JP3303653B2 - Semiconductor laser device, driving method thereof, and optical communication system using the same - Google Patents

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 for optical communication for suppressing dynamic wavelength fluctuation even during high-speed modulation and for stably realizing high-density wavelength-division multiplexed optical communication and the like, and an optical communication device using the same. System and optical communication system.

[0002]

2. Description of the Related Art In recent years, there has been a demand for an increase in transmission capacity in the field of optical communication, and optical frequency multiplexing (optical FDM) transmission in which a plurality of wavelengths or optical frequencies are multiplexed on one optical fiber has been developed. 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 this purpose, it is desirable that the occupied frequency band or the spectral line width of the laser as the light source be small. However, the direct intensity modulation method used in the current optical communication has a spectral line width at the time of modulation of 0.3.
It has been pointed out that this method is not suitable for optical FDM since it spreads to about nm.

As a system in which the spectral line width does not widen during modulation, an external modulation system or a direct polarization modulation system (Japanese Patent Laid-Open No. Sho 62
-42593, JP-A-62-144426, JP-A-2-
159,781). Here, we are trying to deal with the direct polarization modulation system.

[0004] This polarization modulation is performed as shown in FIG.
When the bias current is fixed at the point where the E mode and the TM mode are switched, and I1 is modulated by the minute rectangular current ΔI1, FIG.
This is a method in which the polarization plane switches as shown in FIG. By using this, as shown in FIG.
The ASK is performed by placing the polarizer 1001 at the output terminal of 0 and selectively extracting only one of the polarization planes. According to this method, the wavelength variation is even smaller than that of the external modulation method despite direct modulation by merely devising the structure of a normal DFB laser (distributed feedback laser).

[0005] The principle will be briefly described. By flowing a constant current of I2 to one electrode of the laser 1000 in FIG. 11A and flowing I1 and the modulation current ΔI1 to the other electrode, the phase difference of the round trip in the DFB laser changes. As a result, the loss due to the diffraction grating changes, that is, the threshold gain changes. However, since the effective refractive index of the waveguide differs between the TE mode and the TM mode, the phase difference changes, that is, the threshold gain changes. The way is different. Therefore, TE
The magnitude relationship between the threshold values of the mode and the TM mode can be changed, thereby enabling polarization modulation.

[0006]

The polarization-modulated DFB laser conventionally proposed achieves polarization switching by controlling the phase, and the gain itself cannot be changed much. Therefore, it must be manufactured with high precision so that the gain of TE / TM becomes close to the oscillation wavelength of the DFB mode (near the Bragg wavelength). Specifically, it is necessary to match the gain peak of TE / TM in the active layer, or to match the positional relationship between the gain peak and the Bragg wavelength of the diffraction grating on the order of several nanometers, and it is difficult to manufacture the active layer with good reproducibility. In addition, since the phase shift amount of the diffraction grating and the influence of end face reflection are large, the phase shift amount, the anti-reflection coating, and the like also require accuracy.

[0007] Even if the device is manufactured to a certain degree of accuracy, the manufacturing error causes variations in the TE / TM threshold gain and the initial phase difference of the diffraction grating among the devices, resulting in polarization switching. Because variations occur in the laser bias current and the modulation amplitude of the modulation current,
Productivity is not good.

In view of these problems, an object of the present invention is to
To provide a highly productive polarization-switchable semiconductor laser capable of controlling the confinement of light in an optical waveguide instead of the phase control and changing the magnitude relation between the threshold gains of the TE mode and the TM mode ( In particular, to provide a semiconductor laser capable of more effectively performing confinement control (especially to claim 3), and to provide a semiconductor laser capable of single longitudinal mode oscillation (especially to claims). To provide a semiconductor laser applicable to simple optical communication or space propagation optical communication that does not require wavelength multiplexing (corresponding to items 4, 5, and 14) (especially corresponding to claim 6), to efficiently control confinement. To provide a semiconductor laser having a confinement control layer for performing the operation (particularly corresponding to claims 7, 8 and 9), a TE mode and a TM for efficient polarization switching in the semiconductor laser. Providing a semiconductor laser having means for reducing the gain of the laser diode (especially corresponding to claims 10, 11 and 12), the efficiency of injection into the laser active layer is improved, and the threshold current can be reduced. Providing a semiconductor laser capable of increasing the power of the optical output (particularly corresponding to claim 13), means for applying a current or a voltage independently to the active layer of the laser and the confinement control layer in the vicinity or the buried heterostructure Providing a semiconductor laser having the following and a driving method for performing polarization switching of the semiconductor laser (especially corresponding to claims 15 and 19);
To provide a semiconductor laser having means for applying a current or a voltage independently to an active layer of a laser and a confinement control layer in a vicinity portion or a buried heterostructure while changing an oscillation wavelength, and a driving method for performing polarization switching of the semiconductor laser. (Especially corresponding to claims 16 and 20), to provide a distributed reflection type laser capable of reducing the wavelength fluctuation at the time of modulation and a driving method for polarization switching of this semiconductor laser (especially corresponding to claims 17 and 21). To provide a distributed-reflection laser capable of reducing the wavelength fluctuation at the time of modulation and a driving method for performing polarization switching of the semiconductor laser (especially corresponding to claims 18 and 22);
To provide an optical communication system for easily performing optical communication by driving a semiconductor laser as described above.
4), providing an optical communication system for performing high-density wavelength-division multiplexing optical transmission with the above-described semiconductor laser (particularly corresponding to claims 25 and 26), and using the above-described high-density wavelength-division multiplexing optical transmission. Thus, an optical LAN system is constructed (particularly corresponding to claims 27 and 28).

