FSK+ASK/ASK Operation for Optical 20/10 Gbps Access Networks with Simple Reflective User Terminals Bernhard Schrenk 1 *, Guilhem de Valicourt 2 , Jose A. Lazaro 1 , and Josep Prat 1 1 Dept. of Signal Theory and Comm., Universitat Politecnica de Catalunya, Jordi Girona 1, 08034 Barcelona, Spain (Tel. +34-93-401-7179) 2 Institut Télécom, Télécom ParisTech, LTCI CNRS, 46 rue Barrault,75634 Paris Cedex 13, France *Corresponding author: bernhard.schrenk@tsc.upc.edu Abstract: Doubling of the downstream rate by reusing the reflective upstream modulator is demonstrated. With just two photo-detectors, a RSOA and a power splitter at the ONU, a 20/10 Gbps mode can be provided per wavelength. 2011 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation 1. Introduction Fiber-to-the-Home solutions are continuously evolving and will soon demand higher data rates for the transmission of down- and upstream. Besides the use of traditional downstream detectors such as PIN or avalanche photo diodes (APD) for amplitude shift keyed (ASK) downstream signals with a data rate of 10 Gbps, upstream operation at 10 Gbps has been recently demonstrated with an RSOA-based optical network unit (ONU) [1]. The cost limitations of the components that constitute the ONU are playing an important role for a further increase of the data rates, since they prevent the use of advantageous, though more complex modulation formats that are based on phase shift keying or orthogonal frequency division multiplexing [2], [3]. While these formats are usually compromised by the required integrated delay interferometers or energy-hungry digital signal processing, FSK can be alternatively used as orthogonal modulation format with respect to ASK. However, the required optical filter that acts as FSK-to-ASK demodulator introduces wavelength-specific elements at the ONU, which is intended to be colorless for a mass deployment in a wavelength division multiplexed (WDM) access network with reduced cost. As one solution, the gain ripple of a SOA integrated with an electro-absorption modulator at the ONU can be used as comb filter [4]. In this work we propose to use the gain ripple of a RSOA as additional colorless demodulator for a FSK+ASK downstream signal. We experimentally demonstrate that the downstream data rate can be doubled to 20 Gbps, while the RSOA can be advantageously reused for 10 Gbps ASK upstream transmission. 2. Colorless FSK+ASK Operation The design of the proposed ONU is shown together with the experimental setup for a back-to-back proof in Fig. 1. Fiber-based transmission would additionally require means of dispersion compensation, which can be integrated electronically. While the ASK downstream tributary is received with an APD, the demodulated FSK signal is detected with a PIN diode. The signal for the APD is split off with a 50/50 coupler (C D in Fig. 1) and an isolator was placed in front of the PIN diode to avoid lasing effects from the RSOA due to back-reflections from the photo detector. The RSOA, whose gain ripple is aligned in its spectral transmission function to the incident downstream signal, as will be discussed later, performs the FSK-to-ASK conversion and is also used in half-duplex operation for upstream transmission. At the same time, the natural gain saturation of the RSOA is used to suppress the ASK component of the FSK+ASK downstream signal, which causes severe crosstalk to the FSK detection. With this ONU design the upstream transmitter is advantageously used as FSK demodulator and provides a solution to double the downstream data rate by simply adding a second photo detector at the ONU. A second RSOA could be placed at the ONU to separate the functionalities of FSK demodulation and upstream transmission and, consequently, allow for full-duplex operation. However, half-duplex operation at twice the data rate as in [5] makes sense considering the delivery of large data contents such as for video-on-demand, where mostly downstream transmission is preferred at a given time and bandwidth partitioning will be useful. The buried-ridge RSOA had an optical confinement of Γ~20% which ensures a low noise figure and wider optical bandwidths [6]. An anti-reflection coating was applied to the input facet of the RSOA. The gain ripple was formed via the fiber-pigtail that caused controlled reflections towards the active RSOA waveguide. The gain spectrum of the used RSOA was centered at 1561 nm and had a 3-dB small signal bandwidth of 39 nm. The gain ripple in the transmission function was 6.4 dB for a bias of 100 mA and a wavelength around 1550 nm. With a RSOA length of 700 μm a free spectral range of 0.46 nm was obtained. This transmission function suits to a comb filter with sufficient contrast for a FSK-to-ASK conversion. Due to the dependence of the refractive index on the temperature, the comb function can be tuned and in turn aligned to the downstream signal. The shift of the comb JWA79.pdf OSA/OFC/NFOEC 2011 JWA79.pdf