JPH0398015A - Optical modulator - Google Patents

Abstract

PURPOSE:To obtain the optical modulator of 1.5mum band wavelength which operates on a low voltage by using a strained superlattice structure for a multiple quantum well (MQW) guide layer. CONSTITUTION:The lattice constant of InGaAs is smaller than the lattice constant of InP and lattice mismatching arises. The compsn. ratio of In in the InAlAs which is a barrier layer is increased and the lattice constant is so set as to be made larger than the lattice constant of InP to insert strains in respectively opposite directions between the well layer 8 and the barrier layer 9 by which the growth to the InP substrate 1 is allowed in the case of the MQW. An arbitrary exciton peak wavelength at an arbitrary well thickness is set by controlling the compsn. of the In of InGaAs and InAlAs if the strained superlattice structure of InGaAs/InAlAs is used. The optical modulator which operates on the sufficiently low voltage even at 1.55mum band is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor optical modulator that will be an important element in future optical communication systems and optical information processing systems.

(Prior art and its problems) Optical switches are considered to be one of the key elements of future high-speed optical communication systems and optical information processing systems, and research and development efforts are becoming active in various places. LiN as an optical switch
It is thought that there are two types: one using a dielectric material such as B03, and one using a semiconductor such as GaAs or InP. In recent years, expectations have been increasing for semiconductor optical switches that can be easily miniaturized. Depending on the field of application, these semiconductor optical switches are mainly external modulators that modulate direct current light such as single-power laser beams with one output, gate switches that turn on and off optical signals in optical waveguides, and multi-input multi-output switches. It can be classified as an optical path switching type switch. Among these, external modulators are one of the important devices for high-speed optical communication systems.

Absorption-type optical modulators that utilize the steepness of the absorption edge of a multiple quantum well (MQW) structure and the shift of excitons due to electric fields are attracting attention because of their ability to obtain high extinction ratios at low voltages and to operate at high speeds. has been done. In particular, optical modulators in the 1.5 pm wavelength band are important for high-speed optical communication systems.

Wakita et al. prototyped a modulator using InGaAs/InAlAs MQW (well layer thickness: 67A/barrier layer thickness: 67mm).
An extinction ratio of 15 dB was obtained at a wavelength of 1.53 pm and a voltage of 9 mm.

This is Electronics Letters magazine (Electro
nics Letters) Volume 22, Page 907, 19
It was written in 1986.

In the case of an MQW modulator, the amount of energy shift due to the electric field of the exciton peak, which causes absorption, is proportional to the square of the electric field and the fourth power of the MQW well thickness Lz. Therefore, in order to achieve low voltage operation, Lz must be Larger is preferable. When operating at a wavelength of 1.55 pm, which is important for optical communication systems, it is desirable that the exciton peak wavelength is 1.51 pm or less in order to take into account the skirting of the exciton peak and to reduce waveguide loss in the absence of an electric field. At that time In
The well thickness Lz of an MQW using GaAs as a well layer is required to be 75A or less. In the conventional example described above, it seems that the wavelength and Lz are not optimal, but even if optimization is attempted, Lz is about 75 A, which limits the amount of shift of the exciton peak, making it difficult to lower the voltage sufficiently. .

To solve this problem, Wakita et al.
We prototyped an MQW optical modulator using a AIAs quaternary material. This is described in the 1989 Institute of Electronics, Information and Communication Engineers Spring National Conference Proceedings C-474. By controlling the composition of the quaternary material, it is possible to simultaneously expand Lz and shorten the wavelength of the exciton peak, resulting in a wavelength of 1.55 pm.
It was designed to operate at low voltage.

Compared to when using an InGaAs well layer, Lz is increased to 86 A, and the wavelength is 1.554 pm and the voltage is 6.5 .mu.m.
Although we were able to obtain an extinction ratio of 5 dB, because a quaternary system was used for the well layer, the exciton peak was unclear compared to the InGaAs ternary system, and the exciton peak had a very large tail. However, the insertion loss was as large as 30 dB, making it difficult to use in practice.

