JPH08201740A - Semiconductor quantum well optical modulator and optical modulation method and device using the same - Google Patents

Abstract

(57) [Summary] [Object] To provide a high-performance semiconductor quantum well optical modulator capable of obtaining a large change in absorption and a change in refractive index with a small voltage, and an optical modulation method and device using the same. [Structure] An n-InP clad layer 2, an undoped InGaAsP / InGaAsP (stress compensating) multiple quantum well layer 3, a p-InP clad layer 4, an n-InP and a p-InGaAs layer 5 on an n-InP substrate 1. On the back surface of the n-InP substrate 1, the N-side electrode 6, and the p-I
A P-side electrode 7 is provided on the nGaAs layer 5. A lead wire 8 for applying an electric field is connected to the P-side electrode 7. Incident light 9 is emitted as modulated light 10. Since the absorption peak of TM polarized light is set higher than that of TE polarized light, T
When the modulation operation is performed by M-polarized light, it operates as a high performance optical modulator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type modulator, and more particularly, to controlling the absorption coefficient or refractive index of a multiple quantum well layer constituting the optical waveguide by an externally applied electric field, or the intensity of light passing through the optical waveguide. The present invention relates to a phase-controlled semiconductor quantum well optical modulator. The present invention also relates to an optical modulation method and apparatus for performing optical modulation using such an optical modulator.

[0002]

2. Description of the Related Art In recent years, semiconductor multiple quantum wells (MQW) and superlattice structures have been introduced due to progress in compound semiconductor ultrathin film fabrication techniques such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOVPE). It is possible to significantly improve the characteristics of optoelectronic devices compared to the bulk semiconductors of the above. Among them, the electric field absorption effect of changing the absorption coefficient or the refractive index by applying an electric field to the MQW structure is very remarkable as compared with the bulk semiconductor, and an optical modulator capable of driving at high speed and low voltage is used by this. Has been realized.

[0003]

According to the above-mentioned semiconductor multiple quantum well structure in the prior art, the electric field effect greatly differs depending on the polarization direction of incident light. FIG. 7 shows TE polarization of a multiple quantum well structure having a rectangular potential shape (see FIG.
(A)) and TM polarization (FIG.7 (b)) change of an absorption coefficient are shown. According to these drawings, the absorption spectrum change of the semiconductor multiple quantum well structure is very different depending on whether the polarization direction of the incident light is parallel (called TE polarized light) or perpendicular (called TM polarized light) to the quantum well plane, and TE changes. In polarized light, absorption of excitons composed of light holes and electrons (LH excitons) and absorption of excitons composed of heavy holes and electrons (HH)
While the absorption of excitons) is both observed, only the LH exciton absorption is observed when the polarization direction of the incident light is TM polarized light. In a semiconductor that constitutes a normal quantum well structure, the HH exciton absorption peak appears on the longest wavelength side, and the change in absorption coefficient accompanying the shift of this peak when an electric field is applied is used for device operation. On the other hand, a high performance optical device using TM polarized light has recently been proposed (Japanese Patent Application No. 2-10).
4581). However, the light from the semiconductor laser which is widely used at present is TE polarized light, and most of the control by the TE polarized light is also in the development of the integrated device of the laser and the modulator. The superiority of the control used has not been recognized. In the above-mentioned Japanese Patent Application No. 2-104581, the change in absorption coefficient is T
It is proposed to extract by M-polarized incident light.
The absorption coefficient itself depends closely on the band structure of the valence band (energy region where heavy holes and light holes exist in the wave number space) determined by the quantum well structure. However, in the above-mentioned Japanese Patent Application No. 2-104581,
In the case where the lattice constants of the materials forming the quantum well structure are matched, the ideal parabolic band structure is assumed, so the quantitativeness is poor, and the absorption coefficient control using TM polarization is performed. It is not the basis for suggesting superiority.

