JPH10239646A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an absorption coefficient and its variation and to obtain large refractive index variation by applying tensile strain and forming a barrier layer to such a thickness that two-dimensional excitons are generated in a well layer and a subband zone is formed at the same time. SOLUTION: A core 3 on a clad layer 2 has a superlattice structure consisting of a well layer of InSbAsP and a barrier layer of InP. Then, tensile strain is applied to the well layer so that the top-stage energy band and the bottom- stage energy band among plural energy bands constituting the valence electron band in the energy band structure of electrons for a wave number vector parallel to the well layer are almost in the same shape when the wave number vector is zero or nearly zero. The barrier layer has such a thickness that when light is made incident, two-dimensional excitons are generated in the well layer and a subband zone is generated at the same time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator for controlling, for example, the refractive index of a core constituting an optical waveguide by an externally applied voltage and controlling the intensity or phase of light passing through the optical waveguide.

[0002]

2. Description of the Related Art In recent years, with the development of techniques for forming very thin layers of compound semiconductors, such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOVPE), semiconductor multiple quantum wells (MQWs) and superlattices have been developed. Structure (Super Lattic
e, SL) has emerged, making it possible to significantly improve the characteristics of optoelectronic devices as compared with conventional bulk semiconductors. Among these, there is a characteristic improvement of the electric field absorption effect. This is to change the absorption coefficient or the refractive index by applying an electric field to the MQW structure or the SL structure. The electric field absorption effect of the MQW structure or the SL structure is much more remarkable than that of a bulk semiconductor, and an optical modulator capable of high-speed and low-voltage driving has been realized by using this.

[0003] Optical modulators utilize changes in the optical absorption coefficient or refractive index of a waveguide to modulate a signal. Of these, the one that sets the wavelength (operating wavelength) in a region where the change in the absorption coefficient is small and uses the change in the refractive index there is called a phase modulator. In this phase modulator, it is desired that a large refractive index change can be obtained with an applied voltage (electric field) as low as possible, and at the same time, it is important that the absolute value of the absorption coefficient and the amount of the change are small. If only the refractive index is to be increased, the operating wavelength may be set in a region where the absorption coefficient is large. However, in the case of the waveguide type optical modulator, the light incident on the waveguide is absorbed before the phase of the light is changed by a change in the refractive index. Normally, the operating wavelength of the optical modulator is set in a region where the absorption of light in the waveguide and the fluctuation of its absorption coefficient can be neglected, so that in order to obtain a change in the refractive index, the applied voltage must be increased. It was continuing.

[0004]

Generally, as the wavelength of light to be modulated is farther from the absorption region of the phase light modulator,
The change in the refractive index is also small. That is, if one seeks to obtain a larger refractive index change than is required for an optical modulator, it will be closer to the absorption region of the optical modulator. Therefore, conventionally, in order to achieve high performance of a phase modulator, it is necessary to find answers to seemingly contradictory questions that the absorption wavelength of light is small at the operating wavelength but the refractive index change is larger. Did not.

The present invention has been made to solve the above problems, and provides a high-performance optical modulator capable of obtaining a large refractive index change while sufficiently suppressing the absorption coefficient and its fluctuation. The purpose is to do.

[0006]

An optical modulator according to the present invention comprises:
A quantum well composed of a well layer and a barrier layer is provided as a core, and the well layer has a plurality of energy bands constituting a valence band in an energy band structure of electrons with respect to a wave vector parallel to the well layer. The shape of the uppermost energy band and the shape of the lowermost energy band among the plurality of energy bands constituting the conduction band in the energy band structure are set so that the wave number vector is substantially equal to or near zero. Tensile strain is introduced, and the barrier layer has a thickness such that a two-dimensional exciton is formed in the well layer and a sub-band is formed at the same time. As a result, in the quantum well constituting the core of the optical modulator, the exciton peak disappears when an electric field is applied, and at the same time, the absorption edge moves to a short wavelength.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an outline of the present invention will be described. The current operation of a waveguide-type optical modulator, which has been generally used in the past, mainly uses an MQW structure for a core constituting the waveguide and utilizes the quantum confined Stark effect. FIG. 3 is a characteristic diagram showing a change in an absorption coefficient spectrum due to an applied voltage and a change in the absorption spectrum at that time when TE-polarized light is made incident on such an optical modulator. In FIG. 3, a solid line indicates a case where the applied voltage is 0, a dashed line indicates a case where the voltage is applied, and a dotted line indicates a change in the absorption spectrum.

