Coherent modulation. Enhancing spectral efficiency in modern networks.

From bulky CFP interfaces to compact QSFP-DD optical modules.

Coherent modulation uses three parameters to encode the transmitted data – amplitude, phase and polarization. It requires complicated real time mathematical operations done on the received signal to decode the data stream. Because of that, DSP – Digital Signal Processor – must be used in modules with coherent modulation. Before the QSFP-DD era, coherent modules were bulky and power-intensive, utilizing CFP interfaces.

However, these devices were impractical for routers due to their size and power supply requirements. DSP accounting for nearly half of the coherent module's power consumption, so with advancements in CMOS technology, it has become feasible to develop DSPs that can fit comfortably within the compact housing of QSFP-DD 400G and 800G modules, ideal for achieving high port density in routers and switches.

Nowadays we even have specialized DSP for QSFP28 100G coherent modules, with power consumption below 2W and 5.5 W for the whole optical module. 
According to Shannon law defined more than 70 years ago we are not able to rise the throughput of the transmission channel indefinitely. Limitations are signal-to-noise ratio and the channel width. Coherent modulation is the most efficient way to maximize the spectral efficiency – the data rate of the single channel divided by the channel bandwidth. 

Applications:

• 100G DWDM coherent
• All 400G and all 800G
• 100G, 400G, 800G DWDM
• Short, Metro and Long Haul