JPH09289356A - Semiconductor laser device, driving thereof, and optical communication system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform efficient switching of polarization between a TE mode and a TM mode in a semiconductor laser device which is capable of performing independent current injection to two active layers. SOLUTION: In a semiconductor laser device, an active layer 104 in which a gain of a TM mode is predominant and an active layer 106 in which a TE mode is predominant are stacked, with a barrier layer 105 provided between these active layers. These two active layers 104, 106 are optically coupled but electrically isolated from each other. With such a structure that current injection may be independently performed to the two active layers 104, 106, the laser oscillation mode may be switched between the TE mode and the TM mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source device, such as a light source device for optical communication, which suppresses dynamic wavelength fluctuation even during high-speed modulation and stably realizes high-density wavelength-multiplexed optical communication, and an optical source using the same. A communication system, an optical communication system, etc. are provided.

[0002]

2. Description of the Related Art In recent years, there has been a demand for expansion of transmission capacity in the field of optical communications, and development of optical frequency division multiplexing (optical FDM) transmission in which a plurality of wavelengths or optical frequencies are multiplexed in one optical fiber has been carried out. ing. In the optical FDM, it is important to reduce the wavelength interval in order to increase the transmission capacity as much as possible. For that purpose, it is desirable that the occupied frequency band of the laser as the light source or the line width of the vector is small. However, the direct intensity modulation method used in the current optical communication has a line width of 0.3 nm at the time of modulation.
It has been pointed out that the method spreads to some extent and is not suitable for optical FDM.

As a method for preventing the spread of the vector line width during modulation, an external modulation method or a direct polarization modulation method (Japanese Unexamined Patent Publication No. 62-62160).
-42593, Sho 62-144426, Hei 2-1597
81 etc.) has been devised. Here, we are going to deal with the technology related to the polarization modulation method. This polarization modulation switches between TE (light having an electric field vector only in the direction parallel to the pn junction) and TM mode (light having a magnetic field vector only in the direction parallel to the pn junction) as shown in FIG. the bias current is fixed to the point of, and polarization switching as in the modulating the I ₁ a minute rectangular current △ I ₁ FIG. 14 (c), the polarization at the output end of the laser 1500 as shown in FIG. 14 (a) The ASK is performed by placing the child 1501 and selectively extracting only one of the polarization planes. Even if the direct modulation is performed, the wavelength variation is smaller than that of the external modulation method only by devising the structure of an ordinary DFB laser (distributed feedback laser).

There are various means for realizing such polarization modulation, but an active layer and a TM having a large TE mode gain are used.
It is effective to control the polarization by having two active layers having a large mode gain and independently injecting current into each of them (Japanese Patent Laid-Open Nos. 7-202337 and 1-25738).
6, JP-A-4-346485, JP-B-5-68111, etc.).

In Japanese Unexamined Patent Publication No. 7-202337, as shown in FIG. 15, two types of active layers 16 sandwiching an n-type region layer 1603 are provided.
02 and 1604 are stacked to form each active layer 160.
Since currents can be independently injected into the 2 and 1604 through the p-type region layers 1601 and 1605 by the current sources 1606 and 1607, there is an advantage that the production and the design are easy and the productivity is high. Distributed feedback type (DF
B) It is also possible to use a laser as a single mode. JP-A-4-
Even in 346485, the structure is almost the same, and the oscillation mode can be switched between TE and TM by the current.

[0006]

However, the laminated device of Japanese Patent Laid-Open No. 7-202337 mainly targets a polarization-independent semiconductor optical amplifier, and has a structure for efficiently switching oscillation polarization as a laser. Not mentioned. For example, when operating as a distributed feedback laser with a diffraction grating, the positional relationship between the Bragg wavelength and the gain peak of the active layer is very important, and polarization switching is difficult without this design. In addition, the two active layers 160
The positional relationship between the gain peak wavelengths of 2 and 1604 is also important, and it is necessary to design the strain amount and well thickness of the multiple quantum well. Furthermore, the n-type semiconductor layer 16 sandwiched between two active layers
03 is as thick as 70 nm and operates as an amplifier,
The laser cannot operate in single transverse mode. Further, in Japanese Unexamined Patent Publication No. 4-346485, no specific design guideline is given and the reproducibility of efficient polarization switching or bistable operation is poor.

In view of the above problems, the object of the present invention will be described below corresponding to each claim. The purpose of 1) is a semiconductor laser device capable of independently injecting current into two active layers.
It is intended to provide a laser structure capable of efficiently switching polarization of modes and TM modes. The purpose of 2) is to provide a laser structure capable of efficiently switching polarization of TE mode and TM mode in a distributed feedback laser device capable of independently injecting current into two active layers. The purpose of 3) is to provide a laser structure capable of efficiently switching polarizations of TE mode and TM mode in a distributed reflection laser device capable of independently injecting current into two active layers. The purpose of 4) is to provide a means and a structure which can freely set the difference between the wavelength of TE mode and the wavelength of TM mode in the laser device as in 1) and 2). The purpose of 5) is to provide a laser structure capable of efficiently switching polarization of TE mode and TM mode in a Fabry-Perot type laser device capable of independently injecting current into two active layers. The purpose of 6) is to provide a structure for stable single transverse mode oscillation in the above laser device. The objects 7) and 8) are to provide an active layer structure having a large TE mode gain and an active layer structure having a large TM mode gain. The purpose of 9) is to provide a laser structure capable of independently injecting current into the two active layers. The purpose of 10) and 11) is to provide a structure capable of changing the oscillation wavelength in the above laser device. The object of 12) is to provide a structure capable of improving the single mode property and widening the wavelength variable range in the above laser device. The purpose of 13) is to provide a structure capable of improving the single mode property in the above laser device. The purpose of 14) is to provide an effective driving method for polarization switching in the above laser device. The purpose of 15) is to provide a driving method for performing polarization switching while changing the oscillation wavelength in the above laser device. The purpose of 16) is to provide a driving method for polarization switching while changing the oscillation wavelength in the distributed Bragg reflector laser device as in 3). The purpose of 17) is to provide a method of performing optical communication with less chirping by using a laser device capable of polarization switching. The purpose of 18) is
An object of the present invention is to provide a light source device for signal transmission using a laser device capable of polarization switching. The purpose of 19) is to provide high-density wavelength division multiplexing transmission by polarization modulation transmission. The purpose of 20) is to provide an optical-electrical conversion device for realizing the polarization modulation transmission such as 19). The purpose of 21) is to provide an optical LAN system using such polarization modulation transmission as 19).