[0009]

According to a semiconductor laser device of the present invention which achieves the above object, an optical waveguide including at least a part of an active layer for light emission extends in a resonator direction, and at least a part of the optical waveguide is provided. The optical waveguide is configured so that the distribution of the refractive index in the vicinity of the optical waveguide and the optical waveguide can be controlled, and the gain of the two waveguide modes of the optical waveguide, TE mode and TM mode, is almost equal. It is characterized by.

[0010] In order to achieve the above-mentioned more specific object, the following configuration is also possible. The optical waveguide is formed with a buried heterostructure, and the buried heterostructure has a confinement control layer that is slightly smaller in refractive index than the active layer in the optical waveguide and can control light confinement in the lateral direction of the optical waveguide. It is provided near the extent to which the oscillation light of the laser is coupled. A confinement control layer in a neighboring portion or buried heterostructure is provided adjacent to the active layer of the laser. A semiconductor laser is a distributed feedback laser having a diffraction grating near an active layer. The semiconductor laser is a distributed reflection laser having a distributed reflector having a diffraction grating in series in the resonator direction. The semiconductor laser is a Fabry-Perot laser with cleaved ends. In a semiconductor laser, the confinement control layer in the neighboring portion or the buried heterostructure is a multiple quantum well structure, and the reverse portion or the confining control layer is applied by applying a reverse voltage or injecting a forward current to the neighboring portion or the confining control layer. Is changed, the rate of change for the TE mode is greater than the rate of change for the TM mode. In a semiconductor laser, the confinement control layer in a nearby portion or a buried heterostructure has a multiple quantum well structure, and when the refractive index of the nearby portion or the confinement control layer changes by injecting a forward current into the nearby portion or the confinement control layer. To
The rate of change for the TE mode is greater than the rate of change for the TM mode. In the multiple quantum well, a compressive strain is applied to the well layer. TE mode and TM in active layer
To make the gains of the modes approximately equal is provided near the active layer so that the Bragg wavelength comes close to the wavelength with respect to the energy band gap between the light hole ground level that gives the TM mode gain and the electron ground level. This is a distributed feedback laser that is configured by setting the pitch of the diffraction grating, and is configured so that the threshold gain at the Bragg wavelength is substantially equal in the TE mode and the TM mode. Making the gains of the TE mode and the TM mode substantially equal in the active layer is mainly due to the Bragg wavelength being close to the wavelength for the energy band gap between the light hole ground level and the electron ground level that gives the gain to the TM mode. Is performed by setting the pitch of the diffraction gratings formed in series in the resonator direction so that the threshold gain at the Bragg wavelength is substantially equal in the TE mode and the TM mode. Laser. Making the gains of the TE mode and the TM mode substantially equal in the active layer is made up of a multiple quantum well in which a tensile strain is introduced in the active layer, and the ground level of the heavy hole is substantially equal to the ground level of the light hole. Alternatively, this is performed by making the ground level of the light hole close to the electron ground level. In order to increase the electrical insulation between the confinement control layer and the active layer in the vicinity or the buried heterostructure, a high resistance layer is formed on the bottom and side surfaces between the two. Give the diffraction grating a phase shift. Electrodes are provided so that current injection into the laser active layer and voltage application or current injection to the confinement control layer in the vicinity or the buried heterostructure can be performed independently. It is possible to provide two or more electrodes in the light traveling direction of the optical waveguide and to perform current injection and voltage application non-uniformly in the light traveling direction of the optical waveguide. In the distributed reflector and other optical waveguide portions of the distributed reflection laser, current injection into the optical waveguide of the distributed reflector and voltage application or current injection to a portion near the distributed reflector or the confinement control layer of the buried heterostructure Are provided so as to be able to perform the above operations independently. Electrodes are provided in the distributed reflector of the distributed reflection laser and the other optical waveguide portions so that current injection into the laser active layer and voltage application to the optical waveguide of the distributed reflector can be performed independently. .

According to another aspect of the present invention, there is provided a method of driving a semiconductor laser device, comprising: applying a voltage to a confinement control layer of a nearby portion or a buried heterostructure; Switching between the TE mode and the TM mode is characterized. Further, the method of driving a semiconductor laser device according to the present invention changes the laser oscillation wavelength by non-uniform current injection, and applies a voltage or a forward current injection to a nearby portion or a confinement control layer of a buried heterostructure to one of the optical waveguides. It is characterized in that polarization switching is performed by changing the voltage in the section. Further, in the driving method of the semiconductor laser device according to the present invention, the oscillation wavelength is changed by the current injected into the optical waveguide of the distributed reflector, and the voltage is applied to the portion near the distributed reflector or the confinement control layer of the buried heterostructure or the voltage is sequentially applied. Polarization switching is performed by directional current injection. Further, the driving method of the semiconductor laser device of the present invention is characterized in that polarization switching is performed by a voltage applied to the optical waveguide of the distributed reflector.

