JP2004266095A - Semiconductor optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the polarized wave dependence of the gain of a semiconductor optical amplifier. <P>SOLUTION: The semiconductor optical amplifier has a semiconductor substrate 1, and an active layer 13 including a multiple quantum well (MQW) structure formed over the semiconductor substrate. An incident light 19 from one end face 17a is amplified while traveling a light guide including the active layer 13 formed along the light traveling direction and outputted from the other end face 17b. The multiple quantum well structure has a plurality of well layers 14 and barrier layers 15, and the well layers or the barrier layers have a tensile strain. A thickness of the barrier layer is set to not more than 4 nm. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a semiconductor optical amplifier that amplifies light incident from one end face and emits light from the other end face, and more particularly to a semiconductor optical amplifier that suppresses the polarization dependence of gain in amplification.
[0002]
[Prior art]
In an optical communication system, when an optical signal is transmitted to a remote place via an optical fiber, attenuation of the optical signal occurs. Therefore, it is necessary to provide a repeater and amplify the optical signal by the repeater. 2. Description of the Related Art Semiconductor optical amplifiers have been put to practical use as one of optical amplifiers for directly amplifying optical signals without converting them into electric signals.
[0003]
In such a semiconductor optical amplifier, in order to obtain good light oscillation characteristics, in addition to a bulk structure composed of one uniform material, a plurality of well layers and both sides of each well layer are provided as an active layer. A multiple quantum well structure (hereinafter, abbreviated as MQW structure) in which a plurality of barrier layers are stacked is adopted.
[0004]
FIG. 11A is a cross-sectional view of a semiconductor optical amplifier employing an MQW structure for the active layer, and FIG. 11B is a schematic configuration diagram of an active layer employing the MQW structure.
[0005]
On the upper surface of a semiconductor substrate 1 made of n-type InP, a light confinement layer 2 made of n-type InGaAsP, an active layer 3 made of non-doped InGaAsP, and a light confinement layer 4 made of p-type InGaAsP are sequentially stacked. The upper portion of the semiconductor substrate 1, the optical confinement layer 2, the active layer 3, and the optical confinement layer 4 form a mesa. In the active layer 3 having the MQW structure, a plurality of well layers 6 and a plurality of barrier layers 5 located on both sides of each well layer 6 are stacked.
[0006]
A lower buried layer 7 made of p-type InP is buried on both sides of the mesa portion, and an upper buried layer 8 made of n-type InP is buried on the upper surface of the lower buried layer 7. A cladding layer 9 made of p-type InP is formed to cover the upper surface of the upper buried layer 8 and the upper surface of the optical confinement layer 4 in common. Further, a contact layer 10 made of p-type InGaAsP is formed on the upper surface of the cladding layer 9, a p-electrode 11 is mounted on the upper surface of the contact layer 10, and an n-electrode 11 is mounted on the lower surface of the semiconductor substrate 1.
[0007]
In this semiconductor optical amplifier, the light amplifying action of the semiconductor laser is directly used. Specifically, the number of carriers in active layer 3 is increased by supplying a direct current between p electrode 11 and n electrode 12. Then, stimulated emission occurs in the active layer 3.
[0008]
When signal light enters from one end face of the semiconductor optical amplifier having such an optical amplifying action, the signal light is emitted from the other end face by stimulated emission inside the element until the signal light is emitted from the other end face. Light increases. The gain is defined as the ratio of the light intensity of the emitted light to the light intensity of the incident light.
[0009]
Note that Patent Document 1 discloses a detailed structure of an MQW structure in a super luminescent diode (SLD) employing an MQW structure as an active layer. Further, Non-Patent Document 1 discloses a design method of a multiple quantum well LD.
[0010]
[Patent Document 1]
JP-A-11-284223
[Non-patent document 1]
Yoshihiro Kokubo, "Design of Multiple Quantum Well LD for Optimizing the Envelope Function of Electrons", The Institute of Electronics, Information and Communication Engineers, IEICE Technical Report, 0QE93-90 (1993-09), p51-56
[0012]
[Problems to be solved by the invention]
However, the semiconductor optical amplifier adopting the MQW structure in the active layer 3 shown in FIGS. 11A and 11B has the following problems to be improved.
