Optical Single Sideband Modulator without Second-Order Sidebands

An optical single sideband modulator without generating both the -2nd and +2nd order sidebands is presented. It is based on parallel operation of two dual-drive Mach-Zehnder modulators (DDMZMs) where one is operated as a conventional single sideband (SSB) modulator and the other is operated as a single-drive Mach-Zehnder modulator (MZM) biased at the maximum transmission point. The 2nd order sidebands generated by the SSB modulator and the maximum-biased MZM can be cancelled by controlling the sideband amplitude and phase. Cancelling the 2nd order sidebands eliminates the second order harmonic component presented at the output of a long-reach fibre optic link. The new SSB modulator can be implemented using standard off-the-shelf components and has a wide bandwidth. Experimental results show > 20 dB 2nd order sidebands suppression for different modulation indexes and 22.6 dB reduction in the fibre optic link output second order harmonic component, compared to using a conventional SSB modulator formed by a DDMZM and a 90° hybrid coupler. Results also demonstrate the performance of the new SSB modulator is insensitive to the optical carrier frequency and the input RF signal frequency.