JPH0621578A - Semiconductor integrated modulation light source device - Google Patents

Abstract

(57) [Summary] [Object] To provide a semiconductor integrated modulation light source device capable of operating at higher speed than ever before. A semiconductor laser section (DFB laser section 1) and an optical modulator section (4) for intensity-modulating the emitted light of the semiconductor laser section (1) are monolithically integrated, and the semiconductor laser section (1) is activated. A quantum well structure in which in-plane tensile strain is applied to the well layer is used as the layer (2), and the active layer (light absorbing layer 5) of the optical modulator section (4) is
A quantum well structure is used regardless of the presence / absence of strain or its direction. In this case, an InP substrate is used, and In _1-x Ga is used as the well layer of the quantum well structure forming the active layers of the semiconductor laser section (1) and the optical modulator section (4).
_x As (1 ≧ x ≧ 0.47), In _1-x Ga _x As _{1 -y}
P _y (x and y are 0.1894y−0.4184x +
0.0130xy + 5.8696 [Å] ≤ 56887
The range that satisfies Å) can be used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor integrated modulation light source device.

[0002]

2. Description of the Related Art A semiconductor integrated modulation light source device in which a semiconductor laser section and an optical modulator section for intensity-modulating light are monolithically integrated is promising as an optical modulator for achieving high-speed operation in the technical field of optical communication and the like. It is being watched, and the research and development are underway.

FIG. 1 is a schematic structural explanatory view of a semiconductor integrated modulation light source device. In this figure, 1 is a DFB laser section, 2 is an active layer, 3 is a high resistance InP buried layer, 4 is an optical modulator section, and 5 is a light absorption layer.

In the semiconductor integrated modulation light source device shown in this figure, a forward bias voltage is applied to the active layer 2 defined by the high-resistance InP buried layer 3 of the DFB laser section 1 to cause a current to flow, and laser oscillation is performed. Wake up.

Then, the laser light is guided to the light absorption layer 5 of the optical modulator portion 4 formed monolithically with the optical axis aligned with the active layer of the DFB laser portion 1, and is guided to the optical modulator portion 4. By turning on and off the applied reverse bias voltage, the light emitted from the DFB laser unit 1 is intensity-modulated and is emitted to the outside as modulated light.

FIG. 2 is a diagram for explaining the principle of light modulation in the light modulator section. This figure shows light absorption spectra when the reverse bias voltage is turned on (broken line) and when it is turned off (solid line) in the light absorption layer.

When the reverse bias voltage applied to the light absorption layer of the optical modulator section is turned off, the light absorption spectrum increases in proportion to the square root of the incident energy (see the solid line in FIG. 2). However, when the reverse bias voltage applied to the light absorption layer is increased, the light absorption spectrum gradually tails to the low energy side (left side of the figure) (Franzkeldish effect, see broken line in FIG. 2). .

Therefore, when the oscillation wavelength of the DFB laser section is set to fall within the energy region (arrow) where the solid line light absorption coefficient in FIG. 2 is low, the reverse bias applied to the light absorption layer of the optical modulator section is turned off. Then, the light absorption coefficient of the laser light becomes small, and when the reverse bias is turned on, the light absorption coefficient of the laser light becomes large.Therefore, by turning on / off the reverse bias voltage applied to the light absorption layer of the optical modulator section, the intensity of the laser light is increased. It becomes possible to modulate.

At this time, one of the factors that determines the limit of high-speed modulation is the time from when the reverse bias voltage is turned on until the electric field is applied to the light absorption layer of the light modulation section, that is, the CR time constant of the circuit. . At present, as shown in FIG. 1, the high resistance InP burying layer 3 is embedded around the waveguide which is the active layer 2 to suppress the parasitic capacitance of this portion to be small, thereby substantially eliminating this factor.

[0010]

In practice, what limits the modulation rate is the modulation rate of the external circuit that applies a pulse-like reverse bias voltage to the absorption layer. Therefore, in order to reduce the load on the external circuit as much as possible and obtain an integrated semiconductor modulation light source that can operate at a higher speed, it is an issue to further reduce the operating voltage of the light absorption layer.

It is an object of the present invention to provide an integrated semiconductor modulation light source which can operate at high speed with a lower voltage as compared with the conventional integrated semiconductor modulation light source.

[0012]

In the semiconductor integrated modulation light source device according to the present invention, in order to achieve the above object, a semiconductor laser section and an optical modulator section for intensity-modulating the light emitted from the semiconductor laser section are provided. Monolithically integrated, the quantum well structure in which the in-plane tensile strain is applied to the well layer is used as the active layer of this semiconductor laser section, and the quantum well structure is used as the active layer of this optical modulator section. ing.

