JP4754608B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator that modulates light incident on an optical waveguide and emits it as an optical signal using an electro-optic effect or a thermo-optic effect.

  There is an optical modulator using a dielectric material as a typical optical modulator. In recent years, high-speed and large-capacity optical communication systems have been put into practical use, and development of high-performance optical modulators to be incorporated into such high-speed and large-capacity optical communication systems is required.

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, characteristics and problems of a conventional LN optical modulator using the electro-optic effect of lithium niobate will be considered.

(First prior art)
In recent years, a 40 Gbit / s ultra-high capacity optical communication system that has been developed uses a phase modulator type LN optical modulator such as DPSK or DQPSK whose principle is disclosed in Patent Document 1, for example. Yes.

  For example, FIGS. 8 and 9 show schematic structural diagrams of an LN optical modulator that embodies a method of generating a DQPSK signal disclosed in Patent Document 1. FIG. Here, only the optical waveguide is shown in FIG. 8 for easy understanding. As described above, the optical waveguide of the DQPSK LN optical modulator is composed of two small Mach-Zehnder interference systems (or Mach-Zehnder optical waveguides) and one large Mach-Zehnder interference system that synthesizes the two small Mach-Zehnder interference systems.

In the figure, 1 is a z-cut LN substrate, 2 is a SiO ₂ buffer layer, and 3 is an optical waveguide. Reference numerals 3a, 3b, 3c, and 3d denote optical waveguides in which light interacts with a high-frequency electrical signal propagating through a traveling wave electrode described in FIG. 9, and this interacting region I is defined as an interaction region (or interaction unit). ), The optical waveguides 3a, 3b, 3c, and 3d in this region are called interaction optical waveguides.

9 includes, in addition to the optical waveguide 3 shown in FIG. 8, light propagating through the interaction optical waveguides 3a, 3b, 3c, and 3d and high-frequency electric signals Sa, Sb, Sc, interacting in the interaction region I. A traveling wave electrode for propagating Sd is also shown. For simplification of explanation, the Si conductive layer for suppressing temperature drift is omitted. The thickness of the SiO ₂ buffer layer 2 is generally around 1 μm.

  As a traveling wave electrode applied to an LN optical modulator, a CPW (coplanar line or coplanar-waveguide) structure disclosed in Patent Document 2 is generally widely used. The interaction part of this CPW type traveling wave electrode requires a ground electrode in addition to the center electrode. However, in order to make the explanation easy to understand, only four center electrodes 4a, 4b, 4c and 4d are shown in FIG. The ground electrode is omitted (the ground electrode is shown in detail in FIG. 10 below). Reference numeral 7 denotes a center electrode of the π / 2 shift electrode, and similarly the ground electrode is omitted.

  When the high-frequency electrical signals Sa, Sb, Sc, Sd are input to the LN optical modulator, these high-frequency electrical signals reach the interaction region I via the input-side feed section shown as II in FIG. Interacts with light.

  Now, the problem of the conventional DQPSK type LN optical modulator shown in FIG. 9 will be considered. The length of the LN optical modulator chip is as long as about 50 mm, but in order to obtain many chips from one wafer, its width (perpendicular to the length direction of the interaction optical waveguides 3a, 3b, 3c, and 3d). And the length in the direction parallel to the surface of the z-cut LN substrate 1) is as short as 2 mm to 5 mm. For this reason, the width of this region that can be assigned to the input-side feed unit II in the direction parallel to the surface of the z-cut LN substrate 1 is also relatively narrow.

  10 shows a detailed cross-sectional structure of the input side feed section II in FIG. 9 (perpendicular to the length direction of the interaction optical waveguides 3a, 3b, 3c and 3d and parallel to the surface of the z-cut LN substrate 1). Direction sectional view). As can be seen from the figure, this structure is a planar (or planar) CPW electrode structure.

