Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-12-28
2004-02-17
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S040000
Reexamination Certificate
active
06693934
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of vertical cavity surface emitting laser arrays. More specifically, it relates to vertical cavity surface emitting laser arrays that emit light at different wavelengths, and to a method of producing such arrays binary masks.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. In a VCSEL, optical emission occurs normal to the plane of a PN junction. VCSELs have certain advantages over edge-emitting laser diodes, including smaller optical beam divergence and well-defined, highly circular laser beams. Such advantages make VCSELs well suited for optical data storage, data and telecommunication systems, and laser scanning.
VCSELs can be formed from a wide range of material systems to produce specific characteristics. VCSELs typically have active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or by using molecular beam epitaxy (MBE).
To assist the understanding of VCSELs,
FIG. 1
illustrates a typical VCSEL
10
. As shown, an n-doped gallium arsenide (GaAS) substrate
12
is disposed with an n-type electrical contact
14
. An n-doped lower mirror stack
16
(a DBR) is on the GaAS substrate
12
, and an n-type graded-index lower spacer
18
is disposed over the lower mirror stack
16
. An active region
20
having a plurality of quantum wells is formed over the lower spacer
18
. A p-type graded-index top spacer
22
is disposed over the active region
20
, and a p-type top mirror stack
24
(another DBR) is disposed over the top spacer
22
. Over the top mirror stack
24
is a p-conduction layer
9
, a p-type GaAs cap layer
8
, and a p-type electrical contact
26
.
Still referring to
FIG. 1
, the lower spacer
18
and the top spacer
22
separate the lower mirror stack
16
from the top mirror stack
24
such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack
24
includes an insulating region
40
that is formed by implanting protons into the top mirror stack
24
or by forming an oxide layer. In either event, the insulating region
40
has a conductive annular central opening
42
that forms an electrically conductive path though the insulating region
40
.
In operation, an external bias causes an electrical current
21
to flow from the p-type electrical contact
26
toward the n-type electrical contact
14
. The insulating region
40
and its conductive central opening
42
confine the current
21
flow through the active region
20
. Some of the electrons in the current
21
are converted into photons in the active region
20
. Those photons bounce back and forth (resonate) between the lower mirror stack
16
and the top mirror stack
24
. While the lower mirror stack
16
and the top mirror stack
24
are very good reflectors, some of the photons leak out as light
23
that travels along an optical path. Still referring to
FIG. 1
, the light
23
passes through the p-type conduction layer
9
, through the p-type GaAs cap layer
8
, through an aperture
30
in the p-type electrical contact
26
, and out of the surface of the vertical cavity surface emitting laser
10
.
It should be understood that
FIG. 1
illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, providing a p-type substrate), different material systems can be used, operational details can be varied, and additional structures, such as tunnel junctions, can be added. Furthermore,
FIG. 1
only illustrates one VCSEL.
Producing multiple VCSELs on one substrate can be beneficial. In some applications, such as data and telecommunication systems, it is beneficial to have a VCSEL array that is comprised of multiple individual VCSEL elements that emit light at different wavelengths. Such an array could be used to implement wavelength division multiplexed systems. That is, light of one wavelength could be emitted (and, if required, modulated), then light of another wavelength could be emitted (and, if required, modulated), and so on. Because of the inherent low cost and volume capability of VCSELs, a VCSEL array suitable for wavelength division multiplexing would be highly attractive.
However, despite their many benefits, VCSEL arrays suitable for wavelength division multiplexing are not commercially available. One reason for this has been the unavailability of a low cost method of producing stable wavelength division multiplexed light beams from a single substrate.
In a VCSEL, the wavelength of the light output depends on various factors, one of which (as previously noted) is the separation of the top DBR mirror and the bottom DBR mirror. Thus, the output wavelength can be tuned by controlling the length of the cavity between the top and bottom DBRs. That cavity length is set during the manufacturing process.
FIG. 2
, which illustrates a side view of a simplified VCSEL element
98
of a VCSEL array, is useful for visualizing the cavity length. As shown, the VCSEL element
98
includes a substrate
100
having a backside contact
102
and a backside DBR mirror
104
. An active region
106
is on the backside DBR mirror
104
. A front side DBR
110
is on the active region
106
. Front side electrical contacts
112
are on the front side DBR
110
. Thus, the front side and back side DBR separation is controlled by the width of the active region
106
(and by the reflection depth of the DBRs). Therefore, the output wavelength is controlled by the processes that form the VCSEL element.
Therefore, a process of producing a VCSEL array that emits light beams of different wavelengths would be beneficial. Even more beneficial would be a new VCSEL array that is suitable for wavelength division multiplexing. Still more beneficial would be a low cost lithographic technique of producing VCSEL arrays that emit light beams having different wavelengths.
SUMMARY OF THE INVENTION
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
Accordingly, the principles of the present invention are directed to a method of producing VCSEL arrays, and to VCSEL arrays produced by that method, that are capable of emitting light beams having different wavelengths and that are suitable implementing wavelength division multiplexing in a cost effective manner. According to the principles of the present invention, binary masks are used to control depositions and/or etchings of a spacer that is disposed between top DBR mirrors and an active region. By using the binary masks, the wavelengths of individual VCSEL elements on a common substrate can be controlled.
According to one method that is in accord with the principles of the present invention, a process-controlled spacer is selectively grown on an active region using a sequence of binary masks such that the spacer has multiple thicknesses that are controlled by the binary masks. Then, front side (top) DBR mirrors are disposed over the spacer. Electrical contacts for the individual VCSEL elements are then provided. Additionally, suitable isolation regions are formed, either in the spacer or in the front side DBR mirrors, such that discrete VCSEL elements are formed. Suitable spacers can be formed from regrowth Al
x
Ga
(1−X)
As (or similar materials), a dielectric deposition (such as PECVD SiO
2
Abeyta Andrew A.
Honeywell International , Inc.
Nguyen Dung T
LandOfFree
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