JP4691481B2 - Optical waveguide device - Google Patents

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device that can effectively remove higher-order mode light in an optical waveguide device using a thin plate having a thickness of 20 μm or less.

Various optical processing is performed in optical fiber communication systems and measurement systems. Among them, integration is easy, and optical waveguide elements are used as a means to efficiently control the intensity and phase of light waves. Has been.
Examples of optical waveguide elements include a light intensity modulator in which a ferroelectric material, which is a substrate having an electro-optic effect, is used, and an optical waveguide and a modulation electrode for modulating light propagating through the optical waveguide are formed on the substrate. It has been known. Such an optical waveguide element has a branch waveguide section that branches a specific optical waveguide into a plurality of branch waveguides, for example, a Y branch waveguide, like a Mach-Zehnder type optical waveguide.

On the other hand, as shown in FIG. 1, an optical fiber 5 is connected to one end of the optical waveguide element in order to allow light waves to enter the optical waveguide element. In FIG. 1, 1 is a substrate having an electro-optic effect for forming an optical waveguide element such as lithium niobate, 2 is an incident side waveguide, and 3 and 4 are branch waveguides.
When connecting the optical fiber to the optical waveguide element, incident light is reflected from the incident surface of the optical waveguide element and travels backward in the optical fiber 5, or the other surface (outgoing surface) and incident surface in the optical waveguide element. In order to prevent light from being repeatedly reflected between the optical waveguide elements 6 and interfering in the optical waveguide element, the normal direction of the incident surface of the optical waveguide element and the connection direction of the optical fiber are shifted, or the optical waveguide The direction of the optical waveguide (for example, the incident side waveguide 2) connected to the incident surface and the output surface of the element is shifted from the normal direction of the incident surface and the output surface.

However, in such a configuration, the misalignment of the optical fiber, the manufacturing error of the optical waveguide, the mismatch of the mode field shape between the optical fiber and the optical waveguide in the optical waveguide element, the connection between the optical fiber and the optical waveguide element Due to axial misalignment, etc., higher order mode light is generated in the light propagating through the optical waveguide, and this higher order mode light may propagate through the optical waveguide.
The components of the higher-order mode light and the fundamental mode light generated at the incident portion interfere with each other, and the light distribution fluctuates (hereinafter referred to as “fluctuating light”) as the light propagates. Conventionally, when the fundamental mode light is incident on a symmetric Y-branch waveguide, the branching ratio is 1: 1 due to the symmetry of the waveguide shape. However, when the oscillating light 7 (schematically shown in FIG. 1) enters the symmetric Y-branch waveguide, the branching ratio deviates greatly from 1: 1. Furthermore, because of interference between the fundamental mode and the higher-order mode, the branching ratio varies depending on the length of the incident waveguide, and the branching ratio varies depending on the wavelength. Therefore, it is necessary to remove high-order mode light from the optical waveguide before reaching the branching waveguide portion.

In the conventional optical waveguide element, since the thickness of the substrate is several hundred μm, even if the axial deviation of the optical fiber and the light incident angle are not orthogonal to the end face of the optical waveguide element, the linear length of the incident-side waveguide 2 is about 2 mm. If there is, the higher-order mode light 21 is emitted from the optical waveguide 2 as shown in FIG. In particular, in the case where a diffusion optical waveguide is formed on the surface of a substrate (thick plate) having such a thickness, a lateral direction (a direction parallel to the substrate surface) and a longitudinal direction (direction parallel to the substrate surface) toward the light wave propagation direction of the optical waveguide. In the thickness direction of the substrate), since the diffusion rate is in the same order, light confinement in the vertical direction is weaker than in the horizontal direction. For this reason, when propagating light leaks from the optical waveguide, it leaks from the vertical direction (inner direction) of the substrate.
The higher-order mode light leaking from the optical waveguide 2 propagates in the depth direction of the substrate and does not recombine with the optical waveguide 2, resulting in deterioration of the branching ratio in the Y-branch waveguide. Absent. Reference numeral 20 in FIG. 2A denotes fundamental mode light.

