JPH09311235A - Branching structure of optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide such a branching structure with which the lessening of branching loss is possible in spite of the absence of acute angled parts and fine parts in respective parts constituting a branching part and the need for forming the respective parts with high accuracy is relieved. SOLUTION: This branching structure is provided with a transient waveguide 15 in a separated state between one piece of the input side waveguide 12 and two pieces of the output side waveguides 13, 14. This transient waveguide 15 has an expanding part 16 sharply and slightly flaring in the waveguide width to a serrated shape from the end 12a of the input side waveguide 12 and an extending part 17 extending toward the output side waveguides 13, 14 from the expanding part 16. The waveguide width A of the expanding apart 16 is slightly wider than the width B of the input side waveguide 12. The expanding part 17 is substantially constant in the waveguide width from the expanding part 16 to the output side waveguides 13, 14 or are formed to a shape flaring to a taper shape.

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 Patent Laid-Open No. 3-
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 has a Y-shape between the one waveguide 5 and the two output-side waveguides 6 and 7 provided on the substrate 1, as shown in FIG. The branch portion 8 is formed. 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 M'of the coupling ends of the output side waveguides 6 and 7 in the branch part 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, it is an object of the present invention to reduce the influence of variations in the manufacturing process on the characteristics in a Y-branch structure which is not easily affected by the change in wavelength, and the distance between a pair of output side waveguides is 2 μm or more. An object of the present invention is to provide a branched structure of an optical waveguide in which the loss is small even if secured, and there is no acute angle portion, so that the processing of each portion is easy and the core can be surely wrapped with the cladding glass.

[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
2 arranged in a direction facing the end of the input side waveguide
The output waveguide of the book, the end of the input waveguide, and
An extension portion provided between the output waveguide of the book and the core of the input waveguide and the core of the output waveguide, and the waveguide width of which extends from the end of the input waveguide; And a transition section waveguide having an extension section extending from the extension section toward the output side waveguide.

In the present invention, the transitional section waveguide is not tapered like the branch section of the conventional Y-branch waveguide,
At the interface with the transitional waveguide that is separated from the input side waveguide, there is an extension that abruptly spreads in a stepped manner, and an extension that extends from this extension 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 at the time of transition from the 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. The Fresnel reflection loss, which is considered to occur when the light enters from the side-side waveguide to the transition-side waveguide and from the transition-side waveguide to the output-side waveguide, is so small that it can be ignored. it can. The distance between the input-side waveguide and the transition-side waveguide and the distance between the transition-side waveguide and the output-side waveguide are 2
The range of μm to 8 μm is preferable. If the distance is less than 2 μm, it becomes difficult to manufacture the film with desired accuracy by a normal film forming method. If the interval exceeds 8 μm, the width of the outgoing beam of light becomes too wide, which causes a mode field mismatch and is not suitable for practical use.

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 portion waveguide 15 is formed by the end portion 12a of the core of the input side waveguide 12.
And the ends 13a and 14a of the cores of the output side waveguides 13 and 14 are provided separately from each other via a separating portion G having intervals Y1 and Y2, respectively. The distances Y1 and Y2 between the separating portions G are both 2 μm to 8 μm (for example, 5 μm).
m). The transitional section waveguide 15 has an expansion section 16 in which the width of the waveguide abruptly expands in a step shape at the time of incidence from the end section 12a of the input-side waveguide 12, and the extension section 16 outputs the output-side waveguide 13 , 14 is provided. The end portion 12a of the input side waveguide 12 and the extension portion 16 are parallel to each other or close to each other. Further, the output-side end face 18 of the transitional waveguide 15 and the end portions 13a and 14a of the output-side waveguides 13 and 14 are arranged so as to be parallel or close to each other.

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
The waveguide width C of each of the transitional portion proximal ends 13a, 14a of 3 and 14 is less than half the width B of the input side waveguide 12, and the portions 13b, 14b extending from the transitional portion proximal ends 13a, 14a are , A curve selected from a shape based on a trigonometric function such as an arc or a sine curve. Further, the waveguide widths of the output-side waveguides 13 and 14 gradually widen with increasing distance from the transition portion proximity ends 13a and 14a, and the portions 13c and 14c having a constant waveguide width D are provided.
It has a continuous shape. Transient part proximity ends 13a, 14
The branch waveguide spacing W between a is 2 μm or more.

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 0.32 higher than that of the cladding layer 21 by means such as adding a doping agent to SiO _2.
The core 22 having a heightened percentage 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.
A waveguide having a graded index type refractive index distribution may be formed by a known waveguide manufacturing process (for example, diffusing impurities in a glass substrate) different from the above description.

In the branch structure of the optical waveguide 10 of the above 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 with respect to the input side waveguide 12. Since the expansion portion 16 has a shape that abruptly spreads slightly like a step, 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 and 14 are provided. Almost no mode conversion loss occurs at the interface with. Moreover, when entering the transient section waveguide 15 from the input side waveguide 12 and from the transient section waveguide 15 to the output side waveguides 13, 14
The Fresnel reflection loss, which is considered to occur when the light is incident on, is on the order of 10 ^{−6 with} respect to the input light 1 according to the geometrical optics calculation, and is a value that is practically negligible. For this reason, even if a slight mode conversion loss occurs at the interface, it is possible to suppress the loss of the entire branch portion to be low.

