JPH05158004A - Optical modulator - Google Patents

Abstract

PURPOSE:To provide the optical modulator which is low in driving voltage and suitably reduced in size. CONSTITUTION:The Mach-Zehnder type optical modulator which controls interference at a 2nd branch part 10 by phase variation at branch waveguides 6 and 8 is provided with a delay optical waveguide 18 which is directionally coupled with a 1st branch waveguide 6 at two lengthwise points and then provided with control electrodes 24 and 26 which control a coupling ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator applicable to an external modulation system in an optical communication system.

In many conventional optical communication systems,
A direct modulation method has been adopted in which a current flowing through a semiconductor laser is modulated by a data signal. However, in the direct modulation method, the influence of wavelength fluctuation (chirping) of the output optical signal increases as the transmission speed increases,
Long-distance transmission has become difficult due to wavelength dispersion in optical fibers.

Therefore, an external modulation method using a Mach-Zehnder type optical modulator or other optical modulators, which is in principle less likely to cause chirping, is being studied. A typical optical modulator currently put into practical use utilizes the electro-optical effect of a dielectric crystal such as LiNbO ₃ or LiTaO ₃ . Practical use of this type of optical modulator is that the driving voltage is low, the polarization state of the input light need not be considered, and the versatility with respect to the signal light wavelength is high.

[0004]

2. Description of the Related Art FIG. 10 is a plan view of a conventional general Mach-Zehnder type optical modulator. This optical modulator includes an input-side optical waveguide 2, a first branching unit 4 that splits light propagating through the input-side optical waveguide 2 into two, and a first branching unit that propagates the light split by the first branching unit 4, respectively. A waveguide 6 and a second branching waveguide 8, a second branching section 10 for joining the lights propagating through these branching waveguides, and an output side optical waveguide 12 for propagating the light joined by the second branching section 10. , A first loading electrode 14 and a second loading electrode 16 respectively loaded in the first and second branch waveguides 6 and 8.

In such a configuration, when an appropriate voltage is applied between the first and second loading electrodes 14 and 16, the first and second loading electrodes 14 and 16 are applied.
The propagating lights in the branch waveguides 6 and 8 undergo phase changes in mutually opposite directions. On the other hand, when the electric fields of the lights respectively entering the first and second branch waveguides 6 and 8 to the second branch part 10 are in phase due to the interference action in the second branch part 10, these lights are output side. If the electric fields of the light coupled to the optical waveguide 12 to reach the output end and incident on the second branching section 10 from the first and second branching waveguides 6 and 8 are in opposite phases, these lights are emitted. Is not coupled to the output side optical waveguide 12 and is emitted to the outside through the clad and does not reach the output end. Therefore, the intensity of light can be modulated by controlling the voltage applied to the loading electrodes 14 and 16.

[0006]

In the optical modulator utilizing the phase change of the propagating light in the branch waveguide, the amount of phase change (phase modulation efficiency) per unit applied voltage in the branch waveguide is not necessarily large. In order to perform the intensity modulation operation of No. 2, there is a problem that the driving voltage of the optical modulator becomes high. Although it is possible to reduce the drive voltage by forming the branch waveguide long, in this case, there is a problem that the optical modulator becomes large.

Therefore, an object of the present invention is to provide an optical modulator which has a low driving voltage and is suitable for miniaturization. Also,
In the conventional optical modulator, since the phase modulation efficiency in the branching waveguide differs depending on the polarization mode of the propagating light, only one polarization mode (generally, the polarization mode with the higher phase modulation efficiency) is incident. However, there is a problem that the optical configuration becomes complicated.

In order to eliminate such polarization dependence, a driving voltage for turning on / off the optical modulator for one polarization mode and a turning on / off operation for the other polarization mode are set. The drive voltage of the optical modulator may be set so that the drive voltages match, but the drive voltage satisfying such a condition is generally high and not practical.

Therefore, an object of the present invention is to provide an optical modulator having a low driving voltage and no polarization dependence. Further, since the operating conditions of the optical modulator differ depending on the wavelength of light used, there is a problem that once the drive voltage and the like are set, the optical modulator can be used only for a specific wavelength.