However, in the case of an optical waveguide element using a thin plate having a thickness of 20 μm or less, as shown in FIG. 2B, the boundary surface of the optical waveguide 2 and the lower surface of the substrate 1 (usually on the lower surface of the substrate 1) The adhesive layer for joining the substrate and the reinforcing plate is formed), and the optical confinement in the vertical direction becomes stronger than that of the thick plate due to the change in the refractive index.
For this reason, the linear length of the incident-side waveguide 2 is also insufficient with the conventional 2 mm, and it is required to make the linear length longer. However, if the incident-side waveguide becomes longer, the size of the entire optical waveguide device is reduced. Difficult to do.

In Patent Document 1, as shown in FIG. 3, in a Y-branch optical waveguide device including a straight waveguide 2 formed on a substrate 1 and two branching waveguides 3 and 4, a front of the Y-branch portion is provided. It is disclosed that dummy waveguides 10 and 11 for higher-order mode radiation having an opening angle θ ₁ larger than the opening angle θ ₂ of the Y branch portion are provided in the straight waveguide portion.
JP 2005-181748 A

  By providing such a dummy waveguide, it is possible to stabilize the power distribution ratio in the Y branch portion while shortening the device length. In addition, it is possible to improve the work efficiency by suppressing the deterioration of the distribution ratio due to the influence of the axial misalignment of the incident fiber and the like so that no alignment error occurs during automatic alignment.

  In Patent Document 1, the opening angle of the dummy waveguide, the core refractive index, and the core width are adjusted so that the single mode light is not coupled at the wavelength of the light used in this device. In order to remove more reliably, a plurality of dummy waveguides are arranged, or the angle of each waveguide is changed in the plurality of dummy waveguides.

However, when a single mode optical waveguide is formed on the substrate, the waveguide width of the thick plate is about 4 to 8 μm.
When the technique disclosed in Patent Document 1 is used, a mode conversion loss (incident light is applied to the optical waveguide element) by increasing the ratio of the waveguide widths of the straight waveguide 2 and the dummy waveguides 10 and 11. It means the ratio of the decrease in the light intensity of the fundamental mode component after passing through the three-branch waveguide in which the dummy waveguide is formed with respect to the light intensity of the fundamental mode component when incident. Become.
For this reason, to apply the technique according to Patent Document 1 to an optical waveguide element using a thick plate, it is difficult to create a dummy waveguide with a small optical loss because the width of the straight waveguide is large. When the dummy waveguide is provided, excess loss in the three-branch waveguide in which the dummy waveguide is formed increases.

  The problem to be solved by the present invention is to solve the above-described problems, and even when an optical waveguide is formed on a thin plate having a thickness of 20 μm or less, the oscillating light generated at the incident portion is suppressed and the Y branch It is an object of the present invention to provide an optical waveguide device having an improved branching ratio in a branching waveguide section that branches a specific optical waveguide such as a section into a plurality of branching waveguides.

  As a result of diligent research, the present inventors have found that when an optical waveguide is formed on a thin plate, the optical confinement effect in the vertical direction is enhanced as described above, but higher-order mode removal such as a higher-order mode branching waveguide is eliminated. By providing a means, not only can the incident light fluctuation (the light propagating in the waveguide includes a higher-order mode, the light distribution oscillates with propagation) be eliminated, as well as excess loss due to mode conversion loss can be eliminated. The present invention has been completed by finding out that it is possible to do so.

In order to solve the above problems, an invention according to claim 1 is directed to an optical waveguide device having an electrooptic effect, a substrate having a thickness of 20 μm or less, and an optical waveguide formed on the substrate. reinforcing plate through the adhesive layer is contact wear in, the refractive index difference between the substrate and the adhesive layer is set to substrate optical thickness direction confinement becomes stronger, the optical waveguide, the optical guide propagation and incident portion through which light enters the waveguide, the light shines said input have a branch waveguide section branches into at least two, between the entrance portion and the branch waveguide portions, an optical waveguide High-order mode removal means for removing the higher-order mode light of the light to be emitted is provided , and the higher-order mode removal means is a high-order mode branch waveguide having at least two branch waveguides branched from the optical waveguide. The higher-order mode branching waveguide is provided on the substrate. Plate is characterized that you have been formed on the side which is adhered.

The invention according to claim 2 is the optical waveguide device according to claim 1, the branch waveguides for higher-order mode is characterized by being formed by thermally diffusing impurities to the substrate.