[0008] Here, the MQW structure is InGaAsP.
A 10-nm-thick well layer consisting of
and an 8 nm-thick barrier layer made of sP. In the case of this configuration, the wavelength of the signal light to be modulated is 1.55 μm. Also, as shown in FIG. 3, two peaks at a valence band first level and a valence band second level indicating a transition from a heavy hole band on a shorter wavelength side than the wavelength of the signal light; There are three major exciton absorption peaks, one at the valence band third level, indicating a transition from the light hole band. As can be seen from FIG. 3, when the quantum confined Stark effect is used as described above, the absorption spectrum is shifted to a longer wavelength side by applying a voltage. As a result, the refractive index at a wavelength of 1.55 μm changes. Here, when the absorption change Δα due to the voltage change ΔF is given as a function of λ, the operating wavelength λ ₀ (here, 1.
The refractive index change λn obtained at 55 μm) is determined by the relationship shown in the following equation 1.

[0009]

(Equation 1)

In equation (1), Ρ means to take the main value of integration. By the way, as apparent from FIG. 3, regarding the shift of the absorption of the TE-deflected light due to the application of the voltage, it can be understood that the contribution of the exciton absorption of the first level of the valence band is large. However, as shown by the dotted line in FIG. 3, the change in the absorption coefficient is almost the same in the positive and negative regions, and is offset each other. That is, as is apparent from Equation 1 described above, in the case of this optical modulator, the change in the refractive index due to the application of the voltage is not so large. As can be seen from Equation 1, in order to obtain a large change in the refractive index, in FIG.
When a voltage is applied, the positive (negative) change in the absorption coefficient in the vicinity of the absorption edge only needs to greatly exceed the negative (positive) change in the absorption coefficient.

Here, in the present invention, first, the uppermost energy band among a plurality of energy bands constituting the valence band in the energy band structure of the electron with respect to the wave vector parallel to the well layer is provided in the well layer. The tensile strain is set so that the shape of the band and the shape of the lowest energy band among the plurality of energy bands constituting the conduction band in the energy band structure are almost the same at or near zero wave vector. As introduced. The barrier layer has a thickness such that a two-dimensional exciton is formed in the well layer when light enters, and a sub-band is formed at the same time. With this configuration, in the quantum well serving as the core layer, FIG.
As shown in (1), the exciton peak disappears by applying a voltage, and at the same time, the absorption edge shifts to a short wavelength. For this reason, as shown in FIG. 4, a change in the absorption coefficient in a region near the operating wavelength has a large negative value, and thus a change in the refractive index represented by the above equation 1 has a large value.

Here, the above-mentioned phenomenon will be described. First, as shown in FIG. 5, the energy band of the well layer forming the quantum well includes an energy band 11 representing heavy holes forming a valence band edge. When tensile strain is introduced into the well layer, the energy band 11
The state of the holes in the lighter hole band is mixed, and as a result, as shown in FIG. 5B, an energy band 11a having a downward convex shape near the wave vector k _|| There are cases. Here, since the energy band in the quantum well is considered, FIG. 5 relates to the wave vector k _||

Normally, only the light transition 12 that occurs at the point where the wave vector is zero contributes to the light absorption at the absorption edge of the semiconductor in the state shown in FIG. However, when the energy band 11a is convex downward at the valence band edge as shown in FIG. 5B and the curvature becomes the same as that of the conduction band electron, the light absorption at the absorption edge has a zero wave point. The light transition 13 at a position distant from the light source also contributes. This is because a region 14 where the energy difference between the valence band and the conduction band is equal even when the wave numbers are different is generated. That is, the number of transitions contributing to light absorption at the absorption edge is significantly increased. And
Since the thickness of the barrier layer is set so that the photoexcitation of the secondary exciton occurs, the intensity of the light absorption of the secondary exciton resulting from the transition at the absorption edge becomes larger due to the transition of the absorption edge which has been significantly increased. , Increase significantly. The broken line in FIG. 4 shows the light absorption coefficient in this state.

Here, when an electric field is applied to the quantum well, the degeneracy of the energy band degenerated with respect to the electron spin is released, and the energy band 11a representing heavy holes shown in FIG. As shown, the energy splits into a new energy band 61 and an energy band 62 indicated by a solid line and a broken line. In this quantum well, the energy difference between the lowest levels of the quantum well is 0.8 eV, the thickness of the well layer is 5 nm, the thickness of the barrier layer is 3 nm, and the well layer is 0.5%.
FIG. 6 shows the result of a simulation using these. Since each of these split energy bands 61 and 62 has a different curvature from the energy band of the electron at the conduction band edge, the transition at the absorption edge is remarkably reduced and exciton absorption is not recognized. The light absorption coefficient in this state is shown by the solid line in FIG. As described above, in the quantum well configured according to the present invention, FIG.
As shown in (1), the peak of the exciton disappears when the voltage is applied.

