JP5001310B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator. More particularly, the present invention relates to a Mach-Zehnder type optical modulator.

  With the development of optical communication technology, a high-speed and highly stable optical modulator is required. A Mach-Zehnder type is known as a high-speed optical modulator. In the Mach-Zehnder type optical modulator, the input light is branched, and a phase difference is given to the branched light to combine them, thereby obtaining modulated output light.

FIG. 1 shows a conventional Mach-Zehnder type optical modulator (Patent Document 1). The Mach-Zehnder type optical modulator 100 includes an input waveguide 112 into which input light is incident on an optical substrate 110 (such as lithium niobate crystal (LiNbO ₃ : LN)) having an electro-optic effect, and an optical waveguide from the input waveguide. A Y branching portion 114 that splits light, two arm waveguides 116a and 116b that guide the branched light, and a Y combining portion 118 that combines the light from these two arm waveguides are combined. And an output waveguide 120 that outputs light. These waveguides can be formed by selectively diffusing a metal such as Ti on the optical substrate. Thereafter, a buffer layer such as SiO ₂ is provided on the entire substrate surface, and metal electrodes 122a and 122b such as Au are formed on the arm waveguide. High-frequency signal sources 124a and 124b for modulating the optical modulator are connected to the electrodes 122a and 122b, and termination resistors are connected to the electrode output ends.

  The input light incident on the input waveguide 112 is branched into two at the Y branching portion 114. While the branched light propagates through the arm waveguides 116a and 116b, the phase changes due to the influence of the electro-optic effect by the modulation signal applied to the electrodes 122a and 122b. That is, the phase difference between the arm waveguides can be changed by a signal applied to the electrodes. When the light from the arm waveguides 116 a and 116 b is multiplexed by the Y multiplexing unit 118, light whose intensity is modulated is emitted from the output waveguide 120 in accordance with the phase difference between these two lights. This Mach-Zehnder modulator is configured to control the phase of the arm waveguides 116a and 116b independently by each signal electrode, and is called a two-electrode drive type or a dual drive type. In this configuration, in order to simplify the phase compensation circuit, the input portions of the electrical wirings of the electrodes 122a and 122b are usually brought as close as possible and the electrical wirings input to the arm waveguides are made equal in length. That is, the electrical phase length of the electrical wiring from the input portion of the electrode 122a to the arm waveguide 116a is the same as the electrical phase length of the electrical wiring from the input portion of the electrode 122b to the arm waveguide 116b. . Therefore, in FIG. 1, a U-shaped portion is provided in the electric wiring of the electrode 122b to adjust the phase length.

  On the other hand, as shown in FIG. 4 of Patent Document 2, it is also possible to adjust the phase length of the electrical wiring by devising the layout of the electrical wiring without providing a U-shaped portion.

JP 2002-182172 A JP 2004-029498 A Japanese Patent No. 3867148

  However, in the conventional Mach-Zehnder type optical modulator, the wiring of one electrode (electrode 122a in FIG. 1) straddles the other arm waveguide (waveguide 116b in FIG. 1), so that crosstalk occurs at that portion. There was a problem. This is undesirable for high speed and high stability optical modulators. Further, in a modulator including a plurality of Mach-Zehnder interferometers such as an optical single sideband (SSB) modulator, the number of locations where the wiring of the electrode intersects the arm waveguide increases, so this problem is remarkable. (Patent Document 3). In particular, if the layout of the electrical wiring is devised to adjust the phase length of the electrical wiring, the arm waveguide is crossed a plurality of times (Patent Document 2), and signal degradation increases due to crosstalk. . On the other hand, as shown in FIG. 1, when the U-shaped portion is provided in the electrical wiring and the phase length is adjusted, there is a problem that the size of the substrate increases (Patent Document 1).

  In addition, in order to apply the modulation signal equally to the two arm waveguides of the Mach-Zehnder type optical modulator, it is desirable to design the lengths (action lengths) of the electrodes on the arm waveguides to be equal.

