JPH1022573A - Multiple quantum well semiconductor light device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the energy at the end of a band by constituting a quantum well layer out of an InAs layer of two-atom layer or under and a semiconductor layer smaller in lattice constant than the substrate, and making the lattice constant smaller than that of the substrate. SOLUTION: For a quantum well active layer of a laser which oscillates in TM mode, the well layer is made an In0.25 Ga0.75 As layer which has a lattice constant smaller than that of an InP substrate, and InAs layers are inserted periodically by three layers into the layer. The energy at the end of a band becomes smaller than In0.4 Ga0.6 As being average composition by the insertion of InAs's 3, 5, and 7 being one-atom layers. Hereby, only the energy at the end of the band becomes small, and desired TM gain with a relatively narrow well width can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization-modulated semiconductor laser, a polarization-independent semiconductor optical amplifier capable of switching the polarization of output light requiring substantially equal gain in both TE and TM modes, and a polarization-independent semiconductor optical amplifier. The present invention relates to a semiconductor device such as a laser, and a communication system using the same.

[0002]

2. Description of the Related Art Generally, in a polarization-modulated semiconductor laser or a polarization-independent semiconductor optical amplifier, it is necessary to make gains of a TE mode and a TM mode substantially equal. In this regard, TE
An example in which a polarization-independent semiconductor optical amplifier of about 1.3 μm band is constituted by a laminated structure of a compressive strain quantum well in which gain is dominant and a tensile strain quantum well in which TM gain is dominant is disclosed in Applied.
Physics Letters (Appl. Phys. Let
t. ) Vol. 62, 826 (1993).

[0003] In this example, the TE gain structure has a compression strain of 1%,
It is composed of an InGaAsP layer having a well width of 4.5 nm, and TM
InG with 1% tensile strain and 11 nm well width
It is composed of an aAsP layer. Set the TM gain peak wavelength to TE
In order to make the gain peak wavelength equal to the following, the following is performed. That is, in order to make the TM gain dominant, it is necessary to increase the Ga composition of InGaAsP serving as a quantum well layer to introduce tensile strain, but at the same time, the band edge energy of InGaAsP also increases. Therefore, T
In order to make the gain peak wavelengths of E and TM almost coincide with each other, T
The well width of the M gain structure is set wide (1 as described above).
1 nm).

Further, in order to antagonize the gains of TE and TM, a polarization-independent active layer is formed by using four TE gain wells and three TM gain wells.

[0005]

That is, in order to make the gain peak wavelengths of TE and TM substantially equal and to antagonize the gain of both modes, the width and number of wells of each are limited. Therefore, it is impossible to use an optimum well width and the number of wells as an active layer of a polarization modulation laser or a polarization independent amplifier.

When the Ga composition of the quantum well layer is increased to introduce tensile strain into the well layer, the band edge energy of the well layer increases, so that the energy difference between the conduction band of the well layer and the conduction band of the barrier layer decreases. Also, there is a problem that the injected electrons overflow from the well layer and the reactive current increases.

[0007]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optimum well width and the number of wells for an active layer of a polarization modulation laser, a polarization independent amplifier, etc. An object of the present invention is to provide a semiconductor device having a configuration, a communication system using the same, and the like.

First, the principle of the present invention will be described. An example in which the superlattice structure is composed of InAs and GaAs having two or less atomic layers is described in Japanese Journal of Applied Physics (Japanese Journal of Applied Physics).
Applied Physics) Vo1.23, L
521 (1984), and FIG. 6 is a schematic diagram showing the relationship between the band edge energy of the above structure and the number of layers (n) of InAs and GaAs. In FIG.
Since the number of layers of nAs and GaAs is the same, the average composition is In _0.5 Ga _0.5 As regardless of the number of layers. Therefore, the averaged strain is about −0.2% of the tensile strain with respect to InP. . On the other hand, the band edge energy has a remarkable tendency to rapidly decrease when InAs has a thickness of one atomic layer (n ≧ 1.0) or more. Therefore, even if the average strain is the same by inserting InAs as thin as one atomic layer, the band edge energy can be reduced.

[0009] Each constitution of the present invention which achieves the above object based on the above principle will be described in accordance with each claim.