In order to achieve the above object, in the optical communication system of the present invention, modulation by polarization switching is performed by the above-described method of driving a semiconductor laser device, and either one of them is selected by a polarization selecting means such as a polarizer. It is characterized in that only the polarization mode is extracted and signal detection is performed. Further, in the optical communication system of the present invention, the oscillation wavelength is changed by the driving method of the semiconductor laser device described above, modulation is performed by polarization switching by the driving method, and the desired wavelength is passed through the optical band-pass filter in the receiving device. Is characterized in that only the signal placed on the light is extracted and signal detection is performed. Further, in the optical communication system of the present invention, a plurality of the above-described semiconductor laser devices and receiving devices are connected to one optical fiber to transmit light of a plurality of wavelengths, respectively. Wavelength-division multiplex transmission so that only a signal placed on light having a wavelength of? In the optical communication system of the present invention, the wavelength can be changed by the above-described method of driving a semiconductor laser, modulation is performed by polarization switching, and polarization selecting means such as a polarizer is arranged at the output end. A plurality of optical communication light source devices that take out only the polarization mode of the optical fiber are connected to one optical fiber to transmit light of a plurality of wavelengths, respectively.
The receiver is characterized in that wavelength division multiplex transmission is performed so that only a signal placed on light having a desired wavelength is extracted through a wavelength variable optical bandpass filter and signal detection is performed.

[0013] In order to achieve the above object, an optical-electrical converter according to the present invention is characterized in that a semiconductor laser and a receiver are integrated into one, and optical transmission and reception by the above-mentioned optical communication system is performed. In order to achieve the above object, a wavelength division multiplexing optical transmission system of the present invention is characterized by using the above-mentioned optical-electrical conversion device.

The principle of the present invention will be described using a specific example.
In the present invention, polarization switching can be performed by positively changing the gain of TE / TM instead of changing the phase. FIG. 1 shows a specific device structure according to the present invention.
It is explained along. Although similar to a normal buried heterostructure DFB laser, the buried portion is adjacent to the active layer of the laser and has a higher energy band gap (lower refractive index) near the active layer (containment control layer). Have. When a reverse electric field is applied to this confinement control layer,
The energy band gap becomes low (the refractive index becomes high) and approaches the energy band gap of the laser active layer. Then, the lateral confinement of the laser oscillation light deteriorates, and the oscillation threshold value increases. At this time, if a multiple quantum well (MQW) is used as the confinement control layer, the band gap, that is, the rate of change of the refractive index with respect to the electric field is different (FIG. 2), and greatly changes for the TE mode.
It does not change much for the TM mode. Such M
A change in the energy band gap due to the electric field of the QW is called a quantum confined Stark effect (QCSE). Polarization switching can be performed by utilizing the polarization dependence of the QCSE. That is, if the threshold value of the TE mode is set to be low without applying an electric field, the TE mode oscillates without an electric field, and the threshold value increases only in the TE mode when an electric field is applied to the confinement control layer. As a result, the threshold value of the TM mode becomes lower and oscillation occurs in the TM mode.

There is also a method of passing a current instead of a voltage to the buried layer. The refractive index decreases due to the plasma effect due to the current injection. In particular, the effect on the TE mode is large, the confinement is improved due to the decrease in the refractive index, and the threshold gain decreases. That is, the operation can be performed such that the oscillation transits from the TM mode to the TE mode by applying the current, contrary to the voltage application. The above principle is substantially the same even if the configuration is slightly changed from FIG.

If polarization modulation is performed by such a method, there are two setting parameters, such as a laser bias and an electric field, and a manufacturing error can be covered by a driving method.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 A first embodiment according to the present invention will be described. FIG. 1 is a cross-sectional view of a DFB laser according to the present invention (cross-section perpendicular to the cavity direction). The active layer has a bandgap wavelength of 1.56 μm.
m, SCH-M such that the confinement control layer in the buried heterostructure has a bandgap wavelength of 1.45 μm.
QW. The layer configuration will be described in detail below. FIG.
In the figure, 101 is n-InP serving as a substrate, 102 is an n-InP buffer layer on which a diffraction grating having a depth of 0.05 μm is formed, 103 is n-InP having a thickness of 0.1 μm and a bandgap wavelength λg = 1.25 μm. InGaAsP lower optical guide layer 104 is i-In _0.28 Ga _0.72 As which is a well layer
(Thickness 10 nm), i-InGaAsP barrier layer
(Λg = 1.25 μm, thickness 10 nm) An active layer having a strained multiple quantum well structure consisting of 10 layers, and 105 having a thickness of 0.02 μm
P-InGaAsP (λg = 1.25 μm) upper light guide layer, 106 is a p-InP cladding layer, 107 is p-
In _0.53 Ga _0.47 As contact layer, 108 is p-In
P buried layer, 109 is p-InGa having a thickness of 0.2 μm
AsP (λg = 1.15 μm), i-In _0.53 Ga _0.47
As well layer (4 nm thick) and i-InGaAsP (λg
= 1.15 μm, 10 nm thick) 10 barrier layers and 0.2 μm thick n-InGaAsP (λg = 1.15)
SCH-MQW confinement control layer consisting of
Is an n-InP buried layer, 111 and 112 are Cr / AuZnNi / Au layers as p-side electrodes, 113 and 114 are AuGeNi / Au layers as n-side electrodes, and 115 is an insulating layer. The carrier concentration of the electrode in the p-InP cladding layer 106 was partially increased by Zn diffusion so that an ohmic was easily obtained.

