Dispersion and off-set filtering in RSOA based networks

Next-generation access networks (NGANs) mainly include wavelength division multiplexing (WDM) based 60 GHz Radio over Fibre systems (60G-RoF) and WDM Passive Optical Networks (WDM-PONs). Semiconductor optical amplifier may be the key device of colorless unit in these WDM based systems. The device can be effectively utilized as external modulator because it can perform simultaneously amplification and modulation. However the main limitation is the modulation speed of the system. The modulation bandwidth of SOA is limited because of internal processes of the device. Bandwidth enhancement can be achieved by means of optical equalization thanks to the chirping characteristics of semiconductor optical amplifier. Direct modulation produces optical signal modulated both in amplitude and phase. The limited modulation transfer function can be combined with the converted phase modulation transfer function. It is validated by simulation and experimental results. On the other hand the bandwidth of the transfer function is also limited by the chromatic dispersion of the optical fiber. WDM-PON and WDM-RoF are designed for 20 - 60 km link length and high transmission speed. So attenuation, dispersion and optical nonlinear effects of the optical propagation affect the transmission and cause limitation. So, both limitation effects must be compensated. In the paper theoretical and experimental investigations of combined effect of chirp and chromatic dispersion and off-set filtering are presented with the result of improved modulation bandwidth. In case of long transmission the low-pass response without off-filtering is reduced.