[0008]

In view of the above, the present invention provides a semiconductor device in which two types of active layers are stacked and an independent current is injected into each of the active layers to efficiently perform TE / TM polarization switching in a laser oscillation mode. .

The principle of the present invention is illustrated by using a specific example of the device structure according to the present invention, in which a sectional perspective view of FIG.
Description will be given along the energy band diagram of (b). An optical guide layer having a diffraction grating is formed immediately below the first active layer (active layer 1), and a second active layer (active layer 2) is formed above the optical guide layer via an InP barrier. Similar to the conventional example, the structure is such that current injection into the two active layers can be performed independently. The InP barrier layer is thinned to 10 nm for lateral single mode operation.

This is a TTG (tunable twi).
n guide) type laser (T. Wolf et a.
l. , Electronics Lett. , 2
9, p. 2124, 1993) and TG (twin
Guide) laser (Yamamoto et al., Technical Report of the Institute of Electronics and Communication Engineers OQE91-41, p.55), but these are only for TE mode, and target tunable lasers and modulators. What you are doing is fundamentally different.

The first active layer imparts a tensile strain to the well, and the transition energy between the ground levels of light holes and electrons (Elh
0-Ee0) is smaller than the transition energy (Ehh0-Ee0) between the ground levels of heavy holes and electrons, and the gain for the TM mode is larger than that for the TE mode. Conversely, the second active layer gives compressive strain, and the gain for the TE mode is large.

Therefore, the current I ₁ injected into the first active layer is
If TM is increased, TM mode oscillation can be dominant, and if the current I ₂ injected into the second active layer is increased, TE mode oscillation can be dominant. FIG. 3 shows the state of the gain spectrum when a current is passed through each of them and immediately before oscillation. A gain spectrum of the TM mode of the first active layer,
The TE modes of the second active layer are configured to substantially overlap with each other, and each of them provides a gain for laser oscillation of this device. The pitch of the diffraction grating is set to 240 nm so that the distributed feedback wavelength by the diffraction grating formed in the optical guide layer is in the vicinity of these peak wavelengths.
The structure has a Bragg wavelength of 62 μm and 1.558 μm in the TM mode.

With this structure, when the current I ₁ flowing in the first active layer and the current I ₂ flowing in the second active layer are changed, the TE mode oscillation region and the TM mode are generated as shown in FIG. The oscillation region is completely divided into two. The light output is shown by contour lines.
Therefore, if the bias current is fixed to one of the boundaries a and the modulation current is applied to I ₁ or I ₂ , TE / TM
Can be modulated by switching.

Further, as shown in FIG. 7, a diffraction grating is provided near each of the two active layers, and the pitch of each diffraction grating is set so that the Bragg wavelength by the diffraction grating comes near the gain peak of each active layer. Therefore, the oscillation wavelengths of the TE mode and the TM mode for polarization switching can be freely set. That is, in one type of diffraction grating, the TE / TM wavelength difference is determined by the waveguide structure, and the TM mode is on the short wavelength side by several nm, but in this configuration example, the oscillation wavelength difference is 0 or several tens of nm. It can be a wavelength difference.

A method of designing the active layer so as to operate as described above will be described below. FIG. 13 shows the result of calculation of the relationship between the amount of strain of the multiple quantum well and the gain peak wavelength. This is a case of InGaAsP system, and the barrier layer is fixed as a band gap wavelength of 1.15 μm and a thickness of 10 nm without distortion. The well layer is InGaAs, and the vertical axis represents the ratio of Ga x = Ga / (Ga + As), that is, the amount of strain, and x = 0.47 when there is no strain. The horizontal axis represents the well layer thickness, and the calculation result using the transition energy of Ehh0-Ee0 as a parameter is shown in FIG.
The calculation result using the transition energy of Ee0 as a parameter is shown in FIG. 13B, and the calculation result using the difference between the transition energy of Ehh0-Ee0 and the transition energy of Elh0-Ee0 as a parameter is shown in FIG. 13C. ing. That is, the curves (a) and (b) represent equal wavelength lines.
The region where the transition line is not shown is a region where the lattice is broken and cannot grow normally.

In FIGS. 13A and 13B, the regions are divided by a line having a bandgap wavelength, that is, a gain peak wavelength of 1.55 μm. In FIG. 13C, Ehh0-
The region where the Ee0 energy is smaller, that is, the TE mode gain is larger, and the region where the Ehh0-Ee0 energy is larger, that is, the TM mode gain, is divided. Therefore, from these graphs, the distortion amount (Ga ratio) and the well width can be designed based on the desired gain peak wavelength and the polarization mode.

Furthermore, if the electrodes are divided as shown in FIGS. 8 and 9, the wavelength can be tuned. Further, as shown in FIG. 10, by setting two types of active layers having different aspect ratios of the cross section of the quantum wire, a more efficient polarization modulation laser can be provided.

