JPH0756034A - Crossing optical waveguide - Google Patents

Abstract

PURPOSE:To provide a crossing optical waveguide of small loss where a minimum angle of crossing is reduced without changing the structure of the optical waveguide in crossing parts and the number of crossings can be increased. CONSTITUTION:A first optical. waveguide which is formed on a silicon substrate and consists of a quartz single mode optical waveguide having 0.3% specific refractive index difference and a second optical waveguide which is formed on the silicon substrate so as to cross the first optical waveguide and consists of 50 quartz single mode optical waveguides having 0.3% specific refractive index difference cross each other at 30 deg. angle of crossing at intervals of 30 to 150mum, thereby reducing the excess loss per crossing to <=0.25dB which is an about half of conventional one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical circuit,
In particular, it relates to a crossed optical waveguide used for integrating optical circuits.

[0002]

2. Description of the Related Art Hitherto, as optical components for optical communication and optical signal processing, optical branchers, optical switches, optical multiplexers / demultiplexers, etc. using LiNbO ₃ or a silica optical waveguide have been realized. In addition, 128ch using these optical circuits as components
Large-scale optical circuits such as optical frequency selection switch, 8x8 matrix / optical switch, and 8ch optical frequency multiplexer / demultiplexer have been realized (for example, Opt.Quantum Electron, 22, 3
91, 1990, M. Kawachi). It is effective to use the crossed optical waveguides in order to integrate these large-scale optical circuits with higher density or to integrate a large number of small-scale optical circuits in an array.

FIG. 1 shows the integration of an optical circuit by crossed optical waveguides. In the figure, 1 is a substrate and 2 is the substrate 1.
An optical circuit component 3 constituted by an optical waveguide on the upper side is at least one optical waveguide group connected to the input / output ends of the element 2. Here, assuming that the optical circuit component 2 is an optical multiplexer / demultiplexer composed of a single Mach-Zehnder optical interferometer having two inputs and two outputs, the optical waveguide group 3 is about two optical waveguides. .

FIG. 1 (a) shows a circuit configuration when the crossed optical waveguide is not used. By using the crossed optical waveguide 4 as shown in FIG. 1 (b), the integration density is approximately doubled. It is possible, and as shown in FIG. 1 (c), the crossed optical waveguide portion 5a,
A large number can be integrated in an array by using 5b.

However, in general, an optical waveguide causes excessive loss when crossed. Therefore, even if the excess loss of the optical circuit component 2 itself is less than or equal to the required value, if the excess loss in the cross optical waveguide 4 and the cross optical waveguide portions 5a and 5b is larger than those, the circuit characteristics are deteriorated. It will be. In particular, as the number of crossings increases, the excess loss increases, and it is necessary to fully consider the influence on the circuit characteristics.

The excess loss depends on the structure of the optical waveguide, that is, the relative refractive index difference between the core and the clad, the core diameter, and the like. It is also well known that it depends on the crossing angle of the optical waveguide,
In general, the larger the crossing angle, the smaller the excess loss of the crossed optical waveguide. Thus, the required circuit characteristics limit the minimum crossing angle, which limits the design freedom and consequently the degree of integration of the optical circuit.

FIG. 2 shows an example of a crossed optical waveguide included in a waveguide type optical circuit. In the figure, 11 is a silicon substrate,
Reference numeral 12 is a first silica-based optical waveguide formed on the silicon substrate 11, and 13 is N second silica-based optical waveguides formed on the silicon substrate 11. Here, the optical waveguide 12,
13 has the same height H, width W, and relative refractive index difference Δ between the core and the clad, and each optical waveguide 13 intersects the optical waveguide 12 at a crossing angle θ and an interval 1 between adjacent optical waveguides. It is assumed that

In the crossed optical waveguide shown in FIG. 2, the crossing angle .theta. When the space 1 is set in consideration of the connection between the optical fiber array and the optical circuit, that is, 250 .mu.m which is the fiber space of a generally used optical fiber array. (10 degrees to 90
3 and 4 show the TE and TM modes with a wavelength of 1.31 μm and the TE and TM modes with a wavelength of 1.55 μm.

