Effects of Amplified Spontaneous Emission (ASE) on NRZ, RZ and CSRZ modulation formats in single channel light-wave system

In optical networks, erbium-doped fiber amplifiers (EDFAs) are used to provide a wide and flat gain spectrum. Amplified Spontaneous Emission (ASE) emitted from EDFA adds to signal and grows rapidly. This accumulated ASE noise may limit the effective amplification due to saturation effect and affects the receiver´s bit error ratio (BER). In this paper, we show the effects of ASE noise in transmission fiber. Effects have been shown using three different modulation formats: nonreturn-to-zero (NRZ), return-to-zero (RZ) and carrier suppressed return-to-zero (CSRZ). Conventional NRZ modulation format has been used extensively in many data communications systems mainly because of its signal bandwidth, which is about 50% smaller than the RZ format. But RZ modulation format is found to be more immune to fiber non-linearity and thus achieves better performance. Carrier-suppressed RZ (CSRZ) modulation is one of the recently proposed modulation formats for high bit rate transmission systems, which has been intensively investigated in numerical and experimental works.