JP2897371B2 - Semiconductor waveguide polarization controller - Google Patents

Description

The present invention relates to a polarization control element which is an important element in a polarization diversity receiver or the like of a future coherent optical communication system, and in particular, enables a small and dynamic polarization switching. The present invention relates to a semiconductor directional coupler type polarization beam splitter and a polarization switch.

(Prior Art) A polarization beam splitter is considered as one of the key elements of a polarization diversity receiving system in a future coherent optical communication system, and also has an important component when configuring an optical circulator and the like. However, the conventional polarizing beam splitter is composed of a bulk component having a large size, that is, a so-called micro optic component, and has problems in miniaturization of the device, high-density mounting, productivity, and the like. On the other hand, if a polarization beam splitter is configured using an optical waveguide, the element can be downsized, and high-density mounting by integration with other elements on the same substrate is possible. Since a wafer process technology such as a photolithography technology can be used, it is also advantageous in terms of productivity. For such a polarization beam splitter using waveguide technology, M. Okuno et al. Described a 1990 international conference on optical switching on a waveguide type polarization beam splitter / polarization switch using a silica-based waveguide (19.
90 International Topical Meeting on Photonic Switc
hing) Reported in article number 13A-5. The polarization beam splitter / polarization switch using the silica-based waveguide has the advantage that the existing Si device manufacturing technology can be used and the waveguide-type polarization beam splitter can be manufactured with relatively high productivity. The difficulty of increasing the size and the effect of changing the refractive index etc. are only thermal effects, so high-speed dynamic control cannot be expected, and semiconductor devices such as light-emitting elements and light-receiving elements cannot be monolithically integrated. There are difficulties.

(Problems to be Solved by the Invention) As described above, the waveguide-type polarization beam splitter / polarization switch is promising in terms of miniaturization of the device, high-density mounting, productivity, and the like. A polarizing beam splitter using a system waveguide has disadvantages such as a small element size, not suitable for dynamic control, and monolithic integration with a light emitting element and a light receiving element. The present invention is intended to solve the above-mentioned problems by constructing a polarization beam splitter / polarization switch using a semiconductor waveguide, particularly an optical waveguide made of the same compound semiconductor as the light emitting / receiving element.

(Means for Solving the Problems) In order to solve the above-mentioned problems, a semiconductor waveguide type polarization control element of the present invention comprises a first semiconductor cladding layer, a semiconductor waveguide Means for forming a directional coupler by two adjacent three-dimensional optical waveguides formed on the semiconductor substrate, wherein the two three-dimensional semiconductors are formed on the semiconductor substrate. Means for independently applying an electric field to each of the optical waveguides, such that the guided optical power is completely transferred from one optical waveguide to the other in two adjacent three-dimensional optical waveguides. When the required lengths are L _TE and L _TM for the TE mode and the TM mode, respectively, the element length L of the directional coupler is
_LTE and _LTM are substantially equal.

(Operation) In the present invention, a semiconductor waveguide type directional coupler utilizing the electro-optic effect is used as a polarization control element, in particular, a polarization beam splitter / polarization switch.
The perfect coupling length for the mode and the element length of the directional coupler almost match. Therefore, when no voltage is applied to the directional coupler, both the TE mode and the TM mode exit from the waveguide different from the one on the incident side. On the other hand, when a voltage is applied to only one waveguide and the phase matching between the two waveguides is solved by the electro-optic effect, switching occurs for the TE mode, and the TE mode light is transmitted to the waveguide on the incident side. It comes out more. By the way, in the present invention, (10
0) Since the azimuth substrate is used, in the case of compound semiconductors such as GaAs and InP, the refractive index does not change due to the electro-optic effect in the TM mode. Therefore, even if an electric field is applied to one of the waveguides, switching does not occur in the TM mode, and the TM mode light is emitted from a waveguide different from that on the incident side in the same manner as when no voltage is applied. Therefore, when a certain appropriate voltage is applied to only one of the waveguides, a polarization beam splitter operation can be obtained. This is schematically shown in FIG. As can be seen from FIG. 2, when the present invention is used, when no voltage is applied, the TE mode light and the TM mode light are extracted from the same waveguide, and by applying the voltage, the TE mode light is emitted from one of the waveguides. A polarization switching operation of extracting TM mode light from the other waveguide is also possible. In addition, this polarization switching is very fast because of utilizing the electro-optic effect.

In the present invention, as described above, the polarization beam splitter is configured using the semiconductor waveguide technology. Therefore, a small element, specifically, an element size of the order of mm or 1 mm or less can be realized. Further, as described above, since the high-speed electro-optic effect can be used as a means for changing the refractive index, the polarization switching can be performed at a high speed. Furthermore, since the same compound semiconductor material as the light emitting / receiving element can be used as the waveguide material, the light emitting element,
It is possible to monolithically integrate a light receiving element, a 3 dB coupler, and the polarization beam splitter on the same substrate to form a polarization diversity receiving circuit optical system on one substrate, and has a wide range of application fields.

