JP5991339B2 - Light control element - Google Patents

Description

  The present invention relates to a light control element, and more particularly, to a light control element having two or more signal electrodes in a modulation electrode.

  An optical waveguide and a modulation electrode are formed on a substrate having an electrooptic effect, such as lithium niobate, and a light control element is practically used to modulate a light wave propagating through the optical waveguide with the modulation electrode. In recent years, in order to meet the needs for higher communication speeds and larger communication data capacity in the field of optical communication, etc., there are several methods such as differential quadrature phase shift keying (DQPSK). A Mach-Zehnder type optical waveguide (MZ type optical waveguide) is driven by a plurality of modulation signals. In addition, an integrated modulator including a plurality of signal electrodes for coherent multilevel communication such as a DP-QPSK modulator using polarization synthesis is also used.

  In a light control element having a plurality of optical waveguides and a plurality of signal electrodes, different modulation signals are often inputted to the respective signal electrodes, and different modulation signals are applied to the respective optical waveguides. It is configured. For this reason, when an electric field other than a predetermined modulation signal acts on a specific optical waveguide, optical characteristics of the light control element such as the extinction ratio of the signal light deteriorate. Such a phenomenon is called crosstalk.

  In order to suppress crosstalk between the electrodes, it is effective to widen the distance between the optical waveguides and the distance between the electrodes, but such a countermeasure is not preferable because it causes the size of the light control element to increase. In addition, as shown in Patent Document 1, a method of forming a groove between optical waveguides or signal electrodes has also been proposed, but since a process of providing a groove is added to the manufacturing process of the light control element, the manufacturing time is increased. And the cost will increase. In addition, in the light control element using the substrate having a thickness of 30 μm or less, the groove itself cannot be formed, or even if it can be formed, the substrate is easily damaged.

  Further, as the frequency of the modulation signal becomes wider, a local potential difference corresponding to the operation state is generated between the ground electrodes sandwiching the signal electrode. For this reason, a phenomenon occurs in which the change in the electric field generated between the signal electrode and the ground electrode sandwiching the signal electrode differs between the left and right sides of the signal electrode. When such an electric field is applied to the optical waveguide, the intended modulation operation cannot be expected, and the modulation characteristics of the light control element will be greatly degraded.

  In order to solve such a problem, in the patent document 2, the present applicant, as shown in FIGS. 1 and 2, is a light control element in which two or more signal electrodes (31, 32) are arranged. Proposed bonding the ground electrodes (41, 42, 43) to each other with gold wires (51, 52). As a result, the high-frequency ground potential of the action part is stabilized, and the crosstalk between the electrodes is suppressed, and stable modulation operation at a wideband frequency is possible. 2 is a cross-sectional view taken along one-dot chain line A-A ′ in FIG. 1. Reference numerals other than those described above indicate the bonding connection lines (53, 54) such as the substrate 1, the optical waveguides (21 to 23), the ground electrodes (44 to 46), and the gold wires.

  Usually, means such as wire bonding is used for connection of the ground electrode. The bonding part bonds a bonding wire such as gold with ultrasonic waves (thermal ultrasonic pressure bonding) using a needle-shaped bonding tool. A thin gold wire generally used is 20 to 30 μm, and even in that case, a space of about 100 μm is required as a bonding portion.

  On the other hand, in order to reduce the cost and size of the light control element, it is necessary to narrow the light control element. For this purpose, it is necessary to make the interval between the signal electrodes narrower, and accordingly, the width of the ground electrode between the signal electrodes becomes narrower. For example, the one having a width of 200 μm or less is used. In such a case, a short circuit between the bonding part and the signal electrode may occur due to a positional deviation of the bonding part, or bonding may not be possible due to insufficient space. Such a problem becomes more remarkable in a highly integrated light control element such as a DP-QPSK modulator in which four MZ structures are arranged in parallel.

JP 2009-53444 A Japanese Patent No. 5067464

  The problem to be solved by the present invention is to solve the above-described problems and provide a light control element capable of suppressing crosstalk between electrodes even if the light control element is narrowed. It is.

