JPH11142799A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily obtain the optical modulator employing a quantum well structure having operation characteristics not having much dependency on the polarizing direction of an incident light signal without the need to use very narrow (thin) well width. SOLUTION: Layers 201a and 201b are placed in a state wherein band gap energy less than that of a barrier layer 203 is obtained, and a narrow well layer 202 is placed in a state wherein band gap energy than those of the layers 201a and 201b is obtained, thus constituting a wide well layer 201. Further, the layers 201a and 201b have tension type lattice strain.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light absorption layer having a multiple quantum well structure constituting an optical waveguide, wherein the absorption coefficient of the light absorption layer is controlled by an externally applied electric field to reduce the intensity of light passing through the optical waveguide (light absorption layer). The present invention relates to an electro-absorption optical modulator to be controlled.

[0002]

2. Description of the Related Art In recent years, with the development of very thin compound semiconductor thin films such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOVPE), semiconductor multiple quantum well structures and superlattice structures have been developed. It has come to appear. These structures make it possible to remarkably improve the characteristics of optoelectronic elements, such as the modulation speed, as compared with conventional bulk semiconductors. Among these, the multi-quantum well structure is adopted for the light absorption layer, and the electric field absorption effect of changing the absorption coefficient or the refractive index by applying an electric field to the light absorption layer has a remarkable improvement in characteristics as compared with the bulk semiconductor. Using this, an optical modulator capable of driving at a higher speed and at a lower voltage has been realized.

However, since the semiconductor quantum well structure has a structure in which different semiconductor thin films are stacked in layers, the constituent materials are different between the layer thickness direction and the layer plane direction, and the device performance is also affected by the polarization of the incident original signal. It depends on the direction. This is called polarization dependence. Since the polarization of the original signal rotates irregularly during transmission through the optical fiber,
The original signal after transmission through the optical fiber has the same degree of components (TE polarized light and TM polarized light) whose polarization directions are perpendicular to each other. When the original signal after transmission through the optical fiber is passed through a semiconductor optical modulator to modulate the original signal, the extinction ratio of the optical modulator, etc. The operating characteristics must be independent of the polarization direction of the incoming original signal.

[0004] However, in ordinary quantum well optical modulators, unless special measures are taken, their operation characteristics depend on the polarization direction of the incident optical signal. This is because a normal electro-absorption type optical modulator uses a change in absorption coefficient at a wavelength of an incident optical signal caused by a change in an applied voltage for a light intensity modulation operation. This is because when a quantum well structure is introduced into a layer, a plurality of absorption peaks appear on an absorption spectrum due to a quantum effect, and each absorption peak shows a cumulative dependence on the polarization direction of an incident optical signal.

[0005] As a device for solving this problem, the following method has been considered. First, there is a method of introducing a tensile lattice strain into the quantum well structure. This is an attempt to reduce a plurality of absorption peaks to one. However, when the applied voltage and the wavelength of the incident optical signal are different, the conditions under which the absorption peaks overlap also differ, and thus, in this method, the polarization dependency may be obtained again. To cope with this problem, a structure different from a conventional quantum well having a rectangular potential energy has been proposed (Reference 1: T. Yamaguchi, T. Morimoto, K. Akeura, and K. Ta).
da, IEEE Photonics Technology Letters, Vol. 6, No. 12, 1142-
1444, 1994).

[0006] This is based on the following principle. In the case of a semiconductor multiple quantum well structure, when a voltage is applied perpendicular to the layer direction of the quantum well structure, the energy edge of the conduction band and the valence band changes so that the absorption edge wavelength shifts to the longer wavelength side. This is called the Quantum Confined Stark Effect (QCSE). Assuming that the confinement of electrons and holes in the barrier layer of the quantum well structure is perfect, and the band structure of electrons and holes is given by parabolic approximation, according to the second-order perturbation calculation and variational calculation, , Absorption peak shift amount (energy conversion) Δ
The relationship between E and the structural parameter is “△ E∝mL ⁴ F ² ...
(1) ". Here, m, L, and F represent the effective mass of carriers, the thickness of the quantum well layer, and the applied electric field strength in the direction (quantization axis direction) perpendicular to the quantum well layer, respectively.