The means and actions for achieving the object corresponding to each claim are summarized as follows. 1), an active layer having a dominant TM mode gain and an active layer having a dominant TE mode are stacked with a barrier layer in between, and these two active layers are optically coupled but electrically isolated. The laser oscillation mode is set to TE mode and T mode by making it possible to inject current into the two active layers independently.
Can be switched in M mode.

2) An active layer having a dominant TM mode gain and an active layer having a dominant TE mode are stacked with a barrier layer in between.
The gain peak wavelengths of these two active layers are almost the same,
In a distributed feedback laser having a diffraction grating whose Bragg wavelength is set near the gain peak wavelength, the two active layers are optically coupled but electrically isolated from each other. By allowing current injection independently, the laser oscillation mode can be switched between TE mode and TM mode. That is, the gain peak wavelengths of the two active layers are substantially matched with each other, and an optical guide layer having a diffraction grating is provided in the vicinity of one of the two active layers, and the diffraction grating has a gain of the two active layers. It is a distributed feedback type with a period such that the Bragg wavelength is set near the peak wavelength.

3), an active layer having a dominant TM mode gain and an active layer having a dominant TE mode are stacked with a barrier layer in between.
The gain peak wavelengths of these two active layers are almost the same,
In a distributed Bragg reflector laser having a diffraction grating whose Bragg wavelength is set near the gain peak wavelength, the two active layers are optically coupled but electrically isolated from each other. By allowing current injection independently, the laser oscillation mode can be switched between TE mode and TM mode. That is, the gain peak wavelengths of the two active layers are substantially matched to each other, and a distributed reflector having a diffraction grating is provided in series in the cavity direction with respect to the regions of the two active layers stacked with the barrier layer in between. The diffraction grating is of a distributed reflection type having a period such that the Bragg wavelength is set near the gain peak wavelengths of the two active layers.

4), an active layer having a dominant TM mode gain and an active layer having a dominant TE mode are stacked with a barrier layer in between.
A distributed feedback laser having a diffraction grating in the vicinity of each of the active layers, in which the gain peak wavelengths of these two active layers are different from each other and the Bragg wavelength is set in the vicinity of each of the gain peak wavelengths. Optically coupled but electrically isolated so that current can be independently injected into the two active layers so that the laser oscillation mode switches between TE mode and TM mode with a desired oscillation wavelength difference. it can. That is, the gain peak wavelengths of the two active layers are made different, and an optical guide layer having one diffraction grating is provided near each of the active layers, and the diffraction grating has the gain of the closest active layer. It is a distributed feedback type with a period such that the Bragg wavelength is set near the peak wavelength.

5) The structure as in 1) is constructed as a Fabry-Perot laser. The single mode property is not good, but similarly, the laser oscillation mode can be switched between the TE mode and the TM mode.

6) In the above laser device, the thickness of the barrier layer is set so that it is constituted by an optically strong coupling waveguide structure in which the guided transverse mode does not change in the light guiding direction. It is limited to 5 nm or more and 50 nm or less. When it is less than 5 nm, the two active layers are not electrically isolated by the tunnel effect, and when it is more than 50 nm, the propagation mode of light becomes multimode, which is not preferable.

7), the active layer in which the TM mode gain is dominant is composed of multiple quantum wells with tensile strain applied to the well layer.
The active layer with a dominant mode gain achieves stable polarization switching by being composed of a lattice matching system or multiple quantum wells with compressive strain applied to the well layer.

8) The active layer in which the TM mode gain is dominant is composed of a quantum wire having a vertically long rectangular cross section, and the active layer in which the TE mode gain is dominant is composed of a quantum wire having a horizontally long rectangular cross section. By doing so, stable polarization switching is achieved.

9) The two active layers have the same conductivity type, and a barrier layer having a different conductivity type is sandwiched between the two active layers to make the conductivity type of the buried layer and the barrier layer for forming a waveguide equal. Therefore, the electrode in the buried layer is used as a common electrode,
By forming a pn junction between the active layer and the buried layer (further, the remaining light guide layer and the buried layer), current can be independently injected into the two active layers.

10) Two or more common electrodes in the buried layer are provided in the light traveling direction of the optical waveguide so that the current can be non-uniformly injected in the light traveling direction of the optical waveguide. Therefore, wavelength tunability can be achieved.

11) In the above laser device, multiple electrodes are provided in the cavity direction so that the oscillation wavelength can be continuously changed. Wavelength tunability can be achieved by adopting a configuration in which current can be injected nonuniformly in the light traveling direction of the optical waveguide.

12), a phase shift part is introduced in the diffraction grating. Thereby, the single mode property can be improved and the wavelength variable range can be widened. When the phase shifter is introduced, the oscillation wavelength is surely set to the central Bragg wavelength without unstable hopping at both ends of the stop band, so that the wavelength tunable range can be widened when performing the wavelength tunable operation.

13), a non-reflective coating is applied to the end face of the element having the diffraction grating. Thereby, end face reflection is suppressed and single mode property can be improved.

14) In the laser device having the above-mentioned configuration, polarization switching is achieved by performing polarization switching by changing the current of at least one of the currents applied to the two active layers.

15) The laser oscillation wavelength is changed by nonuniform injection of the current in the light traveling direction of the optical waveguide, and the polarization switching is performed by the current change of at least one of the currents applied to the two active layers. In particular,
In the configuration of 9), two or more common electrodes in the buried layer are provided in the light traveling direction of the optical waveguide so that the current can be nonuniformly injected in the light traveling direction of the optical waveguide. Therefore, wavelength tunability can be achieved.

