JP4408558B2 - Traveling waveform optical modulator and method of manufacturing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a traveling waveform optical modulator and a method for manufacturing the same.
[0002]
[Prior art]
In Japanese Patent Application Laid-Open No. 9-211402, an optical waveguide substrate is bonded to a base fixing substrate, and the optical waveguide is opposed to the fixing substrate. At this time, a groove is provided in the fixing substrate, and the optical waveguide is exposed to the air in the groove. An attempt is made to reduce the effective refractive index of the microwave by reducing the thickness of the optical waveguide substrate by polishing.
[0003]
[Problems to be solved by the invention]
However, in the technique described in Japanese Patent Application Laid-Open No. 9-212402, when the modulator is actually manufactured, it is difficult in processing to reduce the thickness of the substrate. In particular, when the thickness of the substrate is 20 μm or less, particularly 10 μm or less, cracks may occur in the substrate around the optical waveguide, or processing strain may remain in the substrate. Further, when the thickness of the substrate is reduced to 20 μm or less, the light confinement effect in the vertical direction of the optical waveguide becomes strong, and the optical waveguide mode pattern is deformed flat. For this reason, the mode mismatch increases between the external optical waveguide and the optical waveguide mode in the external optical fiber, and the coupling loss increases. Thus, the conventional method has a problem due to the reduction in the thickness of the substrate.
[0004]
It is an object of the present invention to enable high-speed modulation in a traveling waveform optical modulator that modulates light propagating in an optical waveguide in an optical waveguide substrate made of a ferroelectric electro-optic single crystal, and to generate cracks in the substrate. In other words, it is possible to prevent residual distortion and deformation of the optical waveguide mode pattern.
[0005]
[Means for Solving the Problems]
A traveling waveform optical modulator according to the present invention is made of a ferroelectric electro-optic single crystal and is formed on one main surface side of the optical waveguide substrate having a pair of opposing main surfaces. An optical waveguide, at least a pair of electrodes for applying a voltage for modulating light propagating in the optical waveguide, the electrode provided on the other main surface of the optical waveguide substrate, and one of the optical waveguide substrates Adhering one main surface of the fixing substrate and the optical waveguide substrate to the main surface of the optical waveguide substrate and the fixing substrate, covering the optical waveguide, and having a lower refractive index than the electro-optic single crystal An adhesive layer made of an agent is provided.
[0006]
In addition, the manufacturing method of the traveling waveform optical modulator according to the present invention,
Forming an optical waveguide on one principal surface side of a substrate material comprising a ferroelectric electro-optic single crystal and having a pair of opposed principal surfaces;
A step of adhering the fixing substrate to one main surface of the substrate material via an adhesive layer made of an adhesive having a refractive index lower than that of the electro-optic single crystal, and covering the optical waveguide with the adhesive layer,
By processing the other main surface of the substrate material, the thickness of the substrate material is reduced to form an optical waveguide substrate, and light propagating in the optical waveguide is formed on the other main surface of the optical waveguide substrate. A step of forming at least a pair of electrodes for applying a voltage to be modulated is provided.
[0007]
Hereinafter, the present invention will be further described with reference to the drawings.
[0008]
FIG. 1A is a plan view schematically showing a traveling wave optical modulator 1 according to an embodiment of the present invention, and FIG. 1B is an Ib-Ib line schematically showing the modulator 1. It is sectional drawing.
[0009]
The substrate 2 is made of a ferroelectric electro-optic single crystal. Such a crystal is not particularly limited as long as it can modulate light, and examples thereof include lithium niobate, potassium lithium niobate, lithium tantalate, KTP, glass, silicon, GaAs, and quartz. One or more single crystals selected from the group consisting of lithium niobate single crystals, lithium tantalate single crystals, and lithium niobate-lithium tantalate solid solution single crystals are particularly preferred.
[0010]
The substrate 2 includes one main surface 2a and the other main surface 2b. On one main surface 2a, a predetermined shape, for example, a Mach-Zehnder type optical waveguide 4 is formed. The optical waveguide 4 of this example includes an entrance portion 4a, branch portions 4b and 4c, and a coupling portion 4d.
