JPH09199806A - Polarization-modulatable semiconductor laser, light source device using the same, optical communication system, and optical communication system - Google Patents

Abstract

(57) Abstract: A semiconductor laser with good productivity for controlling the polarization mode of laser oscillation by the amount of current injection. A semiconductor laser has a current injection region divided into three or more. In at least one of the current injection regions, the active layers 2 and 3 in the non-current injection state form a horizontal refractive index waveguide structure for TM mode light and a horizontal refractive index for TE mode light. Does not form a waveguide structure. In at least one of the current injection regions, the active layers 2 and 3 in the non-current injection state form a refractive index waveguide structure in the horizontal direction for TE mode light, and in the horizontal direction for TM mode light. No index-guiding structure is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention enables polarization modulation used as a light source device for optical communication for suppressing dynamic wavelength fluctuation even during high-speed modulation and stably realizing high-density wavelength-division multiplexed optical communication. Semiconductor laser, an optical communication system using the same, and an optical communication system.

[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 or spectrum line width of the laser as the light source is small. However, it is pointed out that the direct intensity modulation method used in the current optical communication is not suitable for optical FDM because the spectrum line width at the time of modulation spreads to about 0.3 nm.

As a method for preventing the spread of the vector line width during modulation, an external modulation method or a direct polarization modulation method (JP-A-62-62)
-42593, JP-A-62-144426, JP-A-2-
159781) and the like have been devised. Here, we are trying to deal with the polarization modulation method.

This polarization modulation is as follows. That is, the bias current is fixed to the point of switching TE and TM modes as shown in FIG. 34 (b), when modulating the I ₁ a minute rectangular current △ I _1, is polarization as shown in FIG. 34 (c) switching To do. Then, as shown in FIG. 34A, a polarizer 1001 is placed at the output end of the laser 1000 to selectively extract only one of the polarization planes to perform amplitude shift keying (ASK) modulation. In this method, the wavelength variation is smaller than that in the external modulation method, even though direct modulation is performed, only by devising the structure of an ordinary DFB laser (distributed feedback laser).

The principle will be briefly described. I _{1 in} FIG. 34 (a)
The phase difference of the round trip in the DFB laser 1000 is changed by causing the modulation current ΔI ₁ to flow in superposition. As a result, the loss due to the diffraction grating g changes, that is, the threshold gain changes. However, since the effective refractive index of the waveguide is different between the TE mode and the TM mode, the phase difference is changed, that is, the threshold gain is changed. The way is different. Therefore, it is possible to change the magnitude relation between the threshold values of the TE mode and the TM mode, which enables polarization modulation.

[0007]

By the way, the conventionally proposed polarization modulation DFB laser realizes polarization switching by controlling the phase, and the gain itself cannot be changed so much. Therefore, in the vicinity of the oscillation wavelength of the DFB mode (in the vicinity of the Bragg wavelength), it is necessary to accurately manufacture the TE / TM so that the gain of TE / TM becomes close.
Specifically, it is necessary to match the TE / TM gain peaks in the active layer, or to match the positional relationship between the gain peaks and the Bragg wavelength of the diffraction grating on the order of several nm, which makes it difficult to manufacture with good reproducibility. Moreover, since the influence of the phase shift amount of the diffraction grating and the end face reflection is large, the shift amount,
Precision is also required for non-reflective coatings.

Even if the devices are manufactured with a certain degree of accuracy, the TE / TM threshold gain difference, the initial phase difference, and the like vary among the elements due to manufacturing errors. As a result, the laser bias current for switching the polarization, the modulation amplitude, and the like vary, resulting in poor productivity.

Therefore, an object of the first invention of the present application is to provide a basic semiconductor laser structure for controlling the polarization mode of laser oscillation by the amount of current injection (claim 1, It corresponds to 2, 3, 4).

A second object of the present invention is to provide one means for realizing a refractive index waveguide structure having polarization dependence (corresponding to claim 5).

A third object of the present invention is to provide one means for realizing a refractive index waveguide structure having polarization dependence (corresponding to claim 6).

A fourth object of the present invention is to provide an active layer structure for obtaining a semiconductor laser which oscillates in any polarization mode (corresponding to claim 7).

A fifth object of the present invention is to provide a light source device having the semiconductor laser having the above structure (corresponding to claim 8).

A sixth object of the present invention is to provide an optical communication system having the semiconductor laser having the above structure (corresponding to claim 9).

A seventh object of the present invention is to provide an optical communication method using the semiconductor laser having the above structure (corresponding to claim 10).

[0016]

Therefore, in the semiconductor laser of the present invention, polarization switching can be performed by partially changing the waveguide structure instead of changing the phase.

More specifically, the semiconductor laser of the first invention for achieving the above object has a structure in which the current injection region is divided into three or more, and at least one of the current injection regions is active in the non-current injection state. The layer forms a horizontal refractive index guiding structure for TM mode light, does not form a horizontal refractive index guiding structure for TE mode light, and at least other of the current injection regions. One is that the active layer in the non-current injection state forms a horizontal refractive index waveguide structure for TE mode light and does not form a horizontal refractive index waveguide structure for TM mode light. Is characterized by.

