JP2006317551A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator which can be driven at a high speed with a low voltage and can suppress DC drift, having high reliability and a good yield of manufacturing. <P>SOLUTION: The optical modulator is provided with: a substrate 1 having an electro-optical effect and including at least a face azimuth of x-cut or y-cut; an optical waveguide 3 for wave-guiding the light formed in the substrate; an electrode 4 consisting of a central conductor 4a and grounding conductors 4b, 4c formed on one face side of the substrate for applying voltage to modulate the light; and interactive parts 3a, 3b for the optical waveguide 3 to modulate the phase of the light by applying voltage across the central conductor 4a and the grounding conductors 4b, 4c. The optical modulator is also provided with: a conductive layer 15 which is electrically in contact with a part of the substrate upper surface and is formed so as not to cover the upper surface of the optical waveguide 3; a second buffer layer 16 formed above the conductive layer 15; and the central conductor 4a formed on the second buffer layer 16 in npn-contact with the conductive layer 15 in the interactive parts 3a, 3b. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical modulator, and more particularly, to an optical modulator that has a high driving speed, a low driving voltage, a small DC drift, and a high manufacturing yield.

As is well known, in an optical modulator, a substrate having a so-called electro-optic effect in which a refractive index is changed by applying an electric field, such as lithium niobate (LiNbO ₃ ) (hereinafter, a lithium niobate substrate is referred to as an LN substrate). A traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as an LN optical modulator) in which an optical waveguide and a traveling wave electrode are formed is 2.5 Gbit / s, 10 Gbit / s because of its excellent chirping characteristics. It is applied to large-capacity optical transmission systems.

  Such an LN optical modulator has recently been studied for application to an ultra large capacity optical transmission system of 40 Gbit / s, and is expected as a key device in the large capacity optical transmission system.

  This LN optical modulator includes a type using a z-cut LN substrate and a type using an x-cut LN substrate (or a y-cut LN substrate).

  FIG. 9 shows a perspective view of an x-cut substrate LN optical modulator using an x-cut LN substrate and a coplanar waveguide (CPW) traveling wave electrode as a first prior art. FIG. 10 is a cross-sectional view taken along the line A-A ′ of FIG. 9.

In the figure, reference numeral 1 is an x-cut LN substrate, reference numeral 2 is a transparent SiO ₂ buffer layer having a thickness of about 200 nm to 1 μm in a wavelength region used in optical communication such as 1.3 μm or 1.55 μm, reference numeral 3 Is an optical waveguide formed by thermally diffusing Ti at about 1050 ° C. for 10 hours after vapor deposition of Ti on the x-cut LN substrate 1, and this optical waveguide 3 constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). .

  In the figure, reference numerals 3a and 3b denote optical waveguides (or interaction optical waveguides) at a portion where an electrical signal and light interact (referred to as an interaction portion), that is, two arms of a Mach-Zehnder optical waveguide. The optical waveguide 3 is branched by a Y-branch optical waveguide (not shown).

  In the figure, reference numeral 4 denotes a CPW traveling wave electrode, and this CPW traveling wave electrode 4 is composed of a central conductor 4a and ground conductors 4b and 4c.

Here, the buffer layer 2 is expanded by bringing the effective refractive index n _m of the electric signal or microwaves in the traveling-wave electrode 4 optical waveguide 3a, the equivalent refractive index n ₀ of the light propagating through 3b, and the optical modulation band It plays an important role.

  Next, the operation of the LN optical modulator configured as described above will be described. To operate the LN optical modulator, a direct current bias (hereinafter referred to as a DC bias) is provided between the center conductor 4a and the ground conductors 4b and 4c. And a high-frequency electrical signal (hereinafter referred to as an RF electrical signal) need to be applied.

  In the voltage-light output characteristics shown in FIG. 11, the curve shown by the solid line is the voltage-light output characteristics of the LN optical modulator in a certain state, and Vb is the DC bias voltage at that time.

  As shown in FIG. 11, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

  Generally, a DC bias voltage of 5V to 10V is normally applied between the center conductor 4a and the ground conductors 4b and 4c, but the gap between the center conductor 4a and the ground conductors 4b and 4c is usually about 10 μm to 40 μm. Therefore, an electric field as high as several hundred kV / m to 1000 kV / m is applied between the center conductor 4a and the ground conductors 4b and 4c.

