JPH0430107A - Waveguide type optical star coupler - Google Patents

Abstract

PURPOSE:To obtain a small-sized waveguide type optical star coupler which has a large distribution number and has the high uniformity of branch ratio by making an optical waveguide to constitute a pitch conversion optical waveguide group into a pseudo single mode optical waveguide a little wider than a single mode waveguide in its width, and an addition, setting its munimum radius of curvature smaller than the minimum radius of curvature of the single mode optical waveguide in a Y-type optical branch waveguide group. CONSTITUTION:An input wavguide 2, the Y-type branch waveguide group 3, a width conversion waveguide group 4, a pitch converison waveguide group 5, and plural output waveguide arrays 6 are arranged on a silicon substrate 1, and are connected in order. The input waveguide 2 and the y-type branch waveguide group 3 are constitut ed of quartz type single mode optical waveguides, and the pitch conversion waveguide group 5 and the output waveguide array 6 are constituted of quartz type pseudo single mode optical waveguides a little wider in their widths. Since plural modes can exist in the pseudo single mode waveguide, an unnecessary mode is excited at the time of propagation at a curved part. Then, a tapered shaped width conversion waveguide is arranged between the single mode waveguide and the pseudo single node waveguide different form each other in their widths, and a connection loss is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a waveguide optical star coupler that disperses signal light to multiple locations in an optical communication system.

<Prior Art> In order to spread optical fiber communication to every home, it is desirable for many people to share facilities such as optical fiber transmission lines in order to reduce costs. From this viewpoint, recently, communication network configurations in which a signal light transmitted through a single optical fiber is divided into 8, 16, or 322 branches for use by a large number of people have been actively studied. An 8-branch element, a 16-branch element, a 32-branch element, etc. that play an important role in such a communication network are called star couplers.

Star couplers can be classified into 1) bulk type, 2) fiber type, and 2) waveguide type depending on their form. The bulk type is
Although it is composed of a combination of microlenses, prisms, interference film filters, etc., it takes a long time to assemble and adjust, and there are still problems in terms of price and size. has only just been put into practical use. The fiber type is constructed by using the optical fiber itself as a raw material through polishing, fusing, and stretching processes, and although it has the advantage of being relatively compact, it is basically a so-called (2x2) coupler, so it cannot be used on a large scale. Constructing a coupler requires a complicated process of connecting a large number of (2×2) couplers, resulting in a large size as a whole, resulting in problems such as a lack of productivity. On the other hand, the waveguide type has the advantage of being able to be mass-produced on a flat substrate using a photolithography process, and is attracting the most attention as a future coupler due to its reproducibility and possibility of compact integration. .

A conventional eight-branch waveguide type star coupler is shown in FIG. As shown in the figure, on the substrate 24 are an input waveguide 25, a Y-shaped branch waveguide group 26 in which Y-shaped branch waveguides are connected in three stages, a plurality of stages of pitch conversion waveguide group 27, and an output waveguide. array 2
8 are arranged and connected in order. That is, these waveguides are usually constructed from single-mode glass waveguides formed on glass or silicon substrates, and the input fiber 2
After the signal light guided from 9 propagates through the input waveguide 25,
It is branched into two at the first stage Y-shaped branch waveguide, and then
Furthermore, each wave is branched into two by a second-stage Y-shaped branch waveguide. Generally, by repeating n stages of branching, N
A distribution number of =2·(2 to the n power) can be obtained, which corresponds to the case of n=3 and N=8 in the star coupler of FIG. After being distributed into N (=8) waves in this way, the intervals are widened by the plurality of stages of pitch conversion waveguide group 27 and the waves are connected to output waveguide array 28 . The spacing between the output fiber arrays 30 is normally 250 μm, and the spacing between the output waveguide arrays 28 is set equal to this.

<Problem to be solved by the invention> In the configuration of the waveguide type star coupler shown in FIG.
= 8 or 16 star couplers have been realized,
When trying to realize a large-scale star coupler with even larger N, the following problems were encountered.

That is, if the number of branches N is increased, the length occupied by the plurality of stages of pitch conversion waveguide group 27 increases rapidly, and there is a problem that the entire star coupler cannot be accommodated on a substrate with a limited area. For example, in the case of N=128 (2'), the width occupied by the output waveguide array 28 is approximately 250 μmXN=3
2m+, which is caused by the fact that if the pitch conversion waveguide group 27 is composed of gently curved waveguides, the occupied area will increase.

