JP2006309199A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator using a thin plate having a portion with ≤20 μm thickness of a substrate, in which specified polarized light can be selected from light waves propagating through an optical waveguide in the modulator without deteriorating productivity of the modulator. <P>SOLUTION: The optical modulator includes an X-cut or Y-cut thin plate 1 made of a material having an electro-optic effect, an optical waveguide 3 fabricated on the top or back surface of the thin plate, and a controlling electrode formed on the top surface of the thin plate to modulate light passing through the optical waveguide, wherein an attenuating means 10, 11 which absorbs light having a specified polarizing plane in the light propagating in the waveguide is disposed near the optical waveguide. The attenuating means is preferably a light-absorbing member and is disposed as interposing the optical waveguide at a distance L of 0.5 to 2 times the mode diameter of the light waves propagating in the optical waveguide. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical modulator, and in particular, a thin plate formed of a material having an electro-optic effect and having a thickness of 20 μm or less, and a reinforcing plate bonded to the back surface of the thin plate and having a thickness larger than the thin plate. It is related with the optical modulator containing.

Conventionally, in the optical communication field and the optical measurement field, a waveguide type optical modulator in which an optical waveguide and a modulation electrode are formed on a substrate having an electro-optic effect has been widely used.
In particular, with the development of multimedia, the amount of information transmission is also increasing, and it is necessary to realize a broadband optical modulation frequency. As one of means for realizing these, external modulation methods using a LiNbO ₃ (hereinafter referred to as “LN”) modulator or the like are diversified. However, in order to realize a wide band of the LN modulator, it is necessary to achieve speed matching between the microwave and the light wave that are the modulation signals and to reduce the driving voltage.

Conventionally, as a means for solving the above problems, speed matching has been achieved by providing a buffer layer on a waveguide. However, in recent years, attempts have been made to satisfy the speed matching condition between the microwave and the speed of the light wave and simultaneously reduce the driving voltage by reducing the thickness of the substrate by improving the processing technology of the substrate.
In the following Patent Documents 1 to 3, an optical waveguide and a modulation electrode are incorporated in a thin substrate (hereinafter referred to as a “first substrate”) having a thickness of 30 μm or less, and another substrate having a lower dielectric constant than the first substrate. (Hereinafter referred to as “second substrate”), the effective refractive index with respect to the microwave is lowered, the speed matching between the microwave and the light wave is achieved, and the mechanical strength of the substrate is maintained.
JP-A 64-18121 JP 2003-215519 A Japanese Patent Laid-Open No. 10-133159

On the other hand, optical intensity modulators, optical switches, variable optical attenuators (VOA), phase modulators, and the like have been developed and put into practical use as waveguide type optical modulators. For example, in a VOA element, in a single-arm branching waveguide constituting a Mach-Zehnder interferometer, the refractive index of the waveguide is changed by the electro-optic effect, the thermo-optic effect, etc., and the output light intensity is adjusted.
However, when a substrate having optical anisotropy is used, the applied voltage and the heating amount differ depending on the incident light polarization. For this reason, only specific polarization is guided.

  As a method of selecting a specific polarization, as shown in FIG. 1A, a method of attaching the polarizer 2 to the entrance end face of the substrate 1 of the optical modulator, or an X-cut when an LN substrate is used. For a plate, there is a method for attaching the metal film 4 on the optical waveguide 3 (see FIG. 1B), and for a Z-cut plate, there is a method for applying a slab to the side surface of the optical waveguide.

However, when a thin plate having a thickness of 20 μm or less is used, it is difficult to attach a polarizer to the end face of the substrate with high accuracy as shown in FIG. 1A, and FIG. When the metal film 4 is attached to the surface of the thin plate as described above, there is an increased risk of the substrate being damaged due to the difference in coefficient of thermal expansion between the substrate and the metal film, and the metal film is installed on the optical waveguide 3 in the first place. In such a case, there arises a problem that not only the specific polarized wave is absorbed but also the light wave of another polarization plane is attenuated.
In addition, the method for selecting these specific polarized waves needs to incorporate another process in addition to the process for manufacturing the optical modulator, which complicates the manufacturing process and leads to an increase in manufacturing cost.

  The problem to be solved by the present invention is to solve the above-mentioned problem, and in an optical modulator using a thin plate including a portion having a substrate thickness of 20 μm or less, the productivity of the optical modulator is not deteriorated. It is an object of the present invention to provide an optical modulator capable of selecting a specific polarization from a light wave propagating through an optical waveguide.

