JP3633296B2 - Broadband semiconductor optical amplifier - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor optical amplifier in which light propagating in an active layer is amplified by injecting a current into the active layer, and more particularly, a broadband semiconductor optical amplifier having a flat gain characteristic over a wide band. It is about.
[0002]
[Prior art]
Research and development of high-speed, large-capacity, long-distance transmission systems using wavelength division multiplexing and optical devices used therefor are actively conducted. A broadband optical amplifier is an indispensable device for realizing the above system. As this optical amplifier, a semiconductor optical amplifier that can be expected to be reduced in size and cost is attracting attention, and many proposals regarding a configuration for widening the bandwidth have been made. One of them is shown in FIG. 9 (Japanese Patent Laid-Open No. 4-291983). In the semiconductor optical amplifier, the active layers 43a and 43b and the optical waveguides 42a, 42b, 44a, and 44b are provided with a portion where one or a plurality of refractive indexes change in the middle of the light traveling direction of one or both of these. A composite resonator in which each is optically coupled between the portions where the refractive index changes is formed. Further, electrodes 46a and 46b are provided in each of the above-described composite resonators, and the resonance peak of the composite resonator can be independently controlled by the current injected from each electrode.
[0003]
As another configuration for broadening the bandwidth, as shown in FIG. 10, the active layer 64 has a laminated structure of two or more layers (66, 68), and the layers 66, 68 constituting the active layer are the same. There has been proposed a structure in which the composition is made of materials having different energy gaps, and the film thicknesses of the respective layers are set to the same value or close to each other (Japanese Patent Laid-Open No. 3-76186).
[0004]
Further, JP-A-6-283824 discloses an optical confinement layer having an active region, a plurality of quantum well layers in which at least one layer is different in thickness from other layers and lattice mismatched, and between each quantum well layer. A structure composed of a barrier layer formed in contact with a quantum well layer has also been proposed.
[0005]
[Problems to be solved by the invention]
The conventional method for broadening the bandwidth of a semiconductor optical amplifier has the following problems.
[0006]
(1) Although it is considered that the configuration of FIG. 9 can obtain a certain level of flat gain characteristics in the wavelength range of 20 to 30 nm, it is not possible to expect a wider band than that. This is because the material of the active layer of the first resonator and the material of the active layer of the second resonator are the same. Also, it is difficult to make the materials of these two resonators different from each other in terms of manufacturing method.
[0007]
(2) In the configuration of FIG. 10, the active layer 64 has a laminated structure of two or more layers having different energy gaps, so that the bandwidth is increased. However, the total thickness of the laminated active layers is inevitably increased. Therefore, the light confinement effect in the thickness direction is weakened, and the light output is reduced due to an increase in threshold current.
[0008]
(3) The configuration of JP-A-6-283824 has the same problem as (2).
[0009]
Accordingly, an object of the present invention is to provide a broadband semiconductor optical amplifier that solves the above-described problems and has a flat gain characteristic over a wide band.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the present invention comprises a substrate having a lower clad layer, an active layer and an upper clad layer stacked thereon, an upper electrode loaded on the upper clad layer, and a lower electrode loaded on the substrate. in the semiconductor optical amplifier for amplifying light propagating through the active layer by injecting a current into the active layer from the upper electrode, the active layer is divided into three active layers aligned in the propagation direction of the light, the three The active layer is composed of materials having common composition components and different energy gaps, and the first active layer located on the light incident side is formed with a tapered portion A in which the layer thickness decreases in the light propagation direction, The third active layer located on the light exit side is formed with a tapered portion C whose thickness decreases in the direction opposite to the light propagation direction, and is located between the first active layer and the third active layer. The second active layer has a light propagation direction and A taper portion D in which the layer thickness decreases in the opposite direction and a taper portion B in which the layer thickness decreases in the light propagation direction are formed, and the taper portion A and the taper portion D are stacked in contact with each other so as to be optically coupled, In addition, the tapered portion B and the tapered portion C are laminated in contact with each other so as to be optically coupled.
