JP4961372B2 - Optical waveguide device - Google Patents

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.

In general, an optical waveguide device such as an optical modulator is housed and packaged in a casing for preventing damage and ensuring reliability. Conventionally, in the case of an optical modulator using an optical waveguide substrate that is not thinned as a material of the package housing, a material having a thermal expansion coefficient close to that of the optical waveguide substrate (for example, if the optical waveguide substrate is an LN substrate) Stainless steel (SUS) or the like is used. In the case of the optical modulator of FIG. 3 using a thinned optical waveguide substrate, the holding substrate 102 is made of the same material as the optical waveguide substrate 101, and the optical waveguide substrate 101 is similarly used as the material of the package housing. In addition, a substrate having a thermal expansion coefficient close to that of the holding substrate 102 is used (for example, if the optical waveguide substrate 101 is an LN substrate, the holding substrate 102 is an LN substrate, and the package housing is made of stainless steel (SUS)).
JP 2003-215519 A

  By the way, in recent years, it has been studied to adopt a low-cost resin-made material for the package housing. However, if the optical modulator having the configuration of FIG. 3 in which the optical waveguide substrate is thinned is attached to the resin package housing, the holding substrate 102 (for example, LN substrate) and the package housing (resin) Since the thermal expansion coefficients are different, there is a problem that when the environmental temperature fluctuates, stress is applied to the optical waveguide substrate 101 via the holding substrate 102, and the characteristics of the optical modulator change.

  The present invention has been made in view of the above points, and an object of the present invention is to realize an optical waveguide device that operates in a wide band and can be driven at a low voltage at a low cost without deteriorating its characteristics.

The present invention has been made to solve the above-described problem, and is an optical waveguide device having an optical waveguide substrate having a thickness of 30 μm or less and a holding substrate that holds the optical waveguide substrate, and the holding substrate Is made of a resin having a dielectric constant lower than that of the optical waveguide substrate, and the optical waveguide substrate and the holding substrate are bonded by an adhesive layer having a thickness of 9 μm or less.
Further, the present invention is characterized in that, in the optical waveguide device described above, the holding substrate is attached to a resin casing.

  According to the above configuration, since the dielectric constant of the holding substrate is lower than the dielectric constant of the optical waveguide substrate, it is possible to realize a wide band and a low driving voltage, and the holding substrate is made of resin, so that the package When the housing is made of resin for cost reduction, the thermal expansion coefficients of the holding substrate and the package housing are close to each other, and stress is hardly generated on the optical waveguide substrate. Therefore, an optical waveguide device that operates in a wide band and can be driven at a low voltage can be realized at a low cost without deteriorating its characteristics.

  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, an optical waveguide device that operates in a wide band and can be driven at a low voltage can be realized at a low cost without deteriorating its characteristics.

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 resin substrate 12 that is a holding substrate for holding the optical waveguide substrate 11, an optical waveguide substrate 11 and a resin. An adhesive layer 14 for bonding and fixing the substrate 12, signal electrodes 16 and ground electrodes 17-1 and 17-2 formed on the optical waveguide substrate 11, a package housing 13 for fixing the resin substrate 12, It is comprised including.

  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 used for the purpose of reducing the equivalent refractive index to the microwave as described above and for the purpose of holding the optical waveguide substrate 11. As the material of the resin substrate 12, a material having a dielectric constant lower than that of the optical waveguide substrate 11 is used. 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 such that the microwave electric field generated by the electrodes 16, 17-1, 17-2 leaks greatly into the resin substrate 12 and can hold the optical waveguide substrate 11 firmly. Further, it is sufficiently thick, for example, 50 μm or more, preferably about 0.5 to 1.0 mm.

  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.

  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 package housing 13 is a member that stores the optical waveguide substrate 11, the resin substrate 12, and the portions made of the electrodes 16, 17-1, and 17-2 from the outside so as to prevent damage and ensure reliability. The resin substrate 12 is fixedly attached to a convex portion (pedestal) provided on the inner bottom surface. In FIG. 1, only a part of the bottom surface and the convex portion of the package housing 13 are shown.

The package housing 13 is made of resin in order to reduce the cost of the optical modulator 10. At this time, the resin material of the resin substrate 12 and the package housing 13 is selected so that the thermal expansion coefficient of the resin substrate 12 and the thermal expansion coefficient of the resin material used for the package housing 13 are close to each other. It is even more preferable that the resin substrate 12 and the package housing 13 are made of the same material. By doing in this way, the stress which generate | occur | produces inside the optical waveguide board | substrate 11 when environmental temperature fluctuates can be reduced. Therefore, it is possible to reduce the cost by making the package housing 13 made of resin without deteriorating the characteristics of the optical modulator 10. As a specific example, for example, polycarbonate (thermal expansion coefficient = 70 × 10 ⁻⁶ / K) or noryl (thermal expansion coefficient = 2.5 × 10 ⁻⁶ / K) is used for the resin material of the package housing 13. be able to. When polycarbonate is used, a two-component thermosetting epoxy adhesive (thermal expansion coefficient = 63 × 10 ⁻⁶ / K) can be used as the resin material of the resin substrate 12. When Noryl is used, an ultraviolet curable epoxy adhesive (thermal expansion coefficient = 20 × 10 ⁻⁶ / K) can be used as the resin material of the resin substrate 12.

  The method of fixing the resin substrate 12 and the package housing 13 is not particularly limited in the present invention. For example, the method of bonding and fixing using the same adhesive as the adhesive layer 14 and the resin substrate 12 are fixed. It is assumed that the resin substrate 12 is made of a material that becomes sticky by heating, and the resin substrate 12 is heated and fixed to the package housing 13. The resin substrate 12 and the package housing 13 are mechanically fixed (for example, screwed). Methods, etc. can be applied.

  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 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 changed as appropriate.

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 board | substrate 12 ... Resin board | substrate 13 ... Package housing 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:
The holding substrate is made of a resin having a lower dielectric constant than the optical waveguide substrate,
The optical waveguide device, wherein the optical waveguide substrate and the holding substrate are bonded by an adhesive layer having a thickness of 9 μm or less.   The optical waveguide device according to claim 1, wherein the holding substrate is attached to a resin casing.   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.

Images