[0011]
On the other main surface 2b of the substrate 2, electrodes 3A, 3B, 3C having a predetermined shape are formed. In this example, for example, an X plate and a Y plate of lithium niobate are used as the substrate 2, and therefore, TE mode light is propagated in the optical waveguide. Then, the branch portions 4b and 4c of the optical waveguide are provided in the gap region between the electrodes 3A to 3C.
[0012]
The other main surface 2b of the substrate 2 faces the air layer. One main surface 2 a of the substrate 2 is bonded to the main surface 6 a of the fixing substrate 6 through the adhesive layer 5. Reference numeral 6 b denotes a bottom surface of the fixing substrate 6. The adhesive layer 5 covers the optical waveguides 4b and 4c.
[0013]
The refractive index of the adhesive needs to be lower than that of the electro-optic single crystal of the substrate 2. In addition to this, the dielectric constant of the adhesive is desirably lower than the dielectric constant of the electro-optic single crystal of the substrate 2.
[0014]
Specific examples of the adhesive are not particularly limited as long as the above-described conditions are satisfied. However, the adhesive is comparatively less than a material having an electro-optic effect such as an epoxy adhesive, a thermosetting adhesive, an ultraviolet curable adhesive, or lithium niobate. An example is Aron ceramics C (trade name, manufactured by Toagosei Co., Ltd.) (thermal expansion coefficient 13 × 10 ⁻⁶ / K) having a similar thermal expansion coefficient.
[0015]
An outline of such a substrate manufacturing process will be described with reference to FIGS.
[0016]
A substrate material 2A made of a ferroelectric electro-optic single crystal and provided with a pair of opposing main surfaces 2a and 2c is prepared and cleaned. Then, the optical waveguide 4 is formed on the one main surface 2a side of the substrate material 2A (FIG. 2A). In this case, a known method such as a titanium diffusion method or a proton exchange method can be employed. Next, the fixing substrate 6 is bonded to one main surface 2a of the substrate material 2A through an adhesive layer 5 made of an adhesive having a refractive index lower than that of the electro-optic single crystal constituting the substrate material. At this time, the optical waveguide is covered with the adhesive layer 5 (FIG. 2B).
[0017]
Next, by processing the other main surface 2c of the substrate material 2A, the thickness of the substrate material 6 is reduced, and the optical waveguide substrate 2 is formed (FIG. 2C). Next, at least a pair of electrodes 3A-3C for applying a voltage for modulating light propagating in the optical waveguide 4 is formed on the other main surface 2b of the optical waveguide substrate 2 by vapor deposition or plating ( FIG. 1 (a) and FIG. 1 (b)).
[0018]
Such a traveling waveform optical modulator 1 can make the thickness of the substrate 2 very small, so that high-speed modulation is possible. In addition, in the portions of the optical waveguides 4b and 4c (gap region of the electrodes 3A to 3C), the side opposite to the polishing surface 2b of the substrate is covered and held by the adhesive layer 5, so that the impact during polishing is absorbed. In 2, residual distortion can be prevented. In addition, by covering the optical waveguides 4b and 4c with an adhesive layer, the optical waveguide mode pattern is prevented from becoming excessively flat in the vertical direction, and the coupling loss with an external optical waveguide or optical fiber is increased. Can be prevented.
[0019]
In the present invention, the thickness of the optical waveguide substrate is preferably 20 μm or less, and more preferably 10 μm or less. As a result, the effective refractive index nmw of the microwave can be significantly reduced. The thickness of the optical waveguide substrate is preferably 300 μm or more in terms of processing.
[0020]
The thickness of the adhesive layer is preferably 5 μm or more in order to absorb mechanical stress and vibration during polishing of the substrate material. Further, from the viewpoint of production, it is preferably 100 μm or less.
[0021]
The material of the fixing substrate preferably has a dielectric constant lower than that of the electro-optic single crystal. Such materials include glass such as quartz glass. When the fixing substrate is manufactured using such a material, even when the thickness of the adhesive layer is 20 μm or less, further 10 μm or less, it is possible to prevent an adverse effect on the propagation speed of the microwave by the fixing substrate.
[0022]
Further, the fixing substrate can be formed of a material having a dielectric constant equal to or higher than that of the electro-optic single crystal of the optical waveguide substrate 2. In this case, it is particularly preferable that the fixing substrate is formed of a single crystal of the same type as the single crystal constituting the optical waveguide substrate 2. However, in this case, in order to prevent an adverse effect on the propagation of microwaves, it is particularly preferable that the thickness of the adhesive layer is 20 μm or more.
