Coarse WDM opens the road beyond very-short-reach markets

Oct 1st, 2001
73029

Michael LaHa

In metro and access markets, CWDM will offer solutions for 850-, 1300-, and 1500-nm applications at 10 and 40 Gbit/s. The author argues that, based on a parallel optical subassembly architecture, it will support the eventual 100-Gbit/s Ethernet standards.

Coarse wavelength-division multiplexing (CWDM) is emerging as a robust and economical transceiver solution in an industry traditionally dominated by serial components. The advantage of CWDM technology, as a whole, is its ability to leverage conventional technology to make the jump to next-generation data rates. These jumps, typically taking less time, are pushing system vendors to reconsider the development timelines for equipment using upcoming standards-based optical modules.

CWDM differs from dense wavelength-division multiplexing (DWDM) in that the optical channel spacing between the light sources that are multiplexed into a single fiber is much wider. Additionally, CWDM transceivers use optical multiplexing for achieving serial-equivalent data rates, whereas DWDM multiplexes many serial data streams to achieve bandwidths on the order of hundreds of gigabits per second. Generally speaking, this is achieved by using temperature to control the DWDM channel spacing to 100 GHz. This exact control over the spacing allows up to 80 separate channels to be multiplexed.

The DWDM process is not only impressive, it is expensive. In contrast, typical CWDM spacing is on the order of many nanometers (several terahertz) and does not require temperature control. Consequently, CWDM transceivers with no temperature control, having directly modulated lasers, and incorporating lower-speed components to achieve higher data rates are typically lower in cost, size, and power consumption than their conventional serial counterparts.

Initial implementations of CWDM in optical transceivers focus on the LAN and SONET very-short-reach (VSR) market. Pushing this effort is the availability of low-cost optical sources—namely 850-nm multimode vertical-cavity surface-emitting lasers (VCSELs)—and the significant amount of multimode fiber already installed in the LAN and enterprise environments. Rather than install new single-mode fiber to handle links of less than 2 km, system vendors and site managers seek low-cost, high-data-rate solutions that take advantage of the installed multimode fiber. CWDM fits this bill in its ability to transmit a higher aggregate data rate farther, with lower-speed multimode components (see Fig. 1).

GIGABIT ETHERNET VIA EIGHT WAVELENGTHS
On the transmit side, eight wavelengths from directly modulated VCSELs are multiplexed optically into a single multimode fiber. The eight wavelengths are sent through the multimode fiber to the receiver with the demultiplexer that separates the channels out to individual photodetectors. Each of the eight channels transmits data at 156 Mbit/s. Although the aggregate signal rate is 1.25 Gbit/s, accounting for 8-bit/10-bit encoding, the lower data rate on the individual channels allows data transmission over 2 km on 62.5-µm multimode fiber. As for 850-nm serial transmission of Gigabit Ethernet data, the IEEE specification in the 802.3z standard is only 220 m over 62.5-µm diameter fiber (see Fig. 2).

The wavelengths of the individual lasers are nominally 778, 789, 800, 812, 825, 837, 850, and 864 nm at room temperature. As the temperature changes in the module, the eight wavelengths will not overlap since the optical spacing between each channel is wide. This spacing prevents the overlap of data between adjacent channels. Multiplexing and demultiplexing the eight channels into and from a single fiber is performed via a zig-zag geometry. The eight optical elements, VCSELs at the transmit end, and photodiodes at the receive end, are mounted onto a substrate. Next, the optical multiplexer/demultiplexer (mux/demux) block is placed on top of the substrate (see Fig. 3).

The mux/demux block has eight bandpass filters on the bottom that line up over the active components and a broadband coating on the top to reflect all of the wavelengths. As the laser light zig-zags through the filter block, channels are injected into the data stream in the mux and ejected from the data stream in the demux. The width of the bandpass filters ensures that as the laser wavelength drifts over the transceiver's operating temperature, the entire signal is captured (see Fig. 4).

Putting both transmit and receive optical subassemblies (OSAs) into one package makes a full transceiver. Additionally, because no temperature regulation is required for the device to maintain an exact wavelength and the lasers are directly modulated, the entire transceiver can be packaged in an industry-standard GBIC part. This packaging is relevant in that standard routers and switches with a GBIC interface can be used in campus and enterprise environments that have legacy multimode fiber implemented over distances greater than 200 m.

FOUR-CHANNEL ARCHITECTURE
Coarse wavelength-division multiplexing is also well-suited for transceivers with data rates at 10 Gbit/s. Because of the abundance of low-cost 850-nm VCSELs at 2.5 Gbit/s (OC-48), it makes sense to build a CWDM device based on a four-channel architecture (4 x 2.5 Gbit/s). Using a similar zig-zag mux/demux as described for the eight-channel gigabit per second device, four 850-nm variety VCSELs and photodiodes may be integrated to yield 10-Gbit/s aggregate data transmission. However, in this case, the optical spacing is set at 20 nm between channels. This spacing, in conjunction with the four corresponding bandpass filters, ensures enough "elbow room" for the lasers to move optically with temperature as the mux heats or cools.

An added benefit of the four-channel CWDM transceiver is the convenient electrical interface it provides. For Ethernet LAN applications at 10 Gbit/s, a popular electrical interface between the media access-control layer and the physical layer is the extended attachment-unit interface (XAUI). The XAUI, which is gaining popularity with board designers, is a four-bit-wide interface running at 3.125 Gbit/s per lane. With four-channel CWDM at the physical layer, all that is required to send data through the link is that the four lanes of XAUI interface to the four laser drivers in the CWDM mux (or TIA/limiters in the demux). Naturally, the lasers must operate at 3.125 Gbit/s for this to work; however, this has not posed a problem since several VCSEL manufacturers started shipping lasers that operate with this 25% overhead above the standard 2.5 Gbit/s.

