conventional example.

FIG. 17 is a diagram showing an example of a conventional example.

[Explanation of symbols]

 1 Branch ratio variable coupler 3 180 degree phase switch 5 Phase corrector 7 3dB coupler 9 Controller 11 Voltage conversion circuit 13 Optical waveguide 15 Y branch type 3dB coupler 17 Directional coupler type 3dB coupler 19 Mach-Zehnder type branch ratio variable coupler 21 π / 2 phaser 23 Output variable directional coupler type 3 dB coupler 25 Y branch type variable branching ratio coupler 27 Phase modulation element 29 Directional coupler type variable branching ratio coupler 30 Directional coupler type variable branching ratio coupler

Claims (4)

[Claims] 1. A variable branching ratio coupler for splitting input light into a first output light and a second output light, and a phase variable range of 180 degrees or more arranged in the optical path of the first output light. A first phase modulation element, a second phase modulation element having a phase variable range of 180 degrees or more arranged in the optical path of the second output light, an optical path of the first output light or a second output A phase corrector that is disposed in one of the optical paths of light and corrects the phase difference generated by the variable branching ratio coupler, and is input via the first phase modulator, the second phase modulator, or the phase corrector. And a 3 dB coupler that couples the first output light and the second output light. 2. A variable branching ratio coupler for branching the input light into a first output light and a second output light, and a phase variable range of 180 degrees or more arranged in the optical path of the first output light. A first phase modulation element, a second phase modulation element arranged in the optical path of the second output light and having a phase variable range of 360 degrees or more, the first phase modulation element and the second phase modulation element An optical phase modulator having a 3 dB coupler that couples the first output light and the second output light, which are respectively input via the optical phase modulator. 3. A Mach-Zehnder interferometer type branching ratio variable coupler for branching input light into first output light and second output light, and an optical path for the first output light or the second output light. A phase modulator having a variable phase range of 180 degrees or more, a first output light and a second output light respectively output from the Mach-Zehnder interferometer type variable branch ratio coupler and the phase modulator. An optical phase modulator having a directional coupler type 3 dB coupler for coupling. 4. A Mach-Zehnder interferometer type branching ratio variable coupler for branching input light into a first output light and a second output light, and an optical path for the first output light or the second output light. A phase modulation element having a phase variable range of 180 degrees or more, a π / 2 phaser arranged in the optical path of the first output light or the second output light, and the phase modulation element or π / 2 Y-branch type 3 for coupling the first output light output from the phaser and the second output light
An optical phase modulator having a dB coupler.