Cavity-less vertical semiconductor optical amplifier

Optical: systems and elements – Optical amplifier – Particular active medium

Reexamination Certificate

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C438S022000

Reexamination Certificate

active

06339496

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of semiconductor optical amplifiers and more particularly, to vertically arranged semiconductor structures for amplification of an optical signal where resonant cavities are avoided, which are free of the disadvantages imposed by prior art cavity amplifying structures.
Particularly, the present invention relates to a cavity-less vertical semiconductor optical amplifier which has a broad transmission bandwidth and uses a relatively low injection current for amplification of an optical signal.
Further, the present invention relates to a cavity-less vertical semiconductor optical amplifier based on Periodic Table groups III-V or II-VI material system which includes a relatively thick layer of intrinsic semiconductor material sandwiched between p-doped and n-doped layers of semiconductor materials for amplification of an input optical signal within a strictly confined vertical gain channel extending through a created p-i-n structure. In this manner, a directed flow of p- and n-type carriers is obtained in the active region within the confined cross-section of the vertical gain channel. Additionally, substantial carrier concentration, as well as high current density is achieved within the active region which contributes to amplification of an input optical signal.
Further, the present invention relates to a cavity-less vertical semiconductor optical amplifier which may be arranged into multidimensional architectural structures to form an optical cross-bar switch, spatial light modulator with gain, optical boundary detection device, optical OR gate, and other optical mechanisms.
Still further, the present invention relates to a technological process of manufacturing a cavity-less semiconductor optical amplifier.
BACKGROUND OF THE INVENTION
Optical communication has emerged as one of the most powerful driving forces in present digital systems. The increasing demand for wider bandwidth and the quest for speedier transmission have led to extensive deployment of optical fiber networks for data and voice communications and the concept of parallel optical architecture. As a result, vertical cavity surface emitting lasers (VCSEL) are being actively researched and are used in this type of parallel optical architecture.
PRIOR ART
Vertical cavity surface emitting lasers are explicitedly described in U.S. Pat. Nos. 6,061,381, 6,064,683, 6,067,307, 6,069,905, and 6,069,908. Generally, VCSELs include a resonant cavity formed between top and bottom distributed Bragg reflectors. The resonant cavity contains an active region composed of a bulk semiconductor layer or one or more quantum well layers which are interleaved with barrier layers. On opposite sides of the active regions are mirror stacks (Bragg reflectors) which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength of interest thereby forming the mirrors for the laser cavity. Generally there are opposite conductivity type regions on opposite sides of the active region and the laser is turned on and off by passing the current through the active region.
Problems have arisen in that these prior art devices operate typically in either a transmission mode or a reflection mode. Such prior art devices suffer from a narrow gain band width due to the physics of the vertical cavity surface emitting laser using distributed Bragg reflector mirrors. In order to compensate for the small single pass gain in a typical cavity multiple quantum well gain region, multiple recirculation of the light beam within the resonant cavity is needed. Thus a high Q cavity is needed which is obtained by growing thick distributed Bragg reflectors mirrors on both sides of the cavity. These mirrors are wavelength selective which means that they reflect the light recirculating within the resonant cavity over a very narrow band of wavelength.
By reducing the number of Bragg reflector mirrors on both sides of the cavity it is possible to slightly broaden the bandwidth of the vertical cavity surface emitting lasers however, this approach leads to formation of a “clumsy” and low gain amplifier which still fails to provide operation in sufficient bandwidth spectrum.
As shown in
FIG. 1
, a transmission spectrum for a vertical cavity surface emitting laser amplifier having twenty periods of Al
0.7
Ga
0.3
, As/Al
0.1
Ga
0.9
As mirrors on both sides of the resonant cavity has a very narrow bandwidth at 850 nm of the bandwidth spectrum. These narrow bandwidths of gain of typical vertical cavity surface emitting lasers are not sufficient for spatially-parallel optical communication applications.
It is thus desirable to have a vertical optical amplifier capable of providing much broader bandwidths gain than a typical vertical cavity surface emitting laser, and which would be capable of being turned on and off in a nanosecond time scale.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a vertical semiconductor optical amplifier having the gain bandwidth which is several hundred times higher than that of typical vertical cavity surface emitting lasers, and which would be well suited for multi-dimensional interconnects for optical signal processing in a nanosecond time scale. Additionally, it is an object to provide an optical amplifier which may be used for large parallel interconnect, two dimensional optical signal processing and for implementing a free-space cross connect switch.
It is a further object of the present invention to provide a cavity-less vertical semiconductor optical amplifier, the physics of which does not rely on a resonant cavity sandwiched between distributed Bragg reflector mirrors for amplification of the optical signal.
It is still another object of the present invention to provide a cavity-less vertical semiconductor optical amplifier in which amplification of the signal is achieved in a vertical gain channel of a predetermined confined cross-sectional configuration which includes a p-i-n structure having an active regions of a thick layer of an intrinsic semiconductor material sandwiched between p-doped and n-doped layers of semiconductor materials. This results in a flow of p- and n-type carriers into the active region being confined within the very narrow cross-sectional configuration of the gain channel and results in formation of a substantial carrier concentration, as well as current density within the active region, thus contributing to amplification of an input optical signal.
It is an object of the present invention to provide a cavity-less semiconductor optical amplifier using a thick active region of an intrinsic semiconductor material in combination with confining the injected current over very narrow volume within the thick active region, i.e., to provide an amplification structure having a substantial length for high single pass gain in conjunction with a smaller active volume to lessening the injected current needed to obtain gain of the signal. In this manner, the optimization of performance and operational parameters of the cavity-less semiconductor optical amplifier is obtained.
It is another object of the present invention to provide a method of manufacturing a cavity-less semiconductor optical amplifier which has a wide bandwidth gain and optimized performance and operational parameters.
According to the teaching of the present invention, a cavity-less semiconductor optical amplifier comprises a vertical gain channel of a predetermined confined cross-sectional configuration which includes a p-i-n structure including an active region (thick layer of an intrinsic semiconductor material) sandwiched between a layer of a p-doped semiconductor material on one surface of the active region and a layer of an n-doped semiconductor material on an opposite surface of the active region.
A pair of partially oxidized layers sandwiches the p-i-n structure therebetween. Each of these partially oxidized layers has a current injection path formed therein and arranged

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