In a conventional quantum well optical modulator, a plurality of absorption peaks appear due to different carriers (heavy holes, heavy holes,
Due to the presence of effective masses m _HH , m _{LH of} light holes). Therefore, for these two types of carriers, the condition that the energy shift of the absorption peak is the same regardless of the applied voltage and the wavelength, although the effective masses m _HH and m _LH are different, is obtained from the equation (1). "M _LH / m _HH = (L _HH / L _LH )
⁴ ... (2) "is required. This indicates that quantum well structures having different well widths should be used for carriers having different effective masses.

Based on this principle, a quantum well structure represented by a band edge potential energy distribution as shown in FIG. 6 has been proposed by T. Yamaguchi and others. The feature of this quantum well structure is that a quantum well with a narrow width L _HH is superimposed on a quantum well with a wide width L _LH . The two layers B1 and B2 having the same potential energy as the barrier layer form the narrow quantum well. With this structure,
Good polarization independent properties have been reported. Here, in general, the effective mass m _HH of a heavy hole is the effective mass m of a light hole.
_Heavier than _LH . Accordingly, heavy holes are introduced into the narrow quantum well surrounded by the thin layers (B1, B2 in the figure) of FIG.
Light holes are confined in a wide quantum well.

[0009]

However, there is a problem that the structure is very difficult to manufacture. This means that the thickness of the two layers B1 and B2 forming the quantum well having a narrow well width must be 2 to form the structure.
This is because the thickness must be ３3 atomic layers, in other words, 1 nm or less. At present, it is very difficult to control a film thickness of about 1 nm in a practical semiconductor thin film forming technique such as MOVPE.

FIG. 7 shows a typical relationship between the thickness of a quantum well thin film formed by MOVPE and the half-value width of photoluminescence. In the characteristics shown in FIG. 7, a large half width means that the quality of the thin film is poor.
In other words, the quality of the quantum well thin film formed by the currently practical thin film forming technology is significantly deteriorated when the film thickness is 3 to 4 nm or less. Therefore, good polarization-independent characteristics can be expected from a quantum well structure having a narrow well width, but it has been extremely difficult to form a quantum well structure having a narrow well width in the past. That is, in the related art, it has been difficult to make the operating characteristics of the optical modulator using the quantum well structure as the light absorption layer independent of the polarization direction of the incident optical signal.

The present invention has been made to solve the above problems, and does not require the use of a very narrow (thin) well width as in the prior art. It is an object of the present invention to easily obtain an optical modulator adopting a quantum well structure having operating characteristics that do not depend much on the polarization direction.

[0012]

An optical modulator according to the present invention comprises:
A light absorption layer is a multiple quantum well structure in which a quantum well layer and a barrier layer are alternately stacked, and the quantum well layer constituting the light absorption layer has a band gap energy smaller than the band gap energy of the barrier layer. A first well layer formed in contact with the layer, and a second well formed between the first well layer and having a bandgap energy smaller than the bandgap energy of the first well layer. And layers. Since the quantum well layer is configured in this manner, heavy holes can be confined in the second well layer, and light holes can be formed in the outer first well layer adjacent to the second well layer. It can be confined in the included quantum well layer.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of an optical modulator according to an embodiment of the present invention. Describing the configuration of this optical modulator, first, a lower cladding layer 102 made of n-InP is formed on a substrate 101 made of n-InP. On the lower cladding layer 102, a light absorption layer 103 having a multiple quantum well structure made of non-doped InGaAsP / InGaAsP is formed. On the light absorption layer 103, an upper cladding layer 104 made of p-InP and a buffer layer 105 made of p-InGaAs are sequentially laminated.

