JPH07231133A - Semiconductor laser capable of modulating polarization, and application thereof - Google Patents

Abstract

PURPOSE:To change polarized condition of an output beam by an external control. CONSTITUTION:A semiconductor laser for polarized modulation has active layer formed of two or more active regions 31, 32 haying different gain spectrum within one resonator, at least one active region 32 of the active layer has a gain for TM polarization which is larger than that for the TE polarization and at least the other active region 31 of the active layer has a gain for TE polarization which is larger than that for the TM polarization.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization-modulatable semiconductor laser capable of changing the polarization state of output light under external control and its use.

[0002]

2. Description of the Related Art Conventionally, a semiconductor laser capable of changing the polarization state of output light by external control is disclosed in Japanese Patent Laid-Open No. 2-15.
There is a distributed feedback (DFB) laser described in 9781. This DFB laser changes the phase of internal light by carrier injection for phase adjustment and population inversion formation, and oscillates in a mode having a lower threshold gain of TE or TM modes.

[0003]

However, in the above-mentioned conventional example, in order to change the polarization state of the oscillation light in the phase with respect to the TE light or the TM mode light, the injection current for forming the population inversion is always constant. Was needed.

Further, in the above-mentioned conventional example, the active layer is made of a bulk material, and the gain difference between TE and TM modes is small, but when the quantum well active layer is used, TE and T
The gain difference with respect to the M mode becomes large, the wavelength range in which the polarization state of the output light can be switched by the same operation principle becomes narrow, and the diffraction grating (the oscillation wavelength of the TE and TM modes depending on the period of the diffraction grating is used for the TE and TM modes. The requirements for manufacturing accuracy such as (the wavelength is determined) become stricter.

Therefore, it is an object of the present invention to provide a polarization-modulatable semiconductor laser and a method of using the same that solve the above problems.

[0006]

According to the polarization-modulatable semiconductor laser of the present invention, TM gain is larger than TE gain, for example, a strained quantum well is used as a part of the active layer to change the injection current. To allow the total TE in the resonator to
By changing the gain and the TM gain, the polarization state of the output light can be switched.

Specifically, the polarization-modulatable semiconductor laser of the present invention has an active layer formed of two or more active regions having different gain spectra in one resonator, and at least the active layer. One active region has a gain of TM polarized light larger than that of TE polarized light, and at least one active region of the active layer has a gain of TE polarized light of TM polarized light. Is greater than the gain for.

Specifically, the two or more active regions are one.
The two resonators are arranged along the axial direction. 2 above
One or more active regions are arranged along a direction (horizontal direction of the waveguide and stacking direction) perpendicular to the axial direction within one resonator. It has an electrode structure capable of independently injecting current into a plurality of active regions of the active layer. An active region in which the gain of TM polarized light is larger than that of TE polarized light is composed of a strained quantum well in which tensile strain is introduced. The active regions other than the active regions formed by the strained quantum wells are formed by strained quantum wells into which compressive strain is introduced. Also, it has a distributed reflector, has a distributed feedback structure,
It may have a Fabry-Perot resonator.

According to the method of driving a semiconductor laser capable of polarization modulation of the present invention, in the above semiconductor laser, a current injected into an active region in which the gain for TM polarized light is larger than the gain for TE polarized light. Is modulated (or direct current), and the current injected into another active region is changed to direct current (or modulation). In the semiconductor laser, a current injected into an active region having a gain for TM polarized light larger than a gain for TE polarized light is modulated, and a current injected into another active region is modulated in a phase opposite to that of the above modulation. It is characterized by doing.

Further, the optical transmitter or the optical transmitter / receiver of the present invention comprises the above semiconductor laser, polarization selecting means for transmitting light of one polarization of the output light from the semiconductor laser, and output of the semiconductor laser. It is characterized by comprising control means for polarization-modulating light according to an input signal.

An optical communication system of the present invention is characterized by including at least one of the above optical transmitter or optical transmitter / receiver.

