Connecting Optical Modules on short distance links
Testing is important part before actual deployments as it can save a lot of time in case there are some issues. Therefore, there are many situations that before actual deployment customers would like to make local tests in their lab.
Such kind of test is also important to make sure optical transceivers contain correct coding and links can be made between two sites. If the coding is wrong the transceiver will not be recognized, and links will not go UP. This is not a good situation if you are located at the remote site and hope to launch the link by plug and play.
It is not necessary to utilize full site-to-site fibre infrastructure to test transceivers, one rack-to-rack setup is enough. Mostly because optical fibre can be simulated with attenuators (in this kind of setup non-linear effects are not taken in account).
For example, if we take:
– 80km 10G SFP+ optical transceivers, EDGE Part number: 10G-SFP-80, ( Technical specification):
Then from datasheet, we see following parameters:
Transceiver has maximum transmit power +4dBm, Receiver overload starts at -8dBm, this means we cannot connect one 80km transceiver directly with short patch cord to other transceiver. There is a high chance that receiver side element might get burned from such strong Tx signal.
In this situation if we do lab tests with short patch cable it is strongly recommended to use at least 15dB optical attenuators. They look as in below picture (particular one is for use in modules which has LC connectors) and we plug them inside optical module and then attach patch cable to it:
Why in this mentioned example we need to use 15dB attenuators?
– Maximum Tx is 4dB and if we add 15dB loss to it, then signal will be 4-15dB=-11dB, Rx side receiver overload is -8dB so with help of 15dB attenuators our Tx side will be below this signal and will not cause Rx overload. Attenuators adds attenuation [dB] in connection between transceivers. The increase in attenuation simulates optical fibre distance. The bigger attenuation, the longer optical fibre path can be simulated.
If you have any doubts if you can connect modules you have on hands for short distance connections please feel free to write: firstname.lastname@example.org and we will do best to give our suggestions 🙂