JPH0886926A - Optical branching device - Google Patents

Abstract

PURPOSE: To provide an optical branching device which lessens the influence of the change in wavelength and the variation in production on light propagation characteristics and lessens loss. CONSTITUTION: A glass waveguide 20 formed of a core part and a clad layer is provided with one piece of first waveguide 11 and two pieces of second waveguides 12, 13 so that the light is branched to two ways in a branching part 14. This branching part 14 is provided with a first tapered waveguide 30 extending from the first waveguide 11 and the second tapered waveguides 31, 32 extending from the second waveguides 12, 13. The first tapered waveguide 30 and the second tapered waveguides 31, 32 are in proximate to each other apart a coupling spacing of about several μm and form an optical directional coupler in this proximate part. The coupling length L of the tapered waveguides 30, 31, 32 is preferably 700 to 800μm. The branching angle of the second waveguides 12, 13 to the first waveguide 11 is preferably 0.5 to 1.5 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical branching device for branching or coupling signal lights in optical communication and the like, and more particularly to an optical branching device capable of efficiently transmitting optical power.

[0002]

2. Description of the Related Art An optical integrated circuit (optical I) used for optical communication or the like.
In the device for C), as a means for branching the signal light, Y as described in, for example, Japanese Patent Application Laid-Open Nos. 3-245107 and 5-11130.
Branch waveguides are known. In addition, JP-A-3-1728
Optical directional couplers such as those described in JP-A No. 04 and JP-A-5-119220 are also known.

In a conventional Y-branch waveguide, as shown in FIG. 9, a main waveguide 2 provided on a substrate 1 is branched into a Y-shape, and the main waveguide 2 is divided into two.
The output waveguides 3 and 4 of the book are formed. Since such a Y-branch waveguide has little dependence on the wavelength of light and has a wide wavelength bandwidth of about 1000 angstroms, it is relatively easy to design.

However, in the Y-branch waveguide, since the waveguide width in the branching portion 5 is wider than the widths of the other waveguides 2, 3 and 4, even if the incident light is a single mode, the branching portion 5 is formed. However, there is a tendency for multi-modes to occur due to the generation of higher-order modes. For this reason, there is a drawback in that a part of the optical power is radiated outside the waveguide and the loss becomes large. Moreover, since it is necessary to form the shape of the branch tip 5a into a fine acute angle pattern, it is difficult to form a complete acute pattern, and scattering loss is likely to occur if the shape of the branch tip 5a is incomplete. There was also a problem.

On the other hand, in the conventional optical directional coupler, as shown in FIG. 10, some of the plurality of waveguides 6, 7, and 8 formed on the substrate 1 are parallel and linear to each other. By making them close to each other, the light propagating through the waveguide 6 on the incident side is transferred to the waveguides 7 and 8 on the output side. Since such an optical directional coupler does not have a non-uniform waveguide such as a Y-branch waveguide, it has an advantage that single mode incident light can be propagated as a single mode and a loss is small. Further, since it is not necessary to form a fine acute angle pattern like the tip of the branch portion of the Y branch waveguide,
It is relatively easy to manufacture.

[0006]

However, the above-mentioned optical directional coupler has a strong wavelength dependence because the propagation constant changes sensitively to the change of the wavelength of light. 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 and the length of the coupling portion, and there is a drawback that the characteristics greatly change due to dimensional variations during manufacturing. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical branching device which has a small influence on the optical propagation characteristics due to a change in wavelength and manufacturing variations and has a small insertion loss.

[0007]

The optical branching device of the present invention developed to achieve the above object is a glass waveguide comprising a clad layer formed on a substrate and a core portion embedded in the clad layer. The waveguide has one first waveguide, two second waveguides independent of each other, and a branch portion that optically connects the first waveguide and the second waveguide. In addition, a first tapered waveguide that extends from the first waveguide and narrows the waveguide width toward the tip is provided on the first waveguide side of the branch portion,
On the side of the pair of second waveguides, there are respectively provided second taper waveguides that extend from the second waveguide toward the first taper waveguide and taper toward the tip end in a tapered width. The first taper waveguide and the second taper waveguide are close to each other for a predetermined coupling length to form a directional coupler.

[0008]

In the optical branching device of the present invention, the first and second tapered waveguides are close to each other for a predetermined coupling length to form an optical directional coupler. In the case of the conventional linear optical directional coupler, the loss tends to rapidly increase in the wavelength band on the short wavelength side (particularly 1.4 μm or less), but the present invention has little wavelength dependence, The loss on the short wavelength side can be greatly reduced as compared with the conventional optical directional coupler.

