Comparison of Passive DWDM and Active DWDM System

DWDM (division multiplexing) technology is an ideal solution to address the capacity-hungry issue, which can simply fall into two types, passive DWDM and active DWDM. To greatly expand the bandwidth of the existing fiber system, both passive DWDM and active DWDM systems are designed to multiplex different wavelengths for carrying multiple signals over one single fiber. To better know the features of these two DWDM systems, the following will intend to learn what are the passive DWDM and active DWDM systems, and find their advantages and disadvantages.

Passive DWDM System Overview

Since there is no any active component used in the passive DWDM system, the performance of the passive DWDM link only depends on the optical budget of the DWDM transceivers used in the system. That’s to say, the transmission distance the passive DWDM system supports can’t be extended and is limited to the optical budget of the DWDM transceivers. We can learn from the figure below that shows a common passive DWDM system. Obviously, no active component like fiber amplifier and DCM module, but a pairs of 20 channels DWDM Mux are used. This design allows for high capacity transmission and makes capacity expansion possible. In short, it is very suitable to deploy passive DWDM system in metro networks and high speed and capacity communication lines.

Active DWDM System Overview

Unlike passive DWDM system, active DWDM system can be composed of fiber amplifier, DWDM Mux, DWDM transceiver, DCM module and OEO transponder, which can be also called transponder-based system. Due to its active feature, it is easier to manage and control the optical active DWDM network. Here offers the design of the active DWDM system for your reference.

In the active DWDM system, the transponder usually utilizes short wave 850nm or long wave 1310nm to do the optical-electrical-optical (OEO) DWDM conversion. When the long distance is required in the active DWDM system, several EDFA fiber amplifiers will be inserted along the active DWDM link. What should be noted is that the active DWDM link can’t be extended infinitely, because the number of fiber amplifiers for an active DWDM link is limited to the optical cable type, the channel count, the data transmission rate of each channel, and permissible OSNR value, etc.

Furthermore, the chromatic dispersion occurring in the optical transmission also makes an influence on the transmission distance of the active DWDM link. Hence, when designing the active DWDM system, we should also take the permissible chromatic dispersion values of the link into consideration. If needed, we can insert the dispersion compensators (dcm modules) into the active DWDM link to enhance the optical signals for a longer transmission.

Passive DWDM vs Active DWDM System

It is well known that the passive DWDM system doesn’t need fiber amplifiers or dispersion compensators, which may saves you a lot of time and money. Meanwhile, it is also very easy to deploy due to the uncomplex installation. However, there are also several disadvantages of passive DWDM system. Firstly, the scalability is not so good as the active one. With the development of the passive DWDM system, more passive devices are required. Meanwhile, the passive DWDM system will be difficult to manage with the increasing of passive devices. What’s worse, if a wavelength or connection needs to be changed in the link, the only option is to take it out of link and disconnect the connection.

As for the active DWDM system, it can multiplex more wavelengths over a single fiber pair. Hence, more bandwidth can be provided by the active DWDM system. Furthermore, the active setups make the optical system management easier. And you can directly change the wavelengths or connections in the link without dropping connections. Finally, the active DWDM system is more scalable than the passive one, which makes more wavelengths to be multiplexed over the fiber. But one the other hand, there are also two main disadvantages of active DWDM system. One is the high cost of the active DWDM devices, and the other one is the complex installation.

In conclusion, the active DWDM system can offer greater capacity and higher scalablity, while the the passive DWDM system needs less cost and is easy to deploy. If the passive DWDM system meets your need, you’d better not to choose the active one as it will cost you very high. All in all, there is no the best, but the most suitable for your system. Just choosing the most suitable DWDM for your system according to what your network needs.

EDFA Amplifiers for Building Long-haul DWDM Networks

Clearly different from the traditional repeater, the EDFA amplifier doesn’t need to convert the optical signals into electrical ones, then make the electrical amplification and finally convert the amplified electrical signals into amplified optical signals again. It is an optical amplifier that can directly enhance the optical signals without making additional conversion. By using the EDFA optical amplifier, the attenuated signal power can be amplified into strong signal level to meet the requirement of the long-haul applications, especially the long-haul DWDM networks. To better understand the function of EDFA amplifier, the following will mainly study the working principle of EDFA amplifier works and illustrate how to use it to build the long DWDM network.

What’s the Working Principle of EDFA Amplifier?

