How Does Erbium Doped Fiber Amplifier (EDFA) Benefit WDM Systems

Optical network that involves WDM (wavelength division multiplexing) currently gains in much popularity in existing telecom infrastructure. Which is expected to play a significant role in next generation networks to support various services with very different requirement. WDM technology, together with EDFA (Erbium Doped Fiber Amplifier), allowing the transmission of multiple channels over the same fiber, that makes it possible to transmit many terabits of data over distances from a few hundred kilometers to transoceanic distances, which satisfy the data capacity required for current and future communication networks. This article explains how can WDM system benefit from this technology.

Basics of EDFA

The key feature of EDFA technology is the Erbium Doped Fiber (EDF), which is a conventional silica fiber doped with erbium. Basically, EDFA consists of a length of EDF, a pump laser, and a WDM combiner. The WDM combiner is for combining the signal and pump wavelength, so that they can propagate simultaneously through the EDF. EDFA can be designed that pump energy propagates in the same direction as the signal (forward pumping), the opposite direction to the signal (backward pumping), or both direction together. The pump energy may either by 980nm pump energy or 1480nm pump energy, or a combination of both. The most common configuration is the forward pumping configuration using 980nm pump energy. Because this configuration takes advantage of the 980nm semiconductor pump laser diodes, which feature effective cost, reliability and low power consumption. Thus providing the best overall design in regard to performance and cost trade-offs.

Why EDFA Is Essential to WDM Systems?

We know that when transmitting over long distance, the signal is highly attenuated. Therefore it is essential to implement an optical signal amplification to restore the optical power budget. This is what EDFA commonly used for: it is designed to directly amplify any input optical signal, which hence eliminates the need to firstly transform it to an electronic signal. It simply can amplify all WDM channels together. Nowadays, EDFA rises as a preferable option for signal amplification method for WDM systems, owing to its low-noise and insensitive to signal polarization. Besides, EDFA deployment is relatively easier to realize compared with other signal amplification methods.

4-Channel WDM System With or Without EDFA: What Is the Difference?

Two basic configurations of WDM systems come in two forms: WDM system with or without EDFA. Let’s first see the configuration of WDM system without using it. At the transmitter end, channels are combined in an optical combiner. And these combined multiple channels are transmitted over a single fiber. Then splitters are used to split the signal into two parts, one passes through the optical spectrum analyzer for signal’s analysis. And other passes through the photo detector to convert the optical signal into electrical. Then filter and electrical scope is used to observe the characteristics of signal. In this configuration signals at long distance get attenuated. While this problem can be overcome by using erbium doped fiber amplifier.

As for WDM system which uses EDFA, things are a little bit different. Although the configuration is almost the same as WDM system without it, some additional components are used. These components are EDFAs which are used as a booster and pre-amplifier, and another additional component is optical filter. With the adoption of optical amplifier, this system doesn’t suffer from losses and attenuation. Hence, it is possible to build broadband WDM EDFA which offer flat gain over a large dynamic gain range, low noise, high saturation output power and stable operation with excellent transient suppression. The combination provides reliable performance and relatively low cost, which makes EDFAs preferable in most applications of modern optical networks.

Conclusion

Among the various technologies available for optical amplifiers, EDFA technology proves to be the most advanced one that holds the dominate position in the market. In future, the WDM system integrated with high performance EDFA, as well as the demand for more bandwidth at lower costs have made optical networking an attractive solution for advanced networks.

Source: http://www.fiber-optic-solutions.com/fiber-amplifier-edfa-wdm-systems.html

CWDM and DWDM for Metro Networks: How to Make it Economical?

Fiber exhaust still appears to be a common problem faced by most metro (or metropolitan) networks. Although the cost of fiber optic cable is consistently dropping, the trenching, labor, and other installation costs towards optical fiber stay rather high. This may partially explain why an increasing number of metro networks incline to adopt WDM technology to enhance fiber capacity. It is known that WDM technology used in metro networks generally takes two forms: coarse WDM (CWDM) and dense WDM (DWDM). This article will deliver an overall comparison of CWDM and DWDM in metro networks, from the perspective of the roles each plays and the operating cost. Help you to decide how to reach an economical solution.