A first configuration of the present invention (corresponding to claim 1)
Discloses that a quantum well layer is composed of an InAs layer of 2 atomic layers or less (preferably about 1 atomic layer) and a semiconductor layer sandwiching the InAs layer and having a smaller lattice constant than a substrate. The semiconductor device is characterized in that the TM gain is dominant by making the lattice constant of the layer smaller than the lattice constant of the substrate. That is, in a semiconductor device having a multiple quantum well structure formed on a semiconductor substrate as an active layer, at least one quantum well layer sandwiches this layer with two or less atomic layers of InAs and has a smaller lattice constant than the substrate. A semiconductor device comprising a semiconductor layer, wherein a lattice constant when the quantum well layer is regarded as a uniform composition layer is smaller than a lattice constant of a substrate.

Here, the thickness of the InAs layer does not need to be an integral multiple of one atomic layer.
The s layer may be in a state where the thickness is partially different or in a state where the s layer is not partially covered (this is controlled by the material supply amount).

With this configuration, the average composition and strain state of the quantum well layer are almost the same, and only the band edge energy can be reduced.
Gain. As a result, it is possible to increase the output of the TM oscillation laser, stabilize the high-temperature operation, improve the production yield of the polarization modulation laser, stabilize the switching characteristics, and achieve the polarization-independent characteristics that do not depend on the bias conditions of the polarization-independent optical amplifier. What you get.

A second configuration of the present invention (corresponding to claim 2)
Is to realize a semiconductor device in a wavelength band used in optical communication or the like by using InP as a substrate and using InGaAs or InGaAsP as a semiconductor layer sandwiching two or less atomic layers of InAs.

A third configuration of the present invention (corresponding to claim 3)
Is a device in which an arbitrary well width can be used as necessary by periodically inserting two or less atomic layers of InAs into an InGaAs or InGaAsP layer. That is, the semiconductor optical device is characterized in that the InAs layer having two or less atomic layers is periodically inserted into the semiconductor layer sandwiching the InAs layer having two or less atomic layers.

A fourth configuration of the present invention (corresponding to claim 4)
Discloses a technique in which a quantum well layer in which the TE gain is dominant is constituted by a strain-free or compressive strain well, and the characteristics of the device are improved by combining the quantum well layer in which the TM gain is dominant. In order to make the TE gain dominant, at least one other quantum well layer is formed of a semiconductor layer having a lattice constant substantially equal to or larger than the lattice constant of the substrate.

A fifth configuration of the present invention (corresponding to claim 5)
Is a semiconductor optical device, wherein a plurality of the two types of quantum well layers are respectively arranged with a separation layer interposed therebetween in the stacking direction. Thereby, a suitable polarization modulation laser is formed.

A sixth configuration of the present invention (corresponding to claim 6)
Is a semiconductor optical device, wherein the two types of quantum well layers are alternately arranged in the stacking direction. Thereby, a suitable polarization-independent optical amplifier is configured.

A seventh configuration of the present invention (corresponding to claim 7)
In the resonator direction, a region where the TM gain is superior and a region where the TE gain is superior are arranged in the direction of the resonator, and the polarization modulation and polarization-independent characteristics are further improved by controlling the current injected independently into each region. It is to improve. That is, the light propagation direction of the semiconductor device having the multiple quantum well as the active layer is constituted by the first and second regions, and at least one quantum well layer of the quantum well active layer in the first region is composed of two atoms. Or less, and a semiconductor layer sandwiching this layer and having a smaller lattice constant than the substrate. Further, when the quantum well layer is regarded as a uniform composition layer, the lattice constant is smaller than the lattice constant of the substrate. Wherein at least one quantum well layer of the quantum well active layer in the second region is formed of a semiconductor layer having a lattice constant substantially equal to or larger than the lattice constant of the substrate. Semiconductor optical device.

Eighth configuration of the present invention (corresponding to claim 8)
Is to introduce a strain reverse to that of the well layer into the barrier layer of the quantum well active layer of the device, thereby compensating for the strain in the well layer and making it possible to use a large strain or a thick strained well.
Thereby, the thickness of the quantum well active layer can be increased, and mode gain control and the like can be performed. That is, a semiconductor optical device is characterized in that a strain is introduced into a barrier layer constituting the multiple quantum well layer to form a strain compensation structure.

A ninth configuration of the present invention (corresponding to claim 9)
Is a polarization modulation laser in which the semiconductor optical device switches the polarization state of laser oscillation between TE and TM by an injection current.

According to a tenth aspect of the present invention, there is provided a polarization-modulating semiconductor laser and one of two polarization modes of TE and TM among the light emitted from the polarization-modulating semiconductor laser. A light source device comprising: a polarization selecting unit such as a polarizer that extracts only light due to oscillation.
As a result, an intensity-modulated signal with less chirping can be obtained even during high-speed modulation.

