Analytical formulation framework for directly modulated/detected OOFDM systems

Abstract

Summary form only given. The efficient design of optical communication systems with advanced modulation formats need of an exhaustive mathematical study in order to completely understand the dynamics and design trade-offs involved [1]. The multi-carrier signal's nature of OOFDM signals and the inclusion of the laser chirp into the propagation through a dispersive link makes its rigorous analysis quite challenging, though worthy.We present a comprehensive set of analytical equations which involve the main system design parameters in directly modulated/detected (DM/DD) OOFDM systems. The analytical formulation allows us to understand, for example, the contribution of the laser phase modulation (PM) to the recovery of the information signal, as shown in Fig.1. The total recovered complex symbols in a DM/DD OOFDM system is, thus, the sum of the contribution due to the laser intensity modulation (IM) and that due to the laser PM. We can demonstrate that the latter is proportional to the average optical power, what is right opposite to the widespread affirmation on the optical inefficiency due to the high value of optical carrier needed for DM/DD OOFDM signals [2]. Comprehensive expressions are also presented to characterize the nonlinear distortion due to laser nonlinearity and the interplay between laser chirp and chromatic dispersion, identified as one of the limiting system performance factors in DM/DD OOFDM systems [3]. Comparisons between theoretical expressions and simulated results have shown a great agreement (Fig.2). We have also included in the analytical formulation the performance penalty due to deliberate clipping and inter-symbol & inter-carrier interferences, what has great importance on the correct choice of significant parameters such as the clipping level (CL) and the number of guard samples (Ng). Bit-error-rate (BER) curves obtained through simulations of a DM/DD OOFDM system and the evaluation of the expressions derived (Fig. 3) show a reasonable agreement.