Therefore, an object of the present invention is to provide an optical modulator having high versatility with respect to the wavelength of signal light.

[0011]

One or more of the above technical problems can be solved by the first or second structure of the optical modulator of the present invention.

A first configuration of the optical modulator according to the present invention comprises an input side optical waveguide, a first branching section for branching the light propagating through the input side optical waveguide into two, and a light branched at the branching section. A first branch waveguide and a second branch waveguide to be propagated respectively, a second branch section to merge the light propagating in the first and second branch waveguides, and a light to be merged in the second branch section are propagated. An output side optical waveguide, a first loading electrode and a second loading electrode loaded on the first and second branch waveguides, respectively, and a length of the first and second loading electrodes arranged side by side on at least one of the first and second branch waveguides. The delay optical waveguide is directionally coupled to the branching waveguide at two points in the direction, and a control electrode for controlling the coupling ratio in these directional coupling portions.

A second configuration of the optical modulator according to the present invention is such that an input side optical waveguide, a first branching section for branching the light propagating through the input side optical waveguide into two, and a light branched at the branching section. A first branch waveguide and a second branch waveguide to be propagated respectively, a second branch section to merge the light propagating in the first and second branch waveguides, and a light to be merged in the second branch section are propagated. The output side optical waveguide, the first loading electrode and the second loading electrode respectively loaded on the first and second branch waveguides, and the at least one of the first and second branch waveguides are directionally coupled. A ring type optical waveguide and a control electrode for controlling the coupling ratio in this directional coupling section are provided.

[0014]

According to the first configuration of the present invention, in addition to the configuration of the conventional Mach-Zehnder type optical modulator, a delay optical waveguide is provided in parallel in at least one of the first and second branch waveguides, and this delay optical waveguide is provided. Since two points in the longitudinal direction of the delay optical waveguide are directionally coupled to the branch waveguide (branch waveguide on the side where the delay optical waveguides are arranged in parallel) and the coupling ratio is controlled, the delay optical waveguide Can be used to phase delay one of the lights branched by the first branching unit. Therefore, the first and second
As compared with the conventional configuration in which the phase change is given only by the branch waveguide, if the scale of the optical waveguide is the same, the driving voltage can be significantly reduced.

Also in the case of the second configuration of the present invention, the first
Further, since the ring type optical waveguide is directionally coupled to at least one of the second branching waveguide and the coupling ratio thereof is controlled, the driving voltage can be drastically increased as in the case of the first configuration of the present invention. It can be reduced.

Therefore, according to the present invention, it is possible to provide an optical modulator having a low driving voltage and suitable for miniaturization. still,
Other effects of the first or second configuration of the present invention will be apparent from the description of the embodiments below.

[0017]

The preferred embodiments of the present invention will be described below. FIG. 1 is a plan view of an optical modulator showing an embodiment of the first structure of the present invention, and FIG. 2 is a sectional view taken along the line AA of this optical modulator. 1 is L
a waveguide substrate made of a ferroelectric electro-optical material such as LiNbO ₃ and LiTaO _3, by thermal diffusion from the surface dopant such as Ti in the waveguide substrate, a Y branching path two combined shape optical of A waveguide is formed.

The optical waveguide includes an input side optical waveguide 2, a first branching section 4 for branching the light propagating through the input side optical waveguide 2, and a first branching waveguide 6 for propagating the branched light respectively.
And a second branching waveguide 8, a second branching section 10 for combining the branched lights, and an output side optical waveguide 12 for propagating the combined light.

First and second loading electrodes 14 and 16 are loaded on the first and second branch waveguides 6 and 8, respectively. For example, an input signal in the microwave region is input to the input ends 14a and 16a of the loading electrodes 14 and 16, and the output end 1 of the loading electrode 1 is input.
A termination resistance of, for example, 50Ω is connected to 4b and 16b.

Reference numeral 18 is a curved delay optical waveguide arranged in parallel in the first branch waveguide 6, and both ends of the delay optical waveguide 18 are arranged so as to approach the first branch waveguide 6 in parallel, respectively. Directional coupling portions 20 and 22 are formed.