  As described above, it is desired to minimize the influence of the electrode wiring on the arm waveguide while satisfying the demand for the equal length of the electrode and the electrode wiring.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical modulator capable of reducing the influence of crosstalk due to electrode wiring.

  In order to achieve the above object, the present invention provides a Mach-Zehnder type optical modulator comprising a ferroelectric substrate on which an arm waveguide is formed, and the arm conductor. A non-ferroelectric substrate on which at least one of a Y branching portion and a Y multiplexing portion coupled to the waveguide is formed, and the ferroelectric substrate is a modulation electrode formed on the arm waveguide The non-ferroelectric substrate includes the electrode wiring, and the input-side wiring of the non-ferroelectric substrate has an equal length.

  The invention according to claim 2 is the optical modulator according to claim 1, wherein the arm waveguides formed on the ferroelectric substrate have equal lengths, and the electrodes are The arm waveguide is formed from end to end.

  According to a third aspect of the present invention, there is provided the optical modulator according to the first or second aspect, comprising a plurality of Mach-Zehnder interferometers.

  According to a fourth aspect of the present invention, there is provided the optical modulator according to any one of the first to third aspects, wherein the electrode and the wiring are joined with a conductive adhesive. .

  The invention according to claim 5 is the optical modulator according to any one of claims 1 to 4, wherein the non-ferroelectric substrate is a quartz-based substrate.

  The invention according to claim 6 is the optical modulator according to any one of claims 1 to 5, wherein the ferroelectric substrate is an optical substrate made of a lithium niobate crystal. And

It is a figure which shows the structural example of a Mach-Zehnder type | mold optical modulator. It is a figure which shows the structural example of the Mach-Zehnder type | mold optical modulator by the 1st Embodiment of this invention. It is a figure which shows the structural example of the Mach-Zehnder type | mold optical modulator by the 2nd Embodiment of this invention.

  In the optical modulator according to the present invention, a ferroelectric substrate such as lithium niobate crystal (LN) is combined with a paraelectric substrate such as quartz or silicon, and the ferroelectric substrate is placed on the arm waveguide of the ferroelectric substrate. By arranging the electrode and arranging the electrode wiring on the paraelectric substrate, the influence of the electrode wiring on the arm waveguide can be reduced. The paraelectric substrate is less affected by the electro-optic effect than the ferroelectric substrate. Therefore, even if the electrode wiring intersects the waveguide of the paraelectric substrate, the influence of crosstalk is small.

  Moreover, in the optical modulator according to the present invention, the arm waveguide of the ferroelectric substrate is made equal in length, and an electrode is formed from end to end of the waveguide so that the working length of the waveguide by the electrode is equal. Can be Further, if the electrode wiring is arranged on the paraelectric substrate, the influence of the wiring even if it intersects the waveguide is small, so that there is an advantage that the design for equalizing the wiring is easy. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 2 shows an optical modulator according to the first embodiment of the present invention. The Mach-Zehnder type optical modulator 200 includes a ferroelectric LN substrate 210 and two quartz substrates 230 and 260. The quartz substrate 230 at the previous stage has an input waveguide 232 into which input light is incident, a Y branching unit 234 that branches light from the input waveguide, and two arm waveguides 236a and 236b that guide the branched light. And are formed. The LN substrate 210 is formed with two arm waveguides 212a and 212b that are optically coupled with the two arm waveguides 236a and 236b of the preceding quartz substrate. Electrodes 214a and 214b are provided. The quartz substrate 260 at the rear stage has two arm waveguides 262a and 262b optically coupled to the two arm waveguides 212a and 212b of the LN substrate, and Y that combines light from these two arm waveguides. A multiplexing unit 264 and an output waveguide 266 that outputs the combined light are formed.