Here, in this DFB laser, the active layer 10
4 is a multiple quantum well layer having a tensile strain,
Since the transition energies of Ehh0-Ee0 and Elh0-Ee0 are designed to be equal, the oscillation threshold value in the TM polarization is lower than that of a normal DFB laser, and the polarization switching can be performed efficiently.

In the above configuration, a forward bias current is applied between the electrodes 114 and 111 as shown in the sectional perspective view of FIG. It is shown in FIG. The wavelength corresponding to the transition energy between the ground level of the light hole and the electron (Elh0-Ee0) is 1.56 μm, and the wavelength corresponding to the transition energy between the ground level of the heavy hole and the electron (Ehh0-Ee0) is also 1.56 μm. Become. TE mode (solid line)
And the emission spectrum of the TM mode (broken line) almost overlaps, but the distributed feedback wavelength by the diffraction grating is Elh0-Ee0.
The pitch of the diffraction grating is set to 240 nm so as to be on the shorter wavelength side than the wavelength corresponding to
It has a configuration having a Bragg wavelength of 1.558 μm in the μm and TM modes.

When a forward bias current is applied between the electrodes 114 and 111, oscillation occurs in the TE mode at a threshold value of about 20 mA. Here, when a reverse electric field of 5 V is applied between the electrodes 113 and 112, as shown in FIG. 2, the refractive index of the confinement control layer 109 of the buried heterostructure becomes higher than that in the TE mode, and confinement in the active layer 104 becomes impossible. The threshold value is lower in the TM mode than in the TM mode,
Transition to mode. It is efficient to apply the electric field to both the left and right sides, but only one of them may be used. The electrode 111 and the electrode 1
13 may be short-circuited.

Therefore, an amplitude of 5 V is applied between the electrodes 113 and 112.
Is applied, TE / TM polarization modulation can be performed. FIG. 4 shows a time waveform at this time. In FIG. 4, 1) is a modulation voltage waveform, 2) is laser output light, and 3) is T
E-polarized light and 4) represent TM-polarized light. As shown in this figure, although the laser output light does not change significantly due to the modulation, it can be seen that the laser output light is 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 305 may be placed on the laser emission end face, and either the TE mode or the TM mode may be selected and extracted as intensity modulated light. At this time, the dynamic fluctuation of the oscillation wavelength, that is, chirping, was 0.01 nm or less.

In this embodiment, a tensile strained MQW is used as the active layer 104 of the laser. However, an InGaAsP single layer having a band gap wavelength of 1.55 μm or an MQM having no strain is used.
W may be used to set the Bragg wavelength near the band gap wavelength of Elh0-Ee0. Further, the confinement control layer 109 having the buried hetero structure has a distortion-free MQW structure. However, in order to increase the polarization dependence, the compression distortion MQW
(For example, In _0.7 Ga _0.3 As (2 nm thick) / InG
aAsP). In FIG. 1, the confinement control layer 10 is shown.
9 and the active layer 104 are substantially the same height, but may be slightly shifted or the confinement control layer may be bent. Of course, it is also useful to introduce a λ / 4 shift structure into the diffraction grating, or to improve the single mode by applying an anti-reflection coating to the end face.

In the present embodiment, the relationship between the gain peak of the active layer and the Bragg wavelength and the fabrication accuracy required for the anti-reflection coating and the like are eased as compared with the conventional polarization modulation laser using the phase control method.

Embodiment 2 In a second embodiment of the present invention, a laser having substantially the same configuration (layer composition is slightly different) as that of Embodiment 1 is used, and the refractive index change of the confinement control layer in the buried heterostructure is measured by current injection. It is what you do. The difference from the first embodiment is that the confinement control layer is a layer having a bandgap wavelength of 1.48 μm. As a result, when there is no current injection into the confinement control layer, in the TE mode, the difference in band gap between the active layer and the confinement control layer is small and the effect of optical confinement is small, so that oscillation can be suppressed. Also, the pitch of the diffraction grating is slightly different from that of the first embodiment, and in this layer configuration, the Bragg wavelength is set to 1 near the gain peak in the TM mode so as to oscillate in the TM mode when no current is injected into the confinement control layer. .56 μm and 1.564 μm in the TE mode so that the gain in the TM mode is slightly increased. When a current is injected into the confinement control layer, the refractive index decreases due to the plasma effect. In particular, the lateral confinement of the TE mode is improved, and the threshold gain of the TE mode is reduced. TM
To TE. This enables polarization modulation.

