Selective multi-channel coherent optically preamplified systems: sensitivity assessment

This paper investigates the application of an erbium doped fibre amplifier (EDFA) in a coherent receiver using pulse position modulation (PPM) format. Such a combination can exploit the abundant monomode fibre bandwidth to offer improved sensitivity and improved multichannel selectivity through the exploitation of coherent transmission. Original theoretical results are presented for a system operating at a bit rate of 622 Mbit/s and a wavelength of 1.53 μm. By using a suboptimal matched filter approach and assuming thermal noise dominant conditions, the results demonstrate that the system offers 7.5 dB sensitivity improvement over a homodyne PPM system and 9.4 dB improvement over PSK homodyne. The system investigated also offers a 10.4 dB sensitivity improvement over an equivalent optically preamplified direct detection (DD) PPM system. It was also demonstrated that by using an optical filter in order to reduce the ASE noise the improvement in receiver sensitivity was only 0.3 dB. This greatly reduces the requirement for optical filters in coherent systems and therefore the full bandwidth of the optical amplifiers can be used