Then, the quantum well or the barrier layer is grown so that the lattice constant of the quantum well becomes inconsistent with the lattice constant of the substrate crystal on which the quantum well layer is grown. Experimental attempts have been made to control the intensity and peak position of exciton absorption by generating stress (For InGaAsP / InP multiple quantum well structure, see M. Okamoto et al .: Institute of Electrical and Electronics Engineers, Quantum Electronics, "IEEE"). Journal
of Quantum Electronics, Vol. 27, pp. 143-1469 (1991), InG.
Regarding the aAs / InAlAs system multiple quantum well structure,
Ido et al .: IEICE Research Report, IEICE Technical Report ED
93-61, OQE93-44 (1993-07), 3
3-38). By introducing the lattice strain, the possibility of positively controlling the change of the absorption coefficient has emerged. On the other hand, the band structure of the quantum well structure is greatly changed by a slight lattice strain. It is difficult to judge how much the amount of change in absorption coefficient can be increased by the introduction of strain, and a design guideline for optimizing the quantum well structure for TM polarized light has been demanded.

In view of such circumstances, an object of the present invention is to
It is an object of the present invention to provide a semiconductor quantum well optical modulator which is a high-performance optical waveguide modulator that can obtain a large absorption change and a refractive index change which have not been obtained, and a small voltage, and an optical modulation method and apparatus using the same.

[0006]

In order to achieve the above-mentioned object, a quantum well optical modulator according to a first solution of the present invention is a quantum well consisting of a substrate and two different types of semiconductors formed on the substrate. In an optical modulator having a core of a multi-quantum well structure in which layers and barrier layers are alternately stacked, the core has a lattice constant of a quantum well layer forming the quantum well structure and the layer grown on the quantum well layer. It has a lattice strain caused by a mismatch with the lattice constant of the substrate crystal being made to change, and this lattice strain changes the band structure of the valence band to absorb and absorb light absorption by excitons of the quantum well structure. The peak position is changed so that the light can be controlled by changing the absorption coefficient of the quantum well layer accompanying the voltage application to the optical modulator.

A semiconductor quantum well optical modulator according to a second solving means of the present invention is the semiconductor quantum well optical modulator according to the above first solving means, wherein the lattice strain is tensile strain. .

A semiconductor quantum well optical modulator according to a third solving means of the present invention is the semiconductor quantum well optical modulator according to the above first solving means, wherein the lattice strain is TM of exciton absorption at an absorption edge. It is characterized in that its magnitude is set so that the absorption peak for polarized light is higher than the absorption peak for TE polarized light.

An optical modulation method according to a fourth solution of the present invention uses a semiconductor quantum well optical modulator according to the above first, second or third solution, and has a TM having an electric field component perpendicular to a quantum well layer. It is characterized in that light is modulated by polarized light.

A semiconductor quantum well optical modulator according to the fifth solving means of the present invention is a semiconductor quantum well optical modulator according to the above first, second or third solving means, and the optical modulator has a TM.
A light source for supplying light containing polarized light and a voltage applying means for applying a voltage corresponding to a signal to the optical modulator are provided, whereby the absorption amount and absorption peak position of light absorption by excitons of the quantum well structure are determined. It is possible to control the light by changing the absorption coefficient of the quantum well layer accompanying the voltage application to the optical modulator.

[0011]

