JP2010048929A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator having high performance in the optical modulation characteristics and improved stability. <P>SOLUTION: Disclosed is the optical modulator which includes a substrate 1, an optical waveguide 3, a traveling wave electrode and a ridge portion composed of recess portions, the ridge portion comprising a central conductor ridge portion and a ground conductor ridge portion, wherein the recess portions comprise first to third recess portions, the second and third recess portions are formed in symmetrical positions with respect to a center line of the first recess portion, no ground conductor is formed above the first and second recess portions, a portion 4b<SP>(5)</SP>formed to be thinner than a central conductor 4a and a portion 4b<SP>(5)</SP>having the same thickness as that of the central conductor are alternately formed above the third recess portion in the direction of the optical waveguide, and a portion 4c<SP>(5)</SP>' formed to be thinner than the central conductor and a portion 4c<SP>(5)</SP>having the same thickness as that of the central conductor are alternately formed in the direction of the optical waveguide in a position symmetrical with the third recess portion with respect to the center line of the central conductor in the ground conductor formed on the substrate on the side which is not the side of the central conductor ridge portion adjacent to the second recess portion. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical modulator that uses an electro-optic effect to modulate light incident on an optical waveguide with a high-frequency electrical signal and emit 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.

  Hereinafter, an LN optical modulator using the electro-optic effect of lithium niobate that has been put to practical use or has been proposed will be described.

(First prior art)
FIG. 9 shows a perspective view of a so-called ridge type LN optical modulator disclosed in Patent Document 1, which is configured using a z-cut LN substrate, as a first conventional optical modulator. FIG. 10 is a cross-sectional view taken along line AA ′ of FIG.

  An optical waveguide 3 is formed on the z-cut LN substrate 1. The optical waveguide 3 is an optical waveguide formed by thermally diffusing metal Ti at 1050 ° C. for about 10 hours, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Accordingly, two interaction optical waveguides 3a and 3b, that is, two arms of the Mach-Zehnder optical waveguide are formed at a portion where the electrical signal and light of the optical waveguide 3 interact (referred to as an interaction portion).

An SiO ₂ buffer layer 2 is formed on the upper surface of the optical waveguide 3, 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. Normally, the traveling wave electrode 4 is made of Au, which is an expensive noble metal material. Reference numeral 5 denotes a conductive layer for suppressing temperature drift caused by a pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, and usually a Si conductive layer is used. The width S of the center conductor 4a is about 7 μm, and the gap W between the center conductor 4a and the ground conductors 4b and 4c is about 15 μm. For simplification of explanation, the Si conductive layer 5 for suppressing temperature drift shown in FIG. 9 is omitted in FIG. In the following, the Si conductive layer 5 is omitted and discussed.

  In the first prior art, the recesses 9a, 9b and 9c (or ridges 8a and 8b) are formed by digging the z-cut LN substrate 1 by etching or the like. Here, 10a and 10b are regions or portions where the electromagnetic field of the high-frequency electric signal is reduced in the ground conductor, and are referred to as outer peripheral portions. The ridge portions 8a and 8b are also referred to as a central conductor ridge portion and a ground conductor ridge portion, respectively.

  By adopting this ridge structure, it is possible to realize excellent characteristics in the effective refractive index (or microwave effective refractive index), characteristic impedance, modulation band, driving voltage, and the like of a high-frequency electric signal. In FIG. 10, the depth of the recesses 9a, 9b, and 9c (or the height of the ridges 8a and 8b) is emphasized, but it is actually about 2 to 5 μm, and the center conductor 4a and the ground The value is small compared to the thickness of the conductors 4b and 4c of about 20 μm.

  Now, it has been found that although the first prior art has high modulation characteristics as an LN optical modulator, there is a problem with stability. That is, it has been found that the temperature drift characteristic is poor despite the use of the Si conductive layer 5. The cause is thought to be due to the ridge structure that produces high modulation performance.

  The cause will be described in detail below. As can be seen from FIG. 10, the ridge portion 8a immediately below the central conductor 4a is independent of the ground conductors 4b and 4c, and therefore the ridge portion 8a is pulled in a direction parallel to the surface of the z-cut LN substrate 1. There is no power.

However, in the ridge portion 8b, as described above, the thick ground conductor 4b having a thickness of about 20 μm is formed together with the concave portion 9c and the outer peripheral portion 10b. The Au of the ground conductor 4b on the SiO ₂ buffer layer 2 and the thermal expansion coefficient of the z-cut LN substrate 1 are greatly different from each other. Furthermore, the width of the z-cut LN substrate 1 is as wide as several millimeters (for example, 1 mm to 5 mm). On the other hand, since the gap between the interaction optical waveguides 3a and 3b is as narrow as about 15 μm, the width of the ground conductors 4b and 4c is so wide that it can be said to be about half the width of the z-cut LN substrate 1 (in other words, the outer peripheral portion). 10a and 10b are wide). That is, since the width of the ground conductor 4b in FIG. 10 is wide, stresses such as thermal expansion and thermal contraction due to environmental changes accumulate, and a considerably large stress (or a moment due to a large thickness) is applied to the ridge portion 8b. Furthermore, since the ridge portion 8b protrudes, it is easily affected by stress (especially stress due to moment). Actually, the width of the ground conductor 4c is wide, and this is also affected.

