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21 March 2012
Verizon and NEC Corp. of America have performed what they claim is the highest capacity optical field trial to date. Optical signals with a total capacity of 21.7 Tbps were successfully transmitted over 1,503 km (934 miles) of standard singlemode fibre on Verizon's network in the Dallas area.

Although transmission capacities as high as 100 Tbps have been reported, these experiments took place in the laboratory.  Previously reported fibre capacities in field trials have been below 10 Tbps.

The capacity-distance product (32.6 Pbps*km) is also the highest reported to date in real world conditions in the field, the companies claim.
The results were described in post-deadline paper PDP5D.6 presented at OFC/NFOEC in Los Angeles, and titled “21.7 Tb/s Field Trial with 22 DP-8QAM/QPSK Optical Superchannels Over 1,503-km of Installed SSMF”.
 “It is important to investigate how much capacity network operators can achieve with the deployed fibre infrastructure, since costly large-scale network wide fibre upgrades are not expected any time soon,” the authors wrote in the paper.
The field trial demonstrates how “superchannels” – groups of closely spaced optical carriers – and coherent detection can be used to boost the capacity of installed fibre. The 22 optical superchannels were generated using multiple 12.5-Gbaud subcarriers, with 16 subcarriers on odd superchannels (giving 1.2Tbps capacity) and 14 subcarriers on even superchannels (1.02Tbps). 
This trial also used a flexible approach to modulation. The majority of the subcarriers were modulated using the same format, dual-polarisation with 8-point quadrature amplitude modulation (DP-8QAM). However, the last four channels on the short-wavelength side of the spectrum had a low optical signal-to-noise ratio (OSNR), which could not support DP-8QAM over the required distance, so the modulation format was switched to DP-QPSK (dual-polarisation quadrature phase-shift keying).
“Having the ability to administer different modulation formats based on OSNR performance allows us to use a larger portion of the spectrum with adjustable spectral efficiency, to ensure signal performance, and to obtain the highest total capacity possible,” the researchers said.
The field trial builds on the work done by Verizon and NEC last year when they showed that 100G, 450G and 1T superchannels could coexist on the same fibre (see "Entering the Terabit Era: Verizon, NEC complete high-speed field trial").  Verizon says it continues to deploy 100G on its global network.
By Pauline Rigby