JP3063355B2 - Semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

2. Description of the Related Art Semiconductor lasers oscillating at a wavelength of 1.3 μm to 1.6 μm are extremely important as light sources for optical fiber communication, and research and development of semiconductor lasers capable of high-speed modulation of several tens of Gbit / sec or more have been conducted. Expected. The present invention relates to a semiconductor laser capable of performing such ultra-high-speed modulation.

[0002] oscillates at a wavelength of 1.6μm from a small 1.3μm loss of the optical fiber, In _X Ga _1-X As lattice matched to the InP _{substrate, In X Ga 1-X As} Y P 1-Y system length Wavelength-band semiconductor lasers are used in actual optical fiber communication systems. Recently, many research institutions have studied the use of quantum well structures and / or compression-strained quantum well structures as active layers in order to improve laser characteristics. (For example,
P. J. A. Thijs et al., IEEE Journa
l of Quantum Electronics,
vol. 27, NO. 6, pages 1426 to 1439 (1
991)). FIG. 4 shows a layer structure of a conventional compression-strained multiple quantum well semiconductor laser crystal. In FIG. 4 , the semiconductor laser crystal is composed of an n-type InP substrate 20, an n-type InP cladding layer 21, and InGaAs having a band gap wavelength of 1.25 μm.
sP guide layer 22, In _0.7 Ga _0.3 A having a thickness of 2.5 nm
a multiple quantum well active layer in which an s quantum well layer 23 and an InGaAsP barrier layer 24 having a layer thickness of 15 nm and a band gap wavelength of 1.25 μm are alternately stacked; an InGaAsP guide layer 25 having a band gap wavelength of 1.25 μm;
It comprises a P cladding layer 26 and a p-type In _0.53 Ga _0.47 As cap layer 27. These semiconductor layers are
Except for the n _0.7 Ga _0.3 As quantum well layer 23, all n-type In
Lattice-matched to P substrate 20. The In _0.7 Ga _0.3 As quantum well layer 23 has a larger lattice constant than the n-type InP substrate 20 and has a compressive strain of 1.1%. Compared to a non-strained quantum well, a compression-strained quantum well has a wider energy region where the effective mass of the lowest-order hole subband is smaller, and the effective density of states of the band is smaller. As a result, the compression-strained quantum well semiconductor laser has excellent characteristics such as a lower oscillation threshold and a larger differential gain, as compared with the non-strained quantum well semiconductor laser.

[0003]

The maximum modulation frequency f _T of the semiconductor laser is a frequency at which the response is degraded by 3 dB. However, the maximum modulation frequency f _T inherent to the active layer material excluding the cutoff frequency of the electronic circuit determined by the capacitance and resistance of the semiconductor laser is excluded. The maximum modulation frequency has a relationship with a parameter called a K factor of f _T = 2 ^3/2 π / K (1), and the K factor is K = 4π ² (τ _P + ε / (dg / dN)) (2 ), The photon lifetime τ _P , the nonlinear gain coefficient ε, and the differential gain dg / dN are related. Reducing the K factor leads to an increase in the speed of the semiconductor laser. The photon lifetime decreases as the cavity length decreases,
Since the nonlinear gain coefficient is almost constant in the short cavity region,
Increasing the differential gain is necessary for speeding up. From this viewpoint, the compression-strained quantum well semiconductor laser can be operated at a higher speed than the unstrained quantum well semiconductor laser. However, in the conventional compressive strain quantum well semiconductor laser, the maximum modulation frequency specific to the active layer material is limited to about 40 GHz.

An object of the present invention is to provide a semiconductor laser capable of performing higher-speed modulation using a material system similar to that of the prior art.

[0005]

According to the present invention, there is provided a semiconductor laser comprising:
In this case, an InGaAsP barrier layer is formed on an InP substrate.
In _X Ga _1-X As quantum well layers are alternately stacked.
The quantum well active layer is divided into a P-type cladding layer and an N-type cladding layer.
In a semiconductor laser sandwiched between
The In composition X of the sub well layer is 0.4 or less, and
The bandgap wavelength of the rear layer is 1.15 μm or less.
And modulating p-doping is applied to the quantum well layer.
It is characterized by being.

[0006]

In the semiconductor laser of the present invention, tensile strain
Since the quantum well is used, the oscillator strength of the optical transition is
It becomes larger than the case of. Valence band density of states increases
However, the decrease in differential gain due to this is modulated into the quantum well by p-doping.
To compensate. These two results, compression strain quantum well
Semiconductor laser with differential gain equal to or higher than
User is obtained. However, in this case, the quantum well for the electron
In order to obtain the door effect, the band gap wavelength of the barrier layer is set to 1.
It is 15 μm or less.

[0007]

[0008]

1 shows a tensile strain quantum well semiconductor according to the present invention .
It is sectional drawing which shows the layer structure of a laser.

[0009]

In FIG . 1, a semiconductor laser crystal comprises an n-type InP substrate 9, an n-type InP cladding layer 10, an InGaAsP guide layer 11 having a band gap wavelength of 0.95 μm,
A multiple quantum well active layer in which In _0.3 Ga _0.7 As quantum well layers 12 having a thickness of 10 nm and barrier layers 16 are alternately stacked;
InGaAsP guide layer 17 having a band gap wavelength of 0.95 μm, p-type InP clad layer 18, p-type In _0.53 G
a _0.47 As cap layer 19. The barrier layer 16 has a layer thickness of 2 nm and a band gap wavelength of 0.95.
μm InGaAsP layer 13 and p-type InGaAsP layer 14 having a thickness of 6 nm and a band gap wavelength of 0.95 μm.
And an I of a layer thickness of 2 nm and a band gap wavelength of 0.95 μm.
It is composed of the nGaAsP layer 15 and three semiconductor layers. All of these semiconductor layers are lattice-matched to the n-type InP substrate 9 except for the In _0.3 Ga _0.7 As quantum well layer 12. In
_{The 0.3} Ga _0.7 As quantum well layer 12 has a smaller lattice constant than the n-type InP substrate 9 and has a 1.6% tensile strain. Further, the quantum well layer 12 has a modulated p-doped structure in which holes are supplied from the p-type InGaAsP layer 14 to the quantum well layer 12. The oscillation wavelength is 1.42 μm.