In order to avoid this problem, when the pitch conversion waveguide group 27 is constructed of curved waveguides with a small radius of curvature, a problem arises in which the radiation loss at the curved portion increases rapidly.

Here, in order to maintain a small radius of curvature and suppress the occurrence of radiation loss at the bend, it is necessary to introduce a pseudo-single-mode optical guide that can propagate not only the fundamental (zero-order) mode but also a higher-order mode of the first-order mode. It is known that it is sufficient to employ a waveguide and propagate only the fundamental mode in this pseudo single mode optical waveguide.

However, when a waveguide star coupler is constructed using this quasi-single mode optical waveguide, the branching ratio of each Y-shaped branching waveguide is 1.
This has the disadvantage that it tends to deviate from the pair-to-one relationship, resulting in variations in the intensity of light branched to the output waveguide array. The disadvantages of this are that the power distribution of the fundamental mode tends to deviate from the center of the waveguide in a quasi-single mode optical waveguide, and mode conversion of higher-order modes tends to occur in the Y-shaped branch waveguide, and It is presumed that this is caused by uneven branching ratio in the waveguide.

Therefore, in the conventional waveguide type optical star coupler shown in FIG. 8, it was impossible to simultaneously increase the number of distributions and make the distribution ratio uniform.

The present invention has been made in view of the above-mentioned prior art,
It is an object of the present invention to provide a small-sized waveguide type optical star coupler with a large number of distributions and a highly uniform branching ratio.

<Means for Solving the Problems> The configuration of the present invention that achieves the above object includes a substrate, input optical waveguides arranged on the substrate and connected in sequence, a group of Y-shaped optical branching waveguides having at least one stage, and a pitch. A waveguide type optical star coupler comprising a conversion optical waveguide group and an output optical waveguide array, wherein each optical waveguide constituting the Y-shaped optical branching waveguide group is a single mode optical waveguide, and the pitch conversion optical waveguide group The optical waveguide constituting the is a quasi-single mode optical waveguide that is slightly wider than the single mode optical waveguide, and
The minimum radius of curvature of the pseudo single mode optical waveguide in the pitch conversion optical waveguide group is set smaller than the minimum radius of curvature of the single mode optical waveguide that is compatible with the Y-type optical branching waveguide group. shall be.

Furthermore, it is desirable that a tapered width conversion optical waveguide be installed between the single mode optical waveguide and the quasi-single mode optical waveguide.

<Operation> The pseudo-single mode waveguides constituting the pitch conversion optical waveguide group have a greater optical confinement effect regarding the fundamental mode than single mode waveguides.

Therefore, the radius of curvature can be reduced while suppressing radiation loss in the curved portion of the pitch conversion optical waveguide group.

However, since multiple modes may exist in a quasi-single mode waveguide, unwanted modes are excited when propagating through curved sections. Therefore, the intensity distribution of light in the cross section of the waveguide may not be symmetrical. If this is left unaddressed, it will cause variations in the distribution ratio in the Y-shaped branch waveguide.

Therefore, in such a case, connection loss can be suppressed by arranging a tapered width conversion waveguide between a single mode waveguide and a quasi-single mode waveguide having different widths.

<Examples> The present invention will be described in detail below with reference to examples shown in the drawings.

In the examples below, a silica-based waveguide formed on a silicon substrate is used as an optical waveguide. This is because it is possible to provide a waveguide type optical branching element that is
The present invention is not limited to this.

FIG. 1 shows a first embodiment of the present invention. This embodiment is an example in which the number of branches is 128.

In other words, a silicon substrate! Arranged above are an input waveguide 2, a Y-shaped branch waveguide group 3 with a Y-shaped branch waveguide connected to the upper stage, a width conversion waveguide group 4, a pitch conversion waveguide group 5, and a plurality of output waveguide arrays 6. are concatenated in order. Input waveguide 2
, output waveguide array 6 is connected to input optical fiber 7, respectively.
It is connected to an output optical fiber (not shown). The input waveguide 2 and the Y-type branching waveguide group 3 are composed of silica-based single-mode optical waveguides, and the pitch conversion waveguide group 5 and the output waveguide array 6 are composed of slightly wide silica-based pseudo-single-mode optical waveguides. It is configured.