In order to control the coupling ratio in the directional coupling portions 20 and 22, control polarizations 24 and 26 are loaded in the respective directional coupling portions of the delay optical waveguide 18. still,
As shown in FIG. 2, each electrode is loaded on the optical waveguide via a buffer layer 28. This buffer layer is not shown in FIG. 1 to avoid obscuring the drawing. The control electrodes 24 and 26 are loaded in a traveling wave type like the loading electrodes 14 and 16.

The coupling of optical power in the directional coupler will be described. Now, consider an optical coupling in the case where two optical waveguides are appropriately arranged in parallel as shown in FIG. 3 (A). The two waveguides are respectively connected to the first waveguide 30.
A and second waveguides 30B will be referred to, and modes of light propagating through them will be referred to as a first mode and a second mode, respectively. Then, the mode amplitudes of the first and second modes are set to values a ₁ and a ₂ that are standardized so that twice the square of the absolute value thereof becomes equal to the mode power.

Assuming that the light propagation direction is on the z-axis, a minute distance Δ
The minute changes Δa ₁ and Δa ₂ of the mode amplitudes a ₁ and a ₂ when the light propagates by z are expressed by the following equations.

[0024]

[Equation 1]

Where β ₁ is the propagation constant of the first mode, β
₂ is the propagation constant of the second mode, c ₁₂ is from the second mode to the first
The mutual coupling coefficient to the mode, c ₂₁ is the mutual coupling coefficient from the first mode to the second mode. The following differential equation is derived from the equation (1).

[0026]

[Equation 2]

It will be described that a predetermined relationship is established between the mutual coupling coefficients c ₁₂ and c ₂₁ when both waveguides are lossless. First, the total power P of light propagating through both waveguides is

[Equation 3] Then, if both sides of this equation are differentiated by z,

[Equation 4] Is obtained. In addition, * represents a complex conjugate. The following equation is obtained from the equations (2) and (3).

[0028]

[Equation 5]

When both waveguides are lossless, the right side of equation (4) is equal to zero according to the law of power conservation, and therefore the following equation is obtained.

[0030]

[Equation 6]

Then, a ₂ is deleted from the equation (2),
Using the equation (5), the following differential equation is obtained.

[0032]

[Equation 7]

By solving the equation (6) under the initial condition of the mode amplitude at z = 0, the following equation is obtained.

[0034]

[Equation 8]

Here, βa, βb, and βd are defined as follows, respectively.

[0036]

[Equation 9]

Now, at the position of z = 0, it is assumed that the optical power is input only to the first waveguide and the optical power input to the second waveguide is 0. That is, 2 | a ₁ (0) | ² = 1, 2 | a ₂
It is assumed that (0) | ² = 0. Substituting these into the equation (7), the electric powers P ₁ (z) and P ₂ (z) of the respective waveguides are expressed as follows.

[0038]

[Equation 10]

Here, F is defined by the following equation.

[0040]

[Equation 11]

A graph of the equation (8) is shown in FIG.
It will become Articles for transferring all electric power to the other waveguide
Condition, that is, the condition for perfect coupling (phase matching condition)
Is β ₁= Β₂Is. Also, the length that gives a perfect bond
(Complete bond length) L is

[Equation 12] Becomes After all, the coupling coefficient c ₁₂ in the directional coupler as shown in FIG. 3A is given by the following equation.

[0042]

[Equation 13]

Where c is the speed of light and ε₀Is the permittivity of the vacuum,
λ is the wavelength of light, E₁(X, y) is the mode charge of the first waveguide
Field distribution, E₂(X, y) is the mode electric field component of the second waveguide
Cloth, n _aIs the refractive index of the first waveguide, n_cIs the first and second waveguides
It is the refractive index of the clad between the paths.

As described above, the coupling coefficient c ₁₂ depends on the wavelength and mode of light. In other words, the coupling length can be determined according to the wavelength and mode of light used. On the other hand, in order to reduce F, | β ₁ −β ₂ | /
It is sufficient to increase | c ₁₂ |. Therefore, in order to realize the switching of the coupling ratio between 0% and 100% in the directional coupling portion with a low driving voltage, c ₁₂ should be reduced or the difference between the mode propagation constants in both waveguides should be increased. It is sufficient to change the refractive index as described above.

