JP2004245991A - Optical waveguide device and structure combining the same and optical transmission member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress variations of light outputs according to environmental temperature variations, and to reduce a generation possibility of a loss (a ripple) of a transmission property (S21). <P>SOLUTION: An optical waveguide device 10A comprises a substrate main body 1 made of an electrooptic material, an optical waveguide 2 disposed on one main surface 1a side of the main body 1, and an electrodes 3A-3C for applying electric signals to the waveguide 2. A supporting basic body 6 supports the main body 1. A first adhesive layer 7 is disposed between the other main surface 1b of the main body 1 and the supporting basic body 6. A coated basic body 8 is disposed on the one main surface 1a side of the main body 1. A conductive membrane 9 is disposed on the coated basic body 8, and a second adhesive layer 5 is disposed between the one main surface 1a of the main body 1 and the coated basic body 8. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device and a coupling structure between the optical waveguide device and an optical transmission member.
[0002]
2. Description of the Related Art With the development of multimedia, the demand for broadband communication has increased, and optical transmission systems exceeding 10 Gb / s have been put to practical use, and further speeding up is expected. As a device for modulating an electric signal (microwave signal) of 10 Gb / s or more into light, a lithium niobate optical modulator or a semiconductor electroabsorption modulator (EA modulator) is used.
In the case of a lithium niobate optical modulator, a Mach-Zehnder optical waveguide and a traveling-wave electrode are used. As an important condition for enabling high-speed optical modulation of 10 Gb / s or more, speed matching between microwave and light wave is required. It is necessary to meet the conditions. For this reason, generally, the thickness of the electrode is increased, and furthermore, the effective refractive index of the microwave is reduced by forming a low dielectric constant layer between the substrate and the electrode, thereby satisfying the above speed matching condition. .
On the other hand, as a method for achieving speed matching, a structure in which the thickness of the optical waveguide substrate is reduced to, for example, 10 μm can be considered. However, since the thickness of the substrate of the optical waveguide portion is substantially equal to the size of the optical waveguide (full width at half maximum: 1 / e ² ), the spot shape is flattened, and for example, the coupling loss with the fiber increases, and furthermore, However, there is a problem in that propagation loss due to surface roughness during thin processing increases. For this reason, the present applicant has disclosed in Patent Document 1 that the optical waveguide substrate of the traveling waveform optical modulator has a two-stage structure, and provides a thick portion and a thin portion immediately below the optical waveguide, and the thickness of the thin portion is set to, for example, It has been disclosed that the thickness is reduced to 10 μm. As a result, deterioration of the optical insertion loss can be improved, high-speed optical modulation can be performed without forming a buffer layer made of silicon oxide, and the product of the driving voltage Vπ and the electrode length L (Vπ · L) can be reduced.
[Patent Document 1]
[0005] It is known that the ripple caused by the resonance phenomenon of the optical waveguide substrate is shifted to a high frequency band by reducing the size of the optical waveguide substrate (Patent Document 2, Patent Document 2). Reference 3).
[Patent Document 2]
Patent No. 2669097 [Patent Document 3]
JP-A-5-241115
Further, the present applicant discloses that the thickness of the optical waveguide substrate is reduced, and that a separate reinforcing substrate is joined to the back surface of the optical waveguide substrate to impart mechanical strength. (Patent Document 4). In the optical modulator having only a thin optical waveguide substrate used in the present invention, the loss (ripple) of the transmission characteristic (S21) due to substrate resonance observed in a conventional optical modulator using a thick substrate having a thickness of several hundreds to several mm is used. Avoid problems.
[Patent Document 4]
[0007] However, it has been found that the following problems may occur when such an optical waveguide device is optically coupled to an external optical fiber. This will be described with reference to the schematic diagram of FIG. In FIG. 8, the bottom surface 1 b of the thin optical waveguide substrate 1 is bonded to the surface 6 a of the support base 6 by the adhesive layer 7, and the optical waveguide device 20 is manufactured. An optical waveguide and electrodes (not shown) are formed on the surface 1a side of the substrate body 1. On the other hand, the optical transmission member 12 is held by the holding member 11, and the end surface of the optical waveguide device 20 is brought into contact with the end surface 11 a of the holding member 11, and adhered by the adhesive 21.
However, when an optical waveguide device having such a structure is aligned with an optical fiber, optically coupled to the optical fiber, and passes laser light, the optical output sometimes fluctuates greatly with a change in environmental temperature. In addition, the occurrence probability of the loss (ripple) of the transmission characteristics (S21) tended to increase due to the variation at the time of manufacturing.