The following configuration may be adopted. In a semiconductor laser having a structure capable of independently injecting current into at least three in the cavity direction, one has a quantum well active layer that generates light by current injection, and another one is the gain region. On the same plane as the quantum well active layer, there is a mixed crystal of the average composition of the quantum well active layer in a stripe shape, and a waveguide layer having a quantum well structure other than the stripe-shaped portion is provided. One has a waveguide layer having a quantum well structure in a stripe shape on the same plane as the quantum well active layer in the gain region, and there is a mixed crystal having an average composition of the quantum well active layer except the stripe shape portion. . The above semiconductor laser has a distributed feedback structure (DFB structure) having a diffraction grating in which the current injection region is divided into three or more. In a semiconductor laser having a gain region having a quantum well active layer for generating light by current injection and a diffraction grating region (DBR structure) serving as a distributed reflector, the diffraction grating region sandwiches the gain region in the cavity direction. There are two diffraction grating regions, and one of the diffraction grating regions has a mixed crystal of the average composition of the quantum well active layer in a stripe shape on the same plane as the quantum well active layer of the gain area. And the other diffraction grating region has a quantum well structure in a stripe shape on the same plane as the quantum well active layer of the gain region, and the average of the quantum well active layer except for the stripe-shaped portion. It has a waveguiding layer which is a mixed crystal of composition, and current can be injected into the stripe-shaped portion of the diffraction grating region and a portion on the extension of the stripe in the gain region. Are electrodes separated into three in the resonator direction as is available current controlled independently.

In the semiconductor laser of the second invention for achieving the above object, the refractive index control by partially disordering the quantum well structure of the active layer is used as a means for producing the refractive index guiding structure. Characterize.

In the semiconductor laser of the third invention for achieving the above object, the quantum well structure of the active layer partially has an average composition of the quantum well structure as means for producing the refractive index guiding structure. It is characterized by using refractive index control by replacing with a bulk material.

The semiconductor laser of the fourth invention for achieving the above object is characterized in that a tensile strain quantum well structure is used as the quantum well structure of the active layer of the semiconductor laser.

The light source device of the fifth invention for achieving the above object takes out only the semiconductor laser and the light emitted from the semiconductor laser, and only the light emitted by one of the two polarization modes. And a polarizer.

The optical communication system according to the sixth aspect of the present invention which achieves the above object, is the above semiconductor laser and only the light emitted from one of the two polarization modes of the light emitted from the semiconductor laser. An optical transmitter including a light source device including a polarizer to be extracted, a transmission unit that transmits the light extracted by the polarizing plate, and an optical receiver that receives the light transmitted by the transmission unit. To do.

Further, the optical communication method of the seventh invention for attaining the above object is such that only the light emitted by one of the two polarization modes among the light emitted from the semiconductor laser and the semiconductor laser is emitted. Using a light source device composed of a polarizer to be extracted, a current biased according to a transmission signal is superimposed on a predetermined bias current and supplied to the semiconductor laser, whereby the intensity is modulated from the polarizing plate according to the transmission signal. It is characterized in that the extracted signal light is taken out and the signal light is transmitted to an optical receiver.

The principle will be described in detail with reference to specific examples.
A quantum well active layer and a disordered quantum well active layer or a bulk active layer having the same composition and the same thickness as the quantum well active layer (collectively, a mixed crystal of the average composition of the quantum well active layer). In and, utilizing the fact that the refractive index of light with respect to the polarization mode is different, a refractive index waveguide structure that guides only a specific polarization mode light is provided in a part of the waveguide of the semiconductor laser. Up to now, the change of the refractive index of the quantum well structure and the disordered quantum well structure with respect to the polarization mode has been studied. Suzuki et al. ,
Applied Physics Letters,
p. 2745, Volume 57 (1990) and the like.

According to the paper, it is as follows. The refractive index of the quantum well active layer for TE mode light is n ^TE _QW , the refractive index of the quantum well active layer for TM mode light is n ^TM _QW , and the refractive index of the bulk active layer for TE mode light is n ^TE _Bulk ,
The refractive index of the bulk active layer for TM mode light is n ^TM _Bulk
And In the bulk active layer, the polarization dependence of the refractive index is small, and n ^TE _Bulk = n ^TM _Bulk, which is referred to as n _Bulk . There is the following relationship between these refractive indices. n ^TE _QW > n _Bulk > n ^TM _QW Therefore, when the quantum well active layer is used in the center and the bulk active layer is used in the periphery, TE mode light is confined and guided, and TM mode light is scattered. .

On the contrary, when the bulk active layer is used in the center and the quantum well active layer is used in the periphery, the TM mode light is confined and guided, and the TE mode light is scattered. However, even if this structure is introduced into the semiconductor laser, polarization switching cannot be performed because it only functions as a polarization filter.

Therefore, in the present invention, a gain region having a quantum well active layer as a whole, and a stripe-shaped bulk active layer on the plane of the active layer and a refractive index suitable for the TM mode light constituted by the quantum well active layer in the periphery. A TE having a waveguide region or a diffraction grating region having a refractive index waveguide structure, a stripe-shaped quantum well active layer on the plane of the active layer, and a bulk active layer surrounding the TE region.
A refractive index waveguide region or a diffraction grating region having a refractive index waveguide structure suitable for mode light is provided, and an electrode is provided so that current can be independently injected into the gain region and the refractive index waveguide region or each diffraction grating region. To provide. In the refractive index guiding region or each diffraction grating region,
The current is narrowed so that the current is concentrated in the stripe region.

When current is injected into the refractive index guiding region or each diffraction grating region, the gain guiding exceeds the effect of the refractive index guiding. When gain-guided, there is no polarization dependence of the guided light. That is, when current is injected into the gain region and the TM mode refractive index waveguide region or the diffraction grating region having the TM mode refractive index waveguide structure, a region having no polarization dependence and TE
A semiconductor laser having a refractive index waveguide for mode (or a distribution having a diffraction grating region having a waveguide structure having no polarization dependence and a diffraction grating region having a refractive index waveguide structure for TE mode as a resonator) Since it becomes a reflection type semiconductor laser), TM mode light is scattered and lost in the TE mode refractive index waveguide region or the diffraction grating region having the TE mode refractive index waveguide structure, and the semiconductor laser oscillates in the TE mode. To do.

