JPH04291316A - Refractive index modulation type optical modulator - Google Patents

Abstract

PURPOSE:To generate large optical modulation effect with a low driving voltage and to realize optical modulation which is large in extinction ratio as to the refractive index modulation type optical modulator which is used for an optical communication or optical information processing, etc., and performs high-speed operation. CONSTITUTION:Quantum well layer units each consisting of a 1st quantum well layer W1, a 1st barrier layer B1 which contacts the 1st quantum well layer W1 and has lower electron energy at a point X or L than at a point GAMMA, and a 2nd quantum well layer W2 which contacts the 1st barrier layer B1 and is less in layer thickness than the 1st quantum well layer W1 or small in electron affinity and large in band gap energy are laminated periodically across 2nd barrier layers B2 which are enough to inhibit them from being bonded to adjacent quantum well layer units; and a voltage is applied to the quantum well layers to eliminate excitons and the refractive index to wavelength nearby an exciton peak is varied to modulate light.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a refractive index modulation type optical modulator used for optical communications, optical information processing, etc.

0002

[Background Art] In recent years, in the field of communications, there has been a desire to further improve the transmission speed of information in order to build a highly information-oriented society. is requested.

[0003] Expectations are high for optical communications as a means of achieving high-speed transmission of information, and in order to achieve high-speed transmission of information in optical communications, new devices capable of high-speed modulation must be developed. There is a need. On the other hand, there are active proposals to speed up information processing by using light for information processing, and it is necessary to develop a high-speed optical modulator as one of the functional elements used for this purpose.

Conventional techniques for obtaining modulated light using a semiconductor laser include a direct modulation method in which the light itself emitted by the laser is modulated by changing the current injected into the laser, and a method in which an optical modulator is installed outside the laser. There is an external modulation method that modulates the intensity of the transmitted light by providing a filter.

[0005] When high-speed modulation is performed in the above-mentioned direct modulation, fluctuations in the oscillation wavelength (wavelength chirping) cannot be avoided in principle. Therefore, an external modulation method is desirable for high-speed modulation, and an optical modulator capable of high-speed modulation is required for this purpose. As absorption modulators for external modulation, there are conventionally known ones that use the Franz Keldysh effect and those that use the quantum-confined Stark effect using a multiple quantum well structure. It is considered necessary to reduce the capacity of the device and lower the driving voltage.

[0006] Also, as a refractive index modulation type modulator, Li
There are devices using NbO3 (lithium niobate) crystals, but these have the problem of being larger in size than those using semiconductors and not being able to be integrated with semiconductor lasers.

On the other hand, in refractive index modulation type optical modulators using bulk semiconductors, the amount of change in refractive index with respect to voltage is not sufficient, and in order to achieve a sufficient modulation effect, the driving voltage must be increased or the modulator must be The size of the waveguide (length of the waveguide) must be increased.

However, these problems arise in that it is difficult to apply high-speed modulation and it is difficult to miniaturize the optical modulator. One of the methods for solving the above problems is an optical modulator that uses a multi-quantum well structure and utilizes the quantum-confined Stark effect.

FIGS. 4A and 4B are explanatory diagrams of changes in the optical absorption coefficient and refractive index due to the quantum-confined Stark effect of the quantum well layer. FIG. 4(A) shows the wavelength characteristics of the light absorption coefficient in an electric fieldless state in which no voltage is applied to the quantum well layer and in a state in which a voltage is applied perpendicularly to the quantum well layer.

As seen in this figure, the exciton absorption peak P1, which occurred in the absence of an electric field, moves to P2 on the longer wavelength side when a voltage is applied. When a voltage is applied to the quantum well layer, in addition to changing the light absorption coefficient as described above, the refractive index also changes.

FIG. 4B shows the wavelength characteristics of the refractive index in a state in which no voltage is applied to the quantum well layer and in a state in which a voltage is applied perpendicularly to the quantum well layer. As seen in this figure, the wavelength characteristic of the refractive index of the quantum well layer shifts to the long wavelength side by applying a voltage, and the wavelength characteristic of the refractive index of the quantum well layer shifts to the long wavelength side,
If the wavelength of the incident light is set near P, its refractive index changes from A to B, and this change in refractive index can be used to create a refractive index modulation type optical modulator. In other words, after dividing the light to be modulated into two parts and changing the refractive index of the propagation path of one of the parts to change the propagation speed of the light,
By combining the two lights again, intensity modulation of the lights can be performed by changing the phase relationship of both lights.

