General noise analysis of recirculating delay line structures

Recirculating delay lines have value notably as sensors and optical signal processors. One of their main limitations is that phase noise from the source is converted to amplitude noise at the detector and that intensity noise is also filtered by the system. An expression is derived for the filtering effect that a recirculating delay line topology has on both the source noise and amplified spontaneous emission from in-line amplifiers. From these expressions it is shown that some delay line topologies have far superior noise characteristics compared to others.