In order to solve the above problems, the light control element of the present invention has the following technical features.
(1) In a light control element having a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode that modulates a light wave propagating through the optical waveguide, the modulation electrode includes at least two signals. An electrode and a ground electrode arranged so as to sandwich the signal electrode, and the modulation electrode exerting a modulation action on the light wave is disposed between the two signal electrodes. The ground electrode is electrically connected to the other ground electrode, and an electrical connection means is provided so as to straddle a part of the signal electrode, and the electrical connection means includes at least The plurality of ground electrodes are configured to be connected by a single conductive line, and the length of the conductive line connecting the ground electrodes across the signal electrode is 1 mm or less, and the electrical connection means A plurality of the conductive wires are arranged side by side The distance L between adjacent conductive lines with respect to the direction in which the signal electrode extends, and the length of the straight line connecting the both ends of the conductive lines with respect to the direction, with each conductive line disposed obliquely with respect to the direction. L ′ satisfies the relationship L> L ′. In particular, it is preferable that the distance L between the adjacent conductive lines and the length L ′ of the straight line connecting both ends of the conductive lines satisfy the relationship that L ′ / L is 0.08 or less. .

(2) In the optical control element according to the above (1), the distance L of the adjacent conductive lines, the wavelength λ at the frequency of the modulation signal propagated through the signal electrode, so that less than a quarter It is characterized by being set.

(3) in the above-mentioned light control device described in (1), the distance L of the adjacent conductive lines, the wavelength λ at the frequency of the modulation signal propagated through the signal electrode, so that less than one-tenth It is characterized by being set.

( 4 ) In the light control element according to any one of (1) to ( 3 ), at least a part of a portion where the one conductive line and the ground electrode are connected is a width of the ground electrode. Is 200 μm or less.

The present invention relates to a light control element including a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode that modulates a light wave propagating through the optical waveguide. The modulation electrode includes at least two modulation electrodes. It is composed of a signal electrode and a ground electrode arranged so as to sandwich the signal electrode, and the modulation electrode exerts a modulation action on the light wave, and the modulation electrode is disposed between the two signal electrodes. Electrically connecting means arranged so as to straddle part of the signal electrode, and electrically connecting the grounded electrode and the other grounded electrode, and the electrical connecting means, Since at least a plurality of ground electrodes are connected by a single conductive line, even when the width of the ground electrode is narrow, the conductive lines can be bonded. In addition, compared with a method in which both ends of the conductive wire are bonded to the ground electrode at each location straddling the signal electrode, the number of bonding locations can be greatly reduced in the present invention.
In addition, since the length of the conductive wire that straddles the signal electrodes and connects the ground electrodes is 1 mm or less, a high-frequency potential difference does not occur between the ground electrodes.
In addition, the distance L between adjacent conductive lines and the length L ′ of each straight line connecting the both ends of the conductive lines with respect to the direction L satisfy the relationship L> L ′. By setting so as to satisfy, the potential between the ground electrodes can be maintained substantially the same, and good signal quality with suppressed crosstalk (for example, variation in frequency characteristics is reduced and stability is increased) is obtained. be able to.

  In the light control element of the present invention, the electrical connecting means arranges a plurality of conductive lines, and the interval L between the conductive lines adjacent to the direction in which the signal electrodes extend propagates through the signal electrodes. By setting the wavelength λ at the frequency of the modulation signal to be less than one quarter, more preferably one tenth or less, crosstalk between signal electrodes can be efficiently suppressed. As a result, the frequency characteristic itself is improved.

Na us, also the length L 'as described above, less than one quarter of the wavelength lambda, it more preferably to less than one tenth, conducting the signal electrodes are arranged perpendicular to the direction that extends Crosstalk can be suppressed to the same extent as the line.