In the optical modulator shown in FIG. 1, in order to reduce the driving voltage, the directional coupling section 2 is used when the logic level of the input modulation signal is low for light of a specific wavelength.
The coupling ratio at 0 and 22 is 100%, the electric fields of the lights respectively incident on the second branching section 10 from the first and second branching waveguides 6 and 8 are in opposite phases, and the logic level of the input modulation signal is high. At this time, if the coupling ratio in the directional coupling portions 20 and 22 becomes 0% and the electric fields of the lights respectively incident on the second branching portion 10 from the first and second branching waveguides 6 and 8 have the same phase. Good.

That is, in order to satisfy such a condition,
The optical path length of the delay optical waveguide 18 may be set to control the voltage applied to the first and second loading electrodes 14 and 16 and the control electrodes 24 and 26.

A more specific description is as follows.
The propagation path of light in the optical modulator of FIG.
It changes as follows according to the voltage given to 4, 26 (actually applied between the control electrodes 24, 26 and the first loading electrode 14). FIG. 4 is a diagram for explaining such switching of the propagation route, and in order to facilitate the explanation of the route, the first
The branch portion 4 is A, the directional coupling portion in the first branch waveguide 6 is B, D, the point between B and D is C, and the second branch portion 1 is
0 is E, and B and B are respectively in the second branch waveguide 8.
B ', C', D'corresponding to C, D, and B ', C', D'for the delay optical waveguide 18, respectively.
They are C ″ and D ″.

It is assumed that this optical modulator is turned on when a voltage is applied and turned off when no voltage is applied. The optical path length of each propagation path is designed as follows. The optical path length of A → B → C → D → E is L A → B ′ → C ′ → D ′ → E, and the optical path length of L A → B ″ → C ″ → D ″ → E is L + (half (Length corresponding to wavelength) Furthermore, B → B ″, D ″ → D without voltage applied
The directional coupling portion having the full coupling length is configured so that 100% of the power transfer is generated.

Then, when no voltage is applied, there are the following two propagation paths. A → B ″ → C ″ → D ″ → E A → B ′ → C ′ → D ′ → E Therefore, an optical path difference corresponding to a half wavelength occurs between the propagation path and the propagation path of As a result of multiplexing and interference of propagating light, the output of this optical modulator is turned off.

Next, if a voltage is applied so that the power transfer does not occur in the directional coupling portion, there are the following two propagation paths. A->B->C->D-> E A-> B '->C'-> D '-> E Therefore, the optical path difference between these propagation paths becomes zero and the output of this optical modulator is turned on.

The above description of the operation is given for the case where the optical path difference between the route A → B → C → D → E and the route A → B ′ → C ′ → D ′ → E is equal to zero or an integral multiple of the wavelength. Is something
In this case, it is not necessary to apply a voltage between the first and second loading electrodes 14 and 16, but due to problems in the optical waveguide manufacturing technology,
It may be difficult to always set such an optical path difference. In such a case, this can be dealt with by applying a voltage in an interpolative manner between the first and second loading electrodes 14 and 16.

The conditions for turning on the output of this optical modulator are:
The phase difference of the light entering the second branch is that the phase difference of the light entering the second branch is 2kπ (k is zero or a natural number) and the output of this optical modulator is off. (2k +
1) Since π (k is zero or a natural number), it suffices to apply a voltage for correction between the first and second loading electrodes 14 and 16 so as to satisfy such a condition. In this case, the optical path difference between the first and second branch waveguides can be set so that the applied voltage for correction is sufficiently smaller than the applied voltage for the on / off operation in the conventional configuration. There is no fear that the drive voltage will increase.

In the optical modulator shown in FIG. 1,
When the logical level of the input modulation signal is high for light of a specific wavelength, the coupling ratio in the directional coupling sections 20 and 22 becomes 100%, and the first and second branch waveguides 6 and 8 to When the electric fields of the lights respectively entering the branching units 10 are in opposite phases and the logic level of the input modulation signal is low, the coupling ratio in the directional coupling units 20 and 22 becomes 0% and the first and second branching guiding units. You may make it the electric field of the light which injects into the 2nd branch part 10 from the waveguides 6 and 8 in-phase.