In the structure of the semiconductor optical modulator of the present invention, tensile strain is introduced into the quantum well layer so that the strain-free quantum well, that is, the lattice constant of the quantum well layer material is made equal to that of the substrate material. As compared with the quantum well, it is possible to change the wavelength at which exciton absorption occurs and its absorption amount. Theoretically, the amount of exciton absorption is represented by the exciton oscillator strength. FIG. 1 shows the calculation result of the strain amount dependency of the oscillator strength of the quantum well structure. FIG. 2 is a bandgap energy diagram showing the single quantum well structure used in the calculation. InGaAs is used as a material for both the well layer and the barrier layer.
Assume P. Well width is 6,10,12,14nm
It is assumed that the composition of the well layer is changed to apply a compressive strain of 1.2% to a tensile strain of 0.6% to the well layer. Here, tensile strain refers to the case where the lattice constant of the quantum well layer material is smaller than the lattice constant of the substrate. The bandgap wavelength of the quantum well when no electric field is applied is fixed to a constant value (1.49 μm) regardless of the well width and the amount of strain. Thus, the quaternary material InGaA
By using sP, the band gap wavelength, the well width, and the strain amount can be set independently. On the other hand, the barrier layer was assumed to have a composition with a bandgap wavelength of 1.1 μm. FIG. 1 shows the results of distortion of the oscillator strength for TE polarized light and TM polarized light when no electric field is applied. On the horizontal axis of the figure, positive strain corresponds to compressive strain and negative strain corresponds to tensile strain. The tensile strain amount was calculated up to the maximum strain amount that can be realized according to the well width (the maximum strain amount is limited by the possible composition combinations determined by the bandgap wavelength of the well layer). In a quantum well having a well width exceeding 6 nm, a tensile strain quantum well structure is possible, and the oscillator strength for TM polarized light rapidly increases with the amount of tensile strain and exceeds that of TE polarized light. When the well width is 12 nm,
When the tensile strain is 4%, the strength of the vibrator takes the maximum value. On the other hand, the oscillator strength with respect to the TE polarized light has a small change with respect to the strain amount, and tends to decrease with respect to the tensile strain. It can be considered that the absorption amount of exciton absorption is proportional to the oscillator strength of excitons, which means that when tensile strain is applied to the quantum well layer, the absorption characteristics for TM polarized light are the same as those of the conventional TE.
It shows that it has a great advantage over polarized light. This increase in oscillator strength for TM polarized light is due to a characteristic change in the valence band structure that occurs when tensile strain is applied to the quantum well layer.

FIG. 3 shows the band structure of the conduction band and the valence band of the 0.4% tensile strained quantum well in the case where no electric field is applied. An important operation of the optical modulator is a transition between the first level of the conduction band and the valence band. The first level in the valence band (the level of light holes) due to the addition of tensile strain
And the energy at the wave number zero of the second level (heavy hole level) approaches. As a result of the interaction of these two different levels, the wavenumber-energy curve of the first level in the valence band repels the wavenumber energy curve of the second level in an arcuate manner, Takes an extremum at a wave number other than zero (expressed as "taking an indirect band structure"). This type of band structure strengthens the optical coupling between the electrons in the first level of the conduction band and the light holes in the first level of the valence band (light hole level). .

TM polarized light is absorbed by light holes and electrons (L
Therefore, the absorption amount by TM polarized light is increased by the introduction of tensile strain. As will be described later in Examples, a large absorption change can be obtained even when an electric field is applied, as in the case of an electric field of 0 kV / cm. The above findings are other results obtained by making full use of the original analysis method for the effect of introducing tensile strain on the target quantum well structure, and are supported by quantitative theory.

[0014]

Embodiments of the present invention will be described below with reference to the drawings, but it goes without saying that the present invention is not limited to these embodiments.

(Embodiment) FIG. 4 is a schematic perspective view showing an optical modulator according to an embodiment of increasing the absorption amount for TM polarized light as described above. On the n-InP substrate 1, n-In
P clad layer 2, non-doped InGaAsP / InGa
A multiple quantum well layer 3 made of AsP, a p-InP clad layer 4, an n-InP and a p-InGaAs layer 5 are sequentially stacked, and an N-side electrode 6 is formed on the back surface of the n-InP substrate 1.
However, the P-side electrode 7 is provided on the p-InGaAs layer 5. On the P-side electrode 7, a lead wire 8 for connecting to a voltage application means (not shown) for applying a voltage corresponding to the modulation signal is connected. Incident light 9 from a light source (laser) not shown is emitted as modulated light 10.
The quantum well layer 3 has a multiple quantum well structure including an InGaAsP layer and a stress-compensated InGaAsP layer. In _1-x Ga _x As lattice-matched on an InP substrate
_1-y P _y composition conditions or x, and cause lattice mismatch (distortion) can generate a stress due to lattice strain by changing the y. The magnitude and direction of stress can be freely changed as long as cracks do not occur. A tensile strain of 0.4% is applied to the well layer.

