JP2011191346A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact optical modulator having high performance and capable of preventing electric discontinuity, that is cutting of a center conductor. <P>SOLUTION: In the optical modulator which has an optical waveguide provided with a substrate having a region where polarization is not reversed and a region where polarization is reversed and first and second optical waveguides 3a and 3b having grooves formed on the side parts thereof, and wherein interacting parts wherein light transmitting through the first and second optical waveguides and an electric signal transmitting through a travelling wave electrode made of a center conductor and a grounded conductor interact with each other include first and second interacting parts 5a and 5b polarized in directions different from each other and the center conductor is opposed to the first or second optical waveguide at the first and second interacting parts and which modulates the phase of light transmitting through the first and second optical waveguides at the first and second interacting parts, an optical waveguide shift part is provided between the first and second interacting parts so that relative positions of the center conductor and the grounded conductor and the first and second optical waveguides are interchanged, the center conductor in the interacting part is formed from a straight line and the groove parts are formed at other than a lower part of the center conductor in the optical waveguide shift part. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical modulator that includes a Mach-Zehnder optical waveguide including a ridge optical waveguide including a small and low-loss optical waveguide, and that modulates light incident on the optical waveguide with a high-frequency electric signal and emits it as an optical signal pulse.

  In recent years, high-speed and large-capacity optical communication systems have been put into practical use. There is a demand for the development of a high-speed, small, low-cost, and highly stable optical modulator for incorporation into such a high-speed, large-capacity optical communication system.

As an optical modulator that meets such demands, a light modulator such as lithium niobate (LiNbO ₃ ) is used for a substrate having a so-called electro-optical effect (hereinafter abbreviated as an LN substrate) whose refractive index changes by applying an electric field. There is a traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as an LN optical modulator) in which a waveguide and a traveling wave electrode are formed. This LN optical modulator is applied to a large capacity optical communication system of 2.5 Gbit / s and 10 Gbit / s because of its excellent chirping characteristics. Recently, application to a 40 Gbit / s ultra-high capacity optical communication system is also being studied.

(First prior art)
Patent Document 1 discloses a so-called polarization-inverted zero-chirp optical modulator that realizes a zero-chirp optical modulator by applying polarization inversion to a z-cut LN substrate. Patent Document 2 discloses a so-called planar type LN modulator in which a planar optical waveguide is formed on a z-cut LN substrate. FIG. 6 shows a top view of a polarization inversion zero chirp LN modulator constructed using this planar type LN modulator as an example. FIG. 7 is a cross-sectional view taken along the line AA ′ of FIG.

  Here, 1a is a z-cut substrate (or non-polarization inversion region) that has not undergone polarization inversion, and 1b is a z-cut substrate (or polarization inversion region) that has undergone polarization inversion (collectively referring to 1a and 1b as z). -Called cut substrate 1). An optical waveguide 3 is formed on the z-cut substrate 1. The optical waveguide 3 is an optical waveguide formed by thermally diffusing metal Ti at 1050 ° C. for about 10 hours. ). Reference numerals 3a and 3b denote two arms constituting the Mach-Zehnder optical waveguide, which are also called interactive optical waveguides in which high-frequency electrical signals interact with light. 3a and 3b are referred to as a first optical waveguide and a second optical waveguide, respectively. Reference numeral 40 denotes a boundary between the domain-inverted region 1a and the non-domain-inverted region 1b.

A SiO ₂ buffer layer 2 is formed on the z-cut substrate 1, and a traveling wave electrode 4 is formed on the upper surface of the SiO ₂ buffer layer 2. As the traveling wave electrode 4, a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used. In order to suppress temperature drift due to the pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, a Si conductive layer is used on the entire upper surface of the SiO ₂ buffer layer 2. In the specification, the Si conductive layer is omitted and discussed in order to simplify the description.

  A feature of the first prior art shown in FIG. 6 is that a structure that makes the best use of the characteristics of an electric signal that determines the light modulation band in order to realize a sufficiently wide light modulation band. . That is, in the interaction portion that determines the light modulation characteristics, the center conductor 4a and the ground conductors 4b and 4c, which have small dimensions such as width and gap, are straight or nearly straight so that no discontinuity occurs. The optical waveguides 3a and 3b '(bend optical waveguides) are provided in the optical waveguide 3a and the second optical waveguide 3b to shift each optical waveguide.

