The VCSEL Advantage

There are basically two types of lasers used in fiber optic transmission systems today:  edge-emitters and surface emitters.  The most prevalent high-speed edge-emitters are FP and DFB.  The beam emission for these devices is parallel to the substrate.  In the case of VCSELs, the light is emitted vertical to the substrate.

Short-wavelength VCSELs have been a part of the optical networking world since Honeywell introduced them in the early 1990s.  The devices were adopted quickly to replace unreliable and costly CD lasers in the datacom market.  Due to their inherent low cost, low power and small size, VCSELs became the light source of choice in enterprise networks.  They are also credited with enabling Gigabit transmission in that space and are currently being used in over 90 percent of Fibre Channel and Ethernet transceivers.   By taking what was learned at 850 nm and extending it to 1310 and to 1550-nm wavelengths, companies are now starting to show that they can drastically reduce the cost and size of transmitters.

Historically, longer wavelengths of 1300 to 1700 nm have been more difficult to produce in the VCSEL construction, because of the refractive index of InGaAsP.  This material is generally used for edge-emitters in these wavelengths and does not change very much with composition, which makes it difficult to produce components.  But in the early 2000s, development of different combinations of III-V elements led to long-wavelength VCSELs.  Several manufacturers such as Bandwidth9 with its tunable, 1550-nm VCSEL;  Cielo Communications, E2O and Picolight with their  1310-nm VCSELs had proven that lasers supporting wavelengths higher than 980 nm were possible to produce in volume. In fact, before JDSU acquired Picolight, it had several multi-Gigabit transceivers it produced with 1310-nm VCSELs. JDSU has since discontinued that line and, it seems, its support of 1310-nm VCSELs.

Key advantages of the VCSEL at production-level include the following:

• High Yields. VCSELs can be processed with as many as 20,000 individual lasers on a three-inch wafer.  Even if 20 percent of these are lost due to processing yields (a high number by VCSEL manufacturers standards), this is still a far higher yield than their edge-emitting cousins.
• Testing at the Wafer Level.  VCSELs can be tested before the wafer is diced.  Most edge-emitting lasers (FPs and DFBs) must be cleaved from the wafer and packaged before they can be tested, and are therefore tested individually, which increases processing costs and decreases yields significantly
• Easier Coupling and Packaging.  Another important advantage of the VCSEL structure is that its circular cross-section gives better control over beam size and divergence than for edge-emitters, allowing for much easier coupling of the fiber to the VCSEL output and easier alignment during packaging.

Table I shows a comparison of FP, DFB and VCSEL solutions, an availability status, and a list of some of the component manufacturers.

 

Table I:  Comparison of  VCSEL, FP and DFB Technologies
Attribute	VCSEL	FP	DFB
Cost	Low	High
Optical Output Power
Power Consumption
Size	Small (vertical construction)	Large (planar construction)
Mode Stability	Good	Fair
Testing	On chip	Packaged assembly
Manufacturing	Easy (20, 000 devices on 3 inch wafer)	Difficult
Packaging	Easy
Coupling to Fiber	Efficient	Inefficient
Modulation	Direct up to 12 Gbps	Direct only up to 2.5 Gbps then must be external
Drive circuitry	Simple	Complicated
Monolithic Integration	With receiver and electronic driver components	With other optical components
Suppliers	850 nm:  Agilent, Aerius Photonics, Applied Optoelectronics, Inc., Oclaro, Optowell, Emcore, EpiWorks, FCI/MergeOptics, Finisar, JDSU, Raycan, TE Connectivity, VI Systems 1310 nm:  Alight Technologies, Beam Express, JDSU, Raycan, Vertilas, VI Systems 1550 nm:  Raycan, Princeton Optronics, Vertilas	Agilent, Excelight, Finisar, JDSU, Modulight, Oclaro, OpNext	Agilent, Bookham,  JDSU, Excelight, Finisar, Fujitsu, Oclaro, OpNext

Long-wavelength VCSELs have started to emerge again mainly due to some new process technologies now being leveraged. Two companies stand out to me with their technology developments of long-wavelength VCSELs – Vertilas and VI Systems.

SFP+ - The New Optical RJ45?

For those of you that have been in the industry for what seems to be 100 years, but is really about 25 years, you know that the one “connector” that hasn’t changed much is the RJ45. While there have been improvements by adding compensation for the error that was made way back when AT&T developed the wiring pattern (splitting the pair causing major crosstalk issues), the connector itself has remained intact. Contrastingly, optical connectors for datacom applications have changed several times – ST to SC to MT-RJ to LC. They have finally seemed to settle on the LC and perhaps on a transceiver form factor – the SFP+. The SFP was originally introduced at 1G, was used for 2G and 4G and with slight improvements has become the SFP+ and the dominant form factor now used for 10G. Well, it is in the process of getting some slight improvements again and promises to make it all the way to 32G. That’s six generations of data rates – pretty impressive. But how?

