JP2012068679A - Optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide element comprising an optical waveguide with a plurality of optical waveguide sections arranged parallel with each other as in the case of a nest type optical waveguide, capable of effectively restraining cross talk of a modulating signal applied to each optical waveguide even when the optical waveguide sections are arranged in close proximity to each other.SOLUTION: An optical waveguide element includes a substrate having an electrooptical effect, an optical waveguide formed on the substrate, and a modulating electrode for modulating light waves guided in the optical waveguide. The optical waveguide has a plurality of optical waveguide sections disposed in parallel with each other. An electric field of the modulating electrode is applied to at least one of the optical waveguide sections. The distance between the optical waveguide sections is 300 μm or less with a groove formed between the optical waveguide sections, and a conductive material fills or is adhered to the inside of the groove.

Description

  The present invention relates to an optical waveguide device, and a plurality of optical waveguide portions are arranged in parallel to each other in an optical waveguide like a nested optical waveguide, and an electric field is applied to at least one of the optical waveguide portions by the modulation electrode. The present invention relates to an optical waveguide device having a configuration.

  Conventionally, in recent years, in the optical communication field, the frequency occupation band of an input data signal, such as SSB (Single-Side band) modulation method and DQPSK (differential quadrature phase shift keying) modulation method, can be made compact, and light with high resistance to chromatic dispersion can be obtained. Modulation methods are attracting attention. As a modulator for realizing these methods, an integrated modulator in which a plurality of modulators are integrated on the same substrate is used.

  For example, in the SSB type optical modulator, an optical waveguide device having a so-called nested waveguide structure in which two sub Mach-Zehnder optical waveguides are incorporated in a branch waveguide of a main Mach-Zehnder optical waveguide is used. In such an optical waveguide device, two sub Mach-Zehnder optical waveguides are close to each other and independent modulation signals are applied to each of them. In such an optical waveguide, an electric field of a modulation signal applied to one sub Mach-Zehnder optical waveguide causes a so-called crosstalk phenomenon that affects the other sub-Mach-Zehnder optical waveguide. Such a phenomenon occurs not only in a nested optical waveguide such as an SSB optical modulator but also in a multi-channel optical modulator in which a plurality of optical modulators are arranged on the same substrate.

  As a method for suppressing crosstalk between adjacent optical waveguides, a method of separating the electrodes to which the modulation signal is applied as in Patent Document 1, and a back surface of the substrate portion where the signal electrode is positioned as in Patent Document 2 A method of forming a groove in the optical channel, and a method of reversing the light traveling directions of adjacent optical modulators in a multi-channel optical modulator, as in Patent Document 3, are proposed. . However, in any of the methods, it is difficult to sufficiently suppress the crosstalk when the optical waveguides are arranged close to each other.

  In order to investigate the influence of crosstalk between adjacent optical waveguides, when the strength of the electric field applied to one optical waveguide is 1 (100%), the strength of the electric field exerted on the other optical waveguide by the electric field When the change was calculated corresponding to the waveguide interval, a tendency as shown in FIG. 4 was observed. When the waveguide interval is about 150 μm, the strength of the electric field exerted on the other optical waveguide is about 1%, about 0.2% when about 300 μm, and about 0.2 when about 400 μm. It decreases to about 1%. Therefore, when the distance between the optical waveguides is ensured to be 400 μm or more, the influence of the crosstalk is extremely small. However, if the optical waveguides are arranged so that the distance between the optical waveguides is always 400 μm or more, the entire optical waveguide element is enlarged. Arise. In particular, in the case of a Mach-Zehnder optical waveguide and further a nested optical waveguide, the branching angle of the branching waveguide is set very small in order to suppress the emission of light to the substrate. Then, the entire optical waveguide element becomes longer and larger.

  On the other hand, in an optical waveguide device such as a Mach-Zehnder optical modulator, the modulation curve of the optical modulator shifts due to movement of ions contained in the buffer layer when the substrate temperature changes or a DC electric field is applied, and the DC bias point In some cases, a so-called drift phenomenon may occur. For this reason, it is necessary to control the fluctuation of the operating point corresponding to the shift of the modulation curve. As a commonly used method, there is a bias control method in which a low frequency signal is superimposed, a bias point at that time is detected from an output change, and an appropriate DC signal is applied to maintain the bias point appropriately. In general, a low frequency signal having an amplitude about 1/100 of the modulation signal is applied. However, when the optical waveguides are close to each other due to the influence of the crosstalk described above, the electric field of the modulation signal applied to the adjacent optical waveguides has the same magnitude as the low frequency signal, and the bias It is also the cause of the decrease in control accuracy.

