Assessment of the DC Bias to Mitigate the Clipping Noise in DCO-OFDM, ACO-OFDM; and Non-linear Distortion of DFB Laser Transmitted through Dispersive Single Mode Fibers in IM/DD Systems

Abstract

An analytical investigation on the impact of the DC bias on the clipping noise and the laser non-linearity in Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) and Direct Current--Biased Optical OFDM (DCO-OFDM) is performed. Clipping noise in the OFDM system is introduced either by transmitter non-linearity or due to the low power sensitivity of the receiver. However the clipping noise in the DCO-OFDM is also dependent upon the DC bias applied to make the bipolar OFDM signal unipolar. The addition of this extra DC bias increases the average signal power of the DCO-OFDM signal. The non-linearity of the DFB laser increases with the optical modulation index, which gives rise to intermodulation and harmonic distortions at the receiver. The OFDM signal when transmitted through the Single Mode Fiber (SMF) suffers from group--velocity dispersion (GVD) which limits its performance in the Passive Optical Networks (PON). In this paper, an assessment of the DC bias to reduce the clipping noise in the two systems under consideration is performed. A practical laser model is considered and analysis is performed to estimate the average laser drive power to maintain its non-linearities within a tolerable region for both the OFDM systems. A simulation is performed to transmit 5 and 8 Gb/s (ACO and DCO) OFDM signal over 120 km SMF where the effect of GVD is taken into consideration. An optimum OFDM drive signal power is calculated to maintain the laser transmitter in its linear region such that the effect of second order harmonic distortions (HD2) and third order intermodulation distortion (IMD3) is low. The SNR requirement of DCO-OFDM being higher than the ACO-OFDM leads to more clipping noise, HD2 and IMD3.