Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for optimizing PON efficiency using a variable FEC. In one aspect, an OLT broadcasts a message over an optical network, and receives, from an ONU, a response to the message. The OLT transmits activation parameters specifying that the OLT supports multiple different FEC code lengths when FEC is activated, and receives from the ONU, a set of FEC code lengths supported by the ONU. The OLT also receives performance measures collected by the ONU. The OLT selects, from among the FEC code lengths supported by the ONU, one or more appropriate FEC code lengths for communications between the ONU and the OLT based on the performance measures collected by the ONU and performance measures collected by the OLT. The OLT configures the ONU to utilize the appropriate FEC code length.