**2.1 RSOA evolution**

2 Selected Topics on Optical Amplifiers in Present Scenario

metropolitan network should offer more scalability and flexibility for the next generation of

However the cost mainly drives the deployment of access network and remains the principal issue. Cost effective migration is needed and the cost capital expenditures (CAPEX) per customer has to be reduced. ONU directly impacts on the CAPEX. New optical

For uplink transmission systems using WDM, each ONU requires an optical source, such as a directly modulated laser (DML) (Lelarge et al., 2010). If wavelengths are to be dynamically allocated, one to each ONU, colourless devices are needed in order to minimize the deployment cost. Reflective Semiconductor Optical Amplifier (RSOA) devices can be used as a low-cost solution due to their wide optical bandwidth. The same type of RSOA can be used at different ONUs where they perform modulation and amplification functions. However, cost and compatibility with existing TDM-PONs is still an issue. As a consequence, hybrid (TDM+WDM) architecture is being investigated for next generation access network (An et al., 2004), as a transition from TDM to WDM PONs where some optical splitters could be re-used. Recently, the first commercial hybrid PON based on reflective semiconductor optical amplifiers (RSOA) has been announced (Lee H-H. et al.,

devices are needed in order to obtain high performances and low cost ONU.

2009). Such a network allows serving 1024 subscribers at 1.25 Gbit/s over 20 km.

RSOA must be evaluated considering the trade-offs among the different parameters.

active zone and the operating conditions.

**2. Past history and basic concept of reflective SOA** 

high speed RSOA.

colourless optical transmitters.

Dynamic analysis is then proposed in section 4. The RSOA modulation responses behave as a low-pass filter with a characteristic cut-off frequency. The carrier lifetime turns out to be a key parameter for high speed modulation and a decrease of its value appears to be required. The rates of recombination processes, such as stimulated, spontaneous and non-radiative recombination govern the carrier lifetime. They strongly depend on the position along the

Furthermore, telecommunication networks based on RSOA are studied. We introduce the envisaged architectures of access networks based on RSOA. High gain RSOA is used as colourless transmitter and WDM operations are performed. Laser seeding configuration at 2.5 Gbit/s is realized and error free transmission is obtained for 36 dB of optical budget over 45 km of SMF. Low-chirp RSOAs enable a 100 km transmission at the same bit rate below the Forward Error Correction (FEC) limit. Direct 10 Gbit/s modulation is then realized using

Finally, we summarize the lessons learned in this chapter and conclude on RSOA devices as

In this section, the fundamental properties and the main concepts of SOA and RSOA are introduced in a simple way. We discuss the past history and the evolution of RSOA with the

In this chapter, the basic theory of SOA/RSOA is investigated. The different interactions of light and matter are described. Then, we focus on the device modelling. We develop a multisection model in order to take in account the non-homogeneity of the carrier density. In this approach, we consider a forward and backward propagation as well as the amplified spontaneous emission (ASE) propagation. Longitudinal spatial hole burning (LSHB) strongly affects the average optical gain. An evaluation of the total gain in RSOA devices including the LSHB is proposed. The influence of the optical confinement and the length is described and leads to some design rules. Under the latter analysis, the performance of

optical access network.

The first idea of a Reflective SOA (RSOA) was proposed by Olsson in 1988 to reduce the polarization sensitivity via a double pass configuration using classic SOA.

The first integrated RSOA for optical communication appeared in 1996 where the device was employed for upstream signal modulation at a bit rate of 100 Mbit/s (Feuer et al., 1996). Then several experiments based on RSOAs for local access network were realized where the bit rate was increased to 155 Mbit/s (Buldawoo et al, 1997).

In this chapter, we mainly focus on the wavelength upgrade scenario for WDM-PON systems. While current optical access networks use one single upstream channel to transmit information, WDM systems use up to 32 channels increasing therefore the total transmitted information. RSOA is the perfect candidate because of its wide optical bandwidth, large gain and low cost, therefore fulfilling most of the requirements. For instance, its large optical bandwidth makes it a colourless cost-effective modulator for WDM-PON. The same type of device can be used in different ONUs, which reduces the network cost. Moreover, the large gain provided by an RSOA can compensate link losses without using an extra amplifier, which simplifies the overall solution.

Therefore since 2000, RSOA devices saw a fast growing interest in upstream channels transmission based on reflective ONU for WDM-PON applications. The first re-modulation scheme has been proposed in 2004 where the downlink signal was transmitted at 2.5 Gbit/s and uplink data stream was re-modulated on the same wavelength via the RSOA at 900 Mbit/s (Koponen et al, 2004). Then further investigations were realized and 1.25 Gbit/s remodulation was demonstrated (Prat et al., 2005).

Today, several research groups work on these solutions such as Alcatel-Lucent Bell Labs, III-V Lab, Universitat Politècnica de Catalunya (UPC), Orange labs, ETRI, KAIST, IT and various optical devices manufacturers proposed commercial RSOA devices as CIP, MEL and Kamelian.