Although the x-cut LN substrate 1 does not have a pyroelectric effect, the charge induced in the buffer layer 2 made of an oxide such as SiO ₂ moves under such a high electric field. Since the voltage-light output characteristic changes in the direction indicated by the arrow as shown by the dashed curve, it is necessary to change the setting of the DC bias point from Vb to Vb ′.

  The change of the DC bias point V when the environmental temperature is constant is called DC drift.

  Next, the DC drift phenomenon will be considered using the equivalent circuit diagram of the x-cut substrate LN optical modulator shown in FIG. 9 and FIG. 10 shown in FIG. In FIG. 12, only the left half is shown in consideration of the symmetry of the x-cut substrate LN optical modulator shown in FIGS.

Here, C _B and R _B represent the equivalent capacitance and resistance of the buffer layer 2, and C _LN and R _LN represent the equivalent capacitance and resistance of the LN substrate 1 including the optical waveguide 3a, respectively.

Although the distribution of the voltage at the instant the voltage is applied is determined by the capacitance C _LN capacitance C _B and LN substrate 1 a buffer layer 2, the applied voltage and the resistance R _B of the buffer layer 2 over time, divided by the resistance _{R LN} of LN substrate 1.

For example, when the resistance R _B of the buffer layer 2 is greater than the resistance R _LN of LN substrate 1, since the number of the applied voltage is applied to the resistance R _B of the buffer layer 2, x- cut LN resistor substrate 1 R The voltage drop at _LN is small.

In this case, even if the DC bias voltage Vb is applied, it is not so much applied to the resistor R _LN of the x-cut LN substrate 1. That is, since a large applied voltage does not act on the optical waveguides 3a and 3b, a larger value is required as the DC bias voltage Vb. This is called positive DC drift.

  When the DC bias point Vb 'whose setting has been changed exceeds the range that can be controlled by the electric control circuit, it cannot be electrically controlled. Therefore, it is extremely important to reduce this DC drift.

  In order to solve this DC drift, for the z-cut LN substrate, for example, a configuration example shown in the cross-sectional view of FIG. 13 is disclosed in Patent Document 1 as a second prior art.

Here, reference numeral 5 is a z-cut LN substrate, reference numerals 6a and 6b are SiO ₂ buffer layers, reference numeral 7 is a conductive layer made of a Si film having a thickness of about 30 nm to 100 nm, and reference numerals 8a and 8b are central conductors, respectively. And corresponding to ground conductor.

  In this configuration example, buffer layers 6 a and 6 b are formed only directly above two optical waveguides, and the buffer layers 6 a and 6 b are covered with a conductive layer 7.

  However, when the space between the center conductor 8a and the ground conductor 8b is covered with the conductive layer 7 in this way, the DC drift is caused by the resistance of the conductive layer 7 in the direction perpendicular to the substrate surface of the z-cut LN substrate 5 and the center conductor 8a. And the resistance ratio of the conductive layer 7 between the ground conductor 8b is simply determined.

However, usually, the size of the gap between the center conductor 8a and the ground conductor 8b, the electrical characteristics of the traveling-wave electrode, that is, we should decide from microwave equivalent refractive index n _m and the characteristic impedance Z, FIG. In the configuration of 13, the degree of freedom is small in designing the amount of DC drift, and in general, the resistance ratio is insufficient to sufficiently suppress the DC drift.

  Further, while the structure shown in FIG. 13 is for a z-cut LN substrate, as shown in FIG. 9 and FIG. 10, in the x-cut LN substrate, light is transmitted between the center conductor 4a and the ground conductors 4b and 4c. Since the waveguides 3a and 3b are present, the second prior art cannot be applied.

  In addition, it is not preferable to directly deposit the conductive layer 7 having a large light propagation loss on the optical waveguide from the viewpoint of the light propagation loss to the x-cut LN substrate.

In addition, the resistivity of the conductive layer 7 made of Si or SiOx needs to be set to 10 ⁷ to 10 ¹⁰ Ω · cm. In order to specifically set this value, a heat treatment of several hundred degrees C. is performed after depositing Si. Need to do.

