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30 March 2012
An alternative 100Gbps transmission scheme, based on direct detection and tailored for metro and private networks, is being backed by more system and optical module vendors.


ECI Telecom and chip vendor MultiPhy have developed a 168-pin 5x7-inch 100Gbps direct-detection module while Finisar and Oclaro have announced similar technology in a CFP pluggable format.  The three join ADVA Optical Networking, which late last year announced the industry's first 100Gbps direct detection product. 
Coherent technology is the de-facto standard for 100Gbps long-haul transmission. The 100Gbps direct detection scheme offers a cost-conscious alternative for simple ring metro networks and for linking data centres, with a reach of between 500 and 800km. 
This 100G approach runs well alongside existing 10 and 40Gbps wavelengths. "With this technology, operators can stay with their existing networks yet benefit from 100-Gig high-capacity links," said Shai Stern, CTO of ECI Telecom.
The announced 100Gbps direct detection designs use 4x28Gbps channels and optical duo-binary (ODB) modulation. MultiPhy also promotes an 80km point-to-point variant that uses on-off keying.
The modules use a 10x10Gbps electrical interface input, for a CFP interface or the 168-pin line-side MSA, and a gearbox IC that translates between the 10x10Gbps electrical interface and the four 28Gbps channels feeding the optics. 
The use of optical duo-binary allows 10Gbps transmit optics to be used. MultiPhy's receiver chip runs the maximum likelihood sequence estimation (MLSE) algorithm that compensates for signal distortions when overdriving 10Gbps receive optics (See MultiPhy targets the 100G metro market with MLSE receiver chip). As a result, the chip allows 100Gbps to be implemented using 4x10Gbps transmit and receive optics.
ECI Telecom says that it has yet to decide whether it will adopt 100Gbps direct detection as a product. It will first provide the design to operators for testing and will gauge their interest. Meanwhile ADVA's product is in production, MultiPhy expects its chip to appear in first products in Q3, while Finisar and Oclaro have samples and will ship 100Gbps direct-detection CFP modules by year-end.
By Roy Rubenstein