[0012]

[First Embodiment] FIG. 2 is a sectional view taken along the line AA ′ of FIG. 1 and FIG.
FIG. 4 is a drawing best showing the features of this embodiment having a Fabry-Perot resonator. In the figure, 1 is, for example, n-type G
The substrate made of aAs, 2 is, for example, n-type Al _0.5 Ga _0.5 A
s is a first clad layer, 31 is a first active layer having a large gain for TE polarized light, 32 is a second active layer having a large gain for TM polarized light, and 4 is, for example, p-type Al. _0.5 G
The second cladding layer 5 made of a _0.5 As is, for example, p-type G
a cap layer made of aAs, 6 an insulating film, 7 a first electrode made of, for example, an alloy of gold and germanium formed in contact with the substrate 1, and 8 a first electrode made of, for example, an alloy of gold and chromium. Second electrode used to inject current into active layer 31, 9 is a third electrode used to inject current into second active layer 32, 10 is second electrode 8 and third electrode 9
Is a separation groove for electrically separating the.

FIG. 3 shows the structure (size of the band gap) of the first active layer 31 and the second active layer 32 (only the energy level of the conduction band is shown). In the figure, the numbers indicating the respective areas are the same as those in FIGS. 1 and 2. That is, 2 indicates the first clad layer, and 4 indicates the second clad layer. Also, 311, 312 (layers 3121, 312)
The first active layer 31 is made up of all 313 layer portions. The layers 311 and 313 have a refractive index (energy band gap) gradually increasing from Al _0.5 Ga _0.5 As to Al.
So-called GR that changes to _0.3 Ga _0.7 As (or vice versa)
It is the IN layer. The layer 312 is an active region formed of a quantum well. In FIG. 3, the well layer 31 made of GaAs, for example, is used.
21 and a barrier layer 31 made of, for example, Al _0.3 Ga _0.7 As
It is composed of 22. On the other hand, the second active layer 32 has substantially the same structure as the first active layer 31, but the well layer portion 3
121 is composed of, for example, GaAs _0.8 P _0.2, and is a so-called tensile strain quantum well. In this way, the gain for TM polarized waves is larger than the gain for TE polarized waves.

In this embodiment, for lateral confinement of the waveguide, a ridge waveguide structure is used as shown in FIG. The lateral confinement is not limited to the ridge structure but may be another structure such as a buried structure used in a semiconductor laser.

Further, in this embodiment, in order to separate the two electrodes 8 and 9, the structure in which the electrodes are simply separated into two is used, but the present invention is not limited to this and other structures are used. Configurations (eg, forming deep slits) can also be used.

4 and 5 show a method of driving the semiconductor laser of this embodiment. In FIG. 4, I ₁ is the first active layer 3
1 is a current injected into 1, 1 ₂ is a current injected into the second active layer 32, and 11 is a light output. FIG. 5 shows a method of injecting current into this device. Injection current I ₁ and injection current I ₂₁ (current I ₂
Is set so that the oscillation light from the semiconductor laser of this embodiment becomes TE light. The current I ₂₂ (another amount of the current I ₂ ) is set so that the oscillated light is TM polarized.
The settings made in this way are shown in FIGS. 5 (A) and 5 (B). By this drive, the optical output 11 as shown in FIG.
Is obtained, and the polarization state of the optical output is alternately converted between the TE mode and the TM mode as shown in FIG.

That is, when the current injected into the first active layer 31 and the current injected into the second active layer 32 are I ₁ and I ₂₁ , respectively, a certain wavelength in the longitudinal mode oscillates only in the TE mode. TE-mode oscillation occurs with satisfying the conditions. On the other hand, the current injected into the first active layer 31 and the second active layer 32
When the currents injected into I are I ₁ and I ₂₂ , respectively,
Only in the M mode, a certain wavelength of the longitudinal mode satisfies the oscillation condition and TM mode oscillation occurs.

FIG. 6 shows another driving method of this element. In the driving method of FIG. 5, the optical output (FIG. 5C) changes due to the switching between TE and TM due to the relationship between the injection current and the threshold gain. Therefore, according to the driving method of FIG. 6, by modulating the current I ₁ in a phase opposite to that of the current I ₂ (FIGS. 6A and 6B), there is no optical output fluctuation (FIG. 6).
(C)), only the polarization state can be changed (Fig. 6).
(D)).

5A and 5B are reversed, that is, the current I ₁ injected into the first active layer 31 is modulated and the current I ₂ injected into the second active layer 32 is changed to DC. Good.

[0020]

[Second Embodiment] FIG. 7 shows a second embodiment of the present invention. Second
In the embodiments, the present invention is applied to the distributed Bragg reflector semiconductor laser (DBR-L).
This is an example applied to D). The same members as those in FIG. 1 have the same reference numerals. This embodiment has a structure in which a distributed reflector is provided on one side of the embodiment of FIG. 1 (may be provided on both sides). The distributed Bragg reflector of this embodiment can change its Bragg wavelength by injecting a current.