On the back surface of the substrate 101, an n-side electrode 10 is provided.
6 is formed, and a p-side electrode 10
7 are formed. Then, on the p-side electrode 107,
A lead wire 108 for connecting to voltage applying means (not shown) for applying a voltage corresponding to the modulation signal is connected. And a light source (not shown) such as a laser
Incident light 109 passes through the light absorption layer 103 and is emitted as modulated light 110.

Next, the light absorbing layer 1 will be described in more detail.
Reference numeral 03 denotes a multiple quantum well structure in which InGaAsP having different compositions is used as a quantum well layer and a barrier layer, respectively. The pair of barrier layers and quantum well layers have a layer configuration having a band structure as shown in FIG. That is, a wide well layer (quantum well layer) 201 is formed of a layer (first well layer).
It comprises 201 a and 201 b and a narrow well layer (second well layer) 202.

The layers 201a and 201b have a thickness of 4 nm, a wavelength of 1.4 μm, and a narrow well layer 202.
Has a composition of 1.6 μm with a layer thickness of 9 nm. The barrier layer 203 formed with the wide well layer 201 interposed therebetween has a layer thickness of 5 nm and a wavelength of 1.1 μm. That is, layer 20
The wide well layer 201 is configured such that 1a and 201b have a band gap energy smaller than that of the barrier layer 203, and the narrow well layer 202 has a band gap energy smaller than that of the layers 201a and 201b. In the case of this embodiment, the layer 201
Each of a and 201b has a tensile lattice strain.

By configuring the light absorbing layer 103 in this manner, a structure substantially identical to the state represented by the equation (2) can be realized. That is, in this embodiment, the quantum mechanical tunnel effect caused by complete carrier confinement by the thin layers (B1, B2) shown in FIG. 6 is compensated for by the effect of the spread of the wave function to the layer with low potential energy. It is a substitute. Then, the light absorbing layer 10
The structure 3 does not require the formation of extremely thin layers (B1, B2) as in the structure shown in FIG. 6, and each layer has a thickness that can be formed by a practical thin film forming technique such as MOVPE. it can.

As a result, in this embodiment, heavy holes can be confined in the narrow well layer 202 (FIG. 2) having a narrow layer thickness. Further, light holes can be confined in the wide well layer 201 having a wide layer thickness including the adjacent outer layers 201a and 201b. In this embodiment, the layer 201 forming the wide well layer 201 is formed.
Since the material having tensile strain is used for the layer a and the layer 201b (FIG. 2) as described above, the energy level of light holes can be made closer to the energy level of heavy holes. This is because the energy level of heavy holes is created in the narrow quantum well layer 202, and the energy level of light holes is created in the wide quantum well layer 201.

Summarizing the above, in this embodiment, first, as shown in FIG. 2, the light absorption layer 103 is formed of a narrow well layer 202 and a wide well layer 203 obtained by adding the layers 201a and 201b. To form a quantum well structure. Therefore, as described above, heavy holes and light holes can be confined, respectively. That is, according to this embodiment, heavy holes and light holes can be individually controlled by employing the configuration shown in FIG. As a result, as described above, by using a material having tensile strain for the layers 201a and 201b, the energy level of a light hole can be made closer to the energy level of a heavy hole. .

By making the energy levels closer to each other, it is possible to suppress the polarization dependence as described below. That is, in the light absorption layer of the optical modulator having the configuration as shown in FIG. 1, absorption of incident light occurs due to light transition between electrons and holes.
The absorption of TM polarized light is sensitive to the light transition between electrons and light holes, and the absorption of TE polarized light is sensitive to the light transition between electrons and heavy holes. Therefore, if the energy levels of heavy holes and light holes are significantly different, a difference occurs in the light absorption between TE polarized light and TM polarized light in light absorption.

Therefore, as described above, the light absorbing layer 103
If the energy level of a light hole is made closer to the energy level of a heavy hole in the well layer of the multiple quantum well structure that constitutes the above, the optical modulator becomes less dependent on the polarization direction of the incident optical signal. . The structure of the light absorption layer 103 substantially satisfies the condition required by the expression (2).
As shown in FIG. 3, even when the potential energy of the quantum well is inclined due to the applied voltage, the energy levels of the heavy holes and the light holes are always kept close to each other, and the respective wave functions are maintained. Will be confined in each well layer with their distributions almost overlapping.

