**6.2 Reaching 10 Gbit/s modulation without any electronic processing**

Active research on high bit rate RSOA has led to 10 Gbit/s operation with EDC (Torrientes et al., 2010), OFDM (Duong et al., 2008) or electronic filtering (Schrenk et al, 2010). Bandwidth improvement to 7 GHz small-signal bandwidth with dual-electrode devices have been obtained but no large signal operation (Brenot et al, 2007). However the modulation bandwidth of one-section RSOA is limited to 2 GHz and increasing the modulation bandwidth of RSOA is still a challenge. Since carrier lifetime is mainly governed by stimulated emission rate, we have decided to increase the length of our RSOA to increase photon density, hence reducing carrier lifetime (de Valicourt et al., 2011). This device was chosen because an open eye diagram was obtained when the RSOA was driven by a 27-1 PRBS at 10 Gbit/s (Figure 17. (a)). The experimental set-up used for the 10 Gbit/s modulation is the same as represented in Figure 13. An ECL is used to launch a 4.5 dBm CW signal into the system through an optical circulator (OC). The signal is coupled into the RSOA which is modulated and generates the upstream signal. The RSOA is driven by a 27-1 PRBS at 10 Gbit/s, with a DC bias of 160 mA. The upstream signal propagates on various SMF lengths. A variable optical attenuator is placed in front of the receiver in order to analyze the performance of the system as a function of the received power. BER measurements are done using an APD receiver and an error analyzer. BER measurements without ECL have led to a BER floor of 10-6 (ASE regime).

With optical injection, BER values below the FEC limit in BtB and after 2km transmission are obtained (Figure 17. (b)). Error-free operation can either be obtained with FEC codes, or under certain optical injection regimes. However we can clearly see that the eye diagram tends to be closed due to the chirp over long distances. Multi-electrode devices can be used in order to compensate for this effect as demonstrated in the previous section.

As described in section 4, the modulation speed of RSOA is limited by the carrier lifetime. In the large signal regime, the slow decay is probably governed by the no-stimulated recombination process, which increases the carrier lifetime. A 3 GHz modulation bandwidth can be obtained with 850 µm long RSOA, which has led to the first eye-opening of a RSOA at 10 Gbit/s without electrical equalization or passive electronic filtering. Limitation due to the chirp is observed and further works are underway to overcome this effect using multielectrodes devices. Longer devices and dual-electrode devices will be studied to improve the modulation and transmission properties.

Fig. 17. (a) Eye diagrams at various bit rates of RSOA whose length varies from 500 to 850 µm. The collected power is pure ASE. Red lines are the dark levels. (b) BER value as a function of the received power for 850 µm long RSOA modulated at 10 Gbit/s (de Valicourt et al., 2011)