An eleventh configuration of the present invention (corresponding to claim 11) is a polarization-modulated semiconductor laser and one of two polarization modes of TE and TM among the light emitted from the polarization-modulated semiconductor laser. An optical transmitter including a light source device including a polarization selection unit such as a polarizer that extracts only light by oscillation, a transmission unit that transmits the light extracted by the polarization selection unit, and a light unit that transmits the light transmitted by the transmission unit. An optical communication system comprising an optical receiver for receiving.

A twelfth configuration (corresponding to claim 12) of the present invention is characterized in that the optical receiver includes a semiconductor device configured as the polarization-independent optical amplifier.

According to a thirteenth aspect of the present invention, there is provided a polarization-modulating semiconductor laser and one of two polarization modes of TE and TM among the light emitted from the polarization-modulating semiconductor laser. By using a light source device including a polarization selector such as a polarizer that extracts only light due to oscillation, superimposing a current modulated in accordance with a transmission signal on a predetermined bias current and supplying the current to the semiconductor laser, An optical communication method characterized by extracting signal light whose intensity has been modulated in accordance with a transmission signal from a selection unit, and transmitting the signal light to an optical receiver.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment FIG. 1 shows a quantum well active layer of a laser oscillating in a TM mode according to a first embodiment of the present invention. The well layer is formed of an In _0.25 Ga _0.75 As layer having a smaller lattice constant than that of an InP substrate. In this layer, one atomic layer of InAs layer is periodically inserted in three layers.

In FIG. 1, reference numerals 1 and 9 denote InGaAs having a band edge wavelength of about 1.15 μm and a thickness of 10 nm, which are barrier layers.
sP, 2, 8 are two atomic layers of In _0.25 Ga _0.75 As, 3,
5 and 7 are InAs of one atomic layer, and 4 and 6 are InAs of 4 atomic layer.
_0.25 Ga _0.75 As. The whole well layer is made of In _0.25 Ga
Since it is composed of 12 atomic layers of _{0.75 As and} 3 atomic layers of InAs, the well width is about 4.5 nm, and the average composition is In _0.4 G.
a _0.6 As, which is about 1% tensile strain with respect to InP.

On the other hand, the band edge energy is changed to the average composition of In _0.4
Because it is about 30 meV smaller than Ga _0.6 As,
An oscillation wavelength of approximately 1.55 μm is obtained.

Heretofore, a well layer having a tensile strain of about 1% has been formed with an average composition of In _0.4 Ga _0.6 As, and has an oscillation wavelength of 1.
To obtain 55 μm, a well width of 10 nm or more was required, and a single quantum well structure had to be formed due to the limitation of the critical film thickness.

However, according to the present embodiment, 1% tensile strain can be realized with a well width of about 4.5 nm, so that a multiple quantum well structure can be realized within the limit of the critical film thickness, and high output can be realized. Become. In addition, since the band edge energy of the well layer does not increase, even if tensile strain is introduced, the energy difference between the conduction band of the well layer and the conduction band of the barrier layer hardly changes.
Since the overflow of the injected electrons from the well layer is suppressed, the threshold current value can be reduced and the device can be used at a high temperature.

The structure other than the active layer of this embodiment is the same as that of a normal semiconductor laser.
If it is a type, it will be as shown in FIG.

Second Embodiment FIG. 2 is a cross-sectional perspective view of a polarization modulation laser of the present invention having a quantum well as an active layer in the direction of the resonator.

In the figure, reference numeral 21 denotes a lower common electrode;
Denotes an n-InP substrate and an n-InP lower cladding layer, 23
Is a high-resistance InP buried layer, 24 is an InGaAsP layer having a band-edge wavelength of approximately 1.15 μm serving as an optical waveguide layer, and 25 is T
FIG. 4 shows a multiple quantum well active layer including an M gain quantum well and a TE gain quantum well.

In FIG. 4, the TE / TM separation layer 411
Is about 1.15 μm, the same band edge wavelength as the barrier layer 410.
m, an InGaAsP layer having a thickness of 50 nm, and the quantum well 412 having the TM gain has a tensile strain of about 1% as described above in the configuration shown in FIG. On the other hand, the configuration of the TE gain quantum well 413 is the configuration shown in FIG. 3, and reference numerals 31 and 33 denote barrier layers (InGaAsP having a band edge wavelength of approximately 1.15 μm and a thickness of 10 nm), 32
Is In _0.68 Ga _0.32 As with a thickness of 4.5 nm to be a well layer.
Thus, a compression strain of about 1% is introduced.

