JP2014199354A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator in which the increase in the number of working electrodes is suppressed.SOLUTION: The optical modulator includes a substrate 1 having a plurality of Mach-Zehnder optical waveguides formed thereon and a working electrode which is formed on the substrate and performs phase adjustment for at least one of the Mach-Zehnder optical waveguides. The working electrode comprises three kinds of working electrodes, and first working electrodes (a1 and a2) and second working electrodes (b1 and b2) are mutually different in length along two arms (20 and 21) of the Mach-Zehnder optical waveguides and are configured to perform push-pull operations in cooperation with respective arms, and third working electrodes (c1 and c2) are configured to perform phase adjustment in cooperation with at least a part of the first working electrode (a1) and/or the second working electrode (b2).

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator that includes a plurality of Mach-Zehnder optical waveguides and performs phase adjustment between the Mach-Zehnder optical waveguides.

  In the next-generation long-distance large-capacity optical communication system that requires high speed and large capacity as communication traffic increases, there are many such as differential quadrature phase shift keying (DQPSK) and quadrature phase amplitude modulation (QAM). The introduction of a value modulation / demodulation coding technique is under consideration.

  In an optical modulator that performs such modulation, a plurality of Mach-Zehnder type optical waveguides (MZ type optical waveguides) are formed in parallel on one substrate, and only optical modulation and optical phase difference adjustment in each MZ type optical waveguide are performed. Instead, it is necessary to adjust the optical phase difference between the MZ type optical waveguides.

  Patent Document 1 discloses an optical modulator that generates QAM signals using a plurality of parallel MZ type optical waveguides, and discloses that phase difference adjustment between the MZ type optical waveguides is necessary. It is disclosed that an offset voltage for adjusting the optical phase difference is applied to each arm.

  Further, Patent Document 2 discloses a technique for realizing low power consumption by configuring the electrodes in a nested manner and performing voltage drive push-pull as a configuration for adjusting the optical phase difference between the optical waveguides. ing.

  In an optical modulator in which a plurality of MZ optical waveguides are arranged in parallel, in addition to adjusting the phase difference between two arms (branch waveguides) in a single MZ optical waveguide, each MZ optical waveguide Adjustment of the optical phase difference between the waveguides is required, and as in Patent Document 1, the number of working electrodes for phase difference adjustment increases according to the number of MZ type optical waveguides, and a space for arranging the working electrodes is secured. In addition, the degree of freedom in designing the optical modulator is limited.

JP 2009-244682 A Japanese Patent No. 3806043

  The problem to be solved by the present invention is to solve the above problems and provide an optical modulator in which the space occupied by the working electrode for optical phase difference adjustment is set to a minimum.

In order to solve the above problems, the optical modulator of the present invention has the following technical features.
(1) An optical modulator having a substrate on which a plurality of Mach-Zehnder optical waveguides are formed and a working electrode formed on the substrate and performing phase adjustment on at least one of the Mach-Zehnder optical waveguides. Has three types of working electrodes, and the first working electrode and the second working electrode have different lengths along the two arms of the Mach-Zehnder type optical waveguide and cooperate with each other. Wherein the third working electrode is configured to perform phase adjustment in cooperation with at least a part of the first working electrode and / or the second working electrode. To do.

(2) An optical modulator having a substrate on which a plurality of Mach-Zehnder type optical waveguides are formed and a working electrode formed on the substrate and performing phase adjustment on at least one of the Mach-Zehnder type optical waveguides. Has three types of working electrodes, and the first working electrode and the second working electrode have different lengths along the two arms of the Mach-Zehnder type optical waveguide and cooperate with each other. The third working electrode is configured to perform push-pull operation, and the third working electrode is configured to perform phase adjustment in cooperation with at least a part of the first working electrode and the second working electrode. The two working electrodes are configured to face each other in the first region along the arm, and in the other second region along the arm, the first working electrode and the third working electrode are one of the two working electrodes. Opposite to act on the arm , Characterized in that the second working electrode and the third working electrode is disposed so as to face to act on the other arm.

(3) In the optical modulator described in (2) above, the length of the electrode where the first working electrode and the third working electrode are opposed to each other, and the second working electrode and the third working electrode are It is set so that the length of the electrode of the part which opposes becomes equal.

(4) In the optical modulator according to any one of (1) to (3), the third working electrode is a ground electrode.

(5) The optical modulator according to any one of (1) to (4), wherein the optical modulator operates in a four-phase phase shift keying method or a quadrature amplitude modulation method.

