JPH1164657A - Branching waveguide and production therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sharply reduce the radiant loss of a branching part and to make a branching angle large by providing a hermetically sealed gap in between two lines of proximity cores. SOLUTION: An input waveguide core 3-1, two lines of proximity output waveguide cores 3-3, 3-4 or the like are formed on an SiO2 lower side clad layer 1-2 being on an Si substrate 1-1. However these are covered with an SiO2 upper side clad film 5-1, a hermetically sealed gap (oblique line part) 4 is formed at a part in which the output waveguide core 3-3 and the output waveguide core 3-4 are in close proximity to each other. Here, since the refractive index of the hermetically sealed gap 4 is roughly equal to 1, the difference of refractive indexes between proximity output waveguide cores 3-3, 3-4 at the boundary between them becomes larger than the difference of refractive indexes between the cores and the clad. Thus, the guide mode 6-1 being in the input waveguide core 3-1 is converted into guide modes 6-2, 6-3 being in the output waveguide cores 3-3, 3-4 with low losses by enhancements of light confining effects into the cores.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a branch waveguide and a method for manufacturing the same.

[0002]

2. Description of the Related Art A waveguide type optical device has a possibility that it will be a key device for optical communication and information processing in the future because a complicated optical circuit is formed on the same substrate at a time. The optical circuit is formed on one substrate by combining three-dimensional waveguides of various shapes according to the function of the optical device used.
For example, a bent waveguide for changing the propagation direction of light, a bent waveguide or an S-shaped waveguide, a tapered waveguide for smoothly expanding the waveguide width without mode conversion, optical power splitting, or light multiplexing / interference. For example, a branching waveguide, a crossing waveguide where two waveguides cross each other, a directional coupler for coupling two waveguide modes, and the like are used in an optical circuit. Among the components of these waveguide type optical circuits, the branch waveguides are a waveguide type optical modulator, an optical switch, a 1 × N splitter in which Y branches are coupled in multiple stages, and a plurality of waveguides are arranged in close proximity in an array. It is frequently used in array waveguide type optical multiplexer / demultiplexers and the like, and is the most important element.

FIG. 5A is a plan view showing a conventional example of a branch waveguide, and FIG. 5B is a sectional view taken along line AA of FIG. 5A (Nishihara et al., Optical Integrated Circuit, p41, 1985).

In the branch waveguide, a single-mode input waveguide 3-1 having a width W is connected to an input end of a tapered waveguide 3-2 for smoothly widening the waveguide width without causing mode conversion. , Two single-mode output waveguides 3-3 and 3-4 having a width W close to each other are connected to two output ends of the tapered waveguide 3-2.

At the branch point (Z = 0) of the branch waveguide, the wavefront of the waveguide mode 6-1 in the input waveguide 3-1 is shifted from the waveguide in the output waveguides 3-3 and 3-4. Mode 6-2, 6-3
Is inclined by θ. As the branch angle 2θ increases, the inclination of the wavefront increases, and the waveguide modes 6-1 in the input waveguide 3-1 and the waveguide modes 6-1 and 6-3 in the output waveguides 3-3 and 3-4. Overlap between the electric field distributions at the branch point becomes smaller. At this time, the output waveguides 3-3, 3-4
The optical power that does not couple to the substrate 1
It leaks to the -1 side. This leakage (loss) is generally called radiation loss, and is considered to be an essential loss generated in the branch portion.

On the other hand, an optical circuit having a branch waveguide is manufactured, for example, by a process as shown in FIG. FIG.
6A to 6C are process diagrams showing a conventional method for manufacturing a branch waveguide.

On a silicon substrate 1-1 having a low refractive index layer 1-2 equivalent to the refractive index of a quartz substrate or quartz glass,
iO ₂ -TiO ₂ based material or a core layer 9 of SiO ₂ -GeO ₂ material flame hydrolysis deposition (P-CVD method, an electron beam deposition method or a sputtering method) is formed (FIG. 6 (a)).

An unnecessary portion of the core layer 9 is removed by dry etching, and two adjacent ridge-shaped cores 3 are removed.
3, 3-4 are formed (FIG. 6B).

[0009] Again, by a method such as a flame deposition method, a P-CVD method or a sputtering method, SiO ₂ -P ₂ O ₅ -B ₂ O
The ₃ material composition or to form a clad layer 5-4 pure SiO _2, embedded waveguide is obtained (Figure 6 (c)).