CWDM vs. DWDM: Different Role in Metro Network

As for the major difference between the two WDM technology, their names imply it all: is the channel spacing within the window of the optical spectrum (see the picture below). CWDM has a wider pass-band that spaced at 20 nm apart, which allows for the use of less expensive components like uncooled lasers and thin-film filter technology. The cost advantage of CWDM makes it a more appropriate alternative for the shorter distance typically found in metro access networks.

However, metro networks sometimes demand for longer distance and more wavelengths that CWDM simply cannot satisfy, then DWDM with its narrow channel spacing (0.8 nm) should be put into use. The problem is that the components related to the latter are too expensive for some edge networks. In this case, the best solution is to combine both CWDM and DWDM in metro area networks.

CWDM in the Metro Access

CWDM nowadays commonly supports at least eight of the eighteen ITU-T G.694.2 defined channels over distances of up to 80 km. With simple point-to-point and ring network topology, CWDM eliminates the need for erbium doped-fiber amplifier (EDFA) typically associated with DWDM. CWDM's lower cost and small footprint fit well with customer premises and co-location installations. And due to the readily available of Gigabit interface converters and small form factor pluggable (SFP) transceivers for CWDM platform, it gains in much popularity in enterprise and storage networks. CWDM is most fit in networks with the following features:

• Low channel count of 4 to 8 channels
• Transmission rates of <2.5 Gbits/sec per channel, and short distances of <80 km

DWDM in the Metro Core

Carriers are consistently looking for a cheaper and simpler version of long-haul DWDM, which drives the equipment suppliers to adapt DWDM systems accordingly. Banded wavelength filters, elimination of dispersion compensation, and more tolerant channel spacing were seen as ways to accomplish this goal. Nowadays, DWDM is well suited to high-capacity core networks in the metro, and to regional extensions between metro areas.

Cost Concerning CWDM and DWDM

The cost is still presented as a key difference in metro network systems. DWDM lasers are generally more expensive than those applied in CWDM system, the cooled DFB lasers offer cost-effective solutions for high-capacity large metro rings. And the cost of the this system is amortized over the large number of customers served by the systems. Whereas for metro access networks that demand for lower-cost and lower-capacity systems, it heavily depends on what the customer is willing to pay for broadband service. Since a metro access application would have fewer wavelengths, so based on equipment cost, CWDM is a more profitable solution for metro access points where cost is more important than capacity.

The Future of CWDM and DWDM in Metro Network

Some vendors offering both CWDM and DWDM technologies have merged the system building blocks onto a single platform. This approach allows the de-multiplexed CWDM traffic to be directly connected to DWDM transponders, saving equipment and space. It also enables end-to-end performance monitoring and cost optimization throughout the entire metro network. Then there is no need to choose between these two WDM technologies. The better choice is an integrated solution that makes use of the economies of CWDM for shorter distances, and provides the power of a DWDM network where longer distances and more capacity are needed. The result is an integrated, economical network that doesn't require a carrier to compromise on quality, quantity, or cost. The DWDM building blocks are shown below.

Conclusion

From what we have discussed in this article, we can conclude that metro networks will benefit from the mixture of CWDM and DWDM systems. And metro networks are becoming more flexible over this converged solution: with CWDM fitting the needs of today, and DWDM for the growing demand for increased coverage in the future. Take advantage of both the coarse and dense WDM technology, this integrated metro network that delivers much reliability and flexibility is the trend of the future.

Source: http://www.fiber-optic-solutions.com/cwdm-dwdm-metro-networks-economical.html

How to Use WDM for Fiber Capacity Expansion?