When the logical level of the input modulation signal is low for light of a specific wavelength, the directional coupling section 20,
When the coupling ratio at 22 is 100%, the electric fields of the lights incident on the second branching section 10 from the first and second branching waveguides 6 and 8 are in phase, and the logic level of the input modulation signal is high, The coupling ratio in the directional coupling portions 20 and 22 may be 0%, and the electric fields of the lights incident on the second branching portion 10 from the first and second branching waveguides 6 and 8 may have opposite phases.

Further, when the logical level of the input modulation signal is high for light of a specific wavelength, the directional coupling section 2
When the coupling ratio at 0 and 22 is 100% and the electric fields of the lights entering the second branching section 10 from the first and second branching waveguides 6 and 8 are in phase, the logical level of the input modulation signal is low. In addition, even if the coupling ratio in the directional coupling portions 20 and 22 becomes 0% and the electric fields of the lights respectively incident on the second branching portion 10 from the first and second branching waveguides 6 and 8 have opposite phases. Good.

In the configuration shown in FIGS. 1 and 2,
The propagation ratio of the delay optical waveguide in the directional coupling section is changed according to the voltage applied to the control electrode to control the coupling ratio in the directional coupling section. The coupling ratio can also be controlled by.

FIG. 5 is a plan view of an optical modulator showing another embodiment of the first structure, and FIG. 6 is a sectional view taken along line BB of this optical modulator. In this embodiment, in the directional coupling portions 20 and 22, the control electrodes 2 that are respectively close to the delay optical waveguide 18 are provided.
4'and 26 'are formed in advance, and the control electrodes 24' and 26 'on the waveguide substrate 1 are controlled by the voltage applied to the control electrodes.
The refractive index of the region 32 immediately below is made equal to the refractive index of the delay waveguide 18. With this, when a voltage is applied to the control electrode, the width of the delay optical waveguide is substantially expanded in the directional coupling portion, and the coupling ratio can be controlled.

In this case, since the buffer layer need not be provided, the driving voltage can be further reduced. FIG. 7 is a plan view of an optical modulator showing an embodiment of the second configuration of the present invention. In this embodiment, instead of the delay optical waveguide, the ring type optical waveguide 34 is directionally coupled to the first branching waveguide 6, and the control electrode 38 is partly provided in the directional coupling portion 36. It is loaded. Even with this configuration, the present invention can be implemented in accordance with the operation principle in the above-described embodiments.

That is, when the input modulation signal level is low for light of a specific wavelength, the coupling ratio in the directional coupling section 36 becomes 100%, and the first and second branch waveguides 6, 6.
When the electric fields of the lights respectively entering the second branching section 10 from 8 are in opposite phases and the input modulation signal level is high, the coupling ratio in the directional coupling section 36 becomes 0% and the first and second
The ring type optical waveguide 34 is arranged so that the electric fields of the lights respectively incident from the branching waveguides 6 and 8 to the second branching portion 10 have the same phase.
The optical path length of the first and second loading electrodes 1
By controlling the voltage applied to the control electrodes 4 and 16 and the control electrode 38, the on / off operation at a low driving voltage becomes possible.

By the way, in the case where the optical modulator of FIG. 1 or FIG. 7 is formed of a Z-cut type LiNbO ₃ crystal, it has a polarization plane perpendicular to the substrate surface in order to obtain required operating characteristics. It is necessary to enter TM-mode light. When TE mode light having a plane of polarization parallel to the substrate is incident, its phase modulation efficiency is significantly smaller than that for TM mode light, so it is necessary to set different operating conditions. It is difficult to drive.