An object of the present invention is to provide an optical waveguide substrate having an optical waveguide and an electrode, a support base for supporting the optical waveguide substrate, and an optical waveguide having an adhesive layer for bonding the optical waveguide substrate to the support base. In a device, when an optical waveguide is optically coupled to an external optical transmission member, it is possible to suppress a change in optical output due to a change in environmental temperature.
Another object of the present invention is to reduce the probability of occurrence of a loss (ripple) in the transmission characteristic (S21).
[0011]
According to the present invention, there is provided a substrate body made of an electro-optical material, an optical waveguide provided on one main surface side of the substrate body, and an electrode capable of applying an electric signal to the optical waveguide. An optical waveguide substrate,
A support base for supporting the optical waveguide substrate,
A first adhesive layer provided between the other main surface of the substrate body and the support base,
A coating substrate provided on one main surface side of the substrate body,
A conductive film provided on the coated substrate,
And a second adhesive layer provided between one main surface of the substrate main body and the covering base, and relates to an optical waveguide device.
Further, the present invention relates to a coupling structure comprising the optical waveguide device and an optical transmission member optically coupled to the optical waveguide.
The present inventor has studied the cause of a change in optical output due to a change in environmental temperature after optically coupling an optical waveguide device with an optical transmission member. As a result, in the structure as shown in FIG. 8, attention was paid to the contact portion between the adhesives 21 and 7 and the end surface 11a of the holding member 11. Here, the adhesive 21 has a so-called meniscus shape. When the environmental temperature changes, the adhesives 21 and 7 are asymmetric, and the thickness of the adhesive 21 changes, so that the stress applied to the joint between the optical waveguide and the optical transmission member 12 changes. However, it is considered that this affects the light propagation loss. It has been practically difficult to control the shape of the adhesive 21 to reduce the change in light propagation loss with respect to a change in environmental temperature.
The inventor of the present invention has conceived of providing a coated base on one main surface side (optical waveguide side) of the substrate main body and bonding the one main surface of the substrate main body to the coated base.
Accordingly, the shape of the adhesive with respect to the optical waveguide becomes symmetrical at the contact portion between the adhesive and the end surface 11a of the holding member 11 and the periphery thereof. Thus, even when the environmental temperature changes, the uneven stress applied to the coupling portion between the optical waveguide and the optical transmission member and its surroundings can be reduced, and the change in light propagation loss due to the environmental temperature change can be reduced.
However, in this state, ripples may occur at, for example, 20 to 45 GHz due to the substrate radiation mode from the optical waveguide toward the coated substrate.
Therefore, by providing the conductive film on the surface of the coated substrate, the ripple due to the substrate radiation mode to the coated substrate is shifted to the high frequency side.
[0016]
1 to 4 show a connecting structure according to an embodiment of the present invention. FIG. 1 is a cross-sectional view schematically illustrating an optical waveguide device 10A, and FIG. 2 is a longitudinal cross-sectional view schematically illustrating a coupling structure of the optical waveguide device 10A, a holding member 11, and an optical transmission member 12. . FIG. 3 is a plan view of the coating substrate 8, and FIG. 4 is a plan view schematically showing the optical waveguide substrate 10A.
As mainly shown in FIGS. 1 and 4, the optical waveguide device 10A includes an optical waveguide substrate 18, a support base 6, and a cover base 8. In this example, the support base 6 and the cover base 8 have a flat plate shape, but these shapes are not limited to flat plates. On one main surface 1a of the substrate body 1, predetermined electrodes 3A, 3B, 3C are formed. In this example, a so-called coplanar type (CPW electrode) is used.
Although the electrode arrangement described above is adopted, the arrangement of the electrodes is not particularly limited. In this example, a pair of branch optical waveguides 2 are formed between adjacent electrodes, and a signal voltage is applied to each optical waveguide 2 in a substantially horizontal direction. The optical waveguide 2 constitutes a so-called Mach-Zehnder optical waveguide when viewed in a plane, and this planar pattern is shown in FIG.
A first adhesive layer 7 is provided between a surface 6a of the support base 6 on the substrate main body 1 side and the other main surface 1b of the substrate main body 1, so that the support base 6 and the substrate main body 1 are connected to each other. Glued.
A coating substrate 8 is provided on one main surface 1a of the substrate body 1, and the coating substrate 8 covers the electrodes and the optical waveguide. On the side of the substrate main body side wall surface 8b of the coating base 8, an elongated rectangular groove 8a is provided mainly as shown in FIGS. A pair of notches 13 for connector connection are provided at positions corresponding to the feed-through portions of the electrodes. A conductive film 9 is formed on the entire wall surface 8b of the covering base 8 including the rectangular groove. A gap 4 is formed between the rectangular groove 8a and one main surface 1a of the substrate body 1, and the signal electrode 3B and a part of the ground electrodes 3A and 3C face the gap 4.