According to a second aspect of the present invention, the branch angle of the second waveguide with respect to the extension line of the first waveguide is 0.5 ° to 1.5 °.
When the range is set, the loss hardly changes even if the branch angle changes, and the insertion loss can be suppressed to a low level.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. The optical branching device 10 of this embodiment has one first waveguide (incident side waveguide) 11
And two second waveguides (waveguides on the output side) 12, 13
And a glass waveguide 20 including the branch portion 14 and the like. This glass waveguide 20 comprises a low refractive index clad layers 22 and 23 formed on a substrate 21 made of a Si wafer or quartz, and a high refractive index core portion 24 covered by the clad layers 22 and 23. Then, the signal light propagating through the first waveguide 11 is branched into two in the branch portion 14,
It is configured to lead to the second waveguides 12 and 13.

The branch portion 14 has a tapered first tapered waveguide 30 connected to the first waveguide 11 and a second waveguide 1.
Tapered second tapered waveguides 31 and 3 connected to 2 and 13
2 are provided.

The first tapered waveguide 30 is the first waveguide 11
Is extended in the longitudinal direction of the first waveguide 11, and the waveguide width of the first tapered waveguide 30 is equal to that of the first waveguide 11 in the portion continuous with the first waveguide 11. Is. The first tapered waveguide 30 has a tip 35.
A sharp angle that narrows the waveguide width toward
It is an equilateral triangle.

On the other hand, the second tapered waveguides 31 and 32 are obtained by extending the core portions 24 of the second waveguides 12 and 13 toward the first tapered waveguide 30, respectively. Each second
The tapered waveguides 31 and 32 have tips 36 and 37, respectively.
Is an isosceles triangle with an acute angle such that the waveguide width narrows in a taper shape at a constant rate. In other words, these second tapered waveguides 31 and 32 are the same as the second waveguide 1.
When the waveguide width gradually increases toward 2 and 13 and becomes the same as the width of the second waveguides 12 and 13, the waveguides are connected to the second waveguides 12 and 13.

One hypotenuse 30a of the first tapered waveguide 30
The hypotenuses 31a of one of the second tapered waveguides 31 are close to each other in parallel with a coupling interval of about several μm between them, and the first waveguide 11 and the second waveguide 12 are close to each other.
To form an optical directional coupler that optically couples and.
Similarly, the other hypotenuse 30b of the first tapered waveguide 30 and the hypotenuse 32a of the other second tapered waveguide 32 are each several μm apart.
They are close to each other in parallel with a coupling interval of about m, and an optical directional coupler that optically couples the first waveguide 11 and the second waveguide 13 is formed in this close portion. If the coupling interval is set to, for example, 2 μm to 3.5 μm, the insertion loss can be suppressed to the minimum.

The tips 35, 36, 37 of the tapered waveguides 30, 31, 32 do not necessarily have to be sharpened to a perfect acute angle. For example, as shown in FIG. There is no problem even if there is a roundness 40 of the width W of. Since a relatively large dimple 40 can be allowed as described above, it is easier to manufacture than the conventional Y-branch waveguide (FIG. 9).

The length of the portion where the tapered waveguides 30, 31 and 32 are close to each other, that is, the coupling length L is set in the range of 700 μm to 800 μm for the reason described later. Further, the branch angle θ of the second waveguides 12 and 13 with respect to the extension line 41 in the longitudinal direction of the first waveguide 11 is 0.5 ° to.
It is set in the range of 1.5 °.

To form the above-mentioned glass waveguide 20,
On the surface of the substrate 21 made of Si wafer or quartz, CV
Method D (Chemical Vapor Deposition)
Alternatively, the lower clad layer 22 containing SiO ₂ as a main component is formed by a film forming method such as an FHD method (Flame Hydrolysis Deposition). Further, on the lower clad layer 22, a core layer having a predetermined thickness whose refractive index is higher by about 0.2 to 0.4% than that of the clad layer 22 is formed by SiO ₂ added with a dopant. Next, a predetermined waveguide pattern is formed with photoresist, and RIE (Reacti
ve Ion Etching) or the like to form the core portion 24. After that, again FHD method or CV
The upper cladding layer 23 is formed by the D method so as to fill the core portion 24.

In this way, the glass waveguide 20 having the step index type refractive index distribution is formed. By the known waveguide manufacturing process different from the above description,
A waveguide having a graded index type refractive index distribution may be formed. Although FIG. 2 shows the buried waveguide structure formed by the FHD method, a ridge waveguide structure as shown in FIG. 4 may be formed by the CVD method.

In order to confirm the insertion loss and the like of the optical branching device 10 of the above embodiment, the present inventors conducted a simulation by BPM (beam propagation method). The results will be described below.

First, the dependence of the insertion loss on the wavelength is
As shown by A in FIG. 5, it has a minimum value in the wavelength band of 1.3 μm to 1.4 μm. Ordinary optical directional coupler (Fig. 10)
In the case of, the wavelength characteristics shown by B in FIG.
In particular, the loss tends to increase sharply on the short wavelength side. That is, the optical branching device 10 of this embodiment has little wavelength dependence, and has extremely little loss, especially on the short wavelength side.