Therefore, in the optical modulator of FIG. 1 or FIG. 7, it is not necessary to specify the polarization mode of the incident light as follows. For example, in the configuration of FIG.
For the polarization mode (TE mode) with low phase modulation efficiency in the second branch waveguides 6 and 8, the directional coupling section 20
The coupling ratio at 22 is 100%, and the coupling ratio is 0 for the polarization mode (TM mode) with high phase modulation efficiency.
%, The voltage applied to the control electrodes 24 and 26 is controlled, and the electric fields of both modes incident on the second branching portion 10 from the first branching waveguide 6 are in phase with each other. The optical path length of is set.

According to this structure, the voltage applied to the loading electrodes 14 and 16 enables ON / OFF operation for the TM mode, and for the TE mode, an insufficient phase change can be applied by the delay optical waveguide 18. Therefore, efficient intensity modulation is possible without considering the polarization mode of incident light.

When the optical modulator of FIG. 7 is used, the voltage applied to the control electrode is controlled as in the case of FIG. 1, and both modes of incidence from the first branch waveguide 6 to the second branch section 10 are controlled. The optical path length of the ring-type optical waveguide 34 may be set so that the electric fields of 1 are in phase.

In carrying out the present invention, in order to cope with a manufacturing error in the optical path length of the ring type optical waveguide 34 in FIG. 7, for example, as shown in FIG. Another ring type optical waveguide 3 is provided in the second branch waveguide 8.
4'may be directionally coupled, and the coupling ratio in this directional coupling portion may be controlled by the voltage applied to the control electrode 38 '.

Further, in the configuration of FIG. 1, in order to cope with the manufacturing error of the optical path length of the delay optical waveguide 18, the second
The delay optical waveguide may be directionally coupled to the side of the branch waveguide 8 and the coupling ratio at the directional coupling portion may be controlled.

Next, an example of an optical modulator that can operate with respect to a plurality of specific wavelengths will be described by applying the configuration of FIG. FIG. 9 is a plan view of an optical modulator showing the embodiment. In this example, two delay optical waveguides 18A and 18B, both ends of which are directionally coupled to the first branch waveguide 6, are provided, and control electrodes 24A, 26A and 2 are provided at the respective directional coupling portions.
4B and 26B are provided.

Then, in the directional coupling portion of the delay optical waveguide 18A, light of the first wavelength (for example, 1.3 μm) is directionally coupled at a coupling ratio of 100% when no control voltage is applied, At the directional coupling portion of the delay optical waveguide 18B, a second wavelength different from the first wavelength (for example, 1.5
The light (5 μm) is directionally coupled at a coupling ratio of 100%.

Further, the optical path lengths of the delay optical waveguides 18A and 18B are set so that the ON / OFF operation can be performed for both the first and second wavelength light. With this configuration, intensity modulation can be performed on the light having the first and second wavelengths, and thus the versatility of the optical modulator is increased. Further, by making both the first and second wavelengths of light incident and operating this optical modulator, wavelength division multiplexing transmission and wavelength division bidirectional transmission can be easily implemented.

According to the configuration of FIG. 9, three or more delay optical waveguides may be provided, or a ring type optical waveguide may be provided instead of the delay optical waveguides.

[0070]

As described above, according to the present invention,
It is possible to provide an optical modulator that has a low driving voltage and is suitable for downsizing.

There is also an effect that it is possible to provide an optical modulator having a low driving voltage and no polarization dependency. Further, there is an effect that it becomes possible to provide an optical modulator having high versatility with respect to the wavelength of the signal light.

[Brief description of drawings]

FIG. 1 is a plan view of an optical modulator showing an example of a first configuration of the present invention.

2 is a cross-sectional view taken along the line AA of the optical modulator shown in FIG.

FIG. 3 is an explanatory diagram of optical coupling in a directional coupling section.

FIG. 4 is an explanatory diagram of switching of light propagation paths by directional coupling.

FIG. 5 is a plan view of an optical modulator showing another embodiment of the first configuration of the present invention.

6 is a sectional view taken along line BB of the optical modulator shown in FIG.

FIG. 7 is a plan view of an optical modulator showing an example of the second configuration of the present invention.

FIG. 8 is a plan view of an optical modulator showing another embodiment of the second structure of the present invention.

9 is a plan view of an optical modulator showing an application example of the configuration of FIG. 1. FIG.