[0023]
The traveling waveform optical modulator 1A in FIG. 3 is similar to the modulator 1 in FIG. 1, and thus the description of the components already shown in FIG. 1 is omitted.
[0024]
In the modulator 1A, ridge type optical waveguides 14b and 14c are formed on one main surface 2a of the substrate 2 so as to protrude. Each of the optical waveguides 14b and 14c protrudes and is embedded in the adhesive layer 5.
[0025]
【Example】
Hereinafter, more specific experimental results will be described.
(Manufacture of traveling waveform optical modulator of the present invention)
The main surface of the substrate material made of X-cut LiNbO ₃ single crystal was shaved, and the thickness of the substrate material was set to 300 μm. Next, according to the procedure described with reference to FIGS. 2A to 2C, the traveling waveform optical modulator 1 of FIGS. 1A and 1B was manufactured. Specifically, a Mach-Zehnder type optical waveguide 4 was formed on one main surface 2a of the substrate material by a titanium diffusion process and a photolithography method. This substrate material was bonded to the fixing substrate 6 made of quartz glass with an adhesive having a low dielectric constant. Next, the main surface 2b side of the wafer was polished by a general-purpose polishing apparatus, and the thickness of the optical waveguide substrate was set to 10 μm. Next, the end face of the optical waveguide was optically polished. Next, electrodes 3A-3C made of gold were formed on the main surface 2b by photolithography.
[0026]
The thickness of the fixing substrate 6 is 500 μm, the size of the electrode gap is 26 μm, and the width of the central electrode 3B is 10 μm. The thickness of the adhesive layer is 20 μm.
[0027]
However, in the titanium diffusion process, the width of the titanium pattern formed on the substrate material was changed to 5.5, 6.0, or 6.5 μm.
[0028]
(Manufacture of traveling waveform optical modulator of comparative example)
As a comparative example, a traveling waveform optical modulator 11 having the form shown in FIG. 9 was produced. The substrate material was not adhered to the fixing substrate, and the thickness of the optical waveguide substrate 12 was 500 μm. An optical waveguide 4 and electrodes 3A-3C are formed on one main surface 12a side of the optical waveguide substrate 12. 12b is the other main surface. The material of the optical waveguide substrate, the material of the optical waveguide, the material of the electrode, the dimensions of the electrode, and the like are the same as those of the above-described example of the present invention.
[0029]
(Measurement)
A single-core fiber array holding a 1.5 μm single-mode optical fiber was prepared, and this was coupled to a modulator, the optical fiber and the optical waveguide were aligned, and bonded with an ultraviolet curable resin. The insertion loss was measured for each modulator of the present invention and the comparative example, and the result is shown in FIG. In the example of the present invention, the insertion loss is comparable to the non-polished product of the comparative example even after the polishing is continued until the thickness of the optical waveguide substrate reaches 10 μm. In particular, when the width of the titanium pattern is 5.5 μm, the product of the present invention has much smaller insertion loss and is more stable. This is probably because when the width of the titanium pattern is 5.5 μm, the modulator of the comparative example is close to the cut-off region.
[0030]
FIG. 5 is a graph showing the relationship between the horizontal mode width and the titanium pattern width for the light emitted from the modulators of the present invention and the comparative example, and FIG. It is a graph which shows the relationship with the width | variety of a titanium pattern. When the width of the titanium pattern is within the range of 5.5 to 6.5 μm, the modulator of the present invention has a stable relative ratio between the width of the horizontal mode and the width of the vertical mode as compared with the modulator of the comparative example. You can see that Further, the width of the horizontal mode tends to be slightly larger in the modulator of the example of the present invention than in the modulator of the comparative example, but the deviation is not so large.
[0031]
FIG. 7 is a photograph showing a pattern of light emitted from the modulator of the present invention example when the width of the titanium pattern is 5.5 μm. As is clear from this photograph, a good waveguide mode could be confirmed even when it was close to the cutoff region (when the titanium pattern width was 5.5 μm).
[0032]
Further, TDR measurement was performed for the modulator of the present invention and the modulator of the comparative example, and the results are shown in FIG. As can be seen from the results, the modulator of the present invention has a shorter time required for reflection, and the speed of the microwave propagating through the electrode is faster.