Japanese Patent Laid-Open No. 9-218384 JP 2001-174766 A JP 2001-201725 A JP-A-6-289341

  The problem to be solved by the present invention is to solve the above-mentioned problems and, like a nested optical waveguide, the optical waveguide has a plurality of optical waveguide portions, and the optical waveguide portions are arranged in parallel to each other. In this case, an optical waveguide device capable of effectively suppressing crosstalk of a modulation signal applied to each optical waveguide even when the optical waveguide portions are arranged close to each other is provided. In particular, in the present invention, it is desirable to suppress the crosstalk to about 1/10 or less of the amplitude of a control signal such as a low frequency signal (0.1% of the modulation signal). To provide an optical waveguide device capable of effectively suppressing crosstalk even in the case of 400 μm or less, more preferably 300 μm or less.

  In the invention according to claim 1, an optical waveguide device having a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave guided in the optical waveguide The optical waveguide has a configuration in which a plurality of optical waveguide portions are arranged in parallel to each other, and an electric field by the modulation electrode is applied to at least one of the optical waveguide portions, and the distance between the optical waveguide portions is It is 300 μm or less, and a groove is formed between the optical waveguide portions, and a conductive material is filled or adhered inside the groove.

  The invention according to claim 2 is the optical waveguide element according to claim 1, wherein the depth of the groove is 450 μm or more.

  The reference example of the present invention is characterized in that the width of the groove is 50 μm or more.

  Furthermore, in another reference example of the present invention, the optical waveguide portion is a sub-Mach-Zehnder waveguide of a nested optical waveguide, and the groove is formed between different sub-Mach-Zehnder waveguides.

  According to the first aspect of the present invention, since the distance between the optical waveguide portions is 300 μm or less and the grooves are formed between the optical waveguide portions, the optical waveguide device can be modulated while miniaturizing and integrating. It is possible to suppress the crosstalk due to the electrodes for use.

  Furthermore, since the depth of the groove is 450 μm or more, it becomes possible to suppress the influence of the crosstalk when there is no groove to about half or less.

  According to the second aspect of the present invention, since the depth of the groove is 450 μm or more, it becomes possible to suppress the influence of the crosstalk when there is no groove to about half or less.

  Further, according to the reference example of the present invention, since the groove width is 50 μm or more, it is possible to suppress the influence of the crosstalk when there is no groove to about half or less.

  According to yet another reference example, the optical waveguide portion is a sub-Mach-Zehnder waveguide of a nested optical waveguide, and the groove is formed between different sub-Mach-Zehnder waveguides. Even when it is applied, both crosstalks can be effectively suppressed.

It is a top view of the optical waveguide device to which the present invention is applied. It is sectional drawing at the time of cutting along the dashed-dotted line X of the optical waveguide element shown in FIG. It is a figure which shows the change of the strength of the electric field which concerns on the crosstalk with respect to the shape change of a groove | channel. It is a graph which shows the influence of crosstalk with respect to the change of the distance between waveguides.

Hereinafter, the optical waveguide device according to the present invention will be described in detail.
FIG. 1 is a plan view showing a nested optical waveguide to which the present invention is applicable. FIG. 2 is a cross-sectional view taken along one-dot chain line X in FIG. In FIG. 1, the modulation electrode is omitted to describe the shape of the optical waveguide.

  The optical waveguide element shown in FIG. 1 includes a substrate 1 having an electro-optic effect and optical waveguides 2-0 to 2-9 formed on the surface portion of the substrate 1. In the light wave traveling direction, the optical waveguide includes an input waveguide 2-0, main branch waveguides 2-1 and 2-2 of the main Mach-Zehnder optical waveguide, and sub-branch waveguides 2-3 to 2- of the sub Mach-Zehnder optical waveguide. 6 and main branch waveguides 2-7 and 2-8 of the main Mach-Zehnder optical waveguide, and an output waveguide 2-9.

  In the nest type optical waveguide of FIG. 1, the sub Mach-Zehnder optical waveguides are arranged in parallel to each other and arranged close to each other. As shown in FIG. 2, a signal electrode 3-1 and ground electrodes 4-1 and 4-2 are provided to apply a modulation signal to the sub Mach-Zehnder optical waveguides (2-3 and 2-4). . Similarly, the other sub Mach-Zehnder optical waveguide is provided with a signal electrode 3-2 and ground electrodes 4-3 and 4-4.

