Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-11-25
2001-05-22
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S096000
Reexamination Certificate
active
06236670
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of lasers comprising a stack of semiconductor lasers making it possible to achieve powers of the order of several kilowatts.
DISCUSSION OF THE BACKGROUND
According to the prior art, lasers of this type comprise laser diodes produced by epitaxial growth, on a semiconductor substrate C
n
doped with impurities of a certain type (generally of the n type), of a quantum-well emissive structure PQ then a layer C
p
doped with impurities of the opposite type (generally of the p type).
FIG. 1
illustrates a stack according to the prior art of two laser diodes: metal contacts ME
1
, and ME
2
are deposited on either side of the laser diodes, and the laser diodes are stacked, layer C
n
on layer C
p
, via the metal contacts ME
1
and ME
2
.
Generally, metal mechanical jaws enclose the set of diodes in order to dissipate the heat due to the strong thermal effects which take place in each laser diode. The diodes are electrically connected by soldering between a metallization ME
1
and a metallization ME
2
. The electrons pass by the tunnel effect through the Schottky junction corresponding to the interface of a layer C
n
and a metallization ME
1
while the holes can pass by the tunnel effect through the Schottky junction corresponding to the interface between the layer C
p
and the metallization ME
2
. This is illustrated in
FIG. 2
, which represents a diagram of the energy bands of the layers C
n
, ME
i
and C
p
which are used in a stack of laser diodes according to the prior art.
The quantum-well emissive structures are spaced apart by at least the thickness of a substrate layer (typically a layer C
n
represented in FIG.
1
), that is to say by several hundreds of microns. In this configuration, the diodes have to be driven by pulses which are short enough for the diffusion lengths of the temperature gradients Ld not to overlap, in order to avoid excessive local temperature rises, the length Ld being given by the following equation (1):
Ld
=
(
k
τ
ρ
K
p
)
1
/
2
with k: thermal conductivity
&tgr;: pulse length
&rgr;: semiconductor density
K
p
: semiconductor heat capacity.
For substrates about 400 &mgr;m thick, lengths Ld of the order of 200 &mgr;m are typically obtained for pulse lengths of the order of 70 to 100 microseconds.
These configurations for stacked laser diodes have the drawback that they require the use of external means for carrying out assembly (in particular soldering), and are of a size which it is difficult to minimize further, because of the thickness of the substrate of each elementary diode.
SUMMARY OF THE INVENTION
In order to solve these problems, the invention proposes a laser structure in which the laser diodes are stacked periodically by epitaxial growth of a set of semiconductor layers, requiring no manual intervention.
More precisely, the invention relates to a laser comprising a stack of N laser diodes DL
i
, each having a quantum-well emissive structure. S
i
, characterized in that:
the stack of diodes DL
i
is a stack of epitaxial semiconductor layers which is inserted between two mirrors so as to produce a laser cavity;
the ohmic contact between a laser diode DL
i
and a laser diode DL
i+1
is provided by an Esaki diode junction formed of an n-doped layer with a very high doping level and of a p-doped layer with a very high doping;
the optical electric field of the mode created in the laser cavity is periodically cancelled at the Esaki diode junctions.
More precisely, the junctions being of the Esaki diode type, this laser according to the invention may comprise a stack of N laser diodes, characterized in that the stack of epitaxial layers comprises layers S
i
, n-doped layers C′
ni
, p-doped layers C′
pi
, a layer C′
ni
being adjacent to a layer C′
pi+1
, the doping level t′
ni
of the layers C′
ni
and the doping levels t′
pi
of the layers C′
pi
being such that an electron flow is capable of passing by the tunnel effect from a layer C′
pi
to a layer C′
ni+1
so as to provide the ohmic contact between a diode DL
i
and a diode DL
i+1
.
Since the free carriers, present in very large numbers at the Esaki diode junction, absorb the optical wave which is generated, the invention proposes a laser structure designed so that the optical electric field of the mode is cancelled at the position of the Esaki diode junctions.
This solution is advantageous in so far as it makes it possible to retain laser structures with small dimensions in comparison with a laser structure in which the choice is made to distance the junctions between diodes of the laser emission zone.
According to an alternative embodiment, the stack of diodes (DL
i
) is inserted between two plane mirrors which are parallel to the plane of the epitaxial semiconductor layers, so as to create laser emission perpendicular to the plane of the layers.
Advantageously, the mirrors may be Bragg mirrors, one of which has a reflectivity close to 100% for the optical wave which is created, the other Bragg mirror having a lower reflectivity so as to allow the laser emission to emerge.
According to another variant of the invention, the stack of diodes (DL
i
) is inserted between two plane mirrors which are perpendicular to the plane of the epitaxial semiconductor layers, so as to create laser emission parallel to the plane of the layers.
The optical wave is confined in the stack of diodes by inserting the set of epitaxial layers between two confinement structures or layers.
These confinement structures may in particular consist of Bragg mirrors having close to
100
% reflectivity for the optical wave created in the laser cavity.
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Dupuis et al, “Room-Temperature Operation of Distributed-Bragg-Confinement Ga1-xAlxAs-GaAs Lasers Grown by Metalorganic Chemical Vapor Deposition,” Appl. Phys. Lett., vol. 33, No. 1, pp. 68-69, Jul. 1978.*
Kotaki et al, “GalnAsP/InP Surface Emitting Laser with Two Active Layers,” International Conference on Solid State Devices and Materials, 1984, Tokyo, Japan, pp. 133-136, (no month available), Jan. 1984.
Nagle Julien
Rosencher Emmanuel
"Thomson-CSF"
Arroyo Teresa M.
Menbleau Davienne
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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