JP2005107229A - Optical waveguide element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide element which prevents optical cross talk between closely arranged optical waveguides and suppresses the generation of a wavelength chirping due to the reduction in driving voltage and a modulation . <P>SOLUTION: In the optical waveguide element which has a substrate having an electrooptical effect and a plurality of optical waveguides formed on the substrate, at least two closely arranged optical waveguides are so formed that the widths of the respective optical waveguides in the close region are different from each other. Preferably, the plurality of waveguides have a Mach-Zehnder type optical waveguide 10 having two branched waveguides and the respective optical waveguides in the close region are the two branched waveguides 11 and 12. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device including a plurality of optical waveguides formed on a substrate having an electro-optic effect.

In the field of optical communication or optical measurement, an optical waveguide element is used that forms a plurality of optical waveguides on a substrate having an electrooptic effect such as LiNbO ₃ (hereinafter referred to as LN).
The following Patent Document 1 discloses an example of an optical modulator having a Mach-Zehnder type optical waveguide, which is a kind of optical waveguide element.
As in such an optical modulator, a plurality of optical waveguides are arranged close to each other, so that light waves propagating in each optical waveguide affect light waves propagating in other optical waveguides, and optical cross There is a possibility that a crosstalk state of light waves called talk will occur.
Conventionally, in order to prevent such optical crosstalk, the distance between optical waveguides has been increased to a certain level or more.
JP-A-2-196212

In the optical waveguide element, the crystal direction in which the refractive index can be changed most efficiently by the electro-optic effect is a direction parallel to the surface of the substrate (referred to as “X-cut substrate”) or the direction is the direction of the substrate. A substrate that is perpendicular to the surface (referred to as a “Z-cut substrate”) is used for each application.
When modulating a light wave propagating in the optical waveguide, a control electrode composed of a signal electrode and a ground electrode is disposed in the vicinity of the optical waveguide.
In order to suppress the occurrence of wavelength chirping due to modulation, it is necessary to equalize the amount of phase change due to the modulation of each branch waveguide of the Mach-Zehnder type modulator, and the signal electrode and the ground electrode sandwich the optical waveguide. It is common to use a configuration of an X-cut substrate in which a signal electrode is disposed or a configuration of a two-electrode type Z-cut substrate in which a signal electrode is disposed immediately above an optical waveguide.

For this reason, in the case of an X-cut substrate, since a signal electrode or a ground electrode always exists between adjacent optical waveguides, the distance between the optical waveguides is maintained at a certain level or more, and there is a risk of causing optical crosstalk. However, in the case of a Z-cut substrate, since the signal electrode is arranged directly above the optical waveguide, a plurality of optical waveguides may be arranged close to each other, which may cause optical crosstalk. It was.
In particular, in the case of an X-cut substrate configuration, there is a demand for miniaturization of the optical waveguide element and reduction of the drive voltage applied to the control electrode, and accordingly, in order to increase the modulation efficiency, the signal electrode and the optical waveguide are required. Need to be placed closer together. On the other hand, in order to suppress reflection at high frequencies and increase modulation efficiency, the control electrode needs to control impedance and match the speed between the light wave and the electric signal, and the width of the signal electrode and the distance between the signal electrode and the ground electrode Is limited to a certain number. In a general high-speed LN modulator, the width of the signal electrode is about several μm to 10 μm, and if the signal electrode and the optical waveguide are brought close to each other at this time, the optical waveguide interval tends to be narrowed, resulting in occurrence of optical crosstalk. Deterioration of light modulation characteristics is a problem.

  An object of the present invention is to provide an optical waveguide device that solves the above-described problems, prevents optical crosstalk between optical waveguides arranged close to each other, and suppresses generation of wavelength chirping due to reduction in driving voltage and modulation. That is.

  In order to solve the above-mentioned problem, in the invention according to claim 1, in an optical waveguide element having a substrate having an electro-optic effect and a plurality of optical waveguides formed on the substrate, at least two of them arranged close to each other The optical waveguides are characterized in that the widths of the respective optical waveguides in adjacent regions are different from each other.

  According to a second aspect of the present invention, in the optical waveguide device according to the first aspect, the plurality of optical waveguides include a Mach-Zehnder type optical waveguide having two branching waveguides, and in an adjacent region. Each optical waveguide is the two branch waveguides.

  According to a third aspect of the present invention, in the optical waveguide device according to the second aspect of the present invention, the optical waveguide element has a control electrode for modulating the light wave propagating through the branch waveguide, and the modulation due to the electrode loss of the control electrode It is characterized by adjusting the width of each branch waveguide so that the decrease in efficiency is compensated and the amount of phase change of each branch waveguide becomes substantially equal to each other.