In FIG. 3, H = 8 μm, W = 8 μm, Δ = 0.
3%, that is, an example in the case of a single mode optical waveguide, N =
100, excess loss is shown as a value per crossing. For example, at θ = 30 degrees, the excess loss per intersection is about 0.05.
Although it is dB, it becomes about 5 dB when the number of intersections N = 100,
Only about 32% is transmitted through this crossed optical waveguide. θ = 6
At 0 degrees, the excess loss at N = 100 is about 1 dB, and about 79% can be transmitted.

Further, FIG. 4 shows H = 7 μm and W = 6.5 μm.
m, Δ = 0.75%, that is, an example in the case of a quasi-single-mode optical waveguide, N = 100, and excess loss is shown as a value per one crossing. It can be seen that when the relative refractive index difference Δ is increased, the excess loss is increased even at the same intersection angle θ. For example,
At θ = 30 degrees, the excess loss per crossing is about 0.3 dB, but at N = 100 it is about 30 dB, about 0.1%.
Only it will pass through. If θ = 60 degrees, N =
The excess loss at 100 is about 4 dB and about 40% transmission is possible. Further, when N = 25, the excess loss can be set to about 1 dB, and about 79% can be transmitted. That is, in order to reduce the excess loss to the allowable value or less, it is necessary to limit the minimum intersection angle and the number of intersections.

The excessive loss of the crossed optical waveguides can also be reduced by decreasing the relative refractive index difference Δ of the optical waveguides at the crossing portion, widening the width W, increasing the height H, etc. (for example, in Tech.Dig.OEC'92, 16B4-1, 1992, T. Komi
nato et al.,).

[0012]

As described above, in order to reduce the excess loss to the required value or less, it is necessary to set the intersection angle to a certain angle or more or the number of intersections to a certain number or less.
Therefore, as the number of intersections increases, the minimum intersection angle increases,
There is a problem that the degree of freedom in design is reduced and the degree of integration cannot be increased.

Further, as described above, the excess loss can be adjusted by reducing the relative refractive index difference of the optical waveguide at the intersection, widening the width, increasing the height, or the like. Lowering the height or increasing the height complicates the manufacturing process, and in particular, performing these adjustments with high accuracy in a region of a few hundred μm at a desired position is
There was a problem that the manufacturing process was made more complicated. Also, in order to adjust the relative refractive index difference, width, height, etc. to desired values so that loss does not occur, it is necessary to change these gently enough. There was a problem that would limit the. Further, in this case, it is necessary to change the relative refractive index difference, the width, the height, and the like as the number of intersections increases, and the area for the adjustment becomes larger accordingly, which further limits the integration degree. was there.

In view of the above-mentioned conventional problems, the present invention provides a low-loss crossed optical waveguide in which the minimum crossing angle can be made small and the number of crossings can be increased without changing the structure of the optical waveguide at the crossing portion. The purpose is to

[0015]

According to the present invention, in order to achieve the above object, the relative refractive index difference formed on one substrate is 3%.
A first optical waveguide comprising the following single-mode or quasi-single-mode optical waveguide and at least two relative refractive index differences formed on the one substrate so as to intersect the first optical waveguide have a relative refractive index difference of 3 % Of a single-mode or quasi-single-mode optical waveguide and a second optical waveguide, the crossed optical waveguide having an interval between the first and second optical waveguides of 30 μm to 150 μm. Propose a waveguide.

[0016]

According to the present invention, since the excess loss can be reduced only by limiting the crossing interval at the crossing portion, the minimum crossing angle can be made small and the degree of integration of the optical circuit can be increased. Further, since it is not necessary to change the structure of the optical waveguide, the manufacturing process is not complicated, and excessive loss due to changing the structure of the optical waveguide is not caused. Moreover, since a region for changing the structure of the optical waveguide is not required, higher integration can be achieved.

[0017]

EXAMPLE 1 The structure of the crossed optical waveguide of this example is the same as that shown in FIG. 2, in which the height H is 8 μm and the width W is W.
Is 8 μm, the relative refractive index difference Δ is 0.3%, and a silica single mode optical waveguide is used for the optical waveguides 12 and 13. The crossing angle θ is 30 degrees, the crossing number N is 50, and the crossing interval l is 50 μm. As a result, the excess loss due to the crossing was about 0.2 dB, and 95% could be transmitted.

