JPH08274412A - Polarization modulation semiconductor laser and its driving method - Google Patents

Abstract

PURPOSE: To realize further stable selection of a polarization mode of a device and obtain a semiconductor laser which is not sensitive to an edge face phase and executes switching operation by changing effective reflectance of a laser resonator dynamically. CONSTITUTION: This semiconductor laser enables oscillation in two polarization modes having different polarization surfaces. It has two or more electrodes 113, 114 in a resonance direction and a first optical waveguide path part 11 and a second optical waveguide path part 12 below the electrodes 113, 114 constitute a resonator of a laser. A propagation constant differential of a waveguide path to two polarization modes of different polarization surfaces varies. Two operational states of a state wherein Bragg wavelength of a TE mode coincides in the first optical waveguide path 11 and the second optical waveguide path 12 and a state wherein Bragg wavelength of a TM mode coincides in the first optical waveguide path 11 and the second optical waveguide path 12 are selected alternatively.

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 polarization switching such as optical communication which suppresses dynamic wavelength fluctuation even during high speed modulation and enables driving in a direct modulation system, and a driving method thereof.

[0002]

2. Description of the Related Art Conventionally, as a dynamic single mode semiconductor laser capable of polarization switching,
An element structure has been proposed that enables digital polarization modulation by superimposing a small-amplitude digital signal on an injection current.

This is a DFB laser having a structure in which a distributed reflector composed of a grating is introduced inside a semiconductor laser resonator and the wavelength selectivity thereof is utilized.
Strain is introduced into the quantum well of the active layer in order to make the gain under the injection current around the oscillation threshold value approximately the same for both the TE and TM modes of light with a wavelength near the oscillation wavelength. Alternatively, the Bragg wavelength is set on the shorter wavelength side than the peak of the gain spectrum. And, it is configured to have a plurality of electrodes, and the current is nonuniformly injected to the plurality of electrodes. The non-uniform current injection causes the equivalent refractive index of the resonator to change non-uniformly, and TE mode and TM
Among the modes, it is configured to oscillate in a polarization mode and a wavelength that satisfies the phase matching condition and has the lowest threshold gain. It was possible to change the oscillation wavelength and the polarization mode by changing the competitive relationship of the phase conditions by slightly changing the balance of the nonuniform injection.

In order to asymmetrically bring out the effect of non-uniform injection on the output side and the modulation side, this device can introduce a structural asymmetry of two electrode lengths, which is a one-sided antireflection coating. It was effective.

[0005]

However, in the conventionally proposed DFB polarization switching laser, in order to introduce the asymmetry as described above, one of the end faces is not provided with a non-reflection coating, The choice of oscillation mode was sensitive to the end face phase. Therefore, the dependence of the oscillation wavelength of the device and the polarization mode on the current injection condition exhibits complicated behavior, and the characteristics of the polarization mode vary among the devices.

In order to improve this point, antireflection coating is applied to both end faces to weaken the asymmetry in the waveguide direction of the device, weaken the effect of non-uniform injection, and obtain stable polarization switching. There was a problem that it did not exist.

Therefore, the present invention makes the selection of the polarization mode of the device more stable by dynamically changing the effective reflectivity of the laser cavity, and the anti-reflection coating is applied to both sides to provide the end face phase. It is an object of the present invention to provide a semiconductor laser which is not sensitive to polarization and performs polarization switching operation.

[0008]

In the resonator of a distributed feedback semiconductor laser, the present invention focuses on the light of the oscillation wavelength of the laser in the TE and TM modes, and the forward wave is converted into the backward wave. When the efficiency is considered as an equivalent reflectance, 1) a state in which the TE mode reflectance is higher than the TM mode reflectance and a 2) a state in which the TE mode reflectance is higher than the TM mode reflectance are created. It is a thing.

Specifically, a first optical waveguide portion and a second optical waveguide portion having at least two electrodes in the resonance direction of the distributed feedback semiconductor laser and forming a resonator corresponding to the electrodes. In this section, the difference in the propagation constants of the waveguides with respect to the two polarization modes having different polarization planes is different, so that the difference in the Bragg wavelength of these two polarization modes is made different in the two waveguide sections. .

More specifically, the distributed feedback semiconductor laser of the present invention is a light source that enables oscillation in two polarization modes having different polarization planes, and has two or more electrodes in the resonance direction. Has an independent current injection structure, and the first optical waveguide portion and the second optical waveguide portion under the electrode are a part or all of which constitute a laser resonator, and the first optical waveguide portion The difference between the propagation constants of the waveguide for two polarization modes having different polarization planes is different between the portion and the second optical waveguide portion.

