There are various levels of intelligence in repeaters/regenerators in order to balance the additional gained noise margin with added latency. Simplest repeaters

There are various levels of intelligence in repeaters/regenerators in order to balance the additional gained noise margin with added latency. Simplest repeaters only do optical-electrical conversion to binary signal and then back to optical signal, more complex ones also do realigning of edges and only minority of the infrastructure actually decodes and understands the protocols involved. Most of the design of optical networks traces back to 90's telco requirements and not to computer networking and during that time telcos were extremely sensitive to latency (the idea being that when the latency of the digital part of the network is smaller than some limit the analog-digital transition can be done by somewhat cheaper hardware). The funky interleaved SDH/SONET frame formats are motivated by ability to implement muldexes that introduce less than entire frame (ie. 125us) of latency.