JP2012073328A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator that, even in use of a thin plate having an electro-optical effect and having a thickness of 10 μm or less, suppresses a warp or a crack of the thin plate and improves a product yield, and also suppresses the degradation of high-frequency characteristics.SOLUTION: An optical modulator includes a thin plate having an electro-optical effect and having a thickness of 10 μm or less, an optical waveguide formed on the thin plate, and modulation electrodes for modulating an optical wave transmitted in the optical waveguide. The thin plate is bonded through an adhesion layer to a reinforced substrate which has almost the same linear expansion coefficient as that of the thin plate and a dielectric constant lower than that thereof. In the reinforced substrate, grooves are formed on the surface of the adhesion layer side in parallel to the direction to which the optical waveguide extends.

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator using a thin plate having an electro-optic effect and having a thickness of 10 μm or less.

  In the optical measurement technical field and the optical communication technical field, an optical modulator using a substrate having an electro-optic effect is frequently used. In order to broaden the frequency response characteristics, the thickness of the substrate is reduced, the effective refractive index of the microwave that is the modulation signal is lowered, and the speed matching between the microwave and the light wave is achieved.

  Since such a thin plate has a low mechanical strength, it is used by being bonded to another reinforcing plate. If there is a difference in the coefficient of thermal expansion between the thin plate and the reinforcing plate, a stress is generated in the thin plate as the temperature changes, and a temperature drift phenomenon or the like occurs in the optical modulator.

  In order to suppress such temperature drift, as shown in Patent Document 1, the thermal expansion coefficients of the thin plate and the reinforcing plate are made the same. Patent Document 1 discloses a configuration that suppresses a DC drift phenomenon that occurs in an optical modulator due to a pyroelectric effect that occurs in a reinforcing plate.

  However, as the thickness of the substrate having the electro-optic effect has recently become practical, a substrate having a thickness of 10 μm or less has been put into practical use as the technology advances. In addition, since a modulation electrode having a height of several μm to several tens of μm is formed on the surface of the thin plate, the substrate warps as the temperature changes due to a difference in thermal expansion coefficient between the substrate and the electrode. And cracks have occurred.

  In addition, since the electrode forming process is performed after the reinforcing plate is bonded to the thin plate, even if the warpage generated in the thin plate wafer is cut into chips for each optical modulator, the longitudinal direction or the width direction of the chip There was a problem that warping remained. For this reason, the rate of occurrence of defective products such as deterioration of the characteristics of the optical modulator is high.

  In addition, when the same material as the thin plate such as lithium niobate is used for the material of the reinforcing plate, the dielectric constant of the reinforcing plate is the same as that of the thin plate, so the advantages of the thin plate cannot be exhibited and the high frequency characteristics deteriorate. It will be.

JP 2006-284963 A

  The problem to be solved by the present invention is to solve the above-mentioned problems, have an electro-optic effect, and even when a thin plate having a thickness of 10 μm or less is used, warpage and cracking of the thin plate are suppressed, and the product yield is reduced. It is an object to provide an optical modulator that improves the above. Another object of the present invention is to provide an optical modulator that suppresses deterioration of high-frequency characteristics.

  In order to solve the above problem, the invention according to claim 1 has an electro-optic effect, a thin plate having a thickness of 10 μm or less, an optical waveguide formed on the thin plate, and a light wave propagating in the optical waveguide is modulated. The thin plate is bonded to a reinforcing substrate having a dielectric constant lower than that of the thin plate through an adhesive layer, and is a surface of the reinforcing substrate on the adhesive layer side. A groove is formed below the ground electrode that constitutes a part of the modulation electrode in a direction parallel to the direction in which the optical waveguide extends.

  The invention according to claim 2 is the optical modulator according to claim 1, characterized in that an adhesive constituting an adhesive layer is embedded in the groove.

  The invention according to claim 3 is the optical modulator according to claim 1 or 2, characterized in that the reinforcing substrate has a linear expansion coefficient substantially the same as that of the thin plate.