The following forms are also possible. At least the first and second optical waveguide portions forming the resonator have different widths of lateral optical confinement in the plane of the substrate, so that the propagation constants for polarization modes having different polarization planes are different from each other. I have to. In the first and second optical waveguide portions, the transverse mode of the laser oscillation mode is the 0th order mode. At least the first and second optical waveguide portions forming the resonator have different layer configurations in the stacking direction for the respective optical waveguide portions, so that the propagation constant differences for polarization modes having different polarization planes are different from each other. ing. At least one of the layer thickness and the composition of the active layer in the layer structure in the stacking direction is different for each optical waveguide portion, so that the propagation constant differences for polarization modes having different polarization planes are different from each other. The diffraction grating of the distributed feedback semiconductor laser includes a phase shift. The pitches of the diffraction gratings are different from each other in at least the first and second optical waveguide portions forming the resonator (for example, in the optical waveguide portion having a larger lateral optical confinement width and a larger propagation constant). Under a condition of uniform current injection, the Bragg wavelengths match between the first and second optical waveguide portions for one of two polarization modes having different polarization planes. A quantum well structure is used for a part or all of the active layer in order to cause a large difference in propagation constant for polarization modes having different polarization planes. In order to cause a large difference in propagation constants for polarization modes having different polarization planes, a quantum well structure is used in the portion of the structure of the optical waveguide portion other than the active layer. The first waveguide portion forming the resonator is a portion in which the quantum well structure is mixed crystal,
The second waveguide portion is used as a portion where the quantum well structure is not mixed, and the propagation constant difference for polarization modes having different polarization planes is made different for each optical waveguide portion. The first and second optical waveguide portions have different polarization planes.
The difference between the Bragg wavelengths for the two polarization modes is different, and the amount of the difference between the two optical waveguide portions is larger than the stop bandwidth for one of the polarization modes. In at least the first and second optical waveguide portions forming the resonator, the active layer is a quantum well structure layer in which tensile strain is introduced. In at least the first and second optical waveguide portions forming the resonator, the Bragg wavelength is set near the gain peak of the TM mode on the shorter wavelength side than the gain peak of the TE mode.

Further, the method of driving the distributed feedback semiconductor laser of the present invention is different from the method of driving the distributed feedback semiconductor laser described above, in that the plane of polarization differs depending on the minute modulation current signal to at least one of the independent electrodes. For at least one of the two polarization modes, at least the state where the Bragg wavelengths match between the first and second optical waveguide portions, and for the other of the two polarization modes having different polarization planes, at least the first And a state in which the Bragg wavelengths match between the second waveguide portion and the second waveguide portion.

The characteristic operation of this device is to selectively select the following two operating states by injecting a non-uniform current into the above two electrodes. The two operating states are: 1) a state in which the Bragg wavelengths of the TE mode in the first optical waveguide section and the second optical waveguide section are the same 2) in the first optical waveguide section and the second optical waveguide section This is a state in which the Bragg wavelengths in the TM mode match.

Generally, the Bragg wavelength of the TE mode and the Bragg wavelength of the TM mode are different due to the difference in the propagation constant between the TE mode and the TM mode, but the difference is substantially constant in the device. In the configuration of the present invention, two or more portions having different propagation constant differences between polarization modes are provided,
The difference in the Bragg wavelength difference between the polarization modes in each part is made much larger than that which appears in the normal device due to the effect of nonuniform injection at most, and the reflection loss (or Creates a situation where the equivalent reflectances above) are significantly different.

That is, in the polarization mode in which the Bragg wavelengths are the same between different optical waveguide portions, the reflection loss is small for the light having a wavelength near the Bragg wavelength, and the gain satisfying the laser oscillation gain condition is small. On the other hand, for example, 2
In the polarization mode where the Bragg wavelength in one part is separated by about the stop bandwidth in each part,
Since the light of all wavelengths is separated from the Bragg wavelength of either one by about the stop bandwidth,
Reflection loss increases.

In the device driving method according to the present invention, the equivalent refractive index of one or both of the plurality of waveguides is changed by a minute modulation current signal applied to at least one of the two independent electrodes. , The Bragg wavelength of the waveguide is changed to switch between the two operating states.