Here, 5a in the interaction portion of the electrical signal and the light in the polarization non-reversal region 1a, the length becomes at L _1, referred to as a first interaction portion. 5b is the interaction of the electrical signal and the light in the polarization inversion region 1b, is at L ₂ length, it referred to as a second interaction portion. 5c is a waveguide shift portion provided between the first interacting portion 5a and a second interaction portion 5b, a length consisting of L _3.

The length L ₁ of the first interaction portion 5a for the length L ₂ is the second interaction portion 5b, L ₁ = L to some extent to ₂ Tosureba light pulse generated impart properties zero chirping be able to. However, since the high-frequency electrical signal propagates through the center conductor 4a and the ground conductors 4b and 4c and attenuates, the modulation efficiency in the first interaction unit 5a is higher than the modulation efficiency in the second interaction unit 5c. Considering, it is clear that L ₁ <L ₂ is preferred.

  As can be seen from FIG. 6, in the first interaction portion 5a whose polarization is not reversed, the center conductor 4a is provided on the first optical waveguide 3a, and the ground conductor 4c is provided on the second optical waveguide 3b. There is. On the other hand, in the second interaction portion 5b whose polarization is reversed, the ground conductor 4b is provided on the first optical waveguide 3a, and the center conductor 4a is provided on the second optical waveguide 3b.

  That is, in the first conventional technique, the first optical waveguide 3a and the second optical waveguide 3b are arranged between the first interaction portion 5a and the second interaction portion 5b in the surface direction of the z-cut LN substrate. By shifting the position, the relative positions of the center conductor 4a and the ground conductors 4b and 4c, the first optical waveguide 3a, and the second optical waveguide 3b are switched. By adopting this structure, it is possible to extremely reduce the chirping of the optical signal pulse generated by the LN optical modulator.

In the first prior art, since the traveling wave electrode is straight or almost straight, the width S is about 6 to 11 μm, the narrow center conductor 4a, and the gap W from the center conductor 4a to about 15 to 50 μm. The formed ground conductors 4b and 4c have no electrical discontinuity in the region where light and electrical signals interact. Therefore, the production yield is greatly improved. The reflection characteristics of the interaction portion 5a of the electric signal and an optical, because there is no electrical discontinuities to 5b, the propagation loss and the electrical signals resulting from the discontinuity for S ₂₁ is a transmission characteristic of the electrical signal S ₁₁ is characterized by no deterioration.

(Second prior art)
Patent Document 1 only proposes a concept for realizing a polarization inversion zero chirp, and does not depend on the structure of the optical waveguide. That is, the applied optical waveguide may be a planar type or a ridge type.

  The ridge structure disclosed in Patent Document 3 is known to have higher performance than the planar structure disclosed in Patent Document 2. The top view of the configuration of the optical waveguide 3 of the polarization inversion zero chirp optical modulator and the grooves 8a, 8b, and 8c formed by digging down a part of the LN substrate 1 when the ridge structure is used is shown in a second view in FIG. FIG. 8 shows the prior art. FIG. 9 shows a cross section taken along the line BB ′ of FIG.

  FIG. 10 is a cross-sectional view taken along the line CC ′ of FIG. However, in order to simplify the description, the first optical waveguide 3a and the second optical waveguide 3b are omitted in FIG. Furthermore, only the optical waveguide shift portion 5c and its vicinity are shown. As can be seen from FIG. 10, since the ridge groove 8a is formed, electrical discontinuities such as 4a ′, 4a ″, and 4a ″ are generated in the center conductor.

  Since the disconnection of the central conductor occurs with a fairly high probability, it has been a factor that greatly limits the yield of a polarization-inverted zero chirp type LN optical modulator having a ridge structure.