  In the light control element of the present invention, at least a part of a portion where one conductive line and the ground electrode are connected can achieve a good connection state even when the width of the ground electrode is 200 μm or less. it can. In particular, a sufficient connection region can be ensured by arranging the conductive lines obliquely with respect to the direction in which the signal electrodes extend.

10 is a plan view showing an outline of a light control element disclosed in Patent Document 2. FIG. It is a figure which shows sectional drawing in the dashed-dotted line A-A 'of FIG. It is a top view concerning the light control element of the present invention. It is a figure which shows sectional drawing in the dashed-dotted line AA 'of FIG. It is the figure which expanded a part of modulation electrode of FIG. It is a figure which shows the other Example of the light control element of this invention, and is a figure explaining the arrangement | positioning of the conductive wire which connects ground electrodes in a light control element especially. It is a figure explaining the mode of a modulation electrode in the light control element provided with two signal electrodes used in order to investigate the influence of crosstalk. 8 is a graph showing the influence of crosstalk when the inclination of the conductive line is changed in the direction in which the signal electrode extends in the light control element of FIG. 7.

Hereinafter, the light control element of the present invention will be described in detail using preferred examples. FIG. 3 is a schematic view of a light control element to which the present invention is applied, and FIG. 4 is a cross-sectional view taken along the alternate long and short dash line AA ′ in FIG. FIG. 5 is an enlarged view of the modulation electrode portion of FIG.
As shown in FIGS. 3 to 5, the light control element of the present invention modulates a substrate 101 having an electro-optic effect, optical waveguides (121 to 124) formed on the substrate, and light waves propagating through the optical waveguide. In the light control element having the modulation electrode, the modulation electrode is composed of at least two signal electrodes 130 and a ground electrode 140 disposed so as to sandwich the signal electrode, and the modulation electrode modulates the light wave. The working portion S of the modulation electrode is electrically connected to the ground electrode disposed between the two signal electrodes and the other ground electrode, and a part of the signal electrode. An electrical connection means arranged so as to straddle is provided, and the electrical connection means is configured to connect at least a plurality of ground electrodes with a single conductive line 150.

As the substrate 1 having an electro-optic effect, a single crystal of any one of LiNbO ₃ , LiTaO ₅ or PLZT (lead lanthanum zirconate titanate) can be suitably used. In particular, LiNbO ₃ and LiTaO ₅ frequently used in light control elements such as an optical modulator are preferable. The optical waveguide formed on the substrate is formed, for example, by thermally diffusing a high refractive index material such as titanium (Ti) on a LiNbO ₃ substrate (LN substrate). A ridge-type optical waveguide in which irregularities along the optical waveguide are formed on the substrate can also be used. Further, although the light control element using the X-cut type substrate is illustrated in FIG. 3, the present invention is not limited to this and can be similarly applied to a Z-cut type substrate.

  FIG. 3 illustrates an optical waveguide used for the DP-QPSK modulator. Two second MZ type optical waveguides (122, 123) are inserted into the branching waveguide of the first Mach-Zehnder type optical waveguide (MZ type optical waveguide) 121, and further, the second MZ type optical waveguide is branched. Four third MZ type optical waveguides 124 are inserted in the waveguide. The combining unit 102 of the first MZ type optical waveguide 121 is formed with a polarization combining unit so as to combine two output lights to be combined while changing the polarization plane. The light control element of the present invention is not limited to such an optical waveguide, but a crosstalk phenomenon is likely to occur in a light control element having a plurality of light modulation units, and therefore the configuration of the present invention is applied. Superiority.

The modulation electrode is composed of the signal electrode 130 and the ground electrode 140, and can be formed by forming a Ti / Au electrode pattern on the surface of the substrate and using a gold plating method or the like. Furthermore, if necessary, a buffer layer such as dielectric SiO ₂ can be provided on the surface of the substrate after the optical waveguide is formed, and a modulation electrode can be formed above the buffer layer. It is particularly preferable that the light control element to which the present invention is applied has a configuration in which a ground electrode is disposed between a plurality of signal electrodes.

