JP2003207754A - Resonance type optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a resonance type optical modulator which requires less driving electric power than a conventional resonance optical modulator. <P>SOLUTION: The resonance type optical modulator which has a substrate 1 made of a material with electrooptic effect, an optical waveguide 2 which is formed on the substrate, and a resonance electrode 5 which modulates light traveling in the optical waveguide 2, is characterized in that at least one or more polarization inversion structures 7 having its border at a node position of a standing wave having an electric field distribution generated at the resonance electrode 5 are formed on the substrate including the optical waveguide. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonance type optical modulator, and more particularly to a resonance type optical modulator using a substrate partially having a polarization inversion structure.

[0002]

2. Description of the Related Art In recent years, with the progress of high-speed and large-capacity optical fiber communication systems, high-speed operation is possible using a material having an electro-optical effect such as lithium niobate as a substrate, as represented by an external modulator. A light intensity modulator has been put to practical use. In such an optical modulator, as shown in FIG. 1, an optical waveguide 2 for guiding a light wave is applied to a substrate 1 having an electro-optical effect, and a high speed modulation signal in a microwave band is applied to the light wave. And a modulation electrode composed of the signal electrode 3 and the ground electrodes 4 and 4 '.

Next, the operation of the optical modulator shown in FIG. 1 will be described.
A light wave is incident on the optical waveguide 2 from the end surface of the optically polished substrate. The light wave is generated by the first Y branch 2 formed by the optical waveguide 2.
-1 demultiplexes into two light waves, and each light wave is
Are combined by the Y branch 2-2 and are emitted from the other end of the optical waveguide 2 (the shape of such an optical waveguide is referred to as “Mach-Zehnder type”). Then, the light wave is passing through the optical waveguide 2, particularly the optical waveguide 2 ′ (referred to as “branch waveguide”) while being split by the first Y-branch and being combined by the second Y-branch. The phase of the light wave changes due to the change in the refractive index of the substrate due to the electric signal applied to each electrode, and the combined light wave has a light intensity corresponding to this phase change. As a result, a light wave having a light intensity change corresponding to a change in an electric signal can be generated by the optical modulator.

[0004]

In the optical modulator shown in FIG. 1, an optical modulator corresponding to a frequency in a wide band of 0 to 40 GHz and even 60 GHz has appeared. However, in order to drive these optical modulators, driving power of several hundred milliwatts to several watts is required,
It has been difficult to reduce the cost and size of the entire system related to the optical modulator, such as the optical modulator and its driving power supply circuit.

On the other hand, a resonance type optical modulator as shown in FIG. 2 has been proposed. A T-shaped resonance electrode is formed on a branching waveguide of a Mach-Zehnder type optical waveguide, and the resonance electrode 5 is provided with a feeding electrode (an electrode composed of a feeding portion 5 ′ of the resonance electrode and a ground electrode 6). When a signal voltage (microwave) is applied, a standing wave of an electric field is formed on the open end line (electrode length 1) of a finite length on the upper side of the T-shape by reflection of microwaves at the electrode end, and the resonance electrode 5 It is possible to obtain an amplitude voltage larger than the voltage input to. Therefore, in the resonance type optical modulator, it is necessary to set the frequency of the signal voltage to a specific resonance frequency in order to form a standing wave, and the frequency of the signal voltage is, for example, 19 to 20 GHz.
It will be limited to a narrow band such as 21 GHz. However, due to this standing wave, it is possible to reduce the cost and size of the entire system related to the optical modulator, such as driving with a driving power of several tens of milliwatts.

However, in the resonance type optical modulator, as shown in FIG.
, The electrode length l is 1 = for the wavelength λ of the standing wave.
In the case of the relationship of λ / 2, the resonance electrode is shown in FIG.
As shown in FIG. 3, a standing wave like the one shown in FIG. 3 is generated, which causes light passing through each branch waveguide of the Mach-Zehnder type optical waveguide.
The phase change as shown in (b) will occur. As a result, a phase difference 2Δφ as shown in FIG. 3B is generated, and the combined light has intensity modulation. On the other hand, electrode length l = λ
4A, a standing wave as shown in FIG. 4A is generated in the resonance electrode, and the phase change in this case is x = −λ / 2−− as shown in FIG. 4B. The phase difference increased at λ / 4 becomes 0 at x = 0 and then increases again in the opposite direction, but the final phase difference becomes 0 at x = λ / 2, resulting in Intensity modulation cannot be performed. In this way, even if the electrode length l of the resonance electrode is changed,
Basically, the state shown in FIG. 3 and the state shown in FIG. 4 are alternately repeated, which makes it difficult to reduce the drive power applied to the optical modulator below a certain level.