  In order to solve the above-described problem, in the invention according to claim 1, an X-cut or Y-cut thin plate formed of a material having an electro-optic effect, an optical waveguide formed on the front surface or the back surface of the thin plate, In a light modulator formed on the surface of a thin plate and including a control electrode for modulating light passing through the light guide, a specific polarization plane of a light wave propagating through the light guide is adjacent to the light guide. Attenuating means for absorbing light is disposed, and the attenuating means is composed of a light absorbing member.

According to a second aspect of the present invention, in the optical modulator according to the first aspect, the light absorbing means includes a plurality of light absorbing members, and the light absorbing member propagates through the optical waveguide with the optical waveguide interposed therebetween. It is characterized by being arranged at intervals of 0.5 to 2 times the mode diameter. A plurality of light absorbing members means that a plurality of light absorbing members are provided. Further, when the attenuation means is composed of a plurality of light-absorbing members, each member can be composed not only of the same material but also of different materials as required.

  According to a third aspect of the present invention, in the optical modulator according to the first or second aspect, the attenuation means comprises at least a part of a control electrode. The control electrode in the present invention not only means a combination of a signal electrode and a ground electrode but also includes various electrodes arranged in the vicinity of the optical waveguide, such as a DC electrode.

  According to a fourth aspect of the present invention, in the optical modulator according to the first or second aspect, the attenuating means is formed in at least one optical waveguide portion on the input side or output side of the optical waveguide. And

  According to a fifth aspect of the present invention, in the optical modulator according to the first or second aspect, the attenuation means is formed on at least one of the front surface and the back surface of the substrate.

  According to a sixth aspect of the present invention, in the optical modulator according to any one of the first to fifth aspects, the thin plate has a thickness of 20 μm or less.

According to the first aspect of the present invention, in order to dispose the attenuation means composed of the light absorbing member that absorbs the light of the specific polarization plane of the light wave propagating through the optical waveguide in the vicinity of the optical waveguide, Compared with the case where a metal layer is disposed directly above the optical waveguide, attenuation of the entire light wave propagating through the optical waveguide can be suppressed, and only specific polarization can be efficiently attenuated. In addition, since the attenuation means is disposed in the vicinity of the optical waveguide, avoiding directly above the optical waveguide, there is a risk that the optical waveguide, the substrate, the material different from the attenuation means may be mixed, and the substrate may be damaged due to the difference in each thermal expansion coefficient. It is also possible to suppress the property.
In particular, when a thin plate is used for the substrate, the light wave of a specific polarization plane tends to ooze out from the surface of the substrate as will be described later, so that the oozing and propagating light wave is absorbed using attenuation means. Is possible.
In addition, when a thin plate is used, it is necessary to provide a conventional buffer layer in order to achieve speed matching between the microwave propagating through the signal electrode constituting the control electrode and the light wave propagating through the optical waveguide. Therefore, the attenuation means can be formed directly on the front surface or the back surface of the substrate, and the specific polarization can be absorbed more efficiently.

  According to the invention of claim 2, the attenuating means is composed of a plurality of light-absorbing members, and the light-absorbing member sandwiches the optical waveguide and the mode diameter of the light wave propagating through the optical waveguide is 0.5 to Since it is arranged at twice the spacing, it efficiently absorbs mode light having a polarization plane in the longitudinal direction (direction perpendicular to the substrate surface) of the light wave propagating through the optical waveguide, and laterally (parallel to the substrate surface). It is possible to suppress the attenuation of mode light having a (polarity) polarization plane.

  According to the invention of claim 3, since the attenuation means comprises at least a part of the control electrode, when the control electrode is formed, the attenuation means can be easily formed simply by adjusting the shape of the control electrode. It becomes possible to suppress the complexity of the process. In addition, since at least a part of the control electrode also serves as an attenuating means, the optical modulator itself can be shortened.

  According to the invention of claim 4, since the attenuating means is formed in at least one of the optical waveguide portions on the input side or output side of the optical waveguide, even when non-polarized light is incident on the optical modulator, A light wave having only a wave component can be output from the optical modulator.

  According to the invention of claim 5, since the attenuating means is formed on at least one of the front surface and the back surface of the substrate, the degree of freedom in designing the attenuating means is increased, and an optical waveguide is formed because a thin plate is used. Even on the substrate surface opposite to the surface on which it is positioned, it is possible to efficiently attenuate specific polarized waves.

