JPH07111526B2 - Directional coupler type optical switch - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor optical switch, which is an important element in future optical communication systems and optical information processing systems, and more particularly to a directional coupler type optical switch. .

[Conventional technology]

The optical switch is considered to be one of the key elements for future high-speed optical communication systems, optical information processing systems, etc., and research and development are becoming active in various places. As an optical switch, Li
One using a dielectric such as NbO ₃ and one using a GaAs or InP semiconductor are considered, but it can be integrated with other optical elements such as optical amplifiers and electronic circuits such as FETs. ,Miniaturization,
In recent years, expectations for semiconductor optical switches that can easily be converted to other channels are increasing. Considering the above application fields, such a semiconductor optical switch is required to have a high speed operation, a low power consumption operation, a low voltage operation, a low loss operation, a high degree of integration, and the like. As the method of the semiconductor optical switch, the total reflection type switch that uses the refractive index change due to the band filling effect or the free carrier plasma effect accompanying the current injection, and the refractive index change due to the electro-optical effect due to the electric field application have been used so far. A directional coupler type switch and a total reflection type switch that uses the change in refractive index with the exciton absorption peak shift when an electric field is applied to multiple quantum wells have been experimentally studied. Has a drawback that the operation speed is slow and the power consumption is large. Further, the multi-quantum well structure total reflection type switch has a problem that it is essentially difficult to reduce the loss. On the other hand, the directional coupler switch using the electro-optic effect has a longer element length than the total reflection type switch, but has the advantages of high-speed operation, low power consumption operation, and low loss. Have Regarding the low loss property, in recent years, wavelengths have been determined by E. Kapon et al.
Low loss optical waveguide of 0.15 dB / cm at 1.52 μm is GaA
What can be realized with s / AlGaAs system Applied Physics Letters magazine Volume 50 Volume 2
No. 3, 1628-1630 (1987). Band gap wavelength of GaAs and AlGaAs is 1.3
Since the wavelengths are sufficiently shorter than those in the μm band and the 1.5 μm band, it is possible to realize the low loss optical waveguide as described above. Moreover, since the electro-optic effect has almost no wavelength dependence, the change in the refractive index is the same as in the vicinity of the band gap even when the operating wavelength is away from the band gap.
Therefore, the GaAs / AlGaAs optical waveguide is very promising as a long wavelength band directional coupler type switch material.

The conventional directional coupler type switch had the problem that the element length was as long as several mm, but in recent years RIBE (Reactive Ion Beam
This problem is being solved by the innovation of fine processing technology such as Eching). H. Takeuchi et al. Are GaAs / AlGaA
In the s-directional coupler type optical switch, an element with an element length of 1 mm or less is used for the RIBE technology and MB with excellent film thickness controllability during crystal growth.
It was realized using the E method. It is published in Electronics Letters, Vol. 122, No. 23, page 1241 (1986). The switching voltage of the device is 5V and the crosstalk in each switching state is 14d.
B and 17 dB show relatively good characteristics. However, considering a switch that can be applied to future optical systems,
It is necessary to reduce the size to a few hundreds of μm and to increase the number of channels, and in order to realize this, further controllability and reproducibility of crystal growth technology and process technology are required.

Regarding the process technology, at present, etching of patterns with a width of 1 μm or less is also possible by RIBE, and both the precision and reproducibility of fine processing are sufficient to some extent in the switch manufacturing process.

As for the crystal growth technique, the MBE method, which has excellent controllability of film thickness, is mainly used. The MBE method has the controllability of the growth on the order of angstroms with respect to the film thickness, but in the case of general equipment, Al when growing a mixed crystal of AlGaAs
The controllability of the composition is not so high, and the composition may fluctuate by about ± 0.1. Fluctuations in the composition of Al cause fluctuations in the refractive index of the AlGaAs layer. In the case of a GaAs-based directional coupler type optical switch, fluctuations in the refractive index of AlGaAs mainly appear as a deviation from the set value of the refractive index of the cladding layer. The size is about ± 0.05 at a wavelength of 1.55 μm when the composition of Al deviates by ± 0.1.