Further, reference numeral 26 denotes an InGaAsP layer serving as an upper optical waveguide layer having a band edge wavelength of approximately 1.15 μm, reference numeral 211 denotes a diffraction grating formed on the upper optical waveguide layer 26 at a pitch of approximately 240 nm, and reference numeral 27 denotes a p-InP upper cladding layer. , 28 is p-
InGaAs contact layers 29 and 210 are electrodes for current injection. Although not shown in FIG. 2, a low-reflection film of SiN is formed on both end faces to reduce light returning from the cleaved end faces.

In the above configuration, when a current is applied between the electrodes 21 and 29 to inject carriers into the active layer 25, the electron energy is small in the low injection region, so that the TE gain well 413 (near the n-InP substrate 22) is used. And the gain of the TE mode is increased. Increasing the injection level increases the energy of the electrons,
The injection of TE gain well 413 decreases and the injection of TM gain into well 412 increases.

The injection current between the electrodes 21 and 29 is represented by TE and T
The gain of M is kept almost balanced, and the other electrode 2
When a current flows between 1 and 210, the TE gain increases due to the low injection (I ₂ ) (for the above-mentioned reason), and the laser oscillation in the TE mode occurs together with the gain on the electrode 29 side. Further, when a current is injected (I ₂ + ΔI), the laser oscillation in the TE mode stops with a decrease in the TE gain and an increase in the TM gain,
Switching to laser oscillation in TM mode.

Therefore, an appropriate bias current (I ₂ )
Laser oscillation with a small modulation current (△ I) under TE / T
Switching between M modes, a polarization-modulated laser having a narrow spectral line width can be obtained even in high-speed modulation.

In this embodiment, the width of the TM gain well 412 is 4.5 nm, which is the same as the TE gain well 413.
The quantum effect is also almost the same, and the gains of TE and TM are almost the same. Therefore, the production yield of the polarization modulation laser is greatly improved, and the stability of the switching characteristics between the TE / TM modes of laser oscillation is increased. did.

Third Embodiment FIG. 5 is a structural view of an active layer portion of a polarization independent optical amplifier using a quantum well according to the present invention. The barrier layer 511 has a band edge wavelength of approximately 1.15 μm and a thickness of 10 nm. InGaAs
P, and four TM gain quantum wells and three TE gain quantum wells are alternately stacked.

The TM gain quantum well 512 has the configuration shown in FIG. 1 and has a tensile strain of about 1%. On the other hand, the quantum well 513 having the TE gain has the configuration shown in FIG. 3, and the well layer is made of In _0.68 G having a thickness of 4.5 nm.
At a _0.32 As, the compression strain is about 1%.

In FIG. 5, since the quantum wells 513 of TE gain and the quantum wells 512 of TM gain having the same well width are arranged substantially symmetrically, the state of low injection with small electron energy is changed to the high injection state with large electron energy. Until the state (below the oscillation threshold), the ratio of electrons injected into both the TE / TM wells 513 and 512 is substantially constant, so that the gain relationship between the TE mode and the TM mode does not change depending on the bias condition.
The polarization independence of the polarization independent optical amplifier has been improved.

Fourth Embodiment In a fourth embodiment of the present invention, a region where the TM gain is superior (for example, a region having an active layer including a plurality of wells shown in FIG. 1) and a TE gain are serially arranged in the resonator direction. Advantageous areas (eg, FIG. 2
(A region having an active layer including a plurality of wells shown in FIG. 3), and electrodes are provided in each region so that current can be independently injected. By controlling these injection currents, polarization modulation (when operating as a polarization modulation laser. If more current is injected into a region where TM gain is dominant, TM oscillation occurs,
If more current is injected into the region where TE gain is dominant, TE
Oscillation occurs. ) And polarization-independent (when operated as a polarization-independent optical amplifier; the ratio of electrons injected into the wells of both TE / TM active layers is made substantially constant). It is.

Fifth Embodiment FIG. 7 shows a wavelength division multiplexed optical LAN for a semiconductor laser according to the present invention.
A configuration example of an optical-electrical conversion unit (node) connected to each terminal when applied to a system is shown. FIGS. 8 and 9 show a configuration example of an optical LAN system using the node 701.