On the contrary, when current is injected into the gain region and the TE mode refractive index waveguide region or the diffraction grating region having the TE mode refractive index waveguide structure, there is no polarization dependence and TM mode refractive index region. A semiconductor laser having a refractive index waveguide (or a diffraction grating region having a waveguide structure having no polarization dependence and T
TE becomes a distributed reflection type semiconductor laser in which a diffraction grating region having a refractive index guiding structure for M mode is used as a resonator.
The mode light is scattered and lost in the TM mode refractive index waveguide region or the diffraction grating region having the TM mode refractive index waveguide structure, and the semiconductor laser oscillates in the TM mode.

By utilizing this fact, the injection current value to the gain region is made constant, and the amount of current injection to each refractive index guiding region or each diffraction grating region is alternately modulated, whereby the polarization of the oscillated light is changed. Can be modulated. In this case, in order to reduce the amplitude of the modulation current for modulating the oscillation mode as much as possible, the current injection amount into each refractive index waveguide region or each diffraction grating region is small (not zero) and large. And may be alternately modulated.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1 A first embodiment of the present invention will be described. First, the structure of the semiconductor laser of this embodiment will be described. FIG. 1 is a perspective view showing the basic structure of a Fabry-Perot type semiconductor laser according to the present invention. In FIG. 1, the AA 'cross section is as shown in FIG. The BB 'cross section in FIG. 1 is as shown in FIG. The CC ′ cross section in FIG. 1 is as shown in FIG.

In each drawing, reference numeral 1 denotes a p-type or n-type conductivity type InP substrate, 2 has a disordered quantum well active layer, 3 has disordered quantum well active layers, and 5. In having a conductivity type opposite to that of the InP substrate of No. 1
A P clad layer, 6 is an InGaAs cap layer, 7 is a high resistance InP layer or an InP layer having the same conductivity type as the InP substrate of 1, 8 is an insulating protective film, and 91, 92, 93 and 94 are electrodes.

In order to make the semiconductor laser of this embodiment suitable for polarization modulation, the following structure has been devised. The upper surface electrode of this semiconductor laser is divided into three parts, and the current can be controlled independently for each of the regions under the electrode 91, the electrode 92 and the electrode 93. Further, since this semiconductor laser utilizes a slight difference in the refractive index in the horizontal direction depending on whether or not the quantum well active layers 2 and 3 are disordered, the upper and lower sides of the active layer are made of InP having a substantially uniform thickness as a whole. However, the effective refractive index distribution in the vertical direction is prevented from changing depending on the place.

As shown in FIGS. 2, 3 and 4,
The clad 5 on the upper side of the active layers 2 and 3 is of a ridge type because of current confinement, but the sides thereof are filled with InP7 of high resistance or the same conductivity type as the substrate 1 to be flat to form the active layers 2, 3 The upper part is made of InP having a uniform thickness.

Next, the operation of the semiconductor laser of this embodiment will be described. For the sake of explanation, the region under the electrode 92 is the gain region, the region under the electrode 91 is the TM mode refractive index waveguide region, and the electrode 9 is
The region under 3 is called the TE mode refractive index guiding region. First, the operation of each region in the non-energized state will be described.

In the non-energized state, the polarization dependence of the refractive index distribution near the active layers 2 and 3 in the TM mode refractive index waveguide region under the electrode 91 is as shown in FIG. When current is not injected in the TM mode refractive index guiding region, the vertical refractive index distribution has no polarization dependence (see FIG. 5C). However, the refractive index distribution in the active layers 2 and 3 in the horizontal direction has polarization dependency. In FIG. 5, in the quantum well active layer portion 3 which is disordered in a stripe shape at the center of the active layer and the surrounding undisordered quantum well active layer portion 2, the magnitude relationship of the refractive index is reversed due to polarization. There is. For TE mode light, the surrounding non-disordered quantum well active layer portion 2 has a higher refractive index than that of the central striped quantum well active layer portion 3 as shown in the upper part of FIG. 5 (b). For the TM mode light, the quantum well active layer portion 3 disordered in the central stripe shape is better than the surrounding undisordered quantum well active layer portion 2 in FIG.
As shown in the lower part of (b), the refractive index is large.

Since the guided light is guided to a portion having a large refractive index, in this case, the TE mode light is guided to the surrounding undisordered quantum well active layer portion 2, and the TM mode light is formed in the central stripe shape. Waveguide the disordered quantum well active layer portion 3. Since the surrounding non-disordered quantum well active layer portion 2 has a wide width and can be considered as a slab waveguide,
The E-mode light is scattered and is lost because it is not coupled to the waveguide.
The TM mode light is confined in the quantum well active layer portion 3 which is disordered in the shape of a stripe in the center and is guided, so that no loss occurs.

The polarization dependence of the refractive index distribution in the vicinity of the active layers 2 and 3 in the TE mode refractive index waveguide region under the electrode 93 in the non-energized state is as shown in FIG. Even in the TE mode refractive index waveguide region, the refractive index distribution in the vertical direction has no polarization dependence when no current is injected (see FIG. 6C). However, the refractive index distribution in the horizontal direction in the active layers 2 and 3 has polarization dependence. In FIG. 6, in the striped non-disordered quantum well active layer portion 2 at the center of the active layer and the surrounding disordered quantum well active layer portion 3, the magnitude relationship of the refractive index is reversed due to polarization. There is. For TM mode light, the surrounding disordered quantum well active layer portion 3 has a higher refractive index than the central striped non-disordered quantum well active layer portion 2 as shown in the lower part of FIG. 6 (b). For a TE mode light, the stripe-shaped undisordered quantum well active layer portion 2 in the center of FIG.
As shown in the upper part of (b), the refractive index is large.

