**6.1 Long reach PON using low chirp RSOA**

18 Selected Topics on Optical Amplifiers in Present Scenario

considered. Thus, one of the two 12 dB budget increase can also allow reach extension between the CO and the remote node (including the 25 km reach extension). However, propagation effects such as RBS and dispersion in the fibre would limit this extension. A reduction in RBS level is also needed to improve the performance of this configuration. Different solutions have been studied to reduce the RBS level such as: frequency modulation

Fig. 15. (a) BER as a function of the optical budget. Inset: 2.5 Gbit/s eye-diagram at the output of the RSOA driven at 90 mA and with an input power of -10 dBm (b) BER values for

A cost effective hybrid WDM/TDM-PON which can potentially feed 2048 subscribers (16×16×8 = 2048 subscribers) at a data rate of 2.5 Gbit/s is presented in this section. The large gain and high output power of the RSOA have also allowed extending the link reach up to 45 km instead of the standard 20 km. However, these achievements are obtained at the expense of an increase in deployment and operation costs. We believe this solution is economically viable since these costs are shared between many users, and multi-wavelength sources are becoming cheaper with the advent of Photonic Integrated Circuits (PIC). This 2.5 Gbit/s upstream colourless result allows investigating this solution to achieve in the trunk line a wavelength multiplex of several next generation access solutions (10 Gbit/s down-

Architecture based on single-fibre bidirectional link seems the most interesting and cost efficient approach. ONU becomes a key element for the network evolution. Transparent and flexible architecture based on WDM technology is necessary thus colourless ONU need to be available. High gain should be provided by the transmitter to reach the necessary optical budget and high modulation speed is needed. Bit rates up to 10 Gbit/s (per wavelength) are required to follow the evolution of the 10 Gbit/s GPON. RSOA could be the missing building block to reach this ideal network. However the modulation bandwidth and the

different λ-channels for an optical budget of 40 dB, or a Rx input power of -30 dBm

of the laser source or applying bias dithering at the RSOA.

(de Valicourt et al., 2010a)

and 2.5 Gbit/s up-stream).

**6. Limitations and improvements** 

chirp are still issues that need to be solved.

Another question about Hybrid PON is its property to be compatible with long reach network configuration. It was shown in the previous section that high gain RSOAs enable high optical budget, for instance, up to 36 dB and 45km transmission at 2.5 Gbit/s. A high optical budget is necessary to obtain a long reach PON (compensation of the fibre attenuation). The limitation imposed on the bit rate and distance by the fibre dispersion can dramatically increase depending on the spectral width of the source. This problem can be overcome by reducing the chirp produced by the RSOA device. Chirp reduction was demonstrated using a 2-section RSOA and how it can be used to reduce the transmission penalties (de Valicourt et al., 2010b). We propose an extended reach hybrid PON, taking advantages of a very high gain Reflective Semiconductor Optical Amplifier (RSOA) and the two-electrode configuration operating at 2.5 Gbit/s (de Valicourt et al., 2010c).

Two RSOAs with a cavity length of 500 μm are used in the experimental setup, one with mono and the other with dual-electrode configuration. The dual-electrode RSOA was realized by proton implantation in order to separate both electrodes. The single electrode RSOA was driven at 60 mA and the dual-electrode at 20 mA on the input electrode and 115 mA at the mirror electrode. Both current values correspond to optimized conditions in order to obtain low transmission penalties. It is to be noted that in a dual-electrode RSOA, the AC current is applied to the input/output electrode. In both cases, the CW optical input power was –8.5 dBm. Fig. 16 displays BER measurements performed at 1554 nm and 2.5 Gbps as a function of the received power for one electrode and two-electrode RSOA. The penalties due to 100 km transmission with a single electrode RSOA do not enable to reach the FEC limit. From 25 km to 50 km (100 km), we obtain penalties of 1.2 (3.4) dB. One can see that a BER of 10-4 (FEC limit) has thus been measured with bi-electrode RSOA at a received optical power of -35 dBm over 100 km SMF. The penalties due to extended 25 and 50 km SMF are much lower than with single electrode RSOA (respectively 0.5 and 1.4 dB). These transmission

Fig. 16. Comparison of BER value as a function of the received power for mono-electrode and bi-electrode RSOA over 50, 75 and 100 km at 2.5 Gbps (de Valicourt et al., 2010c).

Next Generation of Optical Access Network Based on Reflective-SOA 21

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

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

Nowadays, research in the telecom area is partly focused on passive optical network architecture. WDM-PON seems to be a promising approach allowing high data bit rate and flexibility. WDM techniques used in long-haul systems are now mature, however the shift to local access networks is more challenging. New requirements appear such as cost reduction, the need for new key devices at the ONU and compatibility with the existing network. Colourless ONU are necessary to obtain cost effective architecture and RSOA is one

The SOA theory has been discussed and applied to Reflective SOA devices. We underline several physical mechanisms that are responsible for the carrier density variation. The stimulated, radiative and non-radiative recombination rates are described. A model has been developed, taking into account several longitudinal sub-sections of the active guide. RSOAs exhibit a non-homogeneous carrier density profile which strongly affects the overall gain. At the input/output of the device, a strong saturation effect is observed. Therefore the net gain needs to be carefully integrated along the device taking in account this nonhomogeneity. All these results confirm the presence of key parameters such as the length

To assess the RSOA dynamics, the carrier lifetime is estimated. The E/O modulation bandwidth mainly depends on this parameter, for instance shorter carrier lifetime induces larger 3 dB E/O modulation bandwidth. The reduction of the carrier lifetime is required to

potential solution. In this chapter, we focused on these devices.

and the optical confinement which should lead to design rules.

the modulation and transmission properties.

et al., 2011)

**7. Conclusion** 

obtain high speed RSOAs.

results clearly show the correlation between the penalty and the chirp. The latter has more pronounced effects over long SMF. In Figure 16, a comparison between single and bielectrode RSOA over 100 km transmission is shown and a difference of 4.1 dB is obtained at the FEC limit. We can clearly see that the eye diagram starts to be closed due to the chirp on single-electrode devices over long distances. This effect is much reduced when using bielectrode RSOAs which confirms BER measurements. The proposed network design allows the use of Dense-WDM (DWDM) which means 62 wavelengths considering the 50 nm optical bandwidth of the RSOA. By considering the passive splitter (four clients), 248 potential suscribers can be feeded. At the FEC level, a variable attenuation of 4 dB is obtained which can be use as a splitter in order to design two parallels WDM PONs (2\*248=496 customers) over 100Km.

It was shown that the large gain of the RSOA and also the low chirp allows a reach extension of the link from standard 20km to 100 km. We demonstrated that penalties due to the transmission over 100 km SMF at 2.5 Gbit/s are reduced using an optimized multielectrode device and a BER below the FEC limit was achieved. We also believe this effect will be even more pronounced when 10 Gbit/s RSOA will be used.