JP 2006-259686 A JP-A-2-51123 JP-A-4-288518

  In a polarization inversion zero chirped LN optical modulator configured to shift the optical waveguide with an optical waveguide using a ridge structure, the center conductor often breaks at the optical waveguide shift portion. Therefore, the development of a technique for solving this problem has been strongly desired.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention is made of a material having an electro-optic effect, and has a substrate having a region where polarization is not reversed and a region where polarization is reversed, and the substrate. A branched optical waveguide for branching incident light, a first optical waveguide and a second optical waveguide for propagating the branched light, and the first optical waveguide; A multiplexing optical waveguide for multiplexing the light propagating through the second optical waveguide is provided, and a groove portion in which a part of the substrate is dug down is formed along the first and second optical waveguides. An interaction portion having a ridge-type optical waveguide, in which light propagating through the first optical waveguide and the second optical waveguide interacts with an electric signal propagating through a traveling wave electrode composed of a center conductor and a ground conductor Are polarized in different directions. Including an interaction portion and a second interaction portion, wherein the central conductor is opposed to the first optical waveguide or the second optical waveguide at the first interaction portion and the second interaction portion, An optical modulator for generating an optical signal pulse by modulating a phase of the light propagating through the first optical waveguide and the second optical waveguide by the first interaction unit and the second interaction unit; In addition, by providing an optical waveguide shift unit between the first interaction unit and the second interaction unit, the optical axis of the first optical waveguide in the first interaction unit and the first The optical axis of the first optical waveguide in the two interaction portions is made different from the optical axis of the second optical waveguide in the first interaction portion and the second in the second interaction portion. Different from the optical axis of the first optical waveguide, and In the second interaction portion, the relative positions of the center conductor and the ground conductor, the first optical waveguide, and the second optical waveguide are switched, and the central conductor in the interaction portion is a straight line. In the interaction portion, the groove portion is formed at a predetermined location other than below the central conductor in the optical waveguide shift portion so that the traveling wave electrode does not have an electrical discontinuity portion in the interaction portion. It is characterized by being.

  In order to solve the above problem, an optical modulator according to claim 2 of the present invention is made of a material having an electro-optic effect, and has a substrate having a region where polarization is not reversed and a region where polarization is reversed, and the substrate. A branched optical waveguide for branching incident light, a first optical waveguide and a second optical waveguide for propagating the branched light, and the first optical waveguide; A multiplexing optical waveguide for multiplexing the light propagating through the second optical waveguide is provided, and a groove portion in which a part of the substrate is dug down is formed along the first and second optical waveguides. An interaction portion having a ridge-type optical waveguide, in which light propagating through the first optical waveguide and the second optical waveguide interacts with an electric signal propagating through a traveling wave electrode composed of a center conductor and a ground conductor Are polarized in different directions. Including an interaction portion and a second interaction portion, wherein the central conductor is opposed to the first optical waveguide or the second optical waveguide at the first interaction portion and the second interaction portion, An optical modulator for generating an optical signal pulse by modulating a phase of the light propagating through the first optical waveguide and the second optical waveguide by the first interaction unit and the second interaction unit; In addition, by providing an optical waveguide shift unit between the first interaction unit and the second interaction unit, the optical axis of the first optical waveguide in the first interaction unit and the first The optical axis of the first optical waveguide in the two interaction portions is made different from the optical axis of the second optical waveguide in the first interaction portion and the second in the second interaction portion. Different from the optical axis of the first optical waveguide, and In the second interaction portion, the relative positions of the center conductor and the ground conductor, the first optical waveguide, and the second optical waveguide are interchanged, and the center conductor in the interaction portion is substantially the same. A part of the central conductor is formed substantially parallel to at least one of the first optical waveguide and the second optical waveguide in the optical waveguide shift portion, and the interaction The groove portion is formed at a predetermined location other than the lower portion of the central conductor in the optical waveguide shift portion so that the traveling wave electrode does not have an electrical discontinuity portion. .

  In order to solve the above-described problem, an optical modulator according to a third aspect of the present invention is the optical modulator according to the first or second aspect, wherein the length of the first interaction unit and the second mutual length are the same. It is characterized in that the length of the action part is substantially equal.

  In order to solve the above problem, an optical modulator according to a fourth aspect of the present invention is the optical modulator according to the first or second aspect, wherein the length of the first interaction unit is the second length. It is characterized by being shorter than the length of the interaction part.

  In order to solve the above-described problem, an optical modulator according to claim 5 of the present invention is the optical modulator according to claim 1 or 2, wherein the region in the interaction unit that does not invert the polarization and the polarization are included. There is a plurality of at least one of the inverted regions, and the optical waveguide shift portion is provided at the boundary between the regions.