Since the guided light is guided to a portion having a large refractive index, in this case, the TM mode light is guided to the surrounding disordered quantum well active layer portion 3, and the TE mode light is disordered in the central stripe shape. The quantum well active layer portion 2 which is not formed is guided. Since the surrounding disordered quantum well active layer portion 3 has a large width and can be considered as a slab waveguide, TM mode light is scattered and is not coupled to the waveguide, and thus is lost. The TE mode light is confined in the central striped non-disordered quantum well active layer portion 2 and guided, so that no loss occurs.

Next, the operation in the energized state will be described. In the TM mode refractive index waveguide region under the electrode 91 and the TE mode refractive index waveguide region under the electrode 93, when a current is applied to the central striped region shown in FIGS. 5 and 6,
Since the difference in the refractive index in the horizontal direction originally caused by disordering the quantum well active layer is small, the effect of the gain guiding by the current injection becomes more remarkable than the refractive index guiding caused by the horizontal confinement due to the refractive index. In the case of gain guiding, since there is no polarization dependence, TE mode light and TM mode light are similarly guided through the central portion of the active layer regardless of whether the quantum well active layer is disordered or not.

The entire semiconductor laser is driven as follows. When oscillating with TM mode light, current is applied to the gain region and the TE mode refractive index waveguide region below the electrode 93,
The TM mode refractive index waveguide region below the electrode 91 is not energized. In this case, T in the TM mode refractive index guiding region
The E-mode light has a large scattering loss, and the semiconductor laser oscillates in the TM mode. When oscillating with TE mode light, the gain region and the TM mode refractive index waveguide region under the electrode 91 are energized, and the TE mode refractive index waveguide region under the electrode 93 is non-energized. In this case, the scattering loss of TM mode light in the TE mode refractive index waveguide region is large, and the semiconductor laser oscillates in TE mode.

Therefore, the gain region is driven with a constant current,
If a current is alternately applied to the TM mode refractive index waveguide region below the electrode 91 and the TE mode refractive index waveguide region below the electrode 93,
Polarization modulation that changes the polarization of the oscillation light of the semiconductor laser can be performed.

In this embodiment, in order to obtain a polarization-dependent refractive index guiding structure, a combination of the non-disordered quantum well active layer 2 and the disordered quantum well active layer 3 was used. However, the same result can be obtained by using a bulk crystal having the average composition of the quantum well active layer 2 as reference numeral 3. That is, a mixed crystal having an average composition is used. When a tensile strained quantum well structure is adopted for the quantum well active layer 2, TE
Since the gain difference between the mode and the TM mode can be reduced, it is possible to obtain a semiconductor laser that is more easily polarization-modulated.

By the way, although the present embodiment is a Fabry-Perot type semiconductor laser, it is also possible to form a DFB structure by providing a guide layer above or below the active layer and forming a diffraction grating there. In this case, the pitch of the diffraction grating in the gain region and each refractive index guiding region usually needs to be changed appropriately. For example, when oscillating in the TM mode without injecting current into the TM mode refractive index guiding region, a gain region into which current is injected (if necessary, a small amount of current is injected into the TE mode refractive index guiding region Also, the pitch of the diffraction grating of each region is changed so that the effective pitch of the diffraction grating of (2) and the effective pitch of the diffraction grating of the TM mode refractive index guiding region in which no current is injected match in the TM mode. At this time, when oscillating in TE mode without injecting current into the TE mode refractive index guiding region, a gain region into which current is injected (if necessary, a small amount of current is injected into the TM mode refractive index guiding region It is preferable that the effective pitch of the diffraction grating in the region (1) and the effective pitch of the diffraction grating in the TE mode refractive index waveguide region in which no current is injected coincide with each other in the TE mode. However, there is a case where the effective pitches of the diffraction gratings in the respective regions may be matched only in the oscillation mode to be used. When the current injection amount is small and the effective pitch of each region is substantially equal in each mode, the pitch of the diffraction grating may be equal in all regions.

Second Embodiment A second embodiment of the present invention will be described. First, the structure of the semiconductor laser of this embodiment will be described. FIG. 7 is a perspective view showing the basic structure of a semiconductor laser having a distributed reflector according to the present invention. In FIG. 7, the AA 'cross section is as shown in FIG. The BB 'cross section in FIG. 7 is as shown in FIG. In FIG. 7, the CC ′ cross section is as shown in FIG.

The structure is almost the same as that of the first embodiment,
It will be as follows when it explains along each figure. In each drawing, reference numeral 101 is a p-type or n-type conductivity type InP substrate, 102 is an undisordered quantum well active layer, 103 is a disordered quantum well active layer, 14
1 is an optical guide layer, 105 is an InP clad layer having a conductivity type opposite to that of the InP substrate 101, and 106 is InGaA.
s cap layer, 107 is a high resistance InP layer or 101
InP layer having the same conductivity type as that of the InP substrate, 108 is an insulating protective film, 191, 192, 193 and 194 are electrodes, and 161 and 163 are diffraction gratings.

Also in the semiconductor laser of this embodiment, the following structure is devised so that it is suitable for polarization modulation. The top electrode of this semiconductor laser is divided into three,
It is possible to independently control the current for each region under the electrode 191, the electrode 192, and the electrode 193. Further, since this semiconductor laser utilizes a slight difference in the refractive index in the horizontal direction depending on whether or not the quantum well active layers 102 and 103 are disordered, InP having a substantially uniform thickness above and below the active layer as a whole.
In order to prevent the effective refractive index distribution in the vertical direction from changing depending on the location.