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 transient section waveguide 15, the fundamental mode M1 is converted to M2. Two output side waveguides 1 in the transitional section waveguide 15
Since the necessary field distribution M3 is created in 3 and 14, the near ends 13a of the output side waveguides 13 and 14 with the transient section waveguide 15 are
In 14a, smooth mode coupling can be realized for the even symmetric mode M4 on the output side. After that, by gradually widening the distance between the two output side waveguides 13 and 14, a mode M5 in which light is divided into two can be created. Further, the near ends 13a and 14a of the two output side waveguides 13 and 14 are
Then, by narrowing each waveguide width to less than half 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-mentioned branch structure, the space Y1 between the input side waveguide 12 and the transient section waveguide 15 and the output side waveguides 13, 1
4 is a distance Y between the proximal ends 13a and 14a of 4 and the transient portion waveguide 15.
Although 2 is set to 2 μm or more and 8 μm or less, the reflection loss at this portion is practically negligible, and since abrupt mode conversion hardly occurs here, such intervals Y1 and Y2 are set. The loss does not matter even if it has a separate shape. In addition, the portion 1 that is connected to the other end side from the near ends 13a and 14a on one end side of the output side waveguides 13 and 14
By using an appropriate shape selected from arcs and curves based on trigonometric functions for 3b and 14b, the bending loss can be reduced to 0.0
It can be suppressed to 01 dB or less.

According to the branched structure of the above embodiment, the mode conversion loss can be reduced without forming a fine pattern or an acute angle pattern of 2 μm or less in the core shape forming each part, and it is necessary to form a fine acute angle portion. Since it is not present, the processing is easy, and the periphery of the core 22 can be completely surrounded and embedded by the upper clad layer 25. That is, one of the causes of the molding failure of the branch portion can be eliminated, and the branch portion can be formed into a shape that can be molded with a high yield, so that 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.

Regarding the branch loss, as shown in FIG. 5, in the above embodiment, the loss is as low as about 0.04 dB in the vicinity of the branch waveguide spacing W = 2 μm. The branch loss tends to increase with an increase in the branch waveguide spacing W, but the degree of increase in the loss is slower in the above-described embodiment than in the conventional Y-branch structure. The branch loss is 0.1 even if the branch waveguide spacing W is increased to about 3.5 μm.
It is suppressed to below dB, and the branch loss is significantly lower than that of the conventional Y branch structure.

On the other hand, in a branch structure having a tapered transient portion such as the conventional Y branch waveguide (see FIG. 8),
Even if the tip of the branch portion is finely formed so that the branch waveguide spacing W is 2 μm, the loss is 0.1 dB or more.
FIG. 5 shows the characteristics at a wavelength of 1.3 μm.
In the branch structure, the branch loss tends to increase rapidly as the spacing between the branch waveguides 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 that causes variations when manufacturing the branch portion of the waveguide, and particularly with respect to the shape of each portion constituting the branch portion, the accuracy can be relaxed, so that the branch portion is low in accuracy. Not only loss can be achieved, but also the allowable range of the branch waveguide spacing can be set to a wide range, which facilitates processing and can dramatically increase the yield in manufacturing the branch portion of the waveguide.

[0029]

According to the present invention, in the transitional portion located between the input side waveguide and the output side waveguide, the waveguide core width is abruptly slightly widened rather than being tapered so that the input side waveguide Although the fundamental mode is mainly at the interface with the transient section waveguide, a slight radiation mode component can be excited, and the length of this transient section waveguide can be adjusted to an appropriate value to improve the fundamental mode and the radiation mode. By properly superimposing the field distributions of the two, it is possible to create a field distribution advantageous for coupling to the two output side waveguides. Here, a slight mode conversion loss occurs when coupling from the input side waveguide to this transient part, but there is almost no loss when coupling from the transient part to the two output side waveguides. It can be suppressed to a loss.

Therefore, branch loss is reduced and light can be efficiently propagated. In addition, the accuracy of the core shape, 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 branch waveguide spacing can be set wide. In addition, the core of the transient waveguide is separated from the core of the input side waveguide and the core of the output side waveguide, and the clad glass has a form that can fully wrap around the entire circumference of the core. The problem of defects can be avoided. For these reasons, 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 widths at the coupling ends of the two output side waveguides are reduced to less than half of the input side waveguides as in the fifth 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 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 branch waveguide interval and loss.

FIG. 6 is a diagram showing the relationship between the branch waveguide spacing 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 20 ... Substrate 21 ... Clad layer 22 ... Core 25 ... Clad layer

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 arranged in a direction facing an end of the input-side waveguide. And between the end of the input-side waveguide and the two output-side waveguides separately from the core of the input-side waveguide and the core of the output-side waveguide, and the waveguide width is A branch structure for an optical waveguide, comprising: a transition section waveguide having an extension section extending from an end section of the input side waveguide and an extension section extending from the extension section toward the output side waveguide. 2. The distance between the input-side waveguide and the transition-side waveguide and the distance between the transition-side waveguide and the output-side waveguide are 2 μm or more and 8 μm or less, respectively. A branched structure of the described optical waveguide. 3. A waveguide width of the extension portion is slightly wider than that of the input side waveguide, and a waveguide width of the extension portion substantially extends to an output end of the extension portion facing the output side waveguide. The branch structure of the optical waveguide according to claim 1, wherein the branch structure is constant. 4. The waveguide width of the extension portion is slightly wider than that of the input side waveguide, and the waveguide width of the extension portion is tapered to reach an output end of the extension portion facing the output side waveguide. The branched structure of the optical waveguide according to claim 1, wherein the branched structure has a shape that spreads out in a regular shape. 5. A waveguide width at an input end of each of the pair of output side waveguides with respect to the transitional section waveguide is half or less of a width of the input side waveguide, and a portion extending from the input end is an arc. Alternatively, the waveguide width is gradually widened as the distance from the input end is increased, and the waveguide width is gradually increased from the input end.
A branched structure of the described optical waveguide.