JP5104805B2 - Light control device - Google Patents

Description

  The present invention relates to a light control device, and more particularly to a light control device with improved broadband characteristics of 20 GHz or higher.

  In the optical communication field and the optical measurement field, an optical waveguide, a signal electrode, and a ground electrode are formed on a substrate having an electro-optic effect, and a high frequency is applied to the signal electrode to modulate a light wave propagating through the optical waveguide. The device is heavily used.

  In order to realize high-speed transfer of a large amount of data, an optical control device having a broadband characteristic of 20 GHz or more is required. Further, in order to reduce the size and power consumption of a driver for driving the light control device, it is required to reduce the drive voltage of the light control device.

For this purpose, it is necessary to solve the following various problems.
(1) In order to achieve speed matching between the speed of the light wave propagating through the optical waveguide and the speed of the microwave that is the modulation signal propagating through the signal electrode, the refractive index of the light wave and the effective refractive index of the microwave should be matched.
(2) To improve the light coupling efficiency when a light wave is introduced from the optical fiber to the light control device or when a light wave is led from the light control device to the optical fiber.
(3) In order to suppress the propagation loss of the microwave applied to the signal electrode, the microwave is prevented from leaking from the signal electrode, and before and after being input to the signal electrode and the electric field of the signal electrode acts on the waveguide. Impedance is matched before and after the action part to suppress reflection of microwaves due to impedance mismatch.
(4) The electric field formed by the signal electrode is efficiently applied to the optical waveguide.

For this reason, as a result of intensive studies, the present inventors use a Z-cut substrate in a substrate having an electro-optic effect, such as a LiNbO ₃ crystal, and form an optical waveguide having a ridge structure on the substrate. For adjusting the effective refractive index and impedance, the shape of the ridge structure (width and height of the ridge), the shape of the signal electrode and the ground electrode (width of the signal electrode, the distance between the signal electrode and the ground electrode) It has been confirmed that optimum numerical values can be obtained by adjusting the height of the signal electrode and the ground electrode.

  On the other hand, in order to realize a wide variety of optical modulation such as differential quadrature phase shift keying (DQPSK) and single sideband modulation (SSB) of Patent Document 1, as shown in FIG. Nested waveguides in which another Mach-Zehnder type waveguide (sub-Mach-Zehnder type waveguide) 3 is incorporated in two branching waveguides 2 constituting a type waveguide (main Mach-Zehnder type waveguide) 1 are used. Yes. In addition, it has also been proposed that the signal electrode 5 is arranged in each of the four branch waveguides 4 of the sub Mach-Zehnder type waveguide 3 to perform optical modulation.

  In the light control device as shown in FIG. 1, the number of signal electrodes provided for modulating the light wave propagating through each waveguide is such that the electric field of the modulation signal is introduced from the electrode pad portion 5 for inputting the modulation signal to the signal electrode. It is required to accurately adjust the timing at which the modulation signal is introduced into each action part by adjusting the length I of the signal electrode to the start point b of the action part S acting on the waveguide. For this reason, as shown in FIG. 2, it is easy to form a portion where a part R of the signal electrode 42 is folded.

  Moreover, in order to reduce the size of the light control device, the radius of curvature at the bent portion of the signal electrode tends to be small. The present inventors examined a light control device in which such a sharply bent portion R exists in the signal electrode, and compared it with a light control device having a signal electrode that has no bent portion or a gentle bend. The microwave propagation loss was found to be extremely large. Moreover, it has been found that this tendency is remarkable when the microwave exceeds 20 GHz, which is an important factor for degrading the broadband characteristics of the light control device.

JP 2008-116865 A JP-A-6-289341

  An object of the present invention is to provide a light control device that solves the above-described problems and has an improved broadband characteristic of 20 GHz or more even in a light control device having a folded portion at a signal electrode.

In order to solve the above problems, an invention according to claim 1 is an optical control 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. In the device, the modulation electrode is a symmetric or asymmetrical CPW electrode composed of a signal electrode and a ground electrode, and is a working unit that modulates a light wave propagating through the optical waveguide from a signal input end of the signal electrode. The signal electrode up to the start point is defined as an input-side signal electrode portion, and at least a part of the input-side signal electrode portion has a bent portion that turns the wiring, and the thickness of the substrate in the region where the bent portion exists is 30 to 30 mm. 100 μm, and the width of the portion of the signal electrode that contacts the substrate including the bent portion is 10 μm or less.

The invention according to claim 2 is a light control device comprising 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. A symmetric or asymmetrical CPW electrode composed of a signal electrode and a ground electrode, and a signal electrode from a signal input end of the signal electrode to a starting point of an action part that modulates a light wave propagating through the optical waveguide. The input-side signal electrode portion has at least a bent portion at which the wiring is folded at least at a part of the input-side signal electrode portion, and the thickness of the substrate in the region where the bent portion exists is 30 to 100 μm . Is composed of a plurality of signal electrodes that transmit independent modulation signals to a plurality of action portions, and the interval between adjacent different signal electrodes in the bent portion is 500 μm or less.