[0010]

However, low cost optical waveguide devices such as optical modulators and optical switches including the above-described branch waveguides, 1 × N splitters, and arrayed waveguide type optical multiplexer / demultiplexers are required. There is a demand for integration and integration of various functional elements on one flat substrate. In order to satisfy this requirement, it is essential to reduce the size of the branch waveguide which is a component of these devices. In order to reduce the size of the branch waveguide, the length of the branch must be reduced by increasing the branch angle.

However, as described above, when the branch angle 2θ is increased, the loss due to radiation increases, and the loss characteristics of the entire device deteriorate.

These radiation losses may be caused by reducing the waveguide width W to reduce the wavefront tilt effect of the guided mode at the branch point, or by reducing the core 3-3, 3-4 and the cladding film 5-5.
4 can be reduced by increasing the refractive index difference between them to increase the confinement of light.

However, when the waveguide width W is reduced or the refractive index difference between the cores 3-3 and 3-4 and the cladding film 5-4 is increased, the coupling loss with an optical fiber or the like is increased. Will be.

[0014] Therefore, there has been a demand for a method of forming a small branch waveguide without increasing losses such as radiation and coupling.

An object of the present invention is to solve the above problems and to provide a low-loss branch waveguide and a method of manufacturing the same.

[0016]

In order to achieve the above object, a branch waveguide according to the present invention comprises: a planar substrate having a low refractive index layer; In a branched waveguide including a core having a rectangular cross-sectional shape and a cladding film having a low refractive index and covering the core, a closed gap is provided between two adjacent cores.

In addition to the above structure, a method of manufacturing a branch waveguide according to the present invention includes a step of forming a core layer having a high refractive index on a flat substrate having a layer of low refractive index, and an unnecessary portion of the core layer by dry etching. At least two adjacent
In a method for manufacturing a branch waveguide including a step of forming a ridge-shaped core and a step of forming a low-refractive-index clad film so as to cover the core, a hermetically sealed state is provided between two adjacent cores. It forms a void.

In addition to the above structure, the core layer in the method for manufacturing a branch waveguide according to the present invention may be formed by any of a flame deposition method, a P-CVD method, an electron beam evaporation method, and a sputtering method.

In addition to the above structure, the clad film in the method for manufacturing a branch waveguide according to the present invention may be formed by any of the P-CVD method and the sputtering method.

In addition to the above structure, the formation of the clad film by the P-CVD method or the sputtering method in the method of manufacturing a branch waveguide according to the present invention is performed with respect to the power applied to the material supply side for supplying the material of the clad film. Preferably, the ratio of the power supplied to the substrate side is set to 50% or less, and the size of the gap is controlled by adjusting the power supplied to the material supply side and the power supplied to the substrate side.

According to the present invention, since there is a structure in which a closed gap is provided between two adjacent waveguide cores near the branch start portion, the refractive index difference between the gap and the core is reduced. Can be much larger than the state. Therefore, the light confinement efficiency of the two cores sandwiching the gap is improved, and the light power radiated between the cores can be significantly reduced.

That is, since the radiation loss at the branch portion can be greatly reduced, the branch angle can be increased, and an optical modulator, an optical switch, a 1 × N splitter, an array waveguide type optical multiplexer / demultiplexer, or the like can be used. The size of the waveguide type optical device can be easily reduced. In particular, in a 1 × N splitter,
There is a merit that the number of N can be increased.

In forming the gap, the upper portion of at least two adjacent cores and the gap between them are sealed by a cladding film grown in an overhanging state to prevent material gas from entering the gap. A closed space between the core and the clad film can be formed intentionally and easily.

Further, the size of the gap can be freely changed by adjusting the bias power and gas pressure applied to the substrate during film formation.

Further, since the closed gap can be formed at the same time as the formation of the cladding film, it is not necessary to largely change the conventional branch waveguide manufacturing process. Therefore, it is possible to reduce the manufacturing cost of the product while improving the performance.

Further, since the air gap is a closed space, it is possible to prevent intrusion of foreign matter, moisture, and the like, which may change the waveguide characteristics.

The branch waveguide and the method of manufacturing the same according to the present invention can be applied to all waveguide devices having branches and all waveguide devices having adjacent waveguides.

[0028]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 (a) is a plan view showing an embodiment of a branch waveguide according to the present invention, in which one output waveguide is connected to two output waveguides at a certain angle. × 2 branch waveguide. FIG. 1B is a cross-sectional view taken along line BB of FIG.

