Adaptive equalisation for broadband predistortion linearisation of optical transmitters

Abstract

Polynomial predistortion is known to have limited signal bandwidth and modulation-depth range above which the improvement gained by linearisation quickly diminishes. This problem has been studied for RF power amplifiers in the context of wireless communications but less so for analogue fibre optic links which could greatly benefit from a broadband, large dynamic range predistortion lineariser. Behavioural models are presented for static and dynamic nonlinearities based on the Volterra series. It is shown that for modulation frequencies much lower than the relaxation oscillation frequency, a simple predistortion architecture that employs only postfiltering is adequate to compensate for memory effects that limit predistortion bandwidth. Design considerations for an analogue adaptive tap delay line filter are also presented along with the general frequency response needed to compensate for memory. The dependence of predistortion parameters on modulation depth is analysed, and it is shown that an adaptive predistorter using instantaneous digital adaptation (IDA) provides both a larger signal-to-noise and distortion ratio (SNDR) and modulation depth range at the transmitter for a given noise floor.