Characterisation of DSB-OCS technique for 40GHz radio over fibre system

The dual sideband optical carrier suppression (DSB-OCS) technique is characterised for its performance in order to be used as a carrier for 1.25Gbps OOK signal in the 40GHz radio over fibre (ROF) system. A dual electrode Mach-Zehnder modulator (DE-MZM) and the minimum transmission bias (MiTB) technique are employed to build the system. The results show that, a 40GHz carrier is successfully generated with the amplitude up to -29dBm and SNR of 35dB. Finally, an error free 40GHz ROF system is constructed with almost no penalty between the back to back and 20km fibre for a BER of 10^-9.

I. Introduction

The millimetre wave (mm-wave) radio over fibre (ROF) system has emerged as the synergy between the optical and wireless technology by offerring gigabit per second (Gbps) wireless access system for future telecommunication use [1][2]. Previously, direct modulation technique is often used to modulate radio frequency (RF) into the optical system to realize the ROF system because of the simplicity in the system design and less costly compared to the external modulation technique using the Mach-Zehnder modulator (MZM). It is done by directly modulating laser diode (LD) current with RF signal. However, there is a limit to the size of bandwidth that can be directly modulated using LD because of the chirp it produces at the frequency of more than 10GHz [3][4]. Therefore, the optical mm-wave generation technique is often adopted to extend the ROF system capability as the frequency is increased to mm-wave region to support Gbps wireless transmission