[0011]

The semiconductor laser of the present invention has
Since the strained quantum well was used, the light hole band
It becomes the bottom hole band, and the heavy hole band
As a result, the energy is reduced. This energy interval is
It becomes larger as the amount of tension strain increases. Quantum well bundle
The binding effect is again the light hole subband and the heavy
Split the hole subband, but the light hole is
Due to the small effective mass for the quantum well constraint,
Light hole sub-band if quantum well layer thickness is not increased
Has lower energy than the heavy hole subband
turn into. Light hole sub-band is base sub-band
The effective mass of the hole is a compressive strain quantum well
6-8 times larger than
The oscillator strength in the maximum direction of the transfer is
Benefits of 4/3 times greater than the transducer strength in the maximum direction
There is. FIG. 2 shows the differential gain with respect to the number of multiple quantum well periods.
And a change in the value of 1.4-
1.5μm tensile strain multiple quantum well semiconductor laser
This is the result of calculating the minute gain. Here, the horizontal axis is the multiple quantum
The confinement coefficient Γ of the laser light in the well. FIG.
n composition X = 0.4 (0.9% tensile strain) of quantum well
Case and X = 0.3 (1.6% tensile strain) quantum well
And two types of barrier layer bandgap waves
The case of long is shown. Quantum well with decreasing X
Conduction band discontinuity between door and barrier layer is reduced
To reduce the band gap of the barrier layer as X decreases.
Otherwise, the electrons will not be bound by the quantum well.
You. When X = 0.4, the barrier layer band gap wavelength
Is about 1.15 μm or less, and 0.95 when X = 0.3.
It is necessary to be about μm or less. In FIG. 2, X =
0.3 has a larger differential gain than X = 0.4
I understand that Semiconductor using this tensile strained quantum well
The differential gain of the body laser is
It is smaller than the maximum differential gain. FIG.
X = 0.00 μm with modulated p-doping in quantum wells.
3 band gap wavelength of tensile strain quantum well / barrier layer
Result of calculating differential gain of 0.95μm semiconductor laser
Is shown. In FIG. 3, the horizontal axis represents the laser in the multiple quantum well.
The light confinement coefficient Γ. Modulated p-doping in quantum wells
The concentration is 0 cm ^-2 , 2 × 10 ¹² cm ^-2 , 8 × 10 ¹² cm ^-2
These three cases are shown. Increased modulation p-doping concentration
It can be seen that the differential gain increases with the increase. p concentration 2 ×
In the case of 10 ¹² cm ^-2 , the differential gain of the compression-strained quantum well laser
Of the maximum value and the same degree, yet p density 8 × 10 ¹² cm ^-2
If so, more. The semiconductor laser of this embodiment is shown in FIG.
3, which is equivalent to the case of p-doping of modulation
It has a differential gain equal to or higher than that of a
The above high-speed modulation is the function.

[0012]

With the above effects, the semiconductor laser of the present invention can perform higher-speed modulation than the conventional semiconductor laser.

[Brief description of the drawings]

FIG. 1 shows a tensile strain quantum well semiconductor laser of the present invention .
Sectional drawing which shows a layer structure.

[2] _{In X Ga 1-X As /} InGaAsP tensile
Optical confinement of differential gain in strained quantum well semiconductor lasers
Number, In composition X and InGaAsP barrier layer band gap
FIG. 4 is a characteristic diagram showing a top wavelength dependency.

FIG. 3. In _0.3 Ga _0.7 As / InGaAsP pull
Of the differential gain of strained quantum well semiconductor lasers
Number and modulation p doping amount dependency (barrier layer band gap
FIG.

FIG. 4 is a sectional view showing the layer structure of a conventional semiconductor laser crystal.
Area view.

[Description of Signs ] 9, 20 n-type InP substrate 10, 21 n-type InP cladding layer 11 0.95 μm wavelength InGaAsP guide layer 12 In _0.3 Ga _0.7 As quantum well layer 13, 15 0.95 μm wavelength InGaAsP barrier layer 14. 95 μm wavelength p-type InGaAsP barrier layer 16 barrier layer 17 0.95 μm wavelength InGaAsP barrier layer 18, 26 p-type InP cladding layer 19, 27 p-type In _0.53 Ga _0.47 As cap layer 22, 25 1.25 μm wavelength InGaAsP guide layer 23 In _0.7 Ga _0.3 As quantum well layer 24 1.25 μm wavelength InGaAsP barrier layer

Claims (1)

(57) [Claims] 1. An InGaAsP barrier on an InP substrate.
Layers and In _x Ga ₁ -x As quantum well layers are alternately stacked.
The multiple quantum well active layer is composed of a P-type cladding layer and an N-type
In the case of a semiconductor laser sandwiched between
The quantum well layer has an In composition X of 0.4 or less and
The barrier layer has a band gap wavelength of 1.15 μm or less.
And the modulation well doping is applied to the quantum well layer.
A semiconductor laser.