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What Is Coherent WDM Technology?

Updated on Jun 1, 2022 by
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As the demand for network bandwidth grows at an unprecedented rate, the telecommunications industry is rapidly adopting advanced technologies to efficiently increase optical bandwidth. One groundbreaking innovation is Coherent Wavelength-Division Multiplexing (WDM). Utilizing sophisticated digital signal processors (DSPs) and cutting-edge photonics, Coherent WDM has transformed Dense Wavelength Division Multiplexing (DWDM) transport, boosting wavelength speeds from 10 Gb/s in the pre-coherent era to astonishing rates of 100 Gb/s, 200 Gb/s, and now even 400 Gb/s and 800 Gb/s with the latest equipment.

What Is Coherent WDM Technology?

Coherent WDM Technology

When WDM was first introduced in the mid-1990s, the typical wavelength data rate was 2.5G, but advancements in high-speed optical modulators and chromatic dispersion management facilitated the transition to 10G wavelengths. Coherent WDM technology has further revolutionized optical communication by employing advanced techniques to modulate light's amplitude and phase, enabling transmission across two polarizations. This innovation significantly enhances data transport capacity over fiber optic cables. By integrating digital signal processing at both ends, coherent WDM ensures cost-effective and highly reliable optical transport in DWDM networks, enabling 100GbE transport and a 10X scaling of network capacity without altering DWDM channel spacing or equipment design.

Coherent WDM Technology VS DWDM Technology

In comparison, coherent optics vs DWDM represent two distinct yet interconnected concepts in optical communication. Coherent optics refers to the use of coherent modulation and detection techniques to transmit and receive optical signals, ensuring high-speed and reliable data transfer over long distances. On the other hand, DWDM is a technology that allows multiple wavelengths or channels to be transmitted simultaneously over a single optical fiber, thereby increasing the overall capacity of the fiber optic network. While coherent optics focuses on signal modulation and detection, DWDM concentrates on multiplexing and demultiplexing multiple signals. However, both coherent optics and DWDM play critical roles in enhancing optical communication systems, often complementing each other to achieve higher transmission speeds and greater network capacity.

coherent technology development

Figure 1: The Technology Enablers for WDM Capacity

What Is the Difference Between Non-Coherent & Coherent?

To understand coherent optical technology , it's crucial to discern Coherent from Non-Coherent optical communication. While both transmit data over optical networks, they diverge notably in signal processing and performance. Before we delve into Coherent WDM intricacies, let's dissect the fundamental differences between Non-Coherent and Coherent communication methods.

Feature
Non-Coherent Systems
Coherent Systems
Overview
Optical systems that do not require coherent local oscillator light.
 
 
Optical systems that use local oscillator light for coherent detection.

 

 
 
Modulation/Detection

 

 
 
Transmitter: Intensity modulation
Receiver: Direct detection

 

 
 
Transmitter: External modulation
Receiver: Coherent detection with local oscillator light

 

 
 
Modulation Techniques

 

 
 
Amplitude Modulation (RZ/NRZ/ODB)
Differential Phase Modulation (DQPSK)

 

 
Phase Modulation (BPSK/QPSK)
Quadrature Amplitude Modulation (QAM)

 

 
 
System Complexity

 

 
 
Simple and straightforward to implement and integrate

 

 
Complex with high technical demands

 

Spectral Efficiency
 
 
Low: Limited single-channel bandwidth due to the inability to utilize frequency and phase information.

 

 
 
High: High single-channel bandwidth by utilizing amplitude, frequency, and phase information in the optical signal.

 

 
 
Dispersion Handling

 

 
 
Low: Requires dispersion compensation modules (DCMs) for dispersion compensation.

 

High: Uses digital signal processing (DSP) to manage fiber dispersion, allowing for long-haul transmission without DCMs.
 
 
ROADM Configuration

 

 
 
Complex: Demultiplexer board needed at the receiver end to filter out specific wavelengths.

 

 
 
Simple: Coherent receivers can directly select wavelengths from the multiplexed signals without a demultiplexer board.

 

 
 
Use Cases

 

 
 
Suitable for 2.5G and 10G transmission, early 40G transmission, and metro 100G sub-wavelength transmission.

 

 
Ideal for 100G and beyond line transmission, including super 100G line transmission.

 

 

Key Technologies for Coherent Optical Communication

Over the past few years, some fundamental coherent WDM technologies have been successfully deployed and applied to DWDM networks.

High-Order Amplitude/Phase Modulation

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).

Polarization Multiplexing

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

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.

DSP Technology

Digital signal processing (DSP) plays a pivotal role in coherent optical communication. It is employed to address signal distortion and latency issues caused by dispersion at the electrical signal layer. DSP technology compensates for polarization mode dispersion (PMD) and chromatic dispersion (CD), significantly enhancing the tolerance to PMD and CD.

Highlights of Coherent WDM Technology

Coherent WDM technology offers higher bit-rates, greater degrees of flexibility, simpler DWDM line systems, and better optical performance.

Simplify Your DWDM Network

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.

Maximize Fiber Capacity

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.

Greater Flexibility

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.

Strong Mitigation to Dispersion

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.

FS 100G Coherent DWDM Solution

The proliferation of cloud-based applications, machine learning, and AR/VR technologies drives the need for upgraded networks with higher bandwidths and longer distances. The FS 100G Coherent DWDM Solution meets this demand by enabling seamless upgrades from 10G/40G to 100G networks without extensive infrastructure changes.

This solution supports high-capacity, long-distance 100G optical transmission with improved spectral efficiency and reliability. It is cost-effective and versatile, suitable for metro, regional, and long-haul applications. Widely recognized and deployed, the FS 100G Coherent DWDM Solution is ideal for bandwidth-intensive cloud applications.

Looking into the Future

Looking into the future, Coherent WDM technologies have laid a solid foundation for efficient WDM transmission, extending optical wavelengths over thousands of kilometers and reducing the need for signal regeneration. This progress highlights the transformative impact of coherent technology on telecommunications. However, the journey continues. Advancements in coherent technologies promise greater line rates and spectral efficiency. Researchers are exploring new modulation formats, enhanced DSP algorithms, and innovative photonic integration techniques. These developments aim to push boundaries, ensuring DWDM networks meet the growing demands for bandwidth and performance. The relentless pursuit of Coherent WDM technology will lead to more efficient, robust, and high-capacity optical networks, shaping the future of global communications.

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