JPH1027948A - Semiconductor optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor optical device such as a polarization modulation laser whose polarization can be switched with a low modulation current and a light amplifier whose polarization independence can be improved. SOLUTION: In a semiconductor laser structure, an active laser 3 employs a multilayer structure which contains a quantum well structure which has a semiconductor of a lattice constant smaller than a substrate 2 as a well layer and a quantum well structure which has a semiconductor of a lattice constant equal to or above that of the substrate 2 as a well layer. A plurality of regions into which a current can be separately injected is constituted, and a loss part in which current injection is partially limited so as to caused loss is formed in one of the areas. By injecting the current into a plurality of region, a region in which the TE gain is dominant and a region in which the TM gain is dominant are formed, and polarization switching of laser oscillation is made possible with a small modulation current.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical device such as a semiconductor laser having a small wavelength chirping even at the time of high-speed modulation used in optical communication, and a communication system using the same. The present invention relates to a polarization-modulated laser in which oscillating TE polarization and TM polarization are switched.

[0002]

2. Description of the Related Art Generally, in a polarization modulation laser, it is necessary to make gains of a TE mode and a TM mode substantially equal. In this regard, a compression strained quantum well with a superior TE gain and a TM
Japanese Patent Application Laid-Open No. 4-346485 describes an example in which a polarization-modulated laser is constituted by a laminated structure of tensile strain quantum wells in which gain is superior.

[0003] An example in which a polarization-independent semiconductor optical amplifier of the 1.3-µm band is constituted by a laminated structure of a compression strain quantum well in which TE gain is superior and a tensile strain quantum well in which TM gain is superior is disclosed in Applied.・ Physics Letters (App
l. Phys. Lett. ) Vol. 62,826 (1
993).

In this example, a TE gain is secured by an InGaAsP layer having a compressive strain of 1% and a well width of 4.5 nm, and a TM gain is obtained by an InGaAsP layer having a tensile strain of 1% and a well width of 11 nm.
Secured by layers. Further, in order to antagonize the gain between TE and TM, the TE gain well has four layers and the TM gain well has three layers.
The layers constitute a polarization-independent active layer.

[0005]

However, when the injection current is modulated in order to switch the polarization of the laser oscillation light in the active layer structure, the magnitude relationship between the TE / TM gains is reversed with a small amplitude modulation current. Can not do. This is because, because of the same active layer, the amount of increase or decrease in current is not injected selectively into either the compressive strain quantum well or the tensile strain quantum well, but is injected substantially equally. Therefore, in order to reverse the magnitude relationship between the gains, a large modulation current is required to cause uneven current injection in each well.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a polarization-modulated laser capable of polarization switching with a small modulation current, a semiconductor optical device such as an optical amplifier capable of improving polarization independence by current control, and the like. An object of the present invention is to provide a communication system and the like using the same.

[0007]

The structure of the present invention will be described below according to each claim.

According to a first structure of the present invention, there is provided a quantum well structure in which an active layer has a well layer of a semiconductor having a lattice constant smaller than that of a semiconductor substrate, and a well having a lattice constant equal to or larger than the lattice constant of the semiconductor substrate. It has a multilayer structure of a quantum well structure as a layer, and is constituted by a plurality of regions in which current can be independently injected in the resonator direction, and current injection is partially limited in one of the plurality of regions. Thus, a loss portion where a loss occurs is formed (corresponding to claims 1 and 2). In this configuration, by injecting current into the plurality of regions, TE
A region where the gain is dominant and a region where the TM gain is dominant are formed,
This enables polarization switching of laser oscillation with a small modulation current.

[0009] In a second configuration of the present invention, one of the plurality of regions is provided.
A current non-injection portion in which no current is injected is provided in one of the two regions to serve as a loss portion (corresponding to claim 3).

[0010] In a third aspect of the present invention, one of the plurality of regions is provided.
The portion in which the current is not injected in the two regions is formed by an insulating layer disposed immediately below the electrode (corresponding to claim 4).

According to a fourth aspect of the present invention, the current non-injection portion forming the partial loss portion is formed by a groove separating the electrodes (corresponding to claim 5).

According to a fifth aspect of the present invention, an electrode is provided so that current can be independently injected into the loss portion, and the loss can be controlled by controlling current injection to the electrode.
Corresponding to).

According to a sixth aspect of the present invention, an electrode provided in the loss section is connected to the same drive power supply as another electrode in the same area via a resistor, so that a power supply for loss control is not required. (Corresponding to claim 7).