The Y-type branch waveguide group 3 is constructed by connecting Y-type branch waveguides 7 in seven stages (n=7), and is arranged as shown in FIG. 2 (al). The branch waveguide 7 has an input-side single-mode waveguide 8, a tapered region 9, and two output-side single-mode waveguides IO, as shown in FIG. 2 (bl), an enlarged view of the vicinity of the branch. It consists of

The single mode waveguide 8 on the input side is shown in Fig. 2 ('b).
The size of the core 13 is determined so that the waveguide satisfies the single mode condition, as shown in FIG. For example, in the case of a silica-based optical waveguide in which the difference between the refractive index of Clara K12 and the refractive index of Core I3 is 0.75%,
The core cross-sectional dimensions are 5 μm×5 μm. At this position, the light intensity distribution is symmetrical with respect to the traveling direction,
After passing through the tapered region 9, the light has an intensity equal to that of the two waveguides 1.
Divided into 0 thirds. In addition, in FIG. 3, 11 is a silicon substrate.

On the other hand, the single mode waveguide 10 on the output side also has the same configuration as the single mode waveguide 8 described above.

That is, except for the tapered region 9, the Y-shaped branch waveguide 7 is composed of a single mode waveguide having the cross-sectional structure shown in FIG. 3, and the distance ratio radius of the bending portion is set to 25 won.

Therefore, the single mode optical waveguides 10 on the output side after passing through the seven stages of Y-shaped branch waveguides 7 are arranged in an array of 128 pieces.
The pitch is set to 25 μm. This 25 μm
The waveguide pitch is the minimum spacing at which coupling between waveguides can be ignored.

Next, in the pitch conversion waveguide group 5, a quasi-single mode waveguide with a wide waveguide width is used, the cross section of which is shown in FIG. This is to reduce the radius of curvature in the pitch conversion waveguide group 5 and to minimize its exclusive area.

The width of this pseudo single mode waveguide is set to be slightly wider than the single mode waveguide in the Y-shaped branch waveguide 7. For example, in the case of a silica-based optical waveguide in which the difference between the refractive index of the cladding 15 and the refractive index of the core 16 is the same as above, the core cross-sectional dimensions are 5 μm×7 μm. Note that 14 in FIG. 4 is a silicon substrate.

Here, the interval on the input side of the pitch conversion waveguide group 5 is 25 μm.
and 250 μm on the output side. The radius of curvature of the curved waveguide used in the pitch conversion waveguide group 5 is large at the center and becomes smaller toward the periphery, and the minimum radius of curvature at the periphery is 5 m+. A group of waveguides whose spacing is increased to 250 μm is connected to an output fiber array (not shown) through an output waveguide array 6.

In this example, the pseudo single mode waveguide is set by keeping the core thickness at 5 μm and increasing only the width from 5 μm to 7 μm. The reason for this is that it is easy to switch between the single mode condition and the pseudo-single mode condition by controlling only the width in the star coupler manufacturing process. That is, if waveguide patterns with different widths are fabricated on a photomask in the manufacturing process of the core part, a single mode waveguide part and a quasi-single mode waveguide part can be formed from the same core film using a known photomask. The desired core width can be cut using lithography and reactive ion etching. If the complexity of the manufacturing process is not a problem, it is possible to create both a single mode waveguide and a quasi-single mode waveguide on the same substrate by changing the core thickness and the difference in refractive index between the core and the cladding. It may be manufactured on top.

Here, the core width of the pseudo-single mode waveguide used in the present invention is 1.2 to 1.6 compared to the core width of the single mode waveguide.
It is desirable to set it to about twice that.

! , if it is about 1 times or less, it cannot withstand a small radius of curvature, and conversely, if it is more than 1.7 times, higher modes other than the fundamental mode are likely to occur, which is also undesirable.

The important point here is the shape of the coupling portion between the single mode waveguide section and the quasi-single mode waveguide section described above. In this connection, it is desirable that the central axes of the single mode waveguide and the quasi-single mode waveguide coincide. Moreover, it is desirable that both waveguides maintain a straight shape and do not bend in a region of at least about 200 μm in length before and after the connecting portion. This is to preferentially excite the fundamental mode of the quasi-single-mode waveguide at the connection portion, and to prevent higher-order modes from being radiated and lost at the sharply curved portion of the pitch conversion waveguide group.

