JP2004020625A - Light controlling element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light controlling element such as an optical modulator which is easy to handle and is operated with high-speed. <P>SOLUTION: The light controlling element is provided with a transparent waveguide layer 11 which makes light propagate along a first direction, a multiple quantum well layer 3 being disposed in a vicinity of, and in parallel to, the transparent waveguide layer and making the light propagate along the first direction and two mutually different conductive regions 6, 7 being disposed in a plane parallel to the multiple quantum well layer so as to sandwich the multiple quantum well layer from a second direction vertical to the first direction. The light propagating through the multiple quantum well layer is in a mode coupling relation with the light propagating through the transparent waveguide layer. It is possible to apply an electric field to the multiple quantum well layer between the two conductive regions along the second direction. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light control element such as a light modulator and a light switch. In particular, the present invention relates to an electroabsorption-type light control element having a multiple quantum well structure.
[0002]
[Prior art]
An electro-absorption optical modulator is used as a light intensity modulation element in the current optical communication. In this electro-absorption type optical modulator, an electric field is applied vertically from the top and bottom of the multiple quantum well layer to shift the absorption line, thereby controlling the intensity of light incident on the optical modulator, and Light intensity modulation is performed. Since the operation limit of this optical modulator is limited by the element capacitance C corresponding to the waveguide length for performing modulation for obtaining an extinction ratio required for communication, modulation at about 40 Gbit / sec is considered to be the operation limit. . Therefore, a higher-speed light intensity modulation element is required to realize the same modulation with a shorter modulation waveguide length.
[0003]
Therefore, an optical modulator that can operate at a higher speed is being developed. For example, Japanese Patent Application Laid-Open No. 8-248363 describes a layer-direction electric field application type optical modulator that applies an electric field in parallel to the layer direction from the left and right sides parallel to the multiple quantum well layer. In this layer-direction electric field application type optical modulator, an electric field is applied in parallel to the layer direction to breach an absorption line to change an absorption coefficient at a signal wavelength, thereby performing light intensity modulation. As a result, a higher degree of modulation can be obtained than in the method of applying an electric field perpendicular to the layer direction. As a result, the same extinction ratio can be obtained even when the modulation waveguide length is shorter than the conventional method, that is, about 100 μm or less. Then, since the modulation waveguide length can be shortened, the device capacity can be reduced and high-speed operation can be performed.
[0004]
[Problems to be solved by the invention]
When the length of the modulation waveguide is shortened as described above, the size of the light control element becomes smaller, so that some problems arise from the viewpoint of handling. For example, it is difficult to perform element indexing from a wafer and handling during mounting. Therefore, in a conventional electroabsorption optical modulator that applies an electric field perpendicular to the layer direction, a transparent waveguide that does not contribute to light modulation is provided before and after the modulation waveguide, and the element length is increased while the modulation waveguide length is kept short. And had improved handling.
[0005]
However, in a layer-direction electric field application type optical modulator in which an electric field is applied in parallel to the layer direction, it is difficult to fabricate a structure that selectively applies an electric field only to the modulation waveguide portion. An electric field is also applied to the wave path portion. For this reason, the capacitance of the transparent waveguide portion is added to increase the device capacitance, and there has been a problem that high-speed operation cannot be performed. In addition, since it is impossible to join the modulation waveguide portion and the transparent waveguide portion completely without loss, there has been a problem that a propagation loss occurs.
[0006]
Therefore, an object of the present invention is to provide an optical control element such as an optical modulator which is easy to handle and can operate at high speed.
[0007]
[Means for Solving the Problems]
The light control element according to the present invention is provided with a transparent waveguide layer that propagates light along a first direction, and is disposed close to and parallel to the transparent waveguide layer, and propagates light along the first direction. A multiple quantum well layer;
Two different conductivity type regions provided in a plane parallel to the layer of the multiple quantum well layer and sandwiching the multiple quantum well layer from a second direction perpendicular to the first direction;
The light propagating through the multiple quantum well layer has a mode coupling relationship with the light propagating through the transparent waveguide layer,
An electric field can be applied to the multiple quantum well layer along the second direction between the two conductivity type regions.
