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Technical Insight

Oxide VCSELs rise to the challenge (Cover Story - VCSELs)

With the latest developments in oxide confinement further boosting performance, VCSELs are looking in good shape to keep pace with the rapid advances in high-speed fiber-optics, writes Ian Aeby of Emcore's Optical Device Division.
In only a decade or so, vertical-cavity surface-emitting lasers (VCSELs) have moved from a technological curiosity to a critical component in powering high-performance, low-cost optical fiber data communications equipment. One key reason is that the highly competitive nature of the global communications market has led to continual technological advances in the fiber-optics arena. One such advance is the switch from VCSELs fabricated using ion implantation to so-called oxide-confined VCSELs. These devices are fabricated with a high aluminum content buried layer, which is subsequently oxidized under conditions of high temperature and high humidity to form the device aperture. Oxide VCSEL devices, which are now commercially available, operate at higher speeds and have lower threshold currents than ion-implanted VCSELs. Another factor driving technological advances in this field is a simultaneous trend towards higher data communications speeds, which has increased the demand for low-cost, high-speed optical interconnects. Examples include the new 2X Gigabit Ethernet and 2X Fibre Channel standards; the new OC-192 very-short-reach (VSR) interconnect designed for low-cost intra-office applications; and the InfiniBand architecture, which has the potential to substantially improve overall data throughput in servers. A radical step VCSELs are revolutionary compound semiconductor microlaser diodes. The device emits light vertically from the surface of a fabricated wafer in a direction perpendicular to the p-n junction. The first VCSELs were made by combining bulk active regions with metal mirrors. Subsequently, other approaches were tried including dielectric mirrors, semiconductor distributed Bragg reflector (DBR) mirrors and air-semiconductor DBR mirrors. Approaches to current confinement include ion-implanted gain-guided lasers, etched air-post lasers, selective etched air-confined lasers, native-oxide-confined lasers and regrown buried heterostructure lasers. VCSEL emission wavelengths can be tailored from visible to near-infrared by a simple materials modification in the laser cavity of the basic structure. The ability to manufacture these lasers using standard microelectronics fabrication methods allows chip-on-board integration of VCSELs with other components without requiring pre-packaging. Advantages of VCSELs VCSELs have a number of important advantages that have catapulted them to the distinctive position of being the technology of choice for a wide range of data communications products. With a low threshold of between 1 and 6 mA, VCSELs offer very efficient power conversion. They can deliver transmission speeds of between 1 and 10 Gbit/s (see ), yet have a modulation swing of only 510 mA, which keeps power consumption low. The latest generation of VCSELs do not require hermetic packaging, yet typical mean lifetimes for well manufactured devices range from 10 to 100 years. At the same time, the circular, low-divergence output beams provided by VCSELs eliminate the need for corrective optics in most applications. A key advantage of VCSELs relative to edge-emitting lasers is that each device can be probe-tested on the wafer before fabricating and packaging. This enables manufacturers to identify defective chips before subassemblies are produced. This allows VCSELs to generally offer much higher yields than edge-emitting lasers. A VCSEL normally has both the cavity mirrors and the gain region grown by a single epitaxial step on a 50 or 75 mm diameter substrate. DBR mirrors are created by growing quarter-wavelength thick layers of semiconductors with alternating refractive indices. After growth, the next step is to fabricate the laser using processing steps that are analogous to integrated circuit manufacturing. The first volume applications of VCSELs almost universally used proton implantation to selectively destroy the epitaxial crystalline structure as part of the laser fabrication process. The damage that was incurred in this process made these devices more difficult to control than is desirable in a high-volume manufacturing process, and resulted in poor device-to-device uniformity. The process also led to high series resistance in the mirror layers, causing less efficient laser current injection into the active region. Move to oxide confinement The latest advancement is the development of oxide-confined VCSELs that offer significant performance improvements. Instead of using protons to define the active region, this approach uses a selective oxide layer (see ). A layer of aluminum gallium arsenide with high aluminum content is laid down and a mesa is etched to expose high oxide layers. The wafer is heated, typically to 400450 C, and steam is introduced to initiate the oxidation process. Oxidation moves from the edges towards the center while the temperature and duration of the process control the width of the oxidized layer. The oxide layer provides both electrical insulation and a light-guiding effect, because the refractive index of aluminum oxide is lower than that of gallium arsenide. The