  In the present invention, “substantially the same linear expansion coefficient” means that even if it is not the same linear expansion coefficient, it is possible to solve the problems of the present invention, even when the linear expansion coefficient is somewhat different from Means acceptable.

The invention according to claim 4 is the optical modulator according to any one of claims 1 to 3, characterized in that the thin plate is either LiNbO ₃ or LiTaO ₅ .

  According to the first aspect of the invention, there is provided a thin plate having an electrooptic effect and having a thickness of 10 μm or less, an optical waveguide formed on the thin plate, and a modulation electrode for modulating a light wave propagating in the optical waveguide. In the optical modulator, the thin plate is bonded to a reinforcing substrate having a dielectric constant lower than that of the thin plate via an adhesive layer, and is a surface on the adhesive layer side of the reinforcing substrate, and constitutes a part of the modulation electrode Since the groove is formed under the ground electrode in a direction parallel to the direction in which the optical waveguide extends, the reinforcing substrate holds the thin plate more firmly at the groove portion and along the groove. For this reason, it is possible to suppress warping and cracking of the thin plate. For this reason, the yield of products can be greatly improved. In addition, since the dielectric constant of the reinforcing substrate is lower than that of the thin plate, the high frequency characteristics of the optical modulator are not deteriorated.

  Furthermore, since the groove of the reinforcing substrate is disposed below the ground electrode among the modulation electrodes formed on the thin plate, it is possible to effectively suppress warpage of the thin plate due to the ground electrode. Since the modulation electrode is composed of a signal electrode and a ground electrode, and the ground electrode occupies most of the surface of the thin plate, the warp of the thin plate by the ground electrode is particularly large.

  According to the invention of claim 2, since the adhesive constituting the adhesive layer is embedded in the groove of the reinforcing substrate, it is possible to increase the adhesive strength between the thin plate and the reinforcing substrate, and the thin plate is deformed along the groove. It is possible to suppress this.

  According to the invention of claim 3, the reinforcing substrate has substantially the same linear expansion coefficient as that of the thin plate. Therefore, even if the thermal stress generated in the reinforcing substrate changes due to the groove formed in the reinforcing substrate, the thin plate Since it also changes at the same linear expansion coefficient, the thin plate is not easily affected by the change in the thermal stress of the reinforcing substrate. Thereby, it can suppress that the excess thermal stress is added to a thin plate by the groove | channel of a reinforcement board | substrate.

According to the invention of claim 4, since the thin plate is either LiNbO ₃ or LiTaO ₅ , the present invention can be applied to many optical modulators currently used.

It is a figure which shows the positional relationship of the arrangement | positioning position of the chip | tip with respect to a thin plate wafer, and the groove | channel formed in a reinforcement board. It is a figure which shows sectional drawing of the state cut | disconnected by the chip | tip of the optical modulator.

Hereinafter, the optical modulator of the present invention will be described in detail using preferred examples.
An optical modulator according to the present invention includes an electro-optic effect, a thin plate having a thickness of 10 μm or less, an optical waveguide formed on the thin plate, and a modulation electrode for modulating a light wave propagating in the optical waveguide. In the optical modulator, the thin plate is bonded to a reinforcing substrate having a dielectric constant lower than that of the thin plate via an adhesive layer, and is a surface on the adhesive layer side of the reinforcing substrate, and constitutes a part of the modulation electrode A groove is formed below the ground electrode to be parallel to the direction in which the optical waveguide extends.