16) In the distributed Bragg reflector type laser device of 3), the oscillation wavelength is changed by the current injected into the distributed reflector portion, and the polarization switching is simultaneously performed by the change in the current in the other optical waveguide portion.

17) A low-chirp optical communication is achieved by performing modulation by polarization switching and extracting only one of the polarization modes by a polarization selecting means such as a polarizer and detecting the signal.

18) A light which comprises the above-mentioned semiconductor laser device and polarization selecting means, performs modulation by polarization switching of the semiconductor laser device, and arranges the polarization selecting means at its output end to extract only one polarization mode. It is a light source device for communication. For example, a wavelength multiplexing optical communication light source device can be provided by simultaneously changing the wavelength and performing modulation by polarization switching and arranging a polarizer at the output end to extract only one polarization mode.

The light from the light source device of 19), 18)
A plurality of optical fibers are connected to each other, and signals having a plurality of wavelengths are loaded respectively for transmission, and only a signal loaded on a light of a desired wavelength is taken out through a wavelength tunable optical bandpass filter in a receiving device to perform signal detection. Thus, high-density wavelength division multiplexing transmission is achieved.

An optical system comprising a light source device for optical communication of 20) and 18) and a receiving device for extracting and detecting only a signal on light of a desired wavelength through a wavelength tunable optical bandpass filter. An electrical conversion device. A wavelength division multiplexing optical transmission system can be constructed by using this to perform optical transmission and reception by the optical communication method such as 19).

A wavelength division multiplexing optical transmission system can be constructed by performing optical transmission / reception by the optical communication method of 19) using the optical-electrical conversion devices of 21) and 20).

[0039]

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment A first embodiment according to the present invention will be described. FIG. 1A is a cross-sectional perspective view of the DFB laser according to the present embodiment. It has a structure in which the active layer region has a bandgap wavelength of 1.56 μm and two types of multiple quantum wells having different strain amounts are separated by a barrier layer. Has become. An energy band diagram near the active layer is shown in FIG.

The layer structure will be described in detail below. 101 is a p-InP substrate having a diffraction grating 102 formed on its surface, 103
Has a thickness of 0. Band gap wavelength λg = 1.15 at 1 μm
The μ-type p-InGaAsP optical guide layer 104 includes a well layer i-In _0.28 Ga _0.72 As (thickness 10 nm) and a barrier layer i-InGaAsP (λg = 1.15 μm, thickness 10).
nm) is composed of 5 layers and a pair of SCH layers i-InGaAsP
(Λg = 1.15 μm, thickness 20 nm) having a tensile strained multi-quantum well (MQW) structure first active layer (active layer 1), 105 denotes a 10 nm-thick n-InP barrier layer, 10
6 is a well layer i-In _0.61 Ga _0.39 As (thickness 3 nm)
And the barrier layer i-InGaAsP (λg = 1.15 μm,
SCH layer i-InGaAsP for the 7 layers of the set of 10 nm thick)
(Λg = 1.15 μm, thickness 20 nm) The second active layer (active layer 2) having a compressive strain multiple quantum well (MQW) structure sandwiched between them, 107 is a p-InP clad layer, and 108 is p.
-In _0.53 Ga _0.47 As contact layer, 109 is n-I
nP buried layer, 110 is p-InP buried layer, 11
1 is an n-InP buried layer, 112 is n-In _0.53 Ga
_0.47 As contact layer, 113 and 114 are Cr / AuZnNi / Au layers which are p-side electrodes, 115 are AuGeNi / Au layers which are n-side electrodes, and 116 is an insulating film S
iN _x .

A method of manufacturing this device will be briefly described with reference to FIG. Pattern formation by the two-beam interference exposure method and RIBE (reactive ion) on the p-InP substrate 101.
The diffraction grating 102 is formed by dry etching by beam etching, and the optical guide layer 103, the first active layer 104, the InP barrier layer 105, the second active layer 106, the cladding layer 107, and the contact layer 108 are sequentially arranged in the CBE (c
It grows by a chemical beam epitaxy method etc. (FIG.4 (a)). Next, a SiO ₂ mask 408 having a width of about 2 μm is formed, and the p-InP substrate 101 is etched by RIBE (FIG. 4).
(B)). Next, n-InP109 and p-InP11
MOVPE (metal organized vap) in the order of 0, n-InP111, and n-InGaAs112.
or phase epitaxy) method or the like to carry out embedded growth (FIG. 4C).

Next, by photolithography and RIBE etching using the photoresist 413 as a mask, n- is formed.
InGaAs 112 is formed in a stripe shape (see FIG. 4).
(D)). Further, the surface serves as a contact layer 108, 1
SiN _x 116 is formed on the surface other than 12 by photolithography (FIG. 4E). Finally the electrode 113,
The formation of 114 and 115 is performed by a lift-off method or the like (FIG. 4F).

Next, the operation principle of the device according to the present embodiment will be described with reference to FIG. 2 which is a cross sectional view. In this device, n-I
Since there is the nP barrier layer 105, the current I ₁ flowing in the first active layer 104 and the current I ₂ flowing in the second active layer 106 can be controlled independently. In the first active layer 104, extension strain is applied to the well, the transition energy between ground levels of light holes and electrons (Elh0-Ee0) is smaller than the transition energy between ground levels of heavy holes and electrons (Ehh0-Ee0), The gain for the TM mode is larger than that for the TE mode, and on the other hand, the second active layer 106 reversely gives a compressive strain, and the gain for the TE mode is large.