The invention according to claim 3, in the optical waveguide device according to claim 1, the branch waveguides for higher-order mode, that is formed by loading a high refractive index materials on the substrate Features.

The invention according to claim 4, in the optical waveguide device according to claim 1, the high order mode branch waveguides, by etching the substrate to form an optical waveguide, for the effective high-order mode branch wherein the waveguides are formed.

According to the first aspect of the present invention, in an optical waveguide element having an electrooptic effect and having a thickness of 20 μm or less and an optical waveguide formed on the substrate, the substrate has an adhesive layer interposed therebetween. a reinforcing plate is against wearing Te, the refractive index difference between the substrate and the adhesive layer is set so that the confinement in the thickness direction of the substrate light becomes stronger, the optical waveguide, light is incident on the optical waveguide the incident portion, the light shines said input have a branch waveguide section branches into at least two, between the entrance portion and the branch waveguide portions, higher order modes of light propagating through the optical waveguide High-order mode removing means, which is a branch waveguide for a higher-order mode, which is formed on the side of the substrate to which the reinforcing plate is bonded and has at least two branch waveguides branched from the optical waveguide for removing light. In order to provide an optical waveguide device using a thin plate, The higher-order mode light generated by the incident end of the waveguide element effectively removed, it is possible to improve the branching ratio in the branched waveguide portions.

According to the invention of claim 2, higher mode branch waveguides, because they are formed by thermally diffusing impurities into the substrate, simultaneously or likewise with the formation of an optical waveguide, for higher mode branch It can form waveguides, nor can complicate the manufacturing process of the optical waveguide device.

According to the invention of claim 3, higher mode branch waveguides, because they are formed by loading a high refractive index materials on board, upon formation of the higher order mode branch waveguides Select a material different from that of the optical waveguide, increase the degree of design freedom such as the thickness of these shapes (thickness in the same direction as the thickness direction of the substrate) and the arrangement on one or both sides of the substrate, and more efficiently Higher order mode light can be removed.

According to the invention of claim 4, the high-order mode branch waveguides, by etching the substrate to form an optical waveguide, since the branch waveguides for effective high-order mode is formed, in the manufacturing process of the optical waveguide device by adding a substrate etching step, it becomes possible to easily form a higher order mode branch waveguides. In the case of forming the optical waveguide of the optical waveguide device as a ridge waveguide when forming the ridge waveguide, it is possible to form a higher order mode branch waveguides together.

Hereinafter, the present invention will be described in detail using examples and reference examples .
The present invention relates to an optical waveguide element having an electro-optic effect and having a thickness of 20 μm or less and an optical waveguide formed on the substrate, wherein the optical waveguide is incident to make the light incident on the optical waveguide. And a high-order mode light of light propagating through the optical waveguide between the incident part and the branching waveguide part. A high-order mode removing means is provided , and the high-order mode removing means is a high-order mode branching waveguide having at least two branching waveguides branched from the optical waveguide. branched waveguides, reinforcing plate of the substrate is characterized that you have been formed on the side which is adhered.

FIG. 4 is a schematic view showing a first reference example according to the optical waveguide element of the present invention.
The substrate 1 is a substrate having an electro-optic effect, and is composed of, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), and a quartz-based material. It is preferable to use lithium niobate (LN) because it is composed of an X-cut plate, a Y-cut plate, and a Z-cut plate, and is particularly easy to configure as an optical waveguide device and has high anisotropy. . In the present invention, a thin plate having a thickness of 20 μm or less is used for the substrate 1.

FIG. 4A shows a part of a top view of the optical waveguide device. The optical waveguide is formed on the substrate 1 by, for example, depositing titanium (Ti) or the like on the substrate and then thermally diffusing it. 2 to 4 are provided. Reference numeral 2 denotes a waveguide (incident side waveguide) arranged on the light incident side of the optical waveguide element. The incident end portion of the incident side waveguide 2 is simply referred to as an “incident portion”. Reference numerals 3 and 4 denote branch waveguides branched from the Y branch portion of the optical waveguide.
A feature of the present invention is that high-order mode branching waveguides 30 and 31 for removing high-order mode light are provided in a part of the incident-side waveguide 2 as high-order mode removal means.
In the optical waveguide device of FIG. 4A, the light propagates through the optical waveguide through a buffer layer made of silicon oxide (SiO ₂ ) or the like for reducing the absorption of light propagating into the electrode layer. A modulation electrode (for example, a signal electrode or a ground electrode) for modulating light is not shown in order to simplify the description. The same applies to the following drawings.

