Fujitsu Laboratories Ltd. and Fujitsu Research and Development Center Co., Ltd. of China have been working on an optical technology that can achieve 100Gbps transmission speeds using commercial components intended for 10Gbps networking. The modulation technique they have adopted is Discrete Multi-Tone (DMT), which is widely used in DSL equipment but is not usually applied to optical systems. Details were presented at OFC/NFOEC in Anaheim, California, in March.
The speed that can be achieved with the simple modulation formats in use today is limited, says Fujitsu. In DMT the data is divided into multiple frequency subcarriers (carrier waves), and multi-level modulation is used on each subcarrier to achieve high transmission speeds.
The modulation level assigned to each subcarrier depends on the noise characteristics at that frequency, to enable robust transmission even at frequencies with poor signal-to-noise ratios. An algorithm was developed that checks the receiving conditions between optical transceivers at the time the system is started up and optimizes the power and other variables to suit the transceivers and line conditions.
This “adaptive bit rate allocation” enables transmitters originally designed for lower speeds to be used. For example, some components have the characteristic of not being able to maintain sufficient linearity. These components can be profiled in advance and compensated for, enabling them to be used in the same way as parts that do maintain linearity, according to Fujitsu.
Fujitsu has applied DMT modulation/demodulation using digital signal processing (DSP) to transmit 100Gbps per channel using components designed for transmission speeds of 10 Gbps per channel. Combining four 100Gbps channels, the company then created a 400 Gigabit Ethernet transceiver, which was shown in a transmission demonstration at OFC/NFOEC. 400GbE transceivers are being developed to enable the next generation of data centres.
Fujitsu says it is turning its attention towards more practical applications of optical DMT technology by developing an integrated DMT modem, along with further improvements for using existing components.
By Pauline Rigby
Fujitsu Develops First Optical Transmission Technology to Achieve 100 Gbps Using 10 Gbps Transmission Components