Therefore, if the current I ₁ from the current source 210 is increased, the TM mode oscillation becomes dominant, and if the current I ₂ from the current source 220 is increased, the TE mode oscillation can be dominant. FIG. 3 shows the state of the gain spectrum when a current is passed through the active layers 104 and 106 immediately before oscillation. TM of the first active layer 104 (active layer 1)
The mode gain spectrum and the TE mode of the second active layer 106 (active layer 2) substantially overlap with each other, and each gives a gain for laser oscillation of this device. Light guide layer 1
Distributed feedback wavelength (T
The pitch of the diffraction grating 102 is set to 24 so that the E / TM Bragg wavelength) is near the peak wavelength of these gain spectra.
The wavelength is set to 0 nm, and the Bragg wavelength is set to 1.562 μm in the TE mode and 1.558 μm in the TM mode.

The characteristics and driving method of this device will be described below.
FIG. 5 shows current-light output characteristics when the horizontal axis is I ₁ and the vertical axis is I ₂ . The light output is shown by contour lines. In this way, the TE oscillation region (TE mode) and the TM oscillation region (TM mode) are
e) is completely divided into two, and if the bias current of I ₁ and I ₂ is fixed at any point on the boundary line a, and the modulation current is added to I ₁ or I ₂ , the TE / TM deviation will occur. Modulation is possible by switching waves.

[0046] For example, when applying a digital signal △ I ₂ amplitude 2mA current I _2, it is polarization modulated in TE / TM. The time waveform at this time is shown in FIG. (1) is the waveform of the modulation current ΔI ₂ , (2) is the output light intensity of the laser,
(3) represents the light intensity of TE light, and (4) represents the light intensity of TM light. As shown in FIG. 5 (2), the laser output light intensity does not change significantly due to the modulation, but it can be seen that the TE light and the TM light are modulated in opposite phases after polarization separation. At this time, the modulation band was DC to 3 GHz.

When transmitting this modulated light, for example, a polarizer is placed on the laser emission end face, and TE mode or TM is used.
One of the modes may be selected and extracted as intensity-modulated light. At this time, the dynamic fluctuation of the oscillation wavelength, that is, the chirping was 0.01 nm or less.

In the present embodiment, the compressive strain MQW is used as the second active layer 106 (active layer 2) of the laser having a dominant TE gain, but an MQW without strain may be used. Of course, it is also useful to introduce a λ / 4 shift structure into the diffraction grating 102 or to provide a non-reflection coating on the end face to improve the single mode property.

In this embodiment, the manufacturing accuracy required for the relationship between the gain peak of the active layer and the Bragg wavelength, the antireflection coating, etc. is relaxed as compared with the conventional polarization modulation laser using the phase control method.

Second Embodiment In the first embodiment, the positions of the gain peaks of the active layer 1 and the active layer 2 are substantially the same. In this case, the oscillation wavelengths of the TE mode and the TM mode had a difference of 4 nm as described in the first embodiment due to the difference in effective refractive index determined by the waveguide. In this embodiment, the difference between the two wavelengths can be freely set. The structure is basically the same as that of the first embodiment, except that the thickness of 0..0 is formed on the second active layer 706 (active layer 2).
p- of bandgap wavelength λg = 1.15 μm at 1 μm
An InGaP second light guide layer 707 (light guide layer 2) is provided, and a second diffraction grating is formed between the InGaP second light guide layer 707 and the cladding layer 708. FIG. 7 shows a band diagram in the vicinity of the active layers 704 and 706 at this time. The element structure is almost the same as that of FIG. In FIG. 7, reference numeral 701 is a lower clad layer in which the first diffraction grating is formed between it and the first light guide layer 703 (light guide layer 1), and 705 is a barrier layer. The driving method and the like of this embodiment are the same as those of the first embodiment.

In this structure, since the first active layer 704 having a large TM mode gain has a high coupling efficiency with respect to the first diffraction grating, the TM mode oscillation wavelength is determined by the first diffraction grating and conversely the second diffraction grating. The oscillation wavelength of the TE mode is determined by the diffraction grating. For example, in order to set the oscillation wavelength to 1560 nm in both TE mode and TM mode, the first diffraction grating is 240.
3 nm, the second diffraction grating was set to a pitch of 239.6 nm.

On the other hand, when the present device is used as a light source for optical communication as in the seventh embodiment described later, either the TE mode or the TM mode is selected by the polarizer, but the extinction ratio is When it is not sufficient, it is effective to use a wavelength filter together. In that case, it is advantageous that the wavelengths of the TE mode and the TM mode are separated from each other from the viewpoint of the accuracy of the filter and the like.
For example, consider a case where the two wavelengths are shifted by 20 nm and oscillate at 1560 nm in the TE mode and 1540 nm in the TM mode. The second active layer 706 on the TE mode side is the first
As in the embodiment, the pitch of the second diffraction grating may be 239.6 nm as already mentioned. On the TM mode side, MQW is used to set the gain peak of the first active layer 704 to 1540 nm.
The thickness of the well layer is changed to 9 nm, which is thinner than that in the first embodiment,
The diffraction grating pitch was set to 237.2 nm.

As described above, according to the present embodiment, it is possible to freely set the oscillation wavelengths of the TE mode and the TM mode when the polarization switch is performed.

Third Embodiment In the third embodiment of the present invention, the laser of the first embodiment has multiple electrodes in the cavity direction. FIG. 8 shows a perspective view of a three-electrode type laser according to the present invention. Only three electrodes 115 and contact layers 112 in FIG. 1 (respectively,
801, 802, 803; 811, 812, 813). A λ / 4 shift is provided on the diffraction grating at the center of the laser resonator, and both end faces are provided with antireflection coating (not shown). The electrode length is 80 at both ends.
1, 803 is 300 μm, and the central 802 is 100 μm. In FIG. 8, the same reference numerals as those in FIG. 1 indicate the same functional units.