In this case, the In _{1 -x} Ga _x As (1 ≧ x ≧ 0..0) is formed in the well layer of the quantum well structure forming the active layer of the semiconductor laser section and the optical modulator section using the InP substrate.
47) can be used.

In this case, the InP substrate is used, and In _1-x Ga _x As _1-y P _y is used for the well layer of the quantum well structure which constitutes the active layer of the semiconductor laser section and the optical modulator section. You can This composition range is a (x, y) = 0.
1894y-0.4184x + 0.0130xy + 5.
In 8696 [Å], x and y satisfy a (x, y) ≦ 5.88687Å.

[0015]

The following two directions can be considered as a method of lowering the operating voltage of the integrated modulation light source device.

The first method for lowering the operating voltage of the integrated modulation light source device is the laser oscillation wavelength (→ mark) in FIG.
And the light absorption edge of the light absorption characteristics (solid line) of the light absorption layer of the optical modulator section when the reverse bias voltage is off. However, according to this method, the laser light is absorbed to some extent by the skirt of the light absorption characteristic even when the reverse bias voltage is off in the absorption layer. Therefore, it is necessary to increase the output of the laser portion to compensate the absorption loss.

A second method of lowering the operating voltage of the integrated modulation light source device is to make the change of the light absorption characteristic of the optical modulator section with respect to the wavelength abrupt and increase the rate of change of the light absorption coefficient by the reverse bias voltage. Is.

Therefore, if the optical output of the laser section is increased, the change of the optical absorption characteristic of the optical modulator section with respect to the wavelength is made rapid, and the rate of change of the optical absorption coefficient by the reverse bias voltage is increased, the integrated modulation light source device The operation can be speeded up.

Here, the principle of the quantum well structure in which tensile strain is added as the active layer in order to increase the output of the laser section as the light source will be described. The semiconductor quantum well structure is
Generally, it has been widely used as an active layer of a semiconductor laser device.

FIG. 3 is a band structure diagram of the quantum well structure of the semiconductor laser device. In this figure, 11, 19 are cladding layers, 12, 14, 16, 18 are well layers, 13,
15, 17 are barrier layers, E _C is the conduction band, E _V is the valence band,
L _Z is the width of the well layer, and L _B is the width of the barrier layer.

The quantum well structure of this semiconductor laser device is
As shown in this band structure diagram, the cladding layer 1
1, 19 between the well layers 12 and 14 having the width L _Z of the well layers,
16, 18 and barrier layers 13, 15, 1 having a barrier layer width L _B
7 are alternately formed, and the interval between the bottom of the conduction band E _{C and} the top of the valence band E _V is alternately changed.

In this semiconductor laser device, the electrons and holes injected into the active layer are confined in the quantum well structure and emit light with high efficiency. In the direction perpendicular to the respective active layers and barrier layers, light has a refractive index difference between the cladding layers 11 and 19 and the quantum well structure (the refractive index of the cladding layers 11 and 19 is smaller than that of the quantum well structure. ) Is trapped by.

Then, the light is repeatedly reflected in the resonator provided in the direction perpendicular to the plane of the drawing to cause stimulated emission. When the injection current is increased, laser oscillation occurs when the gain due to repeated reflection in the resonator exceeds the loss. A semiconductor laser device using this quantum well structure can improve efficiency by applying tensile strain to the well structure.

4A and 4B are dispersion curves of the conduction band and the valence band of the quantum well structure having tensile strain. This figure explains the principle of a quantum well structure having tensile strain, and shows a schematic dispersion curve in the plane of the quantum well structure of the conduction band and the valence band, and FIG. FIG. 4B shows a dispersion curve of a quantum well structure having no strain, and FIG. 4B shows a dispersion curve of a quantum well structure having tensile strain.

FIG. 4A is a dispersion curve of a quantum well structure having no tensile strain. In this case, the heavy hole (HH) band is at a higher energy position than the light hole (LH) band. , The optical transition between the conduction band and the heavy hole band contributes to laser oscillation.

FIG. 4B is a dispersion curve of a quantum well structure having tensile strain. In this case, the light hole band is at a high energy position due to the effect of strain, and the conduction band--
Optical transitions between the light hole bands contribute to laser oscillation.

The difference between the heavy hole band and the light hole band appears in the selection rule of the effective mass and the mode of the optical transition. First, regarding the effective mass, when comparing FIGS. 4A and 4B, the light hole band has a larger effective mass in the quantum well plane, as can be seen from the difference in curvature.