[0008]
The light control element according to the present invention is the light control element, wherein the multiple quantum well layer has a length L along the first direction,
L = 2mπ / (βe-βo)
(Βe is a symmetric fundamental mode propagation constant, βo is an antisymmetric primary mode propagation constant, and m is a natural number).
[0009]
Further, the light control element according to the present invention is the light control element, wherein the length of the multiple quantum well layer along the first direction is equal to the length of the transparent waveguide layer along the first direction. It is characterized by being shorter.
[0010]
Still further, the light control element according to the present invention is the light control element, wherein the multiple quantum well layer has a length along the first direction in a range of 50 to 100 μm.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
An optical modulator according to an embodiment of the present invention will be described with reference to the accompanying drawings. In the drawings, substantially the same members are denoted by the same reference numerals.
[0012]
Embodiment 1 FIG.
An optical modulator according to Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a side view of the optical modulator along a first direction in which light propagates. FIG. 2 is a cross-sectional view taken along line AA ′ of FIG. FIG. 3 is a diagram showing the operation principle of this optical modulator. As shown in FIG. 1, the optical modulator is disposed in parallel with a transparent waveguide layer 11 for propagating light along a first direction, and is disposed adjacent to and in parallel with the transparent waveguide layer 11. And a multiple quantum well layer 3 which is a modulation waveguide layer for propagating along. The multiple quantum well layer 3 has a so-called load-coupled relationship with the transparent waveguide layer 11. In this case, the light propagating through the multiple quantum well layer 3 has a mode coupling relationship with the light propagating through the transparent waveguide layer 11. That is, as shown in FIG. 1, light propagating in the transparent waveguide layer 11 along the first direction repeats coming and going between the multiple quantum well layers 3. In this case, by controlling the length L of the multiple quantum well layer 3 to a predetermined length, the light propagates back to the transparent waveguide layer 11 again when light modulation is not performed. The condition of the length L of the multiple quantum well layer will be described later.
[0013]
The optical modulator also includes two different conductive layers provided in a plane parallel to the layer of the multiple quantum well layer 3 and sandwiching the multiple quantum well layer 3 from a second direction perpendicular to the first direction. Mold regions 6 and 7 are provided. An electric field is applied to the multiple quantum well layer 3 between the two conductivity type regions 6 and 7 along the second direction. In this case, even when an electric field is applied in parallel to the layers of the multiple quantum well layer 3, the transparent waveguide layer 11 is not in the same plane as the multiple quantum well layer 3. Electric field application can be avoided. Thus, the waveguide length of the multiple quantum well layer 3, which is a modulation waveguide layer, can be shortened to reduce the capacity, and high-speed modulation can be performed. In addition, since the size of the optical modulator can be increased by integrally forming it with the transparent waveguide layer 11 while shortening the multiple quantum well layer 3, the handling becomes easy.
[0014]
Next, a cross-sectional structure of the optical modulator will be described with reference to FIG. First, in the central portion where light is propagated along the first direction, from the InP substrate 1, the non-doped InP lower cladding layer 2 having no impurity, the non-doped GaInAsP transparent waveguide layer 11, the non-doped InP intermediate cladding layer 12, the non-doped A GaInAsP multiple quantum well layer 3 and a non-doped InP upper cladding layer 4 are stacked.
[0015]
On the other hand, in the plane parallel to the multiple quantum well layer, on both sides sandwiching the multiple quantum well layer 3 from the second direction perpendicular to the first direction, the non-doped InP lower cladding layer 2 and the central A side cladding layer 12 sandwiching a portion of the transparent waveguide layer 11, two different conductivity type regions 6 and 7 sandwiching the central multiple quantum well layer 3, a GaInAs contact layer 5, and electrodes 8 and 9 are laminated. I have. An electric field is applied in parallel to the multiple quantum well layer 3 by the two conductivity type regions 6 and 7.
