JP2009210634A - Optical waveguide device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve reliability of an optical waveguide device, operating in a wide band and driven at low voltage. <P>SOLUTION: This optical waveguide device includes an optical waveguide substrate having a thickness of 30 μm or less, a holding substrate holding the optical waveguide substrate, a low dielectric constant substrate having a lower dielectric constant than the optical waveguide substrate and interposed between the optical waveguide substrate and the holding substrate, an adhesive layer adhesively bonding the optical waveguide substrate and the low dielectric constant substrate to each other and having a thickness of 9 μm or less, and a fixing means for fixing the low dielectric constant substrate to the holding substrate. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical modulator.

  In recent years, optical communication systems have been increased in speed and capacity, and communication speeds of 40 gigabits / second or more per wavelength have been put into practical use. ing. The traveling wave type optical modulator is an optical modulator that modulates a light wave by the interaction between the light wave traveling through the optical waveguide and the microwave traveling through the electrode provided along the optical waveguide due to the electro-optic effect, By matching the speed between the light wave and the microwave, a broad band can be achieved. As a method for realizing speed matching, a configuration in which an electrode is formed on a low dielectric constant buffer layer provided on an optical waveguide substrate has been conventionally used. In this configuration, an electric field applied to the optical waveguide is not generated. Since the buffer layer becomes small due to the presence of the buffer layer, there is a drawback that the drive voltage cannot be lowered.

In order to improve this drawback, a traveling wave type optical modulator in which the optical waveguide substrate as shown in FIG. 3 is thinned has been proposed (for example, see Patent Document 1). In FIG. 3, the optical waveguide substrate 101 on which the optical waveguide 104 is formed is fixed and held on the holding substrate 102 by the adhesive layer 103. The thickness of the optical waveguide substrate 101 is about 30 μm or less, and is thinner than a normal one (for example, a thickness of 0.5 mm). As the adhesive layer 103, one having a dielectric constant lower than that of the optical waveguide substrate 101 is used, and the thickness thereof is sufficiently thick so that leakage of an electric field applied from the electrode 105 to the adhesive layer 103 becomes large ( For example, 10 μm to 200 μm). In such a configuration, when the electric field from the electrode 105 leaks into the adhesive layer 103 having a low dielectric constant, the equivalent refractive index with respect to the microwave (the value is larger than the equivalent refractive index with respect to the light wave) is reduced. Compared with the case where the thickness of the film is thick, it becomes smaller. Thus, the difference in the value of the equivalent refractive index becomes small, so that the speed of the light wave and the microwave approaches the state of matching, and a wide band is realized. In addition, in this configuration, speed matching is possible without providing a buffer layer on the optical waveguide substrate 101, so that the strength of the electric field applied to the optical waveguide 104 does not decrease, and the drive voltage can be lowered. It can be realized at the same time.
JP 2003-215519 A

  However, in the case of the configuration of FIG. 3, the following points are problematic because the adhesive layer 103 is thick. First, if the adhesive layer is thick, its adhesive strength is reduced. Second, when the adhesive is cured, the temperature is increased by irradiation with ultraviolet rays or heating, and then the stress is generated when the adhesive is cured and then the temperature is decreased. However, when the adhesive layer is thick, the generated stress is also increased. Thirdly, since it is difficult to manufacture a thick adhesive layer, the cost becomes high.

  The present invention has been made in view of the above points, and an object thereof is to improve the reliability of an optical waveguide device that operates in a wide band and can be driven at a low voltage.

  The present invention has been made to solve the above problems, and is an optical waveguide device having an optical waveguide substrate having a thickness of 30 μm or less and a holding substrate for holding the optical waveguide substrate, wherein the optical waveguide A low dielectric constant substrate having a dielectric constant lower than that of the substrate and interposed between the optical waveguide substrate and the holding substrate, and an adhesive having a thickness of 9 μm or less for bonding the optical waveguide substrate and the low dielectric constant substrate And a fixing means for fixing the low dielectric constant substrate to the holding substrate.