As shown in FIGS. 8, 9 and 10, the clad 105 on the upper side of the active layers 102 and 103 is of a ridge type because of current constriction, but the side thereof has a high resistance or is the same as the substrate 101. The active layers 102 and 103 have a uniform thickness by being filled with a conductive type InP so as to be flat.
It is composed of P.

Next, the operation of the semiconductor laser of this embodiment will be described. For the sake of explanation, the area under the electrode 191 is area A (or the TM mode distributed reflection area), the area under the electrode 192 is area B (or the gain area), and the area under the electrode 193 is area C (or TE). This is called a mode distributed reflection area).

First, the operation of each region in the non-energized state will be described. In the non-energized state, the polarization dependence of the refractive index distribution near the active layer in the area B is as shown in FIG. In the area B, the refractive index distribution in the vertical direction has no polarization dependence when no current is injected (see FIG. 11C).
However, the refractive index distribution in the horizontal direction in the active layers 102 and 103 has polarization dependency. In FIG. 11, in the quantum well active layer portion 103 which is disordered in a stripe shape at the center of the active layer and the surrounding undisordered quantum well active layer portion 102, the magnitude relationship of the refractive index is reversed by polarization. ing.
For TE-mode light, the surrounding undisordered quantum well active layer portion 102 is shown in FIG. 11 rather than the central stripe-disordered quantum well active layer portion 103.
As shown in (b), in the case of TM mode light, the quantum well active layer portion 103 which is disordered in the shape of a stripe at the center is better than the surrounding undisordered quantum well active layer portion 102. As shown in 11 (c), the refractive index is large.

Since the guided light is guided to a portion having a large refractive index, in this case, the TE mode light is guided to the surrounding undisordered quantum well active layer portion 102, and the TM mode light is formed in the central stripe shape. Waveguide through the disordered quantum well active layer portion 103. Since the surrounding non-disordered quantum well active layer portion 102 has a wide width and can be considered as a slab waveguide, TE mode light is scattered and is not coupled to the waveguide, and thus is lost. The TM mode light is confined in the quantum well active layer portion 103 which is disordered in the shape of a stripe in the center and is guided, so that it does not suffer a loss.

The active layer 102 in the region C in the non-energized state,
The polarization dependence of the refractive index distribution near 103 is as shown in FIG. Even in area C, if current is not injected,
The vertical refractive index distribution has no polarization dependence (Fig. 12).
(C)). However, the refractive index distribution in the horizontal direction in the active layers 102 and 103 has polarization dependence. In FIG. 12, in the stripe-shaped non-disordered quantum well active layer portion 102 in the center of the active layer and the surrounding disordered quantum well active layer portion 103, the magnitude relationship of the refractive index is reversed by polarization. ing. For TM mode light, the central stripe-shaped non-disordered quantum well active layer portion 102
The surrounding disordered quantum well active layer portion 103 has a larger refractive index as shown in FIG. 12C, and is more centered on the TE-mode light than the surrounding disordered quantum well active layer portion 103. The stripe-shaped non-disordered quantum well active layer portion 102 has a larger refractive index as shown in FIG.

Since the guided light is guided to a portion having a large refractive index, in this case, the TM mode light is guided to the surrounding disordered quantum well active layer portion 103, and the TE mode light is disordered in the central stripe shape. The quantum well active layer portion 102 which has not been converted is guided. Since the surrounding disordered quantum well active layer portion 103 has a wide width and can be considered as a slab waveguide, the TM mode light is scattered and is not coupled to the waveguide, and thus is lost. The TE-mode light is confined in the central stripe-shaped non-disordered quantum well active layer portion 102 and guided, so that no loss occurs.

Next, the operation in the energized state will be described. Regions a and c have a small difference in the refractive index in the horizontal direction originally caused by disordering the quantum well active layer when a current is applied to the central striped region shown in FIGS. The effect of gain guiding due to current injection is more pronounced than the index guiding resulting from horizontal confinement. In the case of gain waveguide, since there is no polarization dependence,
The TE mode light and the TM mode light are the same as in the active layer 10.
Waveguiding is performed through the central portions of 2 and 103 regardless of whether the quantum well active layer is disordered or not.

The entire semiconductor laser is driven as follows. When oscillating with the TM-mode light, the regions b and c are energized, and the region a is de-energized. The light generated in the region B, which is the gain region, is generated by the diffraction grating 1 that constitutes the resonator.
Waveguides 61 and 163 are provided in regions a and c.

As described above, in the area C, there is no polarization dependence and both TE light and TM light are reflected and returned. However, in the area B, TE mode light has a large scattering loss, so TE
The semiconductor laser oscillates in the TM mode because there is almost no feedback of light, and the amount of TM light feedback relatively increases. When oscillating with TE mode light, current is applied to the areas B and A, and no current is applied to the area C. The light generated in the area B, which is the gain area, is transmitted to the diffraction gratings 161 and 16 forming the resonator.
3 propagates to a certain area B and area C. In region B, there is no polarization dependence and both TE light and TM light are reflected and returned, but in region C, there is almost no TM light return because there is a large scattering loss of TM mode light, and TE light is relatively Therefore, the semiconductor laser oscillates in the TE mode.

Therefore, if the region b, which is the gain region, is driven with a constant current and the current is alternately passed through the regions a and c, the polarization modulation in which the polarization of the oscillation light of the semiconductor laser changes can be performed. In this example, the quantum well active layer 10 which is not disordered in order to obtain a refractive index waveguiding structure having polarization dependence.
Although the combination of 2 and the quantum well active layer 103 in which this is disordered was used, the same result can be obtained by using a bulk crystal having the average composition of the quantum well active layer 102 at 103.

