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19 October 2011
 
Infinera has demonstrated its latest DTN-X system transmitting a 100 Gbps light path over a 9,500 km submarine link linking California to Japan. Working with Pacific Crossing, a subsidiary of NTT Communications, the system vendor claims this is the first time 100 Gbps coherent transmission has spanned the Pacific.
 
A challenge with submarine transmission is that the optical signal can only be amplified along the link; optical regeneration using optical-electrical-optical conversions cannot take place undersea. Yet Infinera's systems, based on photonic integrated circuits (PIC), differ from other optical transport systems by using regeneration as the default mode of operation. With 9,500 km transmission, Infinera has demonstrated that its PIC-based coherent system can meet the longest spans.
 
Submarine links are engineered to a higher specification than terrestrial networks, with optical amplifiers spaced 40-km apart typically rather than terrestrial's 80 km. The Pacific Crossing link used for the trial has 200 optical amplifiers.
 
Infinera also used dual-polarisation binary phase-shift keying (DP-BPSK), part of its FlexCoherent technology, for the trial.  Using BPSK approximately doubles the signal's reach but at the expense of halving the data rate when compared to dual-polarisation quadrature phase-shift keying (DP-QPSK), the de facto 100 Gbps modulation scheme. Therefore, DP-BPSK requires two 50GHz channels to send the 100 Gbps signal, rather than one 50GHz channel using DP-QPSK.
 
To further boost reach, the trial used soft-decision forward error correction (SD-FEC), which adds 2-3 dB of optical performance – again near-doubling reach – compared to existing hard-decision FEC. Infinera says SD-FEC will be available in the future on its DTN-X platform.
 
Infinera also trialled sending a 100 Gigabit Ethernet signal over two 40 Gbps light paths using its FlexCoherent transmission. Here Infinera’s existing DTN platform's 40 Gbps optical line module and 100 Gigabit Ethernet client interface were used.
 
The 100 Gbps DTN-X optical transport platform was announced in September 2011 and will become available in the first half of 2012.
 
By Roy Rubenstein