  According to the invention of claim 6, since the thickness of the thin plate has a portion of 20 μm or less, in particular, in the portion of 20 μm or less, the seepage of the polarization plane in the vertical direction to the outside of the substrate becomes remarkable, and the attenuation means efficiently The polarized wave can be absorbed.

Hereinafter, the present invention will be described in detail using preferred examples.
FIG. 2 schematically shows the state of light waves propagating through the optical waveguide 3 formed on the thin plate when a thin plate, which is the substrate 1 having a thickness of 20 μm or less, is used.
As the thickness of the substrate is reduced, a light wave 6 having a polarization plane in a direction parallel to the substrate surface (lateral direction) and a light wave 7 having a polarization plane in a direction perpendicular to the substrate surface (longitudinal direction) are illustrated. However, the flatness of the light wave is different, and the light intensity distribution of each polarization plane changes as 8 (intensity distribution of the polarization plane in the horizontal direction) and 9 (intensity distribution of the polarization plane in the vertical direction) in the figure. In particular, the longitudinal polarization plane has a characteristic that as the thickness of the substrate decreases, the portion that oozes out from the substrate surface increases.

  The inventors pay attention to such a phenomenon, and arrange attenuation means for absorbing a specific polarization (particularly, a light wave having a longitudinal polarization plane) on the front or back surface of the substrate in the vicinity of the optical waveguide 3. Thus, the present invention has been achieved.

FIG. 3 is a diagram showing an embodiment of an optical modulator according to the present invention.
In FIG. 3A, the optical waveguide 3 is formed on the surface of the substrate 1 constituting the optical modulator, and the attenuation means 10 is provided on the surface of the substrate except for the portion directly above the optical waveguide 3 in the vicinity of the optical waveguide. It is arranged.
Further, in FIG. 3B, attenuation means 10 and 11 are arranged on the front surface and the back surface of the substrate 1.

The attenuating means 10 or 11 are arranged at a predetermined interval L or L ′ across the optical waveguide 3. The interval is affected by the thickness of the substrate 1, the shape and material of the optical waveguide 3, and the shape (length and height along the optical waveguide) and material of the attenuation means. 0.5 to 2 times the light intensity distribution of each polarization plane in the light wave, based on the mode diameter (2W: diameter) of the light wave at the position where the maximum intensity attenuates to 1 / e ² (W: radius) It is preferable to set within a range.

When L is a value smaller than W (0.5 times the mode diameter), not only longitudinal polarization but also lateral polarization is absorbed, resulting in a decrease in the intensity of the light wave itself. If L is set to a value larger than 4W (twice the mode diameter), it is difficult to effectively reduce the longitudinal polarization.
In addition, with respect to the distance L ′ of the attenuation means 11 arranged on the back surface of FIG. 3B, the vertical polarization plane tends to bleed out on the back surface of the substrate 1 as the thickness of the substrate 1 decreases. Therefore, the same condition as the distance L between the attenuation means 10 on the surface side is satisfied. Therefore, the relationship of L ′ ≦ L can be set according to the degree of bleeding of the polarization plane in the vertical direction.

  Further, the arrangement of the attenuating means is not limited to that shown in FIG. 3 (a) or (b). The attenuating means 10 shown in FIG. 3 (a) is formed only on one side, or the attenuating means 10 shown in FIG. It is also possible for both and 11 to be only on one side. When the attenuation means is formed only on one side, for example, the arrangement of the attenuation means required when two attenuation means are arranged as shown in FIG. 3A is determined, and then one of the attenuation means is removed. In the state, the remaining damping means is formed only on one side.

  As a material having an electro-optic effect constituting the substrate 1, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, a lithium niobate (LN) crystal having a high electro-optic effect is preferably used. Further, as the substrate 1, an X cut plate or a Y cut plate can be suitably used.

The attenuation means can be formed using a light absorbing material such as a metal film. When using Al or the like, a method of forming a predetermined pattern opening in advance with a photoresist film and vapor-depositing on the opening, or a method of removing other vapor-deposited film leaving a predetermined region on the entire substrate surface after vapor deposition There is. In addition, a method of forming a metal film by a gold plating method after the formation of a Ti / Au electrode pattern, as well as a method of forming a control electrode such as a signal electrode, a ground electrode, or a DC electrode of an optical modulator. is there.
Further, when the attenuation means is composed of a plurality of light-absorbing members, each member can be composed not only of the same material but also of different materials as required.