Example 3 As a modified example of the second example, a modification of the layer structure near the active layer will be described. In the third embodiment of FIG. 13, the light guide layers 141 are formed above and below the active layers 102 and 103, respectively.
142 is provided. By providing an optical guide layer above and below the active layer and making the structures above and below the active layer symmetrical, the vertical intensity peak position of the mode of the guided light can be determined.
It can be around 2,103. As a result, the threshold value can be reduced. A cross section taken along the line AA 'in FIG. 13 is as shown in FIG. The BB 'cross section in FIG. 13 is as shown in FIG. A cross section taken along the line CC 'in FIG. 13 is as shown in FIG.

The polarization dependence of the refractive index distribution under the condition that the current is not injected under the electrode 191 is shown in FIG.
The polarization dependence of the refractive index distribution under the condition that no current is injected under the electrode 193 is shown in FIG.

13 to 18, the same parts as those in FIG. 7 indicate the same functional parts. The other operations are the same as those in the second embodiment.

Embodiment 4 In FIG. 19, the light guide layer 142 is provided only above the active layers 102 and 103, and the diffraction grating 16 is provided on the light guide layer 142.
1, 163 are formed.

The active layers 102 and 103 are formed on the flat substrate 101.
And after forming the light guide layer 142, the diffraction grating 16
Since 1 and 163 are processed, the flatness of the quantum well structure is better than that of the example of FIGS. 7 and 13 in which the active layer is formed on the diffraction grating. As a result, the quantum effect can be more exerted, the threshold value can be made smaller, and the quantum efficiency can be further increased. In FIG. 19, the AA 'cross section is as shown in FIG. The BB 'cross section in FIG. 19 is as shown in FIG. The CC ′ cross section in FIG. 19 is as shown in FIG. FIG. 23 shows the polarization distribution dependence of the refractive index distribution under the condition that no current is injected under the electrode 191.
Is shown in FIG. The polarization dependence of the refractive index profile under the condition that no current is injected under the electrode 193 is shown in FIG.

19 to 24, the same symbols as those in FIG. 7 indicate the same functional units. Other,
The operation and the like are the same as those in the second embodiment.

Example 5 FIG. 25 shows that the light guide layer 14 is formed above and below the active layers 102 and 103.
1, 142 are provided, and the light guide layer 14 above the active layer is provided.
In FIG. 2, diffraction gratings 161 and 163 are formed. By providing the light guide layers 141 and 142 above and below the active layer and making the structures above and below the active layer symmetrical, the vertical intensity peak position of the mode of the guided light can be near the active layers 102 and 103. Enables and is a flat substrate 101
The active layers 102, 103 and the light guide layers 141, 14
Since the diffraction gratings 161 and 163 are processed after forming 2, the quantum well structure has good flatness. As a result,
The effects of the fourth embodiment can be achieved at the same time.

In FIG. 25, the AA ′ cross section is as shown in FIG. The BB 'cross section in FIG. 25 is as shown in FIG. In FIG. 25, C-C '
The cross section is as shown in FIG. The polarization dependence of the refractive index profile under the condition that no current is injected under the electrode 191 is shown in FIG. The polarization dependence of the refractive index profile under the condition that no current is injected under the electrode 193 is shown in FIG.

25 to 30, the same symbols as those in FIG. 7 indicate the same functional units. Other,
The operation and the like are the same as those in the second embodiment.

Next, the setting of the diffraction grating period of the distributed reflector will be described. Usually, when a semiconductor laser is polarization-modulated and used, (a) a polarizer is provided at the emission part of the semiconductor laser to utilize the TE mode light as output light. (Ii) a polarizer is provided at the emission part of the semiconductor laser. There are three modes in which the TM mode light is provided and used as the output light and both polarization modes of the semiconductor laser are used as the output light.

In the case of (a), the wavelength of the desired TE mode light and the waveguide in the gain region where the current is injected, the waveguide in the TM mode distributed reflection region and the waveguide in the TE mode distributed reflection region where the current is not injected are provided. The diffraction grating period of each region is calculated from the effective refractive index. That is, when the semiconductor lasers of the second to fifth embodiments oscillate in TE mode light, current is injected into the TM mode distributed reflector region. The effective refractive index of the waveguide in the distributed reflector region is 0.1% due to current injection.
The degree changes. Therefore, the diffraction grating period may be calculated by estimating this variation.

In the case of (i), the wavelength of the desired TM mode light and the waveguide of the gain region for injecting current, the waveguide of the TE mode distributed reflection region and the waveguide of the TM mode distributed reflection region for no current injection are selected. The diffraction grating period of each region is calculated from the effective refractive index. That is, when the semiconductor lasers of the second to fifth embodiments oscillate in TM mode light, current is injected into the TE mode distributed reflector region. Therefore, the diffraction grating period may be calculated using the effective refractive indices of the waveguide in the TE mode distributed reflector region in the current injection state and the waveguide in the TM mode distributed reflector region in the current non-injection state.

In the case of (u), since the oscillation wavelengths of both polarization modes cannot be exactly matched, either polarization mode is adjusted according to the purpose. In each case, when the current injection amount is relatively small and the effective pitch of each region is substantially equal in each mode, the pitch of the diffraction grating may be equal in both distributed reflection regions.

Here, when it is desired to obtain TM mode light oscillation as in (i) and (u), the gain difference between the TE mode and the TM mode is too large in the ordinary quantum well structure, and the oscillation occurs in the TM mode. It can be difficult. This is because in the ordinary quantum well structure, the electrons in the conduction band mainly combine with the heavy holes in the forbidden band, so that the TE mode gain is high. In such a case, a tensile strain quantum well structure may be adopted for the active layer in the gain region. In the tensile strained quantum well structure, electrons in the conduction band mainly combine with holes in the forbidden band, so that the TM mode gain increases. Depending on the degree of tensile strain applied to the quantum well, the magnitude relationship between the TE mode gain and the TM mode gain can be changed to obtain a semiconductor laser suitable for the purpose.

