Coherent optical transmission or coherent optics (coherent WDM) is a method that uses a variation of the amplitude and phase or segment of light and transmission across two polarizations to transport significantly more information through a fiber optic cable. Coherent optics also provides faster bit rates, greater flexibility, modest photonic line systems, and advanced optical performance.
This technology forms the basis of the industry’s drive to embrace the network transfer speed of 100G, 400G and beyond while delivering terabits of data across one fiber pair. It also allows for increased scalability from 100 to 400G and beyond for every signal carrier. This delivers more data throughput at a relatively lower cost per bit.
Using digital signal processing at both the transmitter and receiver, coherent WDM technology delivers cost-effective and highly reliable optical transport in Dense Wavelength Division Multiplexing (DWDM) networks.
In typical optical fiber communication systems, detection of optical signals is based on a direct measurement of the intensity of the signal – called “Direct Detection” (DD). Similarly, a direct modulation of the signal between an “on” state of intensity, and a lower “off” state is used to encode the information sent by a transmitter – this scheme is referred to as “Intensity Modulation” (IM). Together, this approach is known as “IM-DD”, a method that has been widely used for optical channels with rates up to and including 25Gbps.
Compared with these traditional or legacy IM/DD optical system, the coherent optical communication system has the advantages of high sensitivity, long transmission distance, and large communication capacity. Coherent technology improves the transmission capacity of optical communication systems to a higher level.
Coherent optics are packaged as standardized coherent pluggables into optical systems. Coherent pluggable solutions supporting data rates from 100G to 400G are performance-optimized systems are designed for small spaces and are low power, making them highly attractive for many transport and networking applications from the core to the metro and access edge of communications networks. Examples of optical networking standards utilized in coherent optics pluggables are 400G ZR, Open ROADM, and ZR+ intended for multi-vendor and open optical systems.
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