  However, in this case, it is difficult to control the resistivity and there are problems in cost and yield. In addition, when the resistivity of the conductive layer 7 is significantly lower than the design value, the center conductor 4a and the ground conductor There is a problem that an electrical short circuit occurs between 4b and 4c and the element is destroyed.

  The configuration example shown in the sectional view of FIG. 14 is a z-cut LN substrate according to the third prior art disclosed in Patent Document 2.

In this configuration example, the SiO ₂ buffer layers 6a and 6b cover the optical waveguides 3a and 3b, and the center conductor 8a and the ground conductor 8b are located above the optical waveguide. It is a structure about a cut LN substrate.

  In the figure, reference numerals 12a, 12b, and 12c denote conductive layers. In the third prior art, a plurality of conductive layers 12b are formed between the pair of center conductors 8a and the ground conductor 8b. Insulating portions 13a and 13b are provided.

The LN optical modulator according to the third prior art includes a symmetric coplanar strip (CPS) comprising a center conductor 8a and a ground conductor 8b as traveling wave electrodes. The SiO ₂ buffer layers 6a and 6b cover the optical waveguides 3a and 3b, and the center conductor 8a and the ground conductor 8b are located above the optical waveguide, and the z-cut LN substrate 5 is similar to the configuration of the second prior art. This is the structure.

  In the figure, reference numerals 12a, 12b, and 12c denote conductive layers. In the third prior art, a plurality of conductive layers 12b are formed between the pair of center conductors 8a and the ground conductor 8b. Insulating portions 13a and 13b are provided.

  However, since the conductive layer 12b is electrically floating in the third prior art, the electrical charge induced in the conductive layer 12b is greatly affected by the electrical charge induced in the center conductor 8a and the ground conductor 8b. Instability.

  Furthermore, there are technically very difficult problems to realize the third prior art. In the electrode manufacturing process for realizing the third prior art, first, a central conductor 8a and a ground conductor 8b made of thick gold plating of 20 to 30 μm are formed, and then a conductive layer is deposited on the entire surface. Next, after a photoresist is applied to the entire surface, windows are opened in the photoresist just above and near the center conductor 8a and the ground conductor 8b by exposure and development. The conductive layer at the window portion opened in the photoresist is removed, and the conductive layer is divided into 12a, 12b, and 12c. However, if there is a center conductor 8a and a ground conductor 8b having a thickness of 20 to 30 μm, the resist immediately above the center conductor 8a and the ground conductor 8b is thin (there is a portion where there is almost no photoresist at the edge of the conductor), and A thick resist is accumulated in the vicinity of the conductive layer contacting the central conductor 8a or the ground conductor 8b. As a result, the thickness of the photoresist is greatly different in various places, and it is extremely difficult to pattern with high precision especially in the vicinity of the center conductor 8a and the ground conductor 8b, and there is a big problem in the manufacturing yield.

JP-A-1-302325 JP-A-8-146367

  As described above, the conventionally proposed DC drift suppression structure is for the z-cut substrate LN optical modulator, and an effective DC drift suppression structure for the x-cut substrate LN optical modulator has not yet been proposed.

  Further, with respect to the conventionally proposed z-cut substrate LN optical modulator, the center conductor 8a and the ground conductor 8b are electrically connected by the conductive layer 7 in the second prior art disclosed in Patent Document 1. The effect of suppressing the DC drift is determined by the geometric structure in the horizontal and vertical directions of the surface of the LN substrate 5.

That is, in the second prior art, the DC drift amount is determined by the size of the gap between the center conductor 8a and the ground conductor 8b in which the microwave equivalent refractive index _nm and the characteristic impedance are to be determined. There is a problem that the degree of freedom of design is small.

  Further, in the second prior art, when the resistance of the conductive layer 7 becomes lower than the design value, an electrical short circuit occurs, which destroys the device or requires heat treatment at a high temperature. There are problems in terms of temperature controllability and yield.

  Further, in the third prior art, since the electrically floating conductive layer 12b is provided, it has electrical instability. In addition, a process of opening a gap by patterning the conductive layer in the vicinity of the center conductor 8a and the ground conductor 8b is indispensable. However, since the center conductor 8a and the ground conductor 8b are thick, this process is extremely difficult technically, and is easy to manufacture. And have a problem in terms of yield.

