Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-03-12
2004-04-27
Eckert, George (Department: 2815)
Coherent light generators
Particular active media
Semiconductor
C372S038020, C372S029020, C372S038070, C372S050121, C372S029011, C372S038010
Reexamination Certificate
active
06728280
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to semiconductor laser dies and, more particularly, to a die having a vertical cavity surface emitting laser (“VCSEL”) and providing a balanced output to a drive circuit for optimally driving a VCSEL.
Semiconductor lasers are employed in numerous applications such as pumping solid state lasers, forming laser arrays, serving as sources for optical pick-up in compact disc (CD) players, and coupling to optical fibers in optical communications applications. Traditionally, the most common form of semiconductor laser has been the side or edge-emitting laser, though more recently VCSELs have been used in the above applications and indeed have become the dominant laser source in numerous data communications applications.
In contrast to the edge-emitting laser, in which the active region is positioned within a resonance cavity defined by two reflective layers positioned at opposing sides of the active region, VCSELs, in one form, have a resonance cavity defined by two reflective layers positioned at the top and bottom of the active region to produce a vertical emission, i.e., an emission normal to the junction plane of the active region. The junction plane may be a plane defined by the intersection of an AlGaAs layer and a GaAs layer in a multi-quantum well VCSEL structure, for example. In sum, edge-emitting lasers have a resonance cavity parallel to the junction plane and an emission through the side of the laser, while VCSELs have a resonance cavity orthogonal to the junction plane and emit through a surface of the laser.
Biasing a VCSEL is achieved through contact layers in the form of thin metal layers, where the contact layers, being photo-opaque (i.e., photon absorbing, over the emission spectrum of VCSELs), are positioned at specific locations on the top and bottom of a layered semiconductor structure surrounding the active region. For example, designs include a first metal contact layer over the entire bottom surface of the semiconductor substrate (i.e., between the semiconductor laser and a mounting substrate) and a second metal contact layer that is disposed either over half of the upper surface of the semiconductor substrate, only at a corner of the upper surface, or over all but an emission window of the top surface of the semiconductor laser. Still other designs use the bottom surface of the semiconductor laser as an emission surface with the mounting substrate upon which the VCSEL is mounted being photo-transparent.
In another form known in the art, a vertically-emitting laser is created from an edge-emitting semiconductor laser with an external cleaved surface angled at 45 degrees to the junction plane in the active region. Using materials with a high reflectivity over the principal wavelength(s) emitting from the lasing region, the cleaved surface reflects vertically the horizontally emitted light from the edge of the lasing region. This forms a surface-emitting laser, but without a vertical cavity. More recently, tunable VCSELs that use MEMs (micro-electro-mechanical-structures) to mechanically move the upper reflective region with respect to the active region have been shown. By lengthening or contracting the resonance cavity, one can tune the resonating wavelength of the coherent photonic emission for such VCSELs.
VCSELs have become the dominant laser source in demanding optical data communications systems like the Gigabit Ethernet standard provided for in the IEEE 802.3z protocol and the Fibre Channel standard provided in the ANSI X3.T11 protocol. VCSELs are preferred because they have high modulation bandwidths and can produce high bit transmission rates.
The Gigabit Ethernet standard is designed to improve upon the Ethernet (10 Mbps) and Fast Ethernet (100 Mbps) standards by providing a way to transmit and receive large amounts of data at data rates of 1 Gbps. The Gigabit Ethernet standard is intended for use in such demanding applications as scientific modeling, data warehousing, data mining, internet/extranet access, backing-up networks, and high-quality video conferencing. The Gigabit Ethernet standard achieves higher bandwidth while maintaining the simplicity and the relatively low cost of implementation and maintenance associated with the now-entrenched Ethernet standard. Providing higher bandwidth using low-cost components has made the Gigabit Ethernet standard attractive. With a large percentage of the installed network connections being Ethernet based, the Gigabit Ethernet standard has the advantage of backward compatibility with existing Ethernet backbones, as well. For example, all three Ethernet standards (Ethernet, Fast Ethernet, and Gigabit Ethernet) use the same IEEE 802.3 frame format. An additional feature of the IEEE 802.3z protocol is that it allows for full and half-duplex operation.
Fibre Channel, the ANSI X3.T11 protocol, is used in data storage and access systems in lieu of Small Computer System Interface (SCSI) systems. SCSI systems use an individual SCSI controller for each storage device (e.g., a hard drive) connected to a network. The separate parallel connections that result consume space and, as more storage devices are connected, decrease the I/O processing efficiency of the system, with SCSI applications typically having throughput speeds of less than 100 Mbps. The Fibre Channel standard allows serial I/O connection of numerous devices to a single input of a data processor or server and can achieve throughput in excess of 1 Gbps, and the serial nature of the Fibre Channel standard allows hot-plug connection of storage devices “on the fly”, e.g., without taking the system offline. The Fibre Channel standard also allows access to devices many meters from the processor or server because of the use of optical fiber in place of the copper cable used in SCSI applications.
VCSELs, serving as optical signal sources for these and other optical fiber-based data communication applications, are preferred over edge-emitting lasers for numerous reasons, one reason being the beam shape of the output. The beam shape of the output of the edge-emitting lasers looks approximately like the cross-sectional shape of the active region: the beam shape is elliptical. In contrast, the beam shape of the output of a VCSEL is approximately circular, matching the circular shape of the emission window defined by the upper contact layer and the active region, which produces a uniform photon emission across this window. The output of a VCSEL also has a low numerical aperture. Both of these properties make fiber coupling (in particular single-mode fiber coupling) easier with a VCSEL than with an edge-emitting laser. With a circular beam shape, the VCSEL output can be focused into a single mode optical fiber, using a known ball lens, for example, thus reducing undesirable multimode optical-fiber losses such as intermodal dispersion.
Furthermore, VCSELs are characterized by high power conversion efficiency, even in low input power ranges, and provide wide small-signal modulation bandwidths, with modulation bandwidths in excess of 1 GHz. Both of these advantages demonstrate the ability of VCSELs to be used in Gigabit Ethernet and Fibre Channel applications at a relatively low drive current and, thus, with less power usage and less thermal loss.
Moreover with the vertical emission, VCSELs can be relatively easily packaged, often with a photodetector disposed at the back surface of the VCSEL (or within the substrate upon which the VCSEL is mounted) for power monitoring and feedback control. More recently, VCSELs have been packaged in a transceiver for use in duplex communication.
VCSEL active region layers are deposited by known techniques and are doped to form PN junctions, PIN junctions with an intrinsic layer disposed between the p-type and n-type layers, or double heterostructures. Furthermore, VCSELs have relatively fast transition times, e.g., the VCSEL output can be driven from a binary “0” state to a binary “1” state in a relatively short period of time. Therefore, VCSELs are able to achieve wide
Guenter James Kenneth
Tatum Jim
Abeyta Andrew A.
Eckert George
Honeywell International , Inc.
Nguyen Joseph
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