An SiO ₂ lower cladding layer 1-2 is formed on the Si substrate 1-1. On the SiO ₂ lower cladding layer 1-2, the input waveguide core 3-1 and the tapered waveguide core 3- connected to the output side of the input waveguide core 3-1.
2 and two adjacent output waveguide cores 3-3 and 3-4 connected to the output side of the tapered waveguide core 3-2. These input waveguide core 3-1, the tapered waveguide core 3-2 and the output waveguide core 3-3 and 3-4 are SiO ₂
Although closed by the upper cladding film 5-1, a closed space (hatched portion) 4 is formed in a portion (gap) where the output waveguide cores 3-3 and 3-4 are close to each other. Have been.

6-1 is an electric field intensity distribution of light propagating through the input waveguide core 3-1, and 6-2 and 6-3 are output waveguide cores 3.
3 shows electric field intensity distributions of light propagating in −3 and 3-4, respectively.

The refractive indices of the lower cladding film 1-2 and the upper cladding film 5-1 are 1.4 at a wavelength of 0.633 nm.
58, the refractive index of the waveguide cores 3-1 to 3-4 is 1.469.
6 is set.

Here, the refractive index of the closed space 4 is approximately 1
, The adjacent output waveguide cores 3-3, 3-4
Is larger than the refractive index difference between the core and the clad. Accordingly, the waveguide mode 6-1 in the input waveguide core 3-1 at the branch point is not radiated between the two waveguide cores at the branch point due to the improvement of the light confinement effect in the core, and the output waveguide 3- Guided mode 6-2 in 3, 3-4
The structure is converted to low loss as 6-3.

FIG. 2 is a process chart showing one embodiment of a method for manufacturing a branch waveguide according to the present invention.

On the Si substrate 1-1, a thickness of 1 is formed by thermal oxidation.
An SiO ₂ film having a thickness of 5 μm and a refractive index of 1.458 is formed as the lower clad film 1-2. Then, 10 mol of GeO ₂
1% of SiO ₂ —GeO ₂ material
By F sputtering method, the thickness is 6 μm and the refractive index is 1.4.
A 696 core film is formed, and photolithography and CH
F ₃ gas to remove unnecessary portions by reactive ion etching using to form a ridge-shaped core 3-3 and 3-4. In this embodiment, a thermal oxide film of a Si substrate is used as the lower clad film 1-2, but a quartz substrate is used instead of the Si substrate 1-1 and the lower clad film 1-2 of the thermal oxide film. The substrate itself may be used as the lower cladding film (see FIG. 2).
(A)).

Next, using the SiO ₂ glass material as a target, the cores 3-3 and 3-3 are again formed by RF sputtering.
The upper clad layer 5-1 is formed so as to cover -4. At this time, the substrate temperature is set to a low temperature of 150 ° C. or less, the sputtering gas pressure is set to 0.3 Pa or more, and the bias power applied to the substrate side is set to 50% or less of the power applied to the target side. In other words, the process is performed under the condition that the sputtered atoms are made to easily enter the substrate obliquely, and after the sputtered atoms reach the substrate, they are stationary without moving on the surface. Under such conditions, the film formation ratio at the upper corners of the ridge-shaped cores 3-3 and 3-4 increases, and the growing film becomes overhang. By growing this overhang-like film, the upper part of the gap between the cores 3-3 and 3-4 is closed, so that sputter atoms cannot enter the gap. Therefore, a closed space 4 is formed between the gaps.

The size of the gap 4 depends on the RF bias power applied to the substrate 1-1 and the RF bias applied to the target.
It can be adjusted by controlling the power.
That is, when the ratio of the RF bias power applied to the substrate 1-1 is increased, the growth rate of the overhang is reduced, and the gap 4 can be reduced. Substrate 1
When the ratio of the RF bias power applied to the -1 side is reduced, the growth rate of the overhang increases, and the gap 4 can be increased (FIG. 2B).

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The branch waveguide shown in FIG.
As a result of manufacturing such that θ is 5 deg and the length of the gap in the Z direction is 50 μm, the branch loss is ２ of that in the case where there is no conventional gap, and a low loss branch waveguide is obtained.

Here, the void 4 can be formed by a P-CVD method. In this case as well, the gas pressure is increased, and the power applied to the substrate 1-1 side is changed with respect to the power applied to the source gas (material) supply side electrode, the power applied to the substrate 1-1 side, and By adjusting the electric power to 50% or less, the size of the gap 4 can be adjusted as in the case of the sputtering method.

The 1 × 2 branch waveguides may be connected in multiple stages to form a 1 × 8 splitter as shown in FIG. That is, a small and low-loss 1 × N splitter can be easily configured. FIG. 3 is a plan view showing another embodiment of the branch waveguide of the present invention. 4-1 to 4-7 in circles 7-1 to 7-7 indicated by broken lines in FIG. The input waveguide 3a of this branch waveguide
When the arrow P ₀ direction of the light is incident, the branched light is branched from the output waveguide 3b~3i the arrow P ₁ to P ₈ direction emitted.