  In the light control element of the present invention, a plurality of signal electrodes to which a microwave signal is applied are arranged so that the ground electrode is sandwiched, and the electric field formed by the modulation electrode is applied to the optical waveguide. Thus, the plurality of ground electrodes are continuously connected (wire bonding) with one conductive wire. As shown in FIG. 3 or FIG. 5, one conductive wire 250 is used to connect a plurality of ground electrodes 240, and the conductive wires are bonded on each ground electrode 240 with ultrasonic waves. A plurality of conductive wires connected in this way are arranged in parallel as necessary.

  By continuously bonding a plurality of (at least three or more) ground electrodes with one conductive wire, the bonding distance required for one ground electrode can be reduced. In addition, since the number of bondings is reduced, the manufacturing time of the light control element can be shortened. Furthermore, when a thin plate having a thickness of about several tens of μm is used, the reduction in the number of bondings also reduces damage to the substrate due to bonding.

  As the width of the light control element becomes narrower, the width of the ground electrode sandwiched between the signal electrodes becomes increasingly narrower. In particular, the present invention can be preferably applied to a case where the width of the ground electrode in the portion to which the conductive line is connected is 200 μm or less. This is because the area required for connection can be minimized by connecting with one conductive line. In addition, as shown in FIG. 6, by widening the bonding direction (longitudinal direction of the conductive wire) with respect to the direction in which the signal electrode extends, a wider connection area can be secured. As a result, even in a downsized light control element, the ground potential of the ground electrodes on both sides of the signal electrode is stabilized, crosstalk between the signal electrodes is suppressed, and good modulation signal quality can be obtained.

  As the electrical connection means used in the light control element of the present invention, a conductive wire having a high conductivity such as a gold wire can be used. In order not to cause a high-frequency potential difference between the ground electrodes, it is necessary to reduce the resistance component and inductance component of the conductive wire. For this reason, it is desirable that the conductive wire has as low a loop height as possible (the height of the conductive wire across the signal electrode from the substrate) and the length of the conductive wire is short. For this purpose, the length of the conductive wire between the bonding point and the adjacent bonding point is preferably 1 mm or less, more preferably 0.5 mm or less while maintaining a length that does not cause a short between the conductive wire and the signal electrode. Good.

  As shown in FIGS. 5 and 6, when a plurality of conductive lines are arranged, the interval L between the conductive lines should be set to less than a quarter of the wavelength λ at the frequency of the modulation signal propagating through the signal electrode. Is preferred. Thereby, it is possible to suppress the electric field from the signal electrode from leaking beyond the ground electrode. More preferably, the crosstalk can be reliably suppressed by setting the interval L to 1/10 or less of the wavelength of the electric signal. In addition, since the electric field formed by the signal electrode is efficiently applied to the optical waveguide, a decrease in modulation efficiency at a wideband frequency is suppressed, and the frequency characteristics can be improved.

As shown in FIG. 6, when the longitudinal direction of the conductive line is inclined with respect to the direction in which the signal electrode extends, the distance L between adjacent conductive lines with respect to the direction in which the signal electrode extends The length (projected length) L ′ of the straight line connecting the both ends of each conductive line with respect to the direction (the direction in which the signal electrode extends) L ′ is set so as to satisfy the following relational expression: Is preferred.
(Relational expression) L> L '

  When the length L ′ is larger than the distance L, a potential difference is generated between the ground electrodes, and problems such as a difference in the electric field formed between the right and left signal electrodes and the ground electrode occur. In particular, when a plurality of conductive lines are arranged in parallel, when L ≦ L ′ between the uppermost ground electrode and the lowermost ground electrode in FIG. A phase difference occurs, and the modulation characteristics of the light control element are greatly deteriorated.