Even in the case where the initial state cannot be driven due to the TM oscillation due to a manufacturing error or the like as in the first embodiment, the driving may be performed as in the present embodiment.

Embodiment 3 The third embodiment of the present invention is an embodiment in which the laser of the first embodiment is multi-electrode in the direction of the resonator. FIG. 5 shows a cross-sectional perspective view of a three-electrode laser according to the present invention (the right half of the device is shown). A λ / 4 shift 407 is provided at the center of the electrode 402, and antireflection coating is applied to both end faces. The electrode length is (401, 404), (402, 40)
5), (403, 406) 300 μm, 10
0 μm and 300 μm. The oscillation wavelength can be changed by changing the ratio of each current. In this element, 4
01 current I1, 402 current I2, 403 current I3
By changing the value of I2 / (I1 + I2 + I3) from 0.1 to 0.5, the oscillation wavelength can be continuously increased by about 2
nm. Thus, the element can be used as a light source for wavelength division multiplex transmission. In FIG. 5, the same reference numerals as those in FIG. 1 indicate the same functional units.

Driving of polarization modulation can be performed by applying a voltage signal having an amplitude of 8 V to only the electrode 405.
The reason why the voltage amplitude is larger than that in the first embodiment is that the region for performing the confinement control is short only in the central portion. However, compared to the first embodiment, the parasitic capacitance is reduced, so that the modulation band is extended and modulation up to 10 GHz is possible.

Embodiment 4 A fourth embodiment according to the present invention is applied to a distributed reflection (DBR) laser. FIG. 6 is a cross-sectional view parallel to the resonator of the DBR laser according to the present invention. The electrodes 511, 512, and 513 are divided into three for the laser injection electrode and the electric field application electrode as in the third embodiment, and correspond to the active region, the phase adjustment region, and the DBR region, respectively. When an electric field is applied to the buried layer in the DBR region to weaken the lateral confinement with respect to the TE mode, the amount of light distributed and reflected in the active region becomes larger in the TM mode, and the oscillation mode changes from TE to TM. I do.

In this element, the layer structure of the active region having the active layer 515 is almost the same as that of the first embodiment. In the DBR region, a diffraction grating is formed after etching up to the InP substrate 516, and InGaAs having a band gap wavelength of 1.4 μm is formed.
sP light guide layer 501, p-InP clad layer 502,
The p-InGaAs contact layer 503 has been regrown. Further, unlike the first embodiment, the confinement control layer of the buried layer is made of MQW having a band gap wavelength of 1.3 μm.

In this device, the injection current (between the electrodes 513 and 514 and between the electrodes 512 and
And between the electrodes 514), the oscillation wavelength becomes about 3 nm.
Can be changed. Therefore, it can be used as a light source for wavelength division multiplex transmission as in the third embodiment.

Fifth Embodiment A fifth embodiment according to the present invention employs the same DBR as in the fourth embodiment.
Although it is a laser, it changes the bandgap wavelength of the optical waveguide itself in the DBR region by an electric field. FIG.
FIG. 3 is a cross-sectional view of a DBR laser according to the present invention in the direction of the resonator, in which an optical guide layer 601 having a band gap wavelength of 1.25 μm and InGaAs / InGaAs having a band gap wavelength of 1.45 μm are formed on a diffraction grating formed on a substrate 616.
P10 MQW layer (the confinement control layer 109 of the first embodiment)
602), InP cladding layer 603, InGaAs
The s-contact layer 604 has been regrown. The embedding structure in the horizontal direction is different from the first to fourth embodiments, and is constituted only by high-resistance InP. The layer configuration of the active region including the active layer 615 is the same as that of the first embodiment.

In this device, current is injected into the active region (electrode 611).
And between the electrodes 614) to obtain DBR oscillation. However, when an electric field is applied to the DBR region (between the electrodes 613 and 614), T
The confinement for the E mode is deteriorated, and the amount of distributed reflection is reduced, and the amount of distributed reflection for the TM mode is larger. Therefore, a transition is made from the TE mode to the TM mode.

As described above, in this device, since the polarization modulation is performed by the electric field in the DBR region, the variable wavelength operation is not performed. Also, the phase adjustment region is not adjusted when the wavelength is tuned, but the electrode 612 is used at the initial setting to stabilize the DBR oscillation, or when the reverse electric field is applied to the DBR region, Can be used to prevent the inflow of current.

Embodiment 6 Although the embodiments of the dynamic single mode laser having the diffraction grating have been described in the embodiments 1 to 5, the idea of the present invention is to use a Fabry-Perot laser having no diffraction grating and cleaved at both end faces. Also applicable to The structure is almost the same as that of FIG. 1. The lower light guide layer 103 is made as thin as 0.02 μm, and no diffraction grating is manufactured. Although the single mode property is not good, polarization modulation can be performed as in the first embodiment. The present embodiment can be applied to simple optical communication that does not require wavelength division multiplexing, space propagation optical communication, optical information processing, and the like.

