Optical transmission system with a receiver using full optical clock recovery

Abstract

In an optical transmission system a transmitter ( ) generates a sequence of time domain division multiplexed pulses and transmits them via an optical transmission medium ( ) to a receiver ( ). In the receiver ( ) the received optical multiplex signal is applied to an optical demultiplexer ( ) and to a clock recovery element ( ). The clock recovery element ( ) generates optical clock pulses which are used to control optically controlled optical switches ( ) which demultiplex the optical signal. The optical clock recovery circuit is based on a laser which is mode locked by the received optical signal. The mode locked laser comprises an optical amplifier ( ) having its output coupled to its input by a feedback loop comprising a polarization controller ( ), a saturable absorber ( ), a coupling element ( ) and a adjustable delay ( ). The received optical signal is coupled into the feedback loop by means of the coupling element ( ). The saturable absorber ( ) comprises a 3 dB, 90° coupling element ( ) being coupled to a secondary loop. Said secondary loop comprises an attenuator ( ) and an amplifier ( ). The amplifier ( ) is placed asymmetrically in the secondary loop. Due to the non-linearity in the secondary loop, the optical pulses propagating in the secondary loop interfere destructively for low signal levels, and interfere constructively for high signal levels.