[0013]
That is, in the semiconductor optical amplifier, the TE mode in which the electric field of light propagating in the optical waveguide has a component in the width direction of the active layer, and the TM mode in which the magnetic field of light propagating in the optical waveguide has a component in the width direction of the active layer. There are two polarization states with the mode. In general, the gain is different for each of the two polarization states.
[0014]
On the other hand, when stress is applied to an optical fiber that propagates light incident on the semiconductor optical amplifier due to vibration, temperature change, or the like, the polarization state of light propagating in the optical fiber changes. Therefore, the gain of the optical amplification in the semiconductor optical amplifier fluctuates according to the polarization state of the incident light. Then, the intensity of the emitted light fluctuates due to the polarization dependence of the gain.
[0015]
It has been proposed to increase the thickness of the active layer 3 in order to eliminate the polarization dependence of the gain. That is, it is necessary to adopt not the MQW structure but the bulk structure made of one uniform material as the active layer 3. However, in order to eliminate the polarization dependence by adopting a bulk structure as the active layer 3, the active layer 3 needs to be formed in a rectangular shape of 0.3 to 0.4 μm, and a high precision processing technique is required. There was also a problem with the yield.
[0016]
In order to solve such inconvenience, a material in which tensile strain is applied to the well layer 6 in the MQW structure is employed in the active layer 3 employing the MQW structure. By adjusting the amount of tensile strain to be applied, the gain of the TE mode and the gain of the TM mode can be approximated, and the polarization dependence of the gain can be suppressed.
[0017]
However, when the polarization dependence of the gain in the semiconductor optical amplifier is suppressed only by adjusting the amount of tensile strain applied to the well layer 6, it is necessary to control the amount of applied tensile strain with high accuracy. And the yield of the manufactured semiconductor optical amplifier is reduced.
[0018]
The present invention has been made in view of such circumstances, and even if the amount of tensile strain applied to a well layer is not controlled with high accuracy, the polarization dependence of gain can be significantly suppressed, and the production yield can be reduced. It is an object of the present invention to provide a semiconductor optical amplifier that can be significantly improved.
[0019]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention includes a semiconductor substrate and an active layer including a multiple quantum well structure formed above the semiconductor substrate. In a semiconductor optical amplifier that is amplified in the process of propagating through an optical waveguide including an active layer formed along the surface and is emitted from the other end face, a multiple quantum well structure (MQW structure) has a plurality of sets of well layers and barrier layers. , One of the well layer and the barrier layer has a tensile strain, and the thickness of the barrier layer is set to 4 nm or less.
[0020]
In the semiconductor optical amplifier configured as described above, since a material in which tensile strain is applied to the well layer or the barrier layer in the MQW structure is employed, as described above, the tensile strain applied to the well layer or the barrier layer is used. By adjusting the amount to make the gain of the TE mode and the gain of the TM mode close to each other, the polarization dependence of the gain can be suppressed.
[0021]
Further, since the thickness of the barrier layer is set to 4 nm or less, the wave functions of the electrons confined in the adjacent well layers constituting the MQW structure are connected to each other. Due to the splitting, the quantum effect weakens. For this reason, the allowable range of the amount of tensile strain applied to the well layer or the barrier layer is widened in order to suppress the polarization dependence of the gain to a certain amount. Since the MQW structure is employed as the active layer, the excellent light characteristics of the MQW structure are not impaired.
[0022]
According to yet another aspect, in the semiconductor optical amplifier according to the above aspect, the well layer has a tensile strain, and the barrier layer has a compressive strain.
In such a semiconductor optical amplifier, by applying a compressive strain to the barrier layer, the amount of strain in the entire active layer can be reduced, so that improvement in reliability can be expected.
[0023]
Another aspect of the present invention is the semiconductor optical amplifier according to the above-described invention, further comprising a window portion formed between at least one of the end faces and the active layer by cutting off the active layer near the end face.