The INCITS T11.2 Committee's Fibre Channel Physical Layer – 5 (FC-PI-5) standard was ratified in September. It specifies 16G Fibre Channel. Meanwhile, the top transceiver manufacturers have been demonstrating pre-standard 16G SFP+ SW devices. But, wait a minute – short-wavelength VCSELs were supposed to be very unstable when trying to modulate them at data rates above 10G right? Well, it seems that at least Avago and Finisar have figured this out. New microcontrollers and adding at least one clock and data recovery (CDR) device in the module to help clean up the signals have proven to be keys. Both vendors believe it is possible to do this and not add too much cost to the modules. In fact, both also think that possibly by adding electronic dispersion compensation (EDC) they can push the SFP+ to 32G as well - which is the next step for Fibre Channel - hoping to stop at 20G and 25G to cover developments in Ethernet and InfiniBand.

And what about long wavelength devices? It has always been a challenge fitting the components needed to drive long distances into such a small package mainly because the lasers need to be cooled. But not anymore – Opnext has figured it out. In fact, it was showing its 10km 16G FC SFP+ devices long before any of the SW ones were out (March 2010). Of course, this isn't surprising considering Opnext has already figured out 100G long haul as well.

These developments are important to datacom optical networking for a few of reasons:  

1. They show that Fibre Channel is not dead.
2. The optical connector and form factor "wars" have seemed to subsided so transceiver manufacturers and optical components vendors can focus on cooperation instead of positioning.
3. They will impact the path other networking technologies are taking – Ethernet and InfiniBand are using parallel optics for speeds above 10G – will they switch back to serial?

Stay tuned for more on these points later.

 

25G/40G VCSELs Driving Short-reach Optical Interconnects

Just a few years ago, laser designers were struggling with stability of their 10G VCSELs. But now, at least one, VI Systems GmbH, claims it will have production-ready 40G VCSELs within the next few years. The German start-up has developed two products it believes will take VCSELs beyond 10G applications - a directly-modulated (DM) device and an electro-optic modulated (EOM) DBR VCSEL. Both are short-wavelength (850nm) lasers.

In a recent press release, VI Systems explains that it “developed the VCSEL products at a wavelength of 850 nm along with a range of extremely fast integrated circuits based on the SiGe BiCMOS (silicon-germanium bipolar junction transistors in complementary metal-oxide-semiconductor) technology. The company uses a patent pending micro-assembly platform for the integration of the opto-electrical components and for alignment to a standard high performance multi-mode glass-based fiber.” The start-up has been presenting data supporting its claims of highly stable devices for more than a year now. It gets there by changing the laser active region material and structure to InAs quantum dot (QD).

Not only is VI Systems working on innovative laser structures, it has also developed new electro-optic integration methods to further reduce the cost of these devices.

I’ve noted in previous posts how VCSELs are the key to low-cost optical networks in the data center. These new VCSELs and packaging methods would bring an even more cost-effective “serial” solution for 40/100G. They could also be used for very short-reach optical connections like for chip-to-chip, on-board or board-to-board. Perhaps these inventive products will rival Avago’s MicroPOD and Luxtera’s OptoPHY (also in previous posts). Based on the presentations that VI Systems has released, it sure appears that its management completely understand the needs of both the data center and optical interconnect markets so could very well give incumbents in the industry some competition.

Intel’s Light Peak OR USB 3.0?

After Intel’s Developer’s Forum last week, there is renewed interest in Light Peak. For those of you that don’t remember one of my first blog posts, Light Peak is an Intel-developed optical interconnect techonolgy that uses a new controller chip with LOMF and a 10G 850nm VCSEL-based module with a new optical interface connector that looks very similar to the one used in Avago Technologies MicroPOD transceiver. Light Peak is aimed at replacing all of your external connectors on your PC including USB, IEEE 1394, HDMI, DP, PCIe, etc. It is also targeted at other consumer electronic devices like smart phones and MP3 players.

Many in the industry think Light Peak is intended to replace USB 3.0 even before USB 3.0 is finished being standardized. I tend to disagree. USB 3.0 is a 5G data rate and to me, will bridge the gap between existing USB 2.0 (480 Mbps max) and the 10G that Light Peak can provide. Just because they are being developed at the same time, doesn’t mean they will make it to production simultaneously.

While Intel is now saying that 2011 will be the year for Light Peak to take off, I’m still skeptical. There may be some really high-end applications like video editing that may need this bandwidth, but your run-of-the-mill PC user isn’t going to want to pay the extra money for it when you probably won’t be able to actually detect the improvement. And, what might be more important – what kind of power consumption difference is there and how does this affect battery life?Or is this technology not meant for laptops?  I’m not sure these questions have been sufficiently answered yet.