  In addition, wirings 238a and 238b connected to the electrodes 212a and 212b are provided on the previous stage quartz substrate, and are connected to an external high-frequency signal source. Similarly, wirings 268a and 268b connected to the electrodes 212a and 212b are provided on the subsequent quartz substrate, and are connected to an external termination resistor.

  The input light incident on the input waveguide 232 is branched into two by the Y branching unit 234. The branched light propagates to the arm waveguides 212a and 212b of the LN substrate 210 via the arm waveguides 236a and 236b. In the waveguide of the quartz substrate 230, the propagating light is affected by the modulation signal at the portion intersecting the wirings 238a and 238b, but the electro-optic effect of the quartz substrate is small and the influence is so small that it can be ignored. Therefore, as shown in the figure, the wiring on the quartz substrate 230 can be freely arranged, and the lengths of the respective wirings 238a and 238b can be designed to be equal. As a result, the propagation delays of the modulation signals propagating through both wirings become equal, and good modulation characteristics can be achieved. Furthermore, since a flexible layout allowing crossing with the waveguide is possible, there is no waveform deterioration and the chip size of the optical substrate can be reduced.

  On the other hand, the phase of light propagating through the waveguides 212a and 212b of the LN substrate is modulated by the modulation signal applied to the electrodes 214a and 214b. The amount of phase modulation is determined by the applied voltage and action length of the modulation signal. Here, the working length of the modulation signal can be substantially determined by the lengths of the electrodes 214a and 214b on the waveguide. In order to obtain symmetric phase modulation between the arm waveguides of the optical modulator 200, the lengths of the electrodes 214a and 214b need to be equal (equal length). In the present invention, as shown in the figure, if electrodes 214a and 214b are provided from end to end of the waveguide of the LN substrate, the electrodes are equalized with the same accuracy as that of the waveguide of the LN substrate. Can be lengthened.

  Next, the light propagated through the waveguides 212a and 212b of the LN substrate is coupled to the arm waveguides 262a and 262b of the quartz substrate 260, and is combined by the Y multiplexing unit 264, and the output light is emitted from the output waveguide 266. Is done. In the waveguide of the quartz substrate 260, similarly to the above, the propagating light is affected by the modulation signal at the portions intersecting with the wirings 268a and 268b. Therefore, similarly to the above, wirings on the quartz substrate 260 can be freely arranged, and the lengths of the respective wirings 268a and 268b can be designed to be equal. As a result, the propagation delays of the modulation signals propagating through both wirings become equal, and good modulation characteristics can be achieved. Furthermore, since a flexible layout allowing crossing with the waveguide is possible, there is no waveform deterioration and the chip size of the optical substrate can be reduced.

  Thus, by combining the ferroelectric LN substrate and the paraelectric quartz substrate, in the Mach-Zehnder type optical modulator, it is possible to reduce the crosstalk at the intersection between the wiring and the waveguide. In addition, the length of the electrodes and the length of the wirings are facilitated. Furthermore, the size of the optical substrate does not increase. The electrodes and wirings can be formed on the substrate in a lump after combining the quartz substrate and the LN substrate. Moreover, after forming an electrode and wiring on each board | substrate, a board | substrate can be combined and an electrode and wiring can also be joined with a conductive adhesive. In this case, the adhesive also serves to reinforce the bonding between the substrates.

  FIG. 3 shows an optical modulator according to the second embodiment of the present invention. The Mach-Zehnder optical modulator 300 includes a ferroelectric LN substrate 310 and two quartz substrates 330 and 360. The quartz substrate 330 in the previous stage has an input waveguide 332 into which input light is incident, a Y branch portion 334 that branches light from the input waveguide, and Y branch portions 334-1 and 334 that further branch the branched light. -2 are formed. The LN substrate 310 is formed with four arm waveguides 312-1a, 312-1b, 312-2a, and 312-2b to which light from the Y branch portions 334-1 and 334-2 of the preceding quartz substrate is coupled. The LN substrate arm waveguides are provided with modulation electrodes 314-1a, 314-1b, 314-2a, and 314-2b, respectively. The quartz substrate 360 at the rear stage includes a Y multiplexing unit 364-1 optically coupled to the two arm waveguides 314-1a and 314-1b of the LN substrate, and two arm waveguides 314-2a of the LN substrate. , 314-2b optically coupled to the Y-combining unit 364-2, a Y-combining unit 364 that further combines the light from the two combining units, and an output waveguide that outputs the combined light 366.

