Terabit WDM over multi-mode fibre



Using hyperfine WDM via a multi-port interleaver enables 320 C-band channels.

By Mark Telford

In 1996 wave division multiplexing (WDM) enabled replacement of "out-of-capacity" networks with a new layer of functionality for long-haul. But obtaining 320 channels in the C-band requires seven interleavers and eight 40-channel DWDM filters, costing about USD250,000.

Essex Corp's Hyperfine WDM passive optical mux/demux technology achieves this with a single multi-port interleaver which costs up to 50% less and operates within the same loss budget.

CIR's report Why optical networking will return to its roots, says "service providers will look to maximise usage of the traditional C band with narrower channel spacing." This allows use of existing installed optical amplifiers rather than the parallel amplifiers and supporting band-splitting filters needed for expansion into the L band. Also, at higher speeds (such as a single long-haul 40Gbit/s channel), chromatic and polarisation dispersion become difficult and costly to overcome, and existing amplifiers may not meet the new requirements for signal levels and distance.

Hyperfine WDM is capable of channel spacing from the current 100 or 50GHz to as narrow as 50MHz, increasing capacity by a factor of 16, enabling new services and solving increasingly urgent requirements in long-haul, metro and access networks.

The initial proof-of-concept breadboard was demonstrated at 250MHz, while lab and field trials have been conducted with a 16-channel 6.25GHz product prototype, with a 100GHz free spectrum range.

Long-haul: At 2.5Gbit/s (and 6.25GHz channel spacing), 640 OC-48 C band channels (or sub-bands) can be distributed, enabling 16 channels to be multiplexed for virtual 40Gbit/s OC-768 speeds (but with 256 times less dispersion, which squares with bit rate) into one 100GHz channel on a traditional DWDM filter. This allows greater reach on legacy fibre without regeneration. Multiplexing in a 40-channel system gives 1.6Tbit/s. With a 3.125GHz mux, 1280 GbE channels are possible in the C-band alone.

Access: Used in front of DWDM with 100GHz spacing as a pre-channeliser, Hyperfine WDM is a multi-port interleaver (replacing 15 stacked interleavers), creating up to 640 2.5Gbit/s channels.

Used after DWDM as a post-channeliser, it can sub-channelise a single 100GHz channel into multiple 2.5Gbit/s data streams. Cumulative insertion loss is reduced, as it remains constant regardless of port count.

One 12.5GHz-spaced 16-channel device and a 200GHz Free Spectral Range can be configured as a multi-port interleaver, multiplexing 320 12.5GHz-spaced channels. Each output fibre has 12.5GHz channels spaced by 200GHz. Instead of using 100GHz DWDM multiplexers, such as 40-channel AWGs, each interleaver can be paired with 16 20-channel TFF devices with 200GHz spacing.

Narrowband channelisers allow thousands of wavelengths from a central office to be dedicated to the customer premises.

Metro: Integrated Hyperfine and MEMS or liquid crystal optical switching creates a dynamically configurable add/drop multiplexer. This enables simple, real-time provisioning, offering NxX redundancy and bandwidth-on-demand services.

Market outlook
The first product, a 16-channel demux with 6.25GHz channel spacing, is in customer trials with equipment manufacturers in Europe and North America. Potential partners are looking at 12.5GHz channels for 10Gbit/s.

At Supercomm 2002, Essex Corp says it will demonstrate transmission of multiple OC-48 signals at 6.25GHz spacing using Fiberspace's Gridlocker laser. At this spacing, 1.6Tbit/s of data can be transmitted within the S, C or L bands using existing fibre.

The first alpha unit should be completed in Q3/2002 with integrated beta testing in Q4.

In April, the University of Essex, UK, announced that - with the University of Cambridge - it had employed Essex Corp's hyperfine WDM technology in a test bed to set a "record" throughput of 1.2Tbit/s over 3km of conventional graded-index multi-mode fibre.

A 6.25GHz-spaced filter transmitted OC-48 data with good "eye" diagrams on 12 channels across each 100GHz band of a 40-channel AWG. It was used at the launch end without adjacent channel interference.

"The test shows hyperfine WDM's ability to bring ultra-dense WDM to the large amount of legacy multi-mode fibre on government and commercial LANs and campus network," says CTO Terry Turpin. "The ability of wavelength-per-user applications using multi-mode fibre could be the catalyst for deployment of economic FTTH applications."

In March a co-marketing agreement was reached for Essex's hyperfine WDM and the Gridlocker tunable DFB laser source of Fiberspace Inc, which has an accuracy of ±250MHz based on its Optical Phase Locked Loop (OPLL) stabilisation technology.

The improved performance enables maximum spectral efficiency, lowering sparing costs and leveraging existing network infrastructure.

Furthermore, the University of Central Florida (UCF) School of Optics/CREOL and Sarnoff Corp has reported use of a hyperfine WDM demux to separate discrete wavelengths from a laser configured to generate 45 lines at a spacing of 12.5GHz onto individual fibres. Channel-to-channel stability was 100Hz. "A multiplicity of optical carriers can be generated by single gain-element sources," said Sarnoff's Head of Photonic ICs and Components Joseph H Abeles.

"The possibility of getting many more channels over existing fibre as well as the cost savings of using one laser to service multiple customers would be an important step towards wider deployment of WDM in metro and access markets," says Mark Lutkowitz, VP of Optical Networking Research for CIR Inc.

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