However, although this modulator has a larger refractive index change than conventional optical modulators using bulk semiconductors, it is still not sufficient, and furthermore, the charges (electrons and holes) remaining in the quantum well layer are In particular, when high-speed modulation is applied, there is a concern that the modulation response speed may be reduced. Previously, an exciton quenching optical modulator was proposed as a means to avoid a decrease in response performance due to charges (electrons and holes) remaining in the quantum layer.

FIGS. 5A and 5B are explanatory diagrams of a conventional exciton quenching optical modulator. FIG. 5(A) schematically shows the structure of one period, in which thin first barrier layers B1, B1,
A thick second barrier layer B2, B2 is provided on the outside thereof.

FIG. 5B shows the potential energy of the quantum well portion in FIG. It has a thick second barrier layer B2 having a conduction electron band higher than the electron ground level Ee by Vec. The distance between the hole ground level Eh and the electron ground level Ee in the quantum well layer W is EQabs, and the thick second
The distance between the valence band and conduction band in the barrier layer B2 is E
Babs.

To explain the optical modulation operation using this potential energy diagram, as shown in FIG. 5(B), when no voltage is applied to the quantum well layer, the electrons and holes confined in the quantum well layer W are Exciton is formed by the Coulomb force, and as shown in FIGS. 4A and 4B, a peak of the optical absorption coefficient and refractive index is formed at a wavelength corresponding to the binding energy of the exciton.

When a voltage is applied perpendicularly to this quantum well layer to lower the potential on the right side, the electrons confined within the quantum well layer pass through the thin first barrier layer B1 and spread to B2 on the right, resulting in excitons. quenching, and the absorption coefficient and refractive index of light at the same wavelength change. Light transmitted through the quantum well layer can be modulated by utilizing such changes in the light absorption coefficient and refractive index.

[0017]

[Problems to be Solved by the Invention] In order to drive the above-mentioned optical modulator at a low voltage, it is necessary to reduce the difference between the ground level Ee of electrons in the quantum well layer and the conduction electron band of the thick outer second barrier layer B2. It is necessary to reduce the ground level difference Vec.

However, when Vec is made small, the absorption energy EQab in the quantum well layer W in the absence of voltage decreases.
s and absorbed energy EBabs in barrier layer B2
There is no much difference between the two, and the absorption by the quantum well layer W is buried in the bulk light absorption in the second barrier layer B2, and the exciton absorption peak and the refractive index change due to exciton generation are no longer clear, and the quantum There is a problem that the effectiveness of using the well layer is diminished. An object of the present invention is to provide a refractive index modulation type optical modulator that produces a large optical modulation effect with a low driving voltage, has a large extinction ratio, and can operate at high speed.

[0019]

[Means for Solving the Problems] A refractive index modulation type optical modulator according to the present invention includes a first quantum well layer, a first quantum well layer, and a first quantum well layer.
a first barrier layer in contact with the quantum well layer of which the energy of electrons at the X point or the L point is lower than the energy at the Γ point; or a second quantum well layer with low electron affinity and high bandgap energy; The refractive index modulation layer is a multi-quantum well structure stacked in multiple periods via a . A configuration was adopted in which intensity modulation is performed for light at a wavelength near the exciton absorption peak by changing the refractive index for light at a nearby wavelength.

[0020]

[Operation] FIGS. 1A to 1C are diagrams explaining the principle of the refractive index modulation type optical modulator of the present invention. In this figure, W1
is the first quantum well layer, W2 is the second quantum well layer, B1
is the first barrier layer, B2 is the second barrier layer, Eew
1 is the electronic ground level in the quantum well layer W1 in the absence of an electric field,
EhW1 is the hole ground level, EeW2 is the quantum well layer W
2, EhW2 is the electron ground level, EhW2 is the hole ground level, E
X(L)eB1 is the electronic ground level in X(L) in the barrier layer B1, Eew1' is the electronic ground level in the quantum well layer W1 when bias voltage is applied, EeW2
' is the electronic ground level in the quantum well layer W2, EX(L)
eB1' is the electronic ground level in the barrier layer B1 at the X(L) point.