  10 also shows the ground electrode omitted in FIG. Here, the width of the center electrodes 4a, 4b, 4c and 4d is S, the gap between the center electrodes 4a, 4b, 4c and 4d and the ground electrodes 5a, 5b, 5c, 5d and 5e is W, and the ground electrodes 5b, 5c, Let G be the width of 5d. By making the width G of the ground electrode wider than the gap W, crosstalk between the center electrodes 4a, 4b, 4c, and 4d (or between the high-frequency electrical signals Sa, Sb, Sc, and Sd) can be suppressed. . That is, the ratio between the width G of the ground electrodes 5b, 5c, and 5d and the gap W greatly affects the electrical crosstalk between high-frequency electrical signals. In general, it is desirable that the width G of the ground electrode be 4 to 5 times the gap W in order to sufficiently reduce the electrical crosstalk.

In FIG. 11, the horizontal axis indicates the gap W between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e, and the left vertical axis indicates the characteristic impedance Z of the CPW traveling wave electrode. The right vertical axis indicates the width G of the ground electrodes 5b, 5c, and 5d. As can be seen, the characteristic impedance Z of the traveling wave electrode gap W ₁ in the ground electrode 5b becomes 50Ω matching the characteristic impedance of the external circuit (not shown), 5c, the width G of 5d becomes narrow. In addition, when the width G of the ground electrodes 5b, 5c, and 5d is increased, the gap W between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e becomes narrow (for example, at that time) When W is W ₂ , W ₂ <W ₁ ), and the characteristic impedance Z of the traveling wave electrode is significantly lower than 50Ω.

In FIG. 12, the horizontal axis indicates the gap W between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e, and the vertical axis indicates the high-frequency electrical signals Sa, Sb, and Sc from the traveling wave electrodes. , Sd represents an electric power reflection coefficient (or electric power reflectivity) S ₁₁ that returns to an external electric circuit (not shown). As described in FIG. 11, since the characteristic impedance Z of the traveling wave electrode in the case of the gap W ₁ is 50Ω, the electric power reflection coefficient S ₁₁ becomes extremely small. On the other hand, the center electrode 4a, 4b, 4c, 4d and the ground electrode 5a, 5b, 5c, 5d, the gap between 5e is narrow and _{W 2,} the characteristic impedance Z of the traveling-wave electrode is much lower than 50 [Omega (e.g., 30 [Omega Therefore, as shown in FIG. 12, the electric power reflection coefficient S ₁₁ becomes large at about −10 dB or more, which is not preferable as a traveling wave electrode for an LN optical modulator that performs high-speed optical modulation.

In FIG. 13, the horizontal axis indicates the gap W between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e, and the left vertical axis indicates the center electrode 4a of the CPW traveling wave electrode. 4b, 4c, 4d (or high-frequency electrical signals Sa, Sb, Sc, Sd). As in FIG. 11, the right vertical axis indicates the width G of the ground electrodes 5b, 5c, and 5d. As mentioned in FIG. 11, the characteristic impedance of the traveling wave electrode gap _{W 1} in the ground electrode 5b becomes 50 [Omega, 5c, the width G of 5d becomes narrow. For example, when the width G of the ground electrodes 5b, 5c, and 5d is reduced to the same extent as the widths of the center electrodes 4a, 4b, 4c, and 4d, the electrical crosstalk between the center electrodes 4a, 4b, 4c, and 4d is −10 dB. Deteriorating to a degree or more, it is not preferable as a traveling wave electrode. In addition, although the electrical crosstalk is excellent at −20 dB or less in the gap W ₂ having a characteristic impedance lower than 50Ω shown in FIG. 11, the electrical power reflection coefficient S ₁₁ deteriorates, and this also proceeds. It is not preferable as a wave electrode.