  However, when a stress is applied to the z-cut LN substrate 1, its refractive index changes (stress birefringence). As a result, the refractive index of the interactive optical waveguide 3a changes, and the LN optical modulator operates. The DC bias point when changing is changed. This is a temperature drift phenomenon peculiar to the ridge structure, and results in impairing the stability as the LN optical modulator. Incidentally, when the environmental temperature of the LN optical modulator was changed from room temperature to 80 ° C., the change of the DC bias point in the first prior art was as large as 6V.

(Second prior art)
In order to solve the problem of the first prior art, a cross-sectional view of the interaction portion of the second prior art disclosed in Patent Document 2 is shown in FIG. As can be seen from FIG. 11, the ground conductor 4b 'formed on the ridge portion 8b and the ground conductor 4b''formed on the outer peripheral portion 10b are thick, but the ground conductor formed in the recess 9c. The thickness of 4b ″ ′ is as thin as about 300 nm or less, for example. By reducing the thickness of the ground conductor 4b ′ ″ in the recess 9c in this way, the stress applied to the ridge portion 8b by the ground conductor 4b ″ having a large area can be reduced, so that the temperature stability can be improved. This is the idea.

  In the following, discussion will be made from the viewpoint of temperature drift and propagation of high-frequency electrical signals. In the second prior art, the recesses 9a, 9b, 9c and the optical waveguides 3a, 3b forming the ridges 8a, 8b are symmetrical with respect to the center line I provided between them. Now, the side surfaces of the ridge portions 8a and 8b are inclined (hence, also referred to as inclined surfaces or inclined portions). Since such an inclined surface is not a −z plane, the generation of charges due to the pyroelectric effect is different from the upper surface of the z-cut LN substrate 1 and the recesses 9a, 9b, and 9c, which are the −z plane.

  Therefore, in order to realize excellent temperature drift characteristics, the recesses 9a, 9b, and 9c are substantially symmetrical with respect to the center line I that can be provided between the optical waveguides 3a and 3b (or with the optical waveguide 3a). A structure in which the structure of the z-cut LN substrate 1 including 3b is substantially symmetrical with respect to the center line provided in the middle thereof is desirable. That is, the second prior art can be said to be a structure suitable for temperature drift suppression. In the case of two interaction optical waveguides (optical waveguides 3a and 3b), such as a Mach-Zehnder optical waveguide, the structure in which the optical waveguides 3a and 3b are symmetric with respect to the center line I between them is substantially The number of typical recesses (9a, 9b, 9c) is an odd number.

  However, it can be seen that the second prior art has the following problems from the viewpoint of low-loss and stable propagation of high-frequency electrical signals. As shown in FIG. 9, the high-frequency electric signal that is a microwave propagates through the high-frequency electric signal feed line 6 and a connector (not shown) and is then applied to the traveling wave electrode 4. The electromagnetic field distribution of this connector is axisymmetric with respect to each central conductor. In addition, the electromagnetic field distribution of the high-frequency electromagnetic field is the center of the traveling-wave electrode 4 even at a connection portion (called an input feed-through portion) between a connector (not shown) connected to the high-frequency electric signal feeder 6 and the traveling-wave electrode 4. It is bilaterally symmetric (that is, symmetric with respect to the substrate surface direction) with respect to the conductor (the portion where the central conductor 4a of the interaction portion extends to the connection portion with the core wire of the connector not shown).

  However, in the second prior art, the traveling wave electrode is structurally asymmetric with respect to the line II drawn in the center of the center conductor 4a in FIG. When the structure is asymmetrical, the electromagnetic field distribution is also asymmetrical, so that the matching with the symmetrical electromagnetic field distribution in the input feedthrough portion is not good. As a result, when a high-frequency electric signal propagates through the traveling wave electrode 4 composed of the center conductor 4a and the ground conductors 4b ′, 4b ′ ″, 4b ″, 4c, an asymmetric electromagnetic field from a symmetric electromagnetic field distribution. It must be converted into a distribution, which causes inconveniences such as unstable electromagnetic field distribution or radiation loss (or propagation loss).

(Third prior art)
FIG. 12 shows a top view of the third prior art disclosed in Patent Document 3. As shown in FIG. The width of the z-cut LN substrate 1 is several millimeters, and the gap between the interaction optical waveguides 3a and 3b is about 15 μm. The length of the z-cut LN substrate 1 is about 5 cm to 7 cm.

Here, cross-sectional views along BB ′ and CC ′ are shown in FIGS. 13 and 14. In addition, 11a, 11b, 11c, and 11d are void portions due to the presence of the recesses 9a, 9b, 9c, and 9d, and 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ^{( 5)} and 4c ⁽⁶⁾ are ground conductors. Here, the ground conductor 4b ⁽⁵⁾ connects the ground conductors 4b ⁽⁴⁾ and 4b ⁽⁶⁾ . Reference numeral 10c denotes an outer peripheral portion. Reference numerals 8a, 8b, and 8c denote ridge portions. It can be said that the gaps 11a and 11d are portions where the conductor is missing in the ground conductor (or windows opened in the ground conductor). Reference numerals 13a and 13d denote embedded portions in which the gap portions 11a and 11d are filled with the ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ . In FIGS. 13 and 14, IV is the center line of the center conductor 4a, and the traveling wave electrode is symmetric with respect to the center line IV.

