Testing & Commissioning Optical Fiber Link -Part 1

Calculation of Optical Power Budget 

What is Optical Power Budget ?

The optical power budget identifies how much attenuation or loss is allowed across a fiber span while still maintaining sufficient output power for the receiver.

• Assuming that we are in the planning of installing a singlemode fiber optics cable from point A to point B for our data communication project.
• Prior to actual fiber implementation we need to calculate before hand the allowable maximum loss  or optical power budget or optical power loss.

Figure 1 : Cabling Schematic Diagram

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• For Singlemode fiber optics the common operating wavelengths are both 1310nm and 1550nm.
• For Multimode fiber optics the common operating wavelengths are both 850nm and 1300nm.
• Based upon manufacturers fiber specification, the attenuation or fiber loss at both operating wave length are as follows :​​

Figure 2 : Fiber attenuation for both SM and MM

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How To Calculate Optical Power Budget ?

• The Optical Power Budget Loss is equivalent to the total loss of fiber loss + splicing loss + connector loss.
  
• ​It is illustrated as below :

Figure 3 : Optical Power Budget

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Fiber Loss = Length of fiber (km) x Attenuation (dB/km)
Length of fiber (km) - to get this value by using OTDR
Attenuation (dB/km) -  SM 1310nm - 0.3dB/km  & SM 1550nm - 0.2dB/km 
                                             MM 850nm - 3dB/km & MM 1300nm - 1 dB/km

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Splicing Loss  = No. of splicing  x  Max splicing loss (dB)
No. of splicing  - total number of splicings done from Station A to Station B
Max splicing loss  - as per TIA/EIA recommendation  0.2dB for each splicing
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Connector Loss  = No. of connector x Max connector loss (dB)
No. of connectors  - total number of  connectors from Station A to Station B
Max connector loss  - as per TIA/EIA recommendation  0.75 dB for  connectors
(but we take maximum connector loss at 0.5dB)

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Optical Power Budget Calculation Example

Using above Figure 1 fiber optics schematic diagram followings can be deduced :
a. Total length of fiber is 1km
b. 2 nos of splicings
c. 2 nos of connectors
d. SM operating wavelength of 1310 nm - 0.3dB/km, 1550nm - 0.2dB/km

Operating Wavelength at 1310nm
Maximum Allowable Loss = Fiber Loss + Splicing Loss + Connector Loss
= (1 km x 0.3 dB/km) + ( 2 x 0.2dB) + ( 2 x 0.5dB)
= 1.7dB

Operating Wavelength at 1550nm
Maximum Allowable Loss = Fiber Loss + Splicing Loss + Connector Loss
= (1 km x 0.2 dB/km) + ( 2 x 0.2dB) + ( 2 x 0.5dB)
= 1.6dB

• As such, when we do actual measurement using power meter & light source the measured loss must be less than the calculated allowable maximum loss. 
• If the actual measurement value is higher than calculated maximum loss then most likely there will no communication or transfer data between Station A and Station B.​

On next articles  we will emphasized on

• More calculated examples schematic diagram
• How to measure fiber optics loss using power meter & light source
• How to troubleshoot fiber optic link to find the exact causes of higher fiber loss.