FIG. 2 shows an example of the refractive index dependence of the cladding layer of the completed coupling length l ₀ of the directional coupler. The wavelength was 1.55 μm, and the refractive index of the guide layer at that time was 3.38 with GaAs as the material. The other shape parameters are as shown. In this case, the complete bond length l ₀ is 1 mm when the refractive index of the cladding layer is 3.16. In actual devices, crosstalk is caused by the deviation between the element length and the perfect coupling length l ₀ . Considering only one element of the directional coupler, if you measure the complete coupling length with a prototype device and then cleave the element length to match it, you can get quite good crosstalk, but you can increase the number of channels and integration. Considering this, this method cannot be applied, and it is necessary to design so that the device length and the complete coupling length match.
As shown in FIG. 2, the deviation of the refractive index of the cladding layer causes the deviation of the perfect coupling length from the designed value, that is, the increase of crosstalk. As can be seen from Fig. 2 that the crosstalk required for the device is -15 dB or less, the refractive index of the cladding must be within 3.16 ± 0.04. As described above, the controllability of the refractive index is ± 0.05 from the controllability of Al composition in the current AlGaAs growth.
However, the demands of the device cannot be satisfied with good reproducibility. Therefore, in this situation, it is difficult to reduce the size and improve the performance of the device, the manufacturing precision of which becomes more severe.

[Problems to be Solved by the Invention]

An object of the present invention is to provide a directional coupler type optical switch that solves the above problems.

[Means for Solving the Problems]

A directional coupler type optical switch according to the present invention comprises a first cladding layer of a superlattice structure composed of GaAs and AlAs, a guide layer composed of GaAs, and a superlattice structure composed of GaAs and AlAs on a GaAs substrate. The second clad layer is composed of two adjacent optical waveguides having a layered structure in which the second clad layer is sequentially laminated.

[Action]

In the clad layer of the GaAs / AlAs superlattice structure used in the present invention, the Al composition ratio that determines the refractive index is Ga
It is determined by the ratio of each layer thickness of As and AlAs. In addition, the thickness of each layer can be controlled by the growth time, and with the development of crystal growth equipment such as the MBE method, it has become possible to control to a very high degree of accuracy. Therefore, the proportion of Al in the GaAs / AlAs superlattice structure cladding layer is accordingly changed,
That is, the controllability of the refractive index of the clad layer can be improved. Also, if MBE method is used, RHEED is used to make GaAs
It is possible to monitor the thickness of each growth layer while growing the / AlAs superlattice, and to confirm the Al composition in the cladding layer, that is, the refractive index, at the end of crystal growth. Even if the value and the actual Al composition are different, immediate feedback to the next process is possible, so that the performance of the finally completed optical switch can be improved. Therefore, the directional coupler type optical switch of the present invention in which the clad layer of the waveguide has a GaAs / AlAs superlattice structure can control the refractive index of the clad layer, which greatly affects the characteristics of the switch, with high accuracy. , Switch miniaturization,
It is very useful for high performance.

〔Example〕

 The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of a directional coupler type optical switch according to the present invention. A method of manufacturing an optical switch and its configuration will be described using this.