FIG. 1A shows a schematic configuration of an optical modulator of the present invention, in which a second barrier layer B2 is placed on one side (left side) of the first quantum well layer W1, and a second barrier layer B2 is placed on the other side (right side) of the first quantum well layer W1. A first barrier layer B1, a second quantum well layer W2, and a second barrier layer B2 are stacked on top of each other to form one quantum well structure unit. The configuration and operation of the refractive index modulation type optical modulator of the present invention will be explained based on FIGS. 1(B) and 1(C).

FIG. 1B shows the potential energy of the quantum well structure unit of the refractive index modulation type optical modulator of the present invention in a state where no voltage is applied. In this modulator, as is clear from the figure, the Γ point (k =
0), the first quantum well layer W1 and the second quantum well layer W2 are the first barrier layer B1,
The first quantum well layer W1 is made of a material with higher electron affinity and lower band gap energy than the second barrier layer B2, and the first quantum well layer W1 is thicker than the second quantum well layer W2.

Therefore, in the absence of an electric field, the electron ground level Eew1 and the hole ground level Eh of the first quantum well layer W1 are
W1 is the electronic ground level Ee of the second quantum well layer W2
W2 is smaller than the hole ground level EhW2. The first barrier layer B1 becomes a barrier layer at the Γ point, but at points X or L other than the Γ point, it has a higher electron affinity than the first quantum well layer W1 and the second quantum well layer W2. It is made of a material with a large bandgap energy and a small band gap energy, and at the X point (L point), it becomes a quantum well layer (the broken line is the potential energy when seen at the X (L) point). ). Therefore, the energy of the electronic ground state at point X(L) is EX(L)
eB1.

When no voltage is applied to this quantum well structure unit, the electrons and holes in the first quantum well layer W1 are confined by the potential barrier as shown in FIG. Similar to the exciton quenching type refractive index modulation optical modulator, excitons are formed by Coulomb force, resulting in a peak in the refractive index. A plurality of layers of this quantum well structure unit are stacked, and p
Consider a case where an n-type semiconductor layer is formed on the second quantum well layer W2 side to form a pin structure, and a reverse bias voltage is applied to this pin structure.

FIG. 1C shows the potential energy for electrons when a reverse bias voltage is applied to the quantum well structure unit. In this case, the energy of the electronic ground state in the first quantum well layer W1 is Eew1
', the energy of the electronic ground state at point X(L) in the first barrier layer B1 is EX(L)eB1 ', the energy of the electronic ground state in the second quantum well layer W2 is EeW2
However, if the layer thicknesses and compositions of the first quantum well layer W1, the second quantum well layer W2, and the first barrier layer B1 are appropriately selected, Eew1', Eew1' and E
The first quantum well layer W
1 and the electronic ground state of the second quantum well layer W2, and from the first quantum well layer W1 to the second quantum well layer W2.
It is possible to form a wide range of electronic states spanning .

[0026] Therefore, the binding of electrons and holes becomes weaker and excitons are quenched, so that the refractive index of light having a wavelength corresponding to the exciton absorption peak decreases.

In the case of the present invention, in the absence of an electric field, the first
Even if the electronic ground states of the quantum well layer W1 and the first barrier layer B1 and the second quantum well layer W2 and the first barrier layer B1 are made close to each other to enable low voltage driving, the first quantum well Layer W1 or second quantum well layer W2
The transition from the hole ground state of B1 to the electron ground state of the first barrier layer B1 hardly occurs, and the refractive index is hardly affected by the presence of the first barrier layer B1.

The reason for this is that in the present invention, the electronic state of the first barrier layer B1 is the X point, and the hole state of the first quantum well layer W1 and the second quantum well layer W2 is the Γ point. This is because their symmetries are different, and their wave functions hardly overlap spatially. Therefore, the refractive index of the quantum well layer is not disturbed by bulk absorption and can be modulated with a high extinction ratio at low voltage.

