Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2001-06-04
2004-03-16
Davie, James (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S046012
Reexamination Certificate
active
06707840
ABSTRACT:
The invention concerns an approach to fabricating current blocking regions in a Vertical Cavity Surface Emitting Laser, VCSEL. The approach is simpler and less expensive than those used presently.
BACKGROUND OF THE INVENTION
FIG. 1
is a simplified schematic of a homojunction laser
3
, and is not drawn to scale. A PN junction
4
is formed between a p-plus-type body
6
of gallium arsenide, GaAs, and an n-plus-type body
9
of gallium arsenide. Metal contacts
12
provide entry- and exit paths for current
15
, which is supplied by a voltage source V+. The laser produces laser output
18
, which travels in a plane parallel to the junction
4
. The laser will generally be situated in a resonant optical cavity, which is not shown.
FIG. 2
is a simplified schematic of a different type of laser, namely, the Vertical Cavity Surface Emitting Laser, VCSEL, labeled
30
, and is also not drawn to scale. The VCSEL
30
includes a top mirror
33
and a bottom mirror
34
. These mirrors are constructed of multiple layers L of dielectric material, each layer being ¼ wavelength thick.
Current
35
, indicated by the dashed arrow, flows from a metal contact
36
, through a p-type region
39
, through a gain region
42
, through an n-type region
43
, and to another metal contact
45
. The gain region
42
produces light, and multiple reflections of that light between the top mirror
33
and the bottom mirror
34
induce stimulated emission of laser light, which exits the device as indicated by ray
48
.
A significant feature of the VCSEL
30
is that the laser light travels perpendicular to the plane of the gain region
42
, that is, perpendicular to bottom mirror
34
. Gain region
42
is analogous to junction
4
in
FIG. 1
, in the sense that population inversion occurs in both the gain region
42
and the junction
4
.
In addition, in
FIG. 2
, the light which stimulates emission of photons within the gain region
42
bounces between the top mirror
33
and the bottom mirror
34
. However, stimulated emission only occurs within the gain region
42
. The thickness T of the gain region
42
is very small, of the order of a few hundred angstroms, and is much smaller than the corresponding distance Ti in Figure Thus, since stimulated emission in
FIG. 2
only occurs along a relatively small thickness T, losses must be reduced to a minimum. One source of loss is scattering which would occur at the edge
50
of the top mirror
33
. To reduce this loss, current-blocking regions
53
are fabricated. They block current from flowing near the edge
50
. The absence of current means that photon generation is absent, so that stimulated emission is also absent, at that location.
Fabrication of the current-blocking region
53
is expensive, or at least complex. In one approach, ion implantation is used, wherein the p-type region
39
in
FIG. 3
is bombarded by high-velocity ions, indicated by dashed arrows
54
. These ions bury themselves beneath the surface
55
and generate the current-blocking region
53
in FIG.
4
. Region
53
is generated because the ions
54
compensate the p-type dopants (not shown), effectively converting region
53
into an intrinsic semiconductor, which is low in conductivity, at least at room temperature.
However, this ion implantation technique requires strict process control in order to develop the proper profile
65
in plot
68
in FIG.
4
. Plot
68
indicates ion concentration, as a function of depth in the p-type layer
39
. Also, the overall process requires later annealing of the structure, after the implantation.
In another approach, current blocking region
53
is fabricated through lateral oxidation, wherein the oxidation is begun at regions
70
in
FIG. 3
, and invades the p-layer
39
as indicated by arrows
73
. However, the lateral oxidation process is difficult to control.
In a third approach, shown in
FIG. 5
, a p-type layer
80
in structure A, at the upper left of the Figure, is etched away to form the mesa
83
in structure B. Then, in structure C, the current blocking layer
53
is fabricated, by implantation or surface oxidation. (Intermediate steps required for generation of layer
53
are not indicated.) Next, the p-type layer is expanded in size through crystal regrowth into body
39
, as in Structure D. After that, known process steps are implemented to produce the final structure Z.
However, the processing steps required to convert structure C into structure D are expensive and complex. Specifically, the p-type layer
39
in structure D, as well as the gain region
42
, must all consist of a monocrystalline body of material. Adding a monocrystalline body to the p-layer
83
shown in structure C, to create structure D, is a complex process, as is crystal regrowth generally, which is the process used.
The Inventors have developed a process for producing the current blocking region
53
in
FIG. 2
, but in a simpler manner than described above.
Numerous textbooks exist on laser technology. A good simplified treatment is found in
Optoelectronics. An Introduction,
by Wilson and Hawkes, Third Edition (Prentice Hall, 1998, ISBN 0-13-103961-X). This book is hereby incorporated by reference, partly to show, in simplified terms, the present state of the art.
SUMMARY OF THE INVENTION
In one form of the invention, a film of gold is positioned across the optical gain path of a VCSEL. The gold film delivers electrical current into the semiconductor material within the gain path, and eliminates the need for a crystal re-growth step.
REFERENCES:
patent: 6144682 (2000-11-01), Sun
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