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The use of a spectrally sliced (SS) broad-band source provides a cost effective alternative to laser diode sources for wavelength-division multiplexing (WDM). In this paper the performance of direct detection p-i-n diode intensity modulated SS systems is analyzed using the saddlepoint approximation. Using this approach, based on moment generating functions, the effects of pulse distortion, intersymbol interference (ISI) and nonideal modulator extinction ratios have been included in the analysis of SS systems for the first time. The presence of ISI error floors as the product of optical bandwidth and bit time decreases is demonstrated using a model that agrees with previous work in the case of low bit rate, widely spaced channels. In addition, comparisons with recent experimental results from the literature are made with good agreement. Subsequently, the use of an integrate and dump filter where the integration time starts after the beginning of a bit is investigated and shown to produce significant enhancements to bit-error rate performance. Furthermore, it is shown that a modulator extinction ratio of 20 dB will suffice for 10/sup -9/ bit-error rate (BER) in most cases, with 30 dB offering a performance close to that possible with an ideal modulator. Finally, the likely system power for an SS system is presented.