JP2538955B2 - Design method of directional coupling type optical multiplexer / demultiplexer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention is a method for designing a directional coupling type optical multiplexer / demultiplexer, and particularly reproduces a directional coupling type optical multiplexer / demultiplexer having certain element characteristics. The present invention relates to a design method that enables manufacturing with good performance.

[Conventional technology and its problems]

3A and 3B are diagrams showing the basic structure of the directional coupling type optical multiplexer / demultiplexer, wherein FIG. 3A is a top view and FIG. 3B is a sectional view taken along the line AA ′ in FIG. 1 is a first waveguide, 2 is a second waveguide, 3
Is a coupling part consisting of parallel waveguides with a space, 4-1 and 4-2
Are the entrance and exit ends of the first waveguide 1, 5-1 and 5-2 are the entrance and exit ends of the second waveguide 2, respectively.
Reference numeral 6 is a clad layer, and 7 is a substrate. The waveguide parameters of the first waveguide 1 and the second waveguide 2 are substantially equal. Complete coupling length 2N times of the coupling portion 3 of a length wavelength L lambda ₁ (N is a natural number), and when the 2N + 1 times the coupling length of the wavelength lambda _2, the first incident end 4-1 of the waveguide 1 The light having the wavelengths λ ₁ and λ ₂ incident from the output ends 4- of the first waveguide 1 respectively.
2 and the output end 5-2 of the second waveguide 2, so that the third
The configuration shown in the figure operates as a directional coupling type optical multiplexer / demultiplexer.

Conventionally, the directional coupling type optical multiplexer / demultiplexer is made by using a silica-based waveguide, and the design method is as described below. When designing a directional coupler by arranging two single-mode optical waveguides of about 10 μm square in parallel, first the dimensions (waveguide width W, waveguide thickness D that guarantee the characteristics as a single waveguide) ) Is determined. Then, the complete coupling length with respect to the optical waveguide spacing S is set to L ₁ and L ₂ for predetermined two wavelengths λ ₁ and λ ₂ , and L ₁ and L ₂ are set to L ₁ : L ₂ = 2N + 1: 2N. The interval S is set, and the length L of the connecting portion 3 is L = 2NL ₁ = (2N + 1)
Set to L ₂ . In this way, a directional coupling type optical multiplexer / demultiplexer that operates at two wavelengths λ ₁ and λ ₂ can be designed.

However, in such a designing method, since the design is made without having a fixed relationship between W and S, it is manufactured when W and S change in the actual manufacturing process. The optical multiplexer / demultiplexer has a drawback that the characteristics are not as designed. That is, in the conventional design method of the directional coupling type optical multiplexer / demultiplexer that does not have a specific relationship between W and S,
When the waveguide width W varies from the set value during the etching process of the waveguide, the variation of the waveguide structure parameter becomes large. As a result, the light intensity distribution is greatly affected,
The manufactured directional coupling type optical multiplexer / demultiplexer has a problem that the wavelength characteristics of the device deviate significantly from the designed values.

In order to solve this problem, conventionally, as one method, it has been studied to remarkably improve the processing accuracy of the waveguide in order to improve the reproducibility of the device characteristics. As another method, there was also an adjustment method in which the waveguide structure parameter after processing was measured and the appropriate refractive index n ₂ of the upper cladding layer was selected (K. Imoto, H. Sano and M. .Miyazaki:
“Guided-wave multi / demultiplexers with compensati
on for center wavelength shift by fabrication pro
cess fluctuations ", Electron. Lett., 1987, 23, 14, pp. 73
5-736,). However, this method has a problem that the elements have to be adjusted separately and a problem that both two wavelengths cannot be adjusted completely.

As described above, with the conventional element design, an optical multiplexer / demultiplexer having good wavelength characteristics cannot be obtained when actually manufactured. The improvement is focused on improving the manufacturing method regardless of the improvement of the element design, and no new design method has been proposed.

[Structure of Invention]

According to the invention of the present application, in the design stage, when it is determined that a specific relationship between the waveguide width W and the waveguide spacing S is satisfied, it is possible to determine a structural parameter that satisfies a favorable wavelength characteristic. It is a new design method that was made based on such knowledge.

