Generation of SSB Optical Signals with Dual-EML Modulated with Wideband OFDM Mohamed Essghair Chaibi 1* , Thomas Anfray 2 , Christophe Kazmierski 3 , Christelle Aupetit-Berthelemot 2 , and Didier Erasme 1 1. MINES-TELECOM, TELECOM ParisTech, CNRS LTCI, 46 Rue Barrault, 75634 Paris, France 2. XLIM-CNRS, 16 Rue Atlantis, 87068 Limoges, France 3. III-V Lab-Common laboratory of “Alcatel-Lucent Bell Labs France”,“Thales Research and Technology” and “CEA Leti” Route de Nozay, 91460 Marcoussis, France *chaibi@telecom-paristech.fr Abstract: The generation of optical SSB signals by a Dual-EML is generalized for wideband modulating signals. The feasibility of the proposed technique is demonstrated by transmitting 5.3 GHz baseband OFDM signal in an optical SSB context. © 2014 Optical Society of America OCIS codes: 230.3120, 060.2330. 1. Introduction Access optical networks are generally based on a dispersive Intensity Modulation and Direct Detection (IM/DD) channel. The IM results in a spectrally inefficient double sidebands (DSB) signal. It propagates with a frequency- dependent group velocity through the dispersive fiber. At the receiver side, the beating between the optical carrier and the two sidebands, performed by the DD, leads to frequency fading. The frequency selectivity of the channel is resolved with single sideband (SSB) signals where one of the sidebands is eliminated. SSB signals are commonly generated with I/Q modulators or optical filters, but both of them are bulky and expensive. In [1], the dual modulation of a chirped DFB laser and an external electro-absorption modulator (EAM) to generate SSB signals is introduced by Kim. This technique was matched to Binder conditions [2] to be used in NRZ applications. In analog-optic communication systems, it was only carried out in radio over fiber (RoF) applications for narrowband modulating signals [1]. This paper demonstrates how a Dual-EML consisting of a DFB laser monolithically integrated with an EAM with two independent accesses for modulation is perfectly suited to generating SSB signals according to Kim’s technique and can be used with a wideband OFDM signal input. 2. Operation Principle To generate SSB signals with a Dual-EML, Frequency Modulation (FM) and IM, respectively ruled by the DFB laser and the EAM, have to be either 0 or π phase shifted and the ratio between IM and FM indices (m IM /m FM ) has to be equal to 2 [1]. For narrowband modulating signals, the phase condition can be easily satisfied by adjusting the relative phase shift between the two modulating signals using a tunable delay line (TDL). Electrical attenuators permit to control modulating signals’ RF power and then m IM /m FM . When the Dual-EML is modulated with a wideband signal, the SSB signal is generated if Kim’s conditions are strictly satisfied for each frequency modulating component. Maintaining a phase shift of 0 or π between IM and FM across a large frequency band is not straightforward. This is due to the non-linear phase shift, ΔΨ, between the FM and the laser’s modulating signal and the linear phase shift, ΔΦ, introduced by the difference in electrical length between the two RF access circuits. ΔΨ depends only on the chirp frequency of the DFB laser. In addition, ΔΦ can be easily inferred by characterizing the two circuits using a vector network analyzer. The phase shift between IM and FM, ΔΘ, which turns out to be the sum of ΔΨ and ΔΦ is a non-linear phase shift that can not be compensated with a simple TDL. Apart from that, keeping m IM /m FM = 2 across a large frequency band is also complicated as m FM is a frequency-dependent value. One key idea to satisfy Kim’s conditions for a wideband modulating signal is the insertion of a passive filter in the laser’s access for modulation. While its phase response compensate the known ΔΘ phase shift, the power response of the filter is designed to keep m FM constant for all modulating components. For our experimental setup, the power and the phase spectral responses of the required filter are depicted in Fig. 1. Constructing a real world RF filter which has the characteristics previously shown is under study. Meanwhile, we opt for implementing it numerically.