  Accordingly, an object of the present invention is to provide an optical modulator having a high production yield by eliminating the above-described problems of the prior art and at a high speed, a low driving voltage, a small DC drift.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention has an electro-optic effect and includes at least an x-cut or y-cut plane orientation, and light formed on the substrate. An optical waveguide for guiding light, an electrode made of a central conductor and a ground conductor formed on one side of the substrate, to which a voltage for modulating the light is applied, and the optical waveguide is the central conductor And an electrical contact between the ground conductor and the ground conductor. The optical modulator comprises an interaction unit for modulating the phase of the light by applying the voltage. And a conductive layer formed so as not to cover at least a part of the upper surface of the optical waveguide, and a buffer layer formed above the conductive layer, wherein the central conductor is the conductive layer in the interaction portion. Contact the layer , Wherein the central conductor above the buffer layer is formed.

  According to a second aspect of the present invention, in the optical modulator according to the first aspect, a part of the central conductor and the conductive layer are in electrical contact.

  According to a third aspect of the present invention, there is provided the optical modulator according to the first aspect, wherein a part of the central conductor and the conductive layer are electrically separated.

  According to a fourth aspect of the present invention, in the optical modulator according to the second aspect, a part of the central conductor is other than the interaction portion.

  According to a fifth aspect of the present invention, in the optical modulator according to the third aspect, a part of the central conductor is an interaction portion.

  According to a sixth aspect of the present invention, in the optical modulator according to the second aspect, a part of the center conductor is a DC bias supply section.

  According to a seventh aspect of the present invention, in the optical modulator according to the first to sixth aspects, at least a part of the ground conductor is in electrical contact with the substrate.

  An optical modulator according to an eighth aspect of the present invention is characterized in that, in the first to seventh aspects, a DC bias is applied to the conductive layer via the central conductor.

  According to a ninth aspect of the present invention, in the optical modulator according to the first to seventh aspects, a DC bias is applied to the conductive layer without passing through the central conductor.

  An optical modulator according to a tenth aspect of the present invention is the optical modulator according to the first to ninth aspects, wherein the conductive layer is made of a semiconductor.

  An optical modulator according to an eleventh aspect of the present invention is the optical modulator according to the first to ninth aspects, wherein the conductive layer is made of at least one of a metal or a metal oxide.

  An optical modulator according to a twelfth aspect of the present invention is the optical modulator according to any one of the first to ninth aspects, wherein the conductive layer is made of a combination of a semiconductor and a metal or a metal oxide.

  In the optical modulator according to the present invention, in the x-cut LN substrate and the y-cut LN substrate in which the optical waveguide exists between the central conductor and the ground conductor, the optical modulator is located below the central conductor and between the two optical waveguides. Since a DC bias is applied to the conductive layer in electrical contact with the substrate, the voltage drop in the buffer layer is very small.

  Therefore, according to the optical modulator of the present invention, since most of the voltage applied between the center conductor and the ground conductor is effectively applied to the substrate, in other words, the optical waveguide, it is possible to suppress DC drift. .

  Further, in the interaction portion where the RF electrical signal propagating through the central conductor and the ground conductor and the light propagating through the optical waveguide interact, the central conductor formed above the buffer layer is in electrical contact with the substrate. It is not structurally in contact with the conductive layer formed in the above. Therefore, the structure is simple and the manufacturability and production yield are remarkably good.

  Furthermore, since the conductive layer is provided below the central conductor, that is, in the middle of the two optical waveguides, there is an effect that the DC drift can be suppressed without increasing the propagation loss of light propagating through the optical waveguide.

  Here, when the center conductor and the ground conductor are completely electrically insulated, the effect of suppressing the DC drift is remarkable. In this case, since it is possible to realize that the central conductor and the ground conductor are not conducted by the conductive layer, it is possible to avoid the problem that an electrical short occurs even when the resistance of the conductive layer is low.

  In addition, since the conductive layer provided below the central conductor is in direct electrical contact with the LN substrate, the DC bias voltage can be reduced and long-term reliability due to DC drift can be improved.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the related 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]
FIGS. 1A to 1E show a procedure (step) of a manufacturing process of the first embodiment in the optical modulator according to the present invention. FIG. 2 is a schematic perspective view of the first embodiment of the present invention, and FIGS.