On the other hand, the relationship between the length of the overlapping portions of the tapered waveguides 30, 31, 32 (coupling length L) and the insertion loss is 700 μm to 80 μm as shown by C in FIG.
The loss is almost equal even if it changes in the range of 0 μm. On the other hand, in the case of the conventional linear optical directional coupler, the loss greatly changes depending on the coupling length as shown by D in FIG. 6, and the increase in loss is large particularly at 800 μm or less.

Thus, it can be said that the optical branching device 10 of this embodiment has little dependency on the coupling length. In other words, according to the present embodiment, by adopting the tapered waveguides 30, 31, 32 in the branch portion 14, the coupling length can be shortened without increasing the loss. For this reason,
In the star coupler in which the branched portions 14 of this embodiment are combined in multiple stages, the total length of the device can be shortened.

FIG. 7 shows the branch angle θ of the second waveguides 12 and 13.
And the insertion loss. As can be seen from this figure, the loss has a substantially constant value when the branch angle θ is 0.5 ° to 1.5 °. In the conventional Y-branch waveguide (FIG. 9) or the optical directional coupler (FIG. 10), the loss tends to increase in proportion to the branch angle increasing from 0 °. (Characteristic that loss does not increase between branching angles of 0.5 ° to 1.5 °) cannot be obtained.

As described above, the accuracy of the factors that may vary during manufacturing (for example, the coupling length L and the branch angle θ) can be relaxed, so that the loss at the branch portion 14 can be reduced. Therefore, the number of defective products can be reduced and the yield can be dramatically improved.

The light distribution device 10 of the above embodiment is used.
Is a two-branch type, but by combining a plurality of these two-branch types, a four-branch or eight-branch waveguide can be constructed, and even in that case, a straight waveguide can be constructed with almost no curve. Therefore, it is possible to form a star coupler having almost no loss other than the branched portion.

Further, like the optical branching device 50 shown in FIG. 8, the branching portion 14 of the above-mentioned embodiment is provided at a symmetrical position with respect to the line segment aa, and the switching electrode 51 is provided so that the X-type waveguide is provided. It is also possible to configure an optical switch. This optical branching device 50 has two sets of branching parts 14 and 14 'which are symmetrical to each other, and one branching part 14 has the same second waveguides 12 and 13 and the first waveguides as in the above embodiment.
A tapered waveguide 30 and second tapered waveguides 31 and 32 are provided. The other branch portion 14 'is also provided with the second waveguides 12' and 13 ', the first tapered waveguide 30', and the second tapered waveguides 31 'and 32' having the same configurations as those of the above-described embodiment. .

[0027]

According to the present invention, there is little influence on the optical propagation characteristics due to variations in wavelength and manufacturing variations, and branching loss is small. The yield can be improved by reducing defective products. Further, since the number of curves can be reduced, bending loss can be reduced.

[Brief description of drawings]

FIG. 1 is a plan view of an optical branching device showing an embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II in FIG.

FIG. 3 is an enlarged view of a portion of the optical branching device shown in FIG.

FIG. 4 is a cross-sectional view showing a ridge type waveguide.

FIG. 5 is a diagram showing a relationship between wavelength and insertion loss.

FIG. 6 is a diagram showing a relationship between a coupling length and an insertion loss.

FIG. 7 is a diagram showing a relationship between a branch angle and insertion loss.

FIG. 8 is a plan view of an optical branching device showing another embodiment of the present invention.

FIG. 9 is a plan view showing a conventional Y-branch waveguide.

FIG. 10 is a plan view showing a waveguide structure using a conventional optical directional coupler.

[Explanation of symbols]

 10 ... Optical branching device 11 ... 1st waveguide 12, 13 ... 2nd waveguide 14 ... Branch part 20 ... Glass waveguide 21 ... Substrate 22, 23 ... Clad layer 24 ... Core part 30 ... 1st taper waveguide 31, 32 ... Second tapered waveguide

Claims (2)

[Claims] 1. A glass waveguide comprising a clad layer formed on a substrate and a core portion embedded in the clad layer, the waveguide comprising one first waveguide and two independent waveguides.
A second waveguide of the book, and a branch portion that optically connects the first waveguide and the second waveguide, and extends from the first waveguide to the first waveguide side of the branch portion. Further, a first tapered waveguide having a narrowed waveguide width toward the tip is provided, and each of the pair of second waveguides extends from the second waveguide toward the first tapered waveguide. A second tapered waveguide having a narrowed waveguide width toward the tip is provided, and the first tapered waveguide and the second tapered waveguide are brought close to each other for a predetermined coupling length to form a directional coupler. An optical branching device characterized by the above. 2. The branch angle of the second waveguide with respect to the longitudinal extension line of the first waveguide is set to 0.
The range from 5 ° to 1.5 ° is set.
Optical branching device described.