In the above structure, the crossing interval l (8 μm-
TE and T of wavelength 1.31 μm for 2000 μm)
The excess loss of light in M mode and TE and TM modes of 1.55 μm is shown in FIG. Here, in order to make it easy to understand the change of the excess loss with respect to the crossing interval l, 1.55
A solid line connects measured values in the TE mode of μm. The excess loss is shown by the loss value per intersection.

FIG. 6 is a partially enlarged view of FIG. 5, showing the excess loss when the crossing interval 1 is 200 μm or less. By setting the crossing interval l to be 30 μm to 150 μm, the excess loss per crossing can be 0.025 dB or less, that is, about 1/2 of the excess loss of the conventional crossed optical waveguide, which is sufficient for practical use. In particular, if the crossing interval 1 is 30 μm to 130 μm, the excess loss per crossing can be 0.02 dB or less, and if the crossing interval 1 is 30 μm to 100 μm, the excess loss per crossing is 0.01 dB or less. You can see that

[0020]

Example 2 The structure of the crossed optical waveguide of this example is the same as that shown in FIG. 2, where the height H is 7 μm and the width W is W.
Is 6.5 μm and the relative refractive index difference Δ is 0.75%, and a silica-based pseudo single mode optical waveguide is used for the optical waveguides 12 and 13.
When the intersection angle θ is 30 degrees and the number of intersections N is 100, the intersection interval l
Is 50 μm, the excess loss due to the intersection is about 0.
It was 8 dB and 83% could be transmitted.

In the above structure, the crossing interval l (6.5 μ
FIG. 7 shows the optical excess loss in the TE and TM modes with a wavelength of 1.31 μm and the TE and TM modes with a wavelength of 1.55 μm for m to 2000 μm). Note that here, in order to make it easy to understand the change of the excess loss with respect to the crossing interval 1, 1.
The solid line connects the measured values in the 55 μm TE mode.
The excess loss is shown by the loss value per intersection.

FIG. 8 is a partially enlarged view of FIG. 7, showing the excess loss when the crossing interval 1 is 200 μm or less. By setting the crossing interval l to be 30 μm to 150 μm, the excess loss per crossing can be 0.2 dB or less, that is, the excess loss of the conventional crossing optical waveguide. Particularly, the crossing interval l is set to 30 μm to 130 μm.
If m, the excess loss per intersection is 0.1 dB or less,
That is, it can be seen that the excess loss of the conventional crossed optical waveguide can be set to about ½, and if the crossing interval 1 is set to 30 μm to 100 μm, the excess loss per crossing can be set to 0.05 dB or less.

As described above, in a crossed optical waveguide using a single mode or quasi single mode optical waveguide having a relative refractive index difference Δ of up to 3%, the crossing interval l is set to 30 μm to 150 μm so that one crossing is performed. The excess loss of can be made smaller than that of the conventional crossed optical waveguide. In particular, to make the loss smaller,
It is desirable that the intersection distance l be 30 μm to 100 μm.

The optical waveguide 12 in the first and second embodiments,
Reference numeral 13 is a two-mode optical waveguide capable of propagating the first-order mode at 1.31 μm and 1.55 μm, and is a quasi-single-mode optical waveguide that can be treated in the same way as a single-mode optical waveguide in constructing an optical circuit. However, since it is a two-mode optical waveguide, a large amount of meandering of light is likely to occur, and when the crossing interval l is 200 μm or more, loss fluctuation having a period substantially the same as the meandering period occurs as shown in FIG. 7. Crossing interval l
= 200 μm or more to form the crossed optical waveguide, it is desirable to set the crossing interval l at which excessive loss becomes small. Further, the crossing interval l at which the excess loss becomes small differs slightly depending on the wavelength of light, and it is necessary to consider this.

The term "quasi-single mode optical waveguide" means
Of the optical waveguides that can propagate not only the fundamental mode (0th-order mode) but also the first-order mode and higher modes, it is an optical waveguide that can be treated as a "single-mode optical waveguide" in constructing an optical circuit. ~ Shows an optical waveguide that can propagate up to the second mode. Also, in the optical waveguide capable of propagating up to the third-order mode and higher-order modes, the intersection interval l has an appropriate value similar to that of the present invention.

Further, in the above embodiment, the case where the straight waveguides intersect with each other has been shown, but the present invention is not limited to this. Crossings with bending waveguides are also an object of the invention.