As shown in FIGS. 1 and 2, a second adhesive layer 5 is formed between the peripheral portion of one main surface 1a of the substrate main body 1 and the peripheral portion of the wall surface 8b of the covering substrate 8. As a result, the substrate body 1 and the covering base 8 are bonded to each other.
According to the present embodiment, the shape of the adhesive with respect to the optical waveguide 2 is symmetrical at and around the contact portion between the adhesive and the end surface 11a of the holding member 11. Thereby, even if the environmental temperature changes, the uneven stress applied to the coupling portion between the optical waveguide 2 and the optical transmission member 11 and the periphery thereof can be reduced, and the change in the light propagation loss due to the environmental temperature change can be reduced. In addition, by providing the conductive film on the wall surface 8b of the covering base 8, the ripple due to the substrate radiation mode on the covering base 8 can be shifted to the high frequency side.
In a preferred embodiment, a gap 4 is provided by the coated base 8 and the substrate main body 1, and the optical waveguide 2 faces the gap 4. It is possible to fill the gap 4 with an adhesive, but by providing the gap 4, the effective refractive index of the microwave can be improved, which is advantageous for speed matching.
In a preferred embodiment, at least the signal electrode 3 B faces the gap 4.
In a preferred embodiment, a holding member 11 for holding the optical transmission member 12 is provided, and the first adhesive layer 7 is attached to the opposing surface 11 a of the holding member 11 opposing the substrate body 1. And the second adhesive layer 5 comes into contact. As a result, a change in light propagation loss due to a change in environmental temperature can be further reduced.
In a preferred embodiment, a conductive layer is provided on the surface of the support base on the substrate main body side. As a result, the probability of occurrence of ripples due to the substrate radiation mode in the support base can be reduced.
In a preferred embodiment, a first conductive layer is provided on one side surface of the substrate body and the support base, and a second conductive layer is provided on the other side surface of the substrate body and the support base. And the first conductive layer and the second conductive layer are electrically connected. Thereby, it is possible to reduce a change in light propagation loss due to pyroelectricity due to a temperature change.
FIG. 5 shows an optical waveguide device 10B according to this embodiment. The light transmitting member and the holding member are as shown in FIG.
In the optical waveguide device 10 B of the present embodiment, a conductive layer 25 is provided on the surface 6 a of the support base 6 on the substrate main body 1 side. A first conductive layer 15A is provided on one side surface 1c, 6b of the substrate body 1 and the support base 6, and a second conductive layer 15B is provided on the other side surface 1d, 6c of the substrate body 1 and the support base 6. Is provided. The first conductive layer 15A and the second conductive layer 15B are electrically connected by the conductive layer 14 on the bottom surface 6d of the support base 6.
In this embodiment, the conductive layer 15A extends to and covers the side surface 8c of the covering substrate 8, and the conductive layer 15B extends to and covers the side surface 8d of the covering substrate 8. Such a form is particularly preferred.
The materials of the conductive film 9 and the conductive layer 25 are preferably such that the microwave electric field becomes substantially zero. Specifically, the volume resistivity of the material of the conductive film 9 and the conductive layer 25 is preferably 10 ⁻⁴ Ω · cm or less, and further, a metal material having high conductivity such as gold, silver, and copper, particularly, a noble metal Is preferred.
The thicknesses of the conductive film 9 and the conductive layer 25 are not particularly limited, but may be set in consideration of the skin effect of the microwave at the operating frequency from the viewpoint of reducing the microwave electric field. Desirably, the thickness is 0.05 μm or more in a millimeter wave band. In the microwave band, the thickness is desirably 0.5 μm or more.
The conductive film 9 and the conductive layer 25 are not conductive layers for suppressing the influence of pyroelectricity, but are for cutting the substrate radiation mode of the microwave electric field into the inside of the supporting base or the covering base. It is. For this reason, it is preferable that the conductive film 9 and the conductive layer 25 be formed as a continuous layer on the wall surface of the supporting base or the covering base.
The substrate body constituting the optical waveguide substrate is made of a ferroelectric electro-optic material, preferably a single crystal. Such a crystal is not particularly limited as long as light can be modulated. Examples thereof include lithium niobate, lithium tantalate, lithium niobate-lithium tantalate solid solution, potassium lithium niobate, KTP, GaAs, and quartz. it can.
The electrode is not particularly limited as long as it is a material having low resistance and excellent impedance characteristics, and may be made of a material such as gold, silver, or copper.
