Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2002-07-19
2004-06-08
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular active medium
Reexamination Certificate
active
06747794
ABSTRACT:
FIELD
This patent specification relates to optical amplifiers. More specifically, it relates to a semiconductor optical amplifier that amplifies an optical signal using energy from vertical cavity surface emitting lasers (VCSELs).
BACKGROUND
As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are capable of very high-bandwidth operation.
Optical amplifiers are important components of optical communications links. In general, the two primary types of optical amplifiers are optical fiber based amplifiers, such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers, and semiconductor optical amplifiers (SOAs). EDFAs are widely used in line amplifiers and other applications requiring high output power, high data rates, and low noise. However, EDFAs are quite bulky, having a typical fiber length of about 30 feet, and require the presence of a separate pumping laser to operate. Accordingly, EDFAs are difficult to incorporate into confined spaces and are not amenable to circuit-board-level or chip-level integration.
SOAs, on the other hand, are small in size and conveniently integrated into small devices. However, conventional SOAs generally suffer from pattern-dependent gain fluctuations, which causes crosstalk in multiple-channel optical signals such as those present in wavelength division multiplexed (WDM) networks and dense WDM (DWDM) networks. Amplified spontaneous emission (ASE) noise is another primary troublesome noise source in conventional SOAs. ASE noise arises from random, spontaneous energy state drops in a small fraction of the excited carriers of the gain medium. Efforts continue toward reducing crosstalk effects and ASE noise in SOAs to increase their usefulness in WDM and DWDM networks, and for other applications.
WO 01/28049 (hereinafter the '049 reference) discusses a vertical lasing semiconductor optical amplifier (VLSOA) in which an optical signal travels in a longitudinal direction along an amplifying path, the amplifying path including a semiconductor gain medium, the semiconductor gain medium forming the active medium of a plurality of vertical cavity surface emitting lasers (VCSELs) oriented vertically with respect to the amplifying path. The VCSELs are operated above threshold so as to cause lasing action therein. As the optical signal propagates through the active region, it is amplified by a gain multiplier due to stimulated emission of additional photons. The gain multiplier is substantially constant, i.e., independent of the amplitude of the optical signal, because the laser radiation produced by the VCSELs acts as a ballast to prevent gain saturation.
However, the VLSOA set forth in the '049 reference may experience performance problems due to non-uniformities in the lasing field of the VCSELs. In a non-uniform lasing field, the photon density of the active medium contains undesirable variations that cause the gain experienced by the optical signal to vary, often unpredictably, with time and space within the active medium. One source of such non-uniformity involves the presence of higher-order transverse modes in the vertical cavities, the photon density concentrating in differing spatial patterns depending on which higher-order transverse mode is present. Moreover, when higher-order transverse modes are present, they may be highly unstable. The particular transverse mode dominating at any given instant may vary chaotically with even the smallest variations in excitation current. Operation of the SOA device is compromised in terms of gain multiplier magnitude, gain multiplier stability, and/or saturable power performance.
Another source of non-uniformity in the lasing field arises from practical problems encountered in real-world device fabrication. The growth of “perfect” epitaxial layers being extremely difficult or impermissibly expensive to achieve, real-world devices will have some statistical population of local defects in the semiconductor layers such as crystal dislocations, pitting, voids, etc. Such defects in the epitaxial growth can be a point of lower electrical resistance than the surrounding epitaxial areas. The higher electrical current flowing through these points of lower electrical resistance can create “hot spots” which cause spatially non-uniform currents in the affected areas of the gain medium. The spatially non-uniform currents can adversely affect the lasing action of the VCSEL cavities and cause non-uniform photon densities, again resulting in non-uniform gain and compromised device performance. The area of lasing non-uniformity in the gain medium can extend substantially beyond the immediate region of the local crystal defect. Moreover, the electrical current being funneled through a “hot spot” from the surrounding regions can grow to such a magnitude that overheating and device failure can result.
Accordingly, it would be desirable to provide a vertically lasing semiconductor optical amplifier that is operationally robust in terms of gain multiplier magnitude, gain multiplier temporal stability, and saturable power performance.
It would be further desirable to provide a vertically lasing semiconductor optical amplifier having increased tolerance to local defects that may occur in the epitaxial growth stages of device fabrication.
SUMMARY
A semiconductor optical amplifier (SOA) apparatus and related methods are provided for amplifying an optical signal, the SOA comprising an integrated plurality of vertical cavity surface emitting lasers (VCSELs) having a common gain medium layer, the SOA further comprising a signal waveguide extending horizontally through the VCSELs near the gain medium layer such that the optical signal is amplified while propagating therethrough, wherein each VCSEL is configured and dimensioned to achieve smooth, single-transverse-mode lasing action for promoting spatially uniform and temporally stable gain of the optical signal as it propagates along the signal waveguide. Although integrated onto a common substrate, the VCSELs are functionally isolated from each other, each building up its own distinct lasing field responsive to a distinct electrical pump current therethrough. Each VCSEL is configured and dimensioned to suppress higher order or otherwise uneven lasing modes at nominal bias levels. When each VCSEL is achieving smooth, single-transverse-mode lasing action at its nominal bias levels, the current density in the gain medium is even and temporally stable, thereby resulting in spatially uniform and temporally stable amplification of the optical signal.
According to a preferred embodiment, neighboring VCSELs are functionally isolated from each other by separation zones formed by electrically isolating implants. Preferably, the SOA comprises several dozens to several hundreds of functionally isolated VCSELs positioned along the optical signal path, the gain medium of each VCSEL providing only a small portion of the overall signal gain. Advantageously, if a local defect arises during device fabrication that causes a “hot spot” to occur or that otherwise causes uneven lasing to occur at nominal bias levels, the spatial and operational scope of that defect is limited to its particular VCSEL. Furthermore, because that VCSEL is associated with only a small percentage of the overall signal gain, it is more likely that there will be only minor implications for overall device performance due to that local defect. According to one preferred embodiment, the VCSEL containing the local defec
Cooper & Dunham LLP
Gazillion Bits Inc.
Hellner Mark
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