With the rapid development of optical transport networks and the explosive growth of data services, the bandwidth of traditional PDH and SDH systems can no longer meet current demands. Wavelength Division Multiplexing (WDM) technology has matured and become an excellent method for capacity expansion.
Dense Wavelength Division Multiplexing (DWDM) is a type of WDM technology designed to expand the transmission capacity of existing optical fiber networks. It offers an ideal solution to problems such as limited fiber resources and the difficulty of laying new cables. DWDM can be broadly categorized into two types: passive DWDM and active DWDM. Both systems are designed to multiplex different wavelengths in order to transmit multiple signals over a single fiber, thereby greatly increasing the bandwidth capacity of existing fiber systems.
To better understand the characteristics of these two DWDM systems, we will now explore what passive DWDM and active DWDM systems are, and identify their respective advantages and disadvantages.
In a passive DWDM system, no active components—such as optical amplifiers or dispersion compensators—are used. The transmission distance of such a system is limited by the transmit power of the optical modules, but it offers the advantage of high channel capacity, making it mainly suitable for metropolitan area networks (MANs) and high-channel-capacity, high-speed transmission lines.
Since no active components are deployed in a passive DWDM system, the performance of the passive DWDM link depends solely on the optical budget of the DWDM optical transceivers used in the system. In other words, the supported transmission distance of a passive DWDM system cannot be extended and is constrained by the optical budget of the DWDM transceivers.
From the diagram below, we can see a typical passive DWDM system. Clearly, there are no active components such as optical amplifiers or DCM modules; instead, a pair of 20-channel DWDM Mux units is used. This design enables high-capacity transmission and makes capacity expansion possible. In summary, passive DWDM systems are well-suited for deployment in metropolitan area networks (MANs) and high-speed, high-capacity communication lines.
Unlike passive DWDM systems, an active DWDM system can be composed of optical amplifiers (EDFA), DWDM multiplexers, DWDM optical transceiver modules, DCM dispersion compensation modules, and OTU wavelength converters. It is also referred to as a transponder-based system. Due to its active nature, an optical active DWDM network is easier to manage and control.
In an active DWDM system, the transponder typically performs optical–electrical–optical (O-E-O) conversion, using short wavelengths such as 850 nm or long wavelengths such as 1310 nm, to convert signals into standard DWDM optical signals. When long-distance transmission is required, EDFA optical amplifiers are added along the active DWDM link. It should be noted that the distance of an active DWDM link cannot be extended indefinitely, as the number of optical amplifiers in the link is limited by factors such as fiber type, number of channels, data rate per channel, and the allowable OSNR value.
The transmission distance of an active DWDM system is influenced not only by the number of optical amplifiers and the optical signal-to-noise ratio (OSNR) but also by the dispersion of the optical signal. Therefore, when designing an active DWDM system, optical signal dispersion should be taken into account.
If necessary, a dispersion compensation module (DCM) can be added to the active DWDM system to enhance the optical signal and achieve longer transmission distances. It should be noted, however, that a DCM increases the insertion loss of the fiber transmission link, which can also affect the transmission distance of the active DWDM system.
The passive DWDM system and the active DWDM system differ significantly in terms of transmission distance, capacity, manageability, and cost. Passive systems are suitable for short-distance, cost-sensitive scenarios, such as the aggregation layer in metropolitan area networks (MANs), while active systems are better suited for long-distance, high-capacity networks with dynamic service adjustments, such as backbone networks and large data center interconnects.
Feature |
Passive DWDM |
Active DWDM |
Power Requirement |
No power needed |
Requires power for active components |
Signal Processing |
Optical multiplexing only |
Can amplify, regenerate, and monitor signals |
Distance Support |
Limited by transceiver optical budget |
Extended with optical amplifiers |
Management |
Minimal |
Comprehensive, remote monitoring possible |
Cost |
Lower (simpler equipment) |
Higher (active devices and management features) |
Maintenance |
Low complexity |
Higher complexity, requires skilled operation |
Typical Use Cases |
Short/medium MAN links, DCI |
Long-haul, backbone networks, large-scale MANs |
From the comparison above, it can be seen that passive DWDM systems, compared to active DWDM backbone networks equipped with optical amplifiers and dispersion compensators, can build high-channel-capacity high-speed transmission lines at a lower cost, saving you significant time and money. At the same time, because their installation is simple, deployment is very easy.
Passive DWDM is essentially a plug-and-play system, making it simple and convenient to use. However, passive DWDM systems also have some drawbacks. The number of wavelength channels in a passive DWDM system is limited, and if you want to expand the network, additional passive DWDM devices must be added, which increases system management complexity.
