Tip of the Week – 20 Nov 2013

Tip of the Week – Transceiver Optical Budget

Optical transceivers such as XFPs, SFPs, Xenpaks, etc. need to receive an optical signal power within a specific operating range. If the optical signal is too weak, then the receive signal bit error ratio (BER) will be worse than specified by the manufacturer and/or loss of signal LOS alarm may occur. The lowest valid signal receive level at the transceiver that will meet the manufacturer’s specified BER is referred to as the Receiver’s Sensitivity. Typically 80km, 10G SFP and XFP Receiver Sensitivity is ~ -23 dBm.

Also, the optical signal power cannot be too strong. If it is, the same will occur, poor BER or loss of signal. This hot limit where the receiver will no longer operate within BER specification is referred to as the Receiver’s Overload Threshold. In addition if the signal is very strong it is possible to damage the unit. This is referred to as the Receiver’s Damage Threshold and can be a few dB greater than the Overload Threshold (dependent on transceiver, check specs). Depending on the transceiver the Overload Threshold can be anywhere between -7 dBm to +3 dBm.

The operating optical power range between the Receiver’s Sensitivity and the minimum transmitter output power is referred to as the transceiver’s optical budget.

These transceiver parameters vary greatly among transceivers and are available in the manufacturer specifications. When operating or designing any fiber link, the received optical power needs to be maintained between the Receiver’s Sensitivity and Overload Threshold in order for the link to operate properly with a BER equal to or better that specified by the transceiver manufacturer. It is also wise to leave a few dB margin for laser degradation over time and dirty connectors too.

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To learn more about this topic and other fiber optic network topics, refer to my following books (publisher McGraw-Hill):
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