The oscillation wavelength can be changed by changing the ratio of each current. In this element, the current I ₁ between the electrodes 801-804, the current I ₂ between the electrodes 802-804, and the electrode 80
As the current I ₃ between 3-804, I ₂ / (I ₁ + I ₂ +
By changing the value of I ₃ ) from 0.1 to 0.5,
The oscillation wavelength can be continuously changed by about 2 nm. As a result, it can be used as a light source for wavelength division multiplexing transmission.

The polarization modulation can be driven by applying a current signal with an amplitude of 5 mA only between the electrodes 802 and 805. The amplitude of the current is larger than that of the first embodiment.
This is because the region for current injection is short. But first
Since the parasitic capacitance is reduced as compared with the embodiment, the modulation band is extended and the modulation up to 10 GHz becomes possible.

Fourth Embodiment A fourth embodiment according to the present invention is applied to a distributed reflection (DBR) laser. FIG. 9 is a cross-sectional view parallel to the resonator of the DBR laser according to the present invention, and the cross section perpendicular to this is almost the same as that of the first embodiment (see FIG. 2). Similar to the third embodiment, three electrodes (951, 9) correspond to the electrodes 115 in FIG.
52, 953), and the active region,
It corresponds to the phase adjustment area and the DBR area. Diffraction grating 922
2 active layers 91 similar to those of the first embodiment in the active region without
4 and 916. The DBR region and the phase adjusting region are made of i-InGaAsP (λg = 1.15) having a thickness of 0.3 μm.
(μm) is the light guide layer 901. Polarization modulation is performed by performing current control as in the first embodiment in the active region. In FIG. 9, 901 and 913 are light guide layers and 9
Reference numeral 02 is a clad layer, 908 is a contact layer, 911 is a substrate, 915 is a barrier layer, and 954 is a p-side electrode.

In this device, the oscillation wavelength can be changed by about 3 nm by changing the injection current in the DBR region and the phase adjusting region. Therefore, it can be used as a light source for wavelength division multiplexing transmission as in the third embodiment.

Fifth Embodiment Although the first to fourth embodiments have shown the embodiments of the dynamic single mode laser having the diffraction grating, the Fabry-Perot laser without the diffraction grating and cleaved at both end faces. Can also be applied to. The structure is almost the same as that of FIG. 1, and no diffraction grating is formed on the p-InP substrate 101 below the light guide layer 103.
Although the single mode property is not good, polarization modulation can be performed by the same driving method as in the first embodiment.

The present invention can be applied to simple optical communication which does not require wavelength multiplexing and is not high speed, spatial propagation optical communication, optical information processing and the like.

Sixth Embodiment In the embodiments so far, the two active layers were composed of multiple quantum wells, but this embodiment is composed of quantum wires. The cross-sectional view in the lateral direction is shown in FIG. The first active layer 1004 is composed of a vertically long quantum wire and has a large TM mode gain. On the other hand, the second active layer 1006 is composed of a horizontally long quantum wire and has a large TE mode gain, and the other structure is almost the same as that of the first embodiment. That is, 1011
Is a substrate having a diffraction grating formed on its surface, 1003 is an optical guide layer, 1005 is a barrier layer, 1007 is a cladding layer, 100
8 is a contact layer, 1009 is an n-type buried layer, 101
0 is a p-type buried layer, 1011 is an n-type buried layer, 10
Reference numeral 12 is a contact layer, 1013 and 1014 are Cr / AuZnNi / Au layers which are p-side electrodes, 1015 is an AuGeNi / Au layer which is an n-side electrode, and 1016 is an insulating film SiN _x .

The cross section of the first active layer 1004 has a width of 10 n.
Undoped In _0.53 Ga _0.47 As with m and length of 22 nm
Of InGaAsP (λg = 1.15) at an interval of 20 nm.
μm) It has a structure in which it is arranged in the barrier layer. on the other hand,
The second active layer 1006 has an I of 35 nm in width and 18 nm in length.
n _0.53 Ga _0.47 As is InGaAs at intervals of 20 nm.
It has a structure in which it is arranged in the P (λg = 1.15 μm) barrier layer. Other structures of the optical guide layer, the barrier layer, the electrodes, etc. are almost the same as those of the first embodiment, and the method of setting the Bragg wavelength,
The driving method is also the same.

At present, since the quantum wires are not manufactured in the vacuum part process, the threshold value is high, but the characteristics may be better than those of the MQW structure due to the improvement of the manufacturing process in the future.

In this embodiment, TM is used without introducing distortion.
The gain can be made large, and the differential gain, compared with the MQW in which the number of well layers is conventionally limited by the critical film thickness of strain,
The saturation gain can be increased. Therefore, the quantum efficiency, maximum power, etc. can be made higher than in the first embodiment.

Seventh Embodiment FIG. 11 shows an optical-electrical converter (node) connected to each terminal when the polarization modulation optical transmission using the semiconductor laser according to the present invention is applied to a wavelength division multiplexing optical LAN system. A configuration example is shown, and FIG. 12 shows a configuration example of an optical LAN system using the node.

An optical signal is taken into a node using the optical fiber 1101 connected to each section as a medium, and the branch section 110
2 causes a part of the light to enter the receiving device 1103 including the variable wavelength filter. Although a fiber Fabry-Perot filter is used as the wavelength variable filter, a Mach-Zehnder filter, an interference film filter, or the like may be used instead. The receiving apparatus 1103 extracts only an optical signal of a desired wavelength and performs signal detection. On the other hand, when the optical signal is transmitted from the node, the wavelength tunable DFB laser of the third embodiment or the wavelength tunable DBR laser 1104 of the fourth embodiment is polarization-modulated, and the light converted into the intensity modulation by the polarizer 1105 is used. To the optical transmission line 1101 via the branching unit 1107. At this time, an isolator 1106 may be provided in order to avoid the influence of return light to the laser 1104.