Structural parameters of the directional coupler operates as an optical demultiplexer (refractive index n ₁ of the waveguide core, the refractive index n ₂ of the cladding, the waveguide width W, electrical NamijiAtsu D, waveguide spacing S, bond manager L) The value of
A set of the obtained structural parameters is obtained by calculation so that the demultiplexing is generated for two specific wavelengths, and the obtained set of structural parameters is plotted on a graph with the waveguide spacing S on the horizontal axis and the coupling length L on the vertical axis. , The waveguide width W and the refractive index n ₁ of the waveguide core are represented as parameters in the graph to create a design chart. According to the present invention, on this design chart, the change of the coupling length L with respect to the waveguide spacing S becomes substantially zero, and the variation of the waveguide width W and the variation of the waveguide spacing S have opposite signs and their absolute values are equal. A directional coupling type optical multiplexer / demultiplexer is manufactured with a structural parameter indicated by a point, that is, a point satisfying dL / dS≈0 dW / dS≈−1.

FIG. 2 shows the relationship between (deviation in center wavelength) / (deviation in waveguide width) and − (change in waveguide width) / (change in waveguide interval). n ₁ is 1.53, n ₂ is 1.46, D is 1 μm, and the center wavelength is 1.
It was 3 μm. Together, the positions of the present invention and the prior art are shown in the graph. Generally, when a waveguide is processed on a substrate, the waveguide spacing changes as the waveguide width changes. At this time, the change amounts of the waveguide width and the waveguide interval have opposite signs and have the same absolute value. That is,-(change in waveguide width) /
(Change in waveguide spacing) is almost 1. Nevertheless, as shown in FIG. 2, the prior art does not consider this point, so the value of − (change of waveguide width) / (change of waveguide interval) is not always , Did not meet this condition. On the other hand, in the present invention, − (change in waveguide width) / (change in waveguide interval) is almost 1, and (deviation in center wavelength) / (deviation in waveguide width) at that time.
Is closest to 0. Therefore, by manufacturing the directional-coupling optical multiplexer / demultiplexer with the structural parameters required by the present invention, the directional-coupling optical multiplexer / demultiplexer having desired element characteristics can be reproduced with good reproducibility without increasing processing accuracy. It can be manufactured.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

Structural parameters of the directional coupler operates as an optical demultiplexer (refractive index n ₁ of the waveguide core, the refractive index n ₂ of the cladding, the waveguide width W, electrical NamijiAtsu D, waveguide spacing S, bond manager L) The value of
The set of the obtained structural parameters was obtained by calculation so that the wavelengths of 1.3 μm and 1.55 μm would be combined and demultiplexed, and the abscissa represents the waveguide spacing S and the ordinate represents the coupling length L. On the graph, the waveguide width W and the refractive index n ₁ of the waveguide core are expressed as parameters in the graph, and a design chart is created. Figure 1 shows the refractive index of the cladding n ₂ = 1.
46 shows a design chart when the waveguide thickness D = 1 μm.
On this design chart, the change of the coupling length L with respect to the waveguide spacing S becomes almost zero, and the variation of the waveguide width W and the variation of the waveguide spacing S have opposite signs and their absolute values are equal,
That is, the structural parameter indicated by the point satisfying dL / dS≈0 dW / dS≈−1 is determined. Hereinafter, an example of manufacturing a directional coupling type optical multiplexer / demultiplexer based on the determined structural parameters will be shown. In this example, a directional coupling type optical multiplexer / demultiplexer having the same structure as that shown in FIG. 3 was manufactured by using a combination of structural parameters corresponding to the point A. Si on substrate 7
A wafer was used. 4μm on Si wafer by thermal oxidation method
A SiO ₂ film is formed, a 7059 glass film is formed to a thickness of 1 μm by a sputtering method, and a SiO ₂ film is further formed to a thickness of 4 μm by a sputtering method using a photolithography technique and a reactive ion etching technique. Waveguides 1 and 2 are used. The width W of the waveguides 1 and 2 is 1.2 μm, and the thickness D is 1
μm, waveguide spacing S is 1.44 μm, coupling length L is 554.8 μm
m. The refractive index n ₁ of the waveguide core is 1.53, and the refractive index n _{2 of the} clad is 1.46, and the set wavelength for multiplexing / demultiplexing is 1.3 μm.
And 1.55 μm. In the vicinity of the point A in FIG. 1, the change of the coupling length L with respect to the waveguide spacing S becomes almost zero. Also, n ₁ = 1.
On the line 53, W is 0.5 μm from the value of the waveguide spacing S corresponding to the intersection with the curve of W = 0.5, 1.0, 1.5, 2.0 μm.
When the change of S when it changes is calculated, it is 1.4μ in the region I.
In the region m and region III, the difference between 0.2 μm and the change in W is large,
In the region II including the point A, the change of W is 0.45 μm, which is almost equal to the change of W, and the waveguide spacing S also changes so as to cancel the change of the waveguide width W. Furthermore, a closer examination shows that dW at point A
/ dS≈-1. That is, in consideration of the fact that W + S in actual processing is determined by the mask used for photolithography and is constant in the subsequent element processing step, the sensitivity of each parameter to the element characteristics becomes the minimum in the vicinity of point A. Further, since such a point exists near the minimum value of the coupling portion length L, the element is downsized.