In a seventh configuration of the present invention, a diffraction grating is formed in the optical waveguide layer disposed near the active layer, and at least one of two orthogonal polarizations of the laser oscillation light is a single mode. (Corresponding to claim 8).

An eighth structure of the present invention is characterized in that at least a part of the barrier layer of the active layer is doped with a p-type impurity (corresponding to claim 9).

According to a ninth aspect of the present invention, there is provided a semiconductor device configured as the above-mentioned polarization-modulated semiconductor laser, and one of two polarization modes, TE and TM, of light emitted from the semiconductor device. A light source device comprising a polarization selecting means such as a polarizer for taking out only a light source (corresponding to claim 10). As a result, an intensity-modulated signal with less chirping can be obtained even during high-speed modulation.

According to a tenth aspect of the present invention, there is provided a semiconductor device configured as the above-mentioned polarization-modulated semiconductor laser, and one of two polarization modes, TE and TM, of light emitted from the semiconductor laser. An optical transmitter including a light source device including a polarization selection unit such as a polarizer that extracts only a light, a transmission unit that transmits the light extracted by the polarization selection unit,
And an optical receiver for receiving the light transmitted by the transmission means.

According to an eleventh aspect of the present invention, the optical receiver includes a semiconductor device configured as the above-mentioned polarization independent optical amplifier (corresponding to claim 12).

According to a twelfth aspect of the present invention, there is provided a semiconductor device configured as the above-described polarization-modulated semiconductor laser, and one of two polarization modes, TE and TM, of light emitted from the semiconductor laser. By using a light source device including a polarization selector such as a polarizer that extracts only the light, by superimposing a current modulated in accordance with a transmission signal on a predetermined bias current and supplying the current to the semiconductor laser, Take out the signal light intensity-modulated according to the transmission signal,
An optical communication method characterized by transmitting the signal light to an optical receiver (corresponding to claim 13).

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment FIG. 1 is a perspective view including a partial cross section of a first embodiment of a DFB polarization modulation laser having a ridge structure of two electrodes according to the present invention. In the figure, 1 is a lower common electrode, 2 is n-InP
The substrate and the n-InP lower cladding layer 3 are a multiple quantum well active layer including a TM gain quantum well and a TE gain quantum well, and the configuration is shown in FIG.

In FIG. 2, the configuration of the TE gain quantum well is as follows. The barrier layer 210 has a band edge wavelength of 1.15 μm, an InGaAsP layer having a thickness of 10 nm, and the well layer 211 has a thickness of 4.0 nm.
Approximately 1% compressive strain is introduced by 5 nm of In _0.68 Ga _0.32 As (as shown in FIG. 3, the barrier layer 210 is formed so that the hole is sufficiently filled also in the TE gain well 211 away from the p-side. (It is partially Be-doped.) The TE / TM separation layer 213 has the same band edge wavelength as that of the barrier layer 210, ie, 1.
It is an InGaAsP layer having a thickness of 15 μm and a thickness of 50 nm (as shown in FIG. 3, the TE gain well 2 where the hole is away from the p-side).
11 is Be-doped so as to be sufficiently satisfied.
The quantum well of the TM gain has a barrier layer 220 having the same band edge wavelength of 1.15 μm and an InGaAsP layer having a thickness of 50 nm.
The well layer 221 is made of In _0.4 Ga _0.6 As having a thickness of 11 nm.
About 1% tensile strain has been introduced.

Returning to FIG. 1, 4 is a p-InGaAsP layer having a band edge wavelength of 1.15 μm to be an optical waveguide layer, 5 is a polyimide buried layer, 6 is a p-InP upper cladding layer, 7
Is a p-InGaAs contact layer, 8 is an electrode for injecting current into the first region, 9 is a current non-injection portion or a current injection limiting portion having a length of up to 70 μm in the resonator direction in the first region. Si ₃ N ₄ insulating layer, electrodes for injecting current into the second region 10 for, 11 is the diffraction grating pitch ~240nm formed in the optical waveguide layer. Note that the first region and the second region are electrically separated by the separation groove 12.

