Semiconductor optical amplifier

Details Semiconductor optical amplifier Semiconductor optical amplifier

2002-04-11

2004-06-15

Black, Thomas G. (Department: 2817)

Optical: systems and elements

Optical amplifier

Particular active medium

Reexamination Certificate

active

06751015

ABSTRACT:

The present invention relates to amplifying optical signals. It finds a typical application in fiber optic telecommunication networks. The signals transmitted by fiber optic telecommunication networks consist of pulses carrying information to be transmitted in binary form. The pulses must be amplified to compensate power losses that they suffer during their propagation in said networks. Semiconductor amplifiers constitute a compact means of obtaining such amplification and can be integrated. However, unless specific measures are implemented to prevent it, their gain is sensitive to the state of polarization of the light that they receive, as indicated more simply hereinafter by referring to the polarization-sensitivity of an amplifier.
The invention finds a particular application when it is necessary to eliminate or at least limit polarization-sensitivity, which can be expressed by the following equation: &Dgr;G=G
TE
−G
TM
. The aim is to achieve the condition |&Dgr;G|<1 dB.
The situation in which the sensitivity must be limited or eliminated is frequently encountered and arises when the distance traveled by the optical pulses to be amplified is such that the state of polarization of the pulses has been significantly and randomly affected during their propagation and it is preferable for the amplified pulses to have one or more predetermined power levels.
More generally, the invention finds an application whenever an optical amplifier must have no polarization-sensitivity or a low polarization-sensitivity.
The invention applies more specifically to buried ridge structure (BRS) amplifiers.
A buried ridge structure semiconductor optical amplifier (see
FIG. 1
) includes a wafer
2
made up of layers of semiconductor materials having respective refractive indices and forming a common crystal lattice. In the absence of mechanical stresses, the lattices formed by these materials have respective characteristic dimensions constituting respective lattice parameters of the materials. The layers are in succession in a vertical direction DV forming a right-angle trihedron, defined with respect to the wafer
2
, with two horizontal directions constituting a longitudinal direction DL and a transverse direction DT. The layers form an upward succession in the vertical direction DV from a bottom face
4
to a top face
6
. The wafer
2
includes at least the following layers or groups of layers or parts of layers:
A substrate
8
consisting mainly of a semiconductor basic material having a first type of conductivity. This substrate is sufficiently thick to impose the dimensions of the lattice of the basic material on all of the crystal lattice of the wafer
2
.
An active layer
10
including an active material adapted to amplify light by stimulated recombination of charge carriers of both types injected into the material.
Finally, a top confinement layer
18
consisting of a material having a second type of conductivity which is the opposite of the first type.
The amplifier further includes a bottom electrode
20
and a top electrode
22
respectively formed on the bottom face
4
and the top face
6
of the wafer
2
to enable an electrical current to flow between said faces for injecting said charge carriers of both types into the active material.
The basic materials of prior art semiconductor optical amplifiers are III-V materials, typically indium phosphide and gallium arsenide. The active material is typically a ternary or quaternary material containing the same chemical elements. It is generally required for the width l of the guide active structure
12
which guides the light to be close to one micrometer, to facilitate etching it and most importantly to facilitate integrating the amplifier with other optical components on the same semiconductor wafer. To ensure monomode propagation of light, typically at a wavelength of 1 310 nm or 1 550 nm, the thickness e must then be very much less than the width l. If no special measures are applied to prevent it, this rectangular shape of the section of the guide active structure
12
causes the polarization-sensitivity previously mentioned.
In BRS amplifiers, the active material
10
constituting the active structure
12
guiding the light is surrounded on all sides by a binary semiconductor material
14
,
16
. This material has the advantage of conducting heat well, but its refractive index is only slightly lower than that of the active material. Consider further the situation in which the active material is homogeneous, in which case it is referred to as a bulk material. As a general rule, the section of the buried guide active structure
12
is strongly rectangular. Given the small index difference between the guide structure
12
and the surrounding binary material
14
,
16
, the confinement of a horizontally polarized wave is greater than that of a vertically polarized wave, the difference between the two confinement factors increasing as the ratio of the width l to the thickness e of the guide structure
12
increases. The confinement referred to above in connection with a wave is in a transverse plane. It is equal to the ratio of the power of the wave in the area occupied by the guide structure to the total power of the wave. The confinement factor is defined for each polarization and for each wavelength by the shape and dimensions of the section of the ridge and by the refractive indices of the material of the ridge and the surrounding material. In the case of a rectangular section, it can be considered to be the product of a directional confinement factor in the horizontal direction by a directional confinement factor in the vertical direction, each of the two directional confinement factors depending on the polarization. Given that the phenomenon of amplification of the wave by recombination of carriers and stimulated emission occurs only in the active material, i.e. in the ridge
12
, the gain of the amplifier for a wave increases as the confinement of the wave increases. As a result of this, if the material of the guide structure
12
were a homogeneous material, and also isotropic, and therefore insensitive to polarization, the gain of the amplifier would be greater for horizontally polarized waves than for vertically polarized waves.
Considerable research has been conducted into making these amplifiers insensitive to the polarization of the light to be amplified.
In particular, the applicant's U.S. Pat. No. 5,982,531 proposes an amplifier of this kind that is rendered insensitive to the polarization of the light. The amplifier is characterized in that its active material is subjected to a sufficient tension stress to render its gain insensitive to the polarization of said light to be amplified. This stress generally results from a lattice mismatch between the active material and the basic material of the substrate. The horizontal confinement factor is typically equal to the product of the vertical confinement factor by a confinement asymmetry coefficient.
The present patent application is based on the observation that, even in the presence of a high confinement asymmetry coefficient resulting, for example, from the fact that the guide structure
12
consists of a ridge of strongly rectangular section, the tension stress to be applied to a homogeneous active material forming said ridge to obtain insensitivity to polarization is sufficiently low for the thickness of the ridge to remain less than the corresponding critical thickness relating to dislocations.
The above kind of amplifier has a low sensitivity to polarization. Its main parameters are:
the wavelength of the amplifying active layer: &lgr;=1.57 &mgr;m,
the active material: In
1−x
Ga
x
P
1−y
As
y
,
the tension stress of the active layer: &Dgr;a/a=−0.015,
the thickness of the active layer: e=0.2 &mgr;m, and
the width of the ridge: l=1 &mgr;m.
This kind of structure has drawbacks, however. It has been established experimentally and theoretically that the polarization depends strongly on the con

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