Furthermore, it is desirable that the shape of this connecting portion be gently tapered to prevent even the slightest connection loss from occurring due to the discontinuity of the core shape of the connecting portion.

For this reason, in this embodiment, a width conversion waveguide 4 is provided between the Y-shaped branch waveguide group 3 and the pitch conversion waveguide group 5 to make the connecting portion tapered. This width conversion waveguide 4 is
As shown in FIG. 5, the cross-sectional shape is such that the taper distance necessary to widen the width from 5 μm to 7 μm is set to 300 μm in order to avoid loss due to change in core width. Therefore, the width of the core 19 gradually increases from 5 μm to 7 μm over a distance of 300 μm.

In FIG. 5, 18 is a cladding, and 19 is a substrate.

In the 128-branch star coupler of this embodiment having the above configuration, the dimensions of the substrate 1 are 50IIf11×50wn,
It is large enough to be stored on an inexpensive 3-inch silicon wafer. The lateral length from the input waveguide 2 through the 7-stage Y-shaped branch waveguide 7 until it is divided into 128 branches of 25 μm is approximately 33-, and the lateral length occupied by the pitch conversion waveguide group 5 and the output waveguide array 6 is approximately 33-. The length in the direction is about 15 mm. The width occupied by the output waveguide array 6 is approximately 3211IIn.

The performance of the 128-branch waveguide star coupler of this example is in the 1.3μm wavelength band and the 1.55μm wavelength band, which is the most important in the optical communication field.
In the m wavelength band, the results were good as shown below.

Excessive branching loss ~3 dB (including input/output optical 2-eyebar connection loss) Distribution unevenness ±1 dB For comparison, a star coupler was intentionally formed entirely in a single mode without using a quasi-single mode waveguide. When we prototyped (other conditions are the same as above), the excess branching loss was 8.
It was confirmed that it was as large as ~1 OdB. In particular, 12
Among the 8 branches, the attenuation of the light extracted from the peripheral part was significant. This is because the pitch conversion waveguide group cannot withstand the radius of curvature up to a minimum of 5M, and the signal light is radiated and lost. On the other hand, when we fabricated a prototype consisting entirely of quasi-single mode waveguides, the excess branching loss was ~4d.
Although it was B, the distribution unevenness was as large as ±4, and it could not be said that it was distributed evenly.

Incidentally, in order to construct a low-loss pitch conversion waveguide group using only single mode waveguides, it is necessary to keep the minimum bending radius to about 20 degrees or more from the viewpoint of suppressing the occurrence of radiation loss. In this case, the length of the board would increase.
A large-scale and high-performance waveguide optical star coupler of 50mX
It becomes possible to realize this on a substrate of about 50+ nm.

In this embodiment, the output waveguide array 6 was arranged on one side of the substrate, but the present invention is not limited to this, but the output waveguide array 6 is divided and arranged over two or more sides of the substrate, Furthermore, it is also possible to further increase the number of distributions.

For example, output waveguide arrays may be arranged at three locations as in the second embodiment of the present invention shown in FIG.

That is, the basic configuration of this embodiment is the same as that of the above-mentioned embodiment, but in order to increase the number of distributions to 256, 8
The 256 single mode optical waveguides with a pitch of 25 μm that passed through the Y-shaped branch waveguide group were divided into a central group of 128 and upper and lower groups of 64 each. , respectively, are connected to output waveguide arrays 20 and 21 on three sides of the substrate via a group of pitch conversion waveguides.

In this example, except for the pitch conversion waveguide group, the output waveguide array is also designed as a single mode waveguide, and tapered width conversion waveguides are provided at both the entrance and exit of the pitch conversion waveguide group. There is.

By providing it also on the exit side, it is possible to suppress the occurrence of mode conversion due to an error in optical axis alignment with the output optical fiber array, and further suppress unevenness in connection loss due to optical axis misalignment.

The wavelength of the 256 branch waveguide star coupler of this example is 13μ
The following good performance was shown in the m band and 1.55 μm band.