(Embodiment) A semiconductor waveguide type polarization control device of the present invention, that is, a beam splitter and a polarization switch will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view showing an embodiment of a GaAs / AlGaAs waveguide type polarization beam splitter / polarization switch according to the present invention. In the figure, a rib-type GaAs / AlGaAs waveguide polarization beam splitter / polarization switch formed on a (100) -oriented n ⁺ -GaAs substrate 101 is shown.

First, a method of manufacturing the GaAs / AlGaAs waveguide type polarization beam splitter / polarization switch shown in FIG. 1 will be briefly described. (100) orientation n ⁺ -GaAs substrate 101 on the n-AlGaAs (Al
The composition ratio x = 0.5) of the cladding layer 102 is about 1.5 μm, i-G
When the aAs waveguide layer 103 is 0.2 μm thick, i-AlGaAs (Al composition ratio x =
0.5) The cladding layer 104 is 0.4 μm, the p-AlGaAs (Al composition ratio x = 0.5) cladding layer 108 is 0.6 μm, the p ⁺ -GaAs cap layer 105 is 0.2 μm, and the MBE (Molecular Beam Epitaxy) method is used. Laminate. Then, the p-side electrode material Ti
An / Au film is deposited on the entire surface of the substrate, and this Ti / Au film is processed into two stripes by a normal photolithography method. Further, using the Ti / Au film processed into the stripe shape and the photoresist remaining on the Ti / Au film as a mask, portions other than the stripe portion are formed by p-AlGaAs cladding layers 108 and i by a reactive ion beam etching (RIBE) method. −AlG
It is removed by etching until it reaches the interface of the aAs cladding layer 104 to form two adjacent rib optical waveguides as shown in FIG. Thereafter, the (100) plane n ⁺ -GaAs substrate 101 is polished, AuGeNi / AuNi as an n-side electrode material is deposited, and an electrode alloy is formed. Then, the device is cleaved to a length of 2 mm to complete the device fabrication. Here, the width of the rib optical waveguide is 2.
5 μm and the waveguide spacing is 2.85 μm.

Next, the operation principle of the GaAs / AlGaAs waveguide type polarization beam splitter / polarization switch shown in FIG. 1 will be described.
In the waveguide type polarization switch having the structure shown in FIG.
In the direction perpendicular to the layer, light is transmitted through the i-GaAs waveguide layer 103 because the refractive index of the i-GaAs waveguide layer 103 is higher than that of the upper and lower cladding layers.
Most are confined in 103. On the other hand, in the direction horizontal to the layer, the effective refractive index is higher in the portion where the protruding ribs are formed than in the portion where most of the upper clad layer is removed by etching. Therefore, the guided light is confined three-dimensionally in the i-GaAs waveguide layer 103 immediately below the rib, and the light propagates along the stripe-shaped rib portion. By the way, in the waveguide type polarization switch shown in FIG. 1, since the interval between the two optical waveguides is small, coupling occurs between the two waveguides. Therefore, the light incident on one of the optical waveguides is completely coupled to the other optical waveguide after propagating for a certain length. This length is usually referred to as the complete bond length. The complete coupling length depends on the waveguide preparation conditions such as the thickness of the waveguide layer, the composition, the waveguide width, and the waveguide interval, and also depends on the polarization state of the incident light. This is because the shape of the waveguide is different between the direction horizontal to the layer and the direction perpendicular to the layer. FIG. 3 shows the waveguide spacing dependence of the complete coupling length of the directional coupler polarization switch shown in FIG.
The complete coupling length of the TM mode is longer than the complete coupling length of the TE mode, and the difference between the two tends to increase as the waveguide spacing increases. However, when the waveguide spacing is 2.85 μm, the complete coupling length L _TE for the TE mode is 1.72 mm, and TM
Complete coupling length L _TM responsive to the mode is 2.46 mm, it can be a value close to the element length 2mm both. Therefore, when no voltage is applied between the electrodes, both modes of incident light are almost safely coupled to the other optical waveguide when the element is emitted, so that a so-called state is realized. The crosstalk CT at that time is calculated by the following equation: CT = 10 × LOG ｛P ₁ / (P ₁ + P ₂ )｝ where P ₁ = cos ² (πL / 2L ₀ ) P ₂ = sin ² (πL / L2L ₀ ) where L is the element length, L ₀ is the complete coupling length, It is.