[0016]
Further, the operating principle of the optical modulator will be described with reference to FIG. FIG. 3 is a graph showing a change in the absorption coefficient when an electric field is applied to the multiple quantum well layer 3. In this optical modulator, an electric field is applied in parallel to the layer direction of the multiple quantum well layer 3 to perform light intensity modulation. When an external electric field is applied to the multiple quantum well layer 3, as shown in FIG. 3, the absorption coefficient of light in a specific wavelength range changes as compared with the case where the electric field is 0 (E = 0). Light intensity modulation can be performed using the change in the absorption coefficient. First, when no external electric field is applied (E = 0), the multiple quantum well layer 3 has a sharp absorption line due to excitons as shown by a solid line, and has almost no absorption at the used wavelength. You can think of it as a wave path. On the other hand, when an electric field is applied from the outside, the exciton absorption is effectively reduced even with a small change in the electric field intensity, and the absorption coefficient is increased in a specific wavelength range as shown by a dotted line. Here, the photoluminescence wavelength of the non-doped GaInAsP multiple quantum well layer 3 is, for example, about 1.49 μm when modulating light in the 1.55 μm band so as to be compatible with the wavelength band to be used. By applying an electric field parallel to the layer direction of the multiple quantum well layer 3, an extinction ratio required for the light intensity modulation operation can be obtained with a shorter waveguide length than when an electric field is applied perpendicular to the layer direction. Can be For this reason, since the waveguide length can be shortened, the capacity of the optical modulator can be reduced, and high-speed response can be achieved.
[0017]
Next, the condition of the length L of the multiple quantum well layer 3 will be discussed. The multiple quantum well layer 3 is arranged close to the transparent waveguide layer and parallel to the first direction, and has a so-called load coupling relationship. Thus, light propagating in the multiple quantum well layer 3 has a mode coupling relationship with light propagating in the transparent waveguide layer 11. Here, the relation of the mode coupling means a relation in which the power of the light incident on the transparent waveguide layer 11 can be transferred to and from the multiple quantum well layer 3. In this case, depending on conditions such as the length L of the loaded and coupled multiple quantum well layer 3 and the like, the light propagates back to the transparent waveguide layer 11 and quenches without returning to the transparent waveguide layer 11. There are cases. Here, when an electric field is not applied to the multiple quantum well layer 3 which is a modulation waveguide layer, a condition is that light having a mode coupling relationship returns to the transparent waveguide layer 11. On the other hand, when an electric field is applied to the multiple quantum well layer 3, the absorption coefficient becomes large at the used wavelength, and the absorption is substantially absorbed by the multiple quantum well layer 3.
[0018]
Therefore, a specific length L of the multiple quantum well layer 3 as a modulation waveguide layer is calculated so as to satisfy a condition that light having a mode coupling relationship returns to the transparent waveguide layer 11. First, when light having a frequency ω and a propagation constant β propagates along the z-axis through a waveguide layer having a refractive index n, the electromagnetic field e is represented by the following equation.
e (r, t) = E (x, y) × exp [j (ωt−βz)] (1)
Therefore, of the eigenmodes of light propagating back and forth between the transparent waveguide layer 11 and the multiple quantum well layer 3, the propagation constant of the symmetric fundamental mode is βe, and the propagation constant of the antisymmetric primary mode is βo. Then
βe> βo (2)
There is a relationship. Accordingly, a phase shift occurs in each mode by z (βe−βo) with the propagation, and when this phase difference becomes π, light energy transfers between the waveguide layers. On the other hand, the light returns to the transparent waveguide layer 11 when the phase difference becomes an integral multiple of 2π. Therefore, the length L of the multiple quantum well layer 3 in this case is
2πm = L (βe-βo) (3)
Because it is represented by
L = 2mπ / (βe-βo) (4)
(M = 1, 2, 3, ...)