  In order to solve the above problem, an optical modulator according to a sixth aspect of the present invention is the optical modulator according to the fifth aspect, wherein the region in the interaction unit where the polarization is not inverted and the polarization are inverted. The sum of the number of regions is an odd number.

  In order to solve the above-described problem, an optical modulator according to claim 7 of the present invention is the optical modulator according to claim 6, wherein the region of the interaction unit where the polarization is not inverted and the polarization are inverted. The sum of the lengths of the respective regions is substantially equal to each other.

  In order to solve the above-described problem, an optical modulator according to an eighth aspect of the present invention is the optical modulator according to any one of the first to seventh aspects, wherein the substrate does not reverse the polarization, and It is characterized in that the chirping of the optical signal pulse is smaller than in the case of either one of the regions where the polarization is inverted.

  In the domain-inverted zero chirped LN optical modulator having a ridge structure according to the present invention, the groove is not formed below the central conductor in the optical waveguide shift unit, and is linearly formed across the non-domain-inverted region and the domain-inverted region. Alternatively, a substantially linear ridge is formed. Thereby, electrical discontinuity, that is, disconnection of the central conductor can be prevented, and the yield of the polarization inversion zero chirped LN optical modulator having the ridge structure can be greatly improved.

The top view which shows schematic structure of the 1st Embodiment of this invention The enlarged top view in the area | region I of FIG. Sectional view taken along the line EE ′ of FIGS. 1 and 2 The top view which shows schematic structure of the 2nd Embodiment of this invention. The top view which shows schematic structure of the 3rd Embodiment of this invention. The top view which shows schematic structure about the optical modulator of 1st prior art Sectional drawing in AA 'of FIG. The top view which shows schematic structure about the optical modulator of 2nd prior art Sectional drawing in BB 'of FIG. Sectional drawing in CC 'of FIG.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the related art shown in FIGS. 6 to 10 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

(First embodiment)
A first embodiment of the present invention is shown in FIG. In the present invention, the ridge structure of FIG. 9 shown as the second prior art is applied to the interaction portion where the high-frequency electrical signal and light interact. That is, the sectional view taken along the line DD ′ in FIG. 1 is the same as FIG. FIG. 2 shows an enlarged top view of a portion corresponding to the region I in FIG.

Here, the prior art also, 5a interaction portion of the electrical signal and an optical length in the non-polarization inversion region 1a is in L _1, that is, the first interaction portion. 5b interaction portion of the electrical signal and an optical length in the polarization inversion region 1b is at L _2, that is, the second interaction portion. 5c is provided between the first interacting portion 5a and a second interaction portion 5b, an optical waveguide shift unit length becomes at L _3. In the second prior art, the center conductor is broken over the length L ₃ in the optical waveguide shift portion 5c, which is a problem. The length L ₁ of the first interaction portion 5a for the length L ₂ is the second interaction portion 5b L ₁ <L ₂ is preferred.

  Further, as in the prior art, in the first interaction portion 5a whose polarization is not reversed, the center conductor 4a is provided on the first optical waveguide 3a, and the ground conductor 4c is provided on the second optical waveguide 3b. is there. On the other hand, in the second interaction portion 5b whose polarization is reversed, the ground conductor 4b is provided on the first optical waveguide 3a, and the center conductor 4a is provided on the second optical waveguide 3b. Further, the first optical waveguide 3a and the second optical waveguide 3b are shifted in position between the first interaction part 5a and the second interaction part 5b in the surface direction of the z-cut LN substrate. The relative positions of the center conductor 4a and the ground conductors 4b and 4c, the first optical waveguide 3a, and the second optical waveguide 3b are switched. Thereby, the chirping of the optical signal pulse generated by the LN optical modulator can be extremely reduced.

  As can be seen from FIG. 2, in the present invention, the groove portion 8a of the ridge in the second prior art shown in FIG. 8 is divided (or divided) into two like 9a and 9b. The ridge 10 bridges the domain-inverted region 1a and the non-domain-inverted region 1b (or straddles the boundary 40) and is formed straight. In other words, in the optical waveguide shift portion 5c, no groove is formed below the central conductor 4a, and a groove is formed at a predetermined location other than below the central conductor 4a.