[0017]

First Embodiment A first embodiment according to the present invention will be described with reference to the laser perspective view of FIG. In FIG. 1, 11 is a DFB portion on the light emission side, and 12 is a DFB portion on the modulation electrode side. The layer structure has a depth of 0.3 on the n-InP substrate 101.
I formed a diffraction grating 102 with a pitch of 235 nm and
nP lower optical cladding layer 103, 0.15 μm thick n-I
n _0.73 Ga _0.27 As _0.59 P _0.41 lower optical guide layer 104,
Active layer 105 having a strained quantum well structure, 0.15 μm thick p−
In _0.73 Ga _0.27 As _0.59 P _0.41 Upper optical guide layer 10
6, p-InP clad layer 108, p-In _0.59 Ga
_{A 0.41} As _0.9 P _0.1 contact layer 109 is laminated.
The strained quantum well of the active layer 105 has a well layer of i-In _0.53 G
a _0.47 As (thickness 6 nm), the barrier layer is i-In _0.28 G
a _0.72 As (10 nm thick) 5 quantum wells. Here, tensile strain is introduced into the barrier layer.

The DFB portion 11 on the light emitting side has an active layer 105.
A ridge having a width of 2 μm is formed and embedded with the high resistance InP layer 110. Also, D on the modulation electrode side
In the FB portion 12, a ridge having a width of 5 μm in the active layer 105 is formed and embedded with a high resistance InP layer 111. Here, the electrode separation region 112 is provided by removing even the contact layer 109, and the Cr / AuZnNi / Au layer 113 that is the electrode of the DFB portion 11 on the light emitting side and the electrode of the DFB portion 12 that passes a current with a superimposed signal. Cr / Au
ZnNi / Au layer 114 and AuGeNi of the substrate side electrode
/ Au layer 115 is formed and alloyed. On both ends,
A SiO ₂ film is provided as an antireflection film (not shown).

The operation of this device is shown in FIG.
An explanation will be given by comparing with a conventional example. 2 (b), the conventional
Example device (illustrated in Figure 2 (a))
Show. Bragg wavelength λ_bAnd the propagation constant β at that wavelength₀Is
It is related to the grating pitch Λ by the following formula. β₀= 2πn_eqi/ Λ_b= Π / Λ where n_eqiIs the equivalent refractive index felt by the guided light. Buried
Depending on the shape of the waveguide, TE mode and TM mode transmission
There is a difference in transport constants, and TE module is used for the same grating.
Bragg wavelength λ^TE _bAnd feel of TM mode
Bragg wavelength λ^TM _bAre different. However, FIG.
Front part 21 and rear part 22 shown in FIG.
And the optical waveguide sections below it will be called respectively)
And the difference in Bragg wavelength between both modes is almost the same.
Yes. That is, λ^TE _bf≒ λ^TE _br λ^TM _bf≒ λ^TM _br λ^TE _bf−λ^TE _br≒ λ^TM _bf−λ^TM _br Here, the Bragg wave for the front part 21 and the rear part 22
The length is λ _bf, Λ _brAnd subscripts TE and TM are biased
Represents a wave mode. The laser has TE and TM modes
Near the Bragg wavelength, within the wavelength that satisfies the phase condition
It oscillates at a wavelength that minimizes the gain.

FIG. 2 (c) shows the state of non-uniform implantation of the conventional device. Due to the uneven injection, the front part 2
1, the Bragg wavelength λ for the TE mode in the rear part 22
^TE _bf and λ ^TE _br do not match, but for TM mode,
The Bragg wavelengths λ ^TM _bf and λ ^TM _br do not match between the front portion 21 and the rear portion 22. As shown in FIG. 2C, the detuning amount of the Bragg wavelength in the front portion 21 and the rear portion 22 is almost equal for the TE mode and the TM mode. That is, λ ^TE _bf ≠ λ ^TE _br λ ^TM _bf ≠ λ ^TM _br λ ^TE _bf −λ ^TE _br ≈ λ ^TM _bf −λ ^TM _{br The} TE mode or TM mode oscillates depending on the end face phase and the fineness of the diffraction grating. It was due to asymmetry such as inhomogeneity.

In the state of uniform injection into the device of this embodiment shown in FIG. 3A (FIG. 3B), the front portion 1 is
1, depending on the difference in the ridge width between the rear portion 12 and TE
Not only is there a difference in the propagation constants of the TM and TM modes, but the TE and T modes for the same grating 102
The difference λ ^TE _b −λ ^TM _b of the Bragg wavelengths of the M mode is different from 19 Å in the front part 11 and 23 Å in the rear part 12.
That is, the difference in Bragg wavelength between two polarization modes having different planes of polarization is different, and the amount of difference between the two optical waveguide portions is larger than the stop bandwidth for one of the polarization modes. It is configured. Whether TE mode or TM mode oscillates in this state
It depends on the state of individual devices such as coupling efficiency by the diffraction grating 102, end face phase, and fine nonuniformity of the diffraction grating. In FIG. 3A, the grating 102 is drawn so as to be on the active layer 105 in accordance with the structure of FIG. 2A, but the operation is the same as that of the structure of FIG.

