# Analysis of Power Budget and Link Distance in CWDM System

It can’t be denied that CWDM technology is a cost effective method to increase the capacity in the existing system, which can give different wavelengths to multiple optical signals and multiplex them for transmission through only one single fiber. Different from the DWDM system, the network using CWDM technology are deployed by passive components like passive CWDM Mux Demux, without the need of additional power, which makes CWDM system more commonly used. Do you also plan to build a CWDM system? If yes, you can check the following information for reference, which mainly analyzes the optical power budget in a CWDM system and calculates the CWDM link distance according to the power budget for smoothly deploying a CWDM system.

What’s Optical Power Budget?

Before deploying an optical network, it is very essential to calculate the optical power budget for better deployment. What’s optical power budget? It is just the amount of light available to make a successful fiber connection which can be calculated by analyzing the original output power of the transmitter and the required input power of the receiver. In details, we should firstly learn the optical power that is emitted by the source (also referred to Transmit Power) and the required power of the detector (also called Receiver Sensitivity). Using the first data to subtract the second one, you’ll get the data of the optical power budget which greatly determines the performance of the whole network link.

Here is the equation: Optical Power Budget = Transmit Power – Receiver Sensitivity.

How to Get the Optical Power Budget in a CWDM System?

To estimate the link distance supported by a CWDM system, the optical power budget should be calculated first, which can greatly determine the CWDM link distance. Here will show you a basic CWDM system under an ideal condition to clearly illustrate how to get the optical power budget. In this basic CWDM system, there is a optical transmitter which transmit power is -2 dBm and a optical receiver with -25 dBm receiver sensitivity. Hence, the optical power budget is 23 dB, as shown in the following equation.

Optical Power Budget = Tx Power – Rx Sensitivity = -2 dBm – (-25 dBm) = 23 dB

However, the mentioned CWDM system is just under an ideal condition without loss caused by the signal transmission. In a normal CWDM system, there are many components like passive CWDM Mux Demux, CWDM transceiver inserted. All these components cause insertion loss once they are inserted into the CWDM link. Therefore, when doing the optical power budget, all the loss should be taken into account for calculating the power budget exactly.

Here is more exact equation: Power Budget = Tx Power – Rx Sensitivity – Loss

To get the real power budget of a CWDM system, here offers a simple CWDM link which uses the -2 dBm optical transmitter, -25 dBm optical receiver and four passive CWDM Mux Demux with low insertion loss. Both the stable 4 channel CWDM Mux and stable 4 channel CWDM Demux in the link have 2.0 dB insertion loss, and other two are 8 channel ones feature 2.5dB insertion loss separately, as shown in the figure below. As a result, the total loss caused by the four passive CWDM Mux Demux is 9 dB, resulted from 2.5 dB + 2.0 dB+2.5 dB + 2.0 dB. Then we can get the total power budget, 14 dB. The calculation process is: Power Budget = Tx Power – Rx Sensitivity – Loss = -2 dBm – (-25 dBm) – 9 dB = 14 dB How to Calculate the Link Distance in the CWDM System?

After knowing the optical power budget, let’s calculate the link distance of the CWDM system according to the following equation: Link Distance = Optical Power Budget/Fiber Attenuation. As there may be some other power loss caused by the factors that we didn’t consider like fiber aging, temperature and poor splice, we often subtract 2 dB buffer from the total optical power budget. Meanwhile, the fiber attenuation is changeable according to the wavelength, usually varying from 0.2 to 0.35 dB/km. In this case, we’ll use 0.35 dB/km as a typical data. Then we can get the link distance is about 34 km. The calculation process is Link Distance = Optical Power Budget/Fiber Attenuation = (14 dB- 2 dB)/0.35 dB/km.

Conclusion

This paper intends to illustrate how to calculate the optical power budget and estimate the link distance of a CWDM system according to the optical power budget, which allows for better budget of deploying the CWDM system and eliminates the unwanted or unnecessary issues which may happen in the system deployment. Besides, if you want to make a cost effective CWDM system, you are suggested to buy CWDM components like cheap passive CWDM Mux Demux, CWDM transceivers from FS.COM, which are of good price and quality.

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