Optical fibre systems for signal processing

Abstract

A single-mode optical fibre is an extremely high bandwidth, very low attenuation transmission medium. For signal processing applications, the large information bandwidth which may be modulated onto an optical carrier implies that time delay-bandwidth products of the order of 106 may be realised. This is some two orders of magnitude higher than the TB product which may be achieved using conventional delay line media. Signal bandwidths of perhaps 100 GHz may be contemplated, since the use of the very high (optical) carrier frequency implies very small fractional bandwidth occupancy. An optical fibre delay line signal processor requires that these advantages be exploited via some suitable technology. The exploitation process may take several forms. At its simplest, the data may be (usually intensity) modulated onto the optical carrier and launched into a conventional fibre optic trans mission system. The receiver will then combine predetermined fractions of this data with the input data and retransmit the rest to form a multiple tapped delay line system. The tapping and combining network is, in this case, entirely electronic and this system is distinguished from an identical electrical network only in the use of the fibre delay line which may offer advantages concerning low radiated signal strengths and compact format. Of more interest is an all-optical delay line signal processor in which the tapping and recombining are effected in the optical domain. The speed of this processor is not electronically limited, and most of the discussion in this chapter investigates the properties of this class of processor.