  Further, in the invention according to claim 4, in the optical waveguide device according to claim 3, the width of one branch waveguide is larger from the input side to the output side of the light wave than the width of the other branch waveguide, It is characterized in that it is composed of a part thinner than the width of the other branch waveguide from the middle, and the length of the narrow part is longer than the length of the thick part.

  Further, in the invention according to claim 5, in the optical waveguide device according to claim 3 or 4, the control electrode includes a signal electrode and ground electrodes arranged on both sides of the signal electrode, The distance between the ground electrodes on both sides with respect to the signal electrode is equal.

  According to the first aspect of the present invention, in the region where the optical waveguides are close to each other, the widths of the optical waveguides are different from each other, so that the occurrence of optical crosstalk can be suppressed. As a result, the optical waveguide can be disposed close to the optical waveguide element, and the optical waveguide element can be reduced in size and the driving voltage can be reduced.

  The invention according to claim 2 can prevent optical crosstalk even in an optical waveguide element having a Mach-Zehnder type optical waveguide that is widely used in the field of optical communication or optical measurement, and includes an optical waveguide element. The overall size of the apparatus can be reduced, the driving voltage of the optical waveguide element is reduced, and the optical waveguide element can be driven at high speed.

  Due to the change of the width of each branching waveguide, the mode diameter of the optical waveguide is different, and the modulation efficiency per cross section is asymmetric between the branching waveguides, so that wavelength chirping occurs. According to the invention of claim 3, the electrode loss of each control electrode that modulates the light wave propagating through the branch waveguide is compensated, and the phase change amount of each branch waveguide is made substantially equal to each other. By setting the width, it is possible to suppress the occurrence of chirping.

  According to the invention of claim 4, the width of one branch waveguide is wider from the input side to the output side of the light wave than the width of the other branch waveguide, and the portion narrower than the width of the other branch waveguide from the middle In addition to preventing the optical crosstalk between the branched waveguides, the electrode loss of the control electrode can be reduced. Can be made equal, and the occurrence of chirping can be prevented.

  According to the invention of claim 5, the signal electrode and the ground electrode constituting the control electrode are arranged on both sides of the signal electrode, and the distance between the signal electrode and the ground electrode is equal on both sides. By doing so, it is possible to further suppress the occurrence of chirping. In particular, when each branch waveguide is disposed between the signal electrode and the ground electrode using an X-cut substrate, chirping prevention is effective.

Hereinafter, the present invention will be described in detail using preferred examples. In the following, an optical modulator having a Mach-Zehnder type optical waveguide will be mainly described as an example of the optical waveguide device, but the present invention is not limited to this.
FIG. 1 shows an example of a conventional optical modulator using an X-cut substrate 1.
The substrate 1 is a substrate having an electrooptic effect, and is made of, for example, lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), or PLZT (lead lanthanum zirconate titanate), and in particular, an optical waveguide 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 because it is easy to configure as an element and has high anisotropy.

A Mach-Zehnder type optical waveguide 2 is formed on the surface of the substrate 1. As a method for forming the optical waveguide, any method such as a Ti thermal diffusion method, an epitaxial growth method, and an ion implantation method can be used.
The Mach-Zehnder type optical waveguide includes a branching portion that branches an incident light wave into two, branching waveguides 3 and 4 that propagate the branched light wave, and a combining portion that combines the two separated light waves. Consists of

In the vicinity of the branching waveguides 3 and 4, control electrodes made up of the signal electrode 5 and the ground electrodes 6 and 7 are disposed. These electrodes are made of a conductive material such as Au by vapor deposition and plating, or both. It is formed by using together.
Further, although not shown in FIG. 1, it is possible to provide a buffer layer such as a dielectric SiO ₂ on the substrate 1 in order to reduce the propagation loss of light in the optical waveguide. The buffer layer can be formed from a known material such as SiO _{2 by} a known film forming method such as an evaporation method, a sputtering method, an ion plating method, or a CVD method.

FIG. 1B is a cross-sectional view taken along one-dot chain line A in FIG. Normally, the branch waveguides 3 and 4 and the ground electrodes 6 and 7 are arranged symmetrically with the signal electrode 5 as the center. The widths of the branching waveguides 3 and 4 are the same.
However, if the distance between the branching waveguides 3 and 4 is reduced, optical crosstalk occurs. Therefore, the distance between the branching waveguides 3 and 4 needs to be at least about three times the width W of the branching waveguide conventionally. was there.
For this reason, the branch waveguides 3 and 4 are separated from the signal electrode 5, so that the refractive index change amount due to the electro-optic effect is reduced and the drive voltage of the optical modulator is increased.

In the optical waveguide device according to the present invention, as shown in FIG. 2, by making the widths of the branched optical waveguides 11 and 12 different, even if the distance between the branched waveguides is made closer than before, optical crosstalk is generated. It is possible to suppress it.
In FIG. 2, reference numeral 10 denotes a Mach-Zehnder type optical waveguide formed on the substrate 1, and the width W <b> 1 of the branch waveguide 12 is configured to be larger than the width W <b> 2 of the branch waveguide 11. Further, a signal electrode 13 and ground electrodes 14 and 15 are arranged as control electrodes. FIG. 2B is a cross-sectional view taken along the alternate long and short dash line B in FIG.