(A) Deposition of the first buffer layer 14 In this step, after forming the optical waveguides 3a and 3b manufactured by thermally diffusing Ti on the x-cut LN substrate 1, the first buffer layer 14 such as SiO ₂ is deposited. To do.

(B) Etching the first buffer layer 14 In this step, the first buffer layer 14 is partially etched to open a window.

(C) Deposition of conductive layer 15 and second buffer layer 16 In this step, Si or SiOx is formed on the x-cut LN substrate 1 that is visible from the etched SiO ₂ and SiO ₂ windows opened by etching. A conductive layer 15 and a second buffer layer 16 made of SiO ₂ or the like are deposited. At this time, the conductive layer 15 is in electrical contact with the surface of the x-cut LN substrate 1.

(D) Etching of the second buffer layer 16 and the conductive layer 15 In order to increase the resistance between the center conductor 4a and the ground conductors 4b and 4c formed in the steps described later, the second buffer layer on the optical waveguides 3a and 3b. 16 and the conductive layer 15 are etched. As a result, the optical waveguides 3a and 3b and the x-cut LN substrate 1 are partially exposed.

(E) Formation of electrodes (center conductor 4a, ground conductors 4b, 4c) In this step, the center conductor 4a and the ground conductors 4b, 4c are formed.

  In the present embodiment, the conductive layer 15 is removed between the center conductor 4a and the ground conductors 4b and 4c, so that they are electrically separated and have a high resistance, unlike Patent Document 1.

Further, the first buffer layers 14a ′, 14a ″ made of SiO _{2 and the} like and the second buffer layers 16a, 16b, 16c, in particular, the second buffer layer 16a disposed under the central conductor 4a are provided with an equivalent refractive index of microwaves. It plays an important role in bringing n _m closer to the equivalent refractive index n ₀ of the light propagating through the optical waveguides 3a and 3b.

Thus, the present invention will close the equivalent refractive index n ₀ of the light propagating the optical waveguide 3a and the equivalent refractive index n _m of the microwave, the 3b, conductive layer 15a (15b, is the same even 15c is) Is in electrical contact with the x-cut LN substrate 1.

  FIG. 2 is a schematic perspective view of the first embodiment of the present invention. FIG. 3 is a cross-sectional view of the B-B ′ portion in the output portion of the microwave that is a high frequency (RF) electric signal (generally, the input / output port of the microwave that is the RF electric signal is referred to as a feedthrough portion).

  As can be seen from FIG. 3, the center conductor 4a is electrically connected to the conductive layer 15a in the microwave output portion, and a DC bias voltage is supplied to the conductive layer 15a through the center conductor 4a. In FIGS. 1 and 3, it is assumed that the center conductor 4a is symmetrical with respect to the center conductor 4a. In FIG. 3, the left half is weighted. 1 and 3, the ground conductors 4b and 4c may be electrically connected to the x-cut LN substrate 1 through a conductive layer, or may be in direct contact with the x-cut LN substrate 1.

  Needless to say, it may be left-right symmetric about the center conductor 4a or not left-right symmetric, not only in the first embodiment but also in other embodiments of the present invention.

  As shown in FIG. 1E, also in the interaction portion, the conductive layer 15b and the second buffer layer 16b are provided under the ground conductor 4b, and the conductive layer 15c and the second buffer layer are provided under the ground conductor 4c. Although there is the buffer layer 16c, it has been confirmed that the width of this region (the length in the left-right direction in FIG. 1) is sufficient if it is several times the distance between the center conductor 4a and the ground conductor 4b. However, even if a part or all of the ground conductors 4b and 4c are in direct contact with the x-cut LN substrate 1, the high-speed response characteristic as the optical modulator is not impaired, and the effect of the present invention of suppressing the DC drift is It can be realized even stronger. This also holds true for the feedthrough portion shown in FIG.

  4 is a cross-sectional view taken along the line C-C ′ at the microwave output port of FIG. 2. In this embodiment, the DC bias is applied to the central conductor 4a simultaneously with the conductive layer 15a. Since the conductive layer 15a in contact with the x-cut LN substrate 1 and the central conductor 4a are at the same potential, no potential difference occurs above and below the buffer layer 16a. Further, not only the output port but also the input port side may have the structure shown in FIG.