From the figure below, we can learn the basic configuration of the EDFA amplifier, mainly composed of a length of EDF (erbium doped fiber), a pump laser with 980 nm or 1480 nm, a pump combiner and a simple WDM system. When the attenuated signals around 1550 nm pass through an EDFA amplifier, a pump laser will be generated. Then the DWDM signals and pump laser will be combined by the pump combiner. When they come into the EDF together, the wavelengths of signals and pump laser will be multiplexed and the interaction with the doping ions would enhance the signal power into high level. Thereby, a longer DWDM transmission can be reached.

How to Use EDFA Amplifier for Long DWDM network?

In a long DWDM network, the EDFA amplifier can be put in three different places with different aims. Firstly, we can put it in the transmitter side of the DWDM link to offer high input signal power, so that the DWDM fiber link can be extended. If the EDFA amplifier is deployed in this place, we can also call it EDFA optical booster amplifier. Secondly, we can also place the EDFA amplifier in the receiver side of the DWDM link as optical preamplifier, hence the output signal power can be boosted to meet the necessary receiver level. Finally, when the fiber loss in the transmission process is too high to support the long DWDM network, we can deploy the EDFA amplifier in any intermediate point along the long fiber link to compensate the fiber loss. And this time, we can call it EDFA optical in-line amplifier.

Analysis of Practical Long DWDM Cases with EDFA Amplifiers

Case One: in this case, EDFA optical booster amplifiers are deployed at both transmitter sides of the dual-way DWDM links. We can learn it from the figure below. Two 40 channel DWDM Mux Demux are deployed to multiplex 40 1G signals. Then the two integrated 40G signals from both sides are enhanced by the booster amplifier and can be transmitted up to 170 km over each single fiber.

Case Two: as shown in the following figure, except for the booster amplifiers, EDFA optical optical preamplifiers are also placed at both receiver sides of the dual-way DWDM links. By adding the optical preamplifiers to the CWDM link, the transmission distance is finally extended from 170 km to 200 km.

Case Three: it is highly noted from the following figure that the DWDM transmission distance can be up to 400 km. How to achieve this? Just putting the EDFA amplifiers in the three places mentioned above. As deployed in the figure, a pair of EDFA optical booster amplifiers, optical preamplifiers and optical in-line amplifiers are used for the 400km transmission.

Case Four: If the distance of 400 km still cannot meet our requirement, we can set up more repeater sites to place other optical in-line amplifiers. At present, using these three kinds of EDFA amplifiers already enables 100Gbps bandwidth for realizing up to 1000 km in a point-to-point connection, as shown in the figure below.

Conclusion

When designing a long-haul DWDM network for transmitting big data, EDFA amplifier is an ideal solution for current and future optical system which should be taken into consideration. It can be deployed at the transmitter side, the receiver side and any intermediate point along the DWDM long fiber link, as optical preamplifier, booster amplifier and in-line amplifier, for enhancing the signal power, thereby a long-haul transmission can be deployed.

How to Extend Your Network Transmission Distance?

To face the need for long-haul, high-capacity transmission, experts come up with several DWDM projects including DWDM Mux Demux, EDFA amplifier (erbium-doped fiber amplifier) and DCM module (dispersion compensation module) to expand network capacity and enhance the signal power, which can greatly extend the optical network reach. Do you have the need to deploy a longer fiber optical transmission link? If yes, you can just build a DWDM system with the DWDM projects mentioned above. This paper will introduce three solutions that utilize these DWDM components to extend the optical network transmission distance. Hope these DWDM solutions would be useful for you.

Using DWDM Mux Demux for Long Transmission up to 50 km

DWDM technology plays an important role in building long-haul transmission system, which enables multiple signals with different wavelengths to be transmitted through only one single fiber. To build a long system with DWDM technology, the DWDM Mux Demux is an indispensable component that features low insertion loss and polarization-dependent loss. By using the DWDM Mux Demux in your network, the signal transmission distance can be extended to up to 50 km. To better know the advantage of DWDM Mux Demux, here offers an example that uses two 8 channel DWDM Mux Demux for extending the optical fiber link.

From the figure, we can learn that at the transmit side, eight kinds of signals from different fiber links are multiplexed into an integrated signal by the 8 channel DWDM Mux. Then the integrated signal is transmitted over the single mode fiber (SMF) and the maximum transmission distance can be up to 50 km. At the receiver side, the signal will be demultiplexed into individual signals with their original wavelengths by the 8 channel DWDM Demux and then transmitted to another eight different fiber links. Just by using the DWDM Mux Demux, a 50km long-haul transmission can be simply achieved.