[0012]
The thickness of the incident end of the first active layer and the thickness of the exit end of the third active layer may be equal or approximate to each other.
[0013]
The three active layers may have different energy gaps due to different composition ratios of materials.
[0014]
The energy gap of the first active layer and the energy gap of the third active layer may be substantially equal.
[0015]
A separation groove is provided in the upper electrode of the optical coupling portion between the taper portion A and the taper portion D, and a separation groove is provided in the upper electrode of the optical coupling portion between the taper portion B and the taper portion C. The upper electrode may be divided into three, and current may be injected independently from each of the separated upper electrodes.
[0016]
A window region may be provided at an incident / exit end of the active layer.
[0017]
The active layer may have a bulk structure, a quantum well structure, or a strained quantum well structure.
[0018]
A barrier layer may be provided between the first active layer and the second active layer and between the second active layer and the third active layer.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
[0020]
As shown in FIG. 1, a broadband semiconductor optical amplifier according to the present invention includes an InP (n) clad layer 2, an active layer 3, and an InP as an upper clad layer on an InP (n ⁺ ) substrate 1. (P) A cladding layer 4 and an InGaAsP contact layer 5 are stacked, an upper electrode 6 is loaded on the InGaAsP contact layer 5, and a lower electrode 7 is loaded under the InP (n ⁺ ) substrate 1. Reference numerals 8-1 and 8-2 denote non-reflective coating layers applied to the light incident end face and the light exit end face, and 9 denotes a terminal for injecting a forward current Ik equal to or lower than the threshold current. Reference numeral 10 denotes signal light incident on the incident end face, and reference numeral 11 denotes amplified signal light emitted from the outgoing end face.
[0021]
As a feature of the present invention, the active layer 3 is divided into three active layers 3-1, 3-2 and 3-3 arranged in the light propagation direction. The active layer 3-1 located on the light incident side is called a first active layer, the active layer 3-3 located on the emission side is called a third active layer, and an intermediate between the first active layer and the third active layer. The active layer 3-2 located at is referred to as a second active layer. These three active layers 3-1, 3-2, and 3-3 have a constant layer thickness portion whose upper surface and lower surface are parallel to the InP (n ⁺ ) substrate 1, and a taper portion that tapers continuously to the constant layer thickness portion. A, B, C, D. The first active layer 3-1 is formed with a constant layer thickness portion constituting the incident end and a tapered portion A in which the layer thickness is reduced in the light propagation direction, and the third active layer 3-3 is formed with the constant end portion constituting the emission end. A taper portion C is formed in which the layer thickness decreases in a direction opposite to the light propagation direction, and the second active layer 3-2 has a taper portion D in which the layer thickness decreases in the direction opposite to the light propagation direction. And a constant layer thickness portion and a tapered portion B in which the layer thickness decreases in the light propagation direction. The taper portion A has an upper surface parallel to the InP (n ⁺ ) substrate 1 and a lower surface inclined until the thickness disappears, and the taper portion D is inclined until the thickness disappears and the lower surface parallel to the substrate 1. The taper portions A and D are stacked such that the surfaces parallel to the substrate 1 are aligned with each other. Further, the taper portion C has an upper surface parallel to the InP (n ⁺ ) substrate 1 and a lower surface inclined until the thickness disappears, and the taper portion B has a lower surface parallel to the substrate 1 and no thickness. The taper portions C and B are laminated such that the surfaces parallel to the substrate 1 are aligned with each other. The tips where the thicknesses of the respective taper portions A, B, C, and D are lost reach the constant layer thickness portions of the stacked active layers 3-1, 3-2, and 3-3. Such an optical coupling portion formed by stacking active layers will be referred to as an optical coupling portion.