As described above, the width that can be assigned to the input-side feed unit II (in a direction perpendicular to the length direction of the interaction optical waveguides 3a, 3b, 3c, and 3d and parallel to the surface of the z-cut LN substrate 1) The width to which the input side feed unit II can be allocated is limited. Therefore, the center electrode 4a described in the above, 4b, 4c, to some extent suppressed crosstalk between 4d, and when you suppress the deterioration of the electric power reflection coefficient _{S 11,} the center electrode 4a, 4b, 4c, 4d of the width It is conceivable to narrow S. However, the center electrode 4a, 4b, 4c, the width S of 4d narrow (e.g., in Figure 14 and _{S 1)} Then, a high frequency electric signal Sa, as shown in FIG. 14, Sb, Sc, the propagation loss α of Sd large Therefore, as shown in FIG. 15, the power modulation index | m | ² of the LN optical modulator is significantly deteriorated with the frequency (f) (in other words, the 3 dB optical modulation band is narrowed), and 40 Gbit / s. As an LN optical modulator, it is extremely undesirable.

For example, the center electrode 4a, 4b, 4c, when a 5μm width S of 4d, suppress central electrode 4a, 4b, 4c, both electrical crosstalk and electrical power reflection coefficient _{S 11} between 4d to -20dB or less However, since the propagation loss of the high-frequency electrical signal in this feed portion increases remarkably, the 3 dB band as the LN optical modulator deteriorates to about 10 GHz, and it is difficult to apply to 40 Gbit / s optical transmission. .
JP-T-2004-516743 Japanese Patent No. 2126214

As described above, the input side feed portion of the traveling wave electrode of the LN optical modulator has a planar CPW structure. Therefore, in order to realize an appropriate characteristic impedance (most preferably 50Ω) as the input side feed section, it is necessary to widen the gap (W in FIG. 10) between the center electrode and the ground electrode. As a result, the width G of the ground electrodes 5b, 5c, and 5d shown in FIG. 10 is reduced, and electrical crosstalk occurs between the high-frequency electrical signals that propagate through the traveling wave electrodes. Further, if the width G of the ground electrodes 5b, 5c, and 5d is increased in order to suppress this electrical crosstalk, the gap (W in FIG. 10) between the center electrode and the ground electrode is reduced. As a result, the characteristic impedance Z of the traveling-wave electrode is lowered significantly, the electrical power reflection coefficient S ₁₁ is deteriorated. Furthermore, in order to suppress both the reduction of the characteristic impedance and the deterioration of the electrical crosstalk between the high-frequency electric signals, the characteristic impedance of the traveling wave electrode is set close to 50Ω, and the gap between the center electrode and the ground electrode (see FIG. 10), if the width G of the ground electrodes 5b, 5c, 5d shown in FIG. 10 is set to be wide, the gap W is inevitably narrowed. As a result, the width of the central electrode of the traveling wave electrode is also reduced. It became narrower. When the width of the center electrode is narrowed, the propagation loss of the high frequency electric signal is increased. In other words, if the feed section made of a conventional planar CPW structure is designed to suppress both a decrease in characteristic impedance and a deterioration in electrical crosstalk between high-frequency electrical signals, the propagation loss of high-frequency electrical signals increases. As a LN optical modulator, optical modulation at high speed has been difficult.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an optical modulator that is small in size, has low electrical crosstalk and electrical power reflectivity of a high-frequency electrical signal, and has a wide optical modulation band. And

In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate, an optical waveguide formed on the substrate, a center electrode and a ground electrode formed on the substrate for applying a voltage. and a traveling wave electrode made of, said optical waveguide comprises a plurality of interaction optical waveguides to the interaction portion which is a region of which the refractive index is changed by applying a voltage to said traveling wave electrode, before Symbol progress in the optical modulator frequency electric signal is inputted from the input side feed portion of the wave electrode to the interaction portion of the traveling wave electrode, together with the center electrode of the traveling-wave electrode is formed of a plurality, the plurality of center electrodes are each of the ground electrodes on both sides forming the center electrode of the input side feed portion is formed so as to be folded back relative to the center electrode of the interaction portion, the characteristic impedance of the previous SL traveling wave electrode and the desired value Na As such, dig the thickness of the substrate between the center electrode and the ground electrode are opposed to each other with a gap W'in the region where the central electrode of the input side feed unit is formed as folded The center of the input-side feed section when the gap W ′ is a planar structure in which the characteristic impedance becomes the desired value without the ridge structure is formed. electrode and the narrowly than the gap W between the ground electrode Rutotomoni, width G'of the ground electrode in the region formed such that the center electrode of the input side feed portion is folded back at the plane type structure It becomes What Do wider than the width G of the ground electrode in the input-side feed of a certain case, and, if crosstalk between the high frequency electric signal is of the planar structure As to be reduced, the ratio of the ratio of the width G'said gap W'of Oite the ground electrode to the center area where the electrodes Ru is formed so as to be folded back of the input side feed portion in the case of the planar structure And is configured to be large.