  In the optical modulator of the present invention, the working electrode for adjusting the phase includes three kinds of working electrodes, and the first working electrode and the second working electrode have a length along two arms of the Mach-Zehnder type optical waveguide. The third working electrode is different from each other and configured to perform push-pull operation in cooperation with each arm, and the third working electrode cooperates with at least a part of the first working electrode or the second working electrode to adjust the phase. Since there is no need to arrange another working electrode facing the third working electrode, the space occupied by the working electrode (especially the space in the direction along the arm) is set to a minimum. It becomes possible to do.

It is a figure which shows the structure of the working electrode used for the optical modulator of this invention, and is a figure explaining a 1st Example. It is a figure which shows the structure of the working electrode used for the optical modulator of this invention, and is a figure explaining a 2nd Example. It is a figure which shows the structure of the working electrode used for the optical modulator of this invention, and is a figure explaining a 3rd Example. It is a figure explaining an example of the optical modulator of this invention. It is a figure explaining the other example of the optical modulator of this invention. It is a figure which shows the example of the optical modulator incorporating the optical modulation part shown in FIG.

  Hereinafter, the present invention will be described using preferred examples. 1-3 is a figure explaining the structure of the working electrode used for the optical modulator of this invention.

  The present invention provides an optical modulator having a substrate 1 on which a plurality of Mach-Zehnder optical waveguides are formed, and a working electrode formed on the substrate and performing phase adjustment on at least one of the Mach-Zehnder optical waveguides. The working electrode includes three types of working electrodes. The first working electrode (a1, a2) and the second working electrode (b1, b2) are along the two arms (20, 21) of the Mach-Zehnder type optical waveguide. The third working electrode (c1, c2) is configured to perform a push-pull operation in cooperation with each arm, and the third working electrode (c1, c2) has the first working electrode (a1) and / or the first working electrode. The phase adjustment is performed in cooperation with at least a part of the two working electrodes (b2).

  As a substrate used in the optical modulator of the present invention, it is necessary to use a substrate having an electro-optic effect in a portion where light modulation is performed or a portion where phase difference adjustment is performed. For example, lithium niobate, tantalum Single crystal materials such as lithium oxide and PLZT (lead lanthanum zirconate titanate) and solid solution crystal materials thereof can be used. Semiconductors and polymers can also be used as substrates having an electro-optic effect. On the other hand, it is preferable to use a low-loss substrate with strong light confinement with respect to branching, joining, bending, etc. of the optical waveguide except for the portion that performs optical modulation and phase difference adjustment. For example, it is possible to use a planar lightwave circuit (PLC) that preferably uses quartz or the like.

The optical waveguide can be formed, for example, by injecting or thermally diffusing a high refractive index material such as titanium into the substrate. It is also possible to form a ridge type or rib type optical waveguide by forming irregularities on the substrate. In the light modulation portion, a modulation electrode (signal electrode and ground electrode) is formed in the vicinity of the optical waveguide. In the phase difference adjusting portion, the working electrode is disposed close to the optical waveguide. When an electrode is formed immediately above the optical waveguide as in the case of using a Z-cut substrate, it is made of silicon oxide (SiO ₂ ) or the like in order to suppress absorption of the light wave propagating through the optical waveguide into the electrode layer. The buffer layer can be formed on the optical waveguide or on the substrate.

  When forming the modulation electrode and the working electrode, a base electrode pattern is formed on the substrate with a conductive metal, and an electrode having a necessary thickness is formed by gold plating or the like. In the case of a Z-cut substrate, a signal electrode and a ground electrode are arranged on the optical waveguide. However, in the case of using an X-cut substrate, two working electrodes are arranged to face each other with the optical waveguide interposed therebetween.

  As shown in FIG. 1, the optical modulator of the present invention is characterized by three first working electrodes A (a1, a2) and second working electrodes B (b1, b2) with respect to a single MZ type optical waveguide. ), By providing the third working electrode C (c1, c2), the optical phase difference between the two arms (branch waveguides 20, 21) in the same MZ type optical waveguide and between the other MZ type optical waveguides Adjust.

  A portion of the first working electrode A along the optical waveguide 20 is a1, and its length is L. The portion of the first working electrode A along the optical waveguide 21 is a2, and its length is “L−ΔL−Δl” (in FIG. 1, ΔL is the length of the third working electrode C). Δl is the length of the gap between the third working electrode C (c1) and the second working electrode B (b1).