In FIG. 7, 20 is, for example, Al _0.3 Ga.
A lower region of the grating made of _0.7 As, 21 is an upper region of the grating made of, for example, Al _0.4 Ga _0.6 As, and a lattice (grating) 22 is provided at the interface between the two.
Is formed. Further, 25 is, for example, Al _0.4 Ga _0.6.
Phase adjusting regions 23, 24 made of As are electrodes of the grating regions 20, 21, 22 and the phase adjusting region 25, respectively.

In this embodiment, the method of changing the polarization state is substantially the same as in the first embodiment. That is, in the first control state, the current injected into the first active layer 31, the current injected into the second active layer 32, the grating regions 20, 2
The currents to be injected into the first and the second regions and the currents to be injected into the phase adjusting region 25 are appropriately set to dominate the TE mode gain spectrum, and TE mode oscillation is generated at the TE mode Bragg wavelength which is the gain peak. On the other hand, in the second control state, the current injected into the first active layer 31, the current injected into the second active layer 32, the grating regions 20, 2
The currents injected into Nos. 1 and 22 and the current injected into the phase adjustment region 25 are appropriately set to dominate the TM mode gain spectrum, and TM mode oscillation occurs at the Bragg wavelength of the TM mode, which is the gain peak. At this time, in order to narrow the line width of the oscillation light and change the oscillation wavelength, DB
R is formed.

[0023]

[Third Embodiment] FIG. 8 shows a third embodiment of the present invention. Third
In the embodiment, the present invention is applied to a distributed feedback semiconductor laser (DFB-L).
It is applied to D). The same members as those in FIG. 1 are designated by the same reference numerals. Layers 20 and 21 for forming the grating 22 are newly formed in the same manner as in the second embodiment. That is, 20, 21, and 22 are the same members as in the second embodiment.

Since the operation of this embodiment is substantially the same as that of the first and second embodiments, its explanation is omitted here.

By the way, in the above embodiment, two or more active regions are arranged in series along the axial direction within one resonator, but along the direction perpendicular to the axial direction within the resonator. It may be arranged. That is, for example, two active regions are provided symmetrically on the left and right of the center axis of the waveguide, and two electrodes are formed accordingly, or two active regions are provided above and below in the stacking direction and the upper and lower electrodes are accordingly formed. May be formed and the layer between the two active regions may be grounded so that current can be injected into the two active regions independently from their electrodes.

[0026]

Fourth Embodiment FIG. 9 shows an embodiment in which the semiconductor laser of the present invention is applied to an optical transmitter for optical communication. In FIG. 9, 1
Reference numeral 01 is a control circuit, 102 is the semiconductor laser of the present invention shown in the above embodiment, 103 is a polarizer, 104 is an optical coupling means for coupling light propagating in space to an optical fiber, 105 is an optical fiber, and 106 is Electrical signal sent from the terminal,
107 is a drive signal sent from the control circuit 101 to drive the semiconductor laser 102, and 108 is a drive signal 1.
The optical signal 109 output by driving the semiconductor laser 102 according to 07 is orthogonal to the optical signal 108.
An optical signal that has passed through the polarizer 103 that is adjusted so as to extract only one of the two polarization states, 110 is an optical signal that is transmitted through the optical fiber 105, and 111 is an optical signal that uses the semiconductor laser 102 of the present invention. It is a transmitter. In this embodiment, the optical transmitter 111 is composed of a control circuit 101, a semiconductor laser 102, a polarizer 103, an optical coupling means 104, an optical fiber 105 and the like.

Next, the transmission operation of the optical transmitter 111 of this embodiment will be described. When the electric signal 106 from the terminal is input to the control circuit 101, the drive signal 107 is sent to the semiconductor laser 102 of the present invention according to the modulation method as shown in FIG. 5 or 6. The semiconductor laser 102 to which the drive signal 107 is input outputs an optical signal 108 whose polarization state changes according to the drive signal 107. The optical signal 108 is
An optical signal 109 of one polarization is formed by the polarizer 103, and is further coupled to the optical fiber 105 by the optical coupling means 104. The intensity-modulated optical signal 110 is transmitted for communication.

In this case, since the optical signal 110 is in the intensity-modulated state, the light can be received by the conventionally used optical receiver for intensity modulation. Further, by using the wavelength tunable semiconductor laser having the configuration as shown in the second and third embodiments, it can be used as a transmitter for wavelength division multiplexing communication. In the present embodiment, the case where it is configured as an optical transmitter is shown, but of course it can also be used in the transmitting part in an optical transceiver.