[0024]
By forming the window between each end face and the active layer in this manner, generation of large resonance light in the semiconductor optical amplifier is prevented.
[0025]
According to another aspect of the present invention, in the semiconductor optical amplifier of the above-described aspect, the active layer is formed to be inclined with respect to each end face.
By tilting the active layer with respect to each end face in this manner, light emitted from the end face of the active layer and reflected on each end face is prevented from being incident on the active layer again.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(1st Embodiment)
FIG. 1 is a perspective view showing a schematic configuration of a semiconductor optical amplifier according to a first embodiment of the present invention, and FIG. 2 is a cross-sectional view of the semiconductor optical amplifier shown in FIG. 1 taken along line AA ′. The same parts as those of the conventional semiconductor optical amplifier shown in FIG.
[0027]
On the upper surface of a semiconductor substrate 1 made of n-type InP, a light confinement layer 2 made of n-type InGaAsP, an active layer 13 made of non-doped InGaAsP, and a light confinement layer 4 made of p-type InGaAsP are stacked in this order. The upper portion of the semiconductor substrate 1, the light confinement layer 2, the active layer 13, and the light confinement layer 4 form a mesa. A lower buried layer 7 made of p-type InP is buried on both sides of the mesa portion, and an upper buried layer 8 made of n-type InP is buried on the upper surface of the lower buried layer 7. A cladding layer 9 made of p-type InP is formed to cover the upper surface of the upper buried layer 8 and the upper surface of the optical confinement layer 4 in common. Further, a contact layer 10 made of p-type InGaAsP is formed on the upper surface of the cladding layer 9, a p-electrode 11 is mounted on the upper surface of the contact layer 10, and an n-electrode 11 is mounted on the lower surface of the semiconductor substrate 1.
[0028]
Further, anti-reflection films 18a and 18b are formed on an end face 17a of the semiconductor optical amplifier where the signal light 19 is incident and an end face 17b where the amplified light 20 is emitted, respectively.
[0029]
FIGS. 3A and 3B are schematic configuration diagrams of the active layer 13 having the MQW structure. In the active layer 13 having the MQW structure, a plurality of well layers 14 and a plurality of barrier layers 15 located on both sides of each of the well layers 14 are stacked. Then, in this embodiment, the thickness _{t 1} of the barrier layer 15 is set to 2 nm, the thickness _{t 2} of the well layers 14 is set to 9 nm.
[0030]
Then, as shown in FIG. 3B, the refractive index of the well layer 14 in the active layer 13 is the highest, the refractive index of the barrier layer 15 is the next highest, and subsequently, the optical confinement layers 2, 4,. Further, the semiconductor substrate 1 and the cladding layer 9 follow. Therefore, the active layer 13 and the light confinement layers 2 and 4 on both sides form an optical waveguide that amplifies the light 19 incident from one end face 17a and guides the light 19 to the other end face 17b.
[0031]
Further, in this embodiment, each well layer 14 of the active layer 13 having the MQW structure has a tensile strain. As described above, the amount of tensile strain is set so that the gain in the TE mode matches the gain in the TM mode.
[0032]
In the semiconductor optical amplifier having such a configuration, by supplying a direct current between the p-electrode 11 and the n-electrode 12, the number of carriers in the active layer 13 is increased, and the gain in the active layer 13 is increased.
[0033]
In this state, when the signal light 19 is incident from one end face 17a, it is amplified by the internal gain in the active layer 13, and as a result, the light 20 emitted from the other end face 17b increases.
[0034]
Since the anti-reflection films 18a and 18b are formed on the end faces 17a and 17b, light emitted from the end face of the optical waveguide formed by the active layer 13 and the optical confinement layers 2 and 4 on both sides is emitted. Fabry-Perot resonance that is reflected at 17a and 17b and is incident again on the active layer 13 can be prevented. Therefore, the wavelength characteristic in the semiconductor optical amplifier can be improved without causing a peak at a specific wavelength of the amplified light 20.