As a method of forming the optical waveguide of the optical modulator, it can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method as in the conventional case.
Control electrodes such as a signal electrode and a ground electrode can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. In the present invention, it is preferable not to provide a buffer layer such as dielectric SiO _{2 on} the surface of the substrate 1. As a result, it is possible to efficiently attenuate the longitudinal polarization that oozes onto the substrate surface.

  The method of manufacturing a thin plate including an optical modulator includes the above-described optical waveguide, control electrode, and attenuation means (only when formed on the substrate surface) formed on a substrate having a thickness of several hundreds of μm. Is polished to a thickness of 20 μm or less, for example. The optical waveguide or the control electrode can be formed after the thin plate is formed. However, since there is a risk of damage to the thin plate due to thermal shock when forming the optical waveguide and mechanical shock due to handling of the thin film during various processing, there is a risk that the thin plate will be damaged. It is preferable to do this before. As shown in FIG. 3B, when the attenuation means 11 is formed on the back surface of the substrate 1, it is naturally formed after the substrate 1 is thinned.

Thereafter, the thin plate on which the optical waveguide or the like is formed is joined to a reinforcing plate having higher mechanical strength than the thin plate.
Various materials can be used as the reinforcing plate. For example, in addition to using the same material as the thin plate, a material having a lower dielectric constant than that of the thin plate such as quartz, glass, and alumina is used. It is also possible to use a material having a crystal orientation different from that of the thin plate. However, it is preferable to select a material having a linear expansion coefficient equivalent to that of the thin plate in order to stabilize the modulation characteristics of the optical modulator with respect to temperature changes. If it is difficult to select an equivalent material, a material having a linear expansion coefficient equivalent to that of the thin plate is selected as an adhesive for joining the thin plate and the reinforcing plate as in Patent Document 2.

  For bonding thin plates and reinforcing plates, epoxy adhesives, thermosetting adhesives, UV curable adhesives, solder glass, thermosetting, photocurable or photothickening resin adhesive sheets are used as adhesive layers. It is possible to use various adhesive materials. Further, it is also possible to directly bond the thin plate and the reinforcing plate without using an adhesive by the direct bonding method.

FIG. 4 shows another embodiment of the optical modulator according to the present invention.
A Mach-Zehnder type waveguide 3 is formed on the substrate 1, and various attenuation means can be configured for the optical waveguide as described in symbols A to D.
A or D is an example in which an attenuation unit that sandwiches the optical waveguide 3 is arranged on the input side or output side of the optical waveguide. As a result, the light wave propagating through the optical waveguide can be shaped into a specific polarization when entering the optical modulator or when exiting from the optical modulator. Reference numerals 22 and 23 denote incident and outgoing optical fibers connected to the optical modulator.

B or C indicates that the signal electrode 20 and the ground electrode 21 that are control electrodes are incorporated with attenuation means. In the attenuating means, the interval between these electrodes sandwiching the optical waveguide is set to be 0.5 to 2 times the mode diameter 2W of light as described above. Such attenuating means can be formed very easily only by adjusting the shape of the control electrode when the control electrode is manufactured in the manufacturing process of the optical modulator.
In FIG. 4, attenuating means is provided on a part of the control electrode. However, as shown in FIG. 5, by using all the operating parts of the control electrode as the attenuating means E or F, the absorption effect of the specific polarization plane is further increased. Can be increased.

Next, specific examples and tests related to the optical modulator of the present invention will be described.
Example 1
The thin optical modulator uses an X-cut LN substrate having a thickness of 500 μm as a substrate, and forms a linear optical waveguide (width 4 μm) on the substrate surface by a Ti diffusion process or the like. Next, an Au metal film is formed on both sides of the optical waveguide as attenuating means by a plating process, and the distance between the attenuating means is set to 1.4 times (20 μm) the mode diameter of the light wave propagating through the optical waveguide. The length along the optical waveguide was 2.1 cm. The back surface of the substrate was polished with a polishing machine until the thickness of the substrate became 10 μm, and an ultraviolet curable adhesive was attached to the reinforcing plate as an adhesive layer.

(Example 2)
It was manufactured in the same manner as in Example 1 except that the interval of the attenuation means in Example 1 was set to twice the mode diameter (30 μm).

When the polarization extinction ratio in Examples 1 and 2 was measured with a polarization controller or an optical power meter, it was 4.5 dB in Example 1 and 1.4 dB in Example 2.
From this, it is understood that the attenuation means of the present invention effectively attenuates the mode light of the longitudinal polarization plane.