As can be seen, the widths of the ground conductors 4b ⁽⁴⁾ and 4c ⁽⁴⁾ are as narrow as the ground conductor 4b 'and the center conductor 4a of the second prior art shown in FIG. The ground conductors 4b ⁽⁶⁾ and 4c ⁽⁶⁾ are as wide as the ground conductor 4b '' of the second prior art shown in FIG. Then, this third in the prior art to connect the ground conductor ^4b and ^{(4) 4b (6)} the ground conductor ^{4b (5),} the ground conductor ^4c for connecting with ^{4c (6)} the ground conductor ^{4c (4) (5 )} Is set thicker than the ground conductor 4b '''of the second prior art shown in FIG.

  However, when the third prior art was actually manufactured, it was found that this structure has an important problem that the temperature drift due to the ridge structure cannot be suppressed. The problem will be described below from the viewpoint of the pyroelectric effect.

Ridges 8a, 8b, and 8c (or recesses 9a, 9b, 9c, and 9d) are arranged asymmetrically with respect to the center line III between the optical waveguides 3a and 3b. Since the inclined surfaces that are the side surfaces of the ridges 8a, 8b, and 8c are not the -z plane, the distribution of charges due to the pyroelectric effect accompanying the change in environmental temperature is different from those of the concave portions and the upper surface of the z-cut LN substrate 1. . Therefore, a non-uniform charge distribution (that is, a non-uniform electric field distribution) that changes every moment with a change in the environmental temperature is generated, so that a non-uniform voltage is applied to the optical waveguides 3a and 3b. Since it is necessary to apply a DC bias from an external circuit so as to cancel these non-uniform electric field distributions, it can be concluded that a temperature drift results.
JP-A-4-288518 JP 2004-157500 A JP 2006-84537 A

As described above, in the first conventional technique proposed as the ridge type LN optical modulator, the ground conductor caused by the difference in thermal expansion coefficient between Au constituting the electrode on the SiO ₂ buffer layer and the z-cut LN substrate. Stress (or stress due to moment) caused a temperature drift that changed the optimum DC bias point with temperature. In the second prior art proposed to improve this temperature characteristic, although the temperature drift can be improved, the traveling wave electrode is not symmetric with respect to the central conductor, so that the high frequency electric signal is changed from the symmetric mode to the asymmetric mode. It is disadvantageous from the viewpoint of stable propagation and low loss propagation in mode conversion. In the third prior art, the traveling wave electrode is symmetric with respect to the central conductor, which is advantageous from the viewpoint of stable and low-loss propagation of a high-frequency electrical signal. Since it was asymmetric with respect to the two optical waveguides, the influence of the pyroelectric effect particularly on the inclined surface of the ridge was strong, and there was a problem from the viewpoint of temperature drift. That is, there is an urgent need to develop an optical modulator that can achieve temperature stabilization without sacrificing high speed and low driving voltage as an optical modulator.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optical modulator having high performance in light modulation characteristics and improved stability.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, two optical waveguides formed on the substrate, and a buffer formed on the substrate. A layer, a traveling wave electrode comprising a central conductor and a ground conductor disposed above the buffer layer, and at least part of the substrate in a region where the electric field strength of a high-frequency electric signal propagating through the traveling wave electrode is strong A ridge portion constituted by a concave portion formed by a center conductor ridge portion having the center conductor formed upward and a ground conductor ridge portion having the ground conductor formed upward. In the optical modulator in which one of the two optical waveguides is formed in the ridge portion for the central conductor and another optical waveguide is formed in the ridge portion for the ground conductor, the concave portion is First The second and third recesses, and the second and third recesses are formed symmetrically with respect to the center line of the first recess, and the first recess and the second recess The center conductor ridge is formed between the first recess and the third recess, and the ground conductor ridge is formed between the first recess and the third recess. The ground conductor is not formed above the concave portion 2, and the ground conductor and the central conductor or the central conductor are formed above the third concave portion so that the conductor is thinner than the thickness of the central conductor. A grounding conductor having substantially the same thickness as the opposing grounding conductor is alternately formed in the direction of the optical waveguide, and the substrate on the side that is not the ridge portion for the central conductor adjacent to the second recess The ground conductor formed above has a center line of the center conductor. A ground conductor having a conductor thinner than the thickness of the center conductor at a position symmetrical to the third recess, and a ground conductor having substantially the same thickness as the center conductor or the ground conductor facing the center conductor; Are alternately formed in the direction of the optical waveguide.

  The optical modulator according to claim 2 of the present invention is a ground conductor formed above the third recess, and a portion formed thinner than the thickness of the center conductor is a first thin ground conductor for connection. A position having the same thickness as the center conductor or the ground conductor facing the center conductor is a first thick connecting ground conductor, and is symmetrical with the third recess at the center line of the center conductor. The portion formed to be thinner than the thickness of the center conductor is a second thin connection ground conductor, and is approximately the same thickness as the center conductor or the ground conductor facing the center conductor. When the second thick connecting ground conductor is used as the second thick connecting ground conductor, the first thin connecting ground conductor and the second thin connecting ground conductor are formed symmetrically with respect to the center line of the central conductor. It is characterized by.

