Enhanced reflection tolerance in WDM-PON by chirped RZ modulation L. Banchi, R. Corsini, M. Presi, F. Cavaliere and E. Ciaramella In a central light seeded PON, it is shown that the resilience to the Rayleigh backscattering can be strongly different for NRZ or RZ- direct modulated R-SOAs at the OLT. It is experimentally demon- strated that the RZ modulated signal has an increased tolerance to the in-band crosstalk compared to NRZ format, thanks to the spurious chirp. Error-free uplink communications down to 10 dB signal-to- crosstalk level are obtained. Introduction: Low cost and simple design are two key issues in devel- oping fibre-to-the-home (FTTH) systems. Particularly, wavelength inde- pendent reflective re-modulators, such as reflective semiconductor optical amplifiers (R-SOAs), open the way to the deployment of single-feeder loopback passive optical networks (PONs) with centralised light seeding (CLS). Although this architecture recently attracted a lot of interest, it suffers from a well-known fundamental limitation: the reflec- tions distributed along the fibre produce in-band crosstalk, which can severely reduce the system performance. These reflections arise along the fibre because of the Rayleigh backscattering of the downstream CWs, which are directed towards the optical line terminal (OLT), and of the upstream modulated signals, which are directed towards the optical network unit (ONU). These last can be eventually amplified in the R-SOA and then co-propagate together with the upstream signal [1, 2]. The signal and the crosstalk indeed beat at the OLT receiver strongly affecting the performance of the system. It has been shown that the in-band crosstalk originated by distributed reflections can be tolerated as long as its power is much lower than the power of the signal [3]. Several techniques based on dithering [2], optimal filtering [4] and line coding [5] can increase the tolerance to reflections, moving the crosstalk tolerance level to around 18 dB [2–5]. In this Letter, we propose a novel technique to increase the in-band crosstalk resilience in WDM-PONs using an R-SOA as an uplink modu- lator. We experimentally demonstrate that, by employing an R-SOA as the ONU transmitter and driving it by a return-to-zero (RZ) signal, we dramatically reduce the coherence time of the uplink signal light. This increases the tolerance to the in-band crosstalk up to 10 dB of signal- to-crosstalk power ratio (SCR). CW laser polarisation scrambler 25 km SMF driver APD R-SOA VOA-1 VOA-2 VOA-3 OTF PPG BERT ONU OLT OC-1 phase intensity NRZ RZ t Fig. 1 Experimental setup Inset: Chirping effect in NRZ and RZ formats Experiment and results: To evaluate the impact of the distributed reflec- tions on the proposed modulation format, we used the experimental setup shown in Fig. 1. A laser emits the CW feeding light (l ¼ 1535 nm). This lightwave passes through a polarisation scrambler, driven by a 6 kHz random signal, followed by a variable optical attenu- ator (VOA-1), which controls the feeder power level at the fibre input. The polarisation scrambler is used to average any effect due to the polar- isation dependency of the R-SOA. After an optical circulator (OC-1), the CW feeder is launched into a 25 km-long singlemode fibre (SMF, 6 dB loss). At the output of the fibre, the seeding light passes through VOA-2 and a tunable optical filter (OTF, Gaussian shape, 0.8 nm 3 dB band- width and 4 dB insertion loss), then reaches the R-SOA. The R-SOA is a commercial device providing 26 dB small-signal gain, 6 dBm output saturated power at 75 mA bias current and 1 dB polarisation dependent gain. We use it driven either by a 1.25 Gbit/s NRZ or by a RZ 1.25 Gbit/s PRBS sequence (in both cases a 2 11 2 1 bit-long pattern). The R-SOA driving voltage is quite high (7 V p-p ) because of two main motivations: first, this increases the gain modulation thus chirping significantly the signal; secondly, the high driving voltage partly compensates for the limited E/O bandwidth (around 1.5 GHz) of the R-SOA, which is particularly useful for the RZ signal. Because of the gain-phase coupling in the R-SOA, the intensity modulation always comes with a phase modulation (chirp), which also depends on the R-SOA operating conditions. Therefore increasing the number of intensity transitions in the signal also increases the chirp. When the R-SOA is driven by a non-return-to-zero (NRZ) signal, phase transitions occur only at 0–1 (or 1–0) transitions, whereas a con- stant symbol sequence (e.g. consecutive 1s) comes with no phase change. On the other hand, in a return-to-zero (RZ) signal, we have two opposite intensity transitions at every mark: therefore the signal can be always highly chirped, i.e. with a coherence time significantly reduced compared to the NRZ signal. This difference of the chirp fea- tures in the NRZ and RZ format is illustrated in Fig. 1, where we show the phase changes (dashed line) induced by the amplitude modu- lation (solid line) for the two different modulation formats. As shown, consecutive symbols do not generate any phase modulation in the NRZ stream; however, the RZ stream shows always phase variations, which effectively reduce the signal coherence time and increase the resi- lience to the crosstalk [2]. The signal modulated by the R-SOA is sent upstream over the SMF towards the OLT, where it is extracted by OC-1, attenuated by VOA-3, and received. Our receiver consists of an avalanche photodiode (APD, 235 dBm sensitivity), followed by a lowpass electrical Bessel filter (LPF, 933 MHz bandwidth). The amount of crosstalk can be varied by means of VOA-1, i.e. acting on the power launched into the fibre. We usually set the launch power value at 0 dBm, which corresponds to about 234 dBm backscattered light. By means of VOA-2, we can then change the power at the input of the R-SOA. This changes the gain of the R-SOA and in turn the power at the input of the upstream receiver. But this also changes the SCR value. In our case, since the fibre backscattered light is around 234 dBm when the R-SOA input power is between 218 and 224 dBm, the SCR varies between 24 and 10 dB. Note that in our setup the dominating impairment is due to the reflec- tions of the downstream CW seed, as the effect of the Rayleigh reflec- tions from the upstream signal back into the ONU is negligible. Considering the losses due to VOA-2 and the OTF, the amount of back-reflected power at the ONU is indeed estimated between 248 and 252 dBm. Hence the SCR at the ONU is around 30 dB. Therefore, the SCR degradation at the ONU is negligible compared to the SCR degradation that can be observed at the OLT. The spectral broadening due to RZ direct modulation of the R-SOA is illustrated in Fig. 2 in which we show the optical spectra of the RZ (dashed line) and the NRZ (solid line) outputs, taken with a 20 MHz resolution bandwidth optical spectrum analyser (OSA). As can be seen, the chirp induced largely broadens the spectrum of the RZ signal and makes it around four times wider than the NRZ spectrum. This spectral broadening corresponds to a significant reduction of the coherence time, which is eventually giving an outstanding improvement in the resilience against the in-band crosstalk. power, dBm –30 –40 –50 –60 –70 1534.9 1535.0 l, nm 1535.1 NRZ RZ Fig. 2 Optical spectra of NRZ (solid line) and RZ (dashed line) modulated uplink signals at 1.25 Gbit/s (taken by high resolution OSA) Fig. 3 shows the eye diagrams of the uplink received signal for the two modulation formats for three different SCR values (22, 16 and 10 dB). As can be seen, the RZ signal always shows a good quality ELECTRONICS LETTERS 8th July 2010 Vol. 46 No. 14