The problem to be solved by the present invention is to solve the above-mentioned problems and to provide a new resonance type optical modulator in which the driving power is further reduced as compared with the conventional resonance type optical modulator.

[0008]

In order to solve the above problems, the invention according to claim 1 provides a substrate made of a material having an electro-optical effect, an optical waveguide formed on the substrate, and the optical waveguide. In a resonance type optical modulator having a resonance electrode for modulating light passing through the inside, a polarization inversion structure having a nodal position of a standing wave of an electric field distribution generated in the resonance electrode as a boundary is included in the optical waveguide. At least one or more are formed on the substrate.

According to the first aspect of the invention, since the domain-inverted structure is formed on the substrate with the node position of the standing wave of the electric field distribution generated in the resonance electrode as the boundary, the electric field is formed before and after the node position at the boundary of the domain-inverted structure. Even when the distribution changes from positive to negative or from negative to positive, the phase change of the light passing through the optical waveguide maintains the increasing tendency or the decreasing tendency, so that even if the applied drive power is the same as the conventional one, the result As a result, it is possible to further increase the phase difference, and it is possible to drive the resonant optical modulator with less drive power than in the past.

According to a second aspect of the invention, in the resonance type optical modulator according to the first aspect, the polarization inversion structure is alternately formed at each node position.

As in the invention according to claim 2, the inversion structure is alternately formed for each node position, which is the effect of the invention according to claim 1, that is, the phase change of light tends to increase before and after the node position. Alternatively, the effect of maintaining the decreasing tendency can be continuously generated, the phase change can be continuously increased or decreased in the electrode length direction of the resonance electrode, and the resonance type optical modulator can be driven with less driving power. Can be driven.

According to a third aspect of the present invention, in the resonance type optical modulator according to the first or second aspect, an impedance matching element is provided between the resonance electrode and a power supply electrode for supplying power to the resonance electrode. It is characterized in that it is provided.

According to the third aspect of the present invention, even if the resonance electrode is an open-ended line or a short-circuited line having a finite length (the end of the line is short-circuited to the ground electrode), the feed electrode and the resonance electrode are connected to each other. By performing impedance matching between the two, it becomes possible to efficiently send electric power to the resonance electrode.

According to a fourth aspect of the present invention, in the resonance type optical modulator according to any one of the first to third aspects, the resonance electrode is on one of the branched optical waveguides of the Mach-Zehnder optical waveguide. It is provided in.

According to a fifth aspect of the present invention, in the resonance type optical modulator according to any one of the first to third aspects, the resonance electrode is provided on a single optical waveguide forming a phase modulator. It is characterized by being provided.

According to the invention according to claim 4 or 5, even for a Mach-Zehnder type optical intensity modulator or a single optical waveguide phase modulator, a resonance type optical modulation can be efficiently driven with less drive power than before. It becomes possible to provide a container.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to preferred examples. As a substrate forming the optical modulator, a material having an electro-optical effect, for example, lithium niobate (L
iNbO ₃ ; hereinafter referred to as LN), lithium tantalate (LiTaO ₃ ), PLZT (lead lanthanum zirconate titanate), and a silica-based material, and particularly easy to configure as an optical waveguide device and anisotropic. It is preferable to use a LiNbO ₃ crystal, a LiTaO ₃ crystal, or a solid solution crystal composed of LiNbO ₃ and LiTaO ₃ for the reason that it is large. In this embodiment, an example using lithium niobate (LN) will be mainly described. Also,
In the present invention, it is necessary that the substrate has a domain-inverted structure. In the following, a substrate having a crystal axis direction capable of changing the refractive index most efficiently by the electro-optical effect in a direction perpendicular to the surface (so-called “Z-cut substrate”) will be mainly described. It is not limited to the substrate.

As a method of manufacturing an optical modulator, Ti is thermally diffused on an LN substrate to form an optical waveguide, and then an electrode is formed on the LN substrate without providing a buffer layer over a part or the whole of the substrate. In order to reduce the propagation loss of light in the optical waveguide by directly forming
A buffer layer of ₂ or the like is provided, and a Ti / Au electrode pattern is formed on the buffer layer and a gold plating method is applied to make it several tens μ
There is a method of forming a resonance electrode and a ground electrode having a height of m and indirectly forming the electrodes. It is also possible to have a multilayer structure in which a film body of SiN, Si, or the like is provided on the buffer layer. In general, an optical modulator is manufactured by forming a plurality of optical modulators on a single LN wafer and finally separating the individual optical modulator chips into chips.