lateral oxidation rate can be as high as micrometers per minute using GaAlAs layers as thin as tens of nanometers. shows a plan-view optical micrograph of the complete VCSEL, while is an SEM shot of the central light-emitting region, surrounded by the ring contact. The oxide-confined approach simultaneously increases the current injection efficiency and reduces series resistance. The result is a significant improvement in power conversion efficiency. The bottom line is high-speed performance - data rates of 2.510 Gbit/s can be achieved without compromising other properties. Threshold current, slope efficiency and output power all vary less than 5% from device to device. This allows the fabrication of highly uniform arrays, enabling multichannel, parallel optics applications. Oxide VCSELs also exhibit improved temperature performance. The high slope efficiency of VCSELs produced by these methods provides a very low thermal budget, which aids module design. The advantages offered by oxide VCSELs enable low-cost, higher-speed LAN transceivers. VCSELs produced by oxide-confined methods were first introduced in November 1999, so this approach is now an established and mature technology that has demonstrated high manufacturability and reliability. Use in 10 Gbit/s applications Along with technological improvements, the applications supported by these optical devices have also seen some major developments. VCSELs have achieved a dominant position in the Gigabit Ethernet and Fibre Channel transceiver market, primarily because LEDs encountered obstacles caused by their very wide beam emission and broad spectral emissions. As 10 Gbit/s standards emerge in both the Ethernet and Fibre Channel spaces, nearly all industry observers agree that VCSELs will continue to play a dominant role. The real question is, which type of VCSEL implementation will the market turn to for 10 Gbit/s standards? Three competing implementations each stand a good chance and the odds are that all will co-exist at various stages of the product cycle. Parallel fiber approach has already been used in the first practical low-cost 10 Gbit/s solution to reach the market. On the other hand, wavelength-division multiplexing (WDM) may be another contender in the emerging high-speed data communications markets. However, most industry observers believe that it will be some time before WDM technology migrates from public network applications, where high costs have historically been acceptable, to much more price-sensitive end-user applications. A serial 10 Gbit/s solution doesn t exist in commercial implementations yet. However, many industry experts expect it to emerge as the eventual winner because of its simplicity - a single laser transmitting down a single strand of fiber, combined with the use of time-division multiplexing (TDM). Use in Ethernet and Fibre Channel While these issues shake out, 2X and 4X Ethernet and Fibre Channel implementations are expected to make a significant impact. The key advantage is that there is typically enough margin built into backplanes and edge cards that the speed of existing switches and routers can be doubled simply by switching out the transceivers. Meanwhile, a new generation of parallel optical modules based on VCSEL technology is enabling dramatic improvements in central office (CO) and point of presence (POP) backplane interconnect solutions. Copper interconnects are unable to route data at the speeds required by the latest generation of edge switches used in public networks and customer premise equipment, while current high-speed optical links are too expensive and bulky. With 12 channels each operating at 2.5 Gbit/s over a dedicated fiber strand, the new parallel modules deliver 30 Gbit/s of capacity while using as little as 1.5 inches of board space. Another important advantage of the new modules, which are just beginning to reach the market, is that they cost well under $1000, compared with $5000$10 000 for the 10 Gbit/s modules that they are expected to replace. New applications Finally, we are just now seeing the emergence of a new class of high-speed VCSEL-based solutions: I/O products that are designed to make connections at distances of just a few meters, from microprocessor to disk drives and other peripherals, within switches and routers, etc. An interesting approach is the InfiniBand architecture, which is already supported by 215 members of the InfiniBand trade association. InfiniBand architecture s point-to-point linking technology will be used as the basis for an I/O fabric that will increase the aggregate data rate between servers and storage devices. More important, the I/O fabric of InfiniBand architecture will take on a role similar to that of traditional mainframe channel architecture that used point-to-point cabling technology to maximize overall I/O throughput by handling multiple I/O streams simultaneously. An innovative product in this space is Infineon Technologies 12-channel parallel optical link (PAROLI), which is designed to connect telecommunications and data communications components and equipment for board-to-board or rack-to-rack applications. VCSELs provide a sensible, cost-effective solution for speeding up the transmission of data in the near term while delivering the reliability that both customer premise equipment and public network applications require.
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