  According to this configuration, since the low dielectric constant substrate is provided between the optical waveguide substrate and the holding substrate, it is possible to realize a wide band and a low driving voltage, and the optical waveguide substrate and the low dielectric constant substrate. Since the thickness of the adhesive layer for adhering to each other is 9 μm or less, the adhesive strength is increased and the stress generated during curing is reduced, so that the reliability as an optical waveguide device can be improved. Moreover, since it is easy to form an adhesive layer thinly, manufacturing cost can also be suppressed.

  In the optical waveguide device, the present invention is characterized in that the low dielectric constant substrate has a thickness of 30 μm or more.

  According to this configuration, since the thickness of the low dielectric constant substrate is sufficiently thick, the equivalent refractive index with respect to the microwave is reduced, so that the speed matching state is approached and the broadband characteristics are excellent.

  In the above optical waveguide device, the present invention is characterized in that when the adhesive layer is made of a material having a higher dielectric constant than that of the optical waveguide substrate, the thickness thereof is set to 1 μm or less.

  According to this configuration, even if the adhesive layer is made of a material having a high dielectric constant, the thickness thereof is 1 μm or less. Therefore, the equivalent refractive index with respect to microwaves is the same as when the adhesive layer is made of a material having a low dielectric constant. As a result, there is an advantage that a wide band can be obtained and the selection range of the adhesive used for the adhesive layer is widened (not only a low dielectric constant material but also a high dielectric constant material can be used).

  According to the present invention, it is possible to improve the reliability of an optical waveguide device that operates in a wide band and can be driven at a low voltage.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
1 and 2 are a cross-sectional view and a plan view, respectively, of a traveling wave type optical modulator 10 that is an optical waveguide device according to an embodiment of the present invention. The cross-sectional configuration diagram of FIG. 1 shows a state cut along the line AA ′ in the plan configuration diagram of FIG. 2.

  1 and 2, an optical modulator 10 includes an optical waveguide substrate 11 on which a Mach-Zehnder optical waveguide 15 is formed, a holding substrate 13 that holds the optical waveguide substrate 11, and the optical waveguide substrate 11 and the holding substrate 13. The resin substrate 12 interposed therebetween, the adhesive layer 14 for bonding and fixing the optical waveguide substrate 11 and the resin substrate 12, and the signal electrode 16 and the ground electrodes 17-1 and 17 formed on the optical waveguide substrate 11 -2.

  The optical waveguide substrate 11 is an X-cut substrate that is cut from a mother crystal having an electro-optic effect so that the principal axis P and the substrate surface S are parallel to each other. For example, a lithium niobate (LN) substrate, tantalum acid A lithium (LT) substrate, a lead lanthanum zirconate titanate (PLZT) substrate, or the like can be used. A Mach-Zehnder optical waveguide 15 comprising an input waveguide 15-3, branch optical waveguides 15-1 and 15-2, and an output waveguide 15-4 is connected to the X-cut optical waveguide substrate 11 with the main axis P and the branched light. The waveguides 15-1 and 15-2 are formed so as to be vertical (that is, the main axis P is in the drawing in FIG. 1). The thickness of the optical waveguide substrate 11 is, for example, 30 μm or less, preferably 10 μm or less. When the optical waveguide substrate 11 is thinned in this way, the equivalent refractive index with respect to the microwaves that are excited by the electrodes 16, 17-1, and 17-2 and travel in the optical waveguide substrate 11 is reduced, so that the branched optical waveguide 15- The difference from the equivalent refractive index for light waves traveling through 1 and 15-2 is reduced. As a result, the speed matching between the light wave and the microwave is achieved or close to the speed matching, and the optical modulator 10 has a wider bandwidth.