  FIG. 4B shows a cross-sectional view taken along one-dot chain line A in FIG. Since the substrate 1 is a thin plate, its mechanical strength is weak, and a reinforcing plate 41 is bonded via an adhesive layer 40 to reinforce the substrate 1. Usually, the thickness of the adhesive layer is several tens to several hundreds μm, and the reinforcing plate is several hundreds μm.

  As shown in FIG. 4, the high-order mode branching waveguides 30 and 31 can be formed by depositing Ti or the like on the substrate and thermally diffusing similarly to the optical waveguides 2 to 4. In this case, it is possible to form a branch waveguide for higher-order modes simultaneously with the optical waveguide. Thereby, complication of a manufacturing process can also be suppressed.

FIG. 5 shows a second reference example according to the optical waveguide device of the present invention.
FIG. 5A shows a part of a top view of the optical waveguide device, and FIG. 5B shows a cross-sectional view taken along one-dot chain line B in FIG.
In FIG. 5, high-order mode branching waveguides 50 and 51 are formed on the surface of the substrate 1 as high-order mode removing means. The high-order mode branching waveguide can be formed of a material having a higher refractive index than that of the substrate 1 such as Si.

FIG. 6 is a diagram showing a first embodiment of the optical waveguide device according to the present invention . FIG. 6A is a diagram showing the high-order mode branch waveguide shown in FIG. It is arranged above. Reference numerals 52 and 53 denote branch waveguides for higher-order modes. By effectively utilizing the back surface of the substrate in this manner, it is possible to form a branch waveguide for a higher-order mode having a free shape, regardless of the configuration of the modulation electrode or the like disposed on the substrate surface side. Become.

  FIG. 6B is a combination of FIG. 5B and FIG. 6A, and high-order mode branching waveguides 50 to 53 are arranged on the front surface and the back surface of the substrate 1, respectively. Is. As described above, by arranging many high-order mode branching waveguides in the vicinity of the incident-side waveguide 2, it becomes possible to more efficiently remove high-order mode light.

FIG. 7 shows a third reference example according to the optical waveguide device of the present invention.
FIG. 7A shows a part of a top view of the optical waveguide device, and FIG. 7B shows a cross-sectional view taken along one-dot chain line C in FIG.
In FIG. 7, high refractive index regions 60 and 61 are formed on the substrate 1 as high-order mode removal means. As a method for forming the high refractive index region, it is also possible to form the high refractive index region simultaneously with the optical waveguide in the same manner as described with reference to FIG.

FIG. 8 shows a fourth reference example according to the optical waveguide element of the present invention.
FIG. 8A shows a part of a top view of the optical waveguide device, and FIG. 8B shows a cross-sectional view taken along the alternate long and short dash line D in FIG.
In FIG. 8, as a higher mode removal means, light absorption regions 70 and 71 are formed on the surface of the substrate 1 by thermally diffusing impurities into the substrate. Examples of the impurity material that forms the light absorption region include Ni. The thermal diffusion of impurities can be performed not only from the front surface side of the substrate 1 but also from the back surface side or both.

  As a method for forming the light absorption region, as shown in FIG. 9, it is also possible to load a light absorption material such as metal on at least one of the front surface and the back surface of the substrate 1 to form the light absorption regions 72 and 73. It is. In particular, by effectively using the back surface of the substrate, it is possible to form a light-absorbing region having a free shape regardless of the configuration of the modulation electrode or the like disposed on the front surface side of the substrate.

Further, the shape of the light absorption region is not limited to the shape shown in FIG. 8, and as shown in FIG. 10, the absorption of the fundamental mode light is suppressed by forming a wedge shape in the portion close to the incident side waveguide 2. In addition, it is possible to efficiently absorb only the higher-order mode light.
The shape shown in FIG. 10 can also be adopted as the shape of the high refractive index region in FIG.

