VCELs are really agnostic when it comes to protocals. 10G FC and GE are serial. Short haul metro is parallel. A few companies are touting 1310 VCELs, that is a whole new market for these things...
From Compound Semiconductor Magazine Issue 6 No. 5 (July 2000)
VCSELs: Driving the Cost Out of High Speed
Fiber Optic Data Links
compoundsemiconductor.net
DAVE WELCH
W. L. Gore & Associates
Presented at the Key Conference on Compound Semiconductors
Key West, Florida
March 13-14, 2000
VCSELs offer both performance and cost advantages over traditional edge-emitting lasers. Following strong investment by the U.S. government, universities, and industry in the late eighties and early nineties, VCSELs are now being manufactured in high volume by a number of suppliers. The main application at present is gigabit-speed communications, while technology advances are opening up other markets at higher data rates and longer transmission distances.
The VCSEL Advantage
Essentially, VCSELs are very high performance lasers that can be produced much like low cost LEDs. VCSELs have a low divergence circular output beam and very stable performance characteristics over temperature. They can be modulated at high data rates, and can have threshold currents of less than a milliamp in some cases. In addition, VCSELs offer the manufacturing advantages of wafer-scale fabrication and test, and the practical ability to produce either small or large arrays.
Although VCSELs can be manufactured at relatively low cost, the real advantage comes in the ease of packaging these devices. For example, the low divergence circular beam enables improved coupling tolerances. Also, high-speed operation is achieved at low power, which minimizes problems with crosstalk and heat dissipation.
Another important aspect of VCSELs is that the stable performance over temperature allows the monitor photodiode feedback loop to be eliminated in many applications. This not only eliminates the monitor photodiode, it also simplifies the IC and package designs, with important cost implications. Furthermore, non-hermetic packages are now available and the ability to produce uniform, low cost arrays with good temperature stability has enabled the development of low cost, parallel optical modules (see Figure 1).
VCSELs were successful at entering the communications market because of the strong demand created by the need to transition from 100 Mb/s applications like Fast Ethernet to 1 Gb/s applications like Gigabit Ethernet and Fibre Channel. The performance of LED sources that had been used for 100 Mb/s applications was not able to match the speed requirements of the emerging standards, so a new source technology was required. One interesting point is that VCSELs went head to head with CD lasers in the early stages of the Fibre Channel and Gigabit Ethernet markets. At the time, VCSEL production volumes were very low, while the CD lasers were being produced in high quantities. Despite this, VCSEL manufacturers were able to enter the market and be cost competitive at an early stage. Customers were attracted by the performance and reliability advantages of VCSELs, as well as the promise of scalability to higher data rates. Today, the vast majority of high-speed, short wavelength transceivers use VCSELs, and production is now at the level of million of units per year.
Products and Markets
The first commercial VCSEL product was introduced by Honeywell in 1996. The VCSEL was offered in a traditional TO can, primarily for use in transceiver modules for Fibre Channel and Gigabit Ethernet. In these applications, transmission distance is less than 300 meters. Honeywell conducted early reliability studies that were very helpful in getting VCSELs accepted within the standards community and by customers. By accumulating a large amount of data, Honeywell showed that VCSELs could typically offer reliability improvements over CD lasers. Today Gigabit Ethernet and Fibre Channel transceivers are available from a large number of suppliers.
An important development in the last year or two has been the introduction of small form factor transceivers at approximately half the size of standard products, allowing the incorporation of more transceivers onto each network card.
The gigabit market is growing at a rapid rate, and will continue to do so for the next several years. Recent data from Electronicast states that the gigabit Ethernet transceiver market was worth about $600 million in 1999, and will grow to over $2 billion by 2004. Other projections suggest that this figure will be even higher.
There are a number of emerging opportunities for VCSELs, particularly in the area of multi-gigabit networking, where VCSELs are being developed for emerging standards such as 2X and 4X Fiber Channel and 10 Gigabit Ethernet. Parallel optical products for Very-Short-Reach (VSR) applications have reached the market and are gaining support in industry groups such as the Optical Internetworking Forum (OIF). For these applications, the development of oxide-confined devices and arrays is crucial. Long wavelength VCSELs operating at the single-mode telecom transmission wavelengths (1.3- and 1.55-ìm) will move the cost benefits of VCSELs to longer distance applications. Also, there are a number of emerging applications for VCSELs in areas relating to WDM.