  The portion along the optical waveguide 20 in the second working electrode B is b1, and its length is “L−ΔL−Δl”. In addition, the portion along the optical waveguide 21 in the second working electrode B is b2, and the length thereof is L.

  The first working electrode and the second working electrode used in the optical modulator of the present invention have different lengths of electrodes facing each other with the arms (20, 21) interposed therebetween. In FIG. 1, the electric fields applied to the arm 20 and the arm 21 are different from each other in cooperation with the first working electrode and the second working electrode, and a so-called push-pull operation is achieved. Therefore, the phase of the light wave propagating through the two arms can be adjusted in different directions by the two electrodes, the first working electrode and the second working electrode, and the phase difference can be adjusted efficiently with a small voltage difference. it can.

  To adjust the optical phase difference between the two arms (branch waveguides 20 and 21), a bias voltage of ΔV1 and −ΔV1 is applied to the first working electrode A and the second working electrode B, respectively. good. When there is no need to minimize the bias voltage, it is possible to apply a voltage having a magnitude different from that of ΔV1 and −ΔV1 ′. By applying this voltage, a bias voltage having an action length of “L−ΔL−Δl” is applied to each arm (20, 21).

  Next, phase difference adjustment between MZ type optical waveguides will be described. The third working electrode C (c1) is disposed so as to face the first working electrode A (a1) with the arm 20 interposed therebetween, and the third working electrode C (c2) is arranged to be the second working electrode B (b2). ) With the arm 21 in between.

  If the phase difference relationship between the other MZ type optical waveguides is not adjusted, ΔV1, −V is applied to the first working electrode A and the second working electrode B for adjusting the optical phase difference between the two arms. It is only necessary to apply a bias voltage of ΔV1.

  If the optical phase difference between the other MZ type optical waveguides is adjusted, the optical phase difference between the two arms with respect to the first working electrode A, the second working electrode B, and the third working electrode C. A bias voltage is applied separately from the bias voltage for adjustment.

  In order to perform the bias control as described above, the length (ΔL) of the portion where the first working electrode and the third working electrode are opposed to each other, and the second working electrode and the third working electrode are It is preferable that the length (ΔL) of the electrodes in the facing portion is set to be equal.

  Specifically, bias voltages of ΔV2, ΔV2, and −ΔV3 are applied to the first working electrode A, the second working electrode B, and the third working electrode C, respectively. In this case, since the same voltage is applied to the first working electrode A and the second working electrode B in the region 1, the phase difference is not affected, but in the region 2, the optical phases of the branching waveguides 20 and 21 are the same. Acts to shift in the direction.

  Further, by connecting the third working electrode to the ground electrode, it is possible to reduce the number of types of bias voltage to be fed.

  1 to 3, the third working electrode is disposed so as to face the first working electrode and the second working electrode. However, although the bias voltage control is complicated, the third working electrode is arranged to face only one of the first and second working electrodes, and the bias voltage applied to the first to third working electrodes is comprehensively determined. It is also possible to adjust both the phase difference adjustment between two arms in the same MZ type optical waveguide and the phase difference adjustment between each MZ type optical waveguide.

  When the third working electrode is disposed to face only one of the first and second working electrodes, the length of the first working electrode is set to L and “L−” as shown in FIGS. Not only is the length of the second working electrode composed of two parts, “L-Δl-Δl”, but also the length of the first working electrode is represented by L, for example. The length of the second working electrode can be two portions of “L−ΔL−Δl” and “L−ΔL−Δl”.

  In the embodiment of FIG. 1, the power supply line to the first working electrode is routed between the second working electrode and the third working electrode. In addition to this, as shown in FIG. 2, by dividing the power supply line to the third working electrode into two, the power supply line to the first working electrode is bypassed, and the second working electrode and the third working electrode are It is also possible to reduce the gap.

  In the embodiment of FIG. 3, an example is shown in which the first working electrode and the second working electrode are arranged up to the vicinity of the branch portion of the optical waveguide.

  FIG. 4 shows a part of the optical modulator. Four MZ type optical waveguides (each arm is indicated by reference numerals 36 to 43) are divided into two light waves using the branching waveguides 32 and 33 and 34 and 35. It is a figure which shows the part to multiplex. A method for adjusting the phase difference between the optical waveguides 30 and 31 will be described. The upper half of the optical waveguide is provided with a first working electrode (a1, a2), a second working electrode (b1, b2), and a third working electrode (c1, c2), and includes ΔV2, ΔV2, and By adding −ΔV3 to each, the phase of the light wave emitted from the optical waveguide 30 can be shifted with respect to the light wave emitted from the optical waveguide 31 by the action of the third working electrode.

