Overview
Optics have thresholds for how strong or weak of a light level they can receive before they'll theoretically fail
- The purpose of this page is to help estimate if a particular optic will work on a particular fiber span
Terminology
- Power level
- The power level from optics is measured in microwatts but converted to dBm for ease of reading by humans
- dB is on a logarithmic scale
- The higher the dBm the stronger your receive level (the brighter your receive level)
- The lower the dBm the weaker your receive level
Estimates
- Ballpark estimates:
- 0.25dB loss per kilometer for 1550nm
- 0.5 dB loss per kilometer for 1310nm
- 0.3 dB loss per connector
Tools & Resources
DOM
See our knowledge base page Optical: Digital Optical Monitoring (DOM)
Command line tools to see reported RX and TX light levels of and optic inserted in a router or switch and current levels
Vendor Docs
You should also be able to search your optical vendor's documents to find the rx and tx thresholds for each model optic
RECEIVE MIN. -23.00dBm
RECEIVE MAX. (RECEIVER OVERLOAD) -8.00dBm
TRANSMIT MIN. 0.00dBm
TRANSMIT MAX. 5.00dBm
POWERBUDGET (DB) 23 dB
OTDR
An OTDR is always preferred to accurately determine your path's loss
The calculations performed on this page are intended to help when you don't have an OTDR test to rely on
Power meter
Tool used to report the current light levels (dBm) being received
Pads/Attenuators
Used to attenuate the light power level to be within the threshold of an optic
Always pad optics on the the rx ports of your optics (one pad on the rx port of the hub interface, one pad on the rx port of the cpe interface)
Example image:
Calculating Loss
- Record the TX power of your optic
- Either by using DOM or a power meter
- Subtract the OTDR results (loss in dB)
- If you don't have OTDR test results, subtract the estimated loss based on the distance
- Subtract the connector loss (number of patches)
- Results
- Is the result within your optic's receive min -> max?
- If it's above the max RX add the appropriate pad
- If it's below the max RX you might need a different optic or
- Is the result within your optic's receive min -> max?
Example:
- I'm transmitting at +1dB
- I don't have an OTDR, but the fiber path distance is 27km
- Subtract 0.4 dB loss per kilometer for 1550nm at 27km, subtract 10.8dB
- I'm using two patch cables, one on each side of my fiber span, subtract ~1dB
- Result: −10.8dB
- My result of -10.8dB is within the -8dB to -23dB receive specification for the optic
- I shouldn't need a pad
- My optic should work