  In addition, wirings 338-1a, 338-1b, 338-2a, and 338-2b connected to the electrodes 314-1a, 314-1b, 314-2a, and 314-2b are provided on the previous stage quartz substrate, It is connected to an external high frequency signal source. Similarly, wirings 368-1a, 368-1b, 368-2a, and 368-2b connected to the electrodes 314-1a, 314-1b, 314-2a, and 314-2b are provided on the subsequent quartz substrate. Connected to an external termination resistor.

  The input light incident on the input waveguide 332 is branched into two by the Y branching unit 334. The branched light is further split into two at the Y branch portions 334-1 and 334-2, and propagates to the arm waveguides 312-1a, 312-1b, 312-2a, and 312-2b of the LN substrate 310, respectively. . In the waveguide of the quartz substrate 330, the propagating light is affected by the modulation signal at the portions intersecting with the wirings 338-1a, 338-1b, 338-2a, and 338-2b, but the electro-optic effect of the quartz substrate is small. The impact is negligible. Therefore, as shown in the figure, the wiring on the quartz substrate 330 can be freely arranged, and the lengths of the respective wirings 338-1a, 338-1b, 338-2a, 338-2b are equal. Can be designed. As a result, the propagation delays of the modulation signals propagating through these wirings become equal, and good modulation characteristics can be achieved. Furthermore, since a flexible layout allowing crossing with the waveguide is possible, there is no waveform deterioration and the chip size of the optical substrate can be reduced.

  On the other hand, the light propagating through the waveguides 312-1a, 312-1b, 312-2a, 312-2b of the LN substrate is modulated signals applied to the electrodes 314-1a, 314-1b, 314-2a, 314-2b. Thus, the phase is modulated. The amount of phase modulation is determined by the applied voltage and action length of the modulation signal. Here, the working length of the modulation signal can be substantially determined by the lengths of the electrodes 314-1a, 314-1b, 314-2a, and 314-2b on the waveguide. In order to obtain symmetrical phase modulation between the arm waveguides of the optical modulator 300, the lengths of the electrodes 314-1a, 314-1b, 314-2a, and 314-2b need to be equal (equal length). is there. In the present invention, as shown in the figure, if the electrodes 314-1a, 314-1b, 314-2a, and 314-2b are provided from end to end of the waveguide of the LN substrate, the length of the waveguide of the LN substrate is equal. The electrodes can be made equal in length with the same degree of accuracy.

  Next, the light propagating through the waveguides 312-1a and 312-1b of the LN substrate is multiplexed by the Y multiplexing unit 364-1 of the quartz substrate 360, and passes through the waveguides 312-2a and 312-2b of the LN substrate. The propagated light is multiplexed by the Y multiplexing unit 364-2 of the quartz substrate 360. The lights from the Y multiplexing units 364-1 and 364-2 are further multiplexed by the Y multiplexing unit 364, and output light is emitted from the output waveguide 366. In the waveguide of the quartz substrate 360, similarly to the above, the propagating light is affected by the modulation signal at the portions intersecting with the wirings 368-1a, 368-1b, 368-2a, and 368-2b. The effect is small and the impact is negligible. Therefore, similarly to the above, the wiring on the quartz substrate 360 can be freely arranged, and the lengths of the respective wirings 368-1a, 368-1b, 368-2a, 368-2b are designed to be equal. can do. As a result, the propagation delays of the modulation signals propagating through these wirings become equal, and good modulation characteristics can be achieved. Furthermore, since a flexible layout allowing crossing with the waveguide is possible, there is no waveform deterioration and the chip size of the optical substrate can be reduced.