As shown in the energy band structure in the above configuration, when a current is first injected into the second region, a large amount of electrons are supplied to the n-side (substrate side) TE gain well 211, so that the gain of TE increases. If the current injection is further increased, laser oscillation occurs in the TE mode (FIG. 3A). on the other hand,
When a current is injected into the first region, the gain of the TE increases in a portion where the current is injected. However, since a current non-injection region exists, a gain necessary for laser oscillation cannot be obtained. Further, when the current is increased, the electrons have high energy, and a large amount of electrons are supplied to the p-side (electrode 8 side) TM gain well 221 at the portion where the current is injected in the first region, and the gain of the TM is reduced. Increase (FIG. 3B). Further, when the current is increased, the current flows from the surroundings, so that the loss in the current non-injection region decreases, and the laser oscillates in the TM mode.

When the polarization modulation operation is performed, current is simultaneously injected into the first region and the second region, and the current I ₁ to the first region is increased from a state where the TE / TM gain is almost antagonized. The oscillation mode is set to the TM mode. Next, when the injection current I ₂ to the second region is increased, the second region starts laser oscillation in the TE mode, while the first region is excited by the TE mode light propagating from the second region. Then, the TE mode oscillation occurs, and the TM oscillation stops (TM
The mode gain relates to the transition between the ground electron level and the light hole level of the tensile strain well 221. Since this transition also relates to the TE mode gain, the TE mode light propagated from the second region causes TE mode light in the first region. Mode oscillation occurs and the gain of the TM mode becomes insufficient). That is, the polarization mode of laser oscillation switches from TM to TE with a slight modulation current.

In this embodiment, the first region and the second region have the same active layer 3 having gain in both TE and TM,
Since the current required for the polarization switching is small (the carrier fluctuation is small), the wavelength fluctuation at the time of the switching is small (so-called chirping is small), which is suitable for high-speed operation.

As described above, doping the barrier layer of the active layer 3 with the p-type impurity and filling the wells with holes is very effective in reducing the threshold current value of the laser. .

In the above description of the operation of this embodiment, the modulation current is injected only into the second region. However, by simultaneously injecting the opposite-phase modulation current into the first region, the switching characteristics can be improved.

Second Embodiment FIG. 4 shows another embodiment of the current non-injection section or the current injection limiting section, and portions having the same functions as those in FIG. 1 are denoted by the same reference numerals. The difference from FIG. 1 is that the current non-injection portion in the first region is formed by the electrode separation portion 49 in the stripe portion, and a groove reaching the cladding layer 6 is formed in the separation portion 49. Since the cap layer 7 is removed by this groove, current sneak from an adjacent electrode is small, and a loss necessary for TM mode laser oscillation can be obtained in a relatively short current non-injection region. Other operations and the like are substantially the same as those of the first embodiment.

Third Embodiment FIG. 5 is a partial perspective view of a third embodiment of the present invention. FIG.
Portions of the same function as in are given the same numbers. The difference from FIG. 1 is that an electrode 58 capable of injecting current into the first region more independently is provided.
Is formed, and the electrode 58 is connected to the same drive power supply as the other electrodes 8 in the first region via the resistor 59. The voltage applied to the electrode 58 is the resistance 5
The voltage is lower than the voltage applied to the electrode 8 by the voltage drop of 9. Therefore, as the driving voltage increases, the gain of the dominant TE at the low injection level in the active layer 3 under the electrode 8 increases, but the portion under the electrode 58 functions as an absorption loss section because the applied voltage is low. However, the gain required for laser oscillation cannot be obtained.

Further, when the applied voltage is increased, the electrons become high energy, and in the active layer 3 under the electrode 8, the gain of the dominant TM at the high injection level is increased. On the other hand, as the applied voltage gradually increases also in the portion below the electrode 58, the absorption loss decreases, and the laser oscillates in the TM mode.

Therefore, it is possible to control the current injection to the electrode 58 so as to obtain a value most suitable for polarization switching simply by setting the resistor 59 without requiring a new power supply for loss control. And a polarization-modulated laser excellent in the above.

The operation of this embodiment is substantially the same as that of the first embodiment. That is, when the polarization modulation operation is performed, current is simultaneously injected into the first region and the second region, and TE is injected.
From the state where the / TM gain is almost antagonized, the voltage applied to the first region is increased to set the oscillation state in the TM mode. Next, when the injection current I ₂ to the second region is increased, the second region
Mode starts laser oscillation, while in the first region the second
The TE mode light propagating from the region excites the TE mode light and stops TM oscillation.

By the way, in each of the above embodiments, an appropriate current less than the oscillation threshold is injected into each electrode to create a state in which both the TE and TM modes are in opposition. It can also function as a polarization independent optical amplifier.