So what does this mean? In short, no serializer/deserializer (SERDES) is required between the media access-control layer and the physical layer. In addition, because directly modulated lasers have difficulty running much faster than 10 Gbit/s, a 4:1 XAUI SERDES would not work since the serial transmission rate would be 12.5 Gbit/s of encoded data. For serial solutions to work robustly, the data would first have to be decoded from the four XAUI lanes running at 3.125 Gbit/s and recoded so the serial transmission rate is 10.3 Gbit/s, which is an acceptable rate for directly modulated sources.

Therefore, serial solutions interfacing to XAUI require two additional functions not necessary in CWDM: a 4:1 SERDES and a decoder/encoder function. Similarly, for SONET VSR, which could use the same 4 x 2.5-Gbit/s architecture, CWDM requires less complicated chips at the framer and physical layer interface. For OC-192 VSR, a 16:4 SERDES, rather than 16:1, is required. This criterion means gearbox chips based on standard complementary metal-oxide semiconductor technology, rather than on silicon germanium or indium phosphide, could get the job done. Consequently, CWDM solutions will typically have smaller packages, lower power, and lower total cost—design necessities in today's market.

Coarse wavelength-division multiplexing transceivers provide access to higher bandwidths with conventional active optical components, are lower-power than serial solutions, and typically result in shorter development times. The 850-nm solutions available today range in performance from 1 to 10 Gbit/s and are offered in various industry-standard footprints and electrical interfaces.

WHAT'S NEXT FOR CWDM?
Three variables will define the new CWDM data links coming to the market in the future: longer distance, faster speeds, and higher density.

Short reach is really only the beginning for CWDM. Long-wavelength modules at 1300 and 1500 nm will carry low-cost CWDM solutions into the metro environment, where 10 km will be the minimum link length. In fact, one of the 10 Gigabit Ethernet standards in the IEEE 802.3ae is a four-channel, 1300-nm CWDM solution. While the focus of Ethernet has historically been on the LAN environment, the adoption of single-mode solutions in conjunction with work being done on the wide-area network physical layer will push Ethernet into areas where SONET has traditionally dominated.

In addition, OC-768 VSR for SONET is just around the corner. Coarse WDM, which has the benefits of a single-fiber solution combined with the economy of parallel fiberoptics, will be a dominant player in the OC-768 VSR market. A most likely configuration for 40-Gbit/s CWDM transponders will be a 4 x 10 Gbit/s layout—simply a scaled-up version of the OC-192 VSR. However, with the lasers running at 10 Gbit/s, the transmission distance for 850-nm sources will only reach approximately 70 m.

While a large number of applications can utilize such a budget, a more adequate light source for OC-768 VSR will be 1300- or 1500-nm sources. The 1300-nm sources, whether distributed-feedback lasers or the newly emerging long-wavelength VCSELs, will ensure transmission throughout the VSR link specification. In addition, 1500-nm lasers, robust from years of use in the DWDM market, will propel 40-Gbit/s CWDM out into the metro area with transmission distances in the range of 40 km.

An added benefit of using 1500-nm range lasers in a four-channel CWDM configuration is that the individual channels, each running at 10 Gbit/s, will not face the same physical constraints associated with serial 40 Gbit/s. The problems of chromatic dispersion and polarization-mode dispersion are quantified as if the optical link were running at 10 Gbit/s. Consequently, links of 40 km in a 4 x 10 Gbit/s CWDM environment will have no need for dispersion-compensation equipment. This fact will result in significant savings to carriers implementing 40-Gbit/s CWDM in the metro market.

Last, an impact will be made on the optical backplane market when CWDM is combined with parallel optics. Using the same 10-Gbit/s OSAs described in the earlier example, a high-density data link is possible when the output from each OSA is launched into a ribbon fiber array (see Fig. 5).

These links will be well-suited for transporting large amounts of data in the backplane as required in a terabit router application. Configurations are scalable to the number of fibers in the ribbon and the speed of the OSAs launching into the fiber. For example, using a 12-channel ribbon fiber, 12 CWDM OSAs would be used with four wavelengths being launched into each fiber of the ribbon. The 48 addressable channels would create a 120-Gbit/s parallel link in one compact package. If the OSAs were all 40 Gbit/s (4 x 10 Gbit/s), the 12-channel parallel link would scale to 480 Gbit/s.

THE NEXT STEP
As the line between datacom and telecom continues to blur, CWDM is positioned to help advance the standards in either convention. The technology is accelerating the development of future optical links by leveraging off of conventional components that are readily available. New solutions are turning out to be low cost, low power, and packaged in the same industry standard multisource agreements such as XENPAK, the 300-pin OC-192 module, and XGP.

While not posing an immediate challenge to high-capacity DWDM solutions, CWDM will begin to encroach upon areas previously thought to be controlled by traditional long-haul technology, namely the metro market. Solutions will exist for VSR, whether 850- or 1300-nm-based, and for metro and access applications with 1500-nm devices. The 40-Gbit/s solutions will fit well in the SONET environment, keeping costs and power consumption low, as in the 10-Gbit Ethernet solution, through its more desirable electrical interface. Moreover, CWDM will be ready to usher in the 100 Gigabit Ethernet standard with a parallel OSA architecture.

Michael LaHa is product manager at Blaze Network Products, 5180 Hacienda Drive, Dublin, CA 94568. He can be reached at mlaha@blazenp.com.

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