A feature of the present invention is that the domain-inverted structure is formed on the substrate including the optical waveguide with the nodal position of the standing wave of the electric field distribution generated at the resonance electrode as a boundary. Specifically, as shown in FIG. 5, when the electrode length l = λ (the wavelength of the standing wave related to the electric field distribution), both ends of the resonance electrode 5 λ /
In the substrate region corresponding to 4, the domain-inverted structure 7 which is vertically cut in the vertical direction of FIG. 5 is formed. As a result, even if a standing wave related to the electric field distribution as shown in FIG. 6A is generated in the resonance electrode,
Since a domain-inverted structure is formed at both ends λ / 4 of the resonant electrode, even if a negative electric field is applied to the substrate having the domain-inverted structure, the positive field is not substantially domain-inverted. Change in refractive index (Δn) similar to that applied to
(See FIG. 6B), as a result, the phase change of light depending on the refractive index change is additive such that the phase change amount is always added, as shown in FIG. 6C. A phase change will occur.

In the case of the Mach-Zehnder type optical waveguide as shown in FIG. 5, the effect of the electric field distribution of the resonance electrode 5 causes opposite phase changes in the two branched waveguides (see FIG. 6 ( (Refer to the changes in the graphs of the solid and broken lines in c)), and the phase difference of each light when the branched lights are combined is shown in FIG.
Like 2Δφ in (c), the phase difference is significantly larger than that of the conventional resonant optical modulator, and it is possible to obtain a larger phase change with the same drive power. That is, with the resonance type optical modulator having the polarization inversion structure as shown in FIG. 5, it is possible to reduce the driving power more than ever before.

As a method of forming a domain-inverted structure on a substrate having an electro-optical effect, when it is formed only near the surface of the substrate, a method of diffusing Ti in an LN substrate at a temperature of 1000 ° C. or higher and near the Curie temperature, or a Curie temperature is used. There is a method of utilizing an electric field generated inside the substrate, such as a method of outdiffusing Li ₂ O in the crystal in the vicinity thereof or a method of diffusing impurities or outdiffusion. Further, when forming a domain-inverted structure extending from the front surface to the back surface of the substrate, a method of applying an external electric field is used.
In this method, an electrode corresponding to the polarization inversion portion is formed on the front surface of the LN substrate by photolithography or the like, and on the other hand, the electrode is vapor-deposited or applied on the back surface of the LN substrate. At room temperature,
A pulse voltage of about 20 kV / mm is applied to both electrodes. The application time depends on the area of the domain-inverted portion viewed from the substrate surface side, but is 1 to 30 cm ² and is about 100 μs to 30 μs.
It is about 0 ms. A jig or the like using a liquid electrode may be used instead of the back surface electrode of the LN substrate. After forming the domain-inverted portion, the electrodes used for forming the domain-inverted portion can be removed, and the optical waveguide, the resonance electrode, the ground electrode, etc. can be formed as usual.

As a second example, consider the case where the electrode length 1 is 3/2 times the wavelength λ of the standing wave related to the electric field distribution as shown in FIG. 7A. In this case, since the electric field distribution forms a standing wave as shown in FIG. 7B, the domain-inverted structure 7 whose boundary is the node position of the standing wave is formed as shown in FIG. 7A. As a result, an additive phase change can be realized. The same effect can be achieved by forming the domain-inverted structure in the center of the resonance electrode 5 and setting the two ends of the resonance electrode so as not to form domain-inversion.

In an actual resonance type LN optical modulator, the pattern of the standing wave applied to the electric field distribution is shown in FIG. 8A due to the problem of impedance matching at the feeding point.
As shown in (b), a waveform slightly different from the above-described standing wave is shown. Even in such a case, similarly to the above-described embodiment, by forming the polarization inversion structure (the range in which the polarization inversion structure is formed is indicated by *) at the nodal point where the electric field distribution is 0, the addition is performed. It is possible to realize a phase change.

Next, since the resonance electrode is an open-ended line of finite length, impedance matching between the feeding electrode and the resonance electrode is not achieved. Therefore, as shown in FIG. 9, the matching element 8 is inserted between the ground electrode 6 and the feeding portion of the resonance electrode 5,
Impedance matching is achieved, microwave propagation loss from the feeding electrode to the resonance electrode is kept low, and efficient modulation is possible. An example of the structure of the impedance matching element 8 is a cross-sectional view taken along the alternate long and short dash line A in FIG.
As shown in FIG.
There is one in which a patch type capacitor is formed by forming a film of the dielectric 9 and the upper electrode 10 so as to cover the.