The optical signal is taken into the node 701 by using the optical fiber 700 connected to the outside as a medium,
A part of the light enters a receiving device 703 including a wavelength tunable optical filter and the like. The receiver 703 extracts only an optical signal having a desired wavelength and performs signal detection. This is processed by an appropriate method by the control circuit and sent to the terminal. The receiver 703 can receive weak signal light by using the polarization-independent optical amplifier of the above embodiment as a booster amplifier. On the other hand, when transmitting an optical signal from the node 701, the polarization modulation semiconductor laser 704 of the above-described embodiment is driven by a control circuit in an appropriate method according to the signal, polarization-modulated, and then polarized by the polarization plate 707 (therefore, polarization). The output light (which may further include an isolator) is made to enter the optical transmission line 700 via the junction 706 through the wave modulation signal being converted into the amplitude intensity modulation signal. Further, by providing a plurality of semiconductor lasers and two or more wavelength tunable optical filters, the wavelength tunable range can be expanded. Also, the polarizer 707 is eliminated,
As the laser 704, the TM oscillation laser of the first embodiment may be used.

As a network of the optical LAN system,
FIG. 8 shows a bus type in which nodes 801 to 805 are connected in directions A and B, and a large number of networked terminals and centers 811 to 815 can be installed. However, in order to connect a large number of nodes, it is necessary to arrange optical amplifiers in series on the transmission line 800 to compensate for optical attenuation. In addition, each terminal 811 to 815
By connecting two nodes 801 to 805 to each other and using two transmission paths, bidirectional transmission by the DQDB method becomes possible. Further, as a network system, a loop type (shown in FIG. 9) in which A and B in FIG. 8 are connected, a star type, or a combination thereof may be used.

In FIG. 9, reference numeral 900 denotes an optical transmission line;
906 are optical nodes, and 911 to 914 are terminals.

[0047]

As described above, according to the present invention,
The following effects are obtained.

The quantum well layer having the TM gain is composed of an InAs layer of 2 atomic layers or less and a semiconductor layer sandwiching this layer and having a lattice constant smaller than that of the substrate. By making the lattice constant when considered smaller than the lattice constant of the substrate, the state of strain is almost the same as that in the case where the quantum well layer is formed of the semiconductor layer having the same average composition. A desired TM gain can be obtained with a narrow well width.

By using this quantum well layer as the active layer of the TM oscillation laser, high output and high-temperature operation can be achieved. Further, by using the polarization modulation laser as the TM gain layer of the active layer, it is possible to improve the production yield and to stabilize the switching characteristics. Furthermore, by using the semiconductor optical amplifier as the TM gain layer of the active layer, it has become possible to make the polarization independence such that the gain relationship between the TE mode and the TM mode does not change depending on the bias condition.

Further, by using the above-mentioned polarization-modulated semiconductor laser and polarization selecting means, it is possible to easily realize a light source device with small chirping suitable for a communication system handling an intensity-modulated signal.

Also, an optical transmitter having the above light source device,
A high-performance optical communication system that handles an intensity-modulated signal including a transmission unit and an optical receiver (here, the polarization-independent optical amplifier of the present invention may be used) for receiving light transmitted by the transmission unit is realized. it can.

Further, by controlling the polarization-modulated semiconductor laser according to the transmission signal using the light source device described above, the signal light whose intensity has been modulated according to the transmission signal is extracted from the polarization selecting means. An optical communication method for transmitting signal light to an optical receiver (the polarization-independent optical amplifier of the present invention may be used here) is surely achieved.

[Brief description of the drawings]

FIG. 1 is a diagram of a quantum well structure having a TM gain of the present invention.

FIG. 2 is a sectional perspective view of a polarization modulation laser to which the present invention is applied.

FIG. 3 is a diagram of a conventional quantum well structure having a TE gain.

FIG. 4 is a diagram of an active layer structure of a polarization modulation laser to which the present invention is applied.

FIG. 5 is a diagram of an active layer structure of a polarization independent optical amplifier to which the present invention is applied.

FIG. 6 is the source of the present invention (InAs)
FIG. 9 is a diagram showing a state of a change in band edge energy of an _n (GaAs) _n structure.

FIG. 7 is a schematic diagram showing a configuration example of a node in the systems of FIGS. 8 and 9;

FIG. 8 is a schematic diagram showing a configuration example of a bus-type optical LAN system using the semiconductor device of the present invention.

FIG. 9 is a schematic diagram showing a configuration example of a loop type optical LAN system using the semiconductor device of the present invention.