Although there is a potential to obtain such excellent characteristics, an MQW modulator that uses steep excitons and operates at a wavelength of 1.55 pm with low loss and low voltage has not been obtained so far.

An object of the present invention is to provide an MQW optical modulator that improves these.

(Means for Solving the Problems) An optical modulator according to the present invention is an optical modulator formed on an InP substrate.
A waveguide type optical modulator having a multi-quantum well structure including nGaAs as a well layer and InAlAs as a barrier layer as an optical waveguide layer, and having means for applying an electric field perpendicularly to the optical waveguide layer, the InGaAs/InAlAs The multi-quantum well structure is made of InGaA, which has a composition that has a smaller lattice constant than InP.
I with a composition having a large lattice constant with respect to the s-well layer and InP
It is characterized by having a strained superlattice structure consisting of an nAlAs barrier layer.

(Function) As described in the conventional example, in order to operate at a low voltage in the wavelength band of 1.55 llm, it is necessary to set the exciton peak to an arbitrary wavelength while making the well layer thick to some extent.

The In composition ratio of InGaAs lattice-matched on the InP substrate is 0.53, and the bandgap wavelength at that time is 1.
．． It is 67pm. Therefore, in order to form a quantum well structure and set the exciton peak wavelength to around 1.5 μm, it is necessary to make the well thickness sufficiently thin and raise the quantum level. However, it is theoretically possible to shorten the band gap wavelength in the Hulk by reducing the composition ratio of In in InGaAs compared to the lattice matching condition, so if this is used as a quantum well, it will be possible to Well thickness (e.g. I
The quantum level can be arbitrarily determined by composition or control even when the OOA level is maintained, and the exciton peak wavelength can be set to 1.
．． It becomes possible to set it to around 5 pm.

At this time, the lattice constant of InGaAs is smaller than that of InP, causing lattice mismatch, but in the case of MQW, I
By increasing the composition ratio of In in nAlAs, setting the lattice constant to be larger than that of InP, and applying strain in opposite directions between the well layer and the barrier layer, it is possible to grow onto an InP substrate. Become.

In this way, by using the strained superlattice structure of InGaAs/InAlAs, it is possible to set any exciton peak wavelength at any well thickness by controlling the In composition of InGaAs and InAlAs, and even in the 1.55 pm band, it is possible to set any exciton peak wavelength at a sufficiently low voltage. A working optical modulator becomes possible.

(Example) FIG. 1 (aXb) is a diagram showing an example of the optical modulator according to the present invention, (a) is a perspective view, and (b) is a diagram showing the state of the multiple quantum well structure of the optical modulator. be. First, Figure 1 (aXb)
The manufacturing method of this example will be briefly explained using the following.

An n + -InALAs cladding layer 2 of 0.811 m is formed on an n''-InP substrate 1, and an i-InGaAs/InAlA
s MQW guide layer 3 with a thickness of 0.5 pm, P + -InAl
As cladding layer 4 is 0.811m, P+-InGaA
The s-cap layer 5 is sequentially lengthened by 0.2 pm using the MBE method. At this time, InAlAs cladding layers 2 and 4 and In
A set of GaAs cap layers 5 or In0.52''0.48As+ InO.
53GaO. 47"". InGaAs/InAl
Regarding the As MQW guide layer 3, as mentioned in the function section, the InGaAs well layer 8 has a smaller In composition than the lattice matching condition with InP, that is, the lattice constant is I.
Regarding the InAlAs barrier layer 9, which is smaller than nP, the composition is set so that the lattice constant is larger than that of InP, contrary to the well layer. nO. 48
GaO. 52As+ In■, 53Ga■, 42AS. The thickness of the well layer and the barrier layer are each 100 in consideration of setting the exciton peak wavelength to 1.51 pm and sufficiently confining electrons in the well layer.

