Synchronous, asynchronous, and data rate transparent fiber optic communications link

A circuit for restoring symmetry to an unsymmetrical clock waveform derived from a pulse width modulated optical signal in an optic fiber communications system comprising:

• means for receiving a pulse width modulated input signal;
• means for delaying said pulse width modulated input signal to form a delayed pulse width modulated input;
• means for combining said pulse width modulated input signal with said delayed pulse width modulated input to generate a waveform having one signal value only when one of but not both of said pulse width modulated input signal and said delayed pulse width modulated input are of a first signal value;
• means for gating said pulse width modulated input signal with said waveform to generate another waveform having said one signal value only when both said pulse width modulated input signal and said waveform exhibit said one signal value;
• and
• a voltage-controlled monostable multivibrator connected to an output of said means for gating and functioning to stretch said output of said means for gating into a 50% duty cycle clock waveform.