JPH0656457B2 - Optical switch - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical switch using a semiconductor material.

(Prior art) Improvement of characteristics of optical control elements such as optical switches, etc., in line with advances in systems such as ultra-high speed optical communication systems and coherent transmission, and further advances in research on optical switches, optical computers, etc. Has been strongly desired. The magazine "Electronics-
Letters ", Volume 21, 1985
A waveguide type optical switch utilizing the electric field effect of a multiple quantum well (MQW) structure as reported on pages 693 to 694 is proposed.

This optical switch utilizes the increase in absorption on the long wavelength side of the MQW absorption edge due to the application of an electric field. By using the MQW structure, the absorption edge becomes steep and exciton resonance absorption can be effectively used. Double hetero (DH)
High-efficiency operation is possible compared to the electroabsorption type optical switch using the structured optical waveguide. In addition, it has been reported that ultra-high speed operation is possible due to the operating mechanism that is not governed by the carrier lifetime, and that a short optical pulse of 100 psec (full width at half maximum) is actually created.

(Problems to be solved by the invention) The characteristics of this type of optical switch largely depend on the polarization state of the incident light, and the polarization of the incident light must be aligned in one direction for highly efficient switching. There is. However, since the polarization state of the light transmitted by an ordinary optical fiber is generally random, there is a problem that this type of optical switch / modulator cannot be effectively switched.

An object of the present invention is to solve such problems and provide an optical switch whose characteristics do not depend on the polarization state of incident light.

(Means for Solving the Problems) In the optical switch of the first invention of the present application, the first semiconductor layer having a thickness of about de Broglie wavelength is sandwiched by the second semiconductor having a wider band gap than the semiconductor layer. An optical waveguide including a multiple quantum well structure having multiple quantum well structures in the layer thickness direction, and a thick third semiconductor having a bandgap substantially equal to the bandgap of the multiple quantum well structure in a waveguide layer. And a means for applying an electric field to the optical waveguide, wherein the multiple quantum well structure and the third semiconductor are laminated in the layer thickness direction to form the optical waveguide.

An optical switch according to a second aspect of the present invention is a multiplex structure having multiple quantum well structures sandwiching a first semiconductor layer having a thickness of about de Broglie wavelength and a second semiconductor having a bandgap wider than that of the semiconductor layer in a layer thickness direction. A first optical waveguide including a quantum well structure in the waveguide layer and a second optical waveguide including a thick third semiconductor layer having a bandgap substantially equal to the bandgap of the multiple quantum well structure in the waveguide layer. A semiconductor substrate having a structure in which optical waveguides are optically cascade-connected, and means for applying an electric field to the first and second optical waveguides, wherein the multiple quantum well structure and the third semiconductor are common. It is characterized in that it is formed with the end faces abutting each other in the layer direction above.

(Operation) In order to explain the operation of the present invention, first, the light absorption change due to the electric field in the semiconductor MQW structure and the bulk semiconductor will be described. Although AlGaAs semiconductors are used here as the material, the following discussion is based on direct transition InGaAsP, InGaAs / InAl.
It can be applied to interesting III-V type semiconductor materials such as As type.

Considering the band structure of an AlGaAs semiconductor, at the wavelength k = 0, the conduction band degenerates twice with respect to spin, and the valence band has heavy holes (hh) and light holes (lh) respectively. Is a quadruple degenerate structure with respect to, and has the minimum and maximum values at k = 0, respectively. In the MQW structure, degeneracy of the valence band is resolved, and hh and lh form independent bands. In this MQW structure, the selection rule is different from that of the bulk, and the relative value of the band-to-band transition probability between hh and lh and the electron is 1/4 at hh for 3/4 lh for light whose field vector is parallel to the MQW layer (), For light whose electric field vector is perpendicular (⊥) to the MQW layer, it is 0 at hh and 1 at lh. Therefore, in an optical waveguide using MQW as a waveguide layer, the magnitude of exciton absorption along with hh and lh greatly differs depending on the direction of the electric field vector, that is, the waveguide mode.