  As shown in FIG. 5, the structure in which the DC bias supply unit 17 is electrically separated from the central conductor 4a in the cross section taken along the line C-C 'of FIG. 2 is the second embodiment of the present invention. Also in this case, the DC bias is supplied from the conductive layer 15a. However, since the DC bias supply unit 17 and the central conductor 4a are electrically separated, only the DC bias is supplied to the DC bias supply unit 17 from the central conductor. Only the RF electrical signal is applied to 4a. In the microwave output port portion of FIG. 2, it is convenient to apply the DC bias if the DC bias supply portion 17 is pulled out from the central conductor 4a and provided separately.

  Further, the location where the DC bias is supplied as shown in FIG. 4 may be electrically connected to the central conductor 4a, or may not be connected as shown in FIG. 5, in other embodiments of the present invention. Also holds.

As in the other embodiments of the present invention, the conductive layer 15 is removed between the center conductor 4a and the ground conductors 4b and 4c in the above embodiments. Therefore, the center conductor 4a and the ground conductors 4b and 4c are electrically separated, and the second prior art shown in FIG. 13 and the third prior art shown in FIG. Unlike literature 2, it has high resistance. The resistance of the conductive layers 7 and 12a, 12b, and 12c in the second prior art in FIG. 13 and the third prior art in FIG. 14 is preferably 10 ⁷ to 10 ¹⁰ Ω · cm. In contrast, since the center conductor 4a and the ground conductor 4b (or 4c) can be designed not to be connected to each other by a conductive layer, there is an advantage that other values, for example, smaller resistance values may be used.

  FIG. 6 shows the state of this electrical connection (equivalent circuit). For the sake of simplicity, FIG. 6 shows only the central conductor 4a, the ground conductors 4b and 4c, and the optical waveguides 3a and 3b as components of the optical modulator. In addition, the equivalent circuit shown in the figure shows only the left half in consideration of symmetry.

In the figure, C _C and R _C are capacitance and resistance when the center conductor 4a or the ground conductor 4b (or 4c) is viewed from the surface of the x-cut LN substrate 1.

In general, since a conductive layer such as Si or SiOx has a smaller resistance than SiO ₂ , in particular, the resistance _RC that has a large influence on DC drift is the surface of the x-cut LN substrate 1 and the central conductor 4a or the ground conductor 4b (or 4c) may be considered as the resistance of the conductive layer 15a.

At this time, R _LN >> R _C (1) between the resistance R _LN of the x-cut LN substrate 1 and the resistance R _C of the conductive layer 15a.
If Naritate is related, it is possible to prevent a voltage drop in the first SiO ₂ buffer layer 2 resistors in Figure 9 described in the prior art R _B.

  That is, in this case, almost all of the DC bias voltage applied to the center conductor 4a and the ground conductors 4b and 4c can be applied to the x-cut LN substrate 1, so that there is no movement of charges in the buffer layer, As a result, it is possible to prevent an increase in driving voltage as an optical modulator and to significantly reduce DC drift.

The resistance _{RC of the} conductive layer is preferably 10 ⁷ to 10 ¹⁰ Ω · cm. However, in the present invention, unlike the structure of the second prior art shown as Patent Document 1, the center conductor 4a and the ground conductor 4b ( Alternatively, since 4c) can be designed not to be connected to each other by a conductive layer, there is an advantage that a value other than this, for example, a smaller resistance value may be used.

[Third Embodiment]
FIG. 7 is a cross-sectional view showing a third embodiment of an optical modulator according to the present invention. In the present embodiment, the influence of the conductive layer 15a made of a semiconductor such as Si or SiOx on the RF electrical signal is reduced by reducing the width of the conductive layer 15a. Therefore, it is desirable that the width of the conductive layer 15 is narrower than that of the central conductor 4a. Here, reference numerals 14a ′ and 14a ″ are the first buffer layer.