  The optical modulator according to claim 3 of the present invention is a ground conductor formed above the third recess, and a portion formed thinner than the thickness of the center conductor is a first thin ground conductor for connection. A position having the same thickness as the center conductor or the ground conductor facing the center conductor is a first thick connecting ground conductor, and is symmetrical with the third recess at the center line of the center conductor. The portion formed to be thinner than the thickness of the center conductor is a second thin connection ground conductor, and is approximately the same thickness as the center conductor or the ground conductor facing the center conductor. Of the first thin connecting ground conductor and the second thin connecting ground conductor in the direction of the optical waveguide, the length or the width of the first thin connecting ground conductor At least one is different Made is characterized in that is.

  In the optical modulator according to claim 4 of the present invention, the electromagnetic field of the high-frequency electrical signal of the ground conductor formed on the substrate on the non-ground conductor ridge portion adjacent to the third recess is reduced. The thickness in a region that is a predetermined distance away from the third recess, which is a region, is configured to be thinner than the thickness of the ground conductor in a region other than the region.

  In the optical modulator according to claim 5 of the present invention, the electromagnetic field of the high-frequency electrical signal of the ground conductor formed on the substrate that is adjacent to the second recess and is not on the ridge portion for the central conductor is reduced. The thickness in a region that is a predetermined distance away from the second concave portion that is a region is configured to be thinner than the thickness of the ground conductor in a region other than the region.

  The optical modulator according to claim 6 of the present invention is characterized in that the substrate is made of lithium niobate.

  The optical modulator according to claim 7 of the present invention is characterized in that the substrate is made of a semiconductor.

In the optical modulator according to the present invention, the ridge and the concave portion constituting the ridge are symmetrical with respect to the center line provided in the middle of the two optical waveguides. As a result, the charge distribution (that is, the electric field distribution) due to the pyroelectric effect that occurs in response to a change in the environmental temperature is substantially symmetric with respect to the center line provided between the two optical waveguides. Can be solved. In addition, the traveling wave electrode is symmetric with respect to the center line of the central conductor, and a grounding conductor for connection consisting of a thick region and a thin region of the ground conductor is juxtaposed along the longitudinal direction of the optical waveguide. As a result, it is possible not only to solve the temperature drift, but also to enable stable and low-loss propagation of the high-frequency electric signal, without impairing the high performance from the viewpoint of modulation of the ridge type optical modulator. Therefore, there is an excellent effect that it is possible to provide an LN optical modulator having stable temperature characteristics and excellent high frequency characteristics. The present invention also includes a structure in which the thickness of the outer peripheral conductor, which is a region (or part) where the electromagnetic field distribution of the high-frequency electrical signal is reduced, is reduced in the ground conductor. Due to the structure in which the thickness of the conductor on the outer peripheral portion is reduced, when the environmental temperature changes, the conductor from the wide ground conductor caused by the difference in thermal expansion coefficient between the conductor on the SiO ₂ buffer layer and the z-cut LN substrate Since the stress of the conductor can be relaxed, it not only contributes to further improvement of temperature drift characteristics, but also can reduce the cost of optical modulator because it can reduce the amount of Au, which is an expensive noble metal material It becomes.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the prior art shown in FIGS. 9 to 14 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

(First embodiment)
FIG. 1 shows a top view of the first embodiment of the present invention. Moreover, sectional views taken along DD ′ and EE ′ are shown in FIGS. 2 and 3, respectively. Here, 11b and 11c are gaps. 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ⁽⁵⁾ ', 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁵⁾ ', 4c ⁽⁶⁾ are ground conductors.

The ground conductor 4b ⁽⁵⁾ and the ground conductor 4c ⁽⁵⁾ are thick so that they are thick. On the other hand, the ground conductor 4b ⁽⁵⁾ 'and the ground conductor 4c ⁽⁵⁾ ' are thin, so they are also called thin ground conductors. Reference numerals 10b and 10d are portions where the strength of the high-frequency electrical signal is reduced in the ground conductor, and are referred to as outer peripheral portions. 8a and 8b are ridge portions.

The thick ground conductor 4b ⁽⁵⁾ and the thin ground conductor 4b ⁽⁵⁾ 'are ground conductors 4b ⁽⁴⁾ and 4b ⁽⁶⁾ , and the thick ground conductor 4c ⁽⁵⁾ and the thin ground conductor 4c ⁽⁵⁾ ' are grounded. Conductors 4c ⁽⁴⁾ and 4c ⁽⁶⁾ are connected (thick ground conductor 4b ⁽⁵⁾ and thin ground conductor 4b ⁽⁵⁾ ', and thick ground conductor 4c ⁽⁵⁾ and thin ground conductor 4c ^(5). 'Is also called a connecting ground conductor).

In the present embodiment, the ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ , which are connection ground conductors, are thickened, but this is less susceptible to the skin effect as a high-frequency electrical signal. Because. Further, the thickness of the thin ground conductors 4b ⁽⁵⁾ ′ and 4c ⁽⁵⁾ ′ is set to be as thin as, for example, 50 nm to 500 nm so that the temperature drift becomes sufficiently small, but the material is made of Au having a small electric resistance. Accordingly, the propagation loss of the high-frequency electric signal is smaller than the loss of Au in these portions. Needless to say, the above numerical values for the thicknesses of the thin ground conductors 4b ⁽⁵⁾ ′ and 4c ⁽⁵⁾ ′ are examples and are not limited thereto.