Figure 3 shows the magazine "Abride Physics Letters (Ap
plied Physics Letters) "Volume 47, 1985, 1148-115
T of GaAs / AlGaAs MQW waveguide reported on page 0
The change in the optical absorption spectrum near the absorption edge for the E and TM modes due to the electric field is shown (the electric field intensity is (i) 1.
6 × 10 ⁴ V / cm, (ii) 10 ⁵ V / cm, (iii) 1.3 × 10 ⁵ V / c
m, (iv) 1.8 × 10 ⁵ V / cm, (v) 2.2 × 10 ⁵ V / cm). From the figure, it is clear that for TM mode, the absorption edge is closer to the high energy example because there is no exciton absorption corresponding to hh, and the amount of shift at the absorption edge for the same electric field is smaller than in TE mode. Will fall. This is the reason why the characteristics of the MQW waveguide structure electroabsorption optical switch / modulator depend on the polarization state of the incident light.

On the other hand, in a bulk semiconductor material optical waveguide such as a GaAs / AlGaAsDS waveguide, the degeneracy of the valence band cannot be resolved, and therefore N
In contrast to QW, it has been theoretically clarified that the electric field absorption for TM mode is four times as large as that for TE (Journal "Soviet Physics-Semiconductors (Soviet Physi
cs-Semiconductors) ", Volume 3, 1970, 876-884
page). Therefore, by using MQW and a bulk material together in the waveguide layer, the polarization dependence of each other can be compensated, and an optical switch with a small polarization dependence of the characteristics as a whole can be realized. The present invention utilizes this fact.

(Embodiment 1) FIG. 1 is a perspective view showing an embodiment of the first invention of the present application, and this perspective view shows a sectional structure in a plane perpendicular to the well shaker axis of the embodiment. Again, as an example, GaAs / GaAlAs
The case where a system material is used will be described. First, the production of this embodiment will be described. ^{n +} -GaAs n ⁺ -Ga on the substrate 11
As buffer layer 12, n ⁺ -AlGaAs cladding layer 13 (Al molar ratio x = 0.3, i-AlGaAs guide layer 14 (x = 0.05), i-GaA
s / AlGaAs MQW guide layer 15, i-AlGaAs cladding layer 16
(X = 0.3), p-AlGaAs cladding layer 17 (x = 0.3), p
-The GaAs cap layer 18 is grown by the MBE method. Next, Ti / Pt is vapor-deposited on the wafer shrimp layer side to form a metal stripe 19 which becomes a waveguide pattern with a width of 3 to 8 μm by a photolithography method. Etch off up to layer 17. AuGeNi20 which becomes an ohmic electrode is vapor-deposited on the back surface of the wafer and alloyed by heat treatment. Finally, although not shown, Ti / Au is partially laminated on the bonding pad portion. The entrance and exit end faces were formed by wall opening.

Next, the operation of this embodiment will be described. In MQW, the band gap can be controlled by the potential height between the well and the barrier and the well width, and the refractive index can be controlled by the ratio of the well and barrier formation and the thickness thereof. Therefore, the degree of freedom in designing the waveguide is higher than that of the mixed crystal. Here, the Al of the i-AlGaAs guide layer 14 is
By setting the molar ratio x = 0.05, the barrier x of the i-GaAs / AlGaAs MQW guide layer 15 to x = 0.3, and the well layer l _z = 70Å, the band gap wavelengths λ _{g to} 0.83 μm of both can be matched. At this time, the n ⁺ -AlGaAs cladding layer 13, the i-AlGaAs guide layer 14, and the use wavelength λ = 0.86 μm,
The refractive index of the i-MQW guide layer 15 is 3.4, 3.58, 3.5, respectively.
It will be 5. Therefore, a single mode guide can be realized by setting the thickness of the i-AlGaAs guide layer 14 and the i-MQW guide layer 15 to about 0.2 μm.

