JPH059011B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to an optical switch for switching an optical path, and more particularly to a semiconductor optical switch that can be integrated with a semiconductor laser. (Prior art and its problems) Optical switches switch optical paths in optical communication systems, and are used in light source switching devices and the like. In particular, typical total reflection type optical switches include those using ferroelectric materials such as LiNbO ₃ with a large electro-optic effect and those using PLZT ceramics. However, since the above-mentioned materials cannot be integrated with a semiconductor laser, optical switches using semiconductor materials have attracted attention in recent years. Semiconductor optical switches using semiconductor materials include:
There are methods that utilize refractive index changes due to electro-optic effects caused by an externally applied electric field, and methods that utilize refractive index changes due to carrier effects caused by current injection. However, although semiconductor optical switches that utilize the carrier effect can achieve a large extinction ratio even at low currents, the switching speed is limited by the carrier lifetime, making it difficult to operate the switch at a speed of several hundred megabits or more. On the other hand, semiconductor optical switches that use refractive index changes due to the electro-optic effect are capable of high-speed switching operation, but at the same operating voltage, the electro-optic effect is smaller than that of ferroelectric materials, so crosstalk between output terminals is reduced. increases and the extinction ratio deteriorates. In order to reduce this crosstalk, the crossing angle between the two optical waveguide layers can be increased, but this has the drawback that the operating voltage increases to several tens of volts when the crossing angle is increased. As mentioned above, in conventional semiconductor optical switches,
It has been difficult to operate high-speed switching with a high extinction ratio at low voltage. Therefore, a semiconductor optical switch that satisfies these conditions has been strongly desired. (Objects and Features of the Invention) The present invention was made in view of the above-mentioned drawbacks of the prior art, and provides a semiconductor optical switch that has a large extinction ratio and can perform high-speed switching. The purpose is to The feature of the present invention is that the polarization mode of the incident light, the layer thickness or forbidden band width of the light reflective layer, the traveling direction of light, and the surface orientation of the substrate (direction of the applied electric field) are set to appropriate conditions. The reason is that the amount of change in refractive index due to the electro-absorption effect and the amount of change in refractive index due to the electro-optic effect are superimposed on each other, increasing the absolute value of the amount of change in refractive index. (Principle of the invention) Compound semiconductors represented by GaAs, InP, etc.
When an electric field is applied to incident light with a photon energy hν slightly lower than the band gap energy Eg, an electric absorption effect occurs, and both the absorption coefficient and the refractive index change. For example, when the energy difference ΔEg (=Eg - hν) between the band gap energy Eg and the photon energy hν is in the range of 20 meV to 70 meV, the refractive index due to the electric absorption effect increases when the electric field strength is 150 KV/cm or less, Conversely, if the energy difference ΔEg is 20 meV or less and the electric field strength is
It is known that if the refractive index is set to 150 KV/cm or more, it decreases, and the amount of change in the refractive index is several times the amount of change in the refractive index caused by the electro-optic effect. (TEVan Eck et al.
Appl.Phys.Lett.48, pp451-453, Feb, 1986). On the other hand, the refractive index due to the electro-optic effect changes depending on the polarization mode of the incident light, the direction of application of the electric field, and the direction of incidence. For example, in a semiconductor with a zincblende crystal structure (having icosahedrons and also called a point group 43m crystal) such as InP, the polarization mode of incident light is a TE wave, and the direction of electric field application is <100> parallel to the crystal axis and the traveling direction of the incident light is <011>, the main electric field component of the incident light is <01>.
1> parallel to the crystal axis, and the refractive index caused by the electro-optic effect decreases. Conversely, when the traveling direction of the incident light is set to <011>, the refractive index increases. Therefore, if the polarization mode of the incident light, the direction of application of the electric field, etc. are determined so that the refractive index change due to the electro-absorption effect caused by the interband transition and the refractive index change due to the electro-optic effect are determined, the electro-optic effect can be obtained. A refractive index change several times larger can be obtained with the same operating voltage compared to the case where only the 100% refractive index is used. The present invention uses this principle to construct a semiconductor switch. The present invention will be explained in detail below using the drawings. (Embodiment) FIG. 1 is an embodiment according to the present invention, and is a schematic diagram of a semiconductor optical switch. In Fig. 1, figure a is a schematic diagram, figure b is a vertical sectional view along a-a' of the schematic diagram, and figure c is a schematic diagram taken along line b-a' of the schematic diagram.
It is a longitudinal cross-sectional view along b′, and 1 is a plane orientation of (100).
surface n ⁺ InP substrate, 2 is n ^- InGaAsP optical waveguide layer, 3
is an nInP cladding layer, 4 is a mesa portion formed on the InP substrate 1, 5 is an n ^- InGaAsP light reflection layer, and 6 is a p ⁺ InP
The layer 7 is a p-side electrode, and 8 is an n-side electrode. Explain the operation. First, if no voltage is applied to electrodes 7 and 8, 1.55μ in TE mode from port 1.
The m-band incident light passes through a strip-loaded optical waveguide layer 2.
The light propagates inside, is totally reflected by the light reflection layer 5 at the intersection with the port 2, and is taken out from the port 3. This is because the film thickness of the light reflection layer 5 is slightly thinner than the film thickness of the optical waveguide layer 2, so the effective refractive index of the light reflection region is smaller than the effective refractive index of the optical waveguide region by Δn. This is due to total internal reflection of the incident light. Next, when a voltage is applied, the incident light is TE
mode, the main electric field component of the incident light is <011
> direction (strictly speaking, shifted by the intersection angle θ/2), and both the refractive index due to the electro-absorption effect and the refractive index due to the electro-optic effect increase. Therefore, since the effective refractive index of the light reflective layer is the same as or larger than the effective refractive index of the optical waveguide layer even at a low applied voltage, the incident light that enters from port 1 can go straight and be extracted from port 4. can. For example, for an incident TE wave with a wavelength of 1.55 μm, the layer thickness of the optical waveguide layer 2 is 0.4 μm, the wavelength of the forbidden band energy is λg = 1.48 μm, and the strip width is 3 μm.
If the height of the mesa portion 4 is 0.02 μm and the layer thickness of the light reflecting layer 5 is 0.038 μm, then the effective refractive index difference Δn between the optical waveguide portion and the light reflecting portion is approximately 2 × 10 ^-3 . ,
If we set the intersection angle θ to the critical angle of total reflection, it becomes θ4°. At this time, the length of the light reflecting layer 5 can be shortened to about 90 μm. Also, the light goes straight and the 20dB
The voltage required to obtain the above extinction ratio is 9V. This value is about one-fourth of the value when only the electro-optic effect is used. Note that since the junction capacitance between the electrodes 7 and 8 is as small as 0.5 pF or less, a frequency band of 10 GHz or more can be obtained. In the above explanation, the effective refractive index difference between the optical waveguide part and the light reflection part is realized by changing the layer thicknesses of the optical waveguide layer 2 and the light reflection layer 5, but this is prohibited due to the prohibition of the optical waveguide layer 2. This can also be achieved by making the band width energy slightly larger than the forbidden band width energy of the light reflective layer 5. In addition, conversely, the bandgap energy of the optical waveguide layer 2 is made slightly smaller than the bandgap energy of the optical reflection layer 5, and the layer thickness of the optical reflection layer is made thinner than that of the optical waveguide layer. It can be realized even if it is difficult. Embodiment 2 FIG. 2 shows a second embodiment of the present invention, in which figure a is a schematic diagram of a semiconductor optical switch, figure b is a longitudinal sectional view taken along a-a' of the schematic diagram, and figure c is a schematic diagram of a semiconductor optical switch. FIG. Example 1 is designed to cause total reflection when no voltage is applied, but in this example, when no voltage is applied, the refractive index of the optical waveguide layer 2 and the optical reflective layer 5 are equal, so , TE mode light with a wavelength of 1.55 μm enters from port 1 and is taken out from port 4. When a voltage is applied, the refractive index of the light reflecting layer 5 increases due to the electro-absorption effect and the electro-optic effect. The incident light entering from the port 1 is totally reflected at the boundary between the optical waveguide layer under the n-side electrode 8 where the refractive index is increasing and the optical waveguide layer where the refractive index is unchanged.
Output to port 3. Therefore, also in this example,
As in the first embodiment, high-speed switching is possible with low voltage. In addition, in the above explanation, the case where the refractive index increases due to the electric absorption effect when an electric field is applied is shown, but when the refractive index decreases under a higher electric field, the traveling direction of light in the light reflective layer 5 A similar optical switch can be constructed by setting the angle 90° from the above-mentioned direction so that both the refractive index decreases due to the electro-absorption effect and the electro-optic effect.