Here, no concave portion is formed under the thick ground conductor 4c ⁽⁵⁾ and the thin ground conductor 4c ⁽⁵⁾ ′ on the side where the optical waveguide is not formed. In FIG. 2, V is a center line provided between the optical waveguides 3a and 3b, and the optical waveguides 3a and 3b (or the ridges 8a and 8b or the recesses 9a, 9b, and 9c) are located with respect to the center line V. It has a symmetrical structure. Therefore, V can be said to be an axis of symmetry about the optical waveguide. As described above, the ridge portions 8a and 8b are inclined portions (as described above, the distribution of charges generated by the pyroelectric effect is different from the bottom surfaces of the recesses 9a, 9b, and 9c and the top surface of the z-cut LN substrate 1). 8a and 8b, also called inclined surfaces).

  What is important is that in the first embodiment, the concave portions 9a, 9b, 9c including the inclined surfaces of the optical waveguides 3a, 3b are symmetrical with respect to the center line V. As a result, the charge distribution due to the pyroelectric effect, that is, the electric field distribution is also symmetric with respect to the center line V, so that extremely stable characteristics can be realized with respect to the temperature drift accompanying the environmental change.

  It should be noted that even if the symmetry about the optical waveguides 3a and 3b is broken by forming a recess at a position that does not affect the temperature drift below each ground conductor, it is a change of the position that does not affect the present invention, and therefore the present invention. Belongs. 2 and 3, the number of recesses is three. However, as long as the structure is symmetrical with respect to the center line provided in the middle of the two optical waveguides, a larger number of recesses are provided. However, it can be said to belong to the present invention. Needless to say, these are true for all embodiments of the present invention.

FIG. 4 shows the experimental results for the first embodiment of the present invention when the environmental temperature T is changed from 20 ° C. to 80 ° C. For comparison, the figure also shows the measurement results for the first prior art, the second prior art, and the third prior art. Here, the width S of the center conductor 4a is 7 μm, and the gap W between the center conductor 4a and the ground conductor 4b ⁽⁴⁾ or the ground conductor 4c ⁽⁴⁾ is 15 μm. Further, the width Ww of the thin ground conductor 4b ⁽⁵⁾ ′ is 15 μm, and the length Lw thereof and the length Le of the thick ground conductor 4b ⁽⁵⁾ are 1 mm and 100 μm, respectively. As can be seen from the figure, by adopting this embodiment, the traveling wave electrode is symmetrical with respect to the center line of the center conductor 4a, which is advantageous from the viewpoint of high-frequency light modulation, but the recesses 11a, 11b, 11c. , 11d can be greatly suppressed in temperature drift as compared with the third prior art in which the optical waveguides 3a and 3b are not symmetric, and the idea of the present invention was proved to be correct. Note that the second conventional technique is excellent in terms of temperature drift, but as described above, the second conventional technique has a problem from the viewpoint of mode conversion of a high-frequency electric signal. Here, even if the length Lw of the thin ground conductors 4b ⁽⁵⁾ ′ and 4c ⁽⁵⁾ ′ and the length Le of the thick ground conductor 4b ⁽⁵⁾ are changed to about 30 μm to 3 mm and about ⁵ μm to 500 μm, respectively, efficiency is improved. The temperature drift could be suppressed well.

In the above description, in order to simplify the explanation of the principle of the present invention, the width Ww and the length Lw of the thin ground conductor 4b ⁽⁵⁾ ′ are the same as the width Ww ′ and the length of the thin ground conductor 4c ⁽⁵⁾ ′. It is assumed that the length Le of the thick ground conductor 4b ⁽⁵⁾ is equal to the length Le ′ of the thick ground conductor 4c ⁽⁵⁾ , but the present invention is not limited to this. Further, the number of thin ground conductors 4b ⁽⁵⁾ ′ and thick ground conductors 4b ⁽⁵⁾ may be different from the number of thin ground conductors 4c ⁽⁵⁾ ′ and thick ground conductors 4c ⁽⁵⁾ . This is true for all embodiments of the present invention. Note that the length and width of the ground conductor (connection ground conductor) described above are in the length direction of the interaction optical waveguides 3a and 3b.

Next, it considers from a viewpoint of electromagnetic field distribution and propagation loss of a high frequency electric signal. As can be seen from FIGS. 2 and 3, the center line VI drawn to the center of the center conductor 4a is the center conductor 4a and the ground conductors 4b ⁽⁴⁾ , 4b ⁽⁵⁾ ′, 4b ⁽⁶⁾ , 4c ⁽⁴⁾ , 4c. ⁽⁵⁾ ', 4c This is the axis of symmetry of the traveling wave electrode consisting of ⁽⁶⁾ . In the present embodiment, this symmetry is satisfied even if the ground conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ are included. Thus, the traveling wave electrode in this embodiment has structural symmetry with the center line of the center conductor 4a as the axis of symmetry.

  That the structure of the traveling wave electrode is symmetric means that the electromagnetic field distribution of the high-frequency electrical signal propagating through the traveling wave electrode is also symmetric. Therefore, the conversion from the symmetric mode of the symmetric high-frequency electrical signal of the connector and the input feed-through portion to the asymmetric mode of the traveling-wave electrode, which is necessary in the second prior art shown in FIG. It becomes possible to propagate an electric signal stably and with low loss.