For the device of this embodiment, FIG.
As shown in (c) and FIG. 3 (d), two operating points are considered when non-uniform injection is performed. In the first non-uniform injection state shown in FIG. 3C, the Bragg wavelengths λ ^TE _bf and λ ^TE _br for the TE mode are almost the same in the front portion 11 and the rear portion 12, but the Bragg wavelength for the TM mode is λ ^TM
_bf and λ ^TM _br are different. That is, λ ^TE _bf ≈ λ ^TE _br λ ^TM _bf ≠ λ ^TM _{br In} this case, the light near the Bragg wavelength of the TE mode in which the Bragg wavelengths match has a small reflection loss and a gain that satisfies the gain condition of laser oscillation is small. On the other hand, the light near the Bragg wavelength in the TM mode front portion 11 is far from the Bragg wavelength in the rear portion 12, so that reflection loss occurs and the gain condition for laser oscillation rises. Thus, TE
In the state where the Bragg wavelengths of the modes match, the oscillation is in the TE mode.

The oscillation wavelength is a wavelength that satisfies the phase condition near the Bragg wavelength of the TE mode. In this embodiment, the TE mode Bragg wavelengths λ ^TE _bf and λ ^TE _br are set to 155.
When matched at 3.0 nm, the Bragg wavelength of the TM mode is 1551.1 nm in the front part 11 and 1 in the rear part.
2 becomes about 1550.7 nm.

In the second non-uniform injection state shown in FIG. _3D , the Bragg wavelengths λ ^TM _bf and λ ^TM _br for the TM mode are the same in the front portion 11 and the rear portion 12, but the Bragg wavelength for the TE mode is the same. The wavelengths λ ^TE _bf and λ ^TE _br are different. That is, λ ^TE _bf ≠ λ ^TE _br λ ^TM _bf ≈ λ ^TM _br , the reflection loss in the TE mode is larger and the reflection loss in the TM mode is smaller than that in the state of FIG. In the present device in which a large gain is provided in the TM mode by the mechanism, oscillation occurs in the TM mode.
However, the oscillation wavelength in the TM mode is also a wavelength that satisfies the phase condition in the vicinity of the Bragg wavelength of the TM mode.

By the way, in a commonly known quantum well semiconductor laser, the gain of the active layer is large for the TE mode and small for the TM mode. The loss given is larger than the loss given to the TM mode, and T
It facilitates oscillation in M mode. One is the TM on the shorter wavelength side than the TE mode gain peak.
That is, it was set near the gain peak of the mode. Also,
Generally, the coupling coefficient of Bragg reflection by a diffraction grating is TE
The mode is larger than the TM mode, but the difference is smaller than the difference between the reflectances of TE and TM in the case of end face reflection. In this embodiment, SiO ₂ films are formed on both end faces of the antireflection film. Since this is also provided, this point also applies to TE mode, TM
This contributes to approaching the threshold gain of the mode.
That is, in order to suppress the contribution of the end face reflection, the front portion 11
A non-reflective coating was applied to the end faces.

The product κL of the coupling coefficient by the diffraction grating of the front part 11 and the rear part 12 and the resonator length is about 2. Further, in both the front part 11 and the rear part 12, the transverse mode was the 0th order mode. That is, the high-order mode is cut off to become the single mode.

[0027]

Second Embodiment A distributed feedback laser including a phase shift according to a second embodiment of the present invention will be described with reference to FIG. In FIG. 4, 31 is a DFB portion on the light emitting side, 3
2 is a DFB portion on the modulation electrode side. The layer structure of this embodiment is similar to that of the first embodiment (grating 3
3 is drawn on the active layer, the operation is the same as that of the structure of FIG. 1), but the difference from the first embodiment is
The diffraction grating 33 at the boundary between the DFB portions 31 and 32 is provided with a phase shift 34 called a λ / 4 shift.

Similar to the first embodiment, the DFB portion 31 on the light emitting side forms a ridge having a width of 2 μm in the active layer, and the DFB portion 32 on the modulation electrode side has a width of 5 μm in the active layer. A ridge is formed and embedded with a high resistance InP layer. The formation of the electrodes and the formation of the antireflection film are the same as in the first embodiment.

The operational characteristic of this embodiment is that the λ / 4 shift 34 is introduced so that the oscillation wavelength in each of the TE and TM modes is not outside the stop band but in the vicinity of the Bragg wavelength in the stop band. It is possible to oscillate. Other than that, the operation principle is the same as that of the first embodiment.