[Explanation of symbols]

1,9,31,33,410,511 InGaAsP 2,82 as a barrier layer Tensile strain InGaAs of an atomic layer of 4,82, Tensile strain InGaAs of an atomic layer of 64, InAs 32 of an atomic layer of 3,5,71 Incom 32 compressive strain of InGaAs 21, 29, 210 Electrode 22 InP substrate and InP cladding layer 23 High-resistance InP buried layer 25 Quantum well active layer 24, 26 InGaAsP 27 optical guiding layer InP cladding layer 28 Contact layer 211 Diffraction grating 700, 800, 900 Optical transmission path Reference numerals 701, 801 to 805, 901 to 906 Node 702 Optical branching unit 703 Receiver 704 Semiconductor laser 706 of the present invention 706 Junction unit 707 Polarizer 811 to 815, 911 to 916 Terminal

Claims (13)

[Claims] In a semiconductor device having a multiple quantum well structure formed on a semiconductor substrate as an active layer, at least one quantum well layer sandwiches this layer with two or less atomic layers of an InAs layer and has a lattice structure formed by the substrate. A semiconductor optical device comprising a semiconductor layer having a small constant, wherein a lattice constant when the quantum well layer is regarded as a uniform composition layer is smaller than a lattice constant of a substrate. 2. The semiconductor device according to claim 1, wherein the semiconductor substrate is an InP substrate, and the semiconductor layer sandwiching the InAs layer of 2 atomic layers or less is InG.
2. The semiconductor optical device according to claim 1, wherein the device is aAs or InGaAsP. 3. The semiconductor optical device according to claim 1, wherein the InAs layer having a thickness of 2 atomic layers or less is periodically inserted into the semiconductor layer sandwiching the InAs layer having a thickness of 2 atomic layers or less. device. 4. An active layer having a multiple quantum well structure, wherein at least one quantum well layer other than the quantum well layer is formed of a semiconductor layer having a lattice constant substantially equal to or larger than a lattice constant of a substrate. 2. The semiconductor optical device according to claim 1, wherein: 5. The semiconductor optical device according to claim 4, wherein a plurality of said two types of quantum well layers are respectively arranged with a separation layer interposed therebetween in a stacking direction. 6. The semiconductor optical device according to claim 4, wherein said two types of quantum well layers are alternately arranged in a stacking direction. 7. The light propagation direction of a semiconductor device having a multiple quantum well as an active layer is constituted by first and second regions, and at least one of the quantum well active layers in the first region has a quantum well layer. An InAs layer of 2 atomic layers or less and a semiconductor layer sandwiching this layer and having a lattice constant smaller than that of the substrate. Further, when the quantum well layer is regarded as a uniform composition layer, the lattice constant of the substrate is the lattice constant of the substrate. At least one of the quantum well active layers in the second region is made of a semiconductor layer having a lattice constant substantially equal to or larger than the lattice constant of the substrate. A semiconductor optical device characterized by the above-mentioned. 8. The semiconductor optical device according to claim 1, wherein a strain is introduced into a barrier layer constituting said multiple quantum well layer to form a strain compensation structure. 9. The semiconductor optical device according to claim 1, wherein the polarization state of laser oscillation is switched between TE and TM by an injection current to function as a polarization modulation laser. 10. The polarization-modulated semiconductor laser according to claim 9, and TE out of the light emitted from the polarization-modulated semiconductor laser.
And a polarization selecting means such as a polarizer for extracting only light generated by one of two polarization modes of TM. 11. The polarization-modulated semiconductor laser according to claim 9, wherein TE out of the light emitted from the polarization-modulated semiconductor laser is TE.
An optical transmitter comprising a light source device comprising: a polarization selecting means such as a polarizer for extracting only light generated by one of the two polarization modes of TM and TM; and a transmission means for transmitting the light extracted by the polarization selecting means. And an optical receiver for receiving the light transmitted by the transmission means. 12. An optical communication system according to claim 11, wherein said optical receiver includes a semiconductor optical device configured as the polarization independent optical amplifier according to claim 6. 13. The polarization-modulated semiconductor laser according to claim 9, wherein TE out of the light emitted from the polarization-modulated semiconductor laser is TE.
And a polarization selecting means such as a polarizer that extracts only light generated by one of the two polarization modes of the TM and a polarization mode, and superimposes a current modulated according to a transmission signal on a predetermined bias current. An optical communication method comprising: extracting a signal light whose intensity is modulated according to a transmission signal from the polarization selecting unit by supplying the signal light to the semiconductor laser; and transmitting the signal light to an optical receiver.