The material of the supporting substrate and the covering substrate is not limited, but preferably, the difference between the linear thermal expansion coefficient of the substrate body and the linear thermal expansion coefficient of the supporting substrate or the covering substrate is set to ± 50% or less. It is possible to suppress a change in the degree of light modulation when the environmental temperature changes. In this case, the material of the substrate body and the material of the supporting base or the covering base may be the same or different.
Further, the supporting base or the covering base can be formed of a material having a dielectric constant higher than that of the electro-optic single crystal of the optical waveguide substrate. In this case, it is particularly preferable that the supporting substrate or the covering substrate is formed of a single crystal of the same kind as the single crystal constituting the substrate body.
The material constituting the supporting substrate or the coated substrate, particularly a single crystal, and the material constituting the substrate main body, particularly the single crystal, are of the same kind. It is sufficient if the composition is the same, and the other added components may be different.
The electrode is preferably provided on one main surface side of the substrate main body, may be formed directly on one main surface 1a of the substrate main body, or may be formed on the buffer layer. Known materials such as silicon oxide, magnesium fluoride, silicon nitride, and alumina can be used for the buffer layer.
In the present invention, the optical waveguide is formed on one main surface side of the substrate main body. The optical waveguide may be a ridge-type optical waveguide formed directly on one main surface 1a of the substrate main body, and a ridge-type optical waveguide formed on one main surface 1a of the substrate main body via another layer. It may be an optical waveguide. Further, an optical waveguide formed on the substrate body 1 by an internal diffusion method or an ion exchange method, for example, a titanium diffusion optical waveguide or a proton exchange optical waveguide may be used.
The refractive index of the adhesive forming the adhesive layers 5 and 7 is preferably lower than the refractive index of the electro-optic material forming the substrate body. Further, it is desirable that the dielectric constant of the adhesive is lower than the dielectric constant of the electro-optical material forming the substrate body.
Specific examples of the adhesive are not particularly limited, but a thermal expansion relatively close to a material having an electro-optical effect, such as an epoxy adhesive, a thermosetting adhesive, an ultraviolet curable adhesive, and lithium niobate. "Aron Ceramics C" (trade name, manufactured by Toa Gosei Co., Ltd.) having a coefficient of thermal expansion (13 × 10 ⁻⁶ / K) can be exemplified. Also, the adhesive may be conductive. Examples of such a conductive adhesive include carbon and silver pastes, and further, an anisotropic conductive film.
The present invention can also be applied to an electrode arrangement of a so-called asymmetric coplanar strip line (A-CPS electrode) type. Further, the present invention can be applied to a so-called independent modulation type traveling waveform optical modulator.
The method for electrically connecting the first conductive layer and the second conductive layer is not particularly limited. For example, the connection portion 14 can be provided on the bottom surface 6d of the support base 6.
Further, the first conductive layer and the second conductive layer can be connected by a bonding wire.
The optical transmission member is not limited as long as it can be optically coupled to the optical waveguide, but an optical fiber is preferable. Further, the holding member 11 may be an optical fiber array or a ferrule.
[0045]
Example (Example 1)
The optical modulator 10A shown in FIGS. 1 to 4 was manufactured and coupled to an optical fiber. Specifically, an X-cut 3-inch wafer (LiNbO ₃ single crystal: thickness 0.3 mm) was used. Two X-cut 3-inch wafers were bonded. Specifically, after each wafer is washed and dried, SOG (spin on glass) is spin-coated, and the coated surfaces are superimposed on each other and a load is applied thereto, and the coated surface is maintained at 1070 ° C. in an oxygen atmosphere containing water vapor. And glued. Next, the bonded body was placed on a polishing platen, and the substrate body 1 was processed by horizontal polishing and polishing (CMP) to a thickness of 10 μm. Next, the bonded body was removed from the polishing plate, and washed with an organic solvent and UV. Next, after forming titanium to a thickness of 500 to 1000 angstroms by spalling, a resist pattern was formed by photolithography, a titanium pattern was formed by reactive ion etching, and the resist was peeled off with an organic solvent. Then, the temperature was kept at 1020 ° C. in an oxygen atmosphere containing water vapor to form a Mach-Zehnder type titanium diffused optical waveguide.
Next, CPW electrodes 3A, 3B and 3C were formed by a plating process.
Next, the X-cut 3-inch wafer (LiNbO ₃ single crystal: thickness 0.3 mm) was processed by an excimer laser to form a groove 8a having a width of 500 μm and a depth of 100 μm. Next, a gold film 9 having a thickness of 1000 angstroms was formed by a vapor deposition method, and a coated substrate 8 was manufactured.