Embodiment 6 FIG. 32 shows a semiconductor laser according to the present invention as a wavelength division multiplexed light LA.
FIG. 31 shows a configuration example of an optical-electrical conversion unit (node) connected to each terminal when applied to the N system, and FIG. 31 shows a configuration example of an optical LAN system using the node 281.

An optical signal is taken into the node 281 by using the optical fiber 280 connected to the outside as a medium, and the branch unit 27
2 causes a part of the light to enter the receiving device 273 equipped with a variable wavelength optical filter or the like. The receiving device 273 extracts only an optical signal having a desired wavelength and performs signal detection. on the other hand,
When the optical signal is transmitted from the node 281, the semiconductor laser device 274 of the above embodiment is driven by an appropriate method in accordance with the signal, polarization modulation is performed, and the polarization plate 277 (which causes the polarization modulation signal to be amplitude intensity modulated). (Converted into a signal) and the output light through the isolator 275 are incident on the optical transmission line 280 through the branching unit 276. Further, the wavelength tunable range can be widened by providing two or more semiconductor lasers and a plurality of wavelength tunable optical filters.

As a network of the optical LAN system,
The one shown in FIG. 31 is a bus type, and a large number of networked terminals and a center 282 can be installed by connecting nodes in the directions A and B. However, in order to connect a large number of nodes, it is necessary to arrange optical amplifiers in series on the transmission line 280 in order to compensate for optical attenuation. In addition, by connecting two nodes 281 to each terminal 282 and using two transmission paths, bidirectional transmission by the DQDB method becomes possible. Further, as a network system, a loop type in which A and B in FIG. 31 are connected, a star type, or a combination thereof may be used.

Example 7 FIG. 33 shows a bidirectional light CA using the semiconductor laser of the present invention.
It is a schematic diagram which shows the structural example of a TV system. In FIG. 33, 290 is a CATV center, 292 is a sub-center connected to the center 290 by an optical fiber 291, and 293 is a receiver of each subscriber connected to the sub-center. The center 290 includes the light source device of the present invention,
A plurality of image signals are mounted on signal lights having different wavelengths and transmitted to the receiver 293. The image receiver 293 includes a variable wavelength optical filter and a photodetector, detects only the signal light of a desired wavelength among the incident signal lights, and reproduces an image on the monitor. The subscriber can select a channel and obtain a desired image by changing the transmission wavelength of the variable wavelength optical filter.

Conventionally, it was difficult to use the DFB filter in such a system due to the influence of the dynamic wavelength fluctuation of the DFB laser, but the present invention makes it possible.

Further, the subscriber is provided with an external modulator, and the signal from the subscriber is received by the reflected light from the modulator (one form of the simplified type bidirectional optical CATV, for example, Ishikawa, Furuta "optical CATV subscription" By constructing a star type network as shown in FIG. 33, an LN external modulator for two-way transmission in a human system ", OCS91-82, p.51), a two-way optical CATV becomes possible and the service is highly functionalized. Can be achieved.

[0079]

As described above, the present invention has the following effects. According to the first invention of the present application, it is possible to obtain a highly productive semiconductor laser in which the polarization mode of laser oscillation can be controlled by the amount of current injection.
According to the second and third inventions of the present application, one means for realizing a refractive index waveguide structure having polarization dependence can be obtained. According to the fourth invention of the present application, an active layer structure for obtaining a semiconductor laser that oscillates in any polarization mode can be obtained. According to the fifth, sixth, and seventh inventions of the present application, a light source device and optical communication having good performance are achieved by using a semiconductor laser having high productivity, which can control the polarization mode of laser oscillation by the amount of current injection. The system and the optical communication method can be realized relatively easily.

[Brief description of drawings]

FIG. 1 is a perspective view showing a basic configuration of a Fabry-Perot type semiconductor laser which is a first embodiment according to the present invention.

FIG. 2 is a sectional view taken along line AA ′ of FIG. 1;

3 is a sectional view taken along line BB ′ of FIG.

FIG. 4 is a sectional view taken along line CC ′ of FIG.

FIG. 5 is a view showing the refractive index distributions in the horizontal direction and the vertical direction in the state of not injecting current in the AA ′ cross section of FIG. 1.

6 is a diagram showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the CC ′ cross section of FIG. 1.

FIG. 7 is a perspective view showing the basic structure of a semiconductor laser having a distributed reflector according to a second embodiment of the present invention.

8 is a cross-sectional view taken along the line AA ′ of FIG.

FIG. 9 is a sectional view taken along the line BB ′ of FIG. 7;

10 is a cross-sectional view taken along the line CC ′ of FIG. 7.

11 is a view showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the AA ′ cross section of FIG. 7.

12 is a diagram showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the CC ′ cross section of FIG. 7.

FIG. 13 is a perspective view showing a basic configuration of a semiconductor laser having a distributed reflector that is a third embodiment according to the present invention.

FIG. 14 is a sectional view taken along the line AA ′ of FIG. 13;

FIG. 15 is a sectional view taken along line BB ′ of FIG. 13;

16 is a cross-sectional view taken along the line CC ′ of FIG.

FIG. 17 is a diagram showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the AA ′ cross section of FIG. 13;

FIG. 18 is a view showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the CC ′ cross section of FIG. 13;

FIG. 19 is a perspective view showing the basic structure of a semiconductor laser having a distributed reflector according to a fourth embodiment of the present invention.

FIG. 20 is a sectional view taken along line AA ′ of FIG. 19;

21 is a cross-sectional view taken along the line BB ′ of FIG.

