JP4703158B2 - Light control element - Google Patents

Description

  The present invention relates to a light control element, and more particularly to a light control element for applying the same modulation signal to different optical waveguides.

  Conventionally, in an optical communication technique or an optical measurement technique, a light control element such as an external light modulator is used as a high-frequency light modulation means. In particular, a light control element in which a material having an electro-optic effect such as lithium niobate is used for a substrate and an optical waveguide or an electrode is formed on the substrate is particularly noticeable in terms of operational stability and high-speed operation. Has been.

As an example of the light control element, as shown in FIG. 1, an optical waveguide 2 for guiding a light wave is applied to a substrate 1 having an electro-optic effect, and a high-speed modulation signal in a microwave band is applied to the light wave. The modulation electrode 3 and the ground electrode 4 are formed. FIG. 1B shows a cross-sectional view taken along the alternate long and short dash line in FIG. Although not shown in FIG. 1A, a buffer layer such as SiO ₂ is formed between the substrate and the electrode. The optical waveguide 2 is a Mach-Zehnder type optical waveguide (hereinafter referred to as “MZ type optical waveguide”) having two branched optical waveguides, and a light wave propagating through the optical waveguide is subjected to optical modulation by a single modulation electrode 3. It is configured as follows.

  In the light control element as shown in FIG. 1, since the driving voltage applied to the light modulation is high and the electric field applied to the two branch optical waveguides is not symmetrical, the chirp phenomenon occurs in the light wave emitted from the light control element. It also becomes a cause of limiting long-distance transmission. For this reason, a dual-type light control element using two modulation electrodes 5 as shown in FIG. 2 has been put into practical use as a device for reducing drive voltage and realizing zero chirp. However, in the dual-type light control element shown in FIG. 2, since it is necessary to accurately apply the same signal and opposite phase modulation signals to the two modulation electrodes, the setting adjustment of the apparatus becomes complicated and the number of parts increases. This is also a cause of high costs.

In Patent Document 1, a part of the substrate, in particular, a region of the substrate including one of the branched optical waveguides is polarization-inverted, and a modulation having a branch line in which two modulation electrodes required in FIG. 2 are branched from a single line It consists of electrodes. In such a light control element using polarization inversion, the low drive voltage and zero chirp of the dual light control element are realized, and the modulation signal is applied to the modulation electrode in the same manner as the single modulation electrode in FIG. It can be simplified.
JP 2003-202530 A

On the other hand, as shown in Patent Document 2, in order to shorten the length of the light control element and to make it independent of polarization, a device that folds the optical waveguide formed on the substrate at one end portion of the substrate having the electro-optic effect is also proposed. Has been. As shown in FIG. 3, the optical waveguide 10 formed on the substrate 1 is folded at one end of the substrate (right side in the figure), and modulation electrodes 13 and 15 and ground electrodes 14 and 16 are arranged on each branch optical waveguide. Has been. In the polarization-independent light control element, the reflection mirror 12 and the quarter-wave plate 11 are arranged in the folded portion, and the polarization plane of the propagating light wave is rotated by 90 °.
JP 7-325276 A

  In the light control element having such a folded optical waveguide, in order to apply a modulation signal to the modulation electrodes 13 and 15, for example, it is necessary to use a single modulation electrode 20 as shown in FIG. As in the case of 1, the driving voltage rises and a chirp phenomenon occurs. Reference numerals 21 and 22 denote ground electrodes. If two modulation electrodes as shown in FIG. 2 are used, when connecting the electrodes 13 and 15 and the electrodes 14 and 16, respectively, the signal lines need to be crossed and jumper wires must be used. For this reason, the propagation loss of the modulation signal is remarkably increased.

  The problem to be solved by the present invention is to solve the above-mentioned problem, and to handle a modulation electrode even for a light control element having a complicated optical waveguide such as an optical waveguide folded at one end of a substrate. It is also possible to provide a light control element that can realize a low driving voltage and zero chirp.

The invention according to claim 1 is a Mach-Zehnder type optical waveguide having a substrate made of a material having an electro-optic effect and two branched optical waveguides formed on the substrate, and one of the branched optical waveguides. And a modulation electrode for applying an electric field to the branch optical waveguide. The modulation electrode branches one line into a plurality of lines. And at least two sets of branching / merging lines that merge the branched lines again into one line, and the branched lines in the branching / merging line are identical to different branch optical waveguides. A branching / merging line on the same line is disposed at least in each branching optical waveguide before and after the turn- up, and is set to apply a modulation signal.

According to a second aspect of the present invention, in the light control element according to the first aspect, the substrate includes a part of a substrate including a branched optical waveguide to which an electric field is applied by a branched line of the branching / merging line. The polarization is inverted.