[0022]
Further, as a feature of the present invention, these three active layers 3-1, 3-2 and 3-3 are made of materials having the same composition component and different energy gaps. That is, the first active layer 3-1 includes In, Ga, As, and P as composition components, and is composed of a material In _1-x Ga _x As _1-y P _y having a band gap wavelength of λl. is, the second active layer 3-2, in a composition component, Ga, as, has a P, a material such as a band-gap wavelength is _{λm in 1-q Ga q as} 1-r P r The third active layer 3-3 includes In, Ga, As, and P as composition components, and is composed of a material In _1-v Ga _v As _1-w P _w having a band gap wavelength of λs. Has been. The band gap wavelength λl is longer than the band gap wavelength λm, and the band gap wavelength λm is longer than the band gap wavelength λs.
[0023]
Thus, the wideband semiconductor optical amplifier of the present invention comprises three types of active layers 3-1, 3-2 and 3-3 having different band gap wavelengths in cascade, and the horizontal surface of each taper portion is the active activity of the counterpart. Each active layer is laminated and optically coupled so as to be in contact with the layers. With this configuration, the signal light 10 that enters and propagates into the first active layer 3-1 is efficiently optically coupled to the second active layer 3-2 and propagates through the second active layer 3-2. Further, the signal light propagated in the second active layer 3-2 is optically coupled to the active layer 3-3 efficiently and propagates in the third active layer 3-3. The amplified signal light 11 is emitted from the third active layer 3-3.
[0024]
Since the band gap wavelength of each active layer is different in this wideband semiconductor optical amplifier, each active layer has a gain characteristic of a different center wavelength, and as a whole, a flat gain characteristic is obtained over a wide band of wavelengths. be able to.
[0025]
Three band gap wavelength λl of the active layer 3-1, 3-2, 3-3, lambda] m, as a method of varying the [lambda] s, the material _{In 1-x Ga x As 1} -y P y, materials _{an In 1-q} Ga _q As _{1-r P} r and materials _{u in 1-v Ga v As} 1-w P w of x, y, q, r, v, varying the value of w. That is, the three active layers 3-1, 3-2, and 3-3 are made of materials having common composition components but different energy gaps.
[0026]
Further, as a method of making the band gap wavelengths λl, λm, and λs different, the composition ratio of the materials may be made different.
[0027]
Next, another embodiment of the broadband semiconductor optical amplifier of the present invention will be described.
[0028]
The broadband semiconductor optical amplifier shown in FIG. 2 is obtained by changing the material of the third active layer 3-3 in the configuration of FIG. 1, and the same members as those in FIG. In the configuration of FIG. 2, the same material as that of the first active layer 3-1 is used for the third active layer (reference numeral is changed to 3-1). Since this broadband semiconductor optical amplifier has a bilaterally symmetric structure, even if the incident light 10 is incident from any of the non-reflective coating layers 8-1 and 8-2, the same effect can be obtained. .
[0029]
The broadband semiconductor optical amplifier shown in FIG. 3 is obtained by changing the vertical positional relationship of each active layer in the configuration of FIG. That is, in FIG. 1, the first active layer 3-1 and the third active layer 3-3 are located below the second active layer 3-2, but in FIG. And the third active layer 3-3 are located above the second active layer 3-2. In the manufacturing process for obtaining this configuration, the first active layer 3-1 (or the third active layer 3-3) is formed after forming the second active layer 3-2, and then the third active layer 3-3. (Or the first active layer 3-1) is formed.
[0030]
The broadband semiconductor optical amplifier shown in FIG. 4 uses the same material as that of the first active layer 3-1 for the third active layer (3-1) in the configuration of FIG. 3 as in the configuration of FIG.
[0031]
The broadband semiconductor optical amplifier shown in FIG. 5 has electrode separation grooves in the optical coupling portion between the tapered portion A and the tapered portion D and the upper electrode 6 in the optical coupling portion between the tapered portion B and the tapered portion C in the configuration of FIG. It was separated into three upper electrodes 6-1, 6-2 and 6-3 provided 12-1 and 12-2, injecting a current _{Ik 1,} Ik 2, Ik ₃ independently from each of the upper electrodes separated Terminals 9-1, 9-2, and 9-3 are provided so as to be able to. With this configuration, mutually independent currents Ik ₁ , Ik ₂ , and Ik ₃ are injected into the _three active layers 3-1, 3-2, and 3-3, so that the gain in each active layer can be controlled independently. Can do. As described above, by independently injecting current into each active layer having a different center wavelength of the gain characteristic and controlling the gain independently, it becomes easy to flatten the entire gain characteristic. That is, the gain characteristic of the conventional technique can be expanded to about three times the band.
