ACCURATE BIT ERROR RATE EVALUATION IN OPTICALLY PREAMPLIFIED DIRECT-DETECTION p. Martelli^'^, S. M. Pietralunga^ D. Nicodemi^ and M. Martinelli^'^ CoreCom, Via G. Colombo 81, 20133 Milano, Italy, martelli@corecom.it Politecnico di Milano, Dipartimento di Elettronica e Informazione, Via G. Ponzio 34/5, 20133 Milano, Italy Abstract: We present an accurate method for evaluating the bit error rate in optically preamplified direct-detection, by accounting for both the intersymbol-interference and the exact Laguerre photon-count statistics. A quantum-limited sensitivity of 33.9 photons^it is derived. 1. INTRODUCTION We propose a novel method to evaluate the bit error rate (BER) in optically preamplified intensity-modulated direct-detection (IM-DD) systems, based on the Laguerre photon-count statistics predicted by the theory of the photode- tection [1,2]. The previously published works [3,4] which use the Laguerre distribution to calculate the BER in on-off-keying (OOK) IM-DD systems, as- sume a measurement time Tm - l/R, where R is the bit rate, and do not con- sider the signal distortion due to the optical filtering, so that the intersymbol- interference (ISI) is neglected. Besides, they assume a number of modes M of polarised ASE noise equal to the product between the optical bandwidth and the measurement time. These approximations can be considered as ac- curate whenever the optical bandwidth is much greater than R. Unfortunately, in dense wavelength-division multiplexing (DWDM) systems this condition is typically not verified, as one usually deals with optical and electrical band- widths of comparable extent. The method for evaluating the BER presented in this paper calculates the photon-count statistics of the directly detected noisy amplified signal, while accounting for both the effect of the ISI and the dependence of the ASE mode