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[Table] Tables 1 to 4 show the range to which the present invention is applicable. Tables 1 and 2 show cases where the electro-absorption effect and the electro-optic effect take positive values and overlap with each other, and Table 2 is equivalent to the case where the plane orientation of the substrate is (100) in Table 1. having a plane orientation,
That is, each relationship in the plane orientation having three-fold rotational symmetry along <111> is shown. Similarly, the (111) plane orientation in Table 1 can be applied to all plane orientations that have a four-fold rotation antiimage property along <100>. Tables 3 and 4 show cases where the electro-absorption effect and the electro-optic effect take negative values and overlap with each other, contrary to Table 1 (Table 2). This figure shows the relationships between plane orientations that are equivalent to those when the plane orientation is (100), that is, plane orientations that have three-fold rotational symmetry along <111>. Note that this method can be similarly applied to a substrate having a plane orientation equivalent to the (111) plane orientation. In the above description, the strip-loaded optical waveguide layer 2 was used as an example, but a buried optical waveguide layer or a rib-type optical waveguide layer may also be used. Also, an optical waveguide layer,
The light reflecting layer may have an MQW structure. Furthermore, the present invention is not limited to InP-based materials, but can also be applied to other semiconductors with a zinc blende crystal structure such as GaAs-based materials. (Effects of the Invention) As described above, the present invention superimposes the refractive index changes due to the electro-absorption effect and the electro-optic effect by determining the plane orientation of the substrate, the polarization mode of the incident wave, etc., and calculates the absolute value thereof. As a result, it is possible to realize a semiconductor optical switch that has a large extinction ratio and operates at low voltage with high-speed switching, and its industrial value is extremely large.