  The resin substrate 12 is a resin substrate having a characteristic that the dielectric constant thereof is lower than that of the optical waveguide substrate 11 and is used to reduce the equivalent refractive index with respect to microwaves as described above. As such a low dielectric constant resin material, for example, an acrylic resin (dielectric constant ε = 2.7 to 4.5), an epoxy resin (dielectric constant ε = 2.5 to 6.0), or the like is used. Although it is possible, it is desirable to use a resin material having a dielectric constant as low as possible. The thickness of the resin substrate 12 is sufficiently thick so that the microwave electric field generated by the electrodes 16, 17-1 and 17-2 leaks into the resin substrate 12, for example, 10 μm or more, preferably 30 μm or more. Of thickness. Thereby, the equivalent refractive index with respect to a microwave can be made small.

  The optical waveguide substrate 11 and the resin substrate 12 are bonded and fixed by an adhesive layer 14. As the adhesive forming the adhesive layer 14, an ultraviolet curable adhesive that is cured by irradiating ultraviolet rays or a thermosetting adhesive that is cured by heating can be used.

  The adhesive layer 14 is formed to have a sufficiently thin thickness, for example, 9 μm or less, preferably 1 μm or less, because it is necessary to improve the reliability of the optical modulator 10. In general, the thinner the adhesive layer is, the higher the adhesive strength is. Therefore, by reducing the thickness of the adhesive layer 14 in this way, the optical waveguide substrate 11 and the resin substrate 12 are not problematic in terms of reliability. Adhesive fixation can be performed with sufficient strength. Further, when the adhesive is cured, the temperature is increased by ultraviolet irradiation or heating, and then the temperature is cured and the temperature is decreased to generate a stress. However, when the adhesive layer 14 is thin, the generated stress can be reduced. .

Moreover, when the influence which the thickness of the adhesive bond layer 14 has on thermal drift (change of the drive voltage in measurement temperature -40 degreeC-85 degreeC) was measured, the following result was obtained.
Adhesive layer thickness (μm) Thermal drift (V) Evaluation
1 0.3 ◎
5 1.5 ○
9 2.9 ○
15 4.9 ×
Since the general allowable value of the thermal drift is 3.0 V or less, it can be seen that the thickness of the adhesive layer 14 in which the thermal drift is not a problem is preferably 9 μm or less, and more preferably 1 μm or less.

  As described above, the configuration in which the low dielectric constant thick resin substrate 12 is bonded and fixed to the optical waveguide substrate 11 by the thin adhesive layer 14 is adopted, so that the optical modulator 10 has a wider bandwidth and lower drive voltage ( In addition, the reliability can be improved at the same time, and thermal drift is not a problem.

  The dielectric constant of the adhesive used for the adhesive layer 14 is the same as that of the resin substrate 12 when the thickness of the adhesive layer 14 is thicker than 1 μm (in order to reduce the equivalent refractive index for microwaves). It is necessary that the dielectric constant is lower than 11. This is because if the thickness is greater than 1 μm, the adhesive layer 14 has a great influence on the equivalent refractive index of the microwave. On the other hand, when the thickness of the adhesive layer 14 is 1 μm or less, the influence of the adhesive layer 14 on the equivalent refractive index of the microwave is negligible. Therefore, the dielectric of the adhesive used for the adhesive layer 14 The rate may be higher than the dielectric constant of the optical waveguide substrate 11.

  The holding substrate 13 is a substrate that holds the optical waveguide substrate 11 via the resin substrate 12, and the thickness thereof is sufficiently thick so that the optical waveguide substrate 11 can be firmly held, for example, 200 μm or more, preferably About 0.5 to 1.0 mm. The material of the holding substrate 13 has a thermal expansion coefficient of the optical waveguide substrate 11 so that no stress is generated inside the optical waveguide substrate 11 when the environmental temperature fluctuates or the generated stress is reduced. Use a material close to the coefficient. More preferably, the optical waveguide substrate 11 and the holding substrate 13 are made of the same material. For example, when the optical waveguide substrate 11 is an LN substrate, quartz, alumina, or an LN substrate having a crystal orientation different from that of the optical waveguide substrate 11 can be used as the material of the holding substrate 13.