  Thus, by combining the ferroelectric LN substrate and the paraelectric quartz substrate, in the Mach-Zehnder type optical modulator, it is possible to reduce the crosstalk at the intersection between the wiring and the waveguide. In addition, the length of the electrodes and the length of the wirings are facilitated. Furthermore, the size of the optical substrate does not increase. The electrodes and wirings can be formed on the substrate in a lump after combining the quartz substrate and the LN substrate. Moreover, after forming an electrode and wiring on each board | substrate, a board | substrate can be combined and an electrode and wiring can also be joined with a conductive adhesive. In this case, the adhesive also serves to reinforce the bonding between the substrates.

While the present invention has been described with respect to several specific embodiments, the embodiments described herein are merely illustrative in view of the many possible embodiments to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. For example, in the above embodiment, an LN substrate has been described as an example of a ferroelectric substrate. However, the principle of the present invention is that lithium tantalate (LiTaO ₃ ), KTN (KTa _1-x Nb _x O ₃ ), KTP (KTiOPO ₄ ), PZT (lead zirconate titanate), or the like can be used. Further, although the paraelectric quartz substrate is used for both the input side and the output side of the optical modulator, the effect of the present invention can be obtained in that portion even if it is used for either one. Furthermore, in the above embodiment, a circuit such as a directional coupler, a coupler, or a multimode interference coupler (MMI) may be used instead of the Y branch. As described above, the configuration and details of the embodiment exemplified here can be changed without departing from the gist of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

100, 200, 300 Mach-Zehnder type optical modulator 110 Optical substrate having electro-optic effect 112, 232, 332 Input waveguide 114, 234, 334, 334-1, 334-2 Y branching portions 116a, 116b, 212a, 212b, 236a, 236b, 262a, 262b, 312-1a, 312-1b, 312-2a, 312-2b Arm waveguide 118, 264, 364, 364-1, 364-2 Y multiplexer 120, 266, 366 Waveguide 122a, 122b Metal electrode 124a, 124b High frequency signal source 210, 310 LN substrate 214a, 214b, 314-1a, 314-1b, 314-2a, 314-2b Modulation electrode 230, 260, 330, 360 Quartz substrate 238a , 238b, 268a, 268b, 338-1 , 338-1b, 338-2a, 338-2b, 368-1a, 368-1b, 368-2a, 368-2b wiring

Claims (6)

A Mach-Zehnder type optical modulator,
A ferroelectric substrate on which an arm waveguide is formed;
A non-ferroelectric substrate on which at least one of a Y branch portion and a Y multiplexing portion coupled to the arm waveguide is formed, and
The arm waveguide is formed from end to end of the ferroelectric substrate and has an equal length, and the ferroelectric substrate is on the arm waveguide and from end to end of the ferroelectric substrate. And the wiring connected to the electrode is formed on the non-ferroelectric substrate, and the wiring on the input side of the non-ferroelectric substrate is equal in length. An optical modulator characterized by having a thickness. The optical modulator according to claim 1 , comprising:
An optical modulator comprising a plurality of Mach-Zehnder interferometers. The optical modulator according to claim 1 or 2 ,
The optical modulator, wherein the electrode and the wiring are joined with a conductive adhesive. The optical modulator according to claim 1 or 2,
The optical modulator, wherein the electrodes and the wirings are formed in a lump after combining the ferroelectric substrate and the non-ferroelectric substrate. An optical modulator according to any one of claims 1 to 4,
The light modulator according to claim 1, wherein the non-ferroelectric substrate is a quartz substrate. An optical modulator according to any one of claims 1 to 5,
The optical modulator, wherein the ferroelectric substrate is an optical substrate made of lithium niobate crystal.

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