Next, the above-mentioned phenomenon will be described. As described above, in the present invention, the thickness of the barrier layer is set to be thin so as to generate a sub-band. Therefore, in the state where no voltage is applied, FIG.
As shown in (a), the upper end 7 of the sub-band of the valence band
The transition 72 between 3 and the lower end 74 of the conduction band sub-band forms the absorption edge. Here, in FIG. 7, the horizontal axis indicates position coordinates, and the vertical axis indicates electron energy. The graph shown in the figure shows a valence band 81 and a conduction band 82.
Shows the spatial distribution of the energy of FIG.
7A illustrates a state after coupling without applying a voltage, FIG. 7B illustrates a state before coupling without applying a voltage, and FIG. 7C illustrates a state where a voltage is applied.

A sub-band band 71 shown in FIG. 7A is formed by coupling quantum levels 75 formed in each well layer shown in FIG. 7B showing a state before coupling. is there. However, the upper end 73 and the lower end 74 of the sub-band band 71 are spread to the high energy and low energy sides as compared with the quantum level 75 from which the quantum level 75 is based. Due to this spread, the energy difference 72 between the subbands is smaller than the energy difference 76 between the quantum levels before coupling. Then, when a voltage is applied to the quantum well, as shown in FIG. 7 (c), an energy difference occurs between adjacent quantum well levels 77, so that the coupling is broken and the subband disappears. As a result, the transition energy 76a becomes larger than the transition energy 72, and the light absorption edge moves to a shorter wavelength. further,
As described above, the disappearance of exciton absorption when an electric field is applied is also added to the contribution of the short wavelength shift of the absorption edge, and the absorption edge shifts to a short wavelength as shown in FIG.

By the way, the degree of increase in exciton absorption intensity observed when the voltage is zero is larger for a material having a larger effective mass difference between heavy holes and light holes in the direction perpendicular to the quantum well plane in the well layer material. good. Current quantum well crystal growth technology
It is based on group III and group V compound semiconductor materials. From theoretical calculations, when grown on a (100) substrate,
Group III-V compound semiconductors in which the effective mass difference between heavy holes and light holes in the direction perpendicular to the well layer is most pronounced are InP, I
It has been found that the compound is a ternary or quaternary compound obtained by nAs, InSb, or a combination of these binary compounds. Specifically, InSb, InAs, InAs
P, InSbP, InAsSb, or InAsSb
It is preferable to select the material of the well layer from any one material of P.

A theoretical calculation for simulating the results shown in FIGS. 4 and 6 (reference: "New K.P theory fo
r GaAs / Ga _1-x Al _x As-Type Quantum Well "R.
Eppenga, MFHSchuurmans, S.Colak, .Physical Revue B
, Vol. 36, No.3, pp1544-1564, 1987), the strain amount in the well layer and the thickness of the barrier layer for constituting the quantum well in the present invention can be obtained. Based on the above findings, it is possible to achieve an unprecedented change in refractive index while suppressing an increase in absorption fluctuation.

Hereinafter, embodiments of the present invention will be described. FIG. 1 is a perspective view showing a configuration of an optical phase modulator according to an embodiment of the present invention. As shown in FIG.
On an n-type (100) InP substrate 1, an n-type In
P cladding layer 2, non-doped InSbAsP
A core 3 composed of a superlattice of / InP, a cladding layer 4 composed of p-type InP, and a cap layer 5 composed of p-type InGaAs are sequentially laminated. Here, core 3
Has a thickness of 5 n with a strain of 0.5% made of InSbAsP.
This is a superlattice structure composed of a m-well layer and a 3 nm-thick barrier layer made of InP. Note that these structures may be manufactured by a crystal growth method such as a molecular beam epitaxial growth method or a metal organic chemical vapor deposition method.

As described above, the configuration of the core 3 can be determined as appropriate by theoretical calculation for simulating the results shown in FIGS. Also,
An n-side electrode 6 is formed on the back surface of the substrate 1 and the cap layer 5
A p-side electrode 7 is formed thereon. A lead wire 8 is connected to the p-side electrode 7, and a voltage corresponding to the modulation vibration is applied to the lead wire 8. The incident light 9 entering the core 3 from the near side of the drawing is emitted as modulated light 10 from the opposite surface.

FIG. 2 shows a change in refractive index (a) in the optical phase modulator shown in FIG. 1 and a conventional optical phase modulator.
FIG. 4 is a characteristic diagram showing the voltage dependence of a change in light absorption coefficient (b). As shown in FIG. 2A, the characteristic 21 of the optical phase modulator according to the present embodiment shows a sharp increase in the refractive index compared to the characteristic 22 of the conventional optical phase modulator, The change in the refractive index in the region is 3 to 4
It has been found that a value more than twice as high can be obtained. This is a result of using a superlattice structure having tensile strain in the well layer of the core 3 as described above. FIG.
As shown in (b), the characteristic 23 of the optical phase modulator according to the present embodiment differs from the characteristic 24 of the conventional optical phase modulator in that the absorption coefficient decreases as the applied voltage increases. There is no increase in coefficient variation, and it is possible to suppress a decrease in the intensity of guided light due to an increase in the absorption variable.