FIG. 5 is a diagram showing an absorption spectrum when a tensile strain of 0.4% is added to the quantum well structure of the present invention, and FIG. 5 (a) shows TM polarized light. 5 (b) is the case of TE polarized light. A solid line A represents no electric field applied, and a broken line B represents electric field applied. The arrows in the figure indicate the wavelength of the absorption peak and the operating wavelength of the modulator of 1.55μ, respectively.
m is shown. From these figures, it is understood that a high absorption coefficient value is obtained not only in the zero electric field but also when the electric field is applied (the strength of the applied electric field is 125 kV / cm) in the quantum well in which the tensile strain is introduced. Movement is long 1.5
Regarding the change Δα in absorption coefficient at 5 μm, Δα _TM is far larger than Δα _TE , and the superiority of the modulator operation by TM polarized light can be seen. Here, in order to more specifically estimate the improvement in performance, the figure of merit (figure of m
Estimate the value of erit). In the current device theory, as the figure of merit of the modulator,

[0017]

[Equation 1]

And

[0019]

[Equation 2]

Is used as the most suitable amount. Here, TE (TM) represents TE or TM, and Γ _{TE (TM)} is an optical confinement coefficient (Γ _TE or Γ _TM ) for each polarization, Δ
F is the change in the electric field, Δα _{TE (TM)} is the change in the absorption coefficient with respect to the change in the electric field, and α _{TE (TM)} (0) is the wavelength of 1.55 μm
Is the absorption coefficient (called "absorption loss") at zero electric field at. It can be considered that the former mainly corresponds to the modulation characteristic and the latter mainly corresponds to the extinction ratio. In the case of FIG. 5, ΔF = 125k
V / cm, Δα _TM = 3546 (cm ⁻¹ ), Δα _TE = 14
It is 56 (cm ^-1 ). The optical confinement coefficients of a quantum well structure having a quantum well number of 10 with a barrier layer width of 5 nm are Γ _TM = 0.2494 and Γ _TE = 0.29, respectively.
It becomes 93. Therefore, the final figure of merit is

[0021]

(Equation 3)

It is estimated that The electric field strength of 125 kV / cm in this quantum well structure is about 1.6 V in terms of voltage, and a low voltage of 2 V or less is realized.
In any figure of merit, while realizing low voltage, T
The performance of TM polarization operation can be expected to be improved about twice as much as the performance of E polarization operation. Thus, the increase of Δα at low voltage contributes to the high performance of the device.

FIG. 6 shows 10 nm, 0.3% tensile strain-
Δα _TM and Δ for 12 nm, 0.4% tensile strain −, 14 nm, 0.4% tensile strain − quantum well structure, respectively.
The difference in α _TE is plotted as a function of applied electric field. In all of the quantum well structures, Δα _TM >> Δα _TE, and it can be seen that the tensile-strained quantum well exerts an excellent electroabsorption effect on TM polarized light. Δα
The difference between _TM and Δα _TE increases as the well width is narrowed, but the peak shifts to the high electric field side. Δα _TM and Δα
_As far as only the difference in _TE is seen, a narrower quantum well structure including a 10 nm, 0.3% tensile strain quantum well has higher performance, but low voltage driving is an essential condition for device operation, and 2 V or less is required. In consideration of the above operation, it is difficult to realize it by applying a high electric field of 150 kV / cm or more. Therefore, Δα
Considering both the difference between _TM and Δα _TE and the voltage, it can be said that the optimum structure for TM polarized light is 12 nm and 0.4% tensile strain.