Depending on the terminal, it may be necessary to transmit only a single wavelength. In that case, the DFB laser of the first embodiment is used. On the contrary, when it is necessary to further widen the wavelength variable range, a plurality of wavelength variable lasers may be provided.

As a network of the optical LAN system,
The one shown in FIG. 12 is a bus type, and it is possible to connect nodes in the directions A and B to install a large number of networked terminals and centers. However, in order to connect a large number of nodes, it is necessary to install optical amplifiers in series on the transmission line 1101 in order to compensate for the attenuation of light. Also,
By connecting two nodes to each terminal and using two transmission lines, bidirectional optical transmission by the DQDB system becomes possible.

In the polarization modulation according to the present invention, since the wavelength variation during modulation is 0.01 nm or less as described in the first embodiment, when the wavelength tunable width is 2 nm, 2 / 0.01 = 200 channels. It is possible to construct an optical network system by high-density wavelength division multiplexing transmission. Further, as a network form, a loop type in which A and B in FIG. 12 are connected, a star type,
Alternatively, they may be in a composite form.

[0070]

As described above, the present invention has the following effects corresponding to each claim. According to 1)
A semiconductor laser device capable of independently injecting current into two active layers can provide a laser structure capable of stably and efficiently switching TE-mode and TM-mode polarization. According to 2), a distributed feedback laser capable of independently injecting current into two active layers can provide a laser structure capable of stably and efficiently switching polarization of TE mode and TM mode.
According to 3), it is possible to provide a laser structure capable of stably and efficiently switching polarization of TE mode and TM mode with a distributed reflection laser capable of independently injecting current into two active layers. According to 4), it is possible to provide means and structure for freely setting the difference between the TE mode wavelength and the TM mode wavelength with the laser as in 2), which is effective in wavelength division multiplexing transmission and the like. According to 5), the Fabry-Perot laser structure can be provided by the laser as in 1), which is effective in simple optical transmission and the like. According to 6), a structure for stably oscillating a single transverse mode in the above laser can be provided.
According to 7) and 8), an active layer structure having a large TE mode gain and an active layer structure having a large TM mode gain can be effectively provided. According to 9), it is possible to provide a laser structure in which currents can be independently injected into the two active layers, and it is possible to efficiently perform polarization switching. According to 10) and 11), it is possible to provide a structure capable of changing the oscillation wavelength by the laser or the like as in 9). According to 12) and 13), the single mode property can be improved. According to 14), currents can be independently injected into the two active layers to efficiently perform polarization switching. According to 15), it is possible to provide a driving method for performing polarization switching while changing the oscillation wavelength with the above laser. According to 16), it is possible to provide a driving method for polarization switching while changing the oscillation wavelength with the distributed Bragg reflector laser as in 3). According to 17), it is possible to provide a method for performing optical communication with less chirping by using a laser capable of polarization switching. According to 18), it is possible to provide a light source device for signal transmission using a laser capable of polarization switching. 19)
According to this, it is possible to provide high-density wavelength division multiplexing transmission by polarization modulation transmission. According to 20), it is possible to provide an opto-electric conversion device suitable for a system using polarization modulation transmission as in 19). According to 20), it is possible to provide an optical LAN system using polarization modulation transmission as in 19).

[Brief description of drawings]

FIG. 1 is a sectional perspective view (a) and an energy band structure diagram (b) near an active layer of a DFB laser according to a first embodiment of the present invention.

FIG. 2 is a lateral cross-sectional view for explaining the current injection method of the first embodiment.

FIG. 3 is a diagram showing a positional relationship between a gain profile of each polarization mode and a Bragg wavelength in the first embodiment.

FIG. 4 is a process drawing showing the method of manufacturing the semiconductor laser of the first embodiment according to the present invention.

FIG. 5 is a diagram illustrating an oscillation polarization mode map and a modulation method.

FIG. 6 is a diagram for explaining a polarization modulation waveform and the like.

FIG. 7 is a DF of a second embodiment having two types of diffraction gratings.
It is an energy band structure figure of B laser.

FIG. 8 is a perspective view of a three-electrode type wavelength tunable DFB laser according to a third embodiment of the present invention.

FIG. 9 is a sectional view in the resonance direction of a wavelength tunable DBR laser which is a fourth embodiment of the present invention.

FIG. 10 is a lateral cross-sectional view of a quantum wire laser according to a sixth embodiment of the present invention.

FIG. 11 is an optical LA using a laser according to the present invention.
It is a block diagram which shows the structural example of the photoelectric conversion part used for N system.

FIG. 12 is a diagram illustrating a network of an optical LAN system.

FIG. 13 is a diagram showing a relationship between a strain amount and a well width when designing a quantum well active layer.

FIG. 14 is a diagram showing a conventional example of a polarization modulation laser and a driving method thereof.

FIG. 15 is a diagram showing a conventional example of a semiconductor device having two active layers.