[0033]
【The invention's effect】
As described above, the modulator of the present invention has a good waveguide mode of the emitted light, is stable, does not increase the insertion loss due to distortion, and can achieve a very high microwave propagation speed. It was a thing.
[Brief description of the drawings]
1A is a plan view schematically showing a traveling waveform optical modulator 1 according to an embodiment of the present invention, and FIG. 1B is a cross-sectional view schematically showing the modulator 1;
FIGS. 2A, 2B, and 2C are cross-sectional views schematically showing a manufacturing process of the modulator 1 of FIG.
FIG. 3 is a cross-sectional view schematically showing a traveling waveform optical modulator 1A according to another embodiment of the present invention.
FIG. 4 is a graph showing a comparison of insertion loss for each modulator of the present invention example and a comparative example (FIG. 9).
FIG. 5 is a graph showing the relationship between the width of a horizontal mode and the width of a titanium pattern for each modulator of the present invention and a comparative example.
FIG. 6 is a graph showing the relationship between the width of the vertical mode and the width of the titanium pattern for each modulator of the present invention and the comparative example.
FIG. 7 is a photograph showing a pattern of emitted light for a modulator according to an example of the present invention.
FIG. 8 is a graph showing the relationship between characteristic impedance and reflection time for traveling waveform optical modulators according to the present invention and comparative examples.
FIG. 9 is a cross-sectional view schematically showing a traveling waveform optical modulator of a comparative example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 1A Traveling waveform light modulator 2, 12 Optical waveguide board | substrate 2a One main surface 2b, 2c The other main surface 3A, 3B, 3C Electrode 4 Optical waveguide 4b, 4c, 14b, 14c Branch part of an optical waveguide 5 Adhesive layer 6 Fixing board 11 Progressive waveform optical modulator of comparative example

Claims (8)

An optical waveguide substrate made of a ferroelectric electro-optic single crystal and having a pair of opposing main surfaces, an optical waveguide formed on one main surface side of the optical waveguide substrate, and propagating through the optical waveguide At least a pair of electrodes for applying a voltage that modulates the light to be applied, the electrodes provided on the other main surface of the optical waveguide substrate, and bonded to the one main surface of the optical waveguide substrate The fixing substrate and the one main surface of the optical waveguide substrate are bonded to the fixing substrate, the optical waveguide is covered, and the adhesive has a lower refractive index than the electro-optic single crystal. A traveling wave optical modulator comprising an adhesive layer. The traveling wave optical modulator according to claim 1, wherein TE mode light is propagated through the optical waveguide. 3. The traveling wave optical modulator according to claim 1, wherein the optical waveguide substrate has a thickness of 20 [mu] m or less. The traveling waveform optical modulator according to any one of claims 1 to 3, wherein the fixing substrate is made of a material having a dielectric constant lower than that of the electro-optic single crystal. . The thickness of the adhesive layer is 20 μm or more, and the fixing substrate is made of a material having a dielectric constant equal to or higher than that of the electro-optic single crystal. A traveling waveform optical modulator according to claim 1. The electro-optic single crystal is one or more single crystals selected from the group consisting of a lithium niobate single crystal, a lithium tantalate single crystal, and a lithium niobate-lithium tantalate solid solution single crystal, The traveling waveform optical modulator according to any one of claims 1 to 5. Forming an optical waveguide on one principal surface side of a substrate material comprising a ferroelectric electro-optic single crystal and having a pair of opposed principal surfaces;
A fixing substrate is bonded to the one main surface of the substrate material through an adhesive layer made of an adhesive having a refractive index lower than that of the electro-optic single crystal. At this time, the optical waveguide is formed by the adhesive layer. Coating,
A process of reducing the thickness of the substrate material by processing the other main surface of the substrate material to form an optical waveguide substrate, and propagating in the optical waveguide on the other main surface of the optical waveguide substrate A method of manufacturing a traveling wave optical modulator, comprising the step of forming at least a pair of electrodes for applying a voltage for modulating light to be transmitted. 8. The method of manufacturing a traveling wave optical modulator according to claim 7, wherein the processing is continued until the thickness of the optical waveguide substrate becomes 20 [mu] m or less.

Images