# How to Calculate DWDM System Loss in Long Haul Transmission

Nowadays, high capacity network is needed to deal with large amount of data transfer. The application of DWDM (dense wavelength division multiplexing) system is a commonly used technique to enhance network capacity. Due to its complexity caused by various components like EDFA and dispersion compensating module (DCM), it may be difficult to calculate the loss over the whole links, especially in long haul transmissions. Then, how to calculate the budget loss of a DWDM system in long haul transmission? This post will illustrate the method to solve this problem.

Causes of Loss in Long Haul DWDM System

Loss budget is always one of the crucial problems that need to be considered before deploying a network. Any components in optical links will introduce loss. For example, when add a DWDM mux to a DWDM network, it will cause insertion loss which is the total optical power loss (often measured by dB) caused by the insertion of an optical component. In long haul DWDM system, there are several causes of link loss.

DWDM Muxs

DWDM Muxs are the components that combine several different wavelengths so that they can be transferred on one fiber. And Mux is a passive device that cannot strengthen the light signals. Therefore, there is a big insertion loss of DWDM Mux. The lower the insertion loss is, the less network deployment cost is needed. Although Mux vendors are always endeavoring to reduce the insertion loss, there are still big differences between DWDM Muxs of many vendors. Here is a histogram to provide a direct-viewing comparison. From the graph, we can see the maximum insertion loss of FS.COM 40CH DWDM Mux is only 4.5dB. DCM (Dispersion Compensation Module)

Dispersion compensation module is to fix the optical signals that have been deformed by chromatic dispersion. Therefore it is important to use at the receiver end to recover the signals by reshaping the optical pulse. This component also brings a good amount of insertion loss.

OADM is another device that introduces insertion loss for DWDM networks. Since it allows individual or multiple wavelength channels to be added or dropped from an incoming link, as the signal pass from the common port to add/drop port or from the add/drop port to the common port, insertion loss occurs.

Except for the components that cause loss for the whole links, fiber optic cables also introduce loss which increases as the distance gets longer. Besides, in order to achieve balance signal power for receivers, designers often use EDFA to boost or add gain to optical signals on a fiber optic cable.

How to Calculate the Loss Budget of DWDM System?

In order to illustrate the calculation process clearly, here takes a case from FS.COM as an example. This deployment solution is designed for a client in UK. The following picture just shows part of the solution design. The distance of site A and site B is 132.4km. From the figure, we can see optical components used includes 40CH DWDM Mux/Demux, booster EDFA, Pre-amplifier, 2CH OADM, etc. And there are also optical attenuators which are not show in the figure. Here is a chart indicating the loss or gain value of them which can be found in FS.COM website.

 Components Insertion loss/Gain 40CH DWDM+MON 4.5dB Booster EDFA 23dB Pre-amp EDFA 26dB Attenuator 0-30dB

Now let’s start to calculate the budget loss in this link. Considering the whole solution is so complicated that this calculation is just to give an example of calculating the budget loss between site A and site B. And the calculation starts from site A to site B unidirectional, as the arrow line shows.

• The loss between the 40CH DWDM Mux and the router is -8.5dB, caused by the use of an optical attenuator.
• Then the signals pass through the DWDM Mux, the output power is:-8.5dB-4.5dB =-13dB.
• Since the booster EDFA gain is +23dB, the signal output power of this EDFA is: -13dB+23dB= +10dB.
• Calculation of the fiber optic cable, the total loss is: -0.22dB/km x132.4km = -29.13dB. The input power of the pre-amp EDFA is: +10dB-29.13dB = -19.13dB.
• The pre-amp EDFA gain is +26dB, then the output power of this EDFA is +6.87dB.
• There is an attenuator placed after the pre-amp EDFA, the output power after the attenuator is: +6.87dB-18dB = -11.13dB.
• The output power of the booster EDFA is: +23dB-11.13dB=+10.67dB. That’s the output power of site B.
Summary

For DWDM network system, how to control the power loss is important, which requires network designers calculate the budget loss before deploying the systems. This post gives an example from our client to calculate the loss in a DWDM network. FS.COM offers both necessary optical components and solutions for your networks. If you are interested, please contact us via sales@fs.com.