In the above embodiments, the Mach-Zehnder type optical intensity modulator has been mainly described, but as shown in FIG. 10, the resonance electrode 5 is also used for the phase modulator having the single optical waveguide 11. By forming the domain-inverted structure 7 having a boundary at the node position of the standing wave related to the electric field distribution generated on the substrate, it becomes possible to manufacture a phase modulator having a lower driving power than the conventional one. In addition, even when the place where power is supplied to the resonance electrode 15 is at one end of the resonance electrode as shown in FIG. 11, refer to the shape of the standing wave of the electric field distribution in the resonance electrode as shown in FIG. 11B. In addition, by providing the polarization inversion structure 7, it is possible to efficiently drive the optical modulator.

Regarding the positional relationship between the polarization inversion structure and the optical waveguide, the polarization domain wall, which is the boundary of the polarization inversion structure, is arranged as far as possible from the optical waveguide except that it intersects with the optical waveguide. It is possible to reduce the influence of stress strain of the polarization domain wall on the optical waveguide. In addition, the crossing angle between the polarization domain wall and the optical waveguide is configured to be substantially vertical, so that a region in which the polarization inversion region and the non-polarization inversion region coexist at the same location of the optical waveguide (here The term “mixed” means that, for example, when a polarization domain wall is formed obliquely with respect to the optical waveguide, when a cross section perpendicular to the extending direction of the optical waveguide is taken, polarization inversion occurs on the same cross section of the optical waveguide. (Meaning that there is both a part that does not have a polarization inversion and a part that does not have a polarization inversion), it is possible to efficiently produce the effect of the polarization inversion.
Moreover, since the light propagating in the branching waveguide enters the polarization domain wall substantially perpendicularly, the reflection and scattering effects by the polarization domain wall are reduced, and an optical modulator with less propagation loss can be realized.

The scope of the present invention is not limited to these embodiments, and even when an optical modulator having two or more branch waveguides or a plurality of optical modulators are integrated on the same substrate, these optical modulators are integrated. In the scope of the present invention, a resonance electrode is disposed on at least a part of the substrate, and a domain-inverted structure is formed on the substrate corresponding to the standing wave of the electric field distribution generated on the resonance electrode.

[0028]

As described above, according to the present invention,
By forming a domain-inverted structure at an appropriate place on the substrate that forms the resonant optical modulator, the drive power of the optical modulator can be reduced more than ever, and the overall cost and size of the optical modulator system can be reduced. Can be realized.

[Brief description of drawings]

FIG. 1 is a plan view of a conventional optical modulator.

FIG. 2 is a plan view of a conventional resonant optical modulator.

FIG. 3 is a graph showing the relationship between electric field distribution and phase change when l = λ / 2.

FIG. 4 is a graph showing a relationship between an electric field distribution and a phase change when l = 3/2 × λ.

FIG. 5 is a plan view of a resonant optical modulator in which polarization inversion is formed when l = λ.

FIG. 6 is a graph showing the relationship between the electric field distribution, the refractive index distribution, and the phase change when polarization inversion is formed.

FIG. 7 is a plan view of a resonance type optical modulator in which polarization inversion is formed in the case of l = 3/2 × λ.

FIG. 8 shows an example of a standing wave pattern of an electric field distribution of an actual resonance type LN optical modulator.

FIG. 9 is a diagram showing a structure of an impedance matching element.

FIG. 10 is a plan view of a phase modulator having a single optical waveguide in which polarization inversion is formed.

FIG. 11 is a plan view of a resonance-type optical modulator in which power is supplied from one end of a resonance electrode.

[Explanation of symbols]

1 substrate 2 Optical waveguide 2'branch waveguide 3 signal electrodes 4 ground electrode 5 Resonance electrode 6 ground electrode 7 Polarization inversion structure 8 Impedance matching element

Claims (5)

[Claims] 1. A resonant optical modulator having a substrate made of a material having an electro-optical effect, an optical waveguide formed on the substrate, and a resonant electrode for modulating light passing through the optical waveguide. 2. A resonance type optical modulator, wherein at least one polarization inversion structure is formed on a substrate including the optical waveguide, the polarization inversion structure having a boundary of a node position of a standing wave of an electric field distribution generated in the resonance electrode. 2. The resonant optical modulator according to claim 1, wherein the domain-inverted structures are alternately formed for each node position. 3. The resonance type optical modulator according to claim 1 or 2, wherein an impedance matching element is provided between a resonance electrode and a feeding electrode for feeding the resonance electrode. Light modulator. 4. A resonant optical modulator according to claim 1, wherein the resonant electrode is provided on one of the branched optical waveguides of the Mach-Zehnder optical waveguide. Resonant optical modulator. 5. The resonance type optical modulator according to claim 1, wherein the resonance electrode is provided on a single optical waveguide forming a phase modulator. Resonant type optical modulator.