The optical modulator according to claim 2 of the present invention is etched so that the thickness of the substrate between all the center electrodes in the input side feed section and the ground electrode facing each other is reduced. The ridge structure is used.

The optical modulator according to claim 3 of the present invention is etched so that the thickness of the substrate between a part of the center electrodes in the input-side feed section and the ground electrode facing the center electrode is reduced. And having the ridge structure.
The optical modulator according to claim 4 of the present invention is characterized in that the gap W ′ is not less than 30 μm and not more than 60 μm.
The optical modulator according to claim 5 of the present invention is characterized in that a width S ′ of the center electrode in a portion of the input side feed portion forming the ridge structure is 10 μm or more and 50 μm or less.

  In the present invention, a ridge structure in which a z-cut LN substrate in the vicinity of the center electrode in the input side feed portion of the traveling wave electrode is etched is introduced. Since the ridge structure has the effect of increasing the characteristic impedance of the traveling wave electrode, a characteristic impedance close to 50Ω can be obtained even if the gap W between the center electrode and the ground electrode is relatively narrow. Since the gap W can be narrowed, the width of the ground electrode can be increased, and electrical crosstalk between high-frequency electrical signals can be reduced. Furthermore, the fact that the gap W between the center electrode and the ground electrode can be reduced also means that the width of the center electrode of the input side feed section can be increased. And by widening the width of the center electrode of the input side feed section, it is possible to reduce the propagation loss of the high frequency electric signal in this region, so that the electric power reflectivity and the electric crosstalk are suppressed while being remarkably reduced. An optical modulator capable of high-speed modulation can be realized.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the related art shown in FIGS. 8 to 15 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

(Embodiment)
FIG. 1 shows a top view of one embodiment of the optical modulator of the present invention. Since DQPSK is taken as an example of the optical modulation system, the structure of the optical waveguide is the same as that of the prior art shown in FIG.

  Further, corresponding to FIG. 10, FIG. 2 shows a cross-sectional view of the input side feed section III of FIG. 1 (perpendicular to the length direction of the interaction optical waveguides 3a, 3b, 3c, 3d and z-cut LN 1 is a cross-sectional view in a direction parallel to the surface of the substrate 1. FIG. 2 also shows the ground electrode omitted in FIG. In order to simplify the explanation, the Si conductive layer for suppressing temperature drift is omitted.

  The principle of the present invention will be described in detail with reference to FIG. As can be readily understood by comparing FIG. 2 which is an embodiment of the present invention and FIG. 10 which is the prior art, in the present invention, the center electrodes 4a, 4b, 4c and 4d and the ground electrodes 5a, 5b, 5c, 5d and 5e A so-called ridge structure is formed by etching the z-cut LN substrate 1 therebetween.

  That is, in FIG. 2, 6a, 6b, 6c, 6d, 6e, 6f, 6g, and 6h are etching grooves, and a material having a relative dielectric constant lower than that of the z-cut LN substrate 1 (the relative dielectric constant is z−). Any material may be used as long as it is lower than the cut LN substrate 1, but the characteristic impedance Z of the feed portion is increased by filling it with air that is most effective here. In the present invention, as the depth of the etching grooves 6a, 6b, 6c, 6d, 6e, 6f, 6g, and 6h increases (in other words, the height of the ridge increases), the characteristic impedance as the traveling wave electrode is increased. And the effect of the invention is remarkably exhibited.