In this embodiment, the complete coupling length L _TE for element length L and the TE mode, because almost equal to the coupling length L _TM for TM mode TE mode, crosstalk in either case TM modes -10dB It can be: The above is the case where no voltage is applied between the electrodes.
When a reverse bias voltage is applied between one of the p-side electrodes and the n-side electrode 107, the i-GaAs waveguide layer 10
3 and the refractive index of the i-AlGaAs cladding layer 104 change. GaAs
Is a zinc-blende type crystal. When the (100) crystal orientation is used, the refractive index changes due to the electro-optic effect for the TE mode, whereas the refractive index due to the electro-optic effect is completely generated for the TM mode. No change occurs. Therefore, when the (100) crystal orientation is used as in the case of the present invention, T
In the E mode, the phase difference between the two optical waveguides can be reduced by applying an appropriate reverse bias voltage. In this case, a so-called state in which light incident from one waveguide is not coupled to the other waveguide is realized. In the TM mode, however, no matter how much voltage is applied, the position between the two optical waveguides is reduced. No phase difference occurs and remains in the state. Therefore, according to the present invention, as schematically shown in FIG.
When a bias voltage is applied, the device can be operated as a polarizing beam splitter. Furthermore, by using the present invention, when no voltage is applied, both TE and TM modes are taken out from the same waveguide, and by applying a voltage, TE mode light is emitted from one waveguide and TE mode light is emitted from the other waveguide. A polarization switching operation of extracting TM mode light is also possible. In addition, this polarization switching has a feature that it is very fast because it utilizes the electro-optic effect.

FIG. 4 is a diagram showing switching characteristics when the incident light 120 is input from the optical waveguide on the p-side electrode 106b side in the waveguide type polarization beam splitter / polarization switch shown in FIG. In the figure, the solid line shows the switching characteristics for the TE mode, and the broken line shows the switching characteristics for the TM mode. From FIG. 3, when the reverse bias voltage is 0 V, as described above, the guided light is almost completely transferred from the optical waveguide on the incident side to the other optical waveguide in both the TE mode and the TM mode. I understand. As the reverse bias voltage is increased, the refractive index changes due to the electro-optic effect in the TE mode and reaches a state at a certain voltage (17.5 V in this case). However, since the guided light remains in the state because the refractive index does not change due to the electro-optic effect in the TE mode, the TM mode is still in the state even at a voltage at which the TE mode reaches the state. Therefore,
Reverse bias voltage (1
7.5V), a polarizing beam splitter operation is realized. At this time, the crosstalk between both TE and TM modes is -10 dB
The following can be realized relatively easily. It goes without saying that polarization switching is possible if the reverse bias voltage is changed between 0V and 17.5V.

(Effects of the Invention) As described above, in the present invention, a polarization control element, that is, a beam splitter is formed by using the semiconductor waveguide technology. Therefore, a small device, specifically, on the order of mm or
An element size of 1 mm or less can be realized. Also,
Since a high-speed electro-optic effect can be used as a means for changing the refractive index, it is possible to perform polarization switching at a high speed. Further, since the same compound semiconductor material as the light-emitting / light-receiving element can be used as the waveguide material, the light-emitting element, the light-receiving element, the 3 dB coupler and the polarization beam splitter are provided on the same substrate (not shown in the embodiment). It is also possible to monolithically integrate and configure a polarization diversity receiving circuit optical system on one substrate, and has a wide range of application fields.

Note that the present invention is not limited to the above embodiment. As an embodiment, a GaAs-based waveguide type polarization beam splitter / polarization switch has been described. However, the present invention is not limited to this, and a directional coupler type polarization beam splitter / polarization switch using another semiconductor material such as an InP type. The present invention is similarly applicable to switches. In the embodiment, a rib-type optical waveguide has been described as an example of a waveguide structure for realizing a directional coupler. However, the present invention is not limited to this, and a buried type or the like may be used. Needless to say, the present invention is not limited to the element shapes shown in the examples, that is, the thickness of each layer, the composition of each layer, the waveguide dimensions, and the like.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the structure of a GaAs / AlGaAs waveguide polarization control element (beam splitter / polarization switch) according to one embodiment of the present invention, and FIG. 2 is a waveguide polarization beam splitter / polarization according to the present invention. FIG. 3 is a diagram schematically showing the operation of the switch, FIG. 3 is a diagram showing the waveguide spacing dependence of the complete length of the GaAs / AlGaAs polarization beam splitter / polarization switch and the polarization dependence thereof, and FIG. Is an example of GaAs
FIG. 4 is a diagram showing switching characteristics of a / AlGaAs polarization beam splitter / polarization switch. In the figure, 101 ... (100) orientation n ⁺ -GaAs substrate, 102 ...
... n-AlGaAs cladding layer, 103 ... i-GaAs waveguide layer, 104
... i-AlGaAs cladding layer, 105 ... p ⁺ -GaAs cap layer, 106a, 106b ... p-side electrode, 107 ... n-side electrode, 108 ...
... p-AlGaAs cladding layer, 120 ... incident light, 121a, 121b ...
... Outgoing light.

Claims (1)

(57) [Claims] A first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer which are at least laminated on a (100) -oriented semiconductor substrate, and wherein two adjacent semiconductor cladding layers are formed on the semiconductor substrate; And a means for independently applying an electric field to each of the two three-dimensional semiconductor optical waveguides. one length TE mode necessary for the guided optical power is completely shifted from the optical waveguide to the other optical waveguide of the waveguide, each for the TM mode L _TE, when the L _TM, directional coupler _Wherein the element length L is substantially equal to L _TE and L _TM .