The high refractive index region shown in the third reference example and the light absorption region shown in the fourth reference example can be formed over the entire surface of the substrate, but in this case, it propagates through the optical waveguide. This causes problems such as an increase in the propagation loss of the light wave, and high-order mode light captured in these regions reentering the optical waveguide.

  For this reason, the high refractive index region and the light absorption region are all or one between the incident part that makes light incident on the optical waveguide on the substrate and the branched waveguide part that branches the incident light into at least two or more. Preferably, it is formed in the part to prevent light wave propagation loss and re-incidence.

Figure 11 shows a second embodiment of the optical waveguide element of the present invention.
FIG. 11A shows a part of a top view of the optical waveguide device, FIG. 11B shows a cross-sectional view taken along the alternate long and short dash line E in FIG. 11A, and FIG. FIG. 11A is a cross-sectional view taken along one-dot chain line F in FIG.

  In FIG. 11, as a higher-order mode removing means, a higher-order mode branching waveguide (Y-shaped waveguide, comprising a branch front portion 80 and branch rear portions 81 and 82) on the back surface of the substrate 1. Since the substrate 1 is a thin plate, even if the branch front portion 80 of the branch waveguide for higher-order mode is disposed directly below the incident side waveguide 2, the substrate 1 can be efficiently used from the incident side waveguide 2. In addition, it is possible to remove higher-order mode light.

As a method of forming the higher-order mode branching waveguide and the high refractive index region, a method of thermally diffusing Ti or the like as in the first reference example, or a high refraction of Si or the like as in the second reference example. In addition to the method using the rate material, the substrate 1 can be formed by etching as shown in FIG. The method using etching complicates the optical waveguide device manufacturing process by forming a high-order mode branching waveguide or a high refractive index region in combination with the formation of the optical waveguide as a ridge waveguide. There is nothing. In FIG. 12, 2 ′, 3 ′, and 4 ′ are ridge waveguides, and 30 ′ and 31 ′ are branch waveguides for higher-order modes formed with a ridge structure.

The next, as in the first reference example shown in FIG. 4, as the high-order mode rejection means, higher mode branch waveguide 30 for removing high-order mode light in a portion of the incident side waveguide 2 , 31 were provided, and the improvement of the branching ratio was confirmed by simulation.
In the simulation, the substrate thickness is 8 μm, the width of each of the incident side waveguide 2 and the branching waveguides 3 and 4 is 3.5 μm, the width of each of the higher order mode branching waveguides 30 and 31 is 3 μm, and the incident side waveguide. And an angle formed by the higher-order mode branching waveguide is assumed to be 0.8 °.

Further, assuming that the left end of the incident-side waveguide 2 and the optical fiber (MFD 10 μm) are displaced by 1 μm in the vertical direction of FIG. 4A and 3.5 μm in the direction perpendicular to the paper surface of FIG. A light wave having a constant intensity was incident from the optical fiber, and the intensity of the light wave propagated to the branching waveguides 3 and 4 was calculated. In order to examine the branching ratio, the ratio of the light intensity P1 or P2 of the branched light wave to the output intensity P (the total light intensity of the branched light wave) was calculated.
Note that the wavelength of the incident light wave was changed in the range of 1510 nm to 1630 nm.
The result is shown in FIG. FIG. 13 also shows the ratio of the light intensity of the output light wave to the light intensity of the incident light wave (the output intensity P).

Also, in order to confirm the effects of higher-order mode branch waveguide, the optical waveguide device mentioned above, the case excluding the branching waveguide higher mode also it was simulated under the same conditions. The result is shown in FIG.

  Comparing FIG. 13 and FIG. 14, when a high-order mode branching waveguide is used, P1 / P or P2 / P is in the range of 0.48 to 0.52, depending on the wavelength of the incident light wave. It is understood that the branching ratio is maintained at approximately 1: 1 as a whole although there is a slight change in the branching ratio. On the other hand, it is understood that the branching ratio deviates more than 1: 1 when there is no higher-order mode branching waveguide.