[0032]
The broadband semiconductor optical amplifier shown in FIG. 6 uses the same material as the first active layer 3-1 for the third active layer (3-1) as in the configuration of FIG. the three upper electrodes 6-1, 6-2 and 6-3 are provided with an electrode separation grooves 12-1 and 12-2 formed separately, so that it can inject current _{Ik 1,} Ik 2, Ik ₃ independently It is a thing.
[0033]
The broadband semiconductor optical amplifier shown in FIG. 7 is provided with electrode separation grooves 12-1 and 12-2 in the same manner as in FIG. three upper electrode 6-1, 6-2 and 6-3 were separated form Te is obtained by allowing current injection _{Ik 1,} Ik 2, Ik ₃ independently.
[0034]
The broadband semiconductor optical amplifier shown in FIG. 8 changes the vertical positional relationship of each active layer similarly to the configuration of FIG. 4, and the same material as the first active layer 3-1 in the third active layer (3-1). In the same manner as in the configuration of FIG. 5, the electrode separation grooves 12-1 and 12-2 are provided to separate and form the three upper electrodes 6-1, 6-2, and 6-3, and the current Ik independently. ₁ , Ik ₂ , and Ik ₃ can be injected.
[0035]
In each of the above embodiments, the active layer 3 may use any of a bulk structure, a quantum well structure, or a strained quantum well structure. If a strained quantum well structure is used for the active layer 3, the polarization dependence of the gain can be kept low.
[0036]
A barrier layer may be provided between the two active layers 3-1 and 3-2. By inserting a barrier layer between the two active layers 3-1 and 3-2, it is possible to further increase the bandwidth and make the polarization dependent.
[0037]
1 to 8, an InGaAsP optical waveguide is provided below the active layers 3-1 and 3-3, an InGaAsP optical waveguide is provided above the second active layer 3-2, and The refractive index is preferably set to a value lower than the refractive index of each active layer. With such a configuration, it is possible to efficiently propagate light between the active layers.
[0038]
1 to 8, a so-called window region in which the active layer 3 is interrupted may be formed in the vicinity of the incident end and the emission end of the active layer 3. The window region is continuously and integrally formed with the cladding layer using the same material as the cladding layer sandwiching the active layer 3 in the horizontal direction. By providing the window region, the polarization dependence of gain can be reduced.
[0039]
Further, in the configurations of FIGS. 1 to 8, a tapered structure type optical waveguide for spot size conversion may be added to the entrance end and the exit end of the broadband semiconductor optical amplifier. The optical waveguide section for spot size conversion is obtained by gradually changing the core cross-sectional area of the optical waveguide so as to have a target spot size. For example, the optical waveguide section has the same thickness as the active layer 3 of the broadband semiconductor optical amplifier. The core is gradually changed to approach the spot size of the input / output optical fiber. In this way, the optical coupling between the optical waveguide and the optical fiber is efficient, and the optical coupling between the optical waveguide and the broadband semiconductor optical amplifier is efficient, so the optical coupling between the optical fiber and the broadband semiconductor optical amplifier is efficient. Get better.
[0040]
【The invention's effect】
The present invention exhibits the following excellent effects.
[0041]
(1) By dividing the active layer into three active layers and arranging them in the light propagation direction, it is possible to connect three optical amplification units having different wavelength bands in cascade, and these three optical amplification units are integrated. Therefore, an unprecedented ultra-wideband semiconductor optical amplifier can be realized.