When tensile strain is applied to the quantum well structure,
The optical gain is increased for the reasons described below. The optical gain of the semiconductor laser with respect to the light of energy E is expressed by the following mathematical formula 1.

[0029]

[Equation 1]

When the delta function is integrated in k-space, the following formula 2 is obtained near the band edge.

[0031]

[Equation 2]

As can be seen from this equation, the larger the effective mass of holes, the larger the optical gain, and the quantum well structure having tensile strain has a larger gain. That is, the optical transition from the conduction band to the light hole band (FIG. 4B).
Is the conduction band-heavy hole band optical transition (Fig. 4 (A))
It has a larger optical gain than.

As a result, in a semiconductor laser device using a quantum well structure having tensile strain as an active layer, oscillation occurs at a lower injected carrier density. By reducing the carrier density, the absorption of the valence band and the Auger recombination process, which are the main loss factors of the semiconductor laser device, are suppressed, so that the output of the semiconductor laser device can be increased.

Next, regarding the mode selection rule, the conduction band-
In a quantum well that does not have the usual tensile strain involving heavy hole band transitions, oscillation occurs in the TE mode with the polarization direction in the plane of the quantum well.

On the other hand, in the quantum well structure having a tensile strain, the polarization direction is TM perpendicular to the plane of the quantum well structure.
It oscillates in the mode. By thus oscillating in the TM mode, the modulation of the light absorption layer in the optical modulator is effectively performed as described below.

FIG. 5 is an optical absorption spectrum diagram of the quantum well structure for TE mode and TM mode. This figure shows the optical absorption spectrum of the quantum well structure for TE mode without tensile strain (solid line) and the optical absorption spectrum of quantum well structure for TM mode with tensile strain (broken line) at room temperature.

In this figure, a peak (a) of electron-heavy hole excitons appears at the absorption edge for TE-mode light (solid line) having no tensile strain, and a higher energy side than that. Electron-light hole exciton peak (b)
Appears. On the other hand, with respect to TM mode light (dashed line) having tensile strain, the transition of electron-heavy hole excitons is prohibited, and therefore the absorption peak (c of electron-light hole excitons at the absorption edge (c ) Only appears.

Comparing the electron-heavy hole exciton transition matrix element in the TE mode with the electron-light hole exciton transition matrix element in the TM mode, the latter is about 4/3 times as large as the former, and the light absorption coefficient thereof is also large. It grows by a minute. Therefore,
Since the rate of change of the absorption coefficient when an electric field is applied becomes larger, more effective light modulation can be realized.

As described above, in the absorption layer having the quantum well structure, a strong optical absorption peak due to excitons is obtained, unlike the bulk semiconductor, regardless of whether the quantum well structure has tensile strain or not. Appears, and the absorption characteristic is rapidly increased with wavelength.

[0040]

EXAMPLE An example of the present invention will be described below. [Structure of Semiconductor Laser] In this embodiment, InP is used.
In as a quantum well layer on a (100) substrate_{0. Four}Ga
_0.6The As layer is formed, and the PL wavelength at room temperature is about as a barrier layer.
1.3 nm In _0.72Ga_0.28As_0.6P_0.4Stacking layers
Then formed. In this case, the thickness of the quantum well layer is 16
0Å, with about 0.9% in-plane tensile strain
There is. Also, In_0.72Ga_0.28As_0.6P_0.4From layers
The barrier layer is a lattice match with InP.

For comparison with this, as a quantum well layer I
In _0.53 Ga _0.47 As layer lattice-matched with the nP substrate is used, and In _0.72 G lattice-matched with the InP substrate is used as a barrier layer.
A quantum well structure using a _0.28 As _0.6 P _0.4 layer was formed. For these two types of quantum well structures, the valence band dispersion relation and the optical gain were compared and examined by the kp perturbation method.

FIG. 6 is a graph showing the relationship between the maximum optical gain and the injected carrier density at room temperature of the quantum well structure. According to this figure, it is clearly understood that the optical gain is increased by adding the tensile strain. Assuming that the threshold gain at which lasing occurs is 1000 cm ⁻¹ , the threshold carrier density is lattice-matched to the substrate in the case of a laser device having an active layer in which tensile strain exists, and tensile strain exists. It is reduced to nearly half that of a laser device with an active layer that does not. Due to this reduction in carrier density, light absorption loss is also reduced, so that higher output can be achieved.

