JPS62296129A - Optical switch - Google Patents

Abstract

PURPOSE:To obtain an optical switch which is small in size, in which a light source, photoreceptor, etc., are monolithically integrated in circuit and whose loss variation caused when rays of light are switched to another is small, by utilizing the variation of the refractive index following to the disappearance of an exciton absorption peak. CONSTITUTION:A buffer layer 2, clad layer 3, MQW guide layer 4, clad layer 5, and top layer 6 are successively grown on a base plate 1 and, by etching the layers 6 and 5, crossing waveguides are formed by means of channel guides 7a and 7b. Thereafter, Schottky electrodes 8a and 8b are formed in a symmetrical state and a receiving and emitting end faces are finally formed. When a voltage is applied across the electrodes 8a and 8b, an electric field which is parallel with the layers 2-6 is applied to the gap section between the electrodes 8a and 8b and, when the electric field is applied to the layer 4, an exciton absorption peak disappears and a change in refractive index is induced. As a result, the incident light 9a to the channel guide 7a is totally reflected to the channel guide 7b side due to the induction of a negative change in refractive index of the layer 4.

Description

DETAILED DESCRIPTION OF THE INVENTION Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical switch using a semiconductor material.

(Prior Art) An optical switch that switches the connection of optical signals between two input/output ports is the most important component in optical transmission and optical switching. Such an optical switch can be realized by forming a directional coupler, crossover, branch, etc. using an optical waveguide and changing the refractive index of the part using physical optics effects.

Currently, the primary electro-optic effect (Pockels effect) is most widely used as a means to obtain a change in refractive index, but it can also be practically obtained using a ferroelectric material such as LiNb0q, which has a relatively large electro-optic coefficient. The relative refractive index change is 10-3
The device is small, making it difficult to miniaturize the device.

On the other hand, in recent years, it has been reported that when an electric field is applied perpendicularly to the layers of a semiconductor multiple quantum well (MQW) structure, a large refractive index change occurs at wavelengths near the absorption edge. is proposed. (Transactions of the Institute of Electronics and Communication Engineers E, Vol. 68, pp. 737-739, 1905) Here, the principle of change in refractive index due to an electric field in this MQW structure will be explained.

The MQ-structure is made by forming multiple quantum wells (Q-) in the layer thickness direction, in which a semiconductor layer with a thickness of approximately the wavelength of electron waves (de Broglie wavelength) is sandwiched between semiconductors with a wider band gap. , has attracted attention because it exhibits physical properties that are significantly different from those of bulk materials due to the two-dimensionality of hole waves.

Figures 2 <a> and (b) show how the absorption coefficient and refractive index spectrum of light near the absorption edge change due to the electric field when light propagates in the direction perpendicular to the layers of the MQW structure. . When there is no electric field E perpendicular to the layer, the absorption coefficient spectrum shows an exciton absorption peak corresponding to the transition between the heavy hole (hh), light hole 1th) and electron levels; Large steps can be seen in the refractive index spectrum. As the electric field E is applied, the electron and hole levels shift, causing the absorption edge to move toward the longer wavelength side. At this time, although some broadening occurs, the exciton absorption peak exists stably even in a strong electric field of about 105 V/cm.

Corresponding to the absorption edge shift caused by the application of the electric field E, the step on the refractive index spectrum also moves toward the longer wavelength side. This smooth refractive index change occurs, and its absolute value is 1:X: 10'
It has been reported that the electric field is on the order of 10-2 in an electric field of about V/cm, and the results of designing a cross-waveguide total internal reflection type optical switch using this phenomenon show that the length of the switch length is 100 μm.
It has been reported that the following compact optical switch is possible. Since this optical switch uses a semiconductor material, monolithic integration with a light source and a light receiver is also possible.

(Problems to be Solved by the Invention) The change in refractive index due to the application of an electric field in the direction perpendicular to the MQW layer described here essentially accompanies a change in absorption coefficient. Therefore, in an optical switch that utilizes this phenomenon, a large optical loss occurs when switching light. Also, as can be seen from Figure 2 (b), the signs of the refractive index changes are from the short wavelength side to ■, θ.
, ■, and the width of this region changes with the electric field strength, so this must be taken into consideration when designing the device.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate such problems and to provide an optical switch that is compact, can be monolithically integrated with a light source, a light receiver, etc., and has a small change in loss due to switching of light.

(Means for Solving the Problem) The optical switch according to the present invention includes a directional coupler, crossover, or branch formed by a plurality of optical waveguides, and a change in the refractive index of a part of the directional coupler, crossover, branch, or portion. In the optical switch comprising means for producing the directional coupler,
Of the crossover and branching, at least the part that should cause a change in refractive index has at least a layer thickness of a quantum well in which a first semiconductor layer with a thickness of about the de Broglie wavelength is sandwiched between a second semiconductor layer with a wider band gap. and means for applying an electric field to the quantum wells of the structure in a horizontal direction to each layer of the quantum wells.

