As network bandwidth demand grows at an unprecedented rate, more complex technologies are in place to increase optical bandwidth cost-efficiently through wavelength-division multiplexing (WDM). Using sophisticated digital signal processors (DSPs) and advanced photonics, coherent WDM technology has revolutionized DWDM transport, enabling wavelength speeds to go from 10 Gb/s in the pre-coherent era to 100 Gb/s, 200 Gb/s, and now even 400 Gb/s as well as 800 Gb/s with the latest coherent optical equipment.
Coherent WDM technology refers to advanced optical techniques that use modulation of the amplitude and phase of the light, as well as transmission across two polarizations, so that significantly more information can be transported through a fiber optic cable. Using digital signal processing at both the transmitter and receiver, coherent WDM technology delivers cost-effective and highly reliable optical transport in DWDM networks.
When WDM was first introduced in the mid-1990s, the typical wavelength data rate was 2.5G. The move to 10G wavelengths was enabled by high-speed optical modulators and better chromatic dispersion management.
The advent of coherent WDM technology enables 100GbE transport over backbone optical networks and a 10X scaling of network or fiber capacity without any change in DWDM channel spacing or DWDM common equipment design.
Coherent WDM technology offers higher bit-rates, greater degrees of flexibility, simpler DWDM line systems, and better optical performance.
Coherent WDM technology helps streamline DWDM network planning and deployment, thanks to soft-decision Forward Error Correction (FEC), a method of encoding the original signal with additional error detection and correction overhead information to detect and correct errors that occur in the transmission path.
FEC provides more margin, allowing higher bit-rate signals to go farther distances with fewer regenerator points. This results in simpler photonic lines, less equipment, lower costs, as well as a significantly larger bandwidth in a DWDM network.
Spectral shaping is also a common technique of coherent WDM technology when deploying a DWDM network. It is a way of applying dynamics processing across the frequency spectrum. Thus it can help bring balance to the sound of instruments and voices in a way that traditional compressors and equalizers have not been able to in the past.
Spectral shaping provides greater capacity across cascaded Reconfigurable Optical Add-Drop Multiplexers (ROADMs), enabling increased spectral efficiency for DWDM channels. As a critical technique in flexible WDM grid systems, it allows carriers to be squeezed closer together to maximize capacity.
Coherent WDM technology can be tailor-made for a wide variety of DWDM networks and DWDM applications. Coherent optical line cards can support multiple modulation formats and different baud rates, enabling operators to choose from a variety of line rates. Fully programmable coherent WDM transceivers provide a wide range of tunability options with fine granularity between incremental capacities. Network operators can make use of all available capacity and convert excess margin into revenue-generating services.
When optical signals are transmitted across a fiber cable, there are inevitably fiber impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). By mitigating dispersion effects, the advanced digital signal processors (DSPs) in coherent WDM technology take away the headaches of planning dispersion maps and budgeting for PMD. This also removes the cost and latency of dispersion compensation modules (DCMs).
In addition, coherent processors improve tolerances for Polarization-Dependent Loss (PDL) and track the State of Polarization (SOP) to avoid bit-errors due to cycle slips that would otherwise affect optical performance. As a result, operators can deploy line rates up to 400G per carrier across longer distances than ever. High bit-rate signals can even be deployed on old fiber that previously couldn’t support 10G.
Over the past few years, some fundamental coherent WDM technologies have been successfully deployed and applied to DWDM networks.
In the early 2000s, many optical experiments were aimed at increasing the data rate per WDM channel beyond what was possible using 10G direct detection (IM-DD). Phase shift keying modulation, such as differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK), were favored because compared to IM-DD, there is a significant advantage in the required optical signal-to-noise ratio (OSNR).
In addition, by encoding more amplitude or phase changes in the carrier, it is possible to increase the number of bits carried in each symbol, and the sensitivity to fiber impairments relates to the symbol rate (not directly to the bit rate).
Fiber can be regarded as a circular waveguide and it supports two orthogonal polarizations. By using polarization multiplexed (PM) carriers to selectively transmit modulated signals, we can effectively double the spectral efficiency of a given modulation technique while using the same PM receiver.
By using polarization multiplexing, the effective symbol rate can be reduced to half of that of single polarization transmission. That makes a high-speed DWDM transmission system possible by using lower speed electronics.
Coherent detection originates from radio communications, where a local carrier mixes with the received radio frequency (RF) signal to generate a product term. As a result, the received RF signal can be frequency translated and demodulated.
Used in DWDM networks, coherent detection can not only achieve higher sensitivity than direct detection but can significantly increase the spectral efficiency (encoding more bits on each symbol) as well because it uses phase, amplitude, and polarization of an optical carrier to carry information.
In addition, coherent detection is a linear process, and linear equalization can be employed to effectively compensate for CD and PMD.
Within the WDM industry, coherent WDM technologies form the foundation for efficient WDM transmission. They have extended the reach of coherent optical wavelengths to thousands of kilometers, minimizing the need for signal regeneration.
But that is not the end. To further improve the line rate and spectral efficiency, newer coherent technologies will be looked into to continue delivering better DWDM networks.