[0035]
Next, the operation principle of the semiconductor optical amplifier configured as described above, which can suppress the polarization dependence of the gain, will be described.
In the MQW structure incorporated in the general semiconductor optical amplifier shown in FIGS. 11A and 11B as the active layer 3, the well layer 6 is usually about 4 to 12 nm, and the barrier layer 5 is confined in the well layer 6. The thickness of the electron is generally set to 8 to 12 nm so that the wave function of the electron does not overlap the wave function of the electron confined in the adjacent well layer 6.
[0036]
Therefore, in the semiconductor optical amplifier of the embodiment, the barrier layer 5 is devised so that the wave function of the electrons confined in the well layer 14 and the wave function of the electrons confined in the adjacent well layer 14 are intentionally overlapped. The thickness is set to 2 nm, which is 4 nm or less, which is greatly reduced from 4 to 12 nm. Note that if the thickness of the barrier layer is set to 4 nm or less, the wave functions of the electrons confined in the adjacent well layers are connected to each other, as already described in the above-mentioned Patent Document 1 and Non-Patent Document 1. .
[0037]
As described above, since the wave functions of the electrons confined in the plurality of well layers 14 constituting the MQW structure are connected to each other, even if the number of the well layers 14 constituting the MQW structure is increased, the The carrier uniformity does not deteriorate.
[0038]
On the other hand, since a material in which tensile strain is applied to the well layer 14 in the MQW structure is employed, the polarization dependence of gain can be reduced by adjusting the amount of tensile strain applied to the well layer 14 as described above. Can be suppressed.
[0039]
In this case, as described above, the wave functions of the electrons confined in the plurality of well layers 14 constituting the MQW structure are connected to each other. The effect is weak. Therefore, the allowable range of the amount of tensile strain applied to the well layer 14 is widened in order to suppress the polarization dependence of the gain to a certain amount.
[0040]
FIG. 4 shows a semiconductor optical amplifier employing a conventional MQW structure in which a barrier layer having a thickness of 8 to 12 nm is incorporated in an active layer, and a semiconductor employing an MQW structure in which a barrier layer 15 having a thickness of 4 nm or less is incorporated in an active layer. FIG. 9 is a diagram illustrating polarization dependence of gain when tensile strain is applied to an optical amplifier.
[0041]
As shown in the figure, in the bulk structure, the polarization dependence is due to the polarization dependence of the optical confinement coefficient. Properties can be reduced. On the other hand, when the MQW structure in which the barrier layer 14 having a thickness of 4 nm or less is incorporated in the active layer is adopted, the strain is higher than when the MQW structure in which the barrier layer having a thickness of 8 to 12 nm is incorporated in the active layer. The amount of change in gain (polarization dependent amount) with respect to the amount of change is small.
[0042]
Therefore, even if the amount of tensile strain applied to the well layer 14 is not controlled with high accuracy, the polarization dependence of the gain can be greatly suppressed, and the production yield can be greatly improved. Since the MQW structure is employed as the active layer, the excellent light characteristics of the MQW structure are not impaired.
[0043]
In addition, in addition to applying tensile strain to the well layer 14 in the MQW structure, it is also possible to apply compressive strain to the barrier layer 15. By applying compressive strain to the barrier layer, the amount of strain in the entire active layer 13 can be reduced, so that reliability can be improved.
[0044]
Furthermore, by applying tensile strain only to the barrier layer 15 in the MQW structure, the gain in the TM mode and the gain in the TE mode can be approximated.
[0045]
(2nd Embodiment)
FIG. 5 is a perspective view showing a schematic configuration of the semiconductor optical amplifier according to the second embodiment of the present invention, and FIG. 6 is a horizontal sectional view of the semiconductor optical amplifier shown in FIG. The same parts as those of the semiconductor optical amplifier of the first embodiment shown in FIGS. 1 and 2 are denoted by the same reference numerals, and detailed description of the overlapping parts will be omitted.