  The fixing method of the resin substrate 12 and the holding substrate 13 is not particularly limited in the present invention. For example, a method of bonding and fixing using the same adhesive as the adhesive layer 14, and the resin substrate 12 by heating. It is assumed that the material is made of an adhesive material, and a method of heating and fixing the resin substrate 12 to the holding substrate 13, a method of mechanically fixing (for example, screwing) the resin substrate 12 and the holding substrate 13, and the like are applied. be able to.

  The Mach-Zehnder optical waveguide 15 is, for example, a method of thermally diffusing a metal such as titanium (Ti) into the optical waveguide substrate 11, and protons in the optical waveguide substrate 11 (lithium (Li) atoms in the case of an LN substrate). For example, a method in which the optical waveguide substrate 11 is formed in a ridge shape, and light is guided to the ridge portion.

  The electrodes 16, 17-1 and 17-2 formed on the optical waveguide substrate 11 propagate light waves propagating through the branched optical waveguides 15-1 and 15-2 by causing microwaves to travel through the optical waveguide substrate 11. The signal electrode 16 is disposed between the branch optical waveguides 15-1 and 15-2, and the ground electrodes 17-1 and 17-2 are connected to the branch optical waveguides 15-1 and 15-2, respectively. It is arranged so as to face the signal electrode 16 with being sandwiched. With this arrangement, the microwave electric field has a main component in the principal axis P direction inside the branched optical waveguides 15-1 and 15-2. As described above, since the speed matching is achieved by the resin substrate 12 provided below the optical waveguide substrate 11, the electrodes 16, 17-1 and 17-2 are formed directly on the optical waveguide substrate 11. Yes. For this reason, the strength of the electric field of the microwave applied to the branched optical waveguides 15-1 and 15-2 does not decrease, and the drive voltage can be lowered. The modulation voltage input to each of the electrodes 16, 17-1 and 17-2 is supplied from an external high frequency power supply 20.

  As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to the above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to

For example, the resin substrate 12 may be replaced with a substrate made of a material other than resin having a lower dielectric constant than that of the optical waveguide substrate 11. As an example, a glass substrate made of alumina (dielectric constant ε = 6.0 to 10.0) or quartz (dielectric constant ε = 3.5 to 4.5) can be used.
Further, the specific configurations of the Mach-Zehnder optical waveguide 15 and the electrodes 16, 17-1, and 17-2 are not limited to those described above, and may be appropriately changed as necessary.

1 is a cross-sectional configuration diagram of a traveling wave type optical modulator according to an embodiment of the present invention. FIG. 1 is a plan configuration diagram of a traveling wave type optical modulator according to an embodiment of the present invention. FIG. It is a cross-sectional block diagram of the conventional traveling wave type | mold optical modulator.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Optical modulator 11 ... Optical waveguide substrate 12 ... Resin substrate 13 ... Holding substrate 14 ... Adhesive layer 15 ... Mach-Zehnder optical waveguide 15-1, 15-2 ... Branch optical waveguide 15-3 ... Input waveguide 15-4 ... Output waveguide 16 ... Signal electrode 17-1, 17-2 ... Ground electrode

Claims (3)

An optical waveguide substrate having a thickness of 30 μm or less;
A holding substrate for holding the optical waveguide substrate;
An optical waveguide device comprising:
A dielectric constant lower than that of the optical waveguide substrate, and a low dielectric constant substrate interposed between the optical waveguide substrate and the holding substrate;
An adhesive layer having a thickness of 9 μm or less for bonding the optical waveguide substrate and the low dielectric constant substrate;
Fixing means for fixing the low dielectric constant substrate to the holding substrate;
An optical waveguide device comprising:   2. The optical waveguide device according to claim 1, wherein the low dielectric constant substrate has a thickness of 30 [mu] m or more.   3. The optical waveguide device according to claim 1, wherein when the adhesive layer is made of a material having a higher dielectric constant than the optical waveguide substrate, the thickness thereof is set to 1 μm or less.

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