Imagine turning a cottage into a majestic skyscraper without having to deliver any innovation or construction. This is what wavelength division multiplexing (WDM) allows with your existing fiber optic network. The hunger for bandwidth propels service providers to make a substantial investment in upgrading fiber cabling infrastructure. This can be a challenge both economically and practically. However, the WDM technology offers an alternative to increase capacity on the fiber links that are already in place. Without deploying additional optical fiber, WDM greatly reduces the cost of network expansion.

WDM Technology Explanation

Let’s begin with the most fundamental question: What is WDM technology? Short for wavelength division multiplexing, WDM is a way of transmitting multiple simultaneous data streams over the same fiber. Since this happens simultaneously, WDM does not impact transmission speed, latency or bandwidth. WDM functions as multiplexing multiple optical signals on a single fiber by using different wavelengths, or colors, of laser light to carry different signals. Network managers can thus realize a multiplication effect in their available fiber’s capacity with WDM.

To implement WDM to the infrastructure is rather simple, WDM setup generally consists of the following:

• WDM transmit devices, each operating at a different wavelength
• Multiplexer, a passive device that combines the different light sources into a blended one
• Fiber infrastructure
• De- Multiplexer, a passive device that splits the blended light source into separate ones
• WDM receive devices

What Capacity Increase Can We Expect?

There are two variants of WDM: CWDM (coarse wave-division multiplexing) and DWDM (dense wave-division multiplexing). The only difference between them is the band in which they operate, and the spacing of the wavelengths and thus the number of wavelength or channels that can be used.

When using WDM on existing fiber cabling, you should also consider the fiber type (single-mode or multimode) and loss level. For CWDM, 8 to 18 devices may be possible, whereas for DWDM, up to 40 channels are the most common case, but it is possible to reach up to 160 channels.

Choose the Right Type of WDM

We’ve known that both CWDM and DWDM are available to optimize network capacity. Then, here comes another question: should I choose CWDM or DWDM technology? Let’s make a comparison of them.

Coarse Wave Division Multiplexing (CWDM)

CWDM increases fiber capacity in either 4, 8, or 18 channel increments. By increasing the channel spacing between wavelengths on the fiber, CWDM allows for a simple and affordable method of carrying up to 18 channels on a single fiber. CWDM channels each consume 20 nm of space and together use up most of the single-mode operating range.

Benefits of CWDM:

• Passive equipment that uses no electrical power
• No configuration is necessary, much lower cost per channel than DWDM
• Scalability to grow the fiber capacity as needed
• With little or no increased cost
• Protocol transparent and ease of use

Drawbacks of CWDM:

• 18 channels may not be enough, and fiber amplifier cannot be used with them
• Passive equipment that has no management capabilities
• Not the ideal choice for long-haul networks

Dense Wave Division Multiplexing (DWDM)

DWDM allows many more wavelengths to be combined onto one fiber. DWDM comes in two different versions: an active solution and a passive solution. An active solution requires wavelength management and is well-suited for applications involving more than 32 links over the same fiber. In most cases, passive DWDM is regarded as a more realistic alternative to active DWDM.

Benefits of DWDM:

• Ideal for use in long-haul and areas of greater customer density
• Up to 32 channels can be done passively
• Up to 160 channels with an active solution
• Active solutions involve EDFA optical amplifiers to achieve longer distances

Drawbacks of DWDM:

• DWDM solutions are quite expensive
• Active DWDM solutions require a lot of set-up and maintenance expense
• Very little scalability for deployments under 32 channels, much unnecessary cost is incurred per channel

To sum it up, CWDM can be typically used for applications that do not require the signal to travel great distances and in locations where not many channels are required. While for applications that demand for a high number of channels or for long-haul applications, DWDM is the ideal solution.

Considerations for Deploying WDM

Making sure that the CWDM and DWDM will perform properly is critical, so one should account for the following aspects for when deploying.

1.Before buying a mux or demux for use in an unconditioned cabinet or splice case, verify that the operating temperature will fit the application. And ensure that the CWDM or DWDM will be able to operate within the temperatures in which they will be placed.

2.Take the insertion loss of WDM network into account. Using the maximum insertion loss value in the link budget is a good idea. Calculate the loss for both the mux and demux components.