FIGS. 2A and 2B are wavelength-dependent characteristic diagrams of the refractive index and absorption coefficient of the refractive index modulation type optical modulator of the present invention. FIG. 2(A) is a wavelength characteristic diagram of the refractive index. When no voltage is applied, there is a peak in the refractive index due to excitons at the wavelength λOP, but when a reverse bias voltage Vb is applied, this peak disappears. ing.

FIG. 2B is a wavelength characteristic diagram of the absorption coefficient in the above case, and it can be seen that when the reverse bias voltage Vb is applied, the exciton peak disappears, similar to the refractive index. From these characteristic diagrams, the incident light wavelength is λOP
It can be seen that if set to , large refractive index modulation is possible by the reverse bias voltage, and that at λOP, where the refractive index peaks in the absence of an electric field, there is almost no light absorption and little light loss.

[0031]

[Embodiment] FIGS. 3(A) and 3(B) are explanatory diagrams of the configuration of a refractive index modulation type optical modulator according to an embodiment of the present invention. FIG. 3(A) is a plan view, and FIG. 3(B) is a diagram. FIG. 3A is a sectional view taken along the line X-Y of FIG. 3(A).

The configuration of the refractive index modulation type optical modulator of this embodiment will be explained based on this figure. In this figure, 1 is Ga
As substrate, 2 is AlAs cladding layer, 3 and 5 are AlAs
Non-doped layer, 4 is a mixed crystal layer, 6 is an AlAs cladding layer,
7 is a GaAs contact layer, 8 is a SiO2 film, and 9 is a p
The type electrode 10 is an n-type electrode.

The refractive index modulation type optical modulator of this embodiment is manufactured as follows. 1. First, GaAs doped with donors such as Si and Sn
An AlAs cladding layer 2 is formed on a substrate 1, an AlAs non-doped layer 3 is formed thereon to prevent impurity diffusion, and a first quantum well layer W1 is formed on the AlAs cladding layer 2 to a thickness of 8.
Ga0.6 Al0.4 As layer with a thickness of 0 Å, Ga0.6 Al0.4 with a thickness of 30 Å as the second quantum well layer W2
As layer, AlAs layer with a thickness of 100 Å as the first barrier layer B1, and 150 Å thick as the second barrier layer B2.
10 cycles of AlAs are formed.

2. A resist film in the shape of a waveguide is formed on this multilayer structure, and ions are selectively implanted using this as a mask, or impurities are selectively diffused.
The exposed region is made into a mixed crystal layer 4, and waveguides R1 and R2, which are made of a multiple quantum well layer and are split into two in the middle, are left.

3. And on top of that, non-doped AlA
An s-layer 5 and an AlAs cladding layer 6 doped with an acceptor such as Zn are formed, and highly doped GaAs
A contact layer 7 is formed.

4. Further, a SiO2 film 8 is formed thereon, an opening is provided in one waveguide R2, and a p-type electrode 9 made of Ti/Pt is formed through this opening. 5. Finally, an n-type electrode 10 made of AuGe/Au is provided on the bottom surface of the GaAs substrate 1.

[0037] The formation of each of these layers is performed using the LP method, MO
This can be performed in combination with the VPE method (or MBE method) or the like. In this way, the waveguide R1 divided into two
, R2, and a voltage is applied to only one of the waveguides R2, the refractive index of this waveguide changes, so that the light transmitted through the waveguides R1 and R2 changes. , the phase relationship at their confluence changes.

That is, if the refractive index change and the waveguide length are selected so that the phase of the light transmitted through waveguide R1 and the light transmitted through waveguide R2 are shifted by 180° at this confluence point by a predetermined voltage, then this voltage When voltage is applied, the light output from the optical modulator becomes zero, and when no voltage is applied,
Since they are combined and output in the same phase, this mechanism can be used to modulate light.

[0039]

As described above, the present invention uses a material in which the potential energy of the X point or L point is lower than the Γ point as a barrier layer between quantum well layers, so that it can be driven at a low voltage and the extinction ratio is low. It is possible to realize a refractive index modulation type optical modulator using exciton quenching that is large and capable of high-speed operation.