  The first working electrode (x1, x2) and the second working electrode (y1, y2) arranged on the lower side of FIG. 4 are used to adjust the phase difference of the light wave between the optical waveguides 34 and 35. The

  FIG. 5 shows a configuration in which the above-described working electrode configuration is applied to three MZ type optical waveguides with respect to four parallel MZ type optical waveguides (56 to 63). The phase difference can be adjusted. As a result, the working electrodes arranged in the branch waveguides (32 to 35) shown in FIG. 4 can be reduced.

  The third working electrodes “c1 and c2”, “f1 and f2”, or “i1 and i2” can be applied with independent bias voltages, but the phase difference between the light waves emitted from the optical waveguide 50 and the optical waveguide 51 is determined. In order to adjust, the same bias voltage is applied to “c1 and c2” and “f1 and f2”.

  The electrode structure of the present invention does not need to be applied to all MZ type optical waveguides. For example, when there is no need to adjust the phase difference between the light waves emitted from the optical waveguide 50 and the optical waveguide 51, “c1 and c2”. Alternatively, it may be applied only to a portion where the optical phase difference adjustment function is necessary so that one of “f1 and f2” is not necessary.

  As shown in FIG. 4 or FIG. 5, the optical modulator of the present invention can be suitably applied to a configuration in which a plurality of MZ type optical waveguides are arranged in parallel, and in particular, four-phase phase shift keying. It can be used for a method or a quadrature amplitude modulation method. FIG. 6 shows an outline of an entire optical modulator in which PLCs (Planar Lightwave Circuits) 3 and 4 are connected before and after the modulation circuit shown in FIG.

  However, since the essence of the present invention is characterized in that the phase difference of each light propagating through the optical waveguides arranged in parallel can be adjusted, the configuration other than the parallel MZ type optical waveguides, for example, a coupler structure, etc. Applicable.

  The structure of the present invention can also be applied to cases where the optical path length until light multiplexing is asymmetric.

  Although the above drawings show a configuration in which an X-cut LN substrate is used as a substrate having an electro-optic effect, a Z-cut LN substrate may be used.

  Although the present invention has been described based on the embodiments, it is needless to say that the present invention is not limited thereto and can be appropriately changed in design without departing from the gist of the present invention.

  According to the present invention, it is possible to provide an optical modulator that suppresses an increase in the number of working electrodes.

1 Substrate 2, 20-63 Optical waveguide 3, 4 PLC
a1, a2 first working electrode b1, b2 second working electrode c1, c2 third working electrode

Claims (5)

A substrate on which a plurality of Mach-Zehnder optical waveguides are formed;
In an optical modulator having a working electrode formed on the substrate and performing phase adjustment on at least one of the Mach-Zehnder optical waveguides,
The working electrode includes three types of working electrodes. The first working electrode and the second working electrode have different lengths along the two arms of the Mach-Zehnder optical waveguide, and cooperate with each arm. Configured to work and perform push-pull operation,
The third working electrode is configured to perform phase adjustment in cooperation with at least a part of the first working electrode and / or the second working electrode. A substrate on which a plurality of Mach-Zehnder optical waveguides are formed;
In an optical modulator having a working electrode formed on the substrate and performing phase adjustment on at least one of the Mach-Zehnder optical waveguides,
The working electrode includes three types of working electrodes. The first working electrode and the second working electrode have different lengths along the two arms of the Mach-Zehnder optical waveguide, and cooperate with each arm. Configured to work and perform push-pull operation,
The third working electrode is configured to perform phase adjustment in cooperation with at least a portion of the first working electrode and the second working electrode;
The first working electrode and the second working electrode are configured to face each other in a first region along the arm,
In the other second region along the arm, the first working electrode and the third working electrode are opposed to act on one arm, and the second working electrode and the third working electrode are on the other side. An optical modulator characterized in that the optical modulator is disposed so as to oppose the arm. The optical modulator according to claim 2.
The length of the electrode where the first working electrode and the third working electrode face each other is equal to the length of the electrode where the second working electrode and the third working electrode face each other. An optical modulator characterized by being set. The optical modulator according to any one of claims 1 to 3,
The optical modulator, wherein the third working electrode is a ground electrode. The optical modulator according to any one of claims 1 to 4,
The optical modulator is operated by a four-phase phase shift keying method or a quadrature amplitude modulation method.

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