  In all the embodiments of the present invention including this embodiment, since the conductive layer is in electrical contact with the x-cut LN substrate, the DC bias voltage is directly applied to the optical waveguide without passing through the buffer layer. It becomes possible. On the other hand, this conductive layer functions almost as a dielectric for RF electrical signals. That is, the microwave characteristics for the RF electrical signal, that is, the microwave effective refractive index and the characteristic impedance are determined by the buffer layer and the conductive layer that appears to be almost dielectric, and high-speed optical modulation is possible. In other words, in the present invention, the DC drive voltage can be reduced more than the RF drive voltage (for example, 2V and 5V, respectively), so that the DC bias voltage that greatly affects the long-term reliability caused by DC drift is reduced. It is possible to set, and the DC driving voltage and RF driving voltage are equal, and it is possible to realize a long-term reliability higher than that of a conventional x-cut optical modulator that requires a large DC bias voltage. There is.

[Fourth Embodiment]
FIG. 8 is a cross-sectional view showing a fourth embodiment of an optical modulator according to the present invention. In the present embodiment, by reducing the width of the conductive layer 15a, the influence of the conductive layer 15a made of a semiconductor such as Si or SiOx on the RF electrical signal is reduced, and the first buffer layer in the third embodiment of FIG. 14a 'and 14a''are omitted. Therefore, it is easier to manufacture.

[About each embodiment]
In the optical modulator according to the present invention, the center conductor and the ground conductor are electrically insulated, and the conductive layer is in electrical contact with the substrate. Therefore, as described above, the light due to DC drift suppression and DC bias voltage reduction is reduced. This has the effect of improving long-term reliability as a modulator.

  In this case, the CPW electrode is composed of one center conductor and two ground conductors. However, when the center conductor and one ground conductor are electrically connected by a conductive layer, the effect of the optical modulator according to the present invention is reduced. Needless to say, it is not as effective as possible, but it is more effective than the case where the central conductor and the two ground conductors are electrically connected by the conductive layer.

  In each of the embodiments, the case where the first buffer layer or the second buffer layer between the center conductor and the ground conductor is removed by etching has been described. However, the present invention has an effect even if the etching is not removed. Needless to say.

  The substrate used in the optical modulator according to the present invention has been described using the LN substrate as an example. However, other various substrates having an electro-optic effect such as lithium tantalate or a semiconductor substrate may be used.

Further, although the buffer layer in the optical modulator according to the present invention has been described as SiO ₂ and the conductive layer as Si or SiOx, it may be a buffer layer of other materials such as SiNx or polyimide, and the conductive layer may be Si as described above. Needless to say, in addition to a semiconductor such as SiOx, a metal such as Ti or an oxide thereof, or a combination thereof may be used.

  Metals and metal oxides have a relatively thin optimum thickness of several tens of nanometers, and their patterns may be cut off. However, in the case of a semiconductor such as Si or SiOx, their thickness is about 100 nm. It is relatively thick and is difficult to cause pattern breaks.

  However, metal or metal oxide can be easily patterned by etching or lift-off, but in the case of a semiconductor such as Si or SiOx, pattern formation is not as easy as metal or metal oxide.

  Therefore, it is important to select the material of the conductive layer used in the optical modulator according to the present invention in consideration of the tradeoff between these advantages and disadvantages and available devices.

  Further, although the buffer layer, the conductive layer, or the structure of the optical waveguide in describing each embodiment has been mainly described as being symmetric with respect to the center of the central conductor, it goes without saying that it is not necessarily symmetric.

  Furthermore, although the CPW electrode has been described as an example of the traveling wave electrode, it goes without saying that various traveling wave electrodes such as an asymmetric coplanar strip (ACPS) and a symmetric coplanar strip (CPS), or a lumped constant type electrode may be used.

  In addition to the Mach-Zehnder type optical waveguide, it goes without saying that other optical waveguides such as directional couplers and straight lines may be used as the optical waveguide.

  Further, in each of the above embodiments, the substrate has x-cut and y-cut plane orientations, that is, a crystal x-axis or y-axis in a direction perpendicular to the substrate surface (cut surface). Needless to say, the plane orientation in each of the embodiments described above may be used as the main plane orientation, and other plane orientations may be subordinate to these.

  As described above, the optical modulator according to the present invention is useful as an optical modulator that enables high-speed driving with a low driving voltage and suppression of DC drift, is highly reliable, and has a good manufacturing yield.