In the above embodiment, the InGaAs or InGaAsP mixed crystal system using the InP substrate was described, but the InGaAlAs mixed crystal system using the InP substrate, the InGaAs or InGaAs using the GaAs substrate.
It is clear that the same effect can be obtained with P, InGaAlAs or AlGaAs mixed crystal system.

[0025]

As described above, according to the present invention,
Tensile strain is applied to the quantum well layer in the multiple quantum well structure so that the absorption peak for TM polarized light is higher than the absorption peak for TE polarized light.
By performing the modulation operation with M-polarized light, it is possible to provide a high-performance optical modulator that outperforms the conventional operation with TE-polarized light.

Further, according to the present invention, by using such an optical modulator, it is possible to provide an optical modulation method and device which can obtain a large absorption change and a small refractive index change which are unprecedented with a small voltage.

[Brief description of drawings]

FIG. 1 is a diagram showing a strain amount dependency (calculated value) of an oscillator strength of a quantum well structure.

FIG. 2 is a band gap energy diagram showing a quantum well structure used for calculation of strain dependence of exciton intensity of the quantum well structure of FIG.

FIG. 3 is a diagram showing a band structure of a quantum well structure according to the present invention.

FIG. 4 is a schematic perspective view showing an optical modulator according to an embodiment of the present invention.

FIG. 5 is a diagram showing an absorption spectrum of a quantum well structure according to the present invention, where (a) shows TM polarized light and (b) shows TE polarized light.

FIG. 6 is a diagram showing the difference between the amount of change in absorption coefficient due to TM polarized light and the amount of change in absorption coefficient due to TE polarized light in the quantum well structure according to the present invention as a function of an applied electric field.

FIG. 7 is a diagram showing a change in absorption coefficient due to application of an electric field in a multiple quantum well structure having a rectangular potential shape,
(A) shows the case of TE polarization, (b) shows the case of TM polarization.

[Explanation of symbols]

1 n-InP substrate 2 n-InP clad layer 3 Multiple quantum well layer made of undoped InGaAsP / InGaAsP 4 p-InP clad layer 5 p-InGaAs layer 6 N-side electrode 7 P-side electrode 8 Lead wire 9 Incident light 10 Modulated light

Claims (5)

[Claims] 1. An optical modulator having a substrate and a core having a multi-quantum well structure in which quantum well layers and barrier layers made of two different types of semiconductors formed on the substrate are alternately stacked. There is a lattice strain caused by a mismatch between the lattice constant of the quantum well layer forming the quantum well structure and the lattice constant of the substrate crystal on which the quantum well structure is grown. The band structure of the band is changed to change the absorption amount and the position of the absorption peak of the light absorption by the excitons of the quantum well structure, and the change of the absorption coefficient of the quantum well layer accompanying the voltage application to the optical modulator A semiconductor quantum well optical modulator characterized by being set to a size that enables control. 2. The semiconductor quantum well optical modulator according to claim 1, wherein the lattice strain is tensile strain. 3. The magnitude of the lattice strain is set such that an absorption peak of exciton absorption at an absorption edge for TM polarized light is higher than an absorption peak for TE polarized light. Item 2. The semiconductor quantum well optical modulator according to item 1. 4. An optical modulation method using the semiconductor quantum well optical modulator according to claim 1, wherein optical modulation is performed by TM polarized light having an electric field component perpendicular to the quantum well layer. . 5. The optical modulator according to claim 1, 2, or 3, a light source that supplies light including TM polarized light to the optical modulator, and a voltage that applies a voltage corresponding to a signal to the optical modulator. And applying means for changing the absorption amount and absorption peak position of light absorption by excitons of the quantum well structure,
A semiconductor quantum well optical modulator, wherein light can be controlled by changing the absorption coefficient of the quantum well layer due to application of a voltage to the optical modulator.