Pulse time modulation for subcarrier multiplex (SCM) systems

Subcarrier multiplexing (SCM) is an attractive multiplexing technique to deliver broadband services over optical fibres. The majority of existing systems employ an analogue signal for optical transmission and therefore the performance of these systems are limited. This paper proposes to improve SCM system performance by converting the analogue SCM signal into a pulse time modulated signal for optical transmission. A prototype system has been built where a PAL video signal, two audio signals and a data signal form the composite analogue signal. The pulse time modulation techniques investigated are pulse frequency modulation (PFM) and square wave frequency modulation (SWFM). System measurements and theoretical predictions were obtained and they are in close agreement. We have improved the dynamic range and optical power budget considerably, compared to conventional systems.