  FIG. 1 shows the positional relationship between an optical modulator (indicated by individual rectangular portions in the figure) formed on a wafer composed of a substrate having an electro-optic effect and a groove formed in a reinforcing plate. FIG. The groove is formed on the surface of the reinforcing substrate. For example, in FIG. 1, grooves are formed in a direction parallel to the direction (lateral direction in FIG. 1) in which the optical waveguide (Mach-Zehnder type optical waveguide) of each chip extends. Naturally, the direction of the groove is preferably formed in a direction that effectively suppresses the warp of the thin plate. For example, the warp in the longitudinal direction of the chip is larger than the width direction of the chip. The groove is formed. In addition, since the modulation electrode (signal electrode and ground electrode) is formed along the optical waveguide, the internal stress of the electrode is easily generated in the direction in which the optical waveguide extends. Therefore, a groove along the optical waveguide is formed. It is preferable. In addition, it is more effective that the groove is parallel to the longitudinal direction of the chip and the extending direction of the optical waveguide, but even in a direction intersecting with these directions, for example, unless it is vertical, A certain level of effect can be expected to prevent the warpage of the thin plate.

As a substrate having an electro-optic effect, any single crystal of LiNbO ₃ , LiTaO ₅ or PLZT (lead lanthanum zirconate titanate) can be suitably used. In particular, LiNbO ₃ and LiTaO ₅ frequently used in optical modulators are preferable. The optical waveguide formed on the substrate is formed, for example, by thermally diffusing a high refractive index material such as titanium (Ti) on a LiNbO ₃ substrate (LN substrate).

  FIG. 2 is a view showing a cross section of the wafer of FIG. 1 cut into individual chips (light modulators). An optical waveguide (Mach-Zehnder type optical waveguide) is formed on the thin plate, and a modulation electrode (signal electrode and ground electrode) is disposed on the surface of the thin plate. A reinforcing substrate is bonded to the back surface of the thin plate via an adhesive layer. A groove is formed in a part of the reinforcing substrate. FIG. 2 shows a cross-sectional view of the Mach-Zehnder type optical waveguide that is branched into two branch waveguides and cut in the width direction of the chip.

The modulation electrode is composed of a signal electrode and a ground electrode, and can be formed by forming a Ti / Au electrode pattern on the surface of the substrate and using a gold plating method or the like. Furthermore, a buffer layer such as a dielectric SiO ₂ can be provided on the substrate surface after the formation of the optical waveguide, if necessary.

  As the adhesive for joining the thin plate and the reinforcing substrate, an ultraviolet curable adhesive or the like can be used. However, the thickness of the adhesive layer is preferably 20 μm to 100 μm. If it is thinner than 20 μm, it becomes difficult to achieve speed matching between the microwave that is the modulation signal and the light wave that propagates through the optical waveguide. On the other hand, when the thickness of the adhesive layer is larger than 100 μm, the reinforcing effect by the grooves formed in the reinforcing substrate cannot be exerted so much, and it becomes difficult to effectively suppress the warpage and cracking of the thin plate. Furthermore, since the adhesive is embedded in the groove of the reinforcing substrate, it is possible to increase the adhesive strength between the thin plate and the reinforcing substrate, and to suppress the deformation of the thin plate along the groove.

  Further, when the thickness of the adhesive layer is thin, there may be a case where sufficient speed matching between the microwave and the light wave cannot be obtained. In order to solve such a problem, the dielectric constant of the reinforcing substrate can be made lower than that of the thin plate, and deterioration of the high frequency characteristics of the optical modulator can be suppressed.

  The size (depth or width) of the groove formed in the reinforcing substrate depends on the thickness of the adhesive layer, but when the thickness of the adhesive layer is about 50 μm, the width of the square groove is 250 μm, the depth is 100 μm, or The width of the V groove is 270 μm and the inclination angle of the V groove is about 45 degrees.

  Since the position where the groove is formed in the reinforcing substrate is arranged below the thin ground electrode, the warpage of the thin plate due to the ground electrode can be effectively suppressed. Since the modulation electrode is composed of a signal electrode and a ground electrode, and the ground electrode occupies most of the surface of the thin plate, the warp of the thin plate due to the ground electrode is particularly large. Therefore, the modulation electrode is preferably configured as in the present invention. .