[Brief explanation of drawings]

FIGS. 1 and 2 are configuration diagrams of an embodiment according to the present invention, and in each figure, a is a schematic diagram of a semiconductor optical switch, b is a vertical cross-sectional view along a-a' of the schematic diagram a, and c is a schematic diagram of a semiconductor optical switch.
is a vertical sectional view taken along line b-b' of schematic diagram a. 1... (100) plane n ⁺ InP substrate, 2...
n ^- InGaAsP optical waveguide layer, 3... nInP upper cladding layer, 4... n ⁺ mesa portion on InP substrate, 5...
n ^- InGaAsP light reflection layer, 6...p ⁺ InP layer, 7...p
side electrode, 8...n side electrode.

Claims (1)

[Claims] 1. Two semiconductor optical waveguide layers intersecting at an acute angle,
a semiconductor reflective layer disposed at the intersection of the optical waveguide layer and having an effective refractive index equal to or smaller than the refractive index of the optical waveguide layer; In a semiconductor optical switch in which the traveling direction of incident light is changed by changing the refractive index of the light reflecting layer by applying an external electric field to the layer, when the external electric field is applied, In order to increase the refractive index of the light reflecting layer, the difference between the band gap energy and the photon energy of the light reflecting layer is set to 20 meV to 70 meV, and the intensity of the external electric field application is set to 150 KV/cm or less. , the crystal structure of the substrate and the light reflective layer is zincblende type, and the external electric field is applied to adjust the plane orientation of the crystal structure of the substrate, the crystal axis direction of the light reflective layer, the traveling direction of the incident light, and the polarization mode. When performing this, the refractive index of the light reflecting layer increases due to the electro-optic effect, and the refractive index change amount due to the electro-absorption effect and the refractive index change amount due to the electro-optic effect are superimposed on each other, resulting in a refractive index change. 1. A semiconductor optical switch characterized by being configured such that the absolute value of the amount increases significantly. 2. Two semiconductor optical waveguide layers intersecting at an acute angle,
a semiconductor reflective layer disposed at the intersection of the optical waveguide layer and having an effective refractive index equal to or smaller than the refractive index of the optical waveguide layer; In a semiconductor optical switch in which the traveling direction of incident light is changed by changing the refractive index of the light reflecting layer by applying an external electric field to the layer, when the external electric field is applied, The structure is such that the difference between the band gap energy and photon energy of the light reflective layer is 20 meV or less so that the refractive index of the light reflective layer is small, and the intensity of the applied external electric field is 150 KV/cm or more, The crystal structure of the substrate and the light-reflecting layer is zinc blende type, and the plane orientation of the crystal structure of the substrate, the crystal axis direction of the light-reflecting layer, the traveling direction of the incident light, and the polarization mode are determined by applying the external electric field. When the electro-optic effect occurs, the refractive index of the light reflecting layer becomes smaller, and the refractive index change amount due to the electro-absorption effect and the refractive index change amount due to the electro-optic effect are superimposed on each other, resulting in a refractive index change amount. 1. A semiconductor optical switch characterized by being configured such that the absolute value of is significantly increased.