In this embodiment, the grounding conductors 4b ⁽⁵⁾ and 4c ⁽⁵⁾ , which are connecting grounding conductors, are thickened, but this is unlikely to be affected by the skin effect as a high-frequency electrical signal. It is to make it. Furthermore, although the thickness of the ground conductors 4b ⁽⁵⁾ ′ and 4c ⁽⁵⁾ ′, which are different types of connection ground conductors, is set so as to sufficiently reduce the temperature drift, the material has a small electrical resistance. Au. Accordingly, the propagation loss of the high-frequency electric signal is smaller than the loss of Au in these portions.

Now, it is not correct if the concave portion is not strictly symmetric with respect to the center line V provided between the optical waveguides 3a and 3b. The width of the ground conductor 4b ⁽⁴⁾ may be different from the width of the center conductor 4a by several μm. Including this, the center of the optical waveguides 3a and 3b with respect to the center line V provided in the middle of the optical waveguides 3a and 3b. The center conductor and the ground conductor on the top are symmetrical (or substantially symmetrical). Similarly, the effect of the present invention can be exhibited even if the structure of the traveling wave electrode is not strictly symmetrical with respect to the center line VI. These are true for all embodiments of the invention.

  As described above, in the present embodiment, the structure related to the optical waveguide is symmetric with respect to the center line V provided in the middle of the two optical waveguides, and the structure related to the traveling wave electrode is symmetric with respect to the center line VI of the central conductor. Therefore, as compared with the case where they do not have symmetry, temperature drift due to environmental temperature change is suppressed, and the mode of the high-frequency electric signal is stabilized and propagated with low loss.

(Second Embodiment)
FIG. 5 shows a top view of the second embodiment of the present invention. In addition, cross-sectional views taken along lines HH ′ and II ′ are shown in FIGS. 6 and 7, respectively. Here, 11b and 11c are gaps. 4b ⁽¹⁰⁾ , 4b ⁽¹¹⁾ , 4b ⁽¹¹⁾ ', 4b ⁽¹²⁾ , 4b ⁽¹³⁾ , 4c ⁽¹⁰⁾ , 4c ⁽¹¹⁾ , 4c ⁽¹¹⁾ ', 4c ⁽¹²⁾ , 4c ⁽¹³⁾ and are ground conductors.

The ground conductor 4b ⁽¹¹⁾ and the ground conductor 4c ⁽¹¹⁾ are so thick that they are called thick ground conductors. On the other hand, the ground conductor 4b ⁽¹¹⁾ ′ and the ground conductor 4c ⁽¹¹⁾ ′ are thin because they are thin. Also called a ground conductor. Reference numerals 10b and 10d are portions where the strength of the high-frequency electrical signal is reduced in the ground conductor, and are referred to as outer peripheral portions. 8a and 8b are ridge portions.

The thick ground conductor 4b ⁽¹¹⁾ and the thin ground conductor 4b ⁽¹¹⁾ ′ are ground conductors 4b ⁽¹⁰⁾ and 4b ⁽¹²⁾ , and the thick ground conductor 4c ⁽¹¹⁾ and the thin ground conductor 4c ⁽¹¹⁾ ′ are grounded. Conductors 4c ⁽¹⁰⁾ and 4c ⁽¹²⁾ are connected (thick ground conductor 4b ⁽¹¹⁾ and thin ground conductor 4b ⁽¹¹⁾ ', and thick ground conductor 4c ⁽¹¹⁾ and thin ground conductor 4c ^(11). 'Is also called a connecting ground conductor).

Also in the present embodiment shown in FIG. 6, the structure relating to the optical waveguide is such that the recesses 9a, 9b, 9c are symmetrically arranged with respect to the center line V in the middle between the two optical waveguides 3a and 3b. It is an important factor in having excellent temperature drift characteristics. The center line VI drawn to the center of the center conductor 4a is the center conductor 4a and the ground conductors 4b ⁽¹⁰⁾ , 4b ⁽¹¹⁾ ', 4b ⁽¹²⁾ , 4b ⁽¹³⁾ , 4c ⁽¹⁰⁾ , 4c ^(11). 'Is the axis of symmetry of the traveling wave electrode consisting of 4c ⁽¹²⁾ and 4c ⁽¹³⁾ . This symmetry holds even when the ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ are considered. As described above, since the traveling wave electrode also has structural symmetry with the center of the center conductor 4a as the axis of symmetry, the high-frequency electric signal propagating through the traveling wave electrode is in a symmetric mode. Therefore, it is possible to propagate the high-frequency electric signal in a stable mode and with low loss, with good matching with the symmetrical electromagnetic field distribution of the connector and the input feed-through portion.

What should be noted in the second embodiment of the present invention is that the thickness of the ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ is as thin as 300 nm, for example. When the thickness of the grounding conductor is large, the stress (or stress due to moment) applied to the z-cut LN substrate 1 and eventually the ridges 8a and 8b by the lever principle increases. Therefore, in this embodiment, the stress is reduced by reducing the thickness of the ground conductor 4b ⁽¹³⁾ of the outer peripheral portion 10b. Furthermore, in order to make the effect of the present invention more remarkable, the thickness of the ground conductor 4c ⁽¹³⁾ formed on the outer peripheral portion 10d is also reduced. By incorporating this device, as shown in FIG. 8, the temperature drift of the DC bias with respect to the change in the environmental temperature from 20 ° C. to 80 ° C. can be further suppressed to be smaller than that in the first embodiment.

