ADCs key for high baud-rate coherent systems
Increasing the baud rate of coherent modems benefits optical transport. The higher the baud rate the more data can be sent on a wavelength, reducing the cost-per-bit of traffic.
But engineers have become so good at designing coherent systems that they are now approaching the Shannon limit.
At the OFC show earlier this year, Ciena showcased a coherent module operating at 107 gigabaud (GBd). And last year, Acacia, now part of Cisco, announced its next-generation 1.2 terabits-per-second (Tbps) wavelength coherent module operating at up to 140GBd.
The industry believes that increasing the baud rate to 240+GBd is possible, but each new symbol-rate hike is challenging.
All the components in a modem - the coherent DSP and its digital-to-analogue (DAC) and analogue-to-digital (ADC) converters, the optics, and the analogue drive circuitry - must scale in lockstep.