[Structure of Optical Modulator] As the quantum well of the active layer, any of lattice matching, compressive strain, and tensile strain can be used. In any case, only the electron-light hole excitons react to the TM mode light emitted from the laser. Here, a quantum well made of In _0.53 Ga _0.47 As lattice-matched to the (001) InP substrate and an In _0.72 Ga _0.28 As _0.6 P _0.4 barrier layer also lattice-matched is formed, and the thickness of the well layer is 60 Å And

In the integrated modulation light source in which the semiconductor laser section having the tensile strained quantum well structure as the active layer and the optical modulator section having the lattice-matched quantum well structure as the active layer are integrated, the optical output is In case of 10mW, 10GHz
It was possible to perform high-speed light modulation. This is because the drive voltage could be reduced to about 1.5 V by introducing the active layer.

The semiconductor integrated modulation light source device of the present invention increases the output of the laser by applying tensile strain,
Further, the TM mode light is modulated with a low power by a quantum well to realize a light source capable of high speed modulation. The present invention uses the basic principle that TM-mode light emitted from the tensile strained quantum well is absorbed more strongly in the quantum well. Therefore, a lattice-matched quantum well is used for the active layer. , The performance is significantly improved as compared with the case of oscillating and modulating in the TE mode. Also, because of the integration, low power consumption and high speed have been achieved.

In the case of using the InP substrate, In _1-x Ga _x As (1 ≧ x ≧ 0..0) is used as a well layer of the quantum well structure forming the active layers of the semiconductor laser section and the optical modulator section.
47), or In _1-x Ga _x As _1-y P _y (for this composition, a (x, y) = 0.1894y-0.
4184x + 0.0130xy + 5.8696 [Å]
Then, when a (x, y) ≦ 5.88687Å is satisfied, the same effect as that of the above-described embodiment can be obtained.

When the compositions of the quantum well structures forming the active layers of the semiconductor laser section and the optical modulator section are different,
After forming one quantum well structure, a desired quantum well structure can be formed by removing a part of the quantum well structure and forming the other quantum well structure. Quantum well structures having different compositions or structures can be formed by changing the conditions and growing a semiconductor layer on the entire surface under the same conditions.

[0048]

As described above, according to the present invention,
When a light source composed of a strained quantum well laser section having a quantum well structure in which an active layer is subjected to tensile strain and an optical modulator section having an active layer in which the optical axis is aligned with the light source are used as quantum wells, are combined, Oscillation threshold carrier density is lowered, the output is increased, and the optical absorption coefficient of the modulator is increased, so that it is possible to drive the modulator with a smaller bias voltage, and it also has a great effect on speeding up. .

[Brief description of drawings]

FIG. 1 is a schematic configuration explanatory diagram of a semiconductor integrated modulation light source device.

FIG. 2 is an explanatory diagram of an optical modulation principle of an optical modulator section.

FIG. 3 is a band structure diagram of a quantum well structure of a semiconductor laser device.

4A and 4B are dispersion curves of a conduction band and a valence band of a quantum well structure having tensile strain.

FIG. 5 is an optical absorption spectrum diagram of a quantum well structure for TE mode and TM mode.

FIG. 6 is a graph showing the relationship between maximum optical gain and injected carrier density at room temperature in a quantum well structure.

[Explanation of symbols]

1 DFB laser part 2 Active layer 3 High resistance InP buried layer 4 Optical modulator part 5 Optical absorption layer 11,19 Cladding layer 12,14,16,18 Well layer 13,15,17 Barrier layer E _C conduction band E _V value Electron band L _Z width of well layer L _B width of barrier layer

Claims (3)

[Claims] 1. A semiconductor laser section and an optical modulator section for intensity-modulating emitted light of the semiconductor laser section are monolithically integrated, and in-plane tensile strain is applied to a well layer as an active layer of the semiconductor laser section. A semiconductor integrated modulation light source device, wherein a quantum well structure is used and a quantum well structure is used as an active layer of the optical modulator section. 2. An InP substrate is used, and In _1-x Ga _x As (1 ≧ x ≧ 0.47) is used for a well layer of a quantum well structure forming an active layer of a semiconductor laser section and an optical modulator section. The semiconductor integrated modulation light source device according to claim 1, wherein the modulation light source device is a semiconductor integrated modulation light source device. 3. In _1-x Ga _x As _1-y P _y (x, y are defined as follows) in a well layer of a quantum well structure forming an active layer of a semiconductor laser section and an optical modulator section using an InP substrate. a (x,
y) = 0.1894y-0.4184x + 0.0130
The range is such that xy + 5.8696 [Å] ≦ 5.88687Å is satisfied. ) Is used, the semiconductor integrated modulation light source device according to claim 1.