Furthermore, the applicable optical communication system is not limited to simple two-point optical communication as long as it is a system that handles intensity-modulated signals, and can also be applied to optical CATV, optical LAN, and the like.

[0030]

As described above, according to the present invention,
An active layer formed of two or more active regions having different gain spectra is provided in one resonator, and at least one active region of the active layer has a gain of TE polarized light with respect to TM polarized light. (Composed of strained quantum wells, etc.), and the gain for TE polarized light is larger than the gain for TM polarized light in at least another active region of the active layer. The current can be independently injected into each active region. This allows
It is possible to reduce the ineffective injection current during oscillation in each polarization mode, and it is possible to form a semiconductor laser that can perform polarization modulation in a wider wavelength range than before even when a quantum well is used for the active layer. .

[Brief description of drawings]

FIG. 1 is a perspective view of a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line AA ′ in FIG.

FIG. 3 is a diagram illustrating the configuration of an active layer of the embodiment of FIG.

FIG. 4 is a sectional view for explaining a driving method of the embodiment of FIG.

5A and 5B are graphs for explaining a driving method of the embodiment of FIG.

FIG. 6 is a graph for explaining another driving method of the embodiment of FIG.

FIG. 7 is a sectional view of a second embodiment of the present invention.

FIG. 8 is a sectional view of a third embodiment of the present invention.

FIG. 9 is a block diagram showing an embodiment in which the semiconductor laser of the present invention is applied to an optical transmitter for optical communication.

DESCRIPTION OF SYMBOLS 1 substrate 2,4 clad layer 31 first active layer 32 second active layer 311,313 GRIN (graded indeed)
x) layer 312 quantum well active region 3121 well layer 3122 barrier layer 5 cap layer 6 insulating layer 7, 8, 9, 23, 24 electrode 9 isolation groove 20, 21 grating region 22 grating (grating) 25 phase adjustment region 101 control circuit 102 Semiconductor Laser 103 Polarizer 104 Optical Coupling Means 105 Optical Fiber 111 Optical Transmitter

Claims (13)

[Claims] 1. A resonator has an active layer formed of two or more active regions having different gain spectra, and at least one active region of the active layers has a gain for TM polarized light. Is larger than the gain for the TE polarized light, and at least one other active region of the active layer has a gain for the TE polarized light larger than the gain for the TM polarized light. laser. 2. The semiconductor laser according to claim 1, wherein the two or more active regions are arranged along the axial direction in one resonator. 3. The semiconductor laser according to claim 1, wherein the two or more active regions are arranged along a direction perpendicular to an axial direction within one resonator. 4. The semiconductor laser according to claim 1, wherein the semiconductor laser has an electrode structure capable of independently injecting current into a plurality of active regions of the active layer. 5. The active region in which the gain of TM polarized light is larger than that of TE polarized light is constituted by a strained quantum well in which tensile strain is introduced. Semiconductor laser. 6. The semiconductor laser according to claim 5, wherein the active regions other than the active region composed of the tensile strained quantum well are composed of strained quantum wells into which compressive strain is introduced. 7. The semiconductor laser according to claim 1, further comprising a distributed reflector. 8. The semiconductor laser according to claim 1, which has a distributed feedback structure. 9. The semiconductor laser according to claim 1, further comprising a Fabry-Perot resonator. 10. The semiconductor laser according to claim 1, wherein the TM
Modulating a current injected into one of an active region in which a gain for polarized light is larger than a gain for TE polarized light and an active region in which a gain for TE polarized light is larger than a gain for TM polarized light, 2. The method for driving a semiconductor laser according to claim 1, wherein the current injected into the other active region is DC. 11. The semiconductor laser according to claim 1, wherein the TM
7. A current injected into an active region having a gain for polarized light larger than a gain for TE polarized light is modulated, and a current injected to another active region is modulated in a phase opposite to the above modulation. 2. A method for driving a semiconductor laser according to 1. 12. The semiconductor laser according to claim 1, polarization selection means for transmitting light of one polarization of the output light from the semiconductor laser, and polarization modulation of the output light of the semiconductor laser according to an input signal. An optical transmitter comprising a control means for controlling the optical transmitter. 13. An optical communication system comprising at least one optical transmitter according to claim 12 or an optical transceiver including the optical transmitter.