Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-05-15
2003-09-30
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S096000
Reexamination Certificate
active
06628691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical semiconductor device, and particularly relates to a laser diode having an optical resonator having a diffraction grating.
2. Description of the Related Art
In a fast optical communication network via optical fibers, a DFB (Distributed Feedback) laser diode or a DBR (Distributed Bragg Reflector) laser diode having an optical resonator having a diffraction grating is widely used as a single-mode light source which may be modulated by fast optical modulation.
The laser diode used as a light source for a fast optical communication network is required to operate in a single-mode. Therefore, a DFB laser diode or a DBR laser diode having an optical resonator having a diffraction grating instead of a mirror is commonly used as a light source for such fast optical communication network.
FIG. 1
shows a diagram illustrating a DFB laser diode
10
of the related art.
Referring to
FIG. 1
, the laser diode
10
is formed on an n-type InP substrate
11
. A cladding layer
12
of n-type InP, an SCH (Separate Confinement Heterostructure) layer
13
of undoped InGaAsP, and an active layer
14
of undoped InGaAs are, in turn, epitaxially grown on the InP substrate
11
.
A further SCH layer
15
of undoped InGaAsP is epitaxially grown on the active layer
14
. A DFB diffraction grating
15
A is formed on the SCH layer
15
. Further, a cladding layer
16
of p-type InP and a contact layer
17
of P-type InP are in turn, epitaxially grown on the SCH layer
15
. A p-type electrode
18
is disposed on the contact layer
17
and an n-type electrode
19
is disposed on a lower surface of the substrate
11
.
With the laser diode
10
of the above-mentioned structure, the electrodes
18
and
19
serves to inject carriers into the active layer
14
. Due to recombination caused by the injected carriers, an optical radiation is generated in the active layer
14
. The optical radiation is guided though the SCH layers
13
and
15
and then optically amplified by stimulated emission in the active layer
14
. Thereupon, an optical component tuned to an effective pitch of the diffraction grating
15
A, or, having a wavelength within the range of the Bragg wavelength to the DBF wavelength &lgr;g of the diffraction grating
15
A is repeatedly reflected by the DFB diffraction grating
15
A and is selectively amplified.
However, there is a certain drawback when such a single-mode laser diode is driven by a modulation signal. Since the modulation signal alters the density of the injected carriers in the active layer
14
and thus the refractive index of the active layer, the effective period of the diffraction grating is also altered, and thus it can be said that the oscillation wavelength is altered simultaneous with the modulation signal. This effect is commonly referred to as chirping. Such chirping may cause the wavelength of an optical signal to shift from the optimum transmission band for the optical fibers, so that the transmission distance of the optical signal may be limited.
The magnitude of chirping is determined by a line-width enhancement factor &agr;, which is generally defined by an equation:
α
=
∂
[
Re
{
χ
(
N
)
}
]
/
∂
N
∂
[
Im
{
χ
(
N
)
}
]
/
∂
N
,
(
1
)
where &khgr;(N) is a complex susceptibility of the active layer of the laser diode, N is a carrier density, Re{&khgr;(N)} is the real part of &khgr;(N), and Im{&khgr;(N)} is the imaginary part of &khgr;(N). Re{&khgr;(N)} relates to a refractive index of the active layer and Im{&khgr;(N)} relates to an absorption of the active layer.
Given that a well-known Kramers-Kronig relationship holds between Re{&khgr;(N)} and Im{&khgr;(N)} and that Im{&khgr;(N)} is proportional to the gain g of the laser diode, the line-width enhancement factor &agr; may also be represented by an equation:
α
(
E
,
N
)
=
-
P
∫
-
∞
∞
∂
g
(
E
′
,
N
)
/
∂
N
E
′
-
E
ⅆ
E
′
/
∂
g
(
E
′
,
N
)
/
∂
N
,
(
2
)
where E and E′ represent energies and P is Cauchy's principal value.
With a typical laser diode
10
having the active layer
14
of a bulk structure, the line-width enhancement factor &agr; is generally of an order of 4 to 6 and therefore cannot avoid a substantial chirping effect due to the modulation signals. Whereas with a laser diode having a quantum well layer in the active layer
14
with the SCH layer
15
serving as a barrier layer, the value of the line-width enhancement factor &agr; may be decreased to about 2. With such a quantum well laser diode, by optimizing the material and composition of the quantum well and the laser structure and by combining with the DFB optical resonator, the value of the line-width enhancement factor &agr; may be decreased to about 1.4 to 1.8.
FIG. 2
is a graph showing a relationship between the gain and the line-width enhancement factor of the laser diode of FIG.
1
. Referring to
FIG. 2
, it can be seen that a wavelength at maximum gain is offset from a wavelength where the line-width enhancement factor &agr; is zero and thus the gain is negative at the wavelength where the line-width enhancement factor a is zero. Accordingly, with the quantum well laser diode of the relate art, the material and the composition of quantum wells and the pitch of the DFB diffraction grating
15
are determined such that the laser oscillates at a wavelength where the gain spectrum is positive and the line-width enhancement factor &agr; is as close as possible to zero. However, with such a process, chirping can only be reduced to a limited extent and it is not possible to obtain sufficient gain.
Also, it is known to modify the laser diode of
FIG. 1
by providing the active layer
14
of quantum dots. See, for example, Japanese laid-open patent application No. 9-326506.
FIG. 3
is a diagram illustrating a DFB laser diode
20
of the related art in which quantum dots are used as an active layer.
Referring to
FIG. 3
, the laser diode
20
is formed on a (001) surface of an n-type GaAs substrate
21
. The laser diode
20
includes a cladding layer
22
of n-type AlGaAs having a composition of Al
0.4
Ga
0.6
As which is epitaxially grown on the substrate
21
, an SCH layer
23
of undoped GaAs which is formed on the cladding layer
22
a cladding layer
24
of p-type AlGaAs having a composition of Al
0.4
Ga
0.6
As which is formed on the SCH layer
23
and a contact layer
25
of P-type GaAs formed on the cladding layer
24
. Further, an active layer constituted by a plurality of quantum dots
23
A is formed in the SCH layer
23
. Further, a diffraction grating
23
B is formed on the SCH layer
23
in a direction of axis of the laser diode
20
. A p-type electrode
26
is disposed on the contact layer
25
and an n-type electrode
27
is disposed on a lower surface of the substrate
21
.
With such laser diode
20
using quantum dots, it is expected that, if the zero point of the line-width enhancement factor &agr; is close to the peak of the gain spectrum, chirping can be effectively reduced.
FIG. 4
is a graph showing a relationship between the optical gain and the wavelength for a DFB laser diode. Again spectrum of the laser diode
20
having quantum dots has a thermal dependency of about 0.25 nm/° C., and as can be seen from the graph of
FIG. 4
, the gain spectrum shifts towards longer wavelength side when there is an increase of the temperature of the laser diode. On the contrary, the Bragg wavelength of the DFB diffraction grating
23
B has a thermal dependency of only about 0.1 nm/° C. Therefore, with the quantum dot DFB laser diode
20
of the related art, there is a drawback that the change in operation temperature may cause the Bragg wavelength of the DFB diffraction grating
23
B to shift out of the gain spectrum, which ceases the laser oscillation.
SUM
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Leung Quyen
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