**4. Modulation characteristics and performances**

RSOA devices have limited electro-optical (E/O) bandwidth between 1 to 2 GHz (Omella et al., 2008) compared to laser devices usually between 8 to 10 GHz. The difference can be explained by two effects that are not present in RSOA devices. The first effect is gain clamping. The carrier density stays low even at high electrical input current while the photon density is increasing. This produces a shorter carrier lifetime particularly advantageous for high speed modulation. The second effect is the electron to photon resonance due to the presence of a cavity. The resonance appears in the modulation response increasing the effective -3dB E/O bandwidth.

The absence of cavity in RSOAs limits the modulation speed of this device. The modulation response behaves as a low pass filter with a characteristic cut-off frequency (when the link gain drops by 3dB). One limitation is due to carrier density spatial distribution. High carrier density combined with low photon density induces long carrier lifetime. Furthermore the carrier and photon densities strongly depend on the position z along the device. Therefore a non-homogeneous carrier lifetime is obtained.