  Further, as shown in FIG. 2, from the position below the ground electrode and below the action part (of the optical waveguide) where the electric field formed by the modulation electrode acts on the optical waveguide to the side surface of the reinforcing substrate By forming a groove in between, it is possible to effectively suppress the influence of the warp of the thin plate on the action part. In addition, by forming the groove so as to avoid the vicinity of the optical waveguide, it is possible to avoid the influence of light wave scattering and stray light generation by the groove of the reinforcing substrate. Further, as shown in FIG. 2, in the region where the plurality of optical waveguides are arranged in parallel, outside the optical waveguide working portion and below the ground electrode, particularly with respect to the central axis of the working portion. By arranging the grooves so as to be symmetrical, it is possible to make the distribution of stress applied to each action part (stress accompanying the suppression of warpage of the thin plate) substantially the same, and DC drift due to temperature change Can also alleviate the effects.

  As the material of the reinforcing substrate, since a groove is formed in the reinforcing substrate itself, it is preferable to select a material having substantially the same linear expansion coefficient as that of the thin plate. This is because the grooves formed in the reinforcing substrate cause unevenness in the thermal stress generated in the reinforcing substrate. In order to prevent such a change in thermal stress from being transmitted to the thin plate, it is necessary to configure the linear expansion coefficients of the thin plate and the reinforcing substrate to be substantially the same.

  For example, when lithium niobate or tantalum niobate is used for the thin plate, calcium fluoride, magnesium fluoride, or the like can be suitably used for the reinforcing substrate. In these materials, the linear expansion coefficient of the reinforcing substrate is almost the same as that of lithium niobate, and the dielectric constant of the reinforcing substrate is about one fifth of that of lithium niobate, which is considerably lower than that of the thin plate material. It has become.

  As a method for forming a groove on the reinforcing substrate, it can be formed by machining using a blade. Etching or the like can be used as necessary. 1 and 2, a plurality of grooves are formed in the longitudinal direction of each chip. However, the present invention is not limited to this, and the grooves may be formed in the width direction or the oblique direction of the chip. Various modifications such as a length shorter than the chip size can be made.

  In order to compare the characteristics of an optical modulator with grooves as shown in FIG. 2 and a conventional optical modulator without grooves, a DC drift amount with respect to temperature was measured. The DC drift amount with respect to the temperature is obtained by evaluating the fluctuation range of the bias point in the range of −5 ° C. to 75 ° C. for the optical modulator module in which the chip is accommodated in the case. In the conventional optical modulator, the DC drift amount with respect to the temperature was 0.26V, whereas the two samples to which the present invention was applied were 0.15V and 0.17V. From this, it can be easily understood that the stress accompanying the temperature change is relaxed in the present invention compared to the conventional one, and as a result, the fluctuation of the bias point is also suppressed.

  As described above, according to the present invention, even when a thin plate having an electro-optic effect and a thickness of 10 μm or less is used, an optical modulator that suppresses warping and cracking of the thin plate and improves product yield. It becomes possible to provide. It is also possible to provide an optical modulator that suppresses deterioration of high-frequency characteristics.

Claims (4)

In an optical modulator having an electrooptic effect and having a thin plate having a thickness of 10 μm or less, an optical waveguide formed in the thin plate, and a modulation electrode for modulating a light wave propagating in the optical waveguide,
The thin plate is bonded to a reinforcing substrate having a dielectric constant lower than that of the thin plate through an adhesive layer,
A groove is formed in the direction parallel to the direction in which the optical waveguide extends on the surface of the reinforcing substrate on the adhesive layer side and below the ground electrode that constitutes a part of the modulation electrode. Characteristic light modulator.   2. The optical modulator according to claim 1, wherein an adhesive constituting an adhesive layer is embedded in the groove.   3. The optical modulator according to claim 1, wherein the reinforcing substrate has a linear expansion coefficient substantially the same as that of the thin plate. 4. The optical modulator according to claim 1, wherein the thin plate is one of LiNbO _{3 and} LiTaO _{5. 5} .

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