  FIG. 3 shows a cross-sectional view taken along line EE ′ of FIGS. As described above, in the present invention, the substantially straight ridge portion 10 is formed across the boundary 40 between the domain-inverted region and the non-domain-inverted region, so that the central conductor 4 a is formed in the thickness direction of the z-cut LN substrate 1. There will be no discontinuities.

  In actual manufacturing, even if there are ridge grooves 9a and 9b, the ridge 10 serving as a pedestal of the central conductor 4a is almost straight, so a traveling wave electrode requiring a thickness of several tens of microns is manufactured. It can be said that the structure is very convenient to do. By applying the present invention, it was possible to realize a high yield of almost 100% in the process.

  Although the ridge portion 10 that bridges between the domain-inverted region 1a and the non-domain-inverted region 1b has been described as being almost straight, of course, it is not necessary to be completely straight, and as described in Patent Document 1, it is electrically It goes without saying that it may be bent as long as no discontinuity occurs. This will be described later.

  In FIG. 1, the boundary 40 between the non-polarization inversion region 1a and the polarization inversion region 1b exists in the region of the optical waveguide shift unit 5c, but if the influence on the modulation efficiency is small, the position of the boundary 40 is the position of the optical waveguide. A position slightly deviated from the region of the shift portion 5c (or a position deviated from the ridge portion 10) may be used. And this can be said for all embodiments of the invention.

(Second Embodiment)
FIG. 4 shows a top view of the second embodiment of the present invention. In FIG. 4, traveling wave electrodes are omitted. In the figure, 1a and 1c are regions where the polarization of the z-cut LN substrate is not inverted (non-polarization inversion region), and 1b is a region where the polarization of the z-cut LN substrate is inverted (polarization inversion region). Reference numeral 30 denotes a Mach-Zehnder optical waveguide, and reference numerals 30a and 30b denote two arms (a first optical waveguide and a second optical waveguide) constituting the Mach-Zehnder optical waveguide. Reference numerals 12a, 12b, 12c, 12d, 12e, and 12f denote groove portions of the ridge. Also in the present invention, the ridge grooves 12a, 12d, and 12e are not continuous but are separated from each other.

  In the second embodiment of the present invention shown in FIG. 4 as well, the center conductor and the ground conductor (not shown) are configured to be almost straight so that no discontinuity occurs, and instead, the first optical waveguide 30a and the second optical waveguide 30 The optical waveguide 30b is shifted positionally twice. Here, 11a is an interaction portion between the electric signal and the light in the region 1a where the polarization is not inverted, and is referred to as a first interaction portion. Reference numeral 11b denotes an interaction portion between the electric signal and light in the region 1b where the polarization is inverted, and is referred to as a second interaction portion. Reference numeral 11c denotes an interaction portion between the electric signal and light in the region 1c where the polarization is not reversed, and is referred to as a third interaction portion. 11d is an optical waveguide shift part provided between the first interaction part 11a and the second interaction part 11b, and 11e is an optical light provided between the second interaction part 11b and the third interaction part 11c. It is a waveguide shift part. A center conductor (not shown) is formed on the ridge portions 10 ′ and 10 ″ via the buffer layer 2 and the like.

  As the frequency of the electrical signal increases, the conductor loss of the metal constituting the traveling wave electrode increases. Therefore, the electric field strength of the electric signal acting on the first optical waveguide 30a and the second optical waveguide 30b is the strongest in the first interaction portion 11a, the weakest in the third interaction portion 11c, The second interaction portion 11b is intermediate between the first interaction portion and the third interaction portion.

Thus, for method of determining the L _4, L _5, L ₆ for reducing the chirping, but it is also effective as _{L 4 = L 6 = L 5} /2, to further improve the first in FIG. 1 The same idea as in the first embodiment can be applied.

That is, the length L ₄ of the first interaction portion 11a in the region where the polarization is not reversed, the length L ₅ of the second interaction portion 11b in the region where the polarization is reversed, and the polarization are not reversed. By appropriately setting the length L ₆ of the third interaction part 11c in the region, the sign of the α parameter representing the chirping amount is switched in the low frequency region and the high frequency region near DC, and α at a predetermined frequency The parameter can be zero.