Oxide-Confined VCSELs
First generation VCSELs, including many of today's standard products, are fabricated using ion implantation. This process creates a resistive area that funnels the current into the active region. The second generation of VCSELs use oxide confinement to provide both current guiding and index guiding for improved efficiency. Oxide VCSELs are grown 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 devices, which are now commercially available, operate at higher speeds and have lower threshold currents than the ion implant VCSELs. Oxide VCSELs also exhibit improved temperature performance. The advantages offered by oxide VCSELs enable low cost, higher speed LAN transceivers. These devices also allow the fabrication of highly uniform arrays, enabling multi-channel, parallel optics applications.
Figure 2 illustrates the performance of oxide-confined VCSELs manufactured by Gore. In an uncooled package, the devices can be directly modulated at a rate of 12.5 Gb/s with a very low modulation current. As the networking environment moves towards higher speeds, the transmission distance in standard multi-mode fiber shrinks dramatically (see Figure 3). To address this problem, new multimode fibers have been developed that are optimized for lasers emitting at 850 nm. Gore has been on the leading edge of working with companies including Lucent, Corning and Alcatel, who have been developing this fiber to create next generation multimode systems.
A demonstration combining Gore's 850 nm VCSELs and Lucent's high bandwidth fiber showed 10 Gb/s transmission across a distance of 400 meters. Subsequent experiments have shown that a 12.5 Gb/s signal can be transmitted across nearly a kilometer. These are laboratory-type experiments, but they indicate that robust transmission can be achieved through several hundred meters of multimode fiber using VCSELs.
Parallel Optical Interconnects
One of the major challenges in emerging high-speed networks is the ability to cost-effectively manage the huge amounts of data flowing through the central offices and Internet service providers (ISPs). Parallel optics is a very attractive solution for short-distance (<300 meters) links between routers, switches, and transport equipment. A number of companies have introduced multi-channel transmitter & receiver products, utilizing arrays of VCSELs and detectors connected by ribbon fiber. Early implementations of this technology have 12 channels with 1.25 Gb/s per channel, equating to an array with an aggregate data rate of about 15 Gb/s. It is important to note that this solution costs a few hundred dollars, compared to the thousands of dollars required to implement today's OC-192 (10 Gb/s) serial links.
Gore is in the process of commercializing its nLIGHTENTM parallel modules. The modules are only about three-quarters of an inch wide, allowing many modules to be incorporated onto a card edge (see Figure 1). Both Gore and Infineon produce parallel optical modules compliant with a multi-source agreement that provides for footprint-compatible modules in an effort to speed the market acceptance of these products.
Gore's nLIGHTENTM parallel modules were incorporated into a system displayed by Cisco Systems at the recent Optical Fiber Communications Conference (OFC). The Very-Short-Reach (VSR) parallel optical interface is being promoted within the Optical Internetworking Forum as a leading choice for OC-192 (10 Gb/s) transmission at distances up to 300 m. In this application, an ASIC converts the incoming 16 3 622 Mb/s signal into a 12 3 1.25 Gb/s signal for transmission across the 12-channel ribbon fiber.
A key part of making low cost parallel modules is the availability of a suitable VCSEL array technology. Gore VCSEL arrays are designed to have very stable performance over temperature at the actual operating current of ~5 mA (see Figure 4). This simplifies the system design, and eliminates the need for a monitor photodiode feedback loop. The VCSELs are scalable to high data rates so that in the future, low cost, hundred gigabit connections over very short distances is a viable concept.
Life test data for Gore VCSELs operating at 100°C with a drive current of 5 mA shows no failures after more than 2500 hours of test time. A large-scale reliability test is currently being performed, and the results will be available shortly.
Long Wavelength (1.3 ìm and 1.55 ìm) VCSELs
VCSELs operating at the transmission wavelengths of single-mode fiber (1.3 ìm and 1.55 ìm) will extend the cost benefits of VCSELs into the single-mode domain, enabling longer transmission distances and opening up new applications. This will move VCSELs from local area networks (LANs) with transmission distances of several hundred meters into access networks, campus backbones, metropolitan area networks (MANs) and wide area networks (WANs) with multi-kilometer transmission distances.