Embodiment 7 FIG. 8 is a sectional view of a DFB laser according to a seventh embodiment of the present invention. The structure is almost the same as that of the first embodiment. In this embodiment, in order to increase the electrical insulation between the active layer 104 and the confinement control layer 109, 0.3 μm
In this embodiment, a high-resistance InP layer 901 m is grown on the bottom and side surfaces, and then buried in the same manner as in the first embodiment.
The growth of InP may be based on the fact that the growth rate of the mesa side surface and the bottom surface becomes substantially equal by growing the film by the reduced pressure MOCVD method when the light guide direction of the waveguide is set to the reverse mesa direction.

In this embodiment, since the leakage current can be reduced, the efficiency of laser injection into the active layer 104 is improved, so that the threshold current can be reduced and the optical output can be made higher in power.

Embodiment 8 This embodiment is an example in which intensity-modulated optical transmission using a semiconductor laser according to the present invention is applied to a wavelength division multiplexing optical LAN system. FIG. 9 shows a configuration example of an optical-electrical conversion unit (node) connected to each terminal in this case, and FIG. 10 shows a configuration example of an optical LAN system using the node.

Using an optical fiber 701 connected to each unit as a medium, an optical signal is taken into a node, and a branching unit 702 partially receives a receiving device 70 having a wavelength tunable filter.
3 is incident. 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 receiver 703 extracts only an optical signal of a desired wavelength and performs signal detection. On the other hand, when transmitting an optical signal from the node, the wavelength tunable DFB laser of the third embodiment or the wavelength tunable DBR laser 704 of the fourth embodiment is polarization-modulated, and the light converted into intensity modulation by the polarizer 705 is branched. The light enters the optical transmission path via the unit 707. At this time, an isolator 706 may be inserted to avoid the influence of the return light to the laser. In some cases, only a single wavelength needs to be transmitted depending on the terminal. In that case, the DFB laser of the first embodiment or the DBR laser of the fifth embodiment is used. Conversely, if it is necessary to further widen the wavelength tunable range, a plurality of wavelength tunable lasers may be provided.

As a network of the optical LAN system,
FIG. 10 shows a bus type in which nodes are connected in directions A and B, and a large number of networked terminals and centers can be installed. However, in order to connect a large number of nodes, it is necessary to install an optical amplifier on a transmission line to compensate for optical attenuation. Also, by connecting two nodes to each terminal and using two transmission paths, DQD
Bidirectional optical transmission by the B method becomes possible.

In the polarization modulation according to the present invention, since the wavelength fluctuation at the time of modulation is 0.01 nm or less as described in the first embodiment, when the wavelength variable width is 2 nm, 2 / 0.01 = 200.
An optical network system based on high-density wavelength multiplex transmission of channels can be constructed. Further, as a network form, a loop type connecting A and B in FIG. 10, a star type,
Alternatively, they may be in a composite form.

In this embodiment, signals in both polarization modes are transmitted except for the polarizer (see (3) and (4) in FIG. 4), and a differential output of both signals is obtained on the receiving side for reception. Detection may be performed.

[0043]

According to the configuration of the present invention as described above, the following effects can be obtained. It is possible to manufacture a semiconductor laser capable of performing polarization switching stably with good yield without being largely affected by a deviation from a design value in laser production (especially corresponding to claims 1 and 2). A semiconductor laser capable of performing polarization switching control more effectively can be obtained (particularly corresponding to claim 3). A semiconductor laser capable of single longitudinal mode oscillation can be obtained (especially corresponding to claims 4, 5 and 14).
A confinement control layer for efficiently performing polarization switching in a semiconductor laser can be provided (especially claims 7, 8, and 9).
Corresponding to). In order to efficiently perform polarization switching in the semiconductor laser, the gains of the TE mode and the TM mode can be made close to each other (particularly corresponding to claims 9, 10 and 11). A semiconductor laser capable of improving the injection efficiency into the laser active layer, reducing the threshold current, and increasing the power of the optical output (especially corresponding to claim 13). A current or a voltage can be independently applied to the active layer of the laser and its vicinity or the confinement control layer in the buried heterostructure, so that power consumption required for modulation is small and high-speed driving is facilitated. Further, the degree of freedom of design is widened (especially corresponding to claims 15 and 19). The current or voltage can be independently applied to the active layer of the laser and the confinement control layer in the vicinity or the buried heterostructure while changing the oscillation wavelength, the power consumption required for modulation is small, high-speed driving is easy, and It can be applied as a light source for multiplex light transmission. In addition, the degree of freedom of design is widened (especially in claims 16 and 2,
0). Power consumption required for modulation is small, high-speed driving is easy, and a distributed reflection laser that can reduce wavelength fluctuation during modulation can be realized. Further, the degree of freedom of design is widened (especially corresponding to claims 17 and 21). Further, it is possible to realize a distributed-reflection laser capable of reducing the wavelength fluctuation at the time of modulation, which facilitates fabrication (particularly corresponding to claims 18 and 22). Optical communication can be easily performed by driving the semiconductor laser of the present invention (especially corresponding to claims 23 and 24). With the semiconductor laser of the present invention, wavelength multiplexing optical transmission with high multiplicity can be realized at low cost (particularly corresponding to claims 25 and 26). An optical LAN system with low cost and high throughput can be constructed by using high-density wavelength division multiplexing optical transmission as described above (especially corresponding to claims 27 and 28).

