Crossing optical waveguide

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a waveguide type optical circuit, and more particularly to a crossed optical waveguide used for integrating an optical circuit.

【0002】

[Conventional technology]

Conventionally, LiNbO has been used as an optical component for optical communication and optical signal processing.₃Optical turnouts, optical switches, optical demultiplexers, etc. using quartz-based optical waveguides have been realized. In addition, large-scale optical circuits such as a 128ch optical frequency selection switch, an 8 × 8 matrix / optical switch, and an 8ch optical frequency demultiplexer that use these optical circuits as components have been realized (for example, Opt. See Quantum Electron, 22,391, 1990, M. Kawachi). Then, in order to integrate these large-scale optical circuits at a higher density or to integrate a large number of small-scale optical circuits in an array, it is effective to use a cross-optical waveguide.

【0003】

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

【0004】

Fig. 1 (a) shows the circuit configuration when the crossed optical waveguide is not used, but as shown in Fig. 1 (b), the integration density can be doubled by using the crossed optical waveguide 4. , As shown in Fig. 1 (c), a large number can be integrated in an array by using the crossed optical waveguides 5a and 5b.

【0005】

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

【0006】

The excess loss depends on the structure of the optical waveguide, that is, the difference in the specific refractive index between the core and the cladding, the core diameter, and the like. It is also well known that it also depends on the crossing angle of the optical waveguide, and in general, the larger the crossing angle, the smaller the excess loss of the crossing optical waveguide. Thus, the required circuit characteristics limit the minimum intersection angle, which limits the degree of freedom in design and, as a result, the degree of integration of the optical circuit.

【0007】

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, 12 is a first quartz-based optical waveguide formed on a silicon substrate 11, and 13 is silicon. N second quartz-based optical waveguides formed on the substrate 11. Here, the optical waveguides 12 and 13 have the same height H, width W, and specific refractive index difference Δ between the core and the cladding, and each optical waveguide 13 has an intersection angle θ and an adjacent optical waveguide with respect to the optical waveguide 12. It is assumed that they intersect at an interval l between them.

【0008】

In the crossed optical waveguide of FIG. 2, the crossing angle θ (10 degrees) when the spacing l is the spacing considering the connection between the optical fiber array and the optical circuit, that is, 250 μm, which is the fiber spacing of the commonly used optical fiber array. The excess loss of light 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 with respect to (~ 90 degrees) is shown in FIGS. 3 and 4.

【0009】

FIG. 3 shows an example in the case of H = 8 μm, W = 8 μm, Δ = 0.3%, that is, a single-mode optical waveguide, N = 100, and the excess loss is shown as a value per intersection. For example, at θ = 30 degrees, the excess loss per intersection is about 0.05 dB, but at the number of intersections N = 100, it is about 5 dB, and this crossing optical waveguide transmits only about 32%. When θ = 60 degrees, the excess loss at N = 100 is about 1 dB, and about 79% can be transmitted.

【0010】

Further, FIG. 4 shows an example in the case of H = 7 μm, W = 6.5 μm, Δ = 0.75%, that is, a pseudo single-mode optical waveguide, N = 100, and the excess loss is shown as a value per intersection. It can be seen that when the specific refractive index difference Δ is increased, the excess loss increases even at the same intersection angle θ. For example, at θ = 30 degrees, the excess loss per intersection is about 0.3 dB, but at N = 100, it is about 30 dB, and only about 0.1% is transmitted. When θ = 60 degrees, the excess loss at N = 100 is about 4 dB, which allows about 40% transmission. Also, when N = 25, the excess loss can be reduced to about 1 dB, and about 79% can be transmitted. That is, in order to keep the excess loss below the permissible value, it is necessary to limit the minimum crossing angle and the number of crossing numbers.

【0011】

Further, the excess loss of the crossed optical waveguide can be reduced by reducing the specific refractive index difference Δ of the optical waveguide at the crossing portion, widening the width W, increasing the height H, and the like (for example, in Tech. See Dig. OEC'92, 16B4-1, 1992, T. Kominato et al.).

【0012】

[Problems to be Solved by the Invention]

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

【0013】

Further, as described above, the excess loss can be adjusted by reducing the difference in the specific refractive index of the optical waveguide at the intersection, widening the width, increasing the height, etc., but reducing the difference in the specific refractive index. Increasing the height complicates the fabrication process, and in particular, performing these adjustments with high accuracy in a region of several hundred μm at a desired position causes a problem that the fabrication process becomes more complicated. was there. Further, in order to adjust the specific refractive index difference, width, height, etc. to desired values so as not to cause loss, it is necessary to change these sufficiently gently, and a region for this adjustment is required, and the degree of integration is required. There was a problem that it would limit. Further, in this case, as the number of crossings increases, it is necessary to greatly change the difference in specific refractive index, width, height, etc., and the area for the adjustment becomes larger by that amount, which further limits the degree of integration. was there.