Conclusion

WDM technology provides an ideal solution for fiber exhaust problem that many communication providers are experiencing. It eliminates the need for investing on new fiber construction projects while greatly increases fiber capacity of the existing infrastructure. Hope what presented in the article could help you to choose the right WDM solution.

Source: http://www.fiber-optic-solutions.com/use-wdm-fiber-capacity-expansion.html

Introduction to Optical Add-Drop Multiplexer (OADM)

There exist several different channel routing technologies in the field of optical communications. However, the evolution of single wavelength point-to-point transmission lines to wavelength division multiplexed optical networks has introduced a demand for wavelength selective optical add-drop multiplexers (OADM) to separate or route different wavelength channels. This article provides some fundamentals relevant to OADM.

What Is OADM?

Optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing (WDM) systems. "Add" and "drop" is a capability device to add one or more new wavelength channels to an existing multi-wavelength WDM signal or to drop, which means to remove one or more channels, passing those signals to another network path. OADM can be used at different points along the optical link to insert, remove or route selected channels thus to increase the network flexibility. OADM is particularly important in metropolitan WDM light wave services where offices or sited can be connected by different add-drop channels.

A traditional OADM consists of three stages: an optical demultiplexer, an optical multiplexer, and between them a method of re-configuring the paths between the demultiplexer, the multiplexer and a set of ports for adding and dropping signals. The demultiplexer separates wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a fiber patch panel or by optical switches which direct the wavelengths to the multiplexer or to drop ports. The multiplexer multiplexes the wavelength channels that are to continue on from demultiplexer ports with those from the add ports, onto a single output fiber.

The Functions of OADM

As we have mentioned above, the main function of an optical multiplexer is to couple two or more wavelengths into the same fiber. If place a demultiplexer and properly aligned it back to back with a multiplexer, there would exist two individual wavelength in the area between them. Then, this offers a chance for an enhanced function that individual wavelengths could be removed and inserted as well. The function would be called an optical wavelength drop and add demultiplexer/multiplexer—to make it briefly, optical add-drop multiplexer.

The model of an OADM is clearly shown in the picture below, where F1 signifies a filter selecting wavelength λ1 while passing through all other wavelength, and M1 signifies a multiplexer that multiplexes all wavelengths.

An even better view of OADM function is shown in the following picture. This function is often employed in WDM ring systems as well as in long-haul with drop-add features.

Types of OADM

There are two main types of OADM that can be used in WDM optical networks: fixed OADMs that are used to drop or add data signals on dedicated WDM channels, and re-configurable OADMs that have the ability to electronically alter the selected channel routing through the optical network.

The fixed optical add-drop multiplexer (FOADM) is a traditional wavelength arrangement scheme that can only input or output a single wavelength via the fixed port. FOADMs have pre-assigned channels at static nodes and allowed adding and dropping of individual or multiple wavelength channels from a DWDM.

The re-configurable optical add-drop multiplexer (ROADM), on the other hand, is a dynamic wavelength arrangement scheme, allows for dynamic wavelength arrangement scheme using a wavelength selective switch (WSS). The 8-dimensional cross-connect provided by the WSS enables quick service start-up, remote cross-connect and WDM mesh networking. The ROADM scheme can also achieve inputting/outputting a single wavelength or wavelength group via the fixed port. ROADM can add, block, pass or redirect modulated infrared (IR) and visible light beams of various wavelengths in a fiber optic network. Which featured by providing flexibility in rerouting optical streams, bypassing faulty connections, allowing minimal service disruption and the ability to adapt or upgrade the optical network to different WDM technologies.

Conclusion

To summarize, OADM plays a vital role in improving and optimizing the network performance and reliability. And it is fully compatible with both local area network (LAN) as well as long haul networks. Moreover, OADM also serves as an essential device to meet the requirement of the rapidly developed network.

First published: http://www.china-cable-suppliers.com/introduction-to-optical-add-drop-multiplexer-oadm.html