In each of the above embodiments, a distributed feedback type (DF
B), the present invention relates to a distributed Bragg reflection type (D
BR) and the like.

Fourth Embodiment FIG. 6 is an explanatory view of the case where the polarization modulation laser 610 of the first embodiment is used as an electric / optical conversion unit for optical communication. A polarizer 611 is provided, and only one polarization component is used as an optical signal. The polarization modulation laser 610 has a small modulation current,
There is little change in the intensity of the output light. Therefore, with the intensity modulation method of extracting only one polarized light on the transmission side or the reception side of optical communication, a high-speed modulation operation comparable to the FSK modulation method becomes possible, and the receiving apparatus is greatly simplified as compared with the FSK method. Is done. Further, the wavelength spread is small even at the time of high-speed modulation, so that it is suitable for high-density wavelength multiplexing.

Of course, the second and third embodiments may be used as a polarization modulation laser.

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

An 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 may use the polarization independent optical amplifier of the above embodiment. On the other hand, when an optical signal is transmitted from the node 701, the polarization-modulated semiconductor laser 704 of the above embodiment is driven by a control circuit in an appropriate method according to the signal, and polarization-modulated.
The output light is passed through a polarizing plate 707 (which converts the polarization modulation signal into an amplitude intensity modulation signal) (an isolator may be additionally provided) through a converging section 706 and the optical transmission path 70
0. Further, by providing a plurality of semiconductor lasers and two or more wavelength tunable optical filters, the wavelength tunable range can be expanded.

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.

[0041]

As described above, the following effects can be obtained by each constitution of the present invention.

According to the first configuration (corresponding to claims 1 and 2), the active layer is formed of a quantum well structure having a superior TE gain and a T well.
A quantum well structure in which M gain is superior, and
The resonator direction is constituted by a plurality of regions into which current can be independently injected, and a loss portion exists partially in one of the plurality of regions. And a TM gain dominant region, and a polarization-modulated laser capable of polarization switching with a small modulation current can be easily realized. Since the modulation current is small, chirping can be suppressed.

According to the second configuration (corresponding to claim 3),
This is a very simple and lossless portion that does not require any new control by providing a portion into which no current is injected in the current injection region, thereby realizing a polarization-modulated laser or the like.

According to the third configuration (corresponding to claim 4),
The portion in which the current is not injected in the current injection region is formed by a simple configuration in which an insulating layer is disposed immediately below the electrode.

According to the fourth configuration (corresponding to claim 5),
The current non-injection portion forming the partial loss portion is formed by a separation groove, so that current sneak into the loss portion from an adjacent electrode is small, and the short groove becomes an effective loss portion. .

According to the fifth configuration (corresponding to claim 6),
The loss can be controlled by providing an electrode so that current can be independently injected into the loss portion. Therefore, it is possible to set the injection current to the loss portion to a current value suitable for polarization modulation.

According to the sixth configuration (corresponding to claim 7),
An electrode provided in the loss portion and capable of independently injecting current,
By connecting to the same drive power supply as the other electrodes in the same region via the resistor, a power supply for loss control is not required.

According to the seventh configuration (corresponding to claim 8),
By forming a diffraction grating in the optical waveguide layer disposed near the active layer, at least one of two polarized laser oscillations is converted into a single mode.

According to the eighth configuration (corresponding to claim 9),
At least a part of the barrier layer of the active layer is doped with a p-type impurity. Thereby, the threshold current and the like can be further reduced.

According to the ninth, tenth, eleventh, and twelfth configurations (corresponding to claims 10, 11, 12, and 13), the polarization-modulated laser of the present invention is used for an electric / optical converter for optical communication. (Moreover, by using the polarization-independent optical amplifier of the present invention as a receiver), a high-speed operation equivalent to that of the FSK modulation method can be performed by the intensity modulation method, and the receiving apparatus can be greatly simplified. A communication network can be realized at low cost.

[Brief description of the drawings]

FIG. 1 is a sectional perspective view of a semiconductor optical device used as a polarization modulation laser or the like according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an active layer structure according to a first embodiment of the present invention.

FIG. 3A is a band structure diagram for explaining carrier distribution in a low injection state in the active layer of FIG. 2;
FIG. 2B is a band structure diagram illustrating carrier distribution in the active layer of FIG. 2 in a high injection state.

FIG. 4 is a sectional perspective view of a semiconductor optical device used as a polarization modulation laser or the like according to a second embodiment of the present invention.