Next, P +
-InAlAs cladding layer 4 and i-InGaAs/In
Etching is performed up to the interface of the AlAs MQW guide layer 3 to form a rib-shaped waveguide. The mesa width was 3 pm. Finally, a p-side electrode 6 and an n-side electrode 7 are deposited and cleaved to form input and output end faces. The length in the optical waveguide direction is 30
011m. In this way, the optical modulator shown in FIG. 1 (aXb) is completed.

Next, the operation of this optical modulator and the effects obtained will be explained. Since this is a modulator for optical communication, the wavelength of the light source used is 1.55 pm. The exciton peak wavelength of this MQW is 1.51p from the well layer thickness and composition of the MQW.
m, the absorption loss is sufficiently reduced for a wavelength of 1.55 llm. Therefore, when the electric field is O, the waveguide loss is sufficiently small, 2 to 3 dB/mm, and the incident light 10 to the MQW guide layer 3 is outputted as the output light 11 as it is. At this time, the optical modulator is in the ON state. MQ
When an electric field is applied to W, the exciton peak shifts to the longer wavelength side and absorption increases at 1.5511 m, and the incident light 10 is absorbed within the MQW guide layer 3, and the optical power of the output light becomes almost 0, turning the optical modulator OFF. state. As described in the conventional example section, the energy amount ΔE of the exciton peak shift that causes switching is proportional to the square of the electric field strength F and the fourth power of the MQW well layer thickness Lz. In order to obtain an extinction ratio of about 20 dB required for an optical modulator, if the amount of energy shift toward the long wavelength side of the exciton peak is 11 meV, the MQW well layer thickness of the switch in this example is 1.
Since the total thickness of the MQW guide layer is 0.5 pm, the switching voltage of the modulator is approximately 3 .mu.m.

In contrast, conventional strain-free InGaAs/In
When considering an AlAs MQW, and for comparison with the example, if the exciton peak wavelength in the absence of an electric field is 1.51 pm, the well layer thickness of the MQW is 75 Å. In this case, MQW
In order to obtain an energy shift amount of 11 meV for the exciton peak when the thickness of the guide layer is 0.5 pm, 5.5 V is required.
Is required.

By constructing the MQW guide layer with a strained superlattice in this way, it is possible to increase the MQW well layer thickness at any wavelength, and in the case of the example, the voltage can be reduced to about 1/2. I can do it. Further, in this example, the insertion loss including coupling loss was 15 dB, which was an improvement of more than 10 dB compared to the conventional example. Further, the composition and layer thickness of the MQW well layer according to this embodiment are an example for a wavelength of 1.55 pm, and by changing these two parameters, the exciton peak wavelength can be set to some degree freely in the 1.5 llm band.

(Effects of the Invention) As described above in detail, according to the present invention, by using a strained superlattice structure in the MQW guide layer, it is possible to obtain an optical modulator with a wavelength of 1.5 pm that operates at low voltage.

[Brief explanation of drawings]

FIG. 1 (aXb) is a perspective view of the entire optical modulator according to an embodiment of the present invention and a diagram showing the state of the multiple quantum well structure, respectively. In each figure, 1 is an n''-InP substrate, 2 is an n+
-InAlAs cladding layer, 3 is i-InGaAs/I
nAlAs MQW guide layer, 4 is P + -InAlA
s cladding layer, 5 is p + -? nGaAs cap layer, 6 and 7 electrodes, 8 In■, 4BGaO, 52As well layer, 9 In0.5BA1o, 42As barrier layer, 10
is the incident light, and l1 is the output light.

Claims (1)

[Claims] InGaAs formed on an InP substrate is used as a well layer;
A waveguide type optical modulator having a multi-quantum well structure with AlAs as a barrier layer as an optical waveguide layer, and having means for applying an electric field perpendicularly to the optical waveguide layer, the InGaAs/In
An optical modulator characterized in that the AlAs multiple quantum well structure has a strained superlattice structure consisting of an InGaAs well layer with a composition having a smaller lattice constant relative to InP and an InAlAs barrier layer having a composition with a larger lattice constant relative to InP. .