According to the invention of claim 1, the modulation electrode branches at least two sets of branching / merging lines on the same line by branching one line into a plurality of lines and joining the branched lines again into one line. In addition, each branching line in the branching / merging line is set to apply the same modulation signal to different optical waveguides. )), A branching / merging line is used, and when a modulation electrode is routed from one action part to another action part, a single line can be used. You can avoid the situation. In addition, since there is only one line when the modulation electrode is routed, there is no difference in propagation loss between the lines when a plurality of lines are routed.
The branching / merging line means an electrode having a shape in which one line is electrically branched into a plurality of lines and the branched lines are joined again to one line. In particular, in an electrode using a modulation signal such as a microwave, the length and shape of the branched line are set so that the phases of the microwaves propagating through the branched line coincide at the time of joining.
Further, a part of the branch optical waveguide is folded at the side surface portion of the substrate, and at least the branch optical waveguides before and after the folding are respectively arranged with branching / merging lines on the same line, so that the folding type Even a light control element having a single optical waveguide can be handled by a single modulation electrode.

According to the invention of claim 2, since the substrate is partly inverted including a branched optical waveguide to which an electric field is applied by a branched / merged line, the electrode is branched from the same line. Even when the same modulation signal is applied to different optical waveguides, modulation in a substantially opposite phase can be realized, so that the drive voltage of the light control element can be reduced and zero chirp can be achieved.

The light control element according to the present invention will be described in detail below.
FIG. 5 is a diagram showing the basic principle of the modulation electrode used in the light control element according to the present invention. The modulation electrode includes, for example, an introduction line a, a first branch / merge line b, a single intermediate line c, a second branch / merge line d, and a lead-out line e on a single line. Further branching / merging lines can be added as necessary.

  In the modulation electrode as shown in FIG. 5, the microwave input from the introduction line a is branched into two microwaves A and B in the first branching / merging line b, and then once again in the intermediate line c. Synthesized into microwave C. Further, the second branch / merging line d is similarly branched into two microwaves D and E, and becomes one microwave F on the lead-out line e. Note that, at the final end of the modulation electrode, it is possible to connect the terminator directly to the end of the branched line in the branching / combining line without providing the lead-out line e as shown in FIG.

  Since the branched microwaves A, B, D, and E are individually applied to the optical waveguide, the positional relationship between each optical waveguide and the branched modulation electrode, and the shape (length, width, electrode height) of each modulation electrode. Depending on the strength ratio, etc. Usually, the branched microwaves A and B, or D and E are branched so as to have the same intensity because the applied modulation electrodes have symmetrical shapes. When the branched microwaves are merged, the phase difference of the branched microwaves is adjusted to be substantially zero in order to maximize the intensity of the microwaves. In addition, when modulating with a standing wave, it is also possible to adjust to the phase difference equivalent to the integral multiple of the period of a standing wave.

Next, an embodiment applied to a light control element having a folded optical waveguide as shown in FIG. 3 using the above-described modulation electrode will be described.
In FIG. 6, the MZ type optical waveguide 30 is folded at the end of the substrate 1, and a reflection mirror 35 for increasing the reflection efficiency is provided at the folded portion of the optical waveguide.
The substrate 1 having the electro-optic effect is composed of, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), and a quartz-based material. It is composed of an X-cut plate, a Y-cut plate, and a Z-cut plate. In particular, lithium niobate (LN) is preferably used because it is easily constructed as an optical waveguide device and has high anisotropy. Further, considering the ease of forming polarization reversal described later, FIG. 6 shows an example using a Z-cut plate.

The optical waveguide 30 on the substrate can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method. The modulation electrode, the ground electrode surrounding the modulation electrode, and the like can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Furthermore, if necessary, a buffer layer such as dielectric SiO ₂ is provided on the surface of the substrate after forming the optical waveguide, or a ridge structure is provided so that the electric field formed by the modulation electrode is efficiently applied to the waveguide. Is also possible.

  Before and after the optical waveguide is turned back, the two branch / merging lines 31 and 34 of the modulation electrode are arranged so as to be positioned. In addition, polarization inversion regions 33 and 34 in which the polarization direction of the substrate is reversed are formed in a part of the action part between the branched line of the branching / merging line and the branched optical waveguide of the optical waveguide 30. In the area of each branching / merging line, the same modulation signal is applied to each branching optical waveguide in the same phase by the branched line as in the optical modulator described in Patent Document 1, and With respect to the domain-inverted region, light modulation is performed in an antiphase state. For this reason, it is possible to suppress a decrease in driving voltage and a chirp phenomenon related to light modulation.

Furthermore, since the two branching / merging lines 31 and 34 are connected by a single intermediate line, in the operation unit that performs optical modulation, although there are two modulation electrodes, the line is routed. The tracks do not cross.
FIG. 7 shows a second embodiment in which the MZ type optical waveguides 40 and 41 are folded by the GRIN lens 45 and the reflection mirror 46 attached to one end of the lens. The light wave emitted from the optical waveguide 40 on the input side enters the optical waveguide 41 on the output side while following the locus indicated by 47. For each of the optical waveguides 40 and 41, as in FIG. 6, a modulation electrode having two branching / merging lines 42 and 43 is provided. The domain-inverted region 44 is also formed in a part of each optical waveguide and operates in the same manner as in FIG.