The invention according to claim 3 is the light control device according to claim 1 or 2 , wherein the substrate is a Z-cut substrate, the optical waveguide has a ridge structure at the action portion, and the signal electrode It is arranged on the ridge structure.

According to the first aspect of the present invention, there is provided a light control device 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. A symmetric or asymmetrical CPW electrode composed of a signal electrode and a ground electrode, and a signal electrode from a signal input end of the signal electrode to a starting point of an action part that modulates a light wave propagating through the optical waveguide. The input-side signal electrode portion has at least a bent portion at which the wiring is folded, and the thickness of the substrate in the region where the bent portion exists is 30 to 100 μm. Since the width of the portion of the signal electrode in contact with the substrate is 10 μm or less, the propagation loss of the microwave can be reduced even in the light control device having the bent portion that is folded back to the signal electrode, and the light control device having excellent broadband characteristics. Can provide vice. Further , in general, a signal electrode of 10 μm or less has a light propagation control device that suppresses the propagation loss of the microwave by adopting the technical configuration of the present invention even though the propagation loss of the microwave at the bent portion becomes large. Obtainable.

According to a second aspect of the present invention, there is provided an optical control device comprising 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. A symmetric or asymmetrical CPW electrode composed of a signal electrode and a ground electrode, and a signal electrode from a signal input end of the signal electrode to a starting point of an action part that modulates a light wave propagating through the optical waveguide. The input-side signal electrode portion has at least a bent portion at which the wiring is folded, and the thickness of the substrate in the region where the bent portion exists is 30 to 100 μm. is composed of a plurality of signal electrodes for transmitting independent modulated signals into a plurality of working portions, the bent portion, because the spacing between the adjacent different signal electrodes is 500μm or less, the bent portion folded back to the signal electrode Even in a certain light control device, the propagation loss of microwaves can be reduced, and a light control device with good broadband characteristics can be provided. Furthermore, the signal electrode wiring can be made compact, and the light control device can be further miniaturized. Usually, when the interval between adjacent different signal electrodes is 500 μm or less, crosstalk between the signal electrodes is likely to occur and the propagation loss increases, but in the region where the folded bent portion exists as in the present invention, By setting the thickness of the substrate to 30 to 100 μm, such crosstalk and propagation loss can be suppressed, and signal electrode wiring in which the interval between adjacent different signal electrodes is 500 μm or less is also possible.

According to the invention of claim 3 , since the substrate is a Z-shaped cut substrate, the optical waveguide has a ridge structure in the working portion, and the signal electrode is disposed on the ridge structure, the broadband characteristics are further improved. A light control device can be obtained.

It is a figure which shows an example of the light control device which has a nest type | mold waveguide. It is a figure which shows a part of light control device which has a folding | turning bending part in a signal electrode. It is a figure explaining the improvement state of the wideband characteristic of the light control device of this invention.

The light control device of the present invention will be described in detail below.
A feature of the present invention is that in a light control device 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 a signal A symmetric or asymmetrical CPW electrode composed of an electrode and a ground electrode, and the signal electrode from the signal input end of the signal electrode to the starting point of the action part that modulates the light wave propagating through the optical waveguide The signal electrode portion has at least a bent portion at which the wiring is folded, and the thickness of the substrate in a region where the bent portion exists is 30 to 100 μm.

As the material for the substrate having the electro-optic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, a lithium niobate (LiNbO ₃ ) crystal having a high electro-optic effect is 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 2, it is possible to form an optical waveguide by forming a ridge on the surface of the thin plate 1 in accordance with the shape of the optical waveguide. In the present invention, in order to improve the broadband characteristics, a Z-cut substrate is used, and an optical waveguide having a ridge structure formed by dry etching or the like is preferably used as the optical waveguide. Further, a ridge structure can be further added to the Ti diffusion waveguide.

  Modulation electrodes such as signal electrodes and ground electrodes are generally formed by forming a Ti / Au electrode pattern, a gold plating method, or the like.

  The arrangement state of the signal electrode and the ground electrode is such that the propagation loss of the microwave is reduced, and the ground electrode is sandwiched between the signal electrodes (41, 42) on the substrate as shown in FIG. It is preferable to use a symmetric type coplanar electrode (CPW electrode) in which (61 to 63) are arranged, or an asymmetric coplanar electrode in which a ground electrode is arranged on one of the signal electrodes on the substrate.

  In the input-side signal electrode part I from the signal input end a of the signal electrode to the start point b of the action part that modulates the light wave propagating through the optical waveguide, as shown in FIG. The width of the signal electrode is 10 μm or less. Usually, in order to reduce the propagation loss of the signal electrode, it is preferable to increase the width of the signal electrode. However, when the width of the signal electrode is increased, it is difficult to reduce the size of the light control device. In addition, the length of each signal electrode needs to be optimized in order to match the propagation time of the modulation signal in the input side signal electrode part for each signal electrode. However, if the width of the signal electrode increases, the signal at the bent portion The length of the electrode differs greatly between the inside and outside of the electrode, making it difficult to make an optimal design. For this reason, in the present invention, the thickness of the substrate is set to 30 to 100 μm in order to suppress the propagation loss of the microwave at the folded and bent portion while the width of the signal electrode is 10 μm or less.