As described above, considering that the gap between the interaction optical waveguides 3a and 3b is about 15 μm, the width of the interaction portion where the high-frequency electric signal interacts with the light propagating through the interaction optical waveguides 3a and 3b. Is significantly narrower than the width of the z-cut LN substrate 1 (about 1 mm to 5 mm). Therefore, by reducing the thickness of the ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ , the amount of expensive Au used can be significantly reduced, which can contribute to cost reduction.

The ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ , which are thin but have a large area, establish a firm electrical ground from the viewpoint of high-frequency electrical signals and connect them to the casing that is the electrical ground by wires or ribbons. It is useful from the point of view. This is true for all embodiments of the invention.

Thus, by reducing the thickness of the ground conductor in the outer peripheral portion where the high-frequency electrical signal is small, the idea of improving the temperature drift without impairing the modulation characteristics is not limited to the first embodiment but the grounding of the outer peripheral portion. Needless to say, the present invention is also applicable to other embodiments in which the conductor is thick. In addition, even if only one of the thicknesses of the ground conductors 4b ⁽¹³⁾ and 4c ⁽¹³⁾ is reduced in FIGS. Further, since the electromagnetic field of the high-frequency electric signal is originally small at the outer peripheral portion, even if the asymmetry is broken to this extent, the propagation characteristic of the high-frequency electric signal is hardly affected. This is true for all embodiments of the invention.

(Each embodiment)
In the present invention, the traveling wave electrode has been described as being bilaterally symmetric with respect to the center line of the central conductor, but this is the main structure. For example, in FIG. 1, the thin ground conductors 4b ⁽¹¹⁾ ′ and 4c ⁽¹¹⁾ ′ and the thick ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ are substantially symmetrical with respect to the center line of the center conductor 4a. It is most desirable from the viewpoint of stable propagation of a high-frequency electric signal to be almost symmetrical. For example, even if the thick ground conductors 4b ⁽¹¹⁾ and 4c ⁽¹¹⁾ are displaced from each other (vertically in the plane of FIG. 1 ⁾ . Needless to say, the air gap portions 11b and 11c may be formed to the limit in the longitudinal direction of the interaction portion. That is, it goes without saying that even if the structure is not strictly symmetrical, if it is almost symmetrical at the main part, it also belongs to the present invention.

Although the Mach-Zehnder optical waveguide is used as an example of the branched optical waveguide, it goes without saying that the present invention can be applied to other branched / multiplexed optical waveguides such as directional couplers. The present invention is also applicable to a phase modulator having a single optical waveguide. In the case of the phase modulator, the one optical waveguide and the traveling wave electrode are symmetric with respect to the center line of the central conductor. As a method for forming the optical waveguide, various methods for forming the optical waveguide such as a proton exchange method can be applied in addition to the Ti thermal diffusion method, and various materials other than SiO ₂ such as Al ₂ O ₃ can be applied as the buffer layer.

  Further, although the z-cut LN substrate has been described, an LN substrate having other plane orientation such as x-cut and y-cut may be used, or a lithium tantalate substrate or a substrate made of a different material such as a semiconductor substrate may be used. Furthermore, although the electrode has been described as a traveling wave electrode, in principle, a lumped constant electrode may be used. Therefore, the traveling wave electrode in this specification includes a lumped constant electrode.

  Usually, each recess is formed with the same width, but when etching is performed so that the recess close to the outer periphery becomes very wide (the outer periphery is almost the same height as the bottom of the recess). It is possible to apply the present invention by considering the widely etched portion as the actual outer peripheral portion.

  As described above, in the optical modulator according to the present invention, in the high-performance ridge type optical modulator, the structure related to the recess and the optical waveguide provided in the substrate is symmetrical with respect to the center line provided in the middle of the two optical waveguides. By improving the temperature drift characteristics, the traveling wave electrode has a symmetrical structure with respect to the center line of the central conductor, thereby improving the propagation characteristics of the high-frequency electric signal and reducing the thickness of the outer peripheral Au. Therefore, it is useful as an optical modulator realizing further improvement in temperature drift characteristics and cost reduction.

1 is a top view showing a schematic configuration of an optical modulator according to a first embodiment of the present invention. Sectional drawing in DD 'of FIG. Sectional view taken along line EE 'in FIG. The figure explaining the characteristic of the 1st Embodiment of this invention FIG. 5 is a top view showing a schematic configuration of an optical modulator according to a second embodiment of the present invention. Sectional drawing in HH 'of FIG. Sectional view taken along II 'in FIG. The figure explaining the characteristic of the 2nd Embodiment of this invention The perspective view which shows schematic structure about the optical modulator of 1st prior art Sectional view taken along line AA 'in FIG. Sectional drawing which shows schematic structure about the optical modulator of 2nd prior art 3 is a top view showing a schematic configuration of a third conventional optical modulator. FIG. Sectional drawing in BB 'of FIG. Sectional drawing in CC 'of FIG.