(Function) The present invention utilizes the change in refractive index caused by the disappearance of resonance absorption due to room temperature excitons, which is unique to a multiple quantum well (MQW) structure. First, the principle of this refractive index change will be explained.

FIGS. 3(a) and 3(b) are diagrams showing trends in changes in absorption coefficient and refractive index for light propagating perpendicularly to the layers due to an electric field horizontal to each layer of the MQW II structure. When a horizontal electric field E11 is not applied to the layer, the absorption coefficient spectrum near the absorption edge shows the difference between the heavy hole (hh) at the reference low level, the light hole (Qh) and the electron. Two exciton absorption peaks related to the transition are clearly visible. A large step occurs in the refractive index spectrum corresponding to the exciton absorption peak. On the other hand, when Ell is applied, the quantum well (Q
W) Exciton ionization occurs within the layer, and the absorption peak due to exciton resonance disappears on the absorption coefficient spectrum, resulting in the renormalization of the band gear '7.
occurs. What is important here is that the change in the position of the absorption edge is much smaller than when the electric field is perpendicular to the layer (E2).On the other hand, the step that appears on the piezoelectric index spectrum is due to the presence of exciton absorption. Therefore,
If the exciton absorption peak disappears by applying Ell, the step difference on the refractive index spectrum also disappears. Therefore, a large change in refractive index can be obtained at wavelengths near the absorption edge. As is clear from FIG. 3(b), the sign of this refractive index change is simply negative on the long wavelength side of the absorption edge and positive on the short wavelength side. On the longer wavelength side of the absorption edge, unlike when the electric field is perpendicular to the layer, the absorption edge does not move, so no large loss change occurs. In this way, the change in refractive index caused by the disappearance of the exciton absorption peak is extremely suitable for realizing an optical switch. It is reported in the magazine "Physical Review B (1'Physical Review B)" that the disappearance of the exciton absorption peak is caused by the ionization of excitons due to the electric field in the direction horizontal to the layer, as described here.
J, Vol. 32, pp. 1043-1060 (1985), but it can also be caused by irradiation with light with higher energy than the band gap and by injection of free carriers.

(Magazine “Journal of Optical Society of America A (Journal of 0pt)
icalSociety of AmericaA
) J Vol. 2, pp. 1135-1142 (1905)) The present invention applies the refractive index change caused by the disappearance of the exciton absorption peak to an optical switch. The present invention will be explained in detail below using examples.

(Embodiment) FIG. 1 shows a first embodiment of an optical switch according to the present invention, and this embodiment is suitable for a crossed waveguide total reflection type switch. As for the material system, GaAs/A Q
Although we will explain the case using GaAs-based materials, InG
aAsP/InP, InGaAs/InA Q As
M where stable exciton absorption peaks can be observed at room temperature, such as system
It goes without saying that the present invention is applicable to any material system that can produce a QW structure. Perspective view of the embodiment Fig. 1 (a
), the manufacturing method of this embodiment will be explained first.

A non-doped GaAs buffer layer 2 (thickness O, lμm) is formed on a semi-insulating (S, 1.) GaAs substrate 1.
).

A Q O, 35caO, 65As cladding layer 3 (1μ
m).

GaAs/8Q + to 35Ga0.65As MQ
W guide layer 4 <0.4um).

A Q 0.35GaQ, 65AS cladding layer 5 (
0.5 μm), GaAs top layer 6 (0.1 μm
m) is continuously grown using the MBE method. GaAs/A
Q 0.35Ga (1,65As MQW ・Guide layer 4 is GaAs with a thickness of 100A, A Q g, 35ca
O,65As layers are alternately laminated in 20 periods.

(In the following, the molar ratio of All will be omitted for simplicity.) Next, a mask with a cross pattern of 10 μm in width and a cross angle of 5 to 10 degrees is formed on the shrimp layer side by photolithography, and a reactive ion beam is applied. The GaAs top layer 6 and the AQ GaAs cladding layer 5 other than the mask are etched by an etching method. At this time, etching is A Q Ga
I controlled it so that it stopped in the middle of Askra Sodo layer 5. By this machining and sowing, two loaded channel guides 7
A crossing waveguide is formed by a and 7b. Next, along the bisector A-A'' of the shallow intersecting angle, a 2 μm gap is placed symmetrically on both sides of A-A', and Schottky electrodes 8 are formed using ^U.
Form a and 8b. Finally, entrance and exit end faces were formed by cleaving perpendicular to the straight line AA'.

FIG. 1(b) shows a top view of the device. Schottky electrodes 8a, 8b are crossed channel guides 7a,
A gap is formed at a position parallel to the bisector AA' of the smaller intersection angle 7b and symmetrical to the straight line AA'. Although pads and lead lines for bonding are actually formed, they are omitted from the drawings.