Next, in the case where the thickness of the substrate of Example 1 is set to 8 μm (thin plate) and 100 μm (thick plate), the branched waveguide for higher-order mode when the wavelength of the incident light wave is changed in the range of 1510 nm to 1630 nm. The change of excess loss due to the waveguide was investigated.
FIG. 15 shows changes in excess loss with respect to the wavelength of incident light.

  From FIG. 15, it is understood that the thin plate has less excess loss due to the branch waveguide for higher-order modes than the thick plate. In particular, it was confirmed by the same simulation that the output intensity / input intensity can be maintained at 0.98 or more when a substrate having a thickness of 20 μm or less is used.

  As described above, according to the present invention, even when the optical waveguide is formed on a thin plate having a thickness of 20 μm or less, the oscillating light generated in the incident portion is suppressed and the specific optical waveguide such as the Y branch portion is suppressed. It is possible to provide an optical waveguide device having an improved branching ratio in a branching waveguide section that branches the light into a plurality of branching waveguides.

It is a figure which shows the mode of the light wave which propagates the optical waveguide formed in the board | substrate. It is a figure which shows typically the mode of propagation of the higher order mode in an optical waveguide element. It is a figure which shows the method of removing the higher order mode light disclosed by patent document 1. FIG. It is a figure which shows the 1st reference example of the optical waveguide element of this invention. It is a figure which shows the 2nd reference example of the optical waveguide element of this invention. It is a figure which shows the 1st Example of the optical waveguide element of this invention. It is a figure which shows the 3rd reference example of the optical waveguide element of this invention. It is a figure which shows the 4th reference example of the optical waveguide element of this invention. It is a figure which shows the application example of the 4th reference example of the optical waveguide element of this invention. It is a figure which shows the other application example of the 4th reference example of the optical waveguide element of this invention. It is a figure which shows the 2nd Example of the optical waveguide element of this invention. It is a figure which shows the example of the branch waveguide for high order modes formed by the etching. It is a graph which shows the dependence with respect to the wavelength of the output intensity and branching ratio in the optical waveguide element which has a branched waveguide for higher order modes. It is a graph which shows the dependence with respect to the wavelength of the output intensity and branching ratio in the optical waveguide element which does not have a high-order mode removal means. It is a graph which shows the dependence with respect to the wavelength of the excess loss by the branch waveguide for high order modes in the optical waveguide element (thick board, thin board) which has the branch waveguide for high order modes.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Incident side optical waveguides 3, 4 Branching waveguide 5 Incident optical fiber 6 Incident surface 10, 11 Dummy waveguide 20 Fundamental mode light 21 Higher order mode light 30, 31 Higher order mode branching waveguide 40 Adhesive layer 41 Reinforcing plates 50 to 53 High-order mode branching waveguides 60 and 61 formed on the substrate surface High refractive index regions 70 to 75 Light absorption region 80 Branching front portions 81 and 82 of the Y-shaped waveguide Y-shaped waveguide The rear of the branch

Claims (4)

In an optical waveguide element having an electrooptic effect and having a thickness of 20 μm or less and an optical waveguide formed on the substrate,
The substrate reinforcing plate through the adhesive layer is contact wear, which is configured to confine the thickness direction of the substrate light becomes stronger due to the refractive index difference between the substrate and the adhesive layer,
The optical waveguide has an incident portion for making light incident on the optical waveguide, and a branching waveguide portion for branching the incident light into at least two or more .
Between the entrance portion and the branch waveguide portions, provided higher order modes removing means for removing higher order modes of light propagating through the optical waveguide,
The higher-order mode removing means is a higher-order mode branch waveguide having at least two branch waveguides branched from the optical waveguide, and the higher-order mode branch waveguide is bonded to the reinforcing plate of the substrate. An optical waveguide device, characterized in that the optical waveguide device is formed on the formed side . In the optical waveguide device according to claim 1, the branch waveguides for higher-order modes, an optical waveguide device characterized by being formed by thermally diffusing impurities to the substrate. In the optical waveguide device according to claim 1, the branch waveguides for higher-order modes, an optical waveguide device characterized by being formed by loading a high refractive index materials on the substrate. In the optical waveguide device according to claim 1, the branch waveguides for higher-order mode, by etching the substrate to form an optical waveguide, the branch waveguides for effective high-order mode is formed An optical waveguide device characterized by the above.

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