22 is a cross-sectional view taken along the line CC ′ of FIG.

23 is a diagram showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the AA ′ cross section of FIG. 19;

24 is a diagram showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the CC ′ cross section of FIG. 19;

FIG. 25 is a perspective view showing the basic structure of a semiconductor laser having a distributed reflector that is a fifth embodiment according to the present invention.

26 is a cross-sectional view taken along the line AA ′ of FIG.

27 is a cross-sectional view taken along the line BB ′ of FIG.

28 is a cross-sectional view taken along the line CC ′ of FIG. 25.

29 is a view showing the refractive index distributions in the horizontal direction and the vertical direction in the state of not injecting current in the AA ′ cross section of FIG. 25.

FIG. 30 is a diagram showing the refractive index distributions in the horizontal direction and the vertical direction in the state in which no current is injected in the CC ′ cross section of FIG. 25.

FIG. 31 is a schematic diagram showing a configuration example of an optical LAN system using the semiconductor laser of the present invention.

32 is a schematic diagram showing a configuration example of a node in the system of FIG.

FIG. 33 is a bidirectional light CA using the semiconductor laser of the present invention.
The schematic diagram which shows the structural example of a TV system.

FIG. 34 is a diagram illustrating a conventional example.

[Explanation of symbols]

1, 101 p-type or n-type conductivity type InP substrate 2, 102 non-disordered quantum well active layer 3, 103 disordered quantum well active layer 5, 105 opposite conductivity type to InP substrate Have I
nP clad layer 6, 106 InGaAs cap layer 7, 107 High resistance InP layer or InP layer having the same conductivity type as the InP substrate 8, 108 Insulation protective film 91, 92, 93, 94, 191, 192, 193, 1
94 electrodes 141, 142 optical guide layers 161, 163, g diffraction grating 272, 276 optical branching unit 273 receiver 274 semiconductor laser 275 isolator 277, 1001 polarizer 280, 291 optical transmission line 281 node 282 terminal 290 center 292 of the present invention Sub-center 293 Receiver 1000 Semiconductor laser

Claims (10)

[Claims] 1. A semiconductor laser comprising a current injection region of 3
The active layer in a non-current-injected state has a structure in which at least one of the current-injected regions is divided into two or more, and forms a horizontal refractive index waveguide structure for the TM mode light.
It does not form a horizontal refractive index guiding structure for mode light, and at least one of the current injection regions has an active layer in a non-current injection state, and a horizontal refraction index for TE mode light. A semiconductor laser characterized in that a refractive index waveguide structure is formed and a horizontal refractive index waveguide structure is not formed for TM mode light. 2. A semiconductor laser having a structure capable of independently injecting current into at least three parts in the cavity direction,
One has a quantum well active layer that generates light by current injection, and the other is a mixture of the average composition of the quantum well active layer in a stripe pattern on the same plane as the quantum well active layer of the gain region. And a waveguide layer having a quantum well structure except for the stripe-shaped portion, and another one is a stripe-shaped quantum well structure on the same plane as the quantum well active layer of the gain region. 2. The semiconductor laser according to claim 1, wherein the semiconductor laser has a waveguide layer, and a mixed crystal having an average composition of the quantum well active layer except the stripe-shaped portion. 3. A semiconductor laser comprising a current injection region of 3
2. The semiconductor laser according to claim 1, having a distributed feedback structure having a diffraction grating divided into two or more. 4. A semiconductor laser having a gain region having a quantum well active layer for generating light by current injection and a diffraction grating region serving as a distributed reflector, wherein the diffraction grating region has the gain region in the cavity direction. There are two sandwiched ones, and one of the diffraction grating regions has a mixed crystal of the average composition of the quantum well active layer in a stripe shape on the same plane as the quantum well active layer of the gain area. The structure has a waveguiding layer, and the other diffraction grating region has a quantum well structure in a stripe shape on the same plane as the quantum well active layer of the gain region. It has a waveguide layer which is a mixed crystal with an average composition, and current can be injected into the stripe-shaped portion of the diffraction grating region and a portion on the extension of the stripe in the gain region. The semiconductor laser of claim 1, wherein the independent electrode separated into three in the resonator direction as current control as possible. 5. The refractive index control by partially disordering a quantum well structure of an active layer is used as a means for producing the refractive index guiding structure, according to any one of claims 1 to 4. Semiconductor laser. 6. As a means for producing the refractive index guiding structure, using refractive index control by partially substituting a quantum well structure of an active layer with a bulk material having an average composition of the quantum well structure. 5. The semiconductor laser according to claim 1, wherein the semiconductor laser is a semiconductor laser. 7. The semiconductor laser according to claim 1, wherein a tensile strain quantum well structure is used as a quantum well structure of an active layer of the semiconductor laser. 8. A semiconductor laser according to any one of claims 1 to 7, and a polarizer for extracting only the light emitted from one of the two polarization modes out of the light emitted from the semiconductor laser. A light source device characterized by the above. 9. A semiconductor laser according to any one of claims 1 to 7, and a polarizer for extracting only the light emitted from one of the two polarization modes out of the light emitted from the semiconductor laser. An optical communication system comprising an optical transmitter having a light source device, a transmission means for transmitting the light extracted by the polarizing plate, and an optical receiver for receiving the light transmitted by the transmission means. 10. The semiconductor laser according to claim 1, and 2 of the light emitted from the semiconductor laser.
Using a light source device consisting of a polarizer that extracts only light oscillated in one of the two polarization modes, and supplying to the semiconductor laser by superimposing a current modulated according to a transmission signal on a predetermined bias current, An optical communication method, wherein signal light whose intensity is modulated according to a transmission signal is taken out from the polarizing plate, and the signal light is transmitted to an optical receiver.