【0014】

In view of the above-mentioned conventional problems, it is an object of the present invention to provide a low-loss crossing optical waveguide capable of reducing the minimum crossing angle and increasing the number of crossings without changing the structure of the optical waveguide at the crossing. And.

【0015】

[Means for solving problems]

In the present invention, in order to achieve the above object, a first optical waveguide formed on one substrate and composed of a single-mode or pseudo-single-mode optical waveguide having a specific refractive index difference of 3% or less, and the first optical waveguide. It is composed of at least three optical waveguides formed on the one substrate so as to intersect the optical waveguide and a second optical waveguide composed of a single mode or pseudo single mode optical waveguide having a specific refractive index difference of 3% or less. In a crossed optical waveguide in which at least one of the intersections of the first optical waveguide and the second optical waveguide has an intersection angle of less than 90 °, the intersections are arranged at equal intervals, and the intervals are 30 μm to 150 μm. We propose a cross-optical waveguide.

【0016】

[Action]

According to the present invention, since the excess loss can be reduced only by limiting the intersection interval at the intersection, the minimum intersection angle can be made small and the degree of integration of the optical circuit can be increased. Moreover, since it is not necessary to change the structure of the optical waveguide, the manufacturing process is not complicated and excessive loss is not caused by changing the structure of the optical waveguide. In addition, 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 embodiment is the same as that shown in FIG. 2, where the height H is 8 μm, the width W is 8 μm, and the specific refractive index difference Δ is 0.3%. When the optical waveguide was used for the optical waveguides 12 and 13, the intersection angle θ was 30 degrees, the number of intersections N was 50, and the intersection interval l was 50 μm, the excess loss due to the intersection was about 0.2 dB, and 95% transmission was possible. did it.

【0018】

In the above configuration, the excess loss of light 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 with respect to the crossing interval l (8 μm to 2000 μm) is shown in FIG. Here, in order to make it easier to understand the change in excess loss with respect to the intersection interval l, the measured values in TE mode of 1.55 μm are connected by a solid line. The excess loss is indicated by the loss value per intersection.

【0019】

FIG. 6 is a partially enlarged view of FIG. 5, showing an excess loss when the crossing interval l is 200 μm or less. If the crossing interval l is set to 30 μm to 150 μm, the excess loss per crossing can be 0.025 dB or less, that is, about half of the excess loss of the conventional crossed optical waveguide, which is sufficient for practical use. In particular, if the intersection interval l is set to 30 μm to 130 μm, the excess loss per intersection can be reduced to 0.02 dB or less, and if the intersection interval l is set to 30 μm to 100 μm, the excess loss per intersection can be reduced to 0.01 dB or less. I understand.

【0020】

[Example 2]

The structure of the crossed optical waveguide of this embodiment is the same as that shown in FIG. 2, where the height H is 7 μm, the width W is 6.5 μm, and the specific refractive index difference Δ is 0.75%. When the one-mode optical waveguide was used for the optical waveguides 12 and 13, the intersection angle θ was 30 degrees, the number of intersections N was 100, and the intersection interval l was 50 μm. I was able to.

【0021】

In the above configuration, the excess loss of light 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 with respect to the crossing interval l (6.5 μm to 2000 μm) is shown in FIG. Here, in order to make it easier to understand the change in excess loss with respect to the intersection interval l, the measured values in TE mode of 1.55 μm are connected by a solid line. The excess loss is indicated by the loss value per intersection.

【0022】

FIG. 8 is a partially enlarged view of FIG. 7, showing an excess loss when the crossing interval l is 200 μm or less. If the intersection interval l is set to 30 μm to 150 μm, the excess loss per intersection can be reduced to 0.2 dB or less, that is, the excess loss of the conventional optical waveguide. In particular, if the intersection interval l is set to 30 μm to 130 μm, the excess loss per intersection can be reduced to 0.1 dB or less, that is, about half the excess loss of the conventional optical waveguide, and the intersection interval l is 30 μm to 100 μm. It can be seen that the excess loss per intersection can be reduced to 0.05 dB or less.

【0023】

In this way, in a crossed optical waveguide using a single-mode or pseudo-single-mode optical waveguide with a specific refractive index difference Δ of up to 3%, the excess loss per crossing is achieved by setting the crossing interval l to 30 μm to 150 μm. Can be less than or equal to the conventional crossed optical waveguide. In particular, in order to reduce the loss, it is desirable to set the crossing interval l to 30 μm to 100 μm.

