**4.2 High-frequency experimental set-up and characterization**

We realize a RSOA-based microwave fibre-optic link as depicted in figure 10. All different devices of this experimental set up can be considered as two-port components and classified according to the type of signal present at the input and output ports. E/E, E/O, O/E or O/O are possible classifications where an electrical (E) signal or an optical (O) signal power are modulated at microwave frequencies (Iezekiel et al., 2000).The RSOA is considered as an E/O two-port device which is characterized by the electro-optic conversion process, i.e. the conversion of microwave current to modulated optical power.

Fig. 10. High speed fibre-optic link

A full two-port optical characterisation of the complete set up is important to quantify the system performances. Dynamic characterization allows the measurement of the electrical response of the two-port network. A high-frequency signal is sent to the RSOA and the optical modulation is detected by a photodiode. The |S21|2 parameter (link gain) is measured over a range of frequency from 0 to 10 GHz. Figure 11 shows the electrical response of a typical RSOA device.

Next Generation of Optical Access Network Based on Reflective-SOA 15

The -3 dB E/O bandwidth has been extracted from Figure 11 and plotted in Figure 12-(b). A second approach is proposed by simulating the modulation bandwidth based on the carrier lifetime at z = 0 where the saturation effect is stronger. At low bias current, the first approach fits better with the experimental values. However at high electrical current (I >

The simulations confirmed by the measurements describe why the modulation bandwidth is limited in RSOA devices. It is mainly due to a larger carrier lifetime than in laser which is caused by a smaller photon density. The effective carrier lifetime depends on several recombination rates and strongly on the operating conditions. The stimulated recombination rate can be increased at high input optical power and electrical current. These conditions induce high photon density inside the active zone reducing the carrier lifetime and increasing the -3 dB E/O bandwidth. However these conditions are not suitable for low power consumption networks. Therefore another solution for increasing the photon density seems to be a required condition to push back the RSOA frontiers. A 3 GHz modulation bandwidth can be obtained with 850 µm long RSOA, which has led us to the first eyeopening of a RSOA at 10 Gbit/s without electrical equalization or strong optical injection.

The role of a RSOA as an optical transmitter is to launch a modulated optical signal into an optical fiber communication network. Reflective semiconductor optical amplifier (RSOA) devices have been developed as remote modulators for optical access networks during the past few years and their large optical bandwidth (colorless operation) has placed them in a leading position for the next generation of transmitters in WDM systems. In RSOA devices, the wavelength is externally fixed. Various options have been studied such as using multiwavelength sources (such as tuneable lasers, External cavity laser (ECL) , Photonic Integrated Circuits (PIC) or a set of Directly Modulated Laser (DML) at selected wavelengths), creating a cavity with the active medium of the RSOA, or using filtered white source. Therefore, RSOA devices as colourless transmitters can be used in different

In the laser seeding approach, the multi-wavelength external laser source can be located at the CO (Chanclou et al., 2007) or at the remote node (de Valicourt et al., 2009). From the CO, the optical budget is limited to 25 dB and strong RBS impairments appear. These limits are overcome by locating the laser at the remote node. One laser per remote node is needed,

Another possible architecture is using spectrum-sliced EDFA seeding. An erbium-doped fibre amplifier (EDFA) is used as a broadband source of un-polarised amplified spontaneous emission and this broad spectrum is then sliced by the Arrayed Waveguide Grating (AWG)

Wavelength re-use has been developed by Korean and Japanese companies (Lee W. R. et al., 2005). The downstream source from the CO is re-modulated as an upstream signal at the

thus raising deployment cost, control management and power consumption issues.

80mA), the second model is more adapted.

More details are presented in section 6.2.

Spectrum-sliced EDFA seeding

for each ONU (Healey et al., 2001).

**5. System performances** 

configurations: Laser seeding

 Wavelength re-use Self-seeding

Fig. 11. Direct modulation measurements S21 in 700μm long RSOA device

We simulate the modulation bandwidth depending on the carrier lifetime based on the first order approximation. The carrier lifetime can be estimated along the RSOA but shows a non-homogenous spatial distribution. The first approach consists of considering an average carrier lifetime over the whole device. Simulation and experimental data are compared in Figure 12-(a) for a 700 µm long RSOA at 80 mA. The simulation results fit well with the measurements over a limited range (from 0 to 2GHz). The difference beyond can be explained by the addition of the buried ridge structure (BRS) limitation. In fact, the BRS equivalent electrical circuit exhibits a cut-off frequency around 3 GHz.

Fig. 12. RSOA (a) E/O modulation bandwidth versus frequency at I = 80 mA (b) -3 dB E/O modulation bandwidth versus bias current for 700µm of AZ

The -3 dB E/O bandwidth has been extracted from Figure 11 and plotted in Figure 12-(b). A second approach is proposed by simulating the modulation bandwidth based on the carrier lifetime at z = 0 where the saturation effect is stronger. At low bias current, the first approach fits better with the experimental values. However at high electrical current (I > 80mA), the second model is more adapted.

The simulations confirmed by the measurements describe why the modulation bandwidth is limited in RSOA devices. It is mainly due to a larger carrier lifetime than in laser which is caused by a smaller photon density. The effective carrier lifetime depends on several recombination rates and strongly on the operating conditions. The stimulated recombination rate can be increased at high input optical power and electrical current. These conditions induce high photon density inside the active zone reducing the carrier lifetime and increasing the -3 dB E/O bandwidth. However these conditions are not suitable for low power consumption networks. Therefore another solution for increasing the photon density seems to be a required condition to push back the RSOA frontiers. A 3 GHz modulation bandwidth can be obtained with 850 µm long RSOA, which has led us to the first eyeopening of a RSOA at 10 Gbit/s without electrical equalization or strong optical injection. More details are presented in section 6.2.