  As the substrate 1, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used.

  As a method for forming the optical waveguide, it can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method. Further, as in Patent Document 4, a ridge can be formed on the surface of the substrate 1 in accordance with the shape of the optical waveguide to configure the optical waveguide.

The modulation electrode can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like.
Although not particularly illustrated, a buffer layer may be formed between the substrate 1 and the modulation electrode. Accordingly, it is possible to effectively prevent the light wave propagating through the optical waveguide from being absorbed or scattered by the modulation electrode. In addition, it is possible to improve the speed matching between the modulation signal applied from the modulation electrode and the light wave guided in the optical waveguide.

  The optical waveguide device of the present invention is not limited to the nested optical waveguide as shown in FIG. 1, but a plurality of optical waveguide portions are arranged in parallel with each other as in the sub Mach-Zehnder optical waveguide of FIG. It is possible to suitably use one optical waveguide portion having a configuration in which an electric field is applied to the modulation electrodes (signal electrode and ground electrode).

  As shown in FIG. 4, when the distance between the optical waveguide portions is larger than 400 μm, the influence of the electric field strength due to the crosstalk can be suppressed to 0.1% or less. The feature of the optical waveguide device of the present invention is that even if the distance between the optical waveguide portions is 300 μm or less, the crosstalk caused by the modulation electrode is effectively suppressed by forming a groove between the optical waveguide portions. . Specifically, as shown in FIG. 1 or FIG. 2, the groove 10 is formed between the optical waveguide portions (between the sub Mach-Zehnder optical waveguides).

Next, using FIG. 2 as a calculation model, when the distance L between the optical waveguides is fixed and the width W and the depth H of the groove are changed, the electric field formed by one modulation electrode becomes the other optical waveguide. The so-called crosstalk size was calculated.
The calculation results are shown in FIG. Note that L was 300 μm, W was changed in the range of 1 to 101 μm, and H was changed in the range of 10 to 500 μm. In addition, the calculation result is calculated assuming that the strength of the electric field formed when there is no groove is 1.

  Referring to FIG. 3, when the width W of the groove 10 is 50 μm or more or the depth H of the groove 10 is 450 or more, it is easy to suppress the electric field intensity related to crosstalk to about half or less. Understood. That is, although the distance between the optical waveguide portions is 300 μm, the influence of crosstalk can be suppressed to the same size as when the distance is 400 μm.

  Next, as an application example of the optical waveguide element of the present invention, it is possible to fill the inside of the groove 10 shown in FIG. 1 or 2 with a conductive material or to form a conductive film. With this configuration, the conductive material or the like serves as an electromagnetic shield, and it is possible to suppress the electric field formed by the modulation electrode related to one optical waveguide portion from reaching the other optical waveguide portion.

  The conductive material (or conductive film) is not particularly limited as long as it has conductivity such as gold and aluminum. However, when the conductive material is filled or adhered simultaneously with the formation of the modulation electrode, the modulation is performed. It is also possible to use the same material as that for the electrode.

  As described above, according to the present invention, in an optical waveguide element in which an optical waveguide has a plurality of optical waveguide portions and the optical waveguide portions are arranged in parallel to each other like a nested optical waveguide, the optical waveguide portions are close to each other. Even when the optical waveguide elements are arranged, it is possible to provide an optical waveguide element capable of effectively suppressing crosstalk of modulation signals applied to the respective optical waveguides.

1 Substrate 2-0 to 2-9 Optical waveguide 3-1, 3-2 Signal electrode 4-1 to 4-4 Ground electrode 10 Groove

Claims (2)

A substrate having an electro-optic effect;
An optical waveguide formed on the substrate;
In an optical waveguide device having a modulation electrode for modulating a light wave guided in the optical waveguide,
The optical waveguide has a configuration in which a plurality of optical waveguide portions are arranged in parallel to each other, and an electric field by the modulation electrode is applied to at least one of the optical waveguide portions,
A distance between the optical waveguide portions is 300 μm or less, and a groove is formed between the optical waveguide portions;
An optical waveguide element, wherein a conductive material is filled or adhered inside the groove.   2. The optical waveguide device according to claim 1, wherein the depth of the groove is 450 [mu] m or more.

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