Coherent WDM Technology

What Is Coherent WDM Technology?

Wavelength Division Multiplexing (WDM) is a fiber-optic transmission method that allows multiple light wavelengths (or colors) to carry data over the same medium. Multiple colors of light can travel through a single fiber, enabling several signals to be transmitted simultaneously in an optical waveguide at different wavelengths or frequencies within the optical spectrum.

Coherent WDM technology involves sophisticated optical methods that utilize modulation of both light's amplitude and phase, along with transmission across dual polarizations. These techniques enable a substantial increase in the amount of data that can be transmitted through a fiber optic cable. Coherent WDM technology uses digital signal processing at both the transmitter and receiver to provide cost-effective and highly reliable optical transmission in DWDM networks.

When WDM was initially introduced around the mid-1990s, the standard data rate per wavelength was 2.5G. The advancement to 10G wavelengths became feasible due to advancements in high-speed optical modulators and improved management of chromatic dispersion. As bandwidth requirements continue to challenge hardware capacity, innovation in this space has become a necessity. Fiber capacity has significantly increased, allowing multiple high-bit-rate data streams—such as 40 Gb/s, 100 Gb/s, 200 Gb/s, 400 Gb/s and 800 Gb/s—each carrying distinct throughputs, to be multiplexed over a single fiber.

Benefits of Coherent WDM Technology

Coherent optics technology is the foundation of the industry's ability to achieve 100G and higher transmission speeds. It can increase data transmission rates, enhance flexibility, streamlined DWDM line systems, and improve optical performance. The benefits are as follows:

Simplify DWDM Network

Coherent WDM technology enhances the efficiency of DWDM network planning and deployment through the use of soft-decision Forward Error Correction (FEC). This technique involves encoding the original signal with additional error detection and correction information, enabling the detection and correction of errors that occur during transmission.

FEC provides greater margin, allowing higher bit-rate signals to travel longer distances with fewer regenerator points. This simplifies photonic lines, reduces equipment requirements, lowers costs, and significantly increases the bandwidth capacity of DWDM networks.

Spectral Shaping

Spectral shaping is a widely utilized technique in coherent WDM technology for deploying DWDM networks. It involves applying dynamic processing across the frequency spectrum to achieve a balanced distribution of instruments and voices, surpassing the capabilities of traditional compressors and equalizers.

Additionally, spectral shaping enhances the capacity of cascaded Reconfigurable Optical Add-Drop Multiplexers (ROADMs), thereby increasing spectral efficiency for DWDM channels. This critical technique in flexible WDM grid systems enables carriers to be closely packed together, maximizing network capacity.

Greater Flexibility

Coherent WDM technology is versatile and can be customized for a wide range of DWDM networks and applications. Coherent optical line cards support various modulation formats and baud rates, giving operators flexibility in selecting different line rates. Fully programmable coherent WDM transceivers offer extensive tunability options with precise control over incremental capacities. This allows network operators to utilize all available capacity efficiently and monetize excess margin through revenue-generating services.

Effective Mitigation of Dispersion

When optical signals travel through fiber cables, they encounter unavoidable impairments like chromatic dispersion (CD) and polarization mode dispersion (PMD). Coherent WDM technology's sophisticated digital signal processors (DSPs) effectively mitigate these dispersion effects, eliminating the complexities of planning dispersion maps and budgeting for PMD. Additionally, this approach eliminates the need for dispersion compensation modules (DCMs), thereby reducing costs and latency associated with optical signal transmission.

Furthermore, coherent processors enhance tolerances for Polarization-Dependent Loss (PDL) and monitor the State of Polarization (SOP) to prevent bit-errors caused by cycle slips, which could otherwise degrade optical performance. Consequently, operators can implement line rates up to 400G per carrier over extended distances. This advancement enables high bit-rate signals to be transmitted over older fiber networks that were previously unable to support speeds as high as 10G.

The Implementation of Coherent WDM Technologies

Today, some basic coherent WDM technologies have been successfully deployed and applied to DWDM networks.

High-order Amplitude/Phase Modulation

During the early 2000s, numerous optical experiments aimed to enhance the data rate per WDM channel beyond the limitations of 10G direct detection (IM-DD). Phase shift keying modulations, such as differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK), gained favor due to their ability to achieve better optical signal-to-noise ratio (OSNR) compared to IM-DD.

Moreover, by encoding additional amplitude or phase changes within the carrier signal, it becomes feasible to increase the number of bits conveyed in each symbol. The sensitivity to fiber impairments correlates with the symbol rate rather than directly with the bit rate.

Polarization Multiplexing

Fiber optic cables function akin to circular waveguides and support two orthogonal polarizations. Employing polarization multiplexed (PM) carriers allows for the selective transmission of modulated signals, effectively doubling the spectral efficiency of a given modulation technique while utilizing the same PM receiver.

Through polarization multiplexing, the effective symbol rate can be halved compared to single polarization transmission. This capability enables the implementation of high-speed DWDM transmission systems using lower speed electronics.

Coherent Detection

Coherent detection has its origins in radio communications, where a local carrier mixes with the received radio frequency (RF) signal to produce a product term. This process allows for frequent translation and demodulation of the received RF signal.

In the context of DWDM networks, coherent detection not only achieves higher sensitivity compared to direct detection but also significantly enhances spectral efficiency by encoding more bits per symbol. This is achieved through leveraging the phase, amplitude, and polarization of an optical carrier to convey information.

In addition, The implementation of coherent technologies also includes high-speed analog-to-digital converters (ADCs) and complex digital signal processing in the receiver.