[Brief explanation of the drawing]

FIGS. 1A to 1C are diagrams illustrating the principle of a refractive index modulation type optical modulator of the present invention.

FIGS. 2A and 2B are wavelength-dependent characteristic diagrams of the refractive index and absorption coefficient of the refractive index modulation type optical modulator of the present invention.

FIGS. 3A and 3B are diagrams illustrating the configuration of a refractive index modulation type optical modulator according to an embodiment of the present invention.

FIGS. 4A and 4B are explanatory diagrams of changes in optical absorption rate and refractive index due to the quantum confined Stark effect of a quantum well layer.

FIGS. 5A and 5B are explanatory diagrams of a conventional exciton quenching optical modulator.

[Explanation of symbols]

W1 First quantum well layer W2 Second quantum well layer B1 First barrier layer B2 Second barrier layer Eew1 Electron ground level during no electric field in first quantum well layer W1 EhW1 First quantum well Hole ground level EeW2 in the layer W1 in the absence of an electric field EhW2 Hole ground level in the second quantum well layer W2 in the absence of an electric field EhW2 Hole ground level in the second quantum well layer W2 in the absence of an electric field EX(L)eB1 Electron ground level Eew1' at point X(L) in the first barrier layer B1 when no electric field is present.
The electronic ground level EeW2' in the quantum well layer W1 is the second when a reverse bias voltage is applied.
Ground level of electrons in quantum well layer W2 EX(L)eB1' Ground level of electrons in first barrier layer B1 at point X(L) when reverse bias voltage is applied

Claims (3)

[Claims] Claim 1: a first quantum well layer, a first barrier layer in contact with the first quantum well layer, in which the energy of electrons at the X point or the L point is lower than the energy at the Γ point; A quantum well layer unit that is in contact with a barrier layer and is composed of a second quantum well layer that is thinner than the first well layer or has a smaller electron affinity and a larger band gap energy is an adjacent quantum well layer unit. The refractive index modulation layer has a multi-quantum well structure stacked in multiple periods through a second barrier layer thick enough to inhibit coupling with the quantum well layer, and a voltage is applied perpendicularly to the quantum well layer. It is characterized by performing intensity modulation for light at a wavelength near the exciton absorption peak by extinguishing excitons by applying an electric current, and changing the refractive index of the quantum well layer for light at a wavelength near the exciton absorption peak. Refractive index modulation type optical modulator. 2. A first quantum well layer, a first barrier layer that is in contact with the first quantum well layer and in which the energy of electrons at the X point or the L point is lower than the energy at the Γ point; A quantum well layer unit that is in contact with a barrier layer and is composed of a second quantum well layer that is thinner than the first well layer or has a smaller electron affinity and a larger band gap energy is an adjacent quantum well layer unit. The refractive index modulation layer is a multi-quantum well structure layered in multiple periods through a second barrier layer thick enough to inhibit coupling with the first quantum well layer. layer on the second quantum well layer side.
A type semiconductor layer is formed, and by applying a reverse voltage to the pn junction, excitons are annihilated, and by changing the refractive index of the quantum well layer for light with a wavelength near the exciton absorption peak, A refractive index modulation type optical modulator that performs intensity modulation on wavelength light. 3. A first quantum well layer, a first barrier layer that is in contact with the first quantum well layer and in which the energy of electrons at the X point or the L point is lower than the energy at the Γ point; A quantum well layer unit that is in contact with a barrier layer and is composed of a second quantum well layer that is thinner than the first well layer or has a smaller electron affinity and a larger band gap energy is an adjacent quantum well layer unit. A multi-quantum well structure stacked in multiple periods via a second barrier layer thick enough to prohibit coupling with In the structure,
By applying a voltage perpendicularly to the quantum well layer, excitons are annihilated, and by changing the refractive index of the quantum well layer for light with a wavelength near the exciton absorption peak, light with a wavelength near the exciton absorption peak is A refractive index modulation type optical modulator characterized by performing intensity modulation on a refractive index modulation type optical modulator.