AOCs (Part 2)

Summary of a few more AOC Implementations:

Avago Technologies had a late entry into the AOC market with its 10GBASE-CX4 replacement and QSFP+ products. But they have a rich history in parallel optics so have quickly come up to speed their products. While they may have been somewhat late to market, Avago has an existing customer base to peddle its wares to.

Finisar’s products include Quadwire and Cwire AOCs to address early adoption of 40G and 100G. Quadwire is Finisar’s mainstream product, both in terms of its use of the VCSEL arrays the company produces in volume at its Texas fab, and in terms of its use of the popular QSFP form factor.

The high end of the Finisar product line is designed to exploit anticipated interest in 100G Ethernet and 12-channel QDR InfiniBand. Cwire offers an aggregate data rate of 150 Gbps and a CXP interface. Not only does this represent the direction of high-end enterprise cluster design, but it allows Finisar to utilize the most integrated VCSEL arrays it manufactures. The 12-channel array also represents the most cost-effective per-laser manufacturing option, allowing Finisar to take advantage of its expertise in designing large VCSEL-arrays. The benefit in high channel count can also be seen in power dissipation. While the single serial channel of Laserwire dissipates 500mW per end, the 12-channel Cwire dissipates less than 3W per end – half the power dissipation per channel.

MergeOptics (now part of FCI) was born of the old Infineon which was once a powerhouse in the optical transceiver markets—both telecom and datacom. It emerged in 2006 with its SFP and then SFP+ products and is now one of the first entrants for 40G and 100G AOCs. Unlike most of its competitors, it is focused on 10G and above products so can bring them to market rather quickly. Its technology is being leveraged for InfiniBand and Ethernet applications.

Stay tuned for the next post for just a little more on AOCs.

Laser-optimized Multi-mode Fiber (LOMF)

It occurred to me as I was writing the last post that many of you may not be aware of the different grades of multi-mode fiber and that for the purposes of this blog, it would be good to present the differences so many of my points can be thoroughly understood.

Right now, there are three standardized types of LOMF as well as what most of us in the industry call FDDI-grade fiber, which is not laser optimized. So first, what does laser-optimized actually mean? In basic terms, it just means that the fiber was designed to be used with lasers, and in the case of MMF, typically VCSELs. FDDI-grade fiber pre-dated the use of VCSELs so is not laser-optimized - it was intended for utilization with LEDs. Lasers were adopted as the light source of choice when scientists and engineers realized that LEDs became very unstable when trying to modulate them at data rates beyond 100 Mbps. They originally tried to use the same lasers that were being used in CD players, but these turned out to be unstable at Gigabit data rates as well. In the early 1990s, the development of the VCSEL enabled these higher data rates.

As the light sources evolved, the fiber progressed with them. So, for 850nm operation today we have four choices: 

1. OM1 (FDDI):  Minimum OFL Bandwidth of 200 MHz•km; 10G Minimum Optical Reach of 33m
2. OM2:  Minimum OFL BW of 500; 10G Minimum Optical Reach of 82m
3. OM3: Minimum OFL BW of 1500; 10G Minimum Optical Reach of 300m
4. OM4: Minimum OFL BW of 3500; 10G Minimum Optical Reach of 550m

As you can see, the bandwidth of the fiber is intimately tied to what type of light source is used and the optical reach is dependent on both bandwidth and data rate. And, while OM1 fiber wasn’t necessarily designed to be used with lasers, it works fine with them, albeit at a shorter distance than with LOMF. Of note as well is the fact that there are a few cable manufacturers that also provide what I would call OM1+ cable that is 62.5-micron, but is laser-optimized, so may have some improved bandwidth and reach.

All this leads to a very important point – when specifying a cabling system for your networks and data centers, it is important to understand not only the fiber you’re going to install, but also the equipment you’re trying to connect. Just because you're "only" installing Gigabit systems and you've used OM1 fiber for years, doesn't mean it's the best solution (or even the most economical) for today and tomorrow.

VCSELs – The Enabling Factor of Fiber in the Data Center

Optical technology proponents have argued for many years that fiber is about to take over all of networking. But, time and time again we have seen copper technologies reinvent themselves to serve at least the last 100 meters in LANs. But with the complicated digital signal processing that is needed to enable the 100-meter operation of copper comes a cost – power consumption. And in data center operations, power consumption may be the single most important issue still needing to be solved.

Before the EPA performed their study on data center power consumption and before the creation of the Green Grid, data center managers were worried more about running out of space than out of power. Now, with complicated electronics and the better utilization of server processing through virtualization, lowering power consumption has become more imperative.