  Here, the width of the center electrodes 4a, 4b, 4c, 4d is S ', the gap between the center electrodes 4a, 4b, 4c, 4d and the ground electrodes 5a, 5b, 5c, 5d, 5e is W', the ground electrode 5b, The width of 5c and 5d is G ′. When the width G ′ of the ground electrode is increased compared to the gap W ′, electrical crosstalk between the center electrodes 4a, 4b, 4c, and 4d (or between the high-frequency electrical signals Sa, Sb, Sc, and Sd) is caused. Can be suppressed. That is, the ratio between the width G ′ and the gap W ′ of the ground electrodes 5b, 5c, and 5d has a great influence on the electrical crosstalk between the high-frequency electrical signals. In general, it is desirable that the width G ′ of the ground electrode be 4 to 5 times the gap W in order to sufficiently reduce the electrical crosstalk.

  In FIG. 3, the horizontal axis indicates the gap W ′ between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e, and the left vertical axis indicates the characteristic impedance Z of the CPW traveling wave electrode. The right vertical axis represents the width G ′ of the ground electrodes 5b, 5c, and 5d, and these relationships are shown. In order to explain the effect of the present invention, the characteristic impedance according to the prior art shown in FIG. 11 is indicated by a broken line.

  In the present invention, a ridge structure having an effect of increasing the characteristic impedance of the traveling wave electrode is employed. Therefore, as is apparent from FIG. 3, the characteristic impedance Z of the traveling wave electrode in the present invention is higher than that of the prior art with respect to the gap between the center electrode and the ground electrode.

That is, the gap W ₁ ′ in which the characteristic impedance Z of the traveling wave electrode coincides with the characteristic impedance 50Ω of the external circuit (not shown) can be significantly reduced to, for example, about 30 to 60 μm, and therefore the width G ′ of the ground electrodes 5b, 5c, and 5d. In addition, since the gap W ₁ ′ is small in the first place, the ratio between the width G ′ of the ground electrode and the gap W ₁ ′, which determines the electrical crosstalk between the electrical signals, is determined according to the prior art. (For example, even if the G / W value is about 1 to 2 in the prior art, the G ′ / W ′ value is increased from 4 to 5 in the present invention). Is possible).

In FIG. 4, the horizontal axis indicates the gap W ′ between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e, and the vertical axis indicates the high-frequency electrical signals Sa, Sb, Sc, and Sd. showing the electric power reflection coefficient S ₁₁ to return from the traveling wave electrode to an external electrical circuit (not shown). As described in FIG. 3, in the present invention, even if the value of the gap W ₁ ′ is reduced, the characteristic impedance Z of the traveling wave electrode can be made approximately 50Ω, so that the electric power reflection coefficient S ₁₁ is −20 dB or less. It becomes possible to make it extremely small.

Incidentally, in FIG. 4 shows an electrical power reflection coefficient S ₁₁ according to the prior art by the dashed line. As described with reference to FIG. 12, in the conventional technique, when the electrical crosstalk of the high-frequency electrical signal is sufficiently suppressed, the characteristic impedance Z of the traveling wave electrode is, for example, slightly lower than 30Ω and lower than 50Ω, and the electrical power reflection coefficient S ₁₁ Deteriorated. Thus, by applying the present invention, the characteristic impedance Z of the traveling wave electrode can be reduced even if the gap between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e is reduced. Since it can be set to 50Ω, the electric power reflection coefficient S ₁₁ can be greatly improved.

  In FIG. 5, the horizontal axis indicates the gap W ′ between the center electrodes 4a, 4b, 4c, and 4d and the ground electrodes 5a, 5b, 5c, 5d, and 5e, and the left vertical axis indicates the center electrode 4a of the CPW traveling wave electrode. 4b shows electrical crosstalk between 4b, 4c and 4d. As in FIG. 3, the right vertical axis represents the width G ′ of the ground electrodes 5b, 5c, and 5d.