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a DFB laser (Example 1) according to the present invention.

FIG. 2 is a view for explaining a difference in change in refractive index of a multiple quantum well with respect to an electric field due to polarization.

FIG. 3 is a diagram illustrating a driving method of a DFB laser and an oscillation wavelength according to the present invention.

FIG. 4 is a diagram illustrating a modulation waveform and the like of a DFB laser according to the present invention.

FIG. 5 shows a three-electrode DFB laser according to the present invention (Example 3).
FIG.

FIG. 6 is a longitudinal sectional view of a tunable DBR laser (Example 4) according to the present invention.

FIG. 7 is a longitudinal sectional view of a DBR laser (Example 5) according to the present invention.

FIG. 8 is a cross-sectional view of a DFB laser (Example 7) according to the present invention.

FIG. 9 is a diagram showing a configuration example of an optical-electrical conversion unit used in an optical LAN system using a laser according to the present invention.

FIG. 10 is a diagram illustrating a network of the optical LAN system.

FIG. 11 is a diagram illustrating a conventional example of a polarization modulation laser and a driving method thereof.

[Explanation of symbols]

101,516616 Substrate 102 Buffer layer 103,105,501,601 Light guide layer 104,515,615 Active layer 106,502,603 Cladding layer 107,503,604 Contact layer 108,110 Buried layer 109,602 MQW layer 111, 112, 113, 114, 401, 402, 4
03,404,405,406,511,512,51
3,514,611,612,613,614 Electrode 115 Insulating film 305,1001 Polarizer 407 λ / 4 shift unit 701 Optical fiber 702,707 Optical splitter 703 Wavelength selective receiver 704 Semiconductor laser 705 Polarizer 706 of the present invention Optical isolator 901 High-resistance InP insulating layer 1000 Conventional semiconductor laser

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (28)