FIG. 5 is a partial cross-sectional perspective view of a semiconductor optical device used as a polarization modulation laser or the like according to a third embodiment of the present invention.

FIG. 6 is an explanatory diagram in the case where the polarization modulation laser of the present invention is used for optical communication.

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 optical 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 optical device of the present invention.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 Lower common electrode 2 n-InP substrate and n-InP lower cladding layer 3 Multiple quantum well active layer composed of TM gain quantum well and TE gain quantum well 4 Band edge wavelength to serve as optical waveguide layer p of 1.15 μm
-InGaAsP layer 5 Polyimide buried layer 6 p-InP upper cladding layer 7 p-InGaAs contact layer 8 Electrode for injecting current into first region 9 Si ₃ N ₄ insulating layer for forming current non-injection part 10 Electrode for injecting current into second region 11 Diffraction grating formed in optical waveguide layer 12 Separation groove 49 Non-current injection portion formed by electrode separation groove 58 Electrode for controlling absorption loss in first region 59 Resistance 210 , 220 Barrier layer 211 TE gain quantum well 213 TM / TE separation layer 221 TM gain quantum well 610, 704 Semiconductor laser 611, 707 Polarizer 703 Receiver 706 Junction sections 811 to 815, 911 to 916 Terminal 700, 800 , 900 Optical transmission line 701, 801-805, 901-906 Node 702 Optical branching unit

Claims (13)

[Claims] In a semiconductor optical device having a semiconductor laser structure, an active layer has a quantum well structure having a semiconductor having a lattice constant smaller than that of a semiconductor substrate as a well layer and a quantum well structure having a lattice constant equal to or larger than the lattice constant of the semiconductor substrate. The quantum well structure has a stacked structure of a quantum well structure in which a semiconductor is a well layer. The resonator direction is composed of a plurality of regions in which current can be independently injected, and is partially provided in one of the plurality of regions. A semiconductor optical device, comprising: a loss portion in which current injection is restricted and loss occurs. 2. The semiconductor optical device according to claim 1, wherein the polarization direction of the laser oscillation light is switched between TE and TM by controlling the injection current. 3. The semiconductor optical device according to claim 1, wherein said partial loss portion is formed by a current non-injection portion into which no current can be injected. 4. The semiconductor optical device according to claim 3, wherein the current non-injection portion forming the partial loss portion is formed by an insulating layer disposed immediately below the electrode. 5. The semiconductor optical device according to claim 3, wherein the current non-injection part forming the partial loss part is formed by a groove separating the electrodes. 6. The semiconductor according to claim 3, wherein the partial loss portion has an electrode capable of independently injecting current, and the loss can be controlled by controlling current injection to the loss portion. Optical device. 7. An electrode according to claim 6, wherein the electrode capable of injecting current independently of said partial loss portion is connected to the same drive power supply as another electrode in the same region via a resistor. Semiconductor optical device. 8. A method according to claim 1, wherein a diffraction grating is formed in the optical waveguide layer disposed near the active layer, and at least one of the two orthogonally polarized laser oscillation lights has a single mode. Item 8. A semiconductor optical laser according to any one of Items 1 to 7. 9. At least a part of the barrier layer of the active layer has p
9. The semiconductor optical laser according to claim 1, wherein a type impurity is doped. 10. A semiconductor optical device configured as the polarization-modulated semiconductor laser according to claim 1;
Of the light emitted from the semiconductor device, 2 of TE and TM
A light source device comprising: a polarization selecting unit such as a polarizer that extracts only light generated by one of two polarization modes. 11. A semiconductor optical device configured as the polarization-modulated semiconductor laser according to any one of claims 1 to 9,
An optical transmitter including a light source device including a polarization selection unit such as a polarizer that extracts only light generated by one of two polarization modes of TE and TM out of light emitted from the semiconductor laser; An optical communication system comprising: transmission means for transmitting the light extracted by the means; and an optical receiver for receiving the light transmitted by the transmission means. 12. The 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 1. 13. A semiconductor optical device configured as the polarization-modulated semiconductor laser according to claim 1,
A light source device comprising a polarization selecting means such as a polarizer for extracting only light generated by one of two polarization modes of TE and TM out of light emitted from the semiconductor laser, and transmitting a transmission signal to a predetermined bias current. By superimposing a current modulated according to the above and supplying the superimposed current to the semiconductor laser, a signal light intensity-modulated according to the transmission signal is extracted from the polarization selecting unit, and the signal light is transmitted to the optical receiver. An optical communication method, comprising: