JPH09178964A - Branching structure for optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a branching structure capable of reducing a mode conversion loss even when a dull part exists in a branch part tip and relaxing the necessity precisely forming the branch part tip. SOLUTION: A transient part waveguide 15 is provided between an input side waveguide 12 and two pieces of output side waveguides 13, 14. The transient part waveguide 15 is provided with an expansion part 16 quickly and slightly expanding a waveguide width stepwise from the end part 12a of the input side waveguide 12 and a prolonging part 17 prolonging from the expansion part 16 in the directions of the output side waveguides 13, 14. The waveguide width A of the expansion part 16 is slightly wider than the width B of the input side waveguide 12. The prolonging part 17 is shaped so that the waveguide width is fixed substantially or widened in tapered shape in the interval from the expansion part 16 to the output side waveguides 13, 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide branching structure applied to a device for branching signal light in optical communication or the like.

[0002]

2. Description of the Related Art For example, in a device for an optical integrated circuit (optical IC) used for optical communication, etc., as one means for branching a signal light, for example, JP-A-3-172804 and JP-A-5-119220. Optical directional couplers such as those described in US Pat. In addition, Japanese Unexamined Patent Publication
Y-branch waveguides such as those described in Japanese Patent No. 245107 and Japanese Patent Laid-Open No. 5-11130 are also known.

In a conventional optical directional coupler, as shown in FIG. 7, a part of a plurality of waveguides 2, 3 and 4 formed on a substrate 1 are brought close to each other in parallel and linearly.
The light propagating through the input side waveguide 2 is transferred to the output side waveguides 3 and 4. Since this optical directional coupler has a constant waveguide width, it has the advantage that single mode incident light can be propagated as it is in single mode, and loss is small.

However, the above-mentioned optical directional coupler has a characteristic that its propagation constant is sensitive to changes in the wavelength of light and therefore has strong wavelength dependence. For this reason, the wavelength band width is as narrow as 50 to 100 angstroms, it is difficult to design the distance between the waveguides, the length of the coupling portion, and the like, and there is a drawback that the characteristics greatly change due to dimensional variations during manufacturing. In optical communication, wavelengths of 1.3 μm and 1.55 are mainly used.
Although the light of μm is used, in the branch using the directional coupler, it is necessary to change the coupling length and the like depending on the wavelength to be used. This is a major problem in configuring a communication system.

On the other hand, the Y-branch waveguide is, as illustrated in FIG. 8, formed between the one waveguide 5 provided on the substrate 1 and the two output-side waveguides 6 and 7 in the Y-shape. To form a branch portion 8. Since such a Y-branch waveguide has little dependence on the wavelength of light and has a wide wavelength bandwidth of about 1000 Å, it is relatively easy to design.

[0006]

However, since the width of the waveguide in the branching portion 8 is wider than that of each of the waveguides 5, 6 and 7, the Y-branching waveguide splits even if the incident light is a single mode. There is a tendency for multi-mode to occur due to the generation of higher-order modes in the section 8. Therefore, the Y-branch waveguide has a drawback that a part of the optical power is radiated to the outside of the waveguide and the loss becomes large. Further, although it is necessary to form the branch tip 8a with a fine acute angle pattern, it is difficult to form a complete acute pattern due to processing limitations and the like, and if the shape of the branch tip 8a is incomplete, scattering occurs. There was also the problem of easy loss.

It is known that in the Y-branch waveguide, the branch loss decreases as the width of the branch tip 8a decreases. However, in reality, it is difficult to form the tip 8a of the branch portion finely so as to form a complete acute angle. That is, when the Y-branch waveguide is actually manufactured, the tip 8a of the branch portion does not have the shape as designed due to processing limitations or the like, and a certain amount of "blurring" 9 occurs as shown by the chain double-dashed line in FIG. Will end up. For this reason, the region where the waveguide width expands becomes longer, and in the embedded waveguide, the core is not sufficiently covered by the cladding layer, and a cavity is partially formed, so that the characteristics as designed cannot be obtained. There is.

In order to solve the above problem, it is advisable to secure a blunt width W of 2 μm or more at the tip 8a of the branched portion when designing the branched portion 8. In addition, since it is necessary to accurately process the tip 8a of the branch portion so that the core is completely embedded in the clad layer in manufacturing the embedded waveguide, 2 μ is required.
It is desired to secure a rounded width W of m or more. However, the existence of such a rounded width W becomes a factor for further increasing the mode conversion loss in the branching section 8.

That is, as shown in FIG. 9, in the Y-branch structure having the rounded portion 9, the field distribution in the input side waveguide 5 is subject to a gentle perturbation, so that the light mostly contains the fundamental mode component M1. The distribution spreads symmetrically to the left and right as the light travels. On the other hand, in the branch portion 8, since two symmetrical output side waveguides 6 and 7 are provided, only the even symmetric mode is excited. That is, there is a large difference in the field distribution between the fundamental mode M1 of the light propagating through the input side waveguide 5 and the even symmetric mode M2 at the coupling ends of the output side waveguides 6 and 7 in the branch section 8. This causes a mode conversion loss, which is a part of the branch loss. The specific radiation loss in this case is a value of about 0.1 dB (decibel) or more. In the current waveguide manufacturing process, a certain degree of variation in the waveguide shape is unavoidable, and the round width W tends to increase due to the fact that the core needs to be sufficiently filled with the cladding layer.

Therefore, an object of the present invention is to reduce the influence of variations in the shape of the tip of the branch portion on the characteristics in a Y-branch structure that is not easily affected by changes in wavelength, and to secure a rounded width of 2 μm or more at the tip of the branch portion. Even so, it is to provide a branching structure of an optical waveguide in which mode conversion loss is small and the tip of the branching portion can be easily processed.

[0011]

The present invention, which was developed to achieve the above object, provides an optical waveguide having a core covered with a cladding layer, wherein one input-side waveguide and
Two output-side waveguides arranged to face the ends of the input-side waveguide, and a waveguide formed between the end of the input-side waveguide and the two output-side waveguides There is provided a transition section waveguide having an expansion section whose width extends stepwise from the end section of the input side waveguide and an extension section extending from the expansion section toward the output side waveguide.

In the present invention, the transient section waveguide connected to the input side waveguide is not tapered like the conventional Y-branch waveguide, but is formed in a step shape at the interface between the input side waveguide and the transient section waveguide. It has an expanding portion that abruptly expands slightly and an extending portion that extends from this expanding portion toward the output side waveguide. In this case, the propagating light is mainly in the fundamental mode at the interface between the input side waveguide and the transitional section waveguide, but a slight radiation mode component can be excited. Furthermore, by adjusting the length of the transient section waveguide to an appropriate value and properly overlapping the field distributions of the fundamental mode and the radiation mode, the field distribution advantageous for coupling to the two output side waveguides is transient. It can be created in a partial waveguide. A slight mode conversion loss occurs when the transition is made from this input side waveguide to the transient section waveguide, but there is almost no loss when coupling from the transient section waveguide to the two output side waveguides. As a result, low loss can be suppressed.

The present invention is a bifurcating type bifurcating structure for bifurcating into a Y-type, and by combining a plurality of the Y-branching structures, an 8-branch or 16-branch waveguide can be constructed. However, since the loss per branch is very small, a star coupler having excellent insertion loss characteristics can be obtained. Furthermore, a waveguide type optical switch, a modulator or the like can be formed by applying the above-mentioned branch structure.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to FIGS. Y shown in FIG.
Optical branching device 11 having a branching optical waveguide 10
Is one input side waveguide 12 and two output side waveguides 1
3, 14 and the transition section waveguide 15 and the like. The transition section waveguide 15 is formed between the end 12 a of the input side waveguide 12 and the output side waveguides 13 and 14. The transitional portion waveguide 15 has an expanded portion 16 in which the width of the waveguide abruptly spreads in a step shape at the end 12a of the input side waveguide 12.
And the output side waveguides 13 and 14 from the expansion section 16
An extension portion 17 extending in the direction of is provided. The angle θ formed by the expansion portion 16 with respect to the input side waveguide 12 is a right angle or an angle close thereto.

The waveguide width A of the expanded portion 16 in the illustrated example is, for example, not more than twice the waveguide width B of the input side waveguide 12. The waveguide width of the extension portion 17 is from the extension portion side 16 to the output side waveguide 1.
It is substantially constant between 3 and 14. 2 in FIG.
As shown by the dotted line, the waveguide width of the extension 17 is equal to that of the extension 1.
It is also possible to have a shape that expands in a taper shape from the position 6 to the output side waveguides 13 and 14.

Output side waveguide 1 with respect to the transient section waveguide 15
Waveguide width C of each of coupling ends 13a and 14a
Are each less than or equal to half the width B of the input side waveguide 12, and the portions 13b and 14b extending from the coupling ends 13a and 14a are formed by curves selected from shapes based on trigonometric functions such as arcs or sine curves. In addition, the waveguide widths of the portions 13b and 14b are equal to the coupling end 13a,
The width gradually increases as the distance from 14a increases, and the shape becomes continuous with the portions 13c and 14c having the constant waveguide width D. The tip 18 of the diverging portion is not an acute angle, and a rounded width W of 2 μm or more is secured.

An example of a method of manufacturing the above optical waveguide 10 will be described below. Substrate 20 made of Si wafer or quartz
The CVD method (Chemical Vapor D
eposition: Chemical vapor deposition method or FHD method (Flam
The lower clad layer 21 having SiO ₂ as a main component and having a low refractive index is formed by a film forming method such as eHydrolysis Deposition (flame deposition method). In addition, the lower clad layer 21
The refractive index is 0.2% to 0.4% higher than that of the cladding layer 21 by means such as adding a doping agent to SiO _2.
The core 22 which is slightly elevated is formed. A method of lowering the refractive index of the clad layer 21 may be adopted by adding a dopant that lowers the refractive index to the clad layer 21.

After forming a predetermined waveguide pattern on the surface of the core 22 with a photoresist, RIE (Re
The waveguide core 22 having a predetermined pattern is formed by etching by a method such as active ion etching. After that, the upper cladding layer 25 is formed again by the CVD method or the FHD method so as to fill the core 22. As a result, the optical waveguide 10 having the step index type refractive index distribution is formed. Although FIG. 3 shows a buried type waveguide structure formed by the FHD method, a ridge type waveguide structure as shown in FIG. 4 may be formed by the CVD method.

The graded refractive index distribution is determined by using a waveguide manufacturing method such that the refractive index of the core 22 is set higher in advance by adding a doping agent and the doping agent is thermally diffused by heating. You may form the waveguide which has.
Further, a waveguide having a graded index type refractive index distribution may be formed by a known waveguide manufacturing process different from the above description.

In the branch structure of the optical waveguide 10 of the above-mentioned embodiment, the light propagating at the interface between the input side waveguide 12 and the transient section waveguide 15 is mainly in the fundamental mode, but is connected to the input side waveguide 12. Further, since the expanded portion 16 has a shape that abruptly spreads in a stepped manner, a slight radiation mode component can be excited in the transient portion waveguide 15. Moreover, if the length of the transitional waveguide 15 is adjusted to an appropriate value and the field distributions of the fundamental mode and the radiation mode are properly overlapped, the transitional waveguide 15 and the two output side waveguides 13,
Since almost no loss occurs at the interface with 14,
Even if a slight mode conversion loss occurs at this interface, the overall branch portion can be suppressed to a low loss.

FIG. 2 shows each mode coupling state of the field distribution in the above embodiment. At the interface between the input side waveguide 12 and the transitional section waveguide 15, the fundamental mode M1 is predominant. However, the two waveguides 1 on the output side in the transient waveguide 15
Since the field distributions required for 3 and 14 are created, smooth mode coupling can be realized for the even symmetric mode M2 at the coupling ends 13a and 14a with the transient waveguide 15. Furthermore, the coupling end 13 of these two output side waveguides 13 and 14
In a and 14a, by narrowing each waveguide width to less than half of the core width of the input side waveguide 12, the confinement of light is temporarily weakened and the loss due to mode conversion can be reduced.

In the above branch structure, the output side waveguides 13 and 14 are provided.
The width W of the tip 18 of the branch portion is set to 2 μm or more. However, since abrupt mode conversion hardly occurs at this portion, mode conversion loss is a problem even with a shape having such a round width W. It is small enough not to become. Further, by using an appropriate shape selected from an arc or a curve based on a trigonometric function for the portions 13b and 14b connected from the coupling ends 13a and 14a of the output side waveguides 13 and 14 to the other end side, the bending loss can be reduced to 0. It can be suppressed to 0.001 dB or less.

According to the branch structure of the above embodiment, mode conversion loss can be reduced without forming the width of the tip 18 of the branch portion into an acute angle pattern of 2 μm or less, and it is necessary to form a fine acute angle portion in the branch portion. Since it is not present, the processing is easy and the core 22 can be completely embedded by the upper clad layer 25. That is, the branching portion can be formed into a shape that can be molded with a high yield, and the optical waveguide 10 with less characteristic variation can be obtained.

In order to confirm the operation (insertion loss, etc.) of the optical waveguide 10 of the above-mentioned embodiment, the present inventors conducted a simulation by BPM (beam propagation method). The results will be described below. First, regarding the branch loss,
As shown in FIG. 7, in the above embodiment, the loss is as low as about 0.04 dB in the vicinity of the round width of 2 μm. The branch loss tends to increase as the rounding width increases, but the degree of increase in the loss is slower in the above embodiment than in the conventional Y-branch structure. The branch loss is suppressed to 0.1 dB or less even if the rounding width is increased to about 3 μm, which is a significantly lower branch loss than the conventional Y-branch structure.

On the other hand, in the branch structure having a tapered transient portion like the conventional Y-branch waveguide, the round width is 2 μm.
Even if the tip of the bifurcation is finely molded to be m,
The loss is 0.1 dB or more. FIG. 5 shows the characteristics at a wavelength of 1.3 μm, but in the conventional Y-branch structure, the branch loss tends to increase rapidly as the rounding width increases.

In optical communication, wavelengths of 1.3 μm and 1.55 are mainly used.
Although the light of μm is used, the dependence of the branch loss on the wavelength is small as shown in FIG. 6, and the optical waveguide 10 of the above embodiment can obtain a sufficiently low branch loss at any of the two types of wavelengths. ing.

As described above, with respect to the shape of the branch portion, which becomes a factor of variation when manufacturing the branch portion of the waveguide, and particularly the accuracy of the shape of the tip of the branch portion can be relaxed, the loss at the branch portion can be reduced. Of course, since the allowable range of the rounded width can be set wide, processing becomes easy, and the yield can be dramatically increased in manufacturing the branched portion of the waveguide.

[0028]

According to the present invention, branching loss is reduced and light can be efficiently propagated. In addition, the shape accuracy of the tip of the branch portion, which may vary during manufacturing, can be relaxed due to the processing limit when forming the core of the branch portion, and the allowable range of the rounding width can be set wide. Therefore, it is possible to reduce the influence on the waveguide characteristics due to factors that vary during manufacturing, facilitate manufacturing, reduce the number of defective products, and significantly improve the yield. Moreover, even if the wavelength changes, it has little effect on the light propagation characteristics. Further, since the curve of the output side waveguide at the branch portion can be reduced, the bending loss can be reduced.

When the waveguide width at the coupling end of the two output side waveguides is narrowed to less than half of the input side waveguide as in the fourth aspect, the transition section waveguide and each output side waveguide are It is possible to further reduce the loss due to the mode conversion occurring at the coupling part of the.

[Brief description of the drawings]

FIG. 1 is a plan view of a branch portion of an optical waveguide showing an embodiment of the present invention.

FIG. 2 is a diagram showing a change in the field distribution at the bifurcation shown in FIG.

FIG. 3 is a sectional view taken along line III-III in FIG.

FIG. 4 is a sectional view showing a modification of the waveguide core.

FIG. 5 is a diagram showing a relationship between a rounded width and a loss in the branch portion shown in FIG. 1 and a conventional branch portion.

FIG. 6 is a diagram showing a relationship between rounded width and loss for two types of wavelengths.

FIG. 7 is a plan view showing a conventional optical directional coupler.

FIG. 8 is a plan view showing a conventional Y-branch waveguide.

9 is a diagram showing a change in field distribution in the Y-branch waveguide shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Optical waveguide 11 ... Optical branching device 12 ... Input side waveguide 13, 14 ... Output side waveguide 15 ... Transient part waveguide 16 ... Expansion part 17 ... Extension part

Claims (5)

[Claims] 1. An optical waveguide having a core covered with a cladding layer, wherein one input-side waveguide and two output-side waveguides are arranged so as to face an end portion of the input-side waveguide. An extension portion formed between the end portion of the input side waveguide and the two output side waveguides and having a waveguide width stepwise extending from the end portion of the input side waveguide; An optical waveguide branching structure comprising: a transitional portion waveguide having an extension portion extending in the direction of the output side waveguide. 2. The waveguide width of the extension is slightly wider than that of the input side waveguide, and the waveguide width of the extension is substantially constant from the extension to the output side waveguide. The branch structure of the optical waveguide according to claim 1, wherein 3. A shape in which the waveguide width of the expanded portion is slightly wider than that of the input side waveguide and the waveguide width of the extended portion is tapered so as to extend from the expanded portion to the output side waveguide. The branched structure of the optical waveguide according to claim 1, wherein: 4. The optical waveguide branching structure according to claim 1, wherein the width of the tip of the branching portion of the two output side waveguides coupled to the transitional portion waveguide is set to 2 μm or more. 5. The waveguide width at the coupling end of the output side waveguide to the transitional portion waveguide is half or less of the width of the input side waveguide, and the portion extending from the coupling end is an arc or a triangle. 2. The branch structure of an optical waveguide according to claim 1, wherein the waveguide has a shape that is formed by a curve selected from a shape based on a function and that the width of the waveguide gradually widens as the distance from the coupling end increases.