Further, the structure of the branch waveguide of the present invention is not limited to a branch waveguide that distributes only light energy.
The present invention can also be applied to an array type waveguide element for selectively splitting the wavelength of light as shown in FIG.

This array-type waveguide element has an input waveguide 1
0, an input slab waveguide 11 connected to the input waveguide 10, an output slab waveguide 12, and both slab waveguides 1
A plurality of channel waveguide arrays 13 connected to
And the output waveguide 1 connected to the output side slab waveguide 12
4.

This array type waveguide device has many branching waveguides, and the output waveguides 14 connected to the output side slab waveguide 12 receive light of different wavelengths (λ _{1 to} λ ₄ ). It is designed to be split. In the branch waveguide according to the present invention, light interference hardly occurs between adjacent waveguides. Therefore, a wavelength filter having excellent isolation can be realized. Thus, the present invention can be applied to all waveguide devices having a branch waveguide for the purpose of not only distributing energy but also demultiplexing the wavelength.
FIG. 4A is a plan view showing another embodiment of the branch waveguide of the present invention, and FIG. 4B is a cross-sectional view taken along line CC of FIG. 4A. 13a to 13h denote waveguide cores of the channel waveguide array 13, 4a to 4g denote gaps, and 5-3
Indicates a clad film.

As described above, according to the present invention, by providing a closed gap having a refractive index of about 1 at a branch portion of a waveguide having a branch, the gap functions as a cladding film, and the waveguide core and the gap section are formed. The refractive index difference between the optical waveguide and the optical waveguide increases, the light confinement effect is improved, and various branch waveguides having excellent branch loss and demultiplexing characteristics can be realized. Further, since the branch angle can be increased, the size of the waveguide device can be reduced.

[0045]

In summary, according to the present invention, the following excellent effects are exhibited.

By providing a closed space having a refractive index of about 1 at the branch portion of the branch waveguide, it is possible to provide a low-loss branch waveguide and a method of manufacturing the same.

[Brief description of the drawings]

1A is a plan view showing an embodiment of a branch waveguide according to the present invention, and FIG. 1B is a cross-sectional view taken along line BB of FIG.

FIG. 2 is a process chart showing one embodiment of a method for manufacturing a branch waveguide according to the present invention.

FIG. 3 is a plan view showing another embodiment of the branch waveguide of the present invention.

FIG. 4A is a plan view showing another embodiment of the branch waveguide of the present invention, and FIG. 4B is a cross-sectional view taken along line CC of FIG.

FIG. 5A is a plan view showing a conventional example of a branch waveguide, and FIG. 5B is a cross-sectional view taken along line AA of FIG.

FIGS. 6A to 6C are process diagrams illustrating a conventional method for manufacturing a branch waveguide.

[Explanation of symbols]

 1-1 Substrate 3-1 Input waveguide 3-2 Tapered waveguide 3-3, 3-4 Output waveguide 4 Air gap 5-1 Cladding film

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kentaro Ohira 3550 Kida Yomachi, Tsuchiura City, Ibaraki Prefecture Within the Hitachi Cable Advanced Research Center

Claims (5)

[Claims] 1. A flat substrate having a low refractive index layer, at least two cores having a high refractive index and having a rectangular cross-sectional shape formed on the flat substrate and adjacent to each other, and covering the core and having a low refractive index. A closed waveguide is provided between two adjacent cores in a branched waveguide having a cladding film having a cladding film. 2. A step of forming a core layer having a high refractive index on a flat substrate having a low refractive index layer, and removing at least two ridges adjacent to each other by removing unnecessary portions of the core layer by dry etching. Forming a closed gap between two adjacent cores in a method for manufacturing a branch waveguide, comprising the steps of: forming a core having a low refractive index so as to cover the core; A method for manufacturing a branch waveguide. 3. The method according to claim 1, wherein the core layer is formed by flame deposition, P-CVD.
3. The method according to claim 2, wherein the branch waveguide is formed by any one of a method, an electron beam evaporation method, and a sputtering method. 4. The method according to claim 2, wherein the cladding film is formed by one of a P-CVD method and a sputtering method. 5. A method of forming a clad film by the P-CVD method or the sputtering method, wherein the ratio of the power supplied to the substrate side to the power supplied to the material supply side for supplying the material of the clad film is 50%. 3. The method of manufacturing a branch waveguide according to claim 2, wherein the size of the gap is controlled by adjusting the power applied to the material supply side and the power supplied to the substrate side.