Adding EDFA Amplifier for Transmission Longer Than 50 km

As we know, the longer the transmission distance is, the higher the fiber loss will be. Hence, except for the DWDM Mux Demux, you are suggested to add an EDFA amplifier to the long fiber link if the transmission distance is longer than 50 km. What’s the function of EDFA amplifier? It is mainly designed to amplify the signal power, which enables longer transmission. As shown in the following figure, you can learn that the only difference is the EDFA amplifier in the SMF, compared to the first solution.

When the integrated signal multiplexed by the 8 channel DWDM Demux is transmitted over the SMF, it would become too weak in the transmission process to be transmitted. Then the EDFA amplifier should be placed there to boost the signal power, supporting the transmission longer than 50 km. Once the long transmission is realized, the signal will be also split by the 8 channel DWDM Demux, like the first solution. In short, DWDM Mux Demux and EDFA amplifier are highly suggested if you want to deploy a DWDM system longer than 50 km.

Adding DCM Module for Transmission up to 200 km

With the use of EDFA amplifier, the DWDM fiber link can be extended to 200 km. However, the signal quality is always unsatisfied due to the optical dispersion in long transmission, especially in CATV systems. To meet high requirements of the signal quality in these long transmission systems, an additional optical component, DCM module are needed in the long fiber link, as deployed in the figure below.

From the figure, we can learn it is a long-haul point-to-multipoint CATV system. To extend the transmission distance, 8 channel DWDM Mux Demux, EDFA amplifier are used. Except for that, a DCM module is added to enhance the skew signal for ensuring the whole transmission quality. With the use of DCM module, the accumulated chromatic dispersion issue is solved, without dropping and regenerating the wavelengths on the long fiber link. Thereby, a high-performance 200km system can be reached.

Conclusion

DWDM projects including DWDM Mux Demux, EDFA amplifier and DCM module are key optical components to support long-haul transmission systems. If you want to deploy a long transmission system up to 50 km, then the DWDM Mux Demux is needed. For transmission longer than 50 km, both the DWDM Mux Demux and EDFA amplifier are required for boost the signal power. But once the transmission distance is about 200 km, you should additionally add the DCM module to enhance the signal quality.

How to Enhance the Optical Signals for a Long DWDM System?

As we know, the longer the optical transmission distance is, the weaker the optical signals will be. For a long DWDM system, this phenomenon easily causes transmission error or even failure. Under this case, what can we do for a smooth, long DWDM system? The answer is optical signal enhancement. Only by enhancing the optical signals, can the DWDM transmission distance be extended. In this post, we are going to learn two effective solutions, optical amplifier (OA) and dispersion compensation module (DCM) to enhance the signals, for making a smooth, long DWDM system.

Optical Amplifier Solution

We used to utilize repeater to enhance the signals in fiber optics, which should firstly convert the optical signals into an electrical one, amplify the electrical signals, and then convert the electrical signals into an optical one again. Finally, you can get the enhanced optical signals. However, this method of enhancing signals can not only cause more signal loss, but also add unwanted noises in the actual signal. Taking these issues into account, the optical amplifier is more recommendable.

An optical amplifier is a device that enables direct optical signal enhancement or amplification. Its working principle is not so complicated as that of the repeater, while its performance is much higher. From the following figure, we can learn that the original reach of the DWDM system is limited to 80 km due to the signal loss. But with the optical amplifier, the signals are enhanced and the reach can be extended to 160 km. It is really an ideal option to enhance the signals for a long DWDM system.

At present, there are mainly three major kinds of optical amplifiers, Semiconductor Optical Amplifier (SOA), Doper Fiber Amplifier (DFA), and Raman Amplifier (RA).

Semiconductor Optical Amplifier: as its name implies, the semiconductor in a SOA is used to offer the gain medium. This kind of optical amplifier has a similar structure to the FP laser diode. However, it is designed with anti-reflection elements at the end face that can greatly reduce the end face reflection. Meanwhile, the SOA features small package and low cost that suits for most users to enhance the optical signals.

Doper Fiber Amplifier: in a DFA, the doped optical fiber acts as the gain medium for signal amplification. When the DFA works, the signal to be amplified and a pump laser are multiplexed into the doped fiber. And then the signal is amplified through interaction with the doping ions. The most common DFA is the Erbium Doped Fiber Amplifier (EDFA). Its gain medium is a optical fiber doped with trivalent erbium ions that always enhances the signals near 1550nm wavelength. Undoubtedly, the EDFA is a great choice to enhance the optical signals.