Next, one main surface 1a of the substrate main body 1 and the covering substrate 8 were bonded with an epoxy adhesive. At this time, the adhesive was not filled in the groove 8a.
Next, the end face of the optical waveguide device 10A was optically polished and cut into chips by dicing. The device 10A was connected to the optical fiber 12 held by the ferrule 11, adjusted for the optical axis, and bonded and fixed with a UV curable resin.
(Comparative Example 1)
An optical waveguide device as shown in FIG. 8 was manufactured in the same manner as in Example 1 except for the coated substrate. This optical waveguide device was adhered to the end face 11a of the ferrule 11 with a UV curable resin adhesive 21 and bonded to an optical fiber.
With respect to the structure of each example, the temperature characteristics of the optical power loss from −20 ° C. to 85 ° C. were evaluated. As a result, the variation in Example 1 was 0.2 dB, and the variation in Comparative Example 1 was 0.6 dB. The measurement results in Example 1 are shown in FIG. 6, and the measurement results in Comparative Example 1 are shown in FIG.
The S21 characteristic was measured with a vector network analyzer.
The probe used was a Cascade CPW probe “ACP 50-250”.
For each of Example 1 and Comparative Example 1, 100 devices were manufactured, and the presence or absence of a ripple in a frequency region of 45 GHz or less was confirmed. In Example 1, the probability of occurrence of a ripple in a frequency region of 45 GHz or less was 1%, and in Comparative Example 1, it was 30%.
[0053]
As described above, according to the present invention, an optical waveguide substrate having an optical waveguide and an electrode, a supporting base for supporting the optical waveguide substrate, and bonding the optical waveguide substrate to the supporting base. In an optical waveguide device having an adhesive layer, when an optical waveguide is optically coupled to an external optical transmission member, a change in optical output due to a change in environmental temperature can be suppressed. Further, the probability of occurrence of loss (ripple) in the transmission characteristics (S21) can be reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an optical waveguide device 10A according to the present invention.
FIG. 2 is a longitudinal sectional view showing a coupling structure of an optical waveguide device 10A, a holding member 11, and an optical transmission member 12.
FIG. 3 is a plan view of a coating substrate 8;
FIG. 4 is a plan view of the optical waveguide substrate 18. FIG.
FIG. 5 is a cross-sectional view schematically showing an optical waveguide device 10B according to another embodiment.
FIG. 6 is a graph showing temperature characteristics of optical power in the structure of the first embodiment of the present invention.
FIG. 7 is a graph showing temperature characteristics of optical power in the structure of Comparative Example 1.
FIG. 8 is a longitudinal cross-sectional view schematically illustrating a coupling structure between an optical waveguide device 20 and an optical transmission member 12 according to a comparative example.
DESCRIPTION OF SYMBOLS 1 Substrate main body 1a One main surface of substrate main body 1 1b The other main surface of substrate main body 2 Optical waveguides 3A, 3B, 3C Electrodes 4 Void 5 Second adhesive layer 6 Support base 6a Support base Surface on substrate body side 7 First adhesive layer 8 Covered substrate 8a Groove 8b Wall surface of coated substrate 8 on substrate body side 10A, 10B Optical waveguide device 11 Holding member 11a End face of holding member 11 Optical transmission member 18 Optical waveguide substrate

Claims (7)

A substrate main body made of an electro-optical material, an optical waveguide provided on one main surface side of the substrate main body, and an optical waveguide substrate including an electrode capable of applying an electric signal to the optical waveguide;
A support base supporting the substrate body,
A first adhesive layer provided between the other main surface of the substrate body and the support base,
A coating substrate provided on the one main surface side of the substrate body,
A conductive film provided on the coated substrate,
An optical waveguide device, comprising: a second adhesive layer provided between the one main surface of the substrate main body and the coating substrate. The optical waveguide device according to claim 1, wherein a void is formed by the covering substrate and the substrate body, and the optical waveguide faces the void. The optical waveguide device according to claim 2, wherein the electrode includes a signal electrode and a ground electrode, and the signal electrode faces the gap. The optical waveguide device according to claim 1, further comprising a conductive layer formed on a surface of the support base on the substrate main body side. 5. A first conductive layer is provided on one side surface of the substrate body and the support base, and a second conductive layer is provided on the other side surface of the substrate body and the support base. The optical waveguide device according to any one of claims 1 to 4, wherein a conductive layer and the second conductive layer are electrically connected. The optical waveguide device according to claim 1, wherein a substrate thickness of the substrate body is 30 μm or less. An optical waveguide device and an optical transmission member, comprising: the optical waveguide device according to claim 1; and an optical transmission member optically coupled to the optical waveguide. And connection structure.

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