[0030]

Third Embodiment A third embodiment of the distributed feedback laser including a portion where the grating pitch is changed according to the present invention will be described with reference to FIG. 41 is the DFB on the light emitting side
Reference numeral 42 denotes a DFB portion on the modulation electrode side. The layer structure of this embodiment is similar to that of the first embodiment (in this embodiment as well, the grating 43 is drawn as being on the active layer, but the structure and operation of FIG. No change), first
The difference from the embodiment of FIG.
1 is slightly different from the modulation section 42, and the modulation section 4 having a wide ridge width is 234.8 nm at the emission section 41 having a narrow ridge width.
2 changes to 234.0 nm by about 0.3%.

Similar to the first embodiment, the DFB portion 41 on the light emitting side forms a ridge having a width of 2 μm in the active layer, and the DFB portion 42 on the modulation electrode side has a width of 5 μm in the active layer. A ridge is formed and embedded with a high resistance InP layer. The formation of the electrodes and the formation of the antireflection film are the same as in the first embodiment.

The operational feature of this embodiment is that the Bragg wavelengths of the TM mode of the emitting section 41 and the modulating section 42 are substantially the same in the injection state of a substantially uniform carrier density by changing the grating pitch (see FIG. d)), and a slight non-uniform injection from here will bring the TE mode Bragg wavelengths into agreement (see FIG. 3 (c)). Other than this, the operation principle of this embodiment is the same as that of the first embodiment.

[0033]

[Embodiment 4] A fourth embodiment of a semiconductor laser including two distributed feedback portions having different layer configurations according to the present invention is shown in FIG.
It will be explained by. 51 is the first DFB section, 52 is the second
It is the DFB part of.

The layer structure of the first DFB section 51 is n-In
On P substrate 501, 0.3 μm thick n-In _0.79 Ga
_0.21 As _0.45 P _0.55 Lower optical guide layer 502, i-In _0.59 Ga _0.41 As _0.87 P with a thickness of 0.05 μm to be an active layer
_0.13 503, 0.3 μm thick p-In _0.79 Ga _0.21 As
_0.45 P _0.55 upper light guide layer 504 is formed, pitch 23
A 7 nm diffraction grating 505 is formed. 1.8μ on it
m-thick p-InP clad layer 506, p-In _0.59 Ga
_{A 0.41} As _0.9 P _0.1 contact layer 507 is laminated.

The layer structure of the second DFB portion 52 is n-In.
0.25 μm thick n-In _0.79 Ga is formed on the P substrate 501.
_0.21 As _0.45 P _0.55 Lower optical guide layer 512, 0.15μ
m-thick i-In _0.59 Ga _0.41 As _0.87 P _0.13 active layer 51
3, 0.25 μm thick p-In _0.79 Ga _0.21 As _0.45 P
_0.55 upper light guide layer 514 is formed, pitch 237nm
The diffraction grating 515 is formed. A p-InP clad layer 516 having a thickness of 1.8 μm and a p-In _0.59 Ga _0.41 A layer on top of the
The s _0.9 P _0.1 contact layer 517 is laminated.

In this embodiment, ridges having a width of 3 μm in the active layers 503 and 513 are formed in both DFB portions 51 and 52, and the ridges are filled with high resistance InP. The formation of the electrodes and the formation of the antireflection film are the same as in the first embodiment.

Although the ridge width of each of the portions 51 and 52 is not changed in this embodiment, the T
The difference between the propagation constants of the E mode and the TM mode is different between the first DFB section 51 and the second DFB section 52. The TE mode Bragg wavelength in the first DFB portion 51 of this embodiment is 1550.12 nm, and the TM mode Bragg wavelength is 15.
47.42 nm, the TE mode Bragg wavelength in the second DFB portion 52 is 1566.29 nm, and the TM mode Bragg wavelength is 1561.74 nm.

In order to compensate for the Bragg wavelength difference between the two DFB parts 51 and 52, the grating pitch of the second DFB part 52 is made longer than the grating pitch of the first DFB part 51 by about 1% to 239 nm. You may make it the structure. This makes it possible to reduce the amount of injected current between the first DFB section 51 and the second DFB section 52 until the Bragg wavelengths in the TE mode or the TM mode are matched. Other than this, the operation principle of this embodiment is the same as that of the first embodiment.

[0039]

Fifth Embodiment A semiconductor laser which is a fifth embodiment of the present invention will be described with reference to FIG. 7, which is composed of two distributed feedback portions having different thicknesses of the optical confinement layer.