[Explanation of symbols]

101, 701, 911, 1001 Substrate 102, 922 Diffraction grating 103, 703, 707, 901, 913, 1003
Light guide layer 104, 106, 704, 706, 914, 916 S
CH-MQW active layer 105, 705, 915, 1005, 1016 Barrier layer 107, 708, 902, 1007 Cladding layer 108, 112, 811, 812, 813, 908, 1
008, 1012 Contact layers 109, 110, 111, 1009, 1010, 101
1 buried layer 113, 114, 115, 801, 802, 803, 8
04, 805, 951, 952, 953, 954, 10
13, 1014, 1015 Electrode 116 Insulating film 210, 220, 1606, 1607 Current source 408 SiO ₂ mask 413 Photoresist 1004, 1006 Quantum wire active layer 1101 Optical fiber 1102, 1107 Optical branching device 1103 Wavelength selective receiver 1104, 1500 Semiconductor laser 1105, 1501 Polarizer 1106 Optical isolator 1601, 1605 p-type region layer 1602, 1604 Active layer 1603 n-type region layer

Claims (21)

[Claims] 1. In a semiconductor laser device, an active layer having a dominant TM mode and an active layer having a dominant TE mode are stacked with a barrier layer in between, and the two active layers are optically coupled to each other. However, it is electrically isolated and current can be independently injected into the two active layers, and the laser oscillation mode is switched between the TE mode and the TM mode by controlling the amount of the current injection. Semiconductor laser device. 2. A gain peak wavelength of the two active layers is made to substantially coincide with each other, and an optical guide layer having a diffraction grating is provided in the vicinity of either one of the two active layers, and the diffraction grating has the two optical layers. 2. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is of a distributed feedback type having a period such that the Bragg wavelength is set near the gain peak wavelength of the active layer. 3. A distribution in which the gain peak wavelengths of the two active layers are substantially matched and a diffraction grating is provided in series in the cavity direction with respect to the regions of the two active layers stacked with the barrier layer interposed therebetween. 2. A distributed reflection type having a reflector and having a period such that a Bragg wavelength is set near the gain peak wavelengths of the two active layers.
13. The semiconductor laser device according to claim 1. 4. An optical guide layer having different diffraction peak wavelengths of the two active layers and having one diffraction grating near each of the active layers, wherein the diffraction grating is located at the closest position. 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is of a distributed feedback type having a period such that the Bragg wavelength is set near the gain peak wavelength of the active layer. 5. The semiconductor laser device according to claim 1, which is configured as a Fabry-Perot laser. 6. A waveguide composed of the two active layers and a barrier layer between the two active layers is composed of an optically strongly coupled waveguide structure in which a guided transverse mode does not change in a light guiding direction. 6. The semiconductor laser device according to claim 1, wherein the barrier layer has a thickness of 5 nm or more and 50 nm or less. 7. The active layer in which the TM mode gain is dominant is composed of multiple quantum wells in which extension strain is applied to the well layer, and TE
7. The semiconductor laser device according to claim 1, wherein the active layer having a dominant mode gain is composed of a lattice matching system or a multiple quantum well in which a well layer is subjected to compressive strain. 8. The active layer having a dominant TM mode gain is composed of a quantum wire having a vertically long rectangular cross section, and the active layer having a dominant TE mode gain is composed of a quantum wire having a horizontally long rectangular cross section. 7. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is provided. 9. In order to form a waveguide by injecting currents into the two active layers independently, the two active layers have the same conductivity type and sandwich a barrier layer having a different conductivity type therebetween. By making the conductivity type of the buried layer equal to the conductivity type of the barrier layer, the electrode in the buried layer serves as a common electrode, and a pn junction is formed between the active layer and the buried layer. 9. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is a semiconductor laser device. 10. The common electrode in the buried layer is provided in a plurality of two or more in the light traveling direction of the optical waveguide, and the current can be non-uniformly injected in the light traveling direction of the optical waveguide. The semiconductor laser device according to claim 9. 11. A semiconductor laser device according to claim 1, wherein a multi-electrode is formed in the cavity direction so that the oscillation wavelength can be continuously changed. 12. The semiconductor laser device according to claim 2, wherein a phase shift portion is introduced into the diffraction grating. 13. The semiconductor laser device according to claim 4, wherein the element end face is provided with a non-reflection coating. 14. The semiconductor laser device driving method according to claim 1, wherein polarization switching is performed by a change in current of at least one of the currents applied to the two active layers. Driving method of laser device. 15. The method for driving a semiconductor laser device according to claim 1, wherein the laser oscillation wavelength is changed by non-uniform injection of a current in the light traveling direction of the optical waveguide to change the laser oscillation wavelength. A method for driving a semiconductor laser device, characterized in that polarization switching is performed by a change in at least one of the currents applied to the semiconductor laser device. 16. The method for driving a distributed Bragg reflector semiconductor laser device according to claim 3, wherein the oscillation wavelength is changed by the current injected into the distributed reflector portion, and the polarization is caused by the change in the current in the other optical waveguide portion. A method for driving a distributed Bragg reflector semiconductor laser device characterized by switching. 17. A method for driving a semiconductor laser device according to claim 14, wherein modulation by polarization switching is performed, and only one of the polarization modes is taken out by the polarization selecting means to perform signal detection. An optical communication method characterized by: 18. A semiconductor laser device according to any one of claims 1 to 13 and a polarization selecting means, wherein the semiconductor laser device is modulated by polarization switching and the polarization selecting means is arranged at an output end thereof. A light source device for optical communication, wherein only one polarization mode is extracted. 19. The light from the light source device according to claim 18,
Multiple signals are connected to one optical transmission line and signals of a plurality of wavelengths are loaded respectively for transmission, and only a signal of a desired wavelength of light is taken out through a wavelength tunable optical bandpass filter in a receiving device and signal detection is performed. An optical communication method characterized by performing wavelength division multiplexing transmission. 20. A light source device for optical communication according to claim 18, and a receiving device for collecting and detecting only a signal on a light of a desired wavelength through a wavelength tunable optical bandpass filter. An optical-electrical conversion device characterized by the above. 21. A wavelength division multiplexing optical transmission system, wherein the optical-electrical conversion device according to claim 20 is used to perform optical transmission and reception by the optical communication method according to claim 19.