Given by From the viewpoint of making the length L of the multiple quantum well layer 3 as short as possible, it is desirable that m is small. In addition, from the viewpoint of RC limitation on element capacitance, the length of the waveguide is preferably 150 μm or less in order to perform high-speed modulation exceeding 10 Gbit / sec. Specifically, the range of 50 μm to 100 μm is more preferable. On the other hand, since the length of the transparent waveguide layer 11 does not affect the modulation performance, it is sufficient that the length is large enough to facilitate handling in size. Note that the thickness is preferably 150 μm or more.
[0019]
Here, the case of using as a light intensity modulator has been described, but the application is not limited to the light intensity modulator. For example, it can function as an optical switch. It may be used as another light control element. Further, a semiconductor laser device may be manufactured on the same InP substrate 1, and the semiconductor device and the optical modulator may be integrated.
[0020]
Further, a method for manufacturing the optical modulator will be described. First, in this optical modulator, a central portion for transmitting light is manufactured by the following procedure.
(A) An InP substrate 1 is prepared.
(B) On the first surface of the InP substrate 1, a non-doped InP lower cladding layer 2, a non-doped GaInAsP transparent waveguide layer 11, a non-doped InP intermediate cladding layer 12, a non-doped GaInAsP multiple quantum well layer 3, and a non-doped InP upper cladding in this order. Layer 4 is laminated. The compound semiconductor layers such as InP, GaInAs, and GaInAsP constituting each layer can be epitaxially grown by using a metal organic chemical vapor deposition (MO-VPE) or a molecular beam epitaxy (MBE).
[0021]
On the other hand, the side structure of this optical modulator is manufactured by the following procedure. Here, examples of the side structure include a side clad layer 12 sandwiching both sides of the transparent waveguide layer 11 and two conductive type regions 6 and 7 sandwiching both sides of the quantum well layer 3.
(A) After forming up to the upper cladding layer 4, a stripe having a width of 1 to 3 μm is formed on the upper cladding layer 4 with an insulating film such as SiO ₂ . The width of this insulating film is the width of the transparent waveguide layer 11 and the multiple quantum well layer 3 that guide light.
(B) Next, etching is performed using the insulating film as a mask until the non-doped InP lower cladding layer 2 is exposed.
(C) The side cladding layer 12 is formed on the lower cladding layer 2 so as to sandwich the transparent waveguide layer 11 therebetween. Each semiconductor layer can be formed by selectively regrowing a semiconductor layer to which a conductive impurity is added.
(D) Conductive regions 6 and 7 are formed on side cladding layer 12 so as to sandwich multiple quantum well layer 3. In this case, the conductive type impurities may be added by a method such as diffusion or ion implantation to form the conductive type regions 6 and 7.
(E) A contact layer 5 is formed on each of the conductivity type regions 6 and 7. As the contact layer 5, a GaInAs layer is preferably used for the p-type contact layer.
(F) The electrodes 8 and 9 are formed on the contact layer 5 by vapor deposition.
The optical modulator is manufactured by the above steps.
[0022]
Here, the semiconductor layers such as the lower cladding layer 2, the side cladding layer 12, and the contact layer 5 may be formed, for example, by changing the composition ratio of the GaInAsP layer under the condition of lattice matching with the InP substrate 1. . That is, since the refractive index and the band gap can be controlled by changing the composition ratio, a clad layer having a low refractive index and a quantum well layer having a small band gap can be formed. The compound semiconductor layer used for the transparent waveguide layer and the multiple quantum well layer is not limited to the GaInAsP layer. In addition, for example, a compound semiconductor layer such as an AlGaInAs layer that can lattice-match with the InP substrate can be used. In this case, as long as the refractive index, band gap, and the like have predetermined values, they can be used as a transparent waveguide layer or a multiple quantum well layer.