A 0.2 μm thick n-In _0.86 Ga _0.14 As _0.31 P _0.69 602 is laminated on an n-InP substrate 601, and a diffraction grating 6 having a depth of 0.15 μm and a pitch of 238 nm.
Form 03. On top of this, 0.2 μm thick n-In
_0.79 Ga _0.21 As _0.45 P _0.55 Lower optical guide layer 604, i-In _0.59 Ga _0.41 As having a thickness of 0.05 μm to be an active layer
_0.87 P _0.13 605, 0.4 μm thick p-In _0.79 Ga
_0.21 As _0.45 P _{0.55 An} upper light guide layer 606 is formed.
By patterning by photolithography, the upper light guide layer 6 is partially (DFB portion 62 on the right side of FIG. 7).
06 is etched to a depth of 0.2 μm. A 1.8 μm thick p-InP clad layer 607, p-In
_{A 0.59} Ga _0.41 As _0.9 P _0.1 contact layer 608 is laminated.

In this embodiment, a ridge having a width of 3 μm in the active layer 605 is formed in both DFB portions 61 and 62,
It is embedded by a high resistance InP layer. Formation of electrodes,
The formation of the antireflection film is the same as in the first embodiment.

Although the ridge width for each part is not changed in this embodiment, the difference in the propagation constant between the TE mode and the TM mode may be different from that of the first DFB portion 61 due to the difference in the layer structure.
The DFB section 62 is wrong. First DFB of this embodiment
The TE mode Bragg wavelength in the portion 61 is 1548.
05nm, Bragg wavelength of TM mode is 1546.38
nm, the TE mode Bragg wavelength in the second DFB portion 62 is 1535.53 nm, and the TM mode Bragg wavelength is 1533.53.

Also in this embodiment, the two DFB sections 61 and 62 are used.
Second DFB to compensate for the difference in Bragg wavelength at
The grating pitch of the portion 62 is made longer than the grating pitch of the first DFB portion 61 by about 1% to obtain 240n.
A configuration in which m is set is also conceivable. Thus, the first D
Between the FB portion 61 and the second DFB portion 62, the amount of injected current until the Bragg wavelengths in the TE mode or the TM mode are matched can be reduced. Other than this, the operation principle of this embodiment is the same as that of the first embodiment.

[0044]

Sixth Embodiment A semiconductor laser which is a sixth embodiment according to the present invention and which comprises two distributed feedback portions having different thicknesses of the optical confinement layer will be described with reference to FIG. 71 is the first DF
The section B and 72 are the second DFB section.

The layer structure of the first DFB portion 71 is n-In
On the P substrate 701, 0.3 μm thick n-In _0.79 Ga
_0.21 As _0.45 P _0.55 Lower optical guide layer 702, i-In _0.59 Ga _0.41 As _0.87 P with a thickness of 0.05 μm to be an active layer
_0.13 703, 0.3 μm p-In _0.79 Ga _0.21 As
_0.45 P _0.55 upper light guide layer 704 is formed, pitch 23
A 7 nm diffraction grating 705 is formed. 1.8μ on it
m-thick p-InP clad layer 706, p-In _0.59 Ga
_{A 0.41} As _0.9 P _0.1 contact layer 707 is laminated.

The layer structure of the second DFB portion 72 is n-In
On a P substrate 701, a 0.08 μm thick n-InP buffer 712 and a 0.2 μm thick n-In _0.79 Ga _0.21 As
_0.45 P _0.55 lower optical guide layer 713, i of 0.09 μm thickness
-In _0.59 Ga _0.41 As _0.87 P _0.13 active layers 714, 0.
A 2 μm thick p-In _0.79 Ga _0.21 As _0.45 P _0.55 upper optical guide layer 715 is formed, and a diffraction grating 716 having a pitch of 237 nm is formed. 1.8 μm thick p-InP on it
Cladding layer 717, p-In _0.59 Ga _0.41 As _0.9 P _0.1
The contact layer 718 is laminated.

In this embodiment, a ridge having a width of 3 μm in the active layers 703 and 714 is formed in both DFB portions 71 and 72, and the ridges are filled with a high resistance InP layer. The formation of the electrodes and the formation of the antireflection film are the same as in the first embodiment.

Also in this embodiment, the ridge width of each of the portions 71 and 72 is not changed. However, due to the difference in the layer structure, T
The difference between the propagation constants of the E mode and the TM mode is different between the first DFB section 71 and the second DFB section 72. The TE mode Bragg wavelength is 1550.12 nm and the TM mode Bragg wavelength is 15 in the first DFB section 71 of this embodiment.
47.42 nm, the TE mode Bragg wavelength in the second DFB portion 72 is 1548.23 nm, and the TM mode Bragg wavelength is 1543.72 nm. In this embodiment, the TE mode Bragg wavelengths are relatively close to each other, and the TE mode Bragg wavelengths are almost the same in the injection state of the substantially uniform carrier density. The feature is that the two match. Other than this, the operation principle of this embodiment is the same as that of the first embodiment.