  In the present invention, even if the characteristic impedance of the input side feed section III is set to approximately 50Ω having good matching with an external circuit (not shown), the center electrodes 4a, 4b, 4c, 4d and the ground electrodes 5a, 5b, 5c, 5d, 5e In addition, the gap W ′ can be set narrow, and since the gap W ′ is narrow, the width G ′ of the ground electrodes 5b, 5c, and 5d can be increased (for example, about 200 to 250 μm). Therefore, since the ratio of the width G ′ of the ground electrode and the gap W ′ can be increased to 4 to 5, the electrical crosstalk of the high-frequency electric signal can be greatly improved so as to be −20 dB or less. It becomes possible.

Furthermore, the fact that the gap W ′ between the center electrode and the ground electrode can be reduced also means that the width S ′ of the center electrode of the input side feed section III can be increased. Then, by increasing the width S ′ of the center electrode of the input side feed portion, it is possible to reduce the propagation loss of the high frequency electric signal in this region as shown in FIG. For example, when the width S ′ of the center electrode of the input side feed part III is 10 μm to 50 μm, the high-speed optical modulation that is remarkably improved as compared with the prior art while suppressing the electric power reflectivity and the electric crosstalk. Can be realized. FIG. 6 shows the frequency (f) dependence of the power modulation index | m | ² of the LN optical modulator when the width S ′ of the center electrode is 30 μm. It can be seen that the frequency dependency of the present embodiment is greatly improved as compared with the prior art shown in FIG.

(Each embodiment)
In the above description, the DQPSK optical modulator has been described as an example. However, since the present invention is effective for an LN optical modulator having a plurality of center electrodes in the input side feed section, the DPSK is not limited to DQPSK, but includes a single Mach-Zehnder optical waveguide, It goes without saying that the present invention can also be applied to other phase modulation systems having more Mach-Zehnder optical waveguides than DQPSK, and further to a two-electrode type intensity modulator.

  In addition, the z-cut LN substrate is etched and ridge-processed by sandwiching all the center electrodes of the input-side feed portion in the traveling wave electrode, but only the z-cut LN substrate in the vicinity of some of the center electrodes is ridged. It may be processed. That is, a ridge type and a planar type electrode structure may be combined. In that case, since the equivalent refractive indexes of the high-frequency electrical signals propagating through the traveling wave electrode of the feed portion are different from each other, electrical crosstalk can be further suppressed. Note that the ridge depth of about 2.5 to 5 μm is effective, but for example, when the depth is about 10 μm, the effect is remarkable.

In the above description, although characteristic impedance of the traveling wave electrode case has been described where a 50 [Omega, the effect of the present invention is not limited thereto.

In other words, by introducing at least a portion of the ridge structure of the input side feed portion of the traveling-wave electrode, as long as the characteristic impedance in comparison with the planar structure without a ridge structure is high, characteristics of the traveling-wave electrode Even if the impedance is lower or higher than 50Ω, the effect of the present invention can be exhibited.

For example, to set somewhat lower than 50Ω the characteristic impedance of the traveling wave electrodes, since the gap between the center electrode and the ground electrode can be narrowed, it is possible to set the width of the ground electrode, high-frequency electrical Crosstalk between signals can be improved. Conversely, when the characteristic impedance of the traveling-wave electrode is set higher than 50 [Omega, the current value of the high frequency electric signal propagating through the input-side feed unit is reduced, it is possible to reduce the attenuation due to Joule heat, the As a result, high-speed light modulation is possible.

  Furthermore, different characteristic impedances can be set for the plurality of center electrodes of the input side feed section in the traveling wave electrode. In other words, the fact that the characteristic impedance is different means that the equivalent refractive index of the high-frequency electric signal is also different, so that the electrical crosstalk can be improved from the electromagnetic field coupling theory.

  Note that the characteristic impedance can be further increased by making the thickness of the buffer layer of the input side feed portion thicker than the thickness of the buffer layer of the interaction portion where the high frequency electrical signal and light interact. Therefore, the effect of the present invention can be further enhanced by combining this concept with the present invention.

Moreover, although the z-cut LN substrate has been described, it may be an LN substrate of another cut such as an x-cut, a y-cut, or a cut obtained by mixing them, or another substrate such as a semiconductor substrate. Although the buffer layer has been described as SiO ₂ , it goes without saying that other materials such as Al ₂ O ₃ , SiN, or SiO _x may be used.