[0023]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the light control element which concerns on this invention, the transparent waveguide layer which makes light propagate along a 1st direction, it is arrange | positioned close to and parallel to the transparent waveguide layer, and propagates light along a 1st direction. And a multiple quantum well layer. The multiple quantum well layer has a so-called load-coupled relationship with the transparent waveguide layer, and light propagating through the multiple quantum well layer has a mode-coupled relationship with light propagating through the transparent waveguide layer. The semiconductor device also includes two different conductivity type regions provided in a plane parallel to the multiple quantum well layer and sandwiching the multiple quantum well layer from a second direction perpendicular to the first direction. An electric field can be applied to the multiple quantum well layer along the second direction between the two conductivity type regions. In this case, since the transparent waveguide layer is not in the same plane as the multiple quantum well layer, application of an electric field to the transparent waveguide layer can be avoided. Then, since the waveguide length of the multiple quantum well layer, which is the modulation waveguide layer, can be shortened to reduce the capacity, high-speed modulation becomes possible. In addition, since the size of the optical modulator can be increased by forming the optical modulator integrally with the transparent waveguide layer while shortening the multiple quantum well layer, the handling becomes easy.
[0024]
According to the light control element of the present invention, the multiple quantum well layer has a length L along the first direction,
L = 2mπ / (βe-βo)
(Βe is a symmetric fundamental mode propagation constant, βo is an antisymmetric primary mode propagation constant, and m is a natural number). Accordingly, when no electric field is applied to the multiple quantum well layer, light having a mode coupling relationship propagates back to the transparent waveguide layer.
[0025]
Furthermore, according to the light control element of the present invention, the length of the multiple quantum well layer along the first direction is shorter than the length of the transparent waveguide layer along the first direction. Thereby, high-speed operation can be performed.
[0026]
Still further, according to the light control element of the present invention, the length of the multiple quantum well layer along the first direction is in the range of 50 to 100 μm. This allows high speed operation and a sufficient extinction ratio.
[Brief description of the drawings]
FIG. 1 is a schematic side view of a layer direction electric field application type optical modulator according to a first embodiment of the present invention, taken along a waveguide direction.
FIG. 2 is a sectional view taken along line AA ′ of FIG.
FIG. 3 is a schematic diagram illustrating an operation of the layer-directional electric field application type optical modulator according to the first embodiment of the present invention.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 InP substrate, 2 non-doped InP lower cladding layer, 3 non-doped GaInAsP multiple quantum well layer, 4 non-doped InP upper cladding layer, 5 GaInAs contact layer, 6 first conductivity type region, 7 second conductivity type region, 8, 9 ohmic electrode , 10 laser light, 11 transparent waveguide layer, 12 side cladding layer, 13
Insulating intermediate cladding layer, 20-layer direction electric field application type optical modulator

Claims (4)

A transparent waveguide layer for transmitting light along the first direction;
A multiple quantum well layer disposed in parallel with and close to the transparent waveguide layer and configured to propagate light along the first direction;
Two different conductivity type regions provided in a plane parallel to the layer of the multiple quantum well layer and sandwiching the multiple quantum well layer from a second direction perpendicular to the first direction;
The light propagating through the multiple quantum well layer has a mode coupling relationship with the light propagating through the transparent waveguide layer,
An optical control element, wherein an electric field can be applied to the multiple quantum well layer along the second direction between the two conductivity type regions. The multiple quantum well layer has a length L along the first direction,
L = 2mπ / (βe-βo)
The light control element according to claim 1, wherein β is a symmetric fundamental mode propagation constant, βo is an antisymmetric first-order mode propagation constant, and m is a natural number. The light control device according to claim 1, wherein the length of the multiple quantum well layer along the first direction is shorter than the length of the transparent waveguide layer along the first direction. The light control element according to claim 1, wherein the multiple quantum well layer has a length along the first direction in a range of 50 to 100 μm.

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