Explanation of symbols

1: z-cut LN substrate (LN substrate)
_2, 14, 15: SiO ₂ buffer layer (buffer layer)
3: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b: interaction optical waveguide constituting Mach-Zehnder optical waveguide 4: traveling wave electrode 4a: central conductor 4b, 4b ′, 4b ″, 4b ′ ″, 4b ⁽⁴⁾ , 4b ⁽⁵⁾ , 4b ^{(6 )} , 4b ⁽⁷⁾ , 4b ⁽⁸⁾ , 4b ⁽⁹⁾ , 4b ⁽¹⁰⁾ , 4b ⁽¹¹⁾ , 4b ⁽¹¹⁾ ', 4b ⁽¹²⁾ , 4b ⁽¹³⁾ , 4c, 4c ⁽⁴⁾ , 4c ⁽⁵⁾ , 4c ⁽⁶⁾ , 4c ⁽⁷⁾ , 4c ⁽⁸⁾ , 4c ⁽⁹⁾ , 4c ⁽¹⁰⁾ , 4c ⁽¹¹⁾ , 4c ⁽¹¹⁾ ', 4c ⁽¹²⁾ , 4c ⁽¹³⁾ : Ground conductor 5: Si conductive layer 6: High frequency (RF) electric signal feeder 7: High frequency (RF) electric signal output line 8a: Ridge portion (ridge portion for central conductor)
8b, 8c: Ridge portion (ridge portion for grounding conductor)
9a, 9b, 9c, 9d: recessed portions 10a, 10b, 10c, 10d: outer peripheral portions 11b, 11c: gap portions 13a, 13d: embedded portions

Claims (7)

Progression comprising a substrate having an electro-optic effect, two optical waveguides formed on the substrate, a buffer layer formed on the substrate, a central conductor disposed above the buffer layer, and a ground conductor A wave electrode, and a ridge portion formed by a recess formed by digging at least part of the substrate in a region where the electric field strength of the high-frequency electric signal propagating through the traveling wave electrode is strong, A center conductor ridge portion formed above the center conductor and a ground conductor ridge portion formed above the ground conductor, and one of the two optical waveguides on the center conductor ridge portion. In the optical modulator in which another optical waveguide is formed in the ridge portion for the ground conductor,
The concave portion includes first, second, and third concave portions, and the second concave portion and the third concave portion are formed at symmetrical positions with respect to the center line of the first concave portion. The center conductor ridge portion is formed between the first recess and the second recess, and the ground conductor ridge portion is formed between the first recess and the third recess,
The ground conductor is not formed above the first recess and the second recess, and the conductor is formed above the third recess so that the conductor is thinner than the thickness of the center conductor. And ground conductors having substantially the same thickness as the center conductor or the ground conductor facing the center conductor are alternately formed in the direction of the optical waveguide,
In addition, the ground conductor formed on the substrate on the side that is not the ridge portion for the central conductor adjacent to the second concave portion has a position symmetrical to the third concave portion at the center line of the central conductor. The ground conductor formed thinner than the thickness of the center conductor and the ground conductor having the same thickness as the center conductor or the ground conductor opposite to the center conductor are alternately formed in the optical waveguide direction. Characteristic light modulator. A ground conductor formed above the third recess, the portion formed to be thinner than the thickness of the center conductor is a first thin connection ground conductor, and is opposed to the center conductor or the center conductor. A portion having the same thickness as the ground conductor to be used is the first thick connecting ground conductor,
A ground conductor formed at a position symmetrical to the third recess on the center line of the center conductor, wherein a portion formed thinner than the thickness of the center conductor is a second thin connection ground conductor; When a second thick connecting ground conductor is a portion having substantially the same thickness as the center conductor or the ground conductor facing the center conductor,
2. The optical modulator according to claim 1, wherein the first thin connecting ground conductor and the second thin connecting ground conductor are formed symmetrically with respect to a center line of the center conductor. A ground conductor formed above the third recess, the portion formed to be thinner than the thickness of the center conductor is a first thin connection ground conductor, and is opposed to the center conductor or the center conductor. A portion having the same thickness as the ground conductor to be used is the first thick connecting ground conductor,
A ground conductor formed at a position symmetrical to the third recess on the center line of the center conductor, wherein a portion formed thinner than the thickness of the center conductor is a second thin connection ground conductor; When a second thick connecting ground conductor is a portion having substantially the same thickness as the center conductor or the ground conductor facing the center conductor,
The at least one of the first thin connecting ground conductor and the second thin connecting ground conductor in the direction of the optical waveguide is formed differently. 2. The optical modulator according to 1.   A predetermined distance away from the third recess, which is a region where the electromagnetic field of the high-frequency electrical signal is reduced, on a ground conductor formed on the substrate that is not adjacent to the ridge portion for the ground conductor adjacent to the third recess. 4. The optical modulator according to claim 1, wherein a thickness in the region is smaller than a thickness of the ground conductor in a region other than the region. 5.   A ground conductor formed on a substrate adjacent to the second recess and not the ridge portion for the central conductor is separated from the second recess, which is a region where the electromagnetic field of the high-frequency electric signal is reduced, by a predetermined distance. 5. The optical modulator according to claim 1, wherein a thickness in the region is smaller than a thickness of the ground conductor in a region other than the region.   6. The optical modulator according to claim 1, wherein the substrate is made of lithium niobate.   The optical modulator according to claim 1, wherein the substrate is made of a semiconductor.

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