Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1997-08-27
2001-06-12
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06246708
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
A portion of this work was done under ARPA grant F19628-94-C-0023.
BACKGROUND
The present invention relates to semiconductor lasers. More specifically, the present invention relates to semiconductor lasers having associated electronic components integrally formed therewith.
Semiconductor lasers are important devices used in a variety of applications including printing, scanning, communications, etc. Semiconductor lasers generally fall into two categories: edge-emitting and vertical cavity surface emitting (VCSEL). Each of these types of devices are well known. In an edge emitting semiconductor laser structure, a number of layers are deposited onto a substrate. Following deposition, the edges of the structure are cleaved to form partially transmissive mirrors. One or more of the deposited layers forms an optical cavity, bound at its edges by the mirrors. Lasing occurs within the cavity between the mirrors, and the laser beam exits at one or both of the edges of the laser structure in a direction parallel to the plane of the layers.
Surface emitting lasers are similar in concept, but differ in that the laser beam is emitted orthogonal to the plane of the active layer(s). The mirrors are above and below the optical cavity, as opposed to at each edge of the cavity. For certain applications, a surface emitting laser provides advantages over an edge emitting laser. For example, 2-dimensional arrays of vertical cavity lasers may be produced in wafer form, whereas edge emitting lasers typically must be mechanically jointed to form such arrays. Also, surface emitting lasers typically emit circularly symmetric Gaussian beams, as compared to highly eccentric elliptical beams of edge emitting lasers. Accordingly, today there is much interest and development centered around surface emitting lasers.
Associated with any semiconductor laser are numerous electronic components. For example, the power of a laser is typically controlled by the drive current applied to its electrodes; and one or more electronic components such as transistors, capacitors, diodes, etc. forming drive circuitry may be employed to control the drive current. As another example, components such as transistors are often employed in addressing circuitry for addressing individual lasers in arrays of such devices.
More specifically, in arrays of lasers, it is desirable to be able to independently address each laser. This becomes problematic when dealing with large arrays of such lasers. In such large arrays, the small size and large density of the electrodes to which connection must be made increase the complexity of making connections. Furthermore, the need to produce small size arrays limits the surface area which addressing connections and circuitry are permitted to occupy. To produce such arrays within a practical cost structure, the addressing circuitry and scheme must be relatively simple. Finally, the addressing circuitry and scheme must support rapid addressing of each laser.
While there are many examples of addressing circuitry and schemes in the art, efforts to date have not been successful in producing VCSELs having integrated (i.e., formed either as part of the process of forming the VCSEL or formed above the VCSEL as part of subsequent processing) associated electronic component structures. Rather, addressing circuitry, as well as driver circuitry and other related components, have been built external to the laser structure itself, then interconnected for operation.
For example,
FIG. 1
is an illustration of a laser structure and separately connected voltage source
10
. Laser
12
is formed on a substrate
14
, typically GaAs. A number of thin layers are first deposited to form a lower mirror region
16
, an n-type layer
18
is formed on lower mirror
16
, an intrinsic active layer
20
is formed on n-type layer
18
, a p-type layer
22
is formed on intrinsic active layer
20
, and an upper mirror layer
24
is formed on p-type layer
22
. Typically, a metal, n-material electrode
26
is formed below the substrate
14
, and a metal, p-material electrode
28
is formed above the upper mirror region
18
. Electrode
28
is typically annular in planform, so as to maximize surface contact yet minimize interference with the laser beam B
1
generated by the structure.
A voltage is then applied to electrode
28
from external voltage supply
30
and addressing circuitry
32
. Electrode
26
is typically connected to ground potential. The current through the laser
12
results in the generation of laser beam B
1
. The requirement of external voltage supply, addressing circuitry, and potentially other electronic components associated with laser
12
limits the ability to reduce size, cost, component complexity, etc., and increase speed, efficiency, etc.
In addition, in many laser systems it is necessary to measure and control the power of the beam emitted by the laser. For example, it is necessary in many applications to provide a constant, predetermined beam power, which requires compensation for the laser's temperature, aging, etc. Beam power detection generally involves interposing a detector in a laser beam path. In the case of certain edge emitting lasers, this may be accomplished by detecting one of two beams. That is, where an edge emitting laser is of the type having two beam emissions, one from each edge (facets), one beam is referred to as a forward emission beam and the other as a rear emission beam. The forward emission beam will generally be of a higher power than the rear emission beam. Hence, the forward emission beam is generally the operable beam performing the desired function, such as writing to a photoreceptor, pulsing encoded signals to a transmission line, cutting material, etc., while the rear emission beam is often not used. However, the ratio of the power of the forward emission beam to the power of the rear emission beam can be measured. Thus, by placing a detector in the path of the rear emission beam, and by employing the aforementioned power ratio, the power of the forward emission beam may be determined.
This approach has limited utility for surface emitting laser structures, for several reasons. Typically, surface emitting laser structures include a gallium arsenide (GaAs) substrate, which is opaque for wavelengths shorter than 870 nm. Thus, for most applications, the substrate will be opaque, and the laser structure will be capable of producing only a single, surface emitted laser beam. Second, in general it is desirable to provide as high a beam power as possible, so it is a design goal to produce single beam laser structures.
An existing approach to incorporating a detector into a single beam surface emitting laser (assumed to be single beam herein, unless otherwise stated) is to form the detector in the laser structure. That is, additional layers would be epitaxially grown above the laser structure, but of the same material as the laser structure, which would be appropriately patterned and/or doped, and interconnected to form a detector. The detector is generally coaxial with the laser beam, and relies on partial absorption of the beam to create electron-hole pairs which are detected by methods otherwise known in the art.
The upper detector electrode will have an inside diameter d, which is at its smallest equal to the diameter of the laser beam generated by the underlying laser. In operation, the detector converts photon energy from the laser beam to electron-hole pairs, which migrate to respective electrodes. Beam power is thus measured by measuring the extent of electron-hole pair generation (i.e., current generated in the detector). The speed of the detector is measured by the speed at which the electrons or holes travel to their respective electrodes. Those electrons or holes generated at the center of the annular detector electrode must travel a distance equal to at least d/2. This is a relatively large distance, and results in relatively slow detection.
Third, the detector is es
Street Robert A.
Thornton Robert L.
Davie James W.
Xerox Corporation
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