Excessive branching loss ~3.5 dB (including input/output optical fiber connection loss) Distribution unevenness ±1 dB In the example based on two silica-based optical waveguides, the minimum radius of curvature in the pitch conversion waveguide group is set to 5M. However, if you want to set this radius even smaller, for example, to 4, you can reduce the bending loss by shifting the center axis of the waveguide at the connection part of two curved waveguides with different directions of curvature in the pitch conversion waveguide group. It is also possible to suppress the increase in

FIG. 7 is an example of the above-mentioned axis misalignment shape that can be used in the present invention. Connection part 2 where the curvatures of two curved waveguides 22 are reversed
3, an axis shift of about 0.2 μm is given to the waveguide. This axis misalignment is based on the fact that in a sharply curved waveguide, the optical electric field propagates more toward the outer periphery than the geometric center axis of the waveguide. In this case, the apparent geometrical central axes are shifted so that the substantial central axes of the optical electric field coincide with each other.

In the above example, the relative refractive index difference between the core and the cladding is 0.7
Although it was 5%, the present invention is not limited to this. When the relative refractive index is set larger, the radius of curvature set value in the above embodiments may be relatively increased to increase the size of the substrate.

Conversely, when the relative refractive index is set larger, the radius of curvature may be set relatively smaller to make the substrate smaller.

Furthermore, in the above embodiment, the output waveguide array and the output optical fiber array were connected by the so-called butting method. It is possible to make the connection work with a large number of optical fibers more efficient.

<Effects of the Invention> As specifically explained above based on the embodiments, the waveguide type optical star coupler of the present invention is achieved by skillfully combining a single mode waveguide and a quasi-single mode waveguide. ,
Since we were able to reduce the size and integration of the device and at the same time suppress variations in the branching ratio, it is possible to manufacture a large-scale waveguide optical star coupler on a limited substrate area. Therefore, it is possible to increase the number of branches, which was difficult with conventional fiber type, and it is very useful for building large-scale communication systems. In the above embodiments, 128 branches and 256 branches have been described, but the present invention is not limited to these, but can also be applied to 64 branches or a larger scale star coupler configuration.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the planar configuration of a 128-branch waveguide type optical star coupler according to the first embodiment of the present invention, and FIG. 2(a) (
b) is an overall layout diagram of the Y-shaped branch waveguide group used in the present invention, an enlarged view of the branch part, FIG. 3 is a cross-sectional view of the single mode waveguide used in the present invention, and FIG. FIG. 5 is a cross-sectional view of the pseudo single mode waveguide used in the present invention, and FIG. 6 is a transition diagram of the tapered width conversion waveguide used in the present invention.
The figure is an explanatory diagram of the planar configuration of a 256-branch waveguide type star coupler according to the second embodiment of the present invention, and FIG. 7 is an explanatory diagram of the axis-shifted shape of the curved waveguide section of the pitch conversion waveguide group used in the present invention. 8 are configuration diagrams of a conventional eight-branch waveguide type star coupler. In the drawing, 1 is a waveguide substrate, 2.25 is an input waveguide, 3.26 is a Y-shaped branch waveguide group, 4 is a width conversion waveguide, 5.27 is a pitch conversion waveguide group, and 6.28 is an output. Waveguide array, 7 is a Y-shaped branch waveguide, 8 is an input side single mode waveguide, 9 is a tapered region, 10 is an output side single mode waveguide, 11.14.17 is a silicon substrate, 12.15. 18 is the cladding, +3, 16, 19 is the core, 20.21 is the output waveguide array, 22 is the curved waveguide, 23 is the curvature inversion connection, 24 is the waveguide substrate, 29 is the input fiber 30 is the output fiber array Al.

Claims (2)

[Claims] (1) A substrate, input optical waveguides disposed on the substrate and sequentially connected, and a group of at least one stage of Y-shaped optical branching waveguides;
A waveguide optical star coupler comprising a pitch conversion optical waveguide group and an output optical waveguide array, wherein each optical waveguide constituting the Y-type optical branching waveguide group is a single mode optical waveguide,
The optical waveguides constituting the pitch-converted optical waveguide group are pseudo-single-mode optical waveguides that are slightly wider than the single-mode optical waveguide, and the width of the pseudo-single-mode optical waveguide in the pitch-converted optical waveguide group is A waveguide type optical star coupler, wherein a minimum radius of curvature is set smaller than a minimum radius of curvature of the single mode optical waveguide in the Y-type optical branching waveguide group. (2) A waveguide type optical star according to claim (1), characterized in that a tapered width conversion optical waveguide is installed between the single mode optical waveguide and the pseudo single mode optical waveguide. coupler.