When light having a wavelength longer than the absorption edge of the guide layer 0.83 μm (where λ = 0.86 μm is used) is incident on such an optical guide and a reverse bias is applied between the electrodes 19 and 20, an electric field is applied to the guide layer. Electric field absorption occurs according to the strength. At this time, as described above, the TM mode is absorbed more strongly in the i-AlGaAs guide layer 14 and the TE mode is absorbed more strongly in the i-MQW guide layer 15. Therefore, the polarization dependence that occurs when each of the guide layers exists independently is eliminated. In the prototype device, the voltage required to obtain an extinction ratio of 20 dB with a sample of 500 μm was about 4 V in both TE and TM modes.

(Embodiment 2) FIG. 2 is a sectional view showing an embodiment of the second invention of the present application. First, on the n ⁺ -GaAs substrate 11, the n ⁺ -GaAs buffer layer
12, n ⁺ -AlGaAs cladding layer 13 (x = 0.3), i-GaAs /
AlGaAs MQW guide layer 15 (well: GaAs, thickness 70Å, barrier: AlGaAs (x = 0.3), thickness 80Å, MQW thickness 0.3μ
m), the i-AlGaAs cladding layer 14 was grown by the MBE method. Next, a SiO ₂ mask formed by photolithography is used to form a substantially vertical end face on the wafer up to the i-MQW guide layer 15, and then the i-AlGaAs guide layer 14 (x = 0.05, thickness 0.4
μm), and the i-AlGaAs cladding layer 16a is formed. After removing the SiO ₂ mask, a p ⁺ − region 21 was formed in the i-AlGaAs cladding layer by Zn expansion, and ohmic electrodes 21 and 20 were formed on the p and n sides, respectively. A transverse mode control structure using a rib guide was used in the direction perpendicular to the paper surface of FIG. 2 as in the embodiment shown in FIG. 1, but the process is omitted for simplicity. The end face is formed by wall opening.

In this embodiment, i-AlGaAs having almost the same band gap is used.
Waveguides having 14 and i-MQW15 as guide layers are connected in series. In each waveguide, TM, T
As a whole, the polarization dependence is eliminated because the E mode receives stronger absorption.

Although the embodiment shown here shows an example of application to a GaAs / AlGaAs-based semiconductor, it goes without saying that the present invention can be applied to a semiconductor material having the same crystal structure and band structure as the AlGaAs system.

(Effects of the Invention) As described in detail above, according to the present invention, an optical switch having no polarization dependence in characteristics can be obtained, which is a great advantage in practical use.

[Brief description of drawings]

1 is a perspective view showing an embodiment of the first invention of the present application, FIG. 2 is a sectional view showing an embodiment of the second invention of the present application, and FIG.
The figure shows the electric field absorption characteristics in MQW. In the figure, 11,12,13,14,15,16,17,18,16a, 21 are semiconductors,
19,20 are electrodes.

Claims (2)

[Claims] 1. A multi-quantum well structure having multiple quantum well structures sandwiching a first semiconductor layer having a thickness of about de Broglie wavelength and a second semiconductor having a wider band gap than this semiconductor layer in the layer thickness direction. An optical waveguide including a thick film third semiconductor having a bandgap substantially equal to the bandgap of the multiple quantum well structure in a waveguide layer, and means for applying an electric field to the optical waveguide, An optical switch, wherein the multiple quantum well structure and the third semiconductor are stacked in a layer thickness direction to form the optical waveguide. 2. A multiple quantum well structure having multiple quantum well structures sandwiching a first semiconductor layer having a thickness of about de Broglie wavelength and a second semiconductor having a wider band gap than this semiconductor layer in a layer thickness direction. A first optical waveguide contained in the waveguide layer, and a second optical waveguide containing a thick third semiconductor layer having a bandgap substantially equal to the bandgap of the multiple quantum well structure in the waveguide layer. An optically cascaded structure and a means for applying an electric field to the first and second optical waveguides, wherein the multiple quantum well structure and the third semiconductor are layered on a common semiconductor substrate. An optical switch characterized in that the end faces thereof are butted against each other.