[0049]

Seventh Embodiment A semiconductor laser including a mixed crystal quantum well according to a seventh embodiment of the present invention will be described with reference to FIG.

First, the layer structure shown in FIG. 9A is prepared. That is, 0.3 μm on the n-InP substrate 801.
A thick n-In _0.79 Ga _0.21 As _0.45 P _0.55 lower optical guide layer 802, a strained quantum well structure active layer 803, a 0.2 μm thick p-In _0.79 Ga _0.21 As _0.45 P _0.55 upper optical guide layer 804, and a well layer p-In _0.73 Ga _0.27 As _0.59 P _0.41
(Thickness 6 nm), the barrier layer is p-In _0.86 Ga _0.14 As
_0.31 P _0.69 (thickness 10 nm) A multiple quantum well optical guide layer 805 consisting of 10 layers is laminated to form a diffraction grating 806 having a pitch of 237 nm. On top of this, a 1.8 μm thick p-
InP clad layer 807, p-In _0.59 Ga _0.41 As
_{A 0.9} P _0.1 contact layer 808 is laminated.

Next, as shown in FIG. 9B, the active layer 803 and the quantum well guide layer 805 in a partial region (portion 812) are mixed with each other by implanting hydrogen ions. The part which is not mixed crystal is indicated by 81, and the part which is mixed crystal is indicated by 82.
A ridge having a width of 3 μm in the active layer portion is formed and embedded with a high resistance InP layer, and as in the first embodiment,
The electrodes are formed and the antireflection film is formed.

In the present embodiment, the portion 8 in which the quantum well light guide layer 805 and the quantum well active layer 803 are mixed crystals.
The feature is that the wavelength difference between the TE mode Bragg wavelength and the TM mode Bragg wavelength is different between 2 and the non-mixed portion 81 depending on the presence or absence of a quantum well. The operation and driving method of the device are the same as those of the other embodiments already described.

As a method for forming a mixed crystal in the quantum well, a method of implanting hydrogen ions has been taken as an example. However, mixed crystal is formed by heat treatment after partially forming an insulating film on the surface, and mixed crystal is formed by diffusion of impurities. It is also possible to use a known method such as. In addition, in this embodiment, the active layer 803 and the light guide layer 8 are
Although the case where both the quantum wells of No. 05 are mixed crystal is described, for example, only the quantum well layer used for the optical guide layer 805 may be mixed crystal. Further, in this embodiment, the light guide layer 8
In 05, the layer in which the grating 806 was formed was a quantum well layer, but this is not essential, and the grating is brought below the active layer 803 to activate the active layer 803.
The quantum well layer on the upper side of may be subjected to mixed crystallization.

In order to increase the difference between the Bragg wavelength differences 81 and 82 depending on the polarization mode,
Alternatively, in order to compensate for the difference, as in the case of the first embodiment, the portion 82 for crystallizing the width of the in-plane confinement and the portion 8 for not crystallizing.
You can change it by 1.

In each of the above-described embodiments, the ridge formation and the burying by the high resistance layer have been described as examples, but the current confinement and the light confinement utilizing the reverse bias of the pn junction may be used. Good. Further, the light confining structure in the plane of the waveguide is not limited to the buried (BH) structure, and any structure that confine light in the lateral direction may be used.

[0056]

As described above, according to the present invention, in the multi-electrode DFB laser, the structural nonuniformity is made different depending on the polarization mode, so that the oscillation wavelength and the polarization mode depending on the injection conditions are changed. It is possible to realize a device capable of stable and high-speed polarization modulation with good reproducibility by suppressing complicated behavior and suppressing variations in oscillation wavelength and polarization mode between devices depending on end face phase.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of a semiconductor laser according to the present invention ((a) is a perspective view, (b) is a partial sectional perspective view).

FIG. 2 is a diagram illustrating an operation of oscillating in polarization modes having different polarization planes according to a conventional example.

FIG. 3 is a diagram illustrating an operation of oscillating in polarization modes having different polarization planes according to an example of the present invention.

FIG. 4 is a schematic sectional view showing a second embodiment of the semiconductor laser according to the present invention.

FIG. 5 is a schematic sectional view showing a third embodiment of the semiconductor laser according to the present invention.

FIG. 6 is a sectional view showing a fourth embodiment of the semiconductor laser according to the present invention.

FIG. 7 is a sectional view showing a fifth embodiment of the semiconductor laser according to the present invention.

FIG. 8 is a sectional view showing a sixth embodiment of the semiconductor laser according to the present invention.

FIG. 9 is a sectional view showing the structure and manufacturing method of a seventh example of a semiconductor laser according to the present invention.