(Example 3)
Next, as a thin optical modulator, an X-cut LN substrate having a thickness of 500 μm is used as the substrate in the same manner as in Example 1, and a linear optical waveguide (width 4 μm) is formed on the substrate surface by a Ti diffusion process or the like. Form. Next, a metal film of Au is formed as attenuating means by a plating process after vapor deposition of Ti on both sides of the optical waveguide, and the interval of the attenuating means is set to 1 to 3 of the mode diameter of the light wave propagating through the optical waveguide. A plurality of optical modulators were created with a double range and a length along the optical waveguide of 4.0 cm. The back surface of the substrate was polished with a polishing machine until the thickness of the substrate became 10 μm, and an ultraviolet curable adhesive was attached as an adhesive layer to the reinforcing plate.

When the polarization extinction ratio and the propagation loss (loss) of the plurality of optical modulators created in Example 3 were measured with a polarization controller or an optical power meter, the results shown in the graphs of FIGS. 6 and 7 were obtained. 6 represents the polarization extinction ratio, and FIG. 7 represents the propagation loss. In addition, both horizontal axes of the two graphs indicate the distance (electrode gap) between electrodes serving as attenuation means with respect to the mode diameter (MFD) of the light wave propagating through the optical waveguide.
Moreover, the dotted line in FIG. 6, FIG. 7 has shown the approximate curve with respect to each measured value of Example 3. FIG.

It can be easily understood from the graph of the polarization extinction ratio in FIG. 6 that the polarization extinction ratio increases as the electrode spacing decreases with respect to the MFD. In general, the polarization extinction ratio required for the optical modulator is 7 dB or more, and the electrode gap / MFD is preferably 2 or less from the graph of FIG.
Further, when the electrode gap is made smaller according to FIG. 7, the polarization extinction ratio is improved, but the propagation loss (loss) is increased. This is because not only vertical polarization but also horizontal polarization is absorbed, resulting in a decrease in the intensity of the light wave itself. Therefore, considering the practical limit of propagation loss (for example, about −10 dB) and the production limit of the electrode gap of the optical modulator, the electrode gap / MFD is preferably 0.5 or more. More preferably, a specific polarization can be sufficiently attenuated if the number is 1 or more.

  As described above, according to the present invention, in an optical modulator using a thin plate including a portion having a substrate thickness of 20 μm or less, the productivity of the optical modulator is not deteriorated, and the optical waveguide in the optical modulator is reduced. An optical modulator capable of selecting a specific polarization from a propagating light wave can be provided.

It is a figure which shows the polarization selection means of the conventional optical modulator. It is a figure which shows the light intensity distribution condition for every polarization plane in the optical modulator using a thin plate. It is a figure which shows the 1st Example of the optical modulator which concerns on this invention. It is a figure which shows the 2nd Example of the optical modulator which concerns on this invention. It is a figure which shows the 3rd Example of the optical modulator which concerns on this invention. It is a graph which shows the change of the polarization extinction ratio with respect to the ratio of electrode space | interval and MFD. It is a graph which shows the change of the propagation loss with respect to ratio of electrode space | interval and MFD.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Polarizer 3 Optical waveguide 4 Metal films 10 and 11 Attenuating means 20 Signal electrode 21 Ground electrode

Claims (6)

An X-cut or Y-cut thin plate made of a material having an electro-optic effect, an optical waveguide formed on the surface or the back surface of the thin plate, and light passing through the optical waveguide formed on the surface of the thin plate An optical modulator including a control electrode for modulating,
An attenuation means for absorbing light of a specific polarization plane of a light wave propagating through the optical waveguide is disposed in the vicinity of the optical waveguide, and the attenuation means is composed of a light absorbing member. Light modulator.   2. The optical modulator according to claim 1, wherein the light absorbing means is composed of a plurality of light absorbing members, and the light absorbing member has a mode diameter of 0.5 of a light wave propagating through the optical waveguide with the optical waveguide interposed therebetween. An optical modulator characterized in that the optical modulator is arranged at intervals of ~ 2 times.   3. The optical modulator according to claim 1, wherein the attenuation means includes at least a part of a control electrode.   3. The optical modulator according to claim 1, wherein the attenuation means is formed in at least one optical waveguide portion on the input side or output side of the optical waveguide.   3. The optical modulator according to claim 1, wherein the attenuation means is formed on at least one of the front surface and the back surface of the substrate. 6. The optical modulator according to claim 1, wherein the thickness of the thin plate has a portion of 20 [mu] m or less.

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