  Note that the sum of the number of regions where the polarization is reversed and the number of regions where the polarization is not reversed may be an even number, but is preferably an odd number. In addition, when the sum of the number of regions where polarization is reversed and the number of regions where polarization is not reversed is 3 or more (especially in the case of an odd number), the sum of the lengths of the regions where polarization is reversed and polarization is not reversed. By making the sum of the lengths of the regions substantially equal, good zero chirping can be realized with a simple design.

(Third embodiment)
FIG. 5 shows a top view of the third embodiment of the present invention. In the description of the embodiments so far, the traveling wave electrode 4 has been omitted. However, in order to make the description easy to understand, in the third embodiment, conversely, the center conductor 4a ′ and the ground conductors 4b ′ and 4c ′ are replaced. I'm drawing.

As described above, in the first embodiment of the present invention shown in FIG. 1, the traveling wave electrode is straight in the interaction portion, and only the optical waveguide is shifted in position in the optical waveguide shift portion 5c. However, in the third embodiment, the first optical waveguide 3a and the second optical waveguide 3b are shifted, and the manufacturability of the electrodes, the microwave transmission characteristics (S ₂₁ ), and the reflection characteristics (S ₁₁ ). From this point of view, the traveling wave electrode composed of the central conductor 4a ′ and the ground conductors 4b ′ and 4c ′ is shifted to such an extent that there is no practical problem. It should be noted that excellent electrical characteristics can be realized by making the shift amount of the center conductor 4a 'smaller than the distance between the centers of the first optical waveguide 3a and the second optical waveguide 3b.

By configuring as shown in FIG. 5, light modulation can be performed even in a part of the optical waveguide shift portion 5c, so that the length L ₁ of the _first interaction portion 5a and the second interaction are practically effective. it becomes possible to parts 5b the length L ₂ longer. In other words, a part of the length L ₃ of the optical waveguide shift portion 5c is used for optical modulation. Therefore, more efficient light modulation can be realized as compared with the first embodiment of the present invention in which the traveling wave electrode is straight.

(Each embodiment)
In the above, as a polarization inversion zero chirp type optical modulator having a ridge structure, the first optical waveguide and the second optical waveguide have one or two optical waveguide shift portions (curved portions) in the longitudinal direction. However, it goes without saying that there may be three or more optical waveguide shift portions. In addition to the CPW structure, the electrode may have another structure such as an asymmetric coplanar strip (ACPS).

  Further, although the z-cut LN substrate has been described, a substrate of a different material such as a lithium tantalate substrate may be used. Furthermore, although the electrode has been described as a traveling wave electrode, in principle it may be a lumped constant electrode, and therefore the electrode in this specification includes a lumped constant electrode.

1: z-cut LN substrate (LN substrate)
1a, 1c: non-polarization inversion region 1b: polarization inversion region 2: SiO ₂ buffer layer (buffer layer)
3, 30: Mach-Zehnder optical waveguide (optical waveguide)
3a, 30a: first optical waveguide constituting the Mach-Zehnder optical waveguide 3b, 30b: second optical waveguide constituting the Mach-Zehnder optical waveguide 3a ′, 3b ′: bent portions (bend optical waveguide)
4: Traveling wave electrodes 4a, 4a ′, 4a ″, 4a ″ ″: center conductors 4b, 4c, 4b ′, 4c ′: ground conductors 5a, 11a, 11c: interaction portions 5b, 11b of non-polarized inversion regions : Interaction part 5c, 11d, 11e of polarization inversion region: Optical waveguide shift part 7a, 10, 10 ′, 10 ″: Ridge part 8a, 8b, 8c, 8d, 9a, 9b, 12a, 12b, 12c, 12d , 12e, 12f, 13a, 13b, 13c, 13d: groove 40: boundary between polarization inversion region and non-polarization inversion region

Claims (8)