[0046]
In the semiconductor optical amplifier according to the second embodiment, windows 21a and 21b are provided between the end faces 17a and 17b and the active layer 13, respectively. As shown in FIG. 5, a lower buried layer 7 and an upper buried layer 8 are buried in the windows 21a and 21b.
[0047]
Since the refractive indexes of the lower buried layer 7 and the upper buried layer 8 filling the windows 21a and 21b are smaller than the refractive index of the optical waveguide including the active layer 13, the light output from the active layer 13 is a Gaussian beam. Spread according to. As a result, the density of light reflected on each end face 17a, 17b and returned to the active layer 13 again decreases according to the length of the windows 21a, 21b. Therefore, the light reflectance is equivalently reduced.
[0048]
Therefore, the wavelength characteristic of the semiconductor optical amplifier can be further improved without causing a peak at a specific wavelength of the amplified light 20.
[0049]
(Third embodiment)
FIG. 7 is a horizontal sectional view showing a schematic configuration of the semiconductor optical amplifier according to the third embodiment of the present invention. The same parts as those of the semiconductor optical amplifier according to the second embodiment shown in FIG.
[0050]
In the semiconductor optical amplifier according to the third embodiment, the active layer 13 is formed to be inclined with respect to the end faces 17a and 17b to and from which light 19 and 20 are input and output. Windows 21a and 21b are provided between the end faces 17a and 17b and the active layer 13, respectively. The lower buried layer 7 and the upper buried layer 8 are buried in the windows 21a and 21b. Further, the incident angle of the signal light 19 on the end face 17a is also inclined accordingly. Further, the emission angle of the light 20 emitted from the end face 17b is also inclined.
[0051]
In the semiconductor optical amplifier according to the third embodiment configured as described above, the active layer 13 is formed to be inclined with respect to the end faces 17a and 17b through which the lights 19 and 20 are input and output. The light emitted from the end surfaces of the first and second surfaces 17a and 17b is prevented from being incident on the active layer 13 again. That is, the reflectance of light at the end faces 17a and 17b further decreases.
[0052]
(Fourth embodiment)
FIG. 8 is a horizontal sectional view showing a schematic configuration of a semiconductor optical amplifier according to a fourth embodiment of the present invention. The same parts as those of the semiconductor optical amplifier according to the third embodiment shown in FIG.
[0053]
In the semiconductor optical amplifier according to the fourth embodiment, the active layer 13 is formed so as to be inclined with respect to the end faces 17a and 17b through which the lights 19 and 20 are input and output. In this embodiment, the windows 21a and 21b are not provided, and the end faces of the active layer 13 are in contact with the anti-reflection films 18a and 18b formed outside the end faces 17a and 17b.
[0054]
Also in the semiconductor optical amplifier of the fourth embodiment configured as described above, the light emitted from the end face of the active layer 13 and reflected by the end faces 17a and 17b (the antireflection films 18a and 18b) is again transmitted to the active layer 13. It is prevented from being incident. That is, the reflectance of light at each of the end faces 17a and 17b decreases.
[0055]
(Fifth embodiment)
FIG. 9 is a horizontal sectional view showing a schematic configuration of a semiconductor optical amplifier according to a fifth embodiment of the present invention. The same parts as those of the semiconductor optical amplifier according to the fourth embodiment shown in FIG.
[0056]
In the semiconductor optical amplifier according to the fifth embodiment, the active layer 13 includes a linear portion 13c perpendicular to each of the end faces 17a and 17b through which the light 19 and 20 are input and output, and a pair of inclined portions with respect to each of the end faces 17a and 17b. And the inclined portions 13a and 13b. The end surfaces of the inclined portions 13a and 13b are in contact with the anti-reflection films 18a and 18b formed outside the end surfaces 17a and 17b.
[0057]
Also in the semiconductor optical amplifier of the fifth embodiment configured as described above, the light emitted from the end face of the active layer 13 and reflected by the end faces 17a and 17b (each of the antireflection films 18a and 18b) is again transmitted to the active layer 13. It is prevented from being incident. That is, the reflectance of light at each of the end faces 17a and 17b decreases.