  For the length L ′, in order to have the same potential between the left and right ground electrodes of the signal electrode, as shown in FIG. 5, the conductive wire should be perpendicular to the direction in which the signal electricity extends. Need to be placed. In order to approach this, the length L ′ indicating the inclination of the conductive line with respect to the direction is set to L> L ′, and similarly to the interval L, with respect to the wavelength λ at the frequency of the modulation signal propagating through the signal electrode. It is desirable to set it to less than one quarter, more preferably one tenth or less. As a result, the variation in frequency characteristics can be reduced and the stability can be increased.

  The light control element is usually housed in a metal case and modularized. In such a case, the ground electrode near the side surface of the light control element is connected to the ground side terminal of the signal line introduced from the metal case or the outside.

  Further, as described above, it is preferable that the electrical connection means is provided in a portion where the modulation electrode exerts a modulation action on the light wave. This is because it is necessary to most suppress the crosstalk phenomenon in the action portion. Further, such an action part is present in the vicinity of the center of the substrate of the light control element, and is often located farthest from the place where the ground electrode is grounded to the metal case of the module. For this reason, since it is difficult to ensure a sufficient grounding state, it is necessary to reinforce and stabilize the grounding state using the electrical connection means as in the present invention.

  Next, the influence of crosstalk when two MZ type optical waveguides are arranged in parallel (crosstalk characteristics of 2 × 2 port electrodes) was examined. As shown in FIG. 7, the crosstalk between the signal electrode 130 arranged along one MZ type optical waveguide (not shown) and the signal electrode 131 arranged along the other MZ type optical waveguide (not shown). Measure characteristics.

  The signal S1 is input from the left end of the signal electrode 130 and is absorbed by the terminator at the right end. On the other hand, the left end of the signal electrode 131 is connected to a terminator, and the signal S2 output from the right end is measured. The result is shown in FIG.

  When the conductive line 150 is perpendicular to the direction in which the signal electrode extends (L ′ = 0), the best characteristics are exhibited. A good result close to this is shown when L ′ / L = 0.08 (the inclination with respect to the vertical is about 4.5 degrees). In the case of L '/ L = 0.7 (the inclination with respect to the vertical is about 35 degrees), it can be seen that the deterioration of the characteristics becomes more remarkable as the frequency becomes wider.

  As described above, according to the present invention, it is possible to provide a light control element capable of suppressing crosstalk between electrodes even when the light control element is narrowed.

1, 101, 201 Substrate 21-23, 121-124 Optical waveguide 31, 32, 130, 131 Signal electrode 41-47, 140 Ground electrode 51-54, 150 Conductive line 151 Bonding location

Claims (4)

In a light control element having a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode that modulates a light wave propagating through the optical waveguide,
The modulation electrode is composed of at least two signal electrodes and a ground electrode arranged so as to sandwich the signal electrode,
The ground electrode arranged between the two signal electrodes and the other ground electrode are electrically connected to the working portion of the modulation electrode where the modulation electrode exerts a modulation action on the light wave And electrical connection means arranged so as to straddle part of the signal electrode,
The electrical connection means is configured to connect at least a plurality of ground electrodes with a single conductive wire, and the length of the conductive wire connecting the ground electrodes across the signal electrode is 1 mm or less. ,
Further, the electrical connection means includes a plurality of the conductive lines arranged side by side, an interval L between adjacent conductive lines with respect to a direction in which the signal electrode extends, and each conductive line is disposed obliquely with respect to the direction. And a length L ′ of the straight line connecting both ends of the conductive line with respect to the direction satisfies a relationship in which L ′ / L is 0.08 or less .   2. The light control element according to claim 1, wherein an interval L between the adjacent conductive lines is set to be less than a quarter of a wavelength λ at a frequency of a modulation signal propagating through the signal electrode. A light control element characterized by that.   2. The light control element according to claim 1, wherein an interval L between the adjacent conductive lines is set to be less than 1/10 of a wavelength λ at a frequency of a modulation signal propagating through the signal electrode. A light control element characterized by that. In the optical control element according to any one of claims 1 to 3, at least a part of the portion where the one conductive line and the grounding electrode is connected, the width of the ground electrode is 200μm or less A light control element characterized by the above.

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