Raman Amplifier: different from the SOA and DFA, the signal in a RA is amplified through the nonlinear interaction between the signal and a pump laser within an optical fiber. In details, two kinds, distributed and lumped Raman amplifier (DRA and LRA) are available on the market. The distributed one multiplexes the pump wavelength with signal wavelength through the transmission fiber to enhance the signals, while the amplification of the lumped one is provided by a dedicated, shorter length of fiber.

Dispersion Compensation Solution

Apart from signal amplification, we can also use dispersion compensation to enhance the optical signals. Once the dispersion occurs, the signal will be tended to skew due to the different frequencies, which has a negative effect on the quality of signal transmission. At that moment, we use the dispersion compensating module to enhance the skew signal, for achieving a longer transmission distance. As shown in the figure below, the DWDM system is extended to longer than 80 km with the use of 80km passive dispersion compensation module.

The dispersion compensation module is an important component for a long fiber optical link. It typically connects to the mid-stage of an OA like EDFA, in the long haul transmission system. Except for the 80km DCM mentioned above, FS.COM also provides other DCM modules that allow long transmission distance extension. The compensation distances can range from 10km to 140 km, as shown in the following table.

Module Type	Description
FMT10-DCM	10KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
FMT20-DCM	20KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
FMT40-DCM	40KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
FMT60-DCM	60KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
FMT80-DCM	80KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
FMT100-DCM	100KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
FMT140-DCM	140KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC

Conclusion

The optical amplifier has the ability to directly boost the weak signal, while the dispersion compensation module can reshape the deformed signal and offer a long compensation distance. Considering that the signal strength would become weak as the transmission distance increases, using the optical amplifier and dispersion compensation module to enhance the signals is very necessary when building a long DWDM system.

Original source: http://www.chinacablesbuy.com/enhance-optical-signals-long-dwdm-system.html

Can the Hybrid CWDM-DWDM System Work for Higher Capacity?

When facing the capacity-hungry issue, have you ever hesitated over which WDM system should be choose? As the CWDM system is a more economical solution for limited expanding capacity while the expensive DWDM solution enables much higher capacity, which one should be chose is really a tough decision. In order to solve the issue, can we deploy a Hybrid CWDM-DWDM system, for not choosing a wrong solution to increase the network capacity? Thereby, both the bandwidth shortage with CWDM solution or the potential bankruptcy with DWDM solution can be avoided. Let’s seeking the answer.

Can the Hybrid CWDM-DWDM System Work?

Can the Hybrid CWDM-DWDM system work for higher network capacity? The answer is yes. In fact, it is an ideal solution for boosting the network capacity, which is designed with merging DWDM and CWDM traffic seamlessly at the optical layer, taking full use of the WDM technology. In a hybrid CWDM-DWDM system, more channels can be added to deal with the limited capacity and reach in a CWDM system. That’s to say, the hybrid CWDM-DWDM system utilizes the DWDM technology to empower CWDM system, by integrating CWDM and DWDM equipment, which offers true pay-as-you-grow capacity growth and investment protection.

In short, the hybrid CWDM-DWDM system is a simple, plug-and-play option that enables more DWDM channels interleaved with the existing CWDM channels, for transmitting more data signals. It gets the utmost out of CWDM and DWDM technologies in a single system that greatly reduces the cost, simplifies the installation and keeps the system flexibility for bigger network capacity.

How to Build a Hybrid CWDM-DWDM System?

In general, a normal complete optical connection can be simply done by using a length of fiber patch cable to connect two fiber transceivers and then separately inserting the two transceivers into the ports of two switches. While in a hybrid CWDM-DWDM system, both the CWDM Mux Demux and DWDM Mux Demux should be added offering multiple channels to multiplex and demultiplex the signals. Here offers a typical 44 channel hybrid CWDM-DWDM system information for your reference.

From the figure, we can learn that the original CWDM system uses two 8 channel CWDM Mux Demux with wavelengths from 1470 nm to 1610 nm (20nm channel spacing). In order to add more channels for transmitting larger data signals, two pairs of DWDM multi-channel Mux/Demux are deployed separately under the pass band of the existing CWDM filters. In principle, deploying the DWDM multi-channel Mux/Demux in the 1530nm channel can create 25 100 GHz spaced DWDM channels. However, only 19 DWDM channels circled in the following figure are suitable to be added in the hybrid CWDM-DWDM system. It is also the same to the 1550 channel. Hence, this hybrid CWDM-DWDM system totally offers 6 CWDM channels and 38 DWDM channels with less deployment cost but easier installation.