Further, although the intersection angle θ is set to 30 degrees in the above embodiment, the present invention is not limited to this, and the intersection angle θ of several degrees to 90 degrees is the subject of the present invention. However, when the crossing angle θ is about several degrees, the ratio of optical coupling to the crossed optical waveguides becomes large, so that the excessive loss cannot be expected to be reduced as much as that of the above-described embodiment.

Further, in the above-mentioned embodiment, the case where the crossing angles θ of the respective optical waveguides 13 with respect to the optical waveguide 12 are the same is shown, but the present invention can be applied even if the crossing angles θ are different within a certain range. is there. Further, in the above-mentioned embodiment, the case where the crossed optical waveguides are in the same plane is shown, but the present invention can be applied even if they intersect three-dimensionally and their tilt angles ψ are different within a certain range.

In the above embodiment, the structure of the optical waveguide at the intersection is the same as that of the other optical circuits, but the relative refractive index difference of the optical waveguide at the intersection is reduced, the width is widened, and the height is increased. The present invention may be combined with a configuration for reducing excessive loss, such as when the crossing angle is small.

Further, although the core shape is rectangular in the above-mentioned embodiment, it is not limited to this shape, and it is the electric field distribution that is important. Any optical waveguide having a pseudo unit mode may be used, and for example, the core shape may be circular.

Further, although the crossed optical waveguide is separated from the optical circuit component in the above embodiment, the present invention is not limited to this, and the case where the crossed optical waveguide is included in the optical circuit component is also an object of the present invention. .

Furthermore, in the above-mentioned embodiment, the silica type single mode optical waveguide and the silica type pseudo single mode optical waveguide formed on the silicon substrate are used as the optical waveguide, but the present invention is not limited to this. , Other material based optical waveguides,
For example, an ion diffusion optical waveguide formed by a metal ion diffusion technique on a multi-component glass substrate or a lithium niobate crystal substrate is also applicable to the present invention.

[0033]

As described above, according to the present invention, the first optical waveguide comprising the single mode or quasi single mode optical waveguide formed on one substrate and having a relative refractive index difference of 3% or less. And a second optical waveguide comprising at least two single mode or quasi single mode optical waveguides having a relative refractive index difference of 3% or less formed on the one substrate so as to intersect with the first optical waveguide. In a crossed optical waveguide configured with a waveguide,
The interval at which the first and second optical waveguides intersect is 30 μm to 1
Since the thickness is 50 μm, low excess loss can be achieved even at a small crossing angle, high integration of optical circuits can be achieved, and the product can be downsized. Further, the high integration is advantageous in that a larger-scale optical circuit can be manufactured in the conventional optical circuit manufacturing process.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing how optical circuits are integrated by crossed optical waveguides.

FIG. 2 is a configuration diagram showing an example of a crossed optical waveguide.

3 is a diagram showing excess loss with respect to a crossing angle when the crossing optical waveguide shown in FIG. 2 is configured as a single mode optical waveguide with a crossing interval of 250 μm.

FIG. 4 is a diagram showing excess loss with respect to a crossing angle when the crossing optical waveguide shown in FIG. 2 is configured with a pseudo single mode optical waveguide with a crossing interval of 250 μm.

5 is a diagram showing excess loss with respect to a crossing interval in the case where the crossing optical waveguide shown in FIG. 2 is configured as a single mode optical waveguide with a crossing angle of 30 degrees.

6 is a partially enlarged view of FIG.

7 is a diagram showing excess loss with respect to a crossing interval in the case where the crossing optical waveguide shown in FIG. 2 is configured by a pseudo single mode optical waveguide with a crossing angle of 30 degrees.

FIG. 8 is a partially enlarged view of FIG.

[Explanation of symbols]

11 ... Silicon substrate, 12 ... First silica-based optical waveguide, 1
3 ... A second silica-based optical waveguide.

Front page continuation (72) Inventor Katsumi Kato 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. The relative refractive index difference formed on one substrate is 3
% Or less of a first optical waveguide formed of a single mode or quasi single mode optical waveguide and at least two relative refractive index differences formed on the one substrate so as to intersect the first optical waveguide. In an intersecting optical waveguide composed of a second optical waveguide consisting of 3% or less of single-mode or quasi-single-mode optical waveguides, the interval at which the first and second optical waveguides intersect is 30 μm to 1
A crossed optical waveguide characterized by having a thickness of 50 μm.