Non Linear Effects in a DPSK System

Abstract

Recently much interest has been shown in the advanced data modulation techniques in order to reduce the impairments in the system. Direct detection differential phase-shift-keying modulation has gained a lot of attraction in the recent past. This is accredited to the two main advantages provided by this modulation technique: improved immunity to the fiber nonlinear effects and better sensitivity at the receiver side. It has already been shown, through experiments and in theory, an optically pre-amplified DPSK-balanced receiver requires about half number of photons/bit compared to the ON–OFF-keying (OOK) technique. Thus, the required optical signal-to-noise ratio (OSNR) in DPSK systems is 3 dB less than the conventional OOK systems. Due to this improvement, the transmission distance is extended and the system margin is increased. It also allows lower fiber launch power to avoid the degradation of signals due to the fiber nonlinear effects. The DPSK system also has higher tolerance to fiber nonlinear effects such as cross-phase modulation besides the improved receiver sensitivity. The nonlinear impairment is reduced by the constant amplitude modulation which in turn reduces the nonlinear effects dependent on pattern. In this thesis work, we have verified that DPSK performs better than other modulation techniques like NRZ for long distance communication. The main advantage of DPSK over NRZ is the constant transmission power in the channel due to which the DPSK technique can eliminate SPM and XPM effects. Further, we investigated the performance of two intensity modulators- Electro-absorption modulator (EAM) and Mach Zehnder modulator (MZM) used in the IM-DPSK technique. The high Q-factor and low BER for the system using MZM at all the transmission points proves that MZM is a better choice for intensity modulation in IM-DPSK technique. Finally, we proposed that using mid link spectral inverter in transmission link for IM-DPSK based transmission system makes the system more tolerant to the effects of nonlinearities and is convenient to use since it reduces the need of OEO conversion.