In the above description, the optical phase modulator has been described. However, the present invention is not limited to this, and the present invention may be applied to an optical intensity modulator (absorption type optical modulator). In this case, the composition of the well layer may be adjusted so that the wavelength of the exciton absorption peak when no voltage is applied matches the wavelength of the signal light. In such an optical intensity modulator, compared to the conventional one using the quantum confined Stark effect,
Since the sign of the refractive index change when the absorption coefficient changes is opposite, the modulated light will undergo negative chirping (phase shift).

In the conventional light intensity modulator, the modulated light undergoes positive chirping, so that the width of the modulated light increases while being guided through the optical fiber, and as a result, the frequency band of optical transmission is narrowed. On the other hand, in the optical intensity modulator according to the present invention, the modulated light has a negative chirping sign, so that the width of the modulated light becomes narrower while being guided through the optical fiber. Can be wide. Although the waveguide type optical modulator has been described above, it is apparent that the same effect can be obtained by using the same strained quantum well layer even in a plane type optical modulator.

[0024]

As described above, according to the present invention, a quantum well composed of a well layer and a barrier layer is provided as a core, and the well layer has an electron energy band structure for a wave vector parallel to the well layer. The shape of the uppermost energy band among the plurality of energy bands constituting the valence band and the shape of the lowermost energy band among the plurality of energy bands constituting the conduction band in the energy band structure are represented by wave numbers. The tensile strain is introduced so that the vector becomes almost the same at or near zero, and the barrier layer has a thickness such that a two-dimensional exciton is formed in the well layer and a subband band is formed at the same time. did.
As a result, in the quantum well constituting the core of the optical modulator, the exciton peak disappears, and at the same time, the absorption edge shifts to a short wavelength. For this reason, a change in the absorption coefficient of the core in a region near the operating wavelength has a large negative value, and a change in the refractive index of the core has a large value. That is, according to the present invention, there is an effect that a high-performance optical modulator capable of obtaining a large change in the refractive index can be obtained while sufficiently suppressing the absorption coefficient and its fluctuation.

In the present invention, if the composition of the well layer is adjusted so that the wavelength of the exciton absorption peak when no voltage is applied coincides with the wavelength of the signal light, it can be used as a light intensity modulator. . In this optical intensity modulator, the modulated light that is modulated and emitted and guided through the optical fiber becomes narrower while being guided through the optical fiber, so that the frequency band of optical transmission can be broadened.

[Brief description of the drawings]

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

FIG. 2 is a characteristic diagram showing a voltage dependency of a refractive index change (a) and a light absorption coefficient change (b) in the optical phase modulator shown in FIG. 1 and a conventional optical phase modulator.

FIG. 3 is a characteristic diagram showing a change in an absorption coefficient spectrum due to an applied voltage and a change in the absorption spectrum at that time when TE-deflected light is made incident on a conventional optical modulator.

FIG. 4 is a characteristic diagram showing a change in an absorption coefficient spectrum due to an applied voltage and a change in the absorption spectrum at that time when TE-deflected light is made incident on the optical modulator according to the present invention.

FIG. 5 is a band gap diagram showing an energy band of a well layer.

FIG. 6 is a band gap diagram showing an energy band of a well layer when an electric field is applied.

FIG. 7 is a band gap diagram showing an energy band in the core of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2, 4 ... Cladding layer, 3 ... Core, 5 ... Cap layer, 6 ... N side electrode, 7 ... P side electrode.

Claims (4)

[Claims] 1. A quantum well comprising a well layer and a barrier layer as a core, wherein the well layer has a plurality of valence bands in an energy band structure of electrons with respect to a wave vector parallel to the well layer. The shape of the uppermost energy band in the energy band and the shape of the lowermost energy band in the plurality of energy bands constituting the conduction band in the energy band structure are such that the wave vector is zero or near zero. A tensile strain is introduced so as to be substantially the same, and the barrier layer has a thickness such that a two-dimensional exciton is formed in the well layer and a sub-band is formed at the same time. Light modulator. 2. The optical modulator according to claim 1, wherein the wavelength of the signal light is any one of a wavelength of a two-dimensional exciton absorption peak of the well layer when no voltage is applied to the core and a wavelength in the vicinity thereof. An optical modulator characterized in that: 3. The optical modulator according to claim 1, wherein a wavelength of the signal light is longer than a two-dimensional exciton absorption edge of the well layer when no voltage is applied to the core. An optical modulator characterized by being coincident with a large wavelength. 4. The optical modulator according to claim 1, wherein said well layer is made of InSb, InAs, InAsP, and InSb.
An optical modulator comprising one of SbP, InAsSb, and InAsSbP, or a compound semiconductor obtained by combining them.