  However, due to the change in the width of the branching waveguide, the mode diameter of each branching waveguide is different, and the modulation efficiency per cross section is asymmetric between the branching waveguides. When multiplexed, a new problem of generating wavelength chirping arises.

The present invention further employs a configuration in which the width of the branching waveguide is changed in the middle of the branching waveguide as shown in FIGS.
More specifically, by appropriately combining the number of times each cross-sectional structure of the branching waveguide is changed, the width of the branching waveguide, and the length of each width (referred to as “action length”), the drive signal is used to control the control electrode. It is set so as to compensate for a decrease in the amount of phase change due to electrode loss during propagation.

In FIG. 3, the width of each branch waveguide is changed twice, and each branch waveguide is divided into two sections (31 and 32, or 33 and 34).
The action length L1 of the light wave input side sections 31 and 33 is set short and the action length L2 of the output side sections 32 and 34 is set long to compensate for electrode loss due to signal propagation.
Thereby, at the adjusted frequency of the electrode loss, the absolute value of the phase change amount between the branched waveguides becomes completely equal, and chirping can be prevented.

In FIG. 4, the width of each branch waveguide is changed three times, and each branch waveguide is divided into three sections (41 and 42 and 42 or 44 and 45 and 46).
Then, the first section (41, 44) and the third section (43, 46) have the same cross-sectional structure, and the second section (42, 45) is opposite to the relationship between the waveguide widths of the first and third sections. The structure is as follows.
In such a case, the relationship between the action lengths of the respective sections is approximately L1 + L3 = L2. However, this relationship changes depending on the length of L1.
With such a configuration, it is possible to configure an optical modulator that is driven at a low voltage and has a very low chirp over a wide frequency range.

Further, when the width of the branching waveguide is changed, as shown in FIG. 5A, when the connection portion 52 between the thick portion 50 and the thin portion 51 is changed discontinuously, the light propagates through the optical waveguide. In some cases, a part of the light wave is reflected or leaks outside the optical waveguide.
In order to prevent such a problem, it is also possible to continuously change the connecting portion 62 between the thick portion 60 and the thin portion 61 as shown in FIG.

  As described above, according to the present invention, there is provided an optical waveguide device that prevents optical crosstalk between optical waveguides arranged close to each other, and suppresses generation of wavelength chirping due to reduction in driving voltage and modulation. It becomes possible.

It is a figure which shows the outline of the conventional optical modulator, (a) is a top view, (b) is sectional drawing. It is a figure which shows the example of the optical modulator which concerns on this invention, (a) is a top view, (b) is sectional drawing. It is a figure which shows the example at the time of providing two divisions which change waveguide width. It is a figure which shows the example at the time of providing three divisions which change waveguide width. It is a figure which shows the state of the connection part at the time of changing waveguide width, (a) shows discontinuous connection, (b) shows continuous connection.

Explanation of symbols

1 Substrate 2,10,20,40 Mach-Zehnder type optical waveguide 3,4,11,12 Branched waveguide 5,13,35,47 Signal electrode 6,7,14,15,36,37,48,49 Ground electrode

Claims (5)

In an optical waveguide device having a substrate having an electro-optic effect and a plurality of optical waveguides formed on the substrate,
An optical waveguide element characterized in that at least two optical waveguides arranged close to each other are formed such that the widths of the respective optical waveguides in the adjacent regions are different from each other. The optical waveguide device according to claim 1, wherein the plurality of optical waveguides include Mach-Zehnder type optical waveguides having two branching waveguides,
An optical waveguide element characterized in that each optical waveguide in the adjacent region is the two branched waveguides. The optical waveguide device according to claim 2, further comprising a control electrode for modulating a light wave propagating through the branch waveguide,
An optical waveguide element characterized in that a decrease in modulation efficiency due to an electrode loss of the control electrode is compensated, and a width of each branch waveguide is adjusted so that a phase change amount of each branch waveguide becomes substantially equal to each other.   4. The optical waveguide device according to claim 3, wherein a width of one branching waveguide is wider than the width of the other branching waveguide from the light wave input side to the output side, and a width of the other branching waveguide from the middle. An optical waveguide device characterized in that it is composed of a thinner portion, and the length of the narrower portion is longer than the length of the thicker portion. 5. The optical waveguide device according to claim 3, wherein the control electrode includes a signal electrode and a ground electrode disposed on both sides of the signal electrode, and the ground electrode is provided on both sides of the ground electrode with respect to the signal electrode. An optical waveguide device characterized in that the distances are equal.

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