Conclusion

If you come across the capacity-hungry issue and can’t make the decision about which WDM system should be choose for increasing your network capacity, you are highly recommended to deploy a hybrid CWDM-DWDM system. As an economical and future-proofing solution, the hybrid CWDM-DWDM system can completely deal with the issue of bandwidth shortage when building a CWDM system and avoid the potential bankruptcy for a DWDM system. You can just deploy a CWDM system first. Once the capacity the CWDM system offers can’t meet your requirement, you can add DWDM equipment in for more channels to transmit signals. All in all, the hybrid CWDM-DWDM system is an ideal choice that not only costs less for deployment but keeps the flexibility to increase the network capacity.

Single Fiber Solutions for CWDM and DWDM Networks

It is well known that there are two transmission ways for fiber optical network, single fiber transmission and dual fiber transmission. From the name, it is easy to learn that the fiber amount is the main difference of these two transmission ways. For the dual fiber transmission, there should be totally two optical fibers, one for transmitting signals, and the other for receiving signals. But for the single fiber transmission, it only requires one optical fiber that can transmit and receive signals at the same time. This feature makes the network deployment easier and the network deployment cost lower than that of dual fiber one, especially in WDM network deployment. In this paper, it will mainly introduce the single fiber solution and its applications in CWDM and DWDM networks.

Single Fiber Solution

Single fiber solution can be also called bidirectional (BiDi) solution designed for carrying signals in both sides of one optical fiber simultaneously. When the single fiber network runs, the signals transmitted from the two sides feature different wavelengths to ensure the dual way transmission. Compared to the dual fiber network, the easy-to-deploy single fiber network would be a good choice for those who have limited budgets but need for bigger network capacity. As for its application, it is very popularly used in Point to Point, Ring or linear Add and Drop, where installing new fibers is impracticable or uneconomical. It can be also used for promoting the reliability of an existing dual fiber network, in which there are one optical fiber for work and the other one for protection.

Single Fiber CWDM Network

The single fiber CWDM network enables the signals with different wavelengths to be transmitted through a signal fiber, which results in a network capacity boost in metro and access networks. As each signal beam can be carried via different channel independently, the different data rates and protocols (T1, T3, Ethernet, Serial, etc) can be transmitted for the different users or applications. To better know how does the single fiber CWDM network work, here offers a figure that shows a 8 channel single fiber CWDM network design.

From the figure, we can learn there are two 8 channel CWDM Mux Demux connected by a single fiber for transmitting 16 signals with different wavelengths and the 16 wavelengths are divided into 8 pairs for bidirectional transmission. On the site A, 8 wavelengths are used for transmitting signals and others for receiving signals. While on the site B, the wavelengths for the TX and RX on the CWDM Mux Demux are all reversed to ensure the performance of the single fiber CWDM network. For instance, the 1270 nm on the site A is the first transmitting wavelength but the first receiving wavelength on the site B. As a result, the signal with 1270 nm can be totally transmitted from site A to site B via the single fiber.

Single Fiber DWDM Network

The single fiber DWDM network also takes full use of the wavelength division multiplexing technology that can greatly expand the capacity over the existing optical network, especially for the long transmission network. Compared to the CWDM one, it can be designed with more channels for larger data signals and can support the network with a much longer distance. What’s more, if the transmission distance is too long, the optical amplifier can be used to enhance the signals. On the other hand, all these advantage makes the cost to deploy a single fiber DWDM network much higher than the CWDM one. Besides, here offers the figure of a single fiber DWDM network design for your reference.

The figure shows a complicated a single fiber DWDM network design that uses two 8 channel DWDM Mux Demux in the main link for single fiber transmission. As for the wavelength feature of the TX and RX for the ports on the DWDM Mux Demux, it is similar to the CWDM one, but the channel spacing is denser. What’s more, except for the existing channels in the main single fiber DWDM link, there can be more channels added into the expansion port of the link for carrying larger data signals. In short, although the cost for deploying single fiber DWDM network is so expensive, it is still a good solution that can make full use of the fiber link to transmit more signals at a much longer distance.

Conclusion

Undoubtedly, the single fiber WDM solution is an ideal choice for those who have limited budgets but need for bigger network capacity. As there are two single fiber WDM solutions available, which one to choose just depends on the network need and budget. The single fiber CWDM network would be more economical but can not transmit as much signals as the DWDM one can, while the DWDM is highly recommended for large and long transmission that would cost more.