FIG. 1(c) shows a cross-sectional view taken along a plane including the bisector BB' of the larger intersecting angle and perpendicular to each layer of the device. The operating mechanism of this embodiment will be explained using this figure. Consider a case where a voltage is applied between two Schottky electrodes 8a and Bb.

Here, for example, add Schottky electrode 8aj rule, 8b
If the side is negatively biased, the key electrode 8a will be forward biased and the key electrode 8b will be reverse biased, and the void layer will extend from 8b to 8a. Therefore, in the gap between the Schottky electrodes Ba and 8b, the growth layers 2 and 3
, 4, 5.6. When an electric field parallel to the layer is applied to the MQv guide layer 4, the exciton absorption peak disappears due to exciton ionization, as described above, and a refractive index change is induced. According to the calculation results of the electric field distribution, an electric field of about E·0, 6 V/d is applied to the MQW guide layer 4, where the applied voltage is V and the gap length between the Schottky electrodes i8a and 8b is d. Now d≧0
．． Since there is 2μ■ηte, 3XlO'V/cI at V=10V
A strong electric field on the order of Il can be applied to sufficiently ionize excitons. Therefore, the refractive index changes in the portion 4a of the xqw guide layer 4 directly under the gap between the Schottky electrons i8a and 8b, and the refractive index decreases particularly at wavelengths on the long wavelength side of the absorption edge, so that the cross channel waveguide A portion with a decreased refractive index can be created at the center of the intersection of 7a and 7b, and total reflection can occur.

The actual switch operation will be explained using FIG. 1<b). Here, the wavelength of the light to be switched is MQW ・The absorption edge of the guide layer 4 (band gap wavelength λg・0.B5
0.875 μm was selected considering the wavelength on the longer wavelength side. When no voltage is applied between the horizontal and vertical electrodes [1a and Bb, the light a incident on the channel guide 7a travels straight as it is and is emitted as an emitted light 9b. At this time, the crossing angle of the channel guides 7a and 7b is 5 to 10.

Since the crosstalk to the channel guide 7b is large, the crosstalk to the channel guide 7b is less than -30d[]. Schottky electrodes 13a, B
When a voltage is applied between the Schottky electrode 8a and the Schottky electrode 8a.

A negative refractive index change is induced in the MQW guide layer 4 under the surface of the gear part 8b, whereby the incident light 9a is totally reflected toward the channel guide 71) and becomes an output light 9c. The magnitude of this refractive index change is 10-3 as a relative change △n/n.
It is on the order of ~10-2. Since there is a relationship of sin θ: (1-△n/n) between the total reflection angle θ and Δn/n, Δ
5°~ due to refractive index change of b/n=10-3~10-2
A total internal reflection switch can be obtained using an optical waveguide with a crossing angle of 16°. In fact, with the element we prototyped this time, we confirmed the switching operation with a voltage of 10 V or less and a switch with a cross angle of 10 degrees or less, and the crosstalk during switching was about -20 dB or less. The length of the switch element is 1, including the part for separating the waveguides at the end of the switch in consideration of coupling with optical fibers, etc.
A small one, smaller than a dragon, was obtained. Also, the change in benefits and losses due to switching was only a few percent.

As mentioned here, the change in refractive index is caused by M
It can also be caused by an electric field parallel to the Q- layer, light irradiation, or current injection, but the response speed is limited to several msec because light irradiation or current injection involves a relaxation process of free carriers. . On the other hand, the effect of an electric field parallel to the layer is to cause potential deformation of excitons, so the response speed is determined by the element capacitance C and resistance R.
A number of psec determined by the R time constant is also possible.

This example shows an example in which the present invention is applied to a cross-waveguide total reflection type optical switch, but the point of the present invention is to utilize the refractive index change obtained by applying a horizontal electric field to the MQW layer. Yes, the switch structure is a directional coupler type,
It is clear that the present invention can be applied to various already known structures such as Y-branch type.

(Effects of the Invention) As described in detail above, according to the present invention, it is possible to obtain an optical switch that is small in size, can be monolithically integrated, and has a small change in loss due to switching of light.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram explaining the refractive index change of MQ'IJ, which is the operating principle of a conventional optical switch, and FIG. 3 is a diagram showing the layers used in the present invention. FIG. 3 is a diagram for explaining the refraction of MQW due to an electric field in a horizontal direction. In the figure, 1.2.3, 4.4a, 5.6 are semiconductors, 7
a, 7b are channel guides, Ba, 13b are yacht key electrodes, and 9a, 9b, 9c are lights. of

Claims (1)

[Claims] A directional coupler, crossover or branch formed by a plurality of optical waveguides, and the directional coupler or crossover,
or a means for causing a refractive index change in a part of the branching part, in which at least the part of the directional coupler, or the crossing or the branching, in which the refractive index should be changed is about the de Broglie wavelength. A structure including at least one quantum well in the layer thickness direction in which a first semiconductor layer with a thickness of An optical switch comprising means for applying an electric field in a direction.