FIG. 10 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 2 Input Side Optical Waveguide 4 First Branching Section 6 First Branching Waveguide 8 Second Branching Waveguide 10 Second Branching Section 12 Output Side Optical Waveguide 14 First Loading Electrode 16 Second Loading Electrode 18 Delaying Optical Waveguide 24, 26, 38 control electrode 34 ring type optical waveguide

Claims (6)

[Claims] 1. An input side optical waveguide (2), a first branching section (4) for branching light propagating through the input side optical waveguide into two, and a first branching section for propagating light branched at the branching section, respectively. The branch waveguide (6) and the second branch waveguide (8), the second branch section (10) for joining the lights propagating through the first and second branch waveguides, and the second branch section Output side optical waveguide that propagates the generated light
(12), a first loading electrode (14) and a second loading electrode (16) loaded on the first and second branch waveguides (6, 8), respectively, and the first and second branch waveguides. The delay optical waveguide (18), which is provided in parallel with at least one of (6, 8) and is directionally coupled to the branching waveguide (6) at two locations in the longitudinal direction, and the coupling ratio in these directional coupling portions are An optical modulator comprising: a control electrode (24, 26) for controlling. 2. An input side optical waveguide (2), a first branching section (4) for branching light propagating through the input side optical waveguide into two, and a first branching section for propagating the light branched at the branching section, respectively. The branch waveguide (6) and the second branch waveguide (8), the second branch section (10) for joining the lights propagating through the first and second branch waveguides, and the second branch section Output side optical waveguide that propagates the generated light
(12), a first loading electrode (14) and a second loading electrode (16) loaded on the first and second branch waveguides (6, 8), respectively, and the first and second branch waveguides. A ring type optical waveguide (34) directionally coupled to at least one of (6, 8), and a control electrode (38) for controlling the coupling ratio in this directional coupling portion. Light modulator. 3. When the logic level of the input modulation signal is either low or high for light of a specific wavelength,
The coupling ratio in the directional coupling section is 100%, and the first and second branch waveguides (6, 8) are connected to the second branch section.
When the electric fields of the lights respectively incident on (10) have opposite phases and the logic level of the input modulation signal is either low or high, the coupling ratio in the directional coupling section becomes 0% and the first And the delay optical waveguide (18) or the ring type optical waveguide (34) so that the electric fields of the lights respectively incident from the second branch waveguide (6, 8) to the second branch section (10) are in phase. 3. The optical modulator according to claim 1, wherein the optical path length is set and the voltages applied to the first and second loading electrodes and the control electrode are controlled. 4. When the logical level of the input modulation signal is either low or high for light of a specific wavelength,
The coupling ratio in the directional coupling section is 100%, and the first and second branch waveguides (6, 8) are connected to the second branch section.
When the electric fields of the lights respectively incident on (10) are in phase and the logical level of the input modulation signal is either low or high, the coupling ratio in the directional coupling section becomes 0% and the first and Of the delay optical waveguide (18) or the ring type optical waveguide (34) so that the electric fields of the lights respectively incident from the second branch waveguides (6, 8) to the second branch section (10) have opposite phases. 3. The optical modulator according to claim 1, wherein the optical path length is set and the voltages applied to the first and second loading electrodes and the control electrode are controlled. 5. The specific wavelength is two or more, and the delay optical waveguide (18) or the ring type optical waveguide (34) is selected depending on the number thereof.
A plurality of the above are provided.
The optical modulator according to. 6. For the polarization mode with low phase modulation efficiency in the first and second branch waveguides (6, 8), the coupling ratio in the directional coupling section is 100%, and the phase modulation efficiency is For higher polarization modes, the coupling ratio is 0
%, The voltage applied to the control electrode is controlled so that the delay optical waveguide (18) or the ring optical waveguide (34) of the first and second branch waveguides (6, 8) has a directivity. The delay optical waveguide (18) or the ring type optical waveguide (34) so that the electric fields of both polarization modes incident on the second branch portion (10) from the coupled branch waveguide (6) have the same phase. 3. The optical modulator according to claim 1 or 2, wherein the optical path length of (1) is set.