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A.2 Fiber Optic Calculation
S
TEP
1: A
TTENUATION
C
ONSIDERATIONS
Figure A-1 shows the various elements of the fiber optic link that decrease light
intensity. Depending on the real installation, you must subtract a series of
attenuation (att.) values from the raw optic output level of the fiber optic
transmitter. The remainder of the originally transmitted light at the receiver must
be at least as high as the minimum receiver sensitivity. Generally, you should
project a link margin of at least 3 dB to allow aging and fiber repairs.
Use the formulas described below to calculate the attenuation values for your
system:
• (optical power budget)=(max. of transmitter output power) - (min. of receiver
operation range) The stated optical power budget does not include irregular
light propagation modes. At short distances, these modes increase the optical
power at the receiver. The increase depends mainly on fiber optic mode and
cable quality. Consider the irregular light propagation modes at multimode
only and add about 3 dB. This amount is included in the stated multimode
transmitter output values.
• (total system att.)=sum of all discrete attenuations (as shown in Figure A-1).
This value is the attenuation of the link due to the link components.
• (cable length)=(cable attenuation)/(cable att. factor). This formula is used to
calculate cable length or resulting attenuation. The cable att. factor is provided
by the cable producer.
Example values for calculations below (See Figure A-1):
Fiber optic power budget of the combination unit [1]/unit [7]=17 dB. Cable att.
factor of cables [2], [4], and [6]=3 dB/km. Cable [4]=1 km. Cable [6]=2 km.
Splice [5] loss 1 dB.
Connector loss at patch panel [3]=2 dB.
Example 1: Maximum length of cable [2]:
(cable [4] att.)= 1 km*3 dB/km=3 dB (cable [6] att.)=2 km*3 dB/km=6 dB.
(right part att.)=(Conn. loss [3]) + (cable [4] att.) + (splice [5] loss) + (cable [6]
att.)
(right part att.)= 2 dB + 3 dB + 1 dB +6 dB=12 dB.
APPENDIX: Fiber Optic Planning Guidelines