FIG. 4 shows a change in wavelength characteristic with respect to a change of ± 0.1 μm in the waveguide width W of the optical multiplexer / demultiplexer manufactured based on the designing method of the present invention. FIG. 5 shows changes in wavelength characteristics of the optical multiplexer / demultiplexer manufactured by using the structural parameters at the point B deviated from the optimum point. As can be seen from FIGS. 4 and 5, in the present invention, the wavelength characteristic variation can be suppressed to 1/10 or less with respect to the same variation of the waveguide width W of ± 0.1 μm, which is extremely reproducible and constant. A directional-coupling type optical multiplexer / demultiplexer having the above device characteristics was manufactured.

The effect of the present invention is not limited to the case of the structural parameters of this embodiment, and the refractive index n _{1 of the} waveguide core is
It is clear from FIG. 1 that it is effective even when the values are different.

〔The invention's effect〕

As described above, when the directional coupling type optical multiplexer / demultiplexer is manufactured based on the designing method of the present invention, the influence on the element characteristics due to the variation of the structural parameters described above can be suppressed to the minimum, so that the conventional etching etc. It is possible to manufacture a directional coupling type optical multiplexer / demultiplexer having desired element characteristics with high reproducibility without increasing processing accuracy and without requiring adjustment after processing, and it is possible to improve yield. is there. In addition, there is an advantage that the element is downsized.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a design chart of the present invention, and FIG.
The figure shows the relationship between (deviation in center wavelength) / (deviation in waveguide width) and- (change in waveguide width) / (change in waveguide spacing), and FIG. 3 shows directional coupling type optical multiplexing / demultiplexing. FIG. 4 is a diagram showing the structure of the optical fiber, FIG. 4 is a view showing a change in wavelength characteristic with respect to a change in the waveguide width W of the directional coupling type optical multiplexer / demultiplexer manufactured by the designing method of the present invention, and FIG. It is a figure which shows the wavelength characteristic change with respect to the change of the waveguide width W of the directional coupling type | mold optical multiplexer / demultiplexer manufactured based on the parameter. 1 ... First waveguide, 2 ... Second waveguide, 3 ... Coupling part consisting of parallel straight waveguides, 4-1 ... Injection end of first waveguide 1, 4-2 ... Output end of the first waveguide 1, 5-1 ... second
Of the waveguide 2 of the above, 5-2 ... Outgoing end of the second waveguide 2, 6 ... Clad layer, 7 ... Substrate.

Claims (1)

(57) [Claims] 1. Structural parameters of a directional coupler that operates as an optical multiplexer / demultiplexer for light of two wavelengths (refractive index n ₁ of waveguide core, refractive index n ₂ of clad of waveguide, waveguide width
W, waveguide thickness D, waveguide spacing S, coupling length L) is calculated as
Obtained by calculation so that multiplexing / demultiplexing occurs with respect to wavelength, the obtained set of structural parameters is plotted on a graph in which the horizontal axis represents the waveguide spacing S and the vertical axis represents the coupling length L.
The waveguide width W and the refractive index n ₁ of the waveguide core are represented as parameters in the graph, a design chart is prepared, and dL / dS≈0 dW / dS≈−1 is satisfied on the design chart. A method of designing a directional coupling type optical multiplexer / demultiplexer characterized by determining structural parameters.