Next, in order to confirm the improvement of the propagation loss of the microwave in the light control device of the present invention, a light control device as shown in FIG. 1 was made using a Z-cut LiNbO ₃ substrate. At that time, each of the two sub Mach-Zehnder type waveguides 3 is configured such that the signal electrodes 41 and 42 are arranged in the branching waveguides 32 and 33 of the sub Mach-Zehnder type waveguide as shown in FIG. Reference numeral 31 denotes an input waveguide of a sub Mach-Zehnder type waveguide, and reference numerals 51 and 52 denote electrode pads for inputting a modulation signal to the signal electrodes of the signal electrodes 41 and 42.

  In the working part of the optical waveguide, the shape of the optical waveguide is a ridge shape, and the width of the ridge is 8 μm. The signal electrode was disposed on the ridge, the width of the portion of the signal electrode in contact with the substrate was 7 μm, the height of the signal electrode was 45 μm, and the height of the ground electrode was the same. The distance between the signal electrode and the ground electrode was 50 μm, and the length from the electrode pad to the starting point of the action portion was 50 mm.

  The distance d1 between the signal electrodes before and after the folded bent portion R shown in FIG. 2 is 500 μm or less, and the distance d2 between the two signal electrodes 41 and 42 is also 500 μm or less.

Two light control devices (two types of substrate thickness: 1 mm and 0.1 mm) having different substrate thicknesses with the same arrangement of signal electrodes and the like were prepared, and the electrical response characteristics (S21) related to frequency were tested. The result is shown in FIG. Note that the rear surface of the LiNbO ₃ substrate, bonding the substrate of the same material through the adhesive layer (thickness of about 100 [mu] m) as a reinforcing substrate.

  As can be seen from FIG. 3, the light control device (0.1 mm thickness) with a thin substrate has a significantly higher frequency than that of the conventional light control device (substrate thickness 1 mm) even at 20 GHz or higher. It can be easily understood that the characteristics are significantly improved. A large dip exists in the vicinity of 32 GHz. This is a phenomenon that occurs due to simple measurement by a probe in a chip state, and is not a characteristic of the light control device itself.

  Further, the electrical response characteristics described above were measured by changing the thickness of the substrate, and when the influence of the thickness of the substrate on the signal electrode having the folded portion was examined, the thickness of the substrate was in the range of 30 to 100 μm. It was confirmed that the high frequency characteristics at 20 GHz or higher were improved. The reason why high frequency characteristics cannot be improved when the thickness of the substrate is less than 30 μm is that a difference between the refractive index of the light wave and the effective refractive index of the microwave occurs, and the ridge shape of the optical waveguide and the shape of the signal electrode and the ground electrode This is because it is necessary to readjust the above in consideration of the thickness of the substrate.

  In the light control device of the present invention, as described above, the signal electrode is composed of a plurality of signal electrodes that transmit independent modulation signals to the plurality of action portions, and the adjacent different signal electrodes in the bent portion. Even when the distance between the electrodes (including the distance between the signal electrodes before and after the folded portion) is 500 μm or less, the thickness of the substrate is set to 30 to 100 μm, thereby suppressing the occurrence of crosstalk between the signal electrodes and propagation loss. Can be suppressed. Thereby, the wiring of the signal electrode can be made compact, and the light control device can be further downsized by using the present invention.

  As described above, according to the present invention, it is possible to provide a light control device having improved broadband characteristics of 20 GHz or more even in a light control device having a folded portion at a signal electrode.

DESCRIPTION OF SYMBOLS 1 Main Mach-Zehnder type | mold waveguide 2 Main branching waveguide 3 Sub Mach-Zehnder type | mold waveguide 4,41,42 Signal electrode 5,51,52 Electrode pad 61-63 Ground electrode R Folding bending part

Claims (3)

In a light control device 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 a symmetric or asymmetric CPW electrode composed of a signal electrode and a ground electrode;
The signal electrode from the signal input end of the signal electrode to the start point of the action part that modulates the light wave propagating through the optical waveguide is defined as an input-side signal electrode part.
At least a part of the input-side signal electrode part has a bent part for folding the wiring,
The thickness of the substrate in the region where the bent portion exists is 30 to 100 μm,
A light control device characterized in that a width of a portion of the signal electrode in contact with the substrate including the bent portion is 10 μm or less. In a light control device 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 a symmetric or asymmetric CPW electrode composed of a signal electrode and a ground electrode;
The signal electrode from the signal input end of the signal electrode to the start point of the action part that modulates the light wave propagating through the optical waveguide is defined as an input-side signal electrode part.
At least a part of the input-side signal electrode part has a bent part for folding the wiring,
The thickness of the substrate in the region where the bent portion exists is 30 to 100 μm,
The signal electrode is composed of a plurality of signal electrodes that transmit independent modulation signals to a plurality of action portions, and an interval between different signal electrodes adjacent to each other in the bent portion is 500 μm or less. . In the optical control device according to claim 1 or 2, the substrate is a Z-type cut substrate, the optical waveguide has a ridge structure in the acting portion, the signal electrode is disposed over the ridge structure A light control device.

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