Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2001-09-24
2003-04-08
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular active medium
C257S018000, C257S021000, C257S432000, C372S044010
Reexamination Certificate
active
06545801
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No.2000-367727 filed on Dec. 1, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
This invention generally relates optical semiconductor devices and more particularly to a semiconductor optical amplifier.
An optical-fiber telecommunication system uses an optical amplifier for amplifying optical signals. In recent optical-fiber telecommunication systems that transmit wavelength-multiplexed optical signals, in which a large number of optical elements are used for synthesizing or dividing the optical signals, there is a need of providing a number of semiconductor optical amplifiers of low electric power consumption for compensating for the optical loss that is caused as a result of use of such a large number of optical elements.
In an optical fiber, an optical signal that is transmitted therethrough generally has a random polarization state. Thus, the semiconductor optical amplifier that is used for amplifying optical signals in such an optical fiber has to be a semiconductor optical amplifier of polarization-independent (polarization-dependence free) type.
FIGS. 1A and 1B
show the construction of a typical conventional semiconductor optical amplifier
10
.
Referring to
FIG. 1A
, the semiconductor optical amplifier
10
is formed on an n-type InP substrate
11
and has a layered structure that resembles to the structure of a laser diode. Thus, a first cladding layer
12
of an n-type InP is formed on the substrate
11
, and a first optical confinement layer
13
of undoped InGaAsP is formed on the first cladding layer
12
. Further, an active layer
14
of undoped InGaAs is formed on the first optical confinement layer
13
, and a second optical confinement layer
15
of undoped InGaAsP is formed on the active layer
14
. Further, a second cladding layer
16
p-type InP and a contact layer
16
A of p-type InGaAs are formed consecutively on the second optical confinement layer
15
. Furthermore, a p-type electrode
17
is formed on the contact layer
16
A and an n-type electrode
18
is formed to a bottom surface of the substrate
11
.
Further, the semiconductor optical amplifier
10
has an input end and an output end respectively covered with anti-reflection films
10
A and
10
B. Thus, when an incident optical beam is introduced to the input end through the anti-reflection film
10
A in the state in which a driving bias is applied across the electrodes
17
and
18
, the incident optical beam undergoes optical amplification by stimulated emission as it is guided through the active layer
14
to the output end.
FIG. 1B
shows the semiconductor optical amplifier
10
in an end view.
Referring to
FIG. 1B
, the layered structure formed on the substrate
11
and including the cladding layer
12
, the optical confinement layer
13
, the active layer
14
and the optical confinement layer
15
is subjected to an etching process, and there is formed a mesa stripe that extends in an axial direction of the optical amplifier
10
. At both lateral sides of the mesa stripe, it can be seen that there are formed current confinement layers
11
A and
11
B of n-type InP and current confinement layers
11
C and
11
D of p-type InP.
When using such a semiconductor optical amplifier
10
in an optical-fiber telecommunication system, it is necessary that the optical amplification is obtained irrespective of the polarization state of the incident optical beam as noted previously. Further, the semiconductor optical amplifier for use in an optical-fiber telecommunication system is required to have a large dynamic range so as to be able to deal with large power fluctuation of the input optical signal. In order to meet for these requirements, the semiconductor optical amplifier
10
has to be able to provide a large fiber-coupled saturated optical power. It should be noted that the fiber-coupled saturation optical power is a quantity defined for the entire system including the semiconductor optical amplifier, an input optical fiber coupled to the semiconductor optical amplifier, an optical system cooperating with the input optical fiber, an output optical fiber coupled to the semiconductor optical amplifier and an optical system cooperating with the output optical fiber, and is defined, based on the fiber-to-fiber gain, in which the loss of the optical systems is taken into consideration, as the value of the fiber-coupled optical power that causes a drop of 3 dB in the fiber-to-fiber gain.
In the case of designing a polarization-independent optical semiconductor device based on the semiconductor optical amplifier
10
, the simplest way would be to use a strain-free bulk crystal for the active layer
14
and set the thickness of the active layer
14
to be identical with the width thereof as shown in
FIG. 2A
, wherein it should be noted that
FIG. 2A
is an enlarged view showing a part of the mesa-stripe of FIG.
1
.
With the construction of
FIG. 2A
, it should be noted that polarization-independent operation is guaranteed for the optical amplifier in view of the fact that the optical confinement factor becomes the same in the Te-polarization mode in which the electric field oscillates parallel to the surface of the active layer and in the Tm-polarization mode in which the electric field oscillates vertically to the the active layer (&Ggr;
te
=&Ggr;
tm
), and in view of the fact that the material gain becomes the same in the Te-polarization mode and in the Tm-polarization mode (g
te
=g
tm
). Because of this, the product of the optical confinement factor &Ggr; and the material gain g becomes the same in any of the two polarization modes (&Ggr;
te
·g
te
=&Ggr;
tm
·g
tm
), and this guarantees the above-noted polarization independent operation for the optical amplifier.
In the case the thickness of the active layer
14
is thus formed equally with the width in the semiconductor optical amplifier
10
of
FIG. 1
, on the other hand, it is necessary to form the active layer
14
to have a width of 0.5 &mgr;m or less in order to realize a fundamental-mode optical guiding. However, processing of the active layer to such a small size is difficult, and the production of such an optical amplifier has been difficult.
FIG. 3
shows the relationship between the chip-out saturation power represented in the left vertical axis and the thickness of the active layer
14
obtained by the inventor of the present invention. Further,
FIG. 3
shows a tensile strain to be introduced into the active layer
14
for realizing the polarization independent operation for the optical amplifier. In
FIG. 3
, the optical confinement layers
13
and
15
are assumed to have the thickness of 100 nm in semiconductor optical amplifier
10
of
FIG. 1
, and the calculation was made by setting the width of the active layer
14
to 1.0 &mgr;m. The strain introduced into the active layer
14
will be explained later.
FIG. 3
is referred to.
In the case the thickness of the active layer
14
is decreased, it can be seen from
FIG. 3
that the value of the chip-out saturation power of the semiconductor optical amplifier
10
is increased. This effect reflects the situation in which the saturated output Ps of semiconductor optical amplifier the
10
, represented as
Ps=
(
wd/&Ggr;
)*(
h&ngr;
)/(
&tgr;g′),
Eq.(1)
is increased as a result of increase of the mode cross-sectional area (wd/&Ggr;), which in turn is caused as a result of decrease of thickness d of the active layer
14
and further as a result of increase of the carrier lifetime &tgr;. In Eq.(1), it should be noted that w and d represent the width and thickness of the active layer
14
respectively, &Ggr; represents the optical confinement factor, h represents the Planck constant, &ngr; represents the optical frequency, &tgr; represents the carrier lifetime in the active layer
14
, and g′ represents the differential gain.
In Eq.(1), it should be noted that the value of the param
Armstrong Westerman & Hattori, LLP
Moskowitz Nelson
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