As for active DWDM systems, they support a larger number of wavelength channels, providing higher bandwidth and better fiber utilization. Additionally, active DWDM systems are easier to manage, allowing users to adjust channel wavelengths online without shutting down the system, and network expansion is simpler. Compared to passive DWDM systems, active DWDM systems can cover longer transmission distances, but their deployment costs are higher.
Furthermore, active DWDM systems require equipment such as optical amplifiers and dispersion compensators, making their deployment more complex than that of passive DWDM systems.
Passive DWDM systems and active DWDM systems have different application scenarios in optical communications. Below is a brief analysis of their applications:
Passive DWDM systems do not rely on external power sources. They primarily use passive optical components—such as optical splitters, filters, and AWGs—to achieve wavelength separation and multiplexing. Their main characteristics are simple structure, low cost, and easy maintenance, though their transmission distance and signal processing capabilities are limited.
1. Short-Distance Metropolitan Area Networks (MAN):
Passive DWDM is commonly used in small to medium-sized cities or regional networks to connect data centers, enterprise campuses, or base stations. The typical transmission distance is within tens of kilometers. It is well-suited for scenarios that require low cost and high reliability, such as dedicated enterprise networks.
2. Fiber to the Home (FTTH):
By combining DWDM technology with Passive Optical Networks (PON), multiple users can be provided with high-bandwidth access. This approach is often applied in last-mile fiber networks to reduce deployment costs.
3. Point-to-Point Connections:
Passive DWDM is used for data center interconnects (DCI) or internal enterprise fiber links, transmitting straightforward high-speed data streams efficiently.
4. Power- or Maintenance-Limited Environments:
In areas without power supply or where maintenance is difficult—such as remote locations or industrial settings—passive DWDM offers a clear advantage, as it does not require external power to operate.
Advantages:
Passive DWDM systems consume very little power and do not require an external power source, making them suitable for energy-efficient and green deployments. They are cost-effective in terms of deployment and maintenance, which is ideal for projects with limited budgets. In addition, passive optical components have high reliability, with a low failure rate.
Limitations:
Signal attenuation is relatively high, limiting the transmission distance—typically to less than 80 kilometers. Passive DWDM lacks signal amplification and regeneration capabilities, making it challenging to support complex network topologies.
Active DWDM systems utilize optical amplifiers (such as EDFAs), optical transceivers, and signal regeneration devices to support long-distance, high-capacity transmission, making them suitable for complex network architectures.
1. Long-Haul Backbone Networks:
Active DWDM is widely used in long-distance optical communication networks that span cities, countries, or even continents, such as submarine cables or national backbone networks. Transmission distances can range from hundreds to thousands of kilometers, with optical amplifiers and regenerators employed to overcome signal attenuation.
2. Data Center Interconnect (DCI):
Active DWDM enables high-bandwidth, low-latency interconnections between global data centers of large cloud service providers (e.g., AWS, Google, Alibaba Cloud). It supports dense data transmission at high speeds, such as 100G and 400G channels.
3. 5G Fronthaul and Midhaul Networks:
It provides high-bandwidth, low-latency connections for 5G base stations, meeting the demands of large-scale MIMO and ultra-low latency. Active DWDM is commonly deployed in the fronthaul and midhaul layers of mobile communication networks.
4. Complex Enterprise Networks:
In industries such as finance and telecommunications, active DWDM supports high-capacity, multipoint-to-multipoint network architectures. When combined with ROADMs (Reconfigurable Optical Add-Drop Multiplexers), it enables dynamic wavelength allocation and flexible routing.
Advantages:
Active DWDM systems support ultra-long-distance transmission, ranging from hundreds to thousands of kilometers. They can handle high-capacity, multi-wavelength transmission through the use of optical amplifiers and signal regeneration. In addition, they offer high flexibility, making them well-suited for complex networks that require dynamic adjustments.
Limitations:
Active DWDM systems are complex, resulting in higher deployment and maintenance costs. They rely on a stable power supply, leading to higher energy consumption. Moreover, they require advanced system management and monitoring, as well as professional technical support.
In summary, passive and active DWDM systems each have their own advantages, and the choice of which to deploy should be based on specific application requirements. Regardless of the system type, DWDM multiplexers and demultiplexers are essential components. AscentOptics offers a comprehensive range of DWDM transmission solutions, including DWDM multiplexers/demultiplexers and DWDM optical modules.
Ultimately, there is no “best” system—only the one that best fits your needs. Simply select the DWDM solution that aligns with your network requirements.