[0058]
(Sixth embodiment)
FIG. 10 is a horizontal sectional view showing a schematic configuration of a semiconductor optical amplifier according to a sixth embodiment of the present invention. The same parts as those of the semiconductor optical amplifier according to the fifth embodiment shown in FIG.
[0059]
In the semiconductor optical amplifier according to the sixth embodiment, the pair of inclined portions 13a and 13b in the active layer 13 are inclined in directions opposite to each other.
Also in the semiconductor optical amplifier of the fifth embodiment configured as described above, the light emitted from the end face of the active layer 13 and reflected by the end faces 17a and 17b (each of the antireflection films 18a and 18b) is again transmitted to the active layer 13. It is prevented from being incident. That is, the reflectance of light at each of the end faces 17a and 17b decreases.
[0060]
【The invention's effect】
As described above, in the semiconductor optical amplifier of the present invention, the well layer or the barrier layer of the MQW structure in the active layer has a tensile strain, and the thickness of the barrier layer is set to 4 nm or less. Therefore, even if the amount of tensile strain applied to the well layer or the barrier layer is not controlled with high accuracy, the polarization dependence of the gain can be significantly suppressed without impairing the excellent optical characteristics of the MQW structure, and the manufacturing Yield can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a schematic configuration of a semiconductor optical amplifier according to a first embodiment of the present invention; FIG. 2 is a cross-sectional view of the semiconductor optical amplifier of the first embodiment shown in FIG. FIG. 3 is a detailed configuration diagram of an active layer in the semiconductor optical amplifier according to the first embodiment; FIG. 4 is a diagram illustrating characteristics of polarization dependence of gain of the semiconductor optical amplifier according to the present invention; FIG. 6 is a perspective view showing a schematic configuration of a semiconductor optical amplifier according to a second embodiment of the present invention. FIG. 6 is a horizontal sectional view of the semiconductor optical amplifier according to the second embodiment shown in FIG. FIG. 8 is a horizontal sectional view showing a schematic configuration of a semiconductor optical amplifier according to a third embodiment of the present invention. FIG. 8 is a horizontal sectional view showing a schematic configuration of a semiconductor optical amplifier according to a fourth embodiment of the present invention. FIG. 10 is a horizontal sectional view showing a schematic configuration of a semiconductor optical amplifier according to a fifth embodiment of the present invention. Cross-sectional view showing a schematic configuration of a horizontal cross-sectional view [11] Conventional semiconductor optical amplifier showing a schematic structure of a semiconductor optical amplifier according to a sixth embodiment [Description of symbols]
DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate, 2, 4 ... Light confinement layer, 3, 13 ... Active layer, 5, 15 ... Barrier layer, 6, 14 ... Well layer, 7 ... Lower buried layer, 8 ... Upper buried layer, 9 ... Clad layer, 10 ... Contact layer, 11 ... P electrode, 12 ... N electrode, 17a, 17b ... End face, 18a, 18b ... Non-reflective film, 19 ... Signal light, 20 ... Light, 21a, 21b ... Window

Claims (4)

A semiconductor substrate (1) and an active layer (13) including a multiple quantum well structure formed above the semiconductor substrate, and light (19) incident from one end face (17a) is transmitted in a light propagation direction. In the semiconductor optical amplifier which is amplified in the process of propagating through the optical waveguide including the active layer (13) formed along and exits from the other end face (17b),
The multiple quantum well structure has a plurality of sets of well layers (14) and barrier layers (15), one of the well layers and barrier layers has a tensile strain, and the thickness of the barrier layers is 4 nm. A semiconductor optical amplifier set as follows. 2. The semiconductor optical amplifier according to claim 1, wherein said well layer has a tensile strain and said barrier layer has a compressive strain. Windows (21a, 21b) formed between at least one of the end faces (17a, 17b) and the active layer (13) by breaking the active layer near the end faces (17a, 17b). 3. The semiconductor optical amplifier according to claim 1, comprising: 4. The semiconductor optical amplifier according to claim 1, wherein the active layer is formed to be inclined with respect to each of the end faces. 5.

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