MB ⁺ on n ⁺ -GaAs substrate 1 n ⁺ -GaAs / AlAs superlattice cladding layer 2, p ^-- GaAs guide layer 3, p-GaAs / AlAs superlattice cladding layer 4, p ⁺ -GaAs cap layer 5 is sequentially epitaxially grown. At this time, n ⁺ -GaAs / AlAs superlattice cladding layer 2
The p-GaAs / AlAs superlattice clad layer 4 has GaAs and AlAs film thicknesses of 16Å and 12Å, respectively, and these periods have 360 periods, and the clad layers are laminated by about 1 μm each. The p ⁻ -GaAs guide layer 3 is 0.4 μm, and the p ⁺ -GaAs cap layer 5 is 0.2 μm. After the crystal growth is completed, a p-side electrode metal is formed on the entire surface of the epitaxial surface, and then the ordinary photolithography method and RIBE are used.
The p-side electrode 6, the p ⁺ -GaAs cap layer 5, and the p-GaAs / AlAs superlattice clad layer 4 are 3 μm wide by using the etching method of
m and a depth of 0.92 μm to form two adjacent rib-shaped stripes to form two parallel optical waveguides. The distance between the two optical waveguides is 2.3 μm. Finally, the n-side electrode 7 is formed on the n ⁺ -GaAs substrate 1 side and alloyed. The carrier concentration of each layer is 2 for the n ⁺ -GaAs substrate 1.
× 10 ¹⁸ cm ^-3 , n ⁺ -GaAs / AlAS superlattice cladding layer 2 is 5 × 10 ¹⁷ cm ^-3 , p ^-- GaAS guide layer 3 is 1 × 10 ¹⁵ cm ^-3
The p-GaAs / AlAs superlattice cladding layer 4 is 5 × 10 ¹⁷ cm ^-3
Since the p ⁺ -GaAs cap layer 5 is set to about 2 × 10 ¹⁸ cm ^-3 , an electric field is applied to the p ^-- GaAs guide layer 3 by applying a reverse bias between the p-side electrode 6 and the n-side electrode 7. Can be applied. Further, by setting one p-side electrode of the two optical waveguides to have the same potential as the n-side electrode, it is possible to apply an electric field only to the other waveguide. The above is an example of the method of manufacturing the directional coupler type optical switch according to the present invention and the configuration thereof, and the point that the characteristics of the optical switch obtained by the above configuration are improved as compared with the conventional one will be described below.

As shown in the conventional example and FIG. 2, the characteristics of the directional coupler type optical switch largely depend on the refractive index of the cladding layer.
In the embodiment, since the clad layer has a superlattice structure of GaAs16Å and AlAs12Å, the Al composition ratio x in the clad layer x
Is x = 0.43. Due to variations in the growth rate of each layer, Al
Even if the fluctuation of the layer thickness of the As layer is within ± 0.5Å, the fluctuation of the Al composition ratio is about ± 0.01, and the fluctuation of the refractive index corresponding to this is ± 0.005. Has been greatly improved. Therefore, in the embodiment shown in FIG. 1, since the perfect coupling length is designed to be 1 mm for the light having the wavelength of 1.55 μm, if the element length is set to 1 mm, most of the incident light 8 is extracted as the outgoing light 9a. The output light 9b, which is a crosstalk component, can be suppressed to -20 dB or less. In addition, applying a voltage to one of the two waveguides By shifting them, the outgoing lights 9a and 9b can be switched, and the crosstalk at this time is also good at −20 dB or less.

Although the switch in this embodiment operates at a wavelength of 1.55 μm, it can be designed to operate in the 1.3 μm band, and even in that case, the same effect can be sufficiently obtained.

〔The invention's effect〕

As described above, in the directional coupler type optical switch of the present invention, it is possible to greatly improve the crosstalk characteristic which largely depends on the refractive index of the cladding layer. Therefore, the element length of the device can be obtained as designed without the need to measure the complete coupling length for each device as in the past, which requires precision of each parameter in device fabrication. The present invention is very useful for increasing the number of channels of added switches.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing an example of the refractive index dependence of the cladding layer of the complete coupling length in a directional coupler type optical switch. In the figure, 1 is an n ⁺ -GaAs substrate, 2 is an n ⁺ -GaAs / AlAs superlattice cladding layer, 3 is a p ^-- GaAs guide layer, and 4 is p-GaAs / AlA.
s superlattice cladding layer, 5 is p ⁺ -GaAs cap layer, 6 is p
Side electrodes, 7 is an n-side electrode, 8 is incident light, and 9a and 9b are outgoing lights.

Claims (1)

[Claims] 1. A GaAs substrate on which a first clad layer of a superlattice structure composed of GaAs and AlAs, a guide layer of GaAs, and a second clad layer of a superlattice structure composed of GaAs and AlAs are provided. A directional coupling type optical switch comprising two adjacent optical waveguides having a layered structure which is sequentially laminated.