Despite strong demand for long wavelength VCSELs, the technology has developed relatively slowly due to fundamental materials challenges. Good mirrors can be grown on GaAs substrates, however it is very difficult to grow active regions emitting at 1.3 ìm on GaAs. Conversely, good active regions can be grown on InP substrates, however high reflectivity InP-based mirrors are extremely difficult to fabricate. There has been, and continues to be, a large effort to try to address these problems through fundamental materials research. Thus far, however, the best devices have used wafer fusion to combine materials of different lattice constants into one structure.
Gore has combined wafer fusion with an integrated, optical pump to produce the world's best long wavelength VCSELs (see Figure 5). The Gore structure has several major technical advantages resulting in higher single-mode powers and improved performance over temperature. Gore's 1310 nm VCSELs operate at temperatures approaching 100°C, and about a milliwatt of power can be coupled into fiber even at 70°C. The devices work very well in excess of 2.5 Gb/s and higher speed parts are in development.
Gore's VCSELs are now being sampled to a couple of customers, and will be more broadly available in the near future. At this year's OFC, Gore demonstrated 2.5 Gb/s transmission over a distance of 25 km using its 1300 nm device in a standard package (see Figure 6).
VCSELs are normally associated with multimode operation, however long wavelength devices can be designed such that single longitudinal mode and single transverse mode operation are obtained. The linewidth is similar to that of a DFB laser, so the devices are well suited to high speed, extended distance single-mode fiber applications. Long wavelength VCSELs also have major coupling advantages (see Figure 7). 1300 nm VCSELs have a low divergence, circular beam profile that enables more than 80% of the light to be coupled into single-mode fiber without lensing, creating very obvious advantages with regard to package design.
VCSELs for WDM
There is great potential for using VCSELs in wavelength division multiplexing (WDM) applications, and new concepts are emerging. VCSELs are being applied to coarse or wide WDM, and tunable VCSELs and VCSEL arrays emitting at multiple wavelengths have been demonstrated.
Coarse (or wide) WDM and dense WDM differ in the spacing between adjacent wavelengths. For a DWDM system in which the channels are placed on a 100 GHz ITU grid, the wavelengths are separated by only 0.8 nm. Although this allows 40 channels in the L-band (1530-1560 nm), cooling is typically is required to ensure that the wavelengths of the channels do not drift. Coarse WDM products are focused low cost, local area networking applications and use much wider channel spacings to simplify product design. Channel spacings of 10- to 25-nm are employed, and because VCSELs drift less than 0.1 nm/°C the products can effectively operate uncooled, from 0–70°C without channel overlap.
Blaze Network Products has developed an eight-channel coarse WDM transceiver using VCSELs with wavelengths ranging from 790 nm to 860 nm. Plastic, molded components combined with broad wavelength filters enable the optical mux and demux functions to be accomplished at low cost.
Tunable VCSELs have also been demonstrated. CoreTek (now part of Nortel) has combined a VCSEL technology with a MEMS mirror that can be moved in order to tune the cavity. About 40–50 nm tuning and 5 mW of coupled power has been achieved in this manner.
Finally, Gore has demonstrated VCSEL arrays containing four devices emitting at different wavelengths in the 1550 window. Using this technology, the possibility exists for fabricating low cost, integrated WDM sources with 4, 8 or 16 channels for high bandwidth single-mode links.
Employing VCSELs in WDM applications area should prove to be a fertile ground for innovation and future development.
Summary
For certain applications, VCSELs have fundamental advantages over other types of lasers. No longer lab curiosities, VCSELs are in volume production from multiple suppliers, and technological advances continue to enable new applications. Oxide-confined VCSELs have already enabled higher data rate products and parallel optical interconnects, while longer wavelength devices hold the promise of longer transmission distances over single-mode fiber. Finally, there is a great deal of potential to utilize VCSELs for WDM-related applications.
© 2000 Compound Semiconductor Magazine. All rights reserved.
This technology will bring the needed capability to mass produce optical components and alter the price structure for
the networks of the future.
'Gonna keep my VSCEL seats for a few good years. |