VCSEL-based short-wavelength fiber optic networks may be the answer. As mentioned in a previous post, on a port-by-port basis, 10GBASE-SR devices consume four times less power than 10GBASE-T ones. And when you have thousands of these ports within your data center, the total power consumption adds up quickly. Stay tuned for further quantitative analyses of copper versus fiber in the data center.

Finisar is one of the leading manufacturers of VCSELs, and they sell the short wavelength transceivers to LAN and SAN equipment providers including Brocade, Cisco, QLogic , HP and EMC. While transceivers generally sell at very low margins, they are an essential part of keeping costs down and power consumption low in data center networks.

Contributing Analyst - David Gross.

Quantum Dot Lasers Now Reality

In recent research I conducted for the Optical Interconnect report I wrote for CIR, I found some encouraging news on quantum dot lasers. Measuring 20nm in diameter, a quantum dot (QD) is defined as a semiconductor whose electrons are confined in all three spatial dimensions. As a result, it has properties that lie somewhere between those of bulk semiconductors and those of discrete molecules. QDs have been studied for a wide range of applications such as transistors, solar cells, displays, medical imaging, optical amplifiers, sensors, drug delivery and light emitters (both LEDs and diode lasers). QDs also could be used as the physical "incarnation" of qubits in quantum computing R&D and in quantum encryption systems. All of these applications are based on the fact that QDs are zero dimensional, which gives them superior transport and optical properties. They also need very little power.

QD Laser, a Japanese firm backed by Fujitsu Limited, Mitsui Ventures and Mizuho Capital Co., Ltd, announced what I believe is the first commercially viable QD laser in March 2009. Since then, it has added several products to its portfolio. They include FP/DFB laser chips, TO-can and TOSA and wide-band SOA butterfly components. The lasers have capability to run at data rates up to 10 Gbps. These devices are well suited for datacom and telecom equipment.

It seems that QD lasers may have future applications in chip-to-chip optical connections. They may also have applications outside of telecom in sensors and in future quantum encryption/quantum computing systems. In addition to QD Laser's devices, Taiwanese researchers have built tunable QD VCSELs. Also, VI Systems (VIS), a German-based start-up, is working on QD-enhanced VCSELs. This components company has recently received substantial funding and we note that at recent industry conferences VI Systems presented a paper on 25-Gbps VCSELs that were rendered temperature insensitive with the use of QDs. VIS has recently released a product catalog of TOSAs, ROSAs, VCSELs, PINs, arrays, TIAs, VCSEL drivers and high-speed test boards that utilize its technology. Its products are suitable for 850nm 25G and 40G operation. It seems as though this may be one of the only currently viable solutions for stable operation of VCSELs beyond 10G.

Optical Interconnects for All-Optical Networking May be Closer to Reality than You Think

On-board interconnects have for some time just been handled with copper traces, but with data rates now reaching beyond 10G, this is ripe for change. In fact, it is already changing; evidenced by the big splash IBM and Avago Technologies made at this year's OFC/NFOEC conference. The computer giant and transceiver manufacturer teamed to develop what they are calling "the fastest, most energy-efficient embedded interconnect technology of its kind."

Deemed the MicroPOD™, Avago developed it for IBM's next generation supercomputer, POWER7™. While it was designed for HPC server interconnects, it could be used for on-board or chip-to-chip interconnects as well. The devices use a newly designed miniature detachable connector from US CONEC called PRIZM™ LightTurn™. The system has separate transmitter and receiver modules that are connected through a 12-fiber ribbon. Each lane supports up to 12.5 Gbps. It uses 850nm VCSEL and PIN diode arrays. The embedded modules can be used for any board-level or I/O-level application by either using two PRIZM LightTurn connectors or one PRIZM LightTurn and one MPO.

While these modules are currently for the HPC market, Avago designed something very similar for Intel and its Light Peak interconnect system (see previous post for details) for what some are calling “optical USB.” MicroPOD is targeted at high-density environments so a natural extension of its market reach would be into switches and routers. The market for such devices probably will not become huge in the next few years, but it is exciting to see that companies in this space have started to spend R&D dollars again and that there are at least a few customers willing to employ the technology right out of the gate. Of course, it must be noted that this project was partially funded by DARPA.

But this technology MUST be too expensive for the typical piece of network equipment right? Not so, says Avago, because the manufacturing process is 100-percent automated and with Avago's vertical integration, the prices (at volume) may actually be able to rival those of today's transceivers. I’ll hold judgment until Avago proves it can win more than one big customer, however, I think MicroPOD holds the promise to change the paradigm for on-board, board-to-board and even network-element-to-network-element optical interconnects.