Next, a third embodiment is shown in FIG.
FIG. 8 shows an optical waveguide in which MZ type optical waveguides 50 and 51 are arranged in series on a substrate 1. By using such a plurality of MZ type optical waveguides, the light wave modulated by one MZ type optical waveguide can be optically modulated again by the other MZ type optical waveguide. In the case of modulating with, the extinction ratio characteristic related to the modulation can be improved.

  In FIG. 8, two branching / merging lines 52 and 53 are arranged corresponding to the action portions of the MZ type optical waveguides, and polarization inversion regions 54 and 55 are also formed in a part of each branching optical waveguide. ing. Each branch / merging line 52, 53 is connected by a single intermediate line, which not only facilitates the routing of the line, but also in each MZ type optical waveguide, the modulation signal having the same strength is applied to the two branch optical waveguides. Can be applied. For example, in the case of using two lines straddling each MZ type optical waveguide instead of the modulation electrode of FIG. 8 (two lines from the branched optical waveguide of one MZ type optical waveguide to the branched optical waveguide of the other MZ type optical waveguide) When the distance between the MZ type optical waveguides is increased, a difference in the propagation loss and phase of the microwave between the lines occurs, and an appropriate modulation signal may be applied to the next MZ type optical waveguide. It becomes difficult. On the other hand, when a single intermediate line as described above is used, when a modulation signal is introduced into a new MZ type optical waveguide, it is always introduced with a single line, and then branched / Since it is divided into two branched modulation signals by the merge line, each branch optical waveguide of the MZ type optical waveguide can obtain a modulation signal having a predetermined intensity.

  In addition, as shown in FIG. 8, it is also possible to adjust the timing of the modulation signal entering the next MZ type optical waveguide 51 by forming a delay line 56 of the modulation signal in the intermediate line.

  FIG. 9 shows a fourth embodiment, in which the two MZ type optical waveguides of FIG. 8 are formed by folding. A reflection mirror 66 is formed in the folded portion, and branching / merging lines 62 and 63 and polarization inversion regions 64 and 65 are formed in the MZ optical waveguides 60 and 61, respectively. Further, a delay line 67 can be formed in order to adjust the timing of the light wave entering the next MZ type optical waveguide 61 and the modulation signal.

  FIG. 10 shows a fifth embodiment, and two branch / merging lines 70 and 71 are provided for the two MZ type optical waveguides 73 and 74. In particular, in the branching / merging line 70, the delay line 72 is formed, and the phase difference of the modulation signal applied to each branching optical waveguide is set to be adjustable. Furthermore, another delay line 72 is formed to make the phase difference at the time of merging substantially zero (in the case of modulating with a standing wave, the difference may be an integral multiple of the modulation period). When no delay line is formed, the domain-inverted region 75 can be provided as described above.

  The present invention is not limited to the above-described embodiments. For example, in a branching / merging line, in order to prevent reflection of microwaves at the time of branching, impedances before and after branching are matched, and the like in the technical field. It goes without saying that known techniques can be combined with the present invention.

  As described above, according to the present invention, it is possible to handle the modulation electrode even for a light control element having a complicated optical waveguide such as an optical waveguide folded at one end of the substrate, It is possible to provide a light control element capable of realizing a low driving voltage and zero chirp.

The conventional light control element using a single modulation electrode is shown. 2 shows a conventional light control element using two modulation electrodes. 1 shows a light control element using a conventional folded optical waveguide. A conventional light control element using a folded optical waveguide and a single modulation electrode is shown. 1 shows the basic principle of a modulation electrode according to the present invention. 1 shows a first embodiment of the present invention. 2 shows a second embodiment according to the present invention. 3 shows a third embodiment according to the present invention. 4 shows a fourth embodiment according to the present invention. 5 shows a fifth embodiment according to the present invention.

Explanation of symbols

a Introducing lines b, d, 31, 32 Branching / merging line c Intermediate line e Deriving line 1 Substrate 30 Optical waveguides 33, 34 Polarization inversion region 35 Reflecting mirror

Claims (2)

A Mach-Zehnder type optical waveguide having a substrate made of a material having an electro-optic effect and two branched optical waveguides formed on the substrate , and a part of the branched optical waveguide is a side surface portion of the substrate In a light control element that is configured to be folded and has a modulation electrode that applies an electric field to the branched optical waveguide,
The modulation electrode has at least two sets of branching / merging lines for branching one line into a plurality of lines and joining the branched line again to one line on the same line. Each branched line in the line is set to apply the same modulation signal to different branched optical waveguides, and at least the branched optical waveguides before and after the folding are respectively arranged with branching / merging lines on the same line. A light control element. 2. The light control element according to claim 1, wherein a part of the substrate including a branched optical waveguide to which an electric field is applied by a branched line of the branching / merging line is inverted in polarization. The light control element.

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