[0042]
(2) By separating the upper electrode, the optical amplifiers in the three wavelength bands can be controlled independently, so that a high gain and flat gain characteristic can be obtained.
[0043]
(3) Since the three active layers are coupled by a very efficient coupling method in which the three active layers are stacked in contact with each other, and the reflected light from the optical coupling portion is not generated, the structure is very stable. A working optical amplifier can be realized.
[0044]
(4) Since the structure is simple, manufacturing is easy and cost reduction can be expected.
[Brief description of the drawings]
FIG. 1 is a vertical cross-sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing an embodiment of the present invention.
FIG. 2 is a vertical sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 3 is a vertical cross-sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 4 is a vertical sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 5 is a vertical cross-sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 6 is a vertical sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 7 is a vertical cross-sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 8 is a vertical cross-sectional view along a light propagation direction of a broadband semiconductor optical amplifier showing another embodiment of the present invention.
FIG. 9 is a vertical cross-sectional view along the light propagation direction of a conventional semiconductor optical amplifier.
FIG. 10 is a vertical sectional view perpendicular to the light propagation direction of a conventional semiconductor optical amplifier.
[Explanation of symbols]
1 InP (n ⁺ ) substrate 2 InP (n) cladding layer (lower cladding layer)
3 Active layer 3-1 First active layer (third active layer)
3-2 Second active layer 3-3 Third active layer 4 InP (p) cladding layer (upper cladding layer)
5 InGaAsP contact layers 6, 6-1 and 6-2 Upper electrode 7 Lower electrodes 8-1 and 8-2 Anti-reflective coating layers 12-1 and 12-2 Electrode separation grooves

Claims (8)

A lower clad layer, an active layer and an upper clad layer are laminated on a substrate, an upper electrode is loaded on the upper clad layer, a lower electrode is loaded under the substrate, and current is injected into the active layer from the upper electrode Then, in the semiconductor optical amplifier that amplifies the light propagating in the active layer, the active layer is divided into three active layers arranged in the light propagation direction, and the three active layers have the same composition component and energy. The third active layer is formed of a material having different gaps, and the first active layer located on the light incident side is formed with a tapered portion A whose thickness decreases in the light propagation direction, and is located on the light emission side Is formed with a tapered portion C whose thickness decreases in the direction opposite to the light propagation direction, and the second active layer located between the first active layer and the third active layer has a light propagation direction. A taper portion D in which the layer thickness decreases in the opposite direction; A taper portion B whose thickness decreases in the propagation direction is formed, the taper portion A and the taper portion D are stacked in contact with each other so as to be optically coupled, and the taper portion B and the taper portion C are formed. A broadband semiconductor optical amplifier characterized by being stacked in contact with each other so as to be optically coupled. Claim 1, wherein the broadband semiconductor optical amplifier, characterized in that the thickness of the exit end of the first active layer thickness and the third active layer of the entrance end of the are close or equal to each other. It said three active layers, according to claim 1 or 2 wherein the wideband semiconductor optical amplifier, characterized in that the energy gap differ from each other by the composition ratio of the material differ from each other. Claim 1-3 or according broadband semiconductor optical amplifier and the energy gap of the first active layer and the energy gap of the third active layer is equal to or substantially equal. A separation groove is provided in the upper electrode of the optical coupling portion between the taper portion A and the taper portion D, and a separation groove is provided in the upper electrode of the optical coupling portion between the taper portion B and the taper portion C. , the upper electrode is divided into three, according to claim 1-4 or according broadband semiconductor optical amplifier, characterized in that it injecting current independently from each of the upper electrodes separated. Claim 1-5 or according broadband semiconductor optical amplifier, wherein the window region is provided at the input and output end of the active layer. The active layer is a bulk structure, according to claim 1 to 6 or according broadband semiconductor optical amplifier, characterized in that a quantum well structure or a strained quantum well structure. Claim 1-7, wherein any one, wherein a barrier layer is provided between the third active layer and between the second active layer and said second active layer and the first active layer Broadband semiconductor optical amplifier.

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