(57) [Claims] In a semiconductor laser device, an optical waveguide including at least a part of an active layer for light emission extends in a cavity direction, and a portion near at least a part of the optical waveguide and a portion of the optical waveguide in the optical waveguide are provided. A semiconductor laser device configured to control a distribution state of a refractive index, wherein the active layer has substantially equal gains in a TE mode and a TM mode, which are two waveguide modes of the optical waveguide. 2. The optical waveguide is formed of a buried heterostructure, wherein the buried heterostructure has a slightly lower refractive index than an active layer in the optical waveguide and can control lateral optical confinement of the optical waveguide. 2. The semiconductor laser device according to claim 1, wherein the confinement control layer, which is the vicinity portion, is provided in such a vicinity as to couple the oscillation light of the laser. 3. The semiconductor laser device according to claim 1, wherein a confinement control layer in said vicinity portion or said buried heterostructure is provided adjacent to an active layer of said laser. 4. The semiconductor laser device according to claim 1, wherein said semiconductor laser is a distributed feedback laser having a diffraction grating near said active layer. 5. The semiconductor laser device according to claim 1, wherein said semiconductor laser is a distributed reflection laser having a distributed reflector provided with a diffraction grating in series in a resonator direction. 6. The semiconductor laser device according to claim 1, wherein said semiconductor laser is a Fabry-Perot laser having cleaved end faces. 7. The semiconductor laser, wherein the confinement control layer in the neighboring portion or the buried heterostructure has a multiple quantum well structure, and the vicinity portion or the confining control layer is applied by applying a reverse voltage to the neighboring portion or the confining control layer. 3. The semiconductor laser device according to claim 1, wherein when the refractive index of the confinement control layer changes, the change rate for the TE mode is larger than the change rate for the TM mode. 8. The semiconductor laser, wherein the confinement control layer in the neighboring portion or the buried heterostructure has a multiple quantum well structure, and the vicinity portion or the confining control layer is formed by injecting a forward current into the neighboring portion or the confining control layer. Alternatively, when the refractive index of the confinement control layer changes, TE
3. The semiconductor laser device according to claim 1, wherein the rate of change with respect to the mode is greater than the rate of change with respect to the TM mode. 9. The semiconductor laser according to claim 7, wherein a compressive strain is applied to the well layer in the multiple quantum well. 10. Making the gains of the TE mode and the TM mode substantially equal in the active layer is mainly because the gain of the TE mode and that of the TM mode, which give a gain to the TM mode, depend on the wavelength relative to the energy band gap between the light hole ground level and the electron ground level. This is performed by setting the pitch of the diffraction grating provided in the vicinity of the active layer so that the Bragg wavelength comes in the vicinity, and the threshold gain at the Bragg wavelength is substantially equal in the TE mode and the TM mode. 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is a distributed feedback laser. 11. Making the gains of the TE mode and the TM mode substantially equal in the active layer is mainly because the gain of the TM mode and the energy band gap between the ground level of the electron and the ground level of the electron, which give a gain to the TM mode, are different from each other. This is done by setting the pitch of the diffraction gratings formed in series in the direction of the resonator so that the Bragg wavelength comes close to it, so that the threshold gain at the Bragg wavelength is almost equal between the TE mode and the TM mode. 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is a distributed reflection laser. 12. Making the gains of the TE mode and the TM mode substantially equal in the active layer is constituted by a multiple quantum well in which a tensile strain is introduced in the active layer, wherein the ground level of a heavy hole and the light hole 3. The semiconductor laser device according to claim 1, wherein the ground level is substantially equal or the ground level of the light hole is close to the ground level of the electron. 13. A high-resistance layer is formed on a bottom surface and a side surface between the active layer and the confinement control layer in the neighboring portion or the buried heterostructure in order to enhance electrical insulation. 3. The semiconductor laser device according to claim 1, wherein: 14. The semiconductor laser device according to claim 4, wherein said diffraction grating has a phase shift. 15. An electrode is provided so that current injection into the laser active layer and voltage application or current injection to the vicinity portion or the confinement control layer of the buried heterostructure can be performed independently. The semiconductor laser device according to claim 1. 16. A method according to claim 1, wherein two or more electrodes are provided in the light traveling direction of the optical waveguide, and current injection and voltage application can be performed non-uniformly in the light traveling direction of the optical waveguide. The semiconductor laser device according to claim 1, wherein: 17. In the distributed reflector of the distributed reflector laser and other optical waveguide portions, current is injected into the optical waveguide layer of the distributed reflector and the portion near the distributed reflector or the buried heterostructure is provided. 12. The semiconductor laser device according to claim 5, wherein an electrode is provided so that voltage application or current injection to the confinement control layer can be performed independently. 18. In the distributed reflector of the distributed reflection laser and the other portion of the optical waveguide, current injection into the laser active layer and application of a voltage to the optical waveguide of the distributed reflector can be performed independently. 12. The semiconductor laser device according to claim 5, wherein an electrode is provided as described above. 19. The semiconductor laser device according to claim 15, wherein the laser oscillation mode is switched between a TE mode and a TM mode by applying a voltage or applying a forward current to the confinement control layer of the neighboring portion or the buried heterostructure. A method for driving a semiconductor laser device. 20. The semiconductor laser device according to claim 16, wherein the laser oscillation wavelength is changed by non-uniform current injection, and voltage application or forward current injection to the vicinity portion or the confinement control layer of the buried heterostructure is performed. A method for driving a semiconductor laser device, wherein polarization switching is performed by changing a part of an optical waveguide. 21. The semiconductor laser device according to claim 17, wherein the oscillation wavelength is changed by a current injected into the optical waveguide of the distributed reflector, and the oscillation wavelength is changed to the vicinity of the distributed reflector or the confinement control layer of the buried heterostructure. A polarization switching by applying a voltage or injecting a forward current into the semiconductor laser device. 22. The method of driving a semiconductor laser device according to claim 18, wherein polarization switching is performed by a voltage applied to the optical waveguide of the distributed reflector. 23. A method for driving a semiconductor laser according to claim 19, 20, 21 or 22, wherein modulation by polarization switching is performed, and only one of the polarization modes is taken out by a polarization selection means to perform signal detection. An optical communication system characterized by the following. 24. The method of driving a semiconductor laser according to claim 20 or 21, wherein the wavelength is changed, and modulation is performed by polarization switching by the driving method. An optical communication system characterized in that only a signal placed on light is extracted and signal detection is performed. 25. An optical fiber according to claim 1, wherein:
A plurality of semiconductor laser devices and receiving devices according to any of the above are connected, light of a plurality of wavelengths is respectively transmitted, and only a signal placed on light of a desired wavelength is extracted through a wavelength variable optical bandpass filter in the receiving device. An optical communication system characterized by performing wavelength division multiplexing transmission for signal detection. 26. A method for driving a semiconductor laser according to claim 20, wherein the wavelength can be changed, modulation by polarization switching is performed, and polarization selecting means is provided at an output end of the semiconductor laser, and one of the polarizations is changed. A signal in which a plurality of optical communication light source devices for extracting only the mode are connected to one optical fiber to transmit light of a plurality of wavelengths, respectively, and the receiving device passes the light of a desired wavelength through a wavelength tunable optical bandpass filter. An optical communication system characterized in that wavelength division multiplexing transmission is performed so that only signals are extracted and signal detection is performed. 27. An optical-to-electrical converter, wherein a semiconductor laser and a receiving device are integrated into one, and optical transmission and reception by the optical communication system according to claim 23, 24, 25 or 26 are performed. 28. A wavelength using an optical-electrical conversion device, wherein a semiconductor laser and a receiving device are integrated into one, and optical transmission and reception by the optical communication system according to claim 23, 24, 25 or 26 are performed. Division multiplex optical transmission system.