A substrate made of a material having an electro-optic effect and having a region that does not reverse polarization and a region that reverses polarization,
A branched optical waveguide formed on one side of the substrate for branching incident light, a first optical waveguide and a second optical waveguide for propagating the branched light, and the first optical waveguide A groove portion is provided that includes a combined optical waveguide for combining the light propagating through the waveguide and the second optical waveguide, and a part of the substrate is dug down along the first and second optical waveguides. Comprising a ridge-shaped optical waveguide,
The interaction parts where the light propagating through the first optical waveguide and the second optical waveguide and the electric signal propagating through the traveling wave electrode composed of the center conductor and the ground conductor interact are polarized in different directions. Including one interaction portion and a second interaction portion;
The central conductor is opposed to the first optical waveguide or the second optical waveguide at the first interaction portion and the second interaction portion,
An optical modulator for generating an optical signal pulse by modulating a phase of the light propagating through the first optical waveguide and the second optical waveguide by the first interaction unit and the second interaction unit; There,
By providing an optical waveguide shift portion between the first interaction portion and the second interaction portion,
The optical axis of the first optical waveguide in the first interaction unit is different from the optical axis of the first optical waveguide in the second interaction unit, and the optical axis of the first interaction unit is different from the optical axis of the first interaction unit. Making the optical axis of the second optical waveguide different from the optical axis of the second optical waveguide in the second interaction section;
In the first interaction portion and the second interaction portion, the relative positions of the center conductor and the ground conductor, the first optical waveguide, and the second optical waveguide are switched,
The central conductor in the interaction part is a straight line,
In the interaction portion, the groove portion is formed at a predetermined location other than below the center conductor in the optical waveguide shift portion so that the traveling wave electrode does not have an electrical discontinuity portion. Characteristic light modulator. A substrate made of a material having an electro-optic effect and having a region that does not reverse polarization and a region that reverses polarization,
A branched optical waveguide formed on one side of the substrate for branching incident light, a first optical waveguide and a second optical waveguide for propagating the branched light, and the first optical waveguide A groove portion is provided that includes a combined optical waveguide for combining the light propagating through the waveguide and the second optical waveguide, and a part of the substrate is dug down along the first and second optical waveguides. Comprising a ridge-shaped optical waveguide,
The interaction parts where the light propagating through the first optical waveguide and the second optical waveguide and the electric signal propagating through the traveling wave electrode composed of the center conductor and the ground conductor interact are polarized in different directions. Including one interaction portion and a second interaction portion;
The central conductor is opposed to the first optical waveguide or the second optical waveguide at the first interaction portion and the second interaction portion,
An optical modulator for generating an optical signal pulse by modulating a phase of the light propagating through the first optical waveguide and the second optical waveguide by the first interaction unit and the second interaction unit; There,
By providing an optical waveguide shift portion between the first interaction portion and the second interaction portion,
The optical axis of the first optical waveguide in the first interaction unit is different from the optical axis of the first optical waveguide in the second interaction unit, and the optical axis of the first interaction unit is different from the optical axis of the first interaction unit. Making the optical axis of the second optical waveguide different from the optical axis of the second optical waveguide in the second interaction section;
In the first interaction portion and the second interaction portion, the relative positions of the center conductor and the ground conductor, the first optical waveguide, and the second optical waveguide are switched,
The central conductor in the interaction portion is substantially straight,
A part of the central conductor is formed substantially parallel to at least one of the first optical waveguide and the second optical waveguide in the optical waveguide shift portion;
In the interaction portion, the groove portion is formed at a predetermined location other than below the center conductor in the optical waveguide shift portion so that the traveling wave electrode does not have an electrical discontinuity portion. Characteristic light modulator.   3. The optical modulator according to claim 1, wherein a length of the first interaction unit is substantially equal to a length of the second interaction unit. 4.   The optical modulator according to claim 1, wherein a length of the first interaction unit is shorter than a length of the second interaction unit.   The at least one of a region in which the polarization is not reversed and a region in which the polarization is reversed in the interaction portion are plural, and the optical waveguide shift portion is provided at a boundary between the regions. The optical modulator according to 2.   6. The optical modulator according to claim 5, wherein the sum of the number of the non-inverted regions and the inverted regions in the interaction unit is an odd number.   The optical modulator according to claim 6, wherein the sum of the lengths of the region where the polarization is not inverted and the region where the polarization is inverted in the interaction unit is substantially equal to each other. 8. The chirping of the optical signal pulse is smaller than that in the case where the substrate is one of a region where the polarization is not reversed and a region where the polarization is reversed. The optical modulator according to item.

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