(A)-(e) is sectional drawing which shows the procedure of the manufacturing process of 1st Embodiment in the optical modulator by this invention, respectively. It is a perspective view of 1st Embodiment and 2nd Embodiment of this invention, and is a figure explaining the structure of a feedthrough part. It is a structure of a feedthrough part and is sectional drawing in B-B 'in FIG. It is also a diagram for explaining a method of applying a DC bias. It is a structure of a feedthrough part and is sectional drawing in C-C 'in FIG. It is also a diagram for explaining a method of applying a DC bias. It is a structure of a feedthrough part and is sectional drawing in C-C 'in FIG. It is also a diagram for explaining a method of applying a DC bias. It is an equivalent circuit diagram shown in order to demonstrate the mode of the electrical connection of the optical modulator by 1st Embodiment of this invention shown by FIG. It is sectional drawing which shows the optical modulator by 3rd Embodiment of this invention. It is sectional drawing which shows the optical modulator by 4th Embodiment of this invention. It is a perspective view which shows the structure of the LN optical modulator comprised using the x-cut LN board | substrate by a 1st prior art. FIG. 10 is a cross-sectional view taken along line A-A ′ of FIG. 9. FIG. 11 is a voltage-light output characteristic curve diagram for explaining the operation principle of the LN optical modulator according to the first prior art shown in FIGS. 9 and 10. It is an equivalent circuit diagram shown in order to demonstrate the mode of electrical connection about the LN optical modulator shown by FIG. 9 and FIG. It is sectional drawing which shows the LN optical modulator currently disclosed by patent document 1 as 2nd prior art. It is sectional drawing which shows the LN optical modulator currently disclosed by patent document 2 as 3rd prior art.

Explanation of symbols

1: x-cut LN substrate (LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Optical waveguide 3a, 3b: Optical waveguide of the interaction part (optical waveguide)
4: traveling wave electrode 4a: central conductor 4b, 4c: ground conductor 5: z-cut LN substrate (LN substrate)
6a, 6b: SiO ₂ buffer layer (buffer layer)
7: Conductive layer 8a: Center conductor 8b: Ground conductors 12a, 12b, 12c: Conductive layers 13a, 13b: Insulation locations 14: First buffer layers 14a, 14a ′, 14a ″: First buffer layers 15: Conductive layers 15a , 15b, 15c: conductive layer 16: second buffer layer 16a, 16b, 16c: second buffer layer 17: DC bias supply unit

Claims (12)

A substrate having an electro-optic effect and including at least an x-cut or y-cut plane orientation, an optical waveguide for guiding light formed on the substrate, and a voltage for modulating the light are applied. And an electrode composed of a central conductor and a ground conductor formed on one surface of the substrate, and the optical waveguide applies the voltage between the central conductor and the ground conductor to thereby adjust the phase of the light. An optical modulator comprising an interaction unit for modulating,
A conductive layer formed in electrical contact with a portion of the upper surface of the substrate and not covering at least a portion of the upper surface of the optical waveguide; and a buffer layer formed over the conductive layer, and the interaction The optical modulator includes the central conductor formed above the buffer layer, the central conductor being not in contact with the conductive layer.   The optical modulator according to claim 1, wherein a part of the center conductor and the conductive layer are in electrical contact.   2. The optical modulator according to claim 1, wherein a part of the central conductor and the conductive layer are electrically separated.   The optical modulator according to claim 2, wherein a part of the central conductor is other than the interaction part.   The optical modulator according to claim 3, wherein a part of the central conductor is an interaction portion.   The optical modulator according to claim 2, wherein a part of the central conductor is a DC bias supply unit.   The optical modulator according to claim 1, wherein at least a part of the ground conductor is in electrical contact with the substrate.   The optical modulator according to claim 1, wherein a DC bias is applied to the conductive layer through the center conductor.   The optical modulator according to claim 1, wherein a DC bias is applied to the conductive layer without passing through the central conductor.   The optical modulator according to claim 1, wherein the conductive layer is made of a semiconductor.   The optical modulator according to claim 1, wherein the conductive layer is made of at least one of a metal or a metal oxide.   The optical modulator according to claim 1, wherein the conductive layer is made of a combination of a semiconductor and a metal or a metal oxide.