FIG. 4 shows the voltage dependence of the extinction ratio (corresponding to a change in the absorption coefficient) with respect to the applied voltage at the operating wavelength of 1.55 μm in the optical modulator according to this embodiment. In FIG. 4, the solid line shows the characteristic for TE polarized light, and the dotted line shows the characteristic for TM polarized light. As is apparent from FIG. 4, almost completely polarization-independent characteristics are realized for incident lights having different polarizations.

FIG. 5 shows the polarization difference of the extinction ratio at different operating wavelengths under a constant applied voltage. The polarization difference is the absolute value of the difference between the extinction ratio for TE polarized light and the extinction ratio for TM polarized light in dB (decibel) units. In FIG. 5, the solid line shows the characteristics of the optical modulator according to this embodiment. Also, the dotted line is
9 shows characteristics of a conventional modulator having a quantum well structure having a simple rectangular potential energy distribution. As is clear from FIG. 5, in the optical modulator according to this embodiment, polarization independence with a polarization difference of 1 dB is realized over a wide operating wavelength. On the other hand, in the conventional optical modulator, the region where the polarization independence is obtained is a waveguide wavelength width of about 15 nm.
Limited to a degree.

[0024]

As described above, according to the present invention, the quantum well layer has the bandgap energy smaller than the bandgap energy of the barrier layer and the first well layer formed in contact with the barrier layer. And a second well layer formed between the first well layers and having a band gap energy smaller than that of the first well layers. Since the quantum well layer is configured in this manner, heavy holes can be confined in the second well layer, and light holes can be formed in the outer first well layer adjacent to the second well layer. It can be confined in the included quantum well layer. That is, since the heavy holes and the light holes can be respectively controlled, the energy levels of the heavy holes and the light holes can be reduced by providing the first well layer with a tensile lattice strain. As a result, it is possible to reduce the dependence of the light absorption layer having this configuration on the polarization direction of the incident optical signal.

[Brief description of the drawings]

FIG. 1 is a configuration of an optical modulator according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a band structure of a part of a light absorption layer 103 of the optical modulator of FIG.

FIG. 3 is an explanatory diagram showing a band structure when a voltage is applied to a part of a light absorption layer 103 of the optical modulator of FIG. 1;

FIG. 4 shows an optical modulator according to an embodiment of the present invention.
FIG. 9 is a characteristic diagram showing the voltage dependence of the extinction ratio with respect to an applied voltage at an operating wavelength of 1.55 μm.

FIG. 5 is a correlation diagram showing the polarization difference of the extinction ratio at different operating wavelengths under a constant applied voltage.

FIG. 6 is an explanatory view showing a band structure of a part of a light absorption layer of a conventional optical modulator.

FIG. 7 is a correlation diagram showing a typical relationship between the thickness of a quantum well thin film formed by MOVPE and the half-width of photoluminescence.

[Explanation of symbols]

101: substrate, 102: lower cladding layer, 103: light absorbing layer, 104: upper cladding layer, 105: buffer layer,
106 n-side electrode, 107 p-side electrode, 108 lead wire, 109 incident light, 110 modulated light, 201 wide well layer (quantum well layer), 201a, 201b layer (first well layer) , 202 ... narrow well layer (second well layer), 20
3 ... Barrier layer.

Claims (2)

[Claims] 1. An optical modulator using a multiple quantum well structure in which a quantum well layer and a barrier layer are alternately stacked as a light absorption layer, wherein the quantum well layer has a band gap energy smaller than a band gap energy of the barrier layer. A first well layer formed in contact with the barrier layer; and a band gap energy smaller than a band gap energy of the first well layer, the first well layer being sandwiched between the first well layers. An optical modulator, comprising: a second well layer formed by forming a second well layer. 2. The optical modulator according to claim 1, wherein the first well layer has a tensile lattice strain.