Loss Budget Analysis of Fiber Optic Cables

28 December, 2013 (16:26) | Fiber Optic Cables | By: fiberopticis

Loss budget analysis is the calculation and verification of a fiber optic system’s operating characteristics. This encompasses items such as routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and bandwidth are the key parameters for budget loss analysis.

Analyze Fiber Optic Link Loss In The Design Stage
Prior to designing or installing a fiber optic system, a loss budget analysis is reccommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the budget loss calculation. Passive loss is made up of fiber loss, connector loss, and splice loss. Don’t forget any couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range. Prior to system turn up, test the circuit with a source and FO power meter to ensure that it is within the loss budget.

The idea of a loss budget is to insure the network equipment will work over the installed fiber optic link. It is normal to be conservative over the specifications! Don’t use the best possible specs for fiber attenuation or connector loss – give yourself some margin!

The best way to illustrate calculating a loss budget is to show how it’s done for a 2 km multimode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle. See the drawings below of the link layout and the instantaneous power in the link at any point along it’s length, scaled exactly to the link drawing above it.


Fiber Optic Cable Plant Passive Component Loss

Step 1. Fiber loss at the operating wavelength

Cable Length 2.0 2.0    
Fiber Type Multimode   Singlemode  
Wavelength (nm) 850 1300 1300 1550
Fiber Atten. dB/km 3 [3.5] 1 [1.5] 0.4 [1/0.5] 0.3 [1/0.5]
Total Fiber Loss 6.0 [7.0] 2.0 [3.0]

(All specs in brackets are maximum values per EIA/TIA 568 standard. For singlemode fiber, a higher loss is allowed for premises applications. )

Step 2. Connector Loss
Multimode connectors will have losses of 0.2-0.5 dB typically. Singlemode connectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors may have losses as high as 0.5-1.0 dB. Let’s calculate it at both typical and worst case values.

Connector Loss 0.3 dB (typical adhesive/polish conn)  0.75 dB (TIA-568 max acceptable)
Total # of Connectors 5  5
Total Connector Loss 1.5 dB  3.75 dB

(All connectors are allowed 0.75 max per EIA/TIA 568 standard)

Step 3. Splice Loss
Multimode splices are usually made with mechanical splices, although some fusion splicing is used. The larger core and multiple layers make fusion splicing abut the same loss as mechanical splicing, but fusion is more reliable in adverse environments. Figure 0.1-0.5 dB for multimode splices, 0.3 being a good average for an experienced installer. Fusion splicing of singlemode fiber will typically have less than 0.05 dB (that’s right, less than a tenth of a dB!)

Typical Splice Loss 0.3 dB
Total # splices 1
Total Splice Loss 0.3 dB

(All splices are allowed 0.3 max per EIA/TIA 568 standard)

Step 4. Total Passive System Attenuation
Add the fiber loss, connector and splice losses to get the link loss.

  Best Case  TIA 568 Max
   850 nm  1300 nm  850 nm 1300 nm
Total Fiber Loss (dB) 6.0 2.0  7.0 3.0
Total Connector Loss (dB) 1.5 1.5  3.75  3.75
Total Splice Loss (dB) 0.3 0.3  0.3  0.3
Other (dB) 0 0  0 0
Total Link Loss (dB) 7.8 3.8  11.05  7.05

Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.

Equipment Link Loss Budget Calculation: Link loss budget for network hardware depends on the dynamic range, the difference between the sensitivity of the receiver and the output of the source into the fiber. You need some margin for system degradation over time or environment, so subtract that margin (as much as 3dB) to get the loss budget for the link.

Step 5. Data From Manufacturer’s Specification for Active Components (Typical 100 Mb/s link)

Operating Wavelength (nm) 1300
Fiber Type MM
Receiver Sens. (dBm@ required BER) -31
Average Transmitter Output (dBm) -16
Dynamic Range (dB) 15
Recommended Excess Margin (dB) 3


Step 6. Loss Margin Calculation

Dynamic Range (dB) (above) 15  15
Cable Plant Link Loss (dB) 3.8 (Typ)  7.05 (TIA)
Link Loss Margin (dB) 11.2  7.95

As a general rule, the Link Loss Margin should be greater than approximately 3 dB to allow for link degradation over time. LEDs in the transmitter may age and lose power, connectors or splices may degrade or connectors may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration.

NOTE: Many techs forget when doing a loss budget that the connectors on the end of the cable plant must be included in the loss budget. When the cable plant is tested, the reference cables will mate with those connectors and their loss will be included in the measurements.

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