[Explanation of symbols]

11, 31, 41, 51 Light emission side DFB section (front section) 12, 32, 42, 52 Modulation side DFB section (rear section) 34 λ / 4 shift 43 Pitch modulating grating 61, 71 First DFB section 62, 72 Second DFB part 81 Non-mixed crystal part 82 Mixed crystal part 101, 501, 601, 701, 801 Substrate 102, 505, 515, 603, 705, 716, 8
06 diffraction grating 103, 108, 506, 516, 602, 607, 7
06, 717, 807 Clad layer 104, 502, 512, 604, 702, 713, 8
02 Lower optical guide layer 105, 603, 803 Strained quantum well structure active layer 106, 504, 514, 606, 704, 715, 8
04 Upper light guide layer 109, 507, 517, 608, 707, 718, 8
08 contact layer 110, 111 high resistance layer 112 electrode separation region 113, 114, 115 electrode 503, 513, 605, 703, 714 active layer 805 multiple quantum well optical guide layer

Claims (14)

[Claims] 1. A light source capable of oscillating in two polarization modes with different planes of polarization, having two or more electrodes in the resonance direction, and a first optical waveguide portion and a second optical waveguide portion below the electrodes. Is a part or all of which constitutes a resonator of a laser, and a waveguide for two polarization modes having different polarization planes is provided between the first optical waveguide part and the second optical waveguide part. A distributed feedback semiconductor laser, characterized in that the propagation constants of the two differ. 2. A propagation constant difference between polarization modes having different planes of polarization due to different widths of lateral optical confinement in the plane of the substrate in at least the first and second optical waveguide portions constituting the resonator. 2. The distributed feedback semiconductor laser according to claim 1, wherein the two are different from each other. 3. The distributed feedback semiconductor laser according to claim 2, wherein in the first and second optical waveguide portions, the transverse modes of the laser oscillation mode are 0th-order modes. 4. At least the first and second optical waveguide portions forming the resonator have different layer configurations in the stacking direction for each optical waveguide portion, so that a difference in propagation constant between polarization modes having different polarization planes is obtained. 2. The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor lasers are different from each other. 5. In the layer structure in the stacking direction, at least one of the layer thickness and the composition of the active layer is different for each optical waveguide portion, so that the propagation constant differences for polarization modes having different polarization planes are different from each other. 5. The distributed feedback semiconductor laser according to claim 4, wherein 6. The distributed feedback semiconductor laser according to claim 2, wherein the diffraction grating of the distributed feedback semiconductor laser includes a phase shift. 7. The pitch of the diffraction grating is different from each other in at least the first and second optical waveguide portions constituting the resonator, and the polarization planes are different under the condition of uniform current injection.
5. The distributed Bragg reflector semiconductor laser according to claim 2, wherein the Bragg wavelengths of the first and second optical waveguide portions are the same for one of the two polarization modes. 8. The distribution according to claim 2, wherein a quantum well structure is used for a part or the whole of the active layer in order to cause a large difference in propagation constant between polarization modes having different polarization planes. Feedback semiconductor laser. 9. A quantum well structure is used in a portion of the structure of the optical waveguide portion other than the active layer in order to cause a large difference in propagation constant between polarization modes having different polarization planes. 4. The distributed feedback semiconductor laser described in 4. 10. A first waveguide portion constituting the resonator is a portion in which the quantum well structure is mixed crystal, and a second waveguide portion is formed.
10. The waveguide part of 1 is used as a part where the quantum well structure is not mixed, and the propagation constant difference for polarization modes having different polarization planes is made different for each optical waveguide part. Distributed feedback semiconductor laser. 11. The first and second optical waveguide portions have different Bragg wavelength differences with respect to two polarization modes having different polarization planes, and the difference in the difference between the two optical waveguide portions is any amount. 2. The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor laser is configured so as to have a width larger than a stop band width for one polarization mode. 12. At least a first component of the resonator.
2. The distributed feedback semiconductor laser according to claim 1, wherein in the second optical waveguide portion, the active layer is a quantum well structure layer having tensile strain introduced therein. 13. At least a first component of the resonator.
2. The distributed feedback semiconductor laser according to claim 1, wherein the Bragg wavelength is set near the gain peak of the TM mode on the shorter wavelength side than the gain peak of the TE mode in the second optical waveguide portion. 14. The method of driving a distributed feedback semiconductor laser according to claim 1, wherein two polarization modes having different polarization planes are generated by a minute modulation current signal applied to at least one of the independent electrodes. On the other hand, for at least one of the two polarization modes having different polarization planes, and at least the first and the second waveguides having the same Bragg wavelength. A method of driving a semiconductor laser, characterized in that switching is performed between a portion and a state in which the Bragg wavelengths match each other.