SFP40G and 100G device modulation mode debate

　　There are four modulation schemes currently used for 40 Gbps: fiber duobinary, differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK), and the emerging dual-polarization quadrature phase shift keying (DP- QPSK). DPSK and DQPSK offer excellent reachability and dispersion performance, but their designs are complex and expensive. DP-QPSK has been adopted by several operators, and Mintera Corporation and JDS Uniphase Corporation have developed it into a 300-pin LFF transponder. Mintera has launched its 300-pin LFF adaptive transcoder, and claims that DPSK will dominate the 40 Gbps module market, "DQPSK has entered the Japanese and Chinese markets, but 90% of current shipments are still DPSK."

　　Opnext offers four types of 40 Gbps transponders: duobinary, DPSK, continuous mode DPSK variants, and DQPSK. According to the Ovum survey, duobinary has the lowest cost, followed by DPSK. But the question for transponder vendors is what to do next, should you bet on DQPSK or 40 Gbps coherent DP-QPSK designs?

　　However, material cost is only one of the bottlenecks, and fiber performance is the more important constraint. DQPSK has excellent dispersion performance, but if used concurrently with 10 Gbps, the position of the 40 Gbps coherent channel must be managed.

　　Another concern is 100 Gbps modules. DP-QPSK is an industry-recognized modulation scheme for 100 Gbps, and given the commonality between 40 Gbps and 100 Gbps coherent designs, the relative cost of the two is the winning factor. Rafik Ward, vice president of marketing at Finisar, said the focus is on the economics of 40G vs 100G. "If more affordable 100G coherent designs come out, is there still room for 40G?"

　　At the same time, designers are continuing to shrink the size of existing 40 Gbps modules in an effort to dramatically increase their system capacity. A 7x5-inch 300-pin LFF transponder requires its own line card, so a 40 Gbps link requires two system line cards: one for short reach for the client interface and one for the line-side transponder.

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