【0024】

The optical waveguides 12 and 13 in Examples 1 and 2 are two-mode optical waveguides capable of propagating the primary mode at 1.31 μm and 1.55 μm, but can be treated in the same manner as a single-mode optical waveguide in configuring an optical circuit. It is a pseudo single mode optical waveguide. However, since it is a two-mode optical waveguide, large meandering of light is likely to occur, and when the crossing interval l is 200 μm or more, a loss fluctuation having almost the same period as the meandering period occurs as shown in FIG. When a crossing optical waveguide is configured with a crossing interval l = 200 μm or more, it is desirable to set a crossing interval l that reduces excess loss. In addition, the crossing interval l at which the excess loss is small differs slightly depending on the wavelength of light, and this must be taken into consideration.

【0025】

Note that the "pseudo single mode optical waveguide", among the optical waveguide may propagate the fundamental mode (0 order mode) only Not first mode or modes, "in order to constitute the optical circuit monomode optical guide It is an optical waveguide that can be treated as a "waveguide", and mainly shows an optical waveguide that can propagate from the primary to the secondary mode. Further, even in the optical waveguide capable of propagating to the higher-order mode of the third-order mode or higher, the intersection interval l has an appropriate value similar to that of the present invention.

【0026】

Further, in the above embodiment, the case of the intersection of the linear waveguide and the linear waveguide is shown, but the present invention is not limited to this, and the intersection of the bent waveguide and the linear waveguide and the bending waveguide and the bending waveguide are not limited to this. The intersection with is also an object of the present invention.

【0027】

Further, in the above embodiment, the crossing angle θ is set to 30 degrees, but the present invention is not limited to this, and those having a crossing angle θ of several degrees to 90 degrees are the objects of the present invention. However, when the crossing angle θ is about several degrees, the ratio of optical coupling to the crossed optical waveguide becomes large, so that the reduction of excess loss cannot be expected as in the above-described embodiment.

【0028】

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

【0029】

Further, 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 difference in the specific refractive index of the optical waveguide at the intersection is reduced, the width is widened, the height is increased, and the like. The present invention may be combined with a configuration for reducing excess loss, which is particularly effective when the intersection angle is small.

【0030】

Further, although the core shape is rectangular in the above embodiment, the core shape is not limited to this shape, and what is important is the electric field distribution, which is a single mode or a pseudo unit mode with a specific refractive index difference within the claimed range. Any optical waveguide may be used, for example, the core shape may be circular.

【0031】

Further, in the above embodiment, the crossed optical waveguide is separated from the optical circuit component, but 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.

【0032】

Furthermore, in the above-described embodiment, a quartz-based single-mode optical waveguide and a quartz-based pseudo-single-mode optical waveguide formed on a silicon substrate are used as the optical waveguide, but the optical waveguide is not limited to this. Material-based optical waveguides, such as ion diffusion optical waveguides formed on a multi-component glass substrate or lithium niobate crystal substrate by metal ion diffusion technology, are also applicable to the present invention.

【0033】

[Effect of the invention]

As described above, according to the present invention, a first optical waveguide formed on one substrate and composed of a single-mode or pseudo-single-mode optical waveguide having a specific refractive index difference of 3% or less, and the first optical waveguide. It is composed of at least three optical waveguides formed on the one substrate so as to intersect the optical waveguides of the above, and a second optical waveguide composed of a single mode or pseudo single mode optical waveguide having a specific refractive index difference of 3% or less. In a crossed optical waveguide in which at least one of the intersections of the first optical waveguide and the second optical waveguide has an intersection angle of less than 90 °, the intersections are arranged at equal intervals, and the intervals are 30 μm ~. Since the thickness is 150 μm, low excess loss can be achieved even at a small intersection angle, the optical circuit can be highly integrated, and the product can be miniaturized. Further, since high integration can be achieved, there is an advantage that a larger-scale optical circuit can be manufactured by a conventional optical circuit manufacturing process.

[Simple explanation of drawings]

[Figure 1]

Schematic diagram showing how optical circuits are integrated by crossed optical waveguides [Figure 2]

Configuration diagram showing an example of crossed optical waveguide [Fig. 3]

The figure which shows the excess loss with respect to the crossing angle when the crossing optical waveguide shown in FIG. 2 is configured with a single mode optical waveguide with a crossing interval of 250 μm. [Fig. 4]

The figure which shows the excess loss with respect to the crossing angle when the crossing optical waveguide shown in FIG. 2 is configured by the pseudo single mode optical waveguide with a crossing interval of 250 μm. [Fig. 5]

The figure which shows the excess loss with respect to the crossing interval when the crossing optical waveguide shown in FIG. 2 is configured by the single mode optical waveguide with a crossing angle of 30 degrees. [Fig. 6]

Partially enlarged view of Fig. 5 [Fig. 7]

The figure which shows the excess loss with respect to the crossing interval when the crossing optical waveguide shown in FIG. 2 is configured by the pseudo single mode optical waveguide with a crossing angle of 30 degrees. [Fig. 8]

Partially enlarged view of Fig. 7 [Explanation of symbols]

11 ... silicon substrate, 12 ... first quartz-based optical waveguide, 13 ... second quartz-based optical waveguide.