  As the electrode configuration, a configuration using a CPW electrode having a symmetrical structure has been described. However, a CPW electrode having an asymmetric structure may be used, and other configurations such as an asymmetric coplanar strip (ACPS) or a symmetric coplanar strip (CPS) may be used. good. Needless to say, the traveling wave electrode structure in the interaction portion may be a ridge structure or a planar structure.

  As described above, according to the present invention, it is possible to provide an optical modulator that is significantly improved with respect to characteristic impedance, that is, electrical power reflectivity, and electrical crosstalk, and also propagation loss of high-frequency electrical signals, that is, optical modulation band. .

1 is a top view including a traveling wave electrode constituting an optical modulator according to an embodiment of the invention Cross section of the input side feed section in FIG. The figure explaining the principle of this invention The figure explaining the principle of this invention The figure explaining the principle of this invention The figure explaining the principle of this invention The figure explaining the principle of this invention Top view showing configuration of optical waveguide for DQPSK optical modulator according to prior art Top view including traveling wave electrodes for a prior art DQPSK optical modulator Cross section of the input side feed section in FIG. Diagram explaining the problems of the prior art Diagram explaining the problems of the prior art Diagram explaining the problems of the prior art Diagram explaining the problems of the prior art Diagram explaining the problems of the prior art

Explanation of symbols

1: z-cut LN substrate (substrate)
2, 9: SiO ₂ buffer layer 3, 30: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b, 3c, 3d: interaction optical waveguides 4a, 4b, 4c, 4d, 7: center electrodes 5a, 5b, 5c, 5d, 5e: ground electrodes 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h: Etching groove

Claims (5)

A substrate, an optical waveguide formed on the substrate, and a traveling wave electrode formed on the substrate and including a center electrode and a ground electrode for applying a voltage, and the optical waveguide has a voltage applied to the traveling wave electrode comprising a plurality of interaction optical waveguides to the interaction portion which is a region of which the refractive index is changed by applying a high frequency electric interaction of the traveling wave electrode from the input side feed portion of the front Symbol traveling wave electrode In an optical modulator to which a signal is input,
The traveling-wave electrode includes a plurality of the center electrodes, and the ground electrodes are formed on both sides of the plurality of center electrodes, and the center electrode of the input-side feed unit is connected to the center electrode of the interaction unit. Formed to be folded
As the characteristic impedance of the previous SL traveling wave electrode becomes a desired value, and the center electrode are opposed to each other with a gap W'in the region where the central electrode of the input side feed unit is formed as folded A plane in which the thickness of the substrate between the ground electrode and the ground electrode is dug down to form a ridge structure, and the gap W ′ does not have the ridge structure and the characteristic impedance becomes the desired value. It said center electrode and said narrowly than the gap W between the ground electrode Rutotomoni, an area where the central electrode of the input side feed unit is formed as folded on the input side feed portion of the case is a mold structure the width G'of the ground electrode is made I Do wider than the width G of the ground electrode in the input-side feed unit when a said planar structure in, and the high frequency As crosstalk between the gas signal is reduced as compared with the case of the planar structure, the width G'of the center electrode Oite the ground electrode formed Ru region so as to be folded back of the input side feed unit An optical modulator characterized in that the ratio of the gap W ′ is made larger than that in the case of the planar structure.   All the center electrodes in the input-side feed section are etched so that the thickness of the substrate between the center electrode and the ground electrode opposite to the center electrode is reduced, thereby forming the ridge structure. The optical modulator according to claim 1.   A part of the center electrode in the input-side feed section is etched so that the thickness of the substrate between the center electrode and the ground electrode facing the center electrode is reduced, thereby forming the ridge structure. The optical modulator according to claim 1.   The optical modulator according to any one of claims 1 to 3, wherein the gap W 'is not less than 30 µm and not more than 60 µm.   5. The light according to claim 1, wherein a width S ′ of the center electrode in a portion of the input-side feed portion forming the ridge structure is 10 μm or more and 50 μm or less. Modulator.

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