**1. Introduction**

Communication networks have evolved in order to fulfil the growing demand of our bandwidth-hungry world. First, the coaxial cable has replaced the copper cable since 1950 for long- and medium-range communication networks. The Bit rate-distance product (BL) is commonly used as figure of merit for communication systems, where the B is the bit rate (bit/sec) and L is the repeater spacing (km). A suitable medium for transmission needed to be available and optical fibres were selected as the best option to guide the light (Kao & Hockham, 1966). A radical change occurred, the information was transmitted using pulses of light. Thus further increase in the BL product was possible using this new transmission medium because the physical mechanisms of the frequency-dependent losses are different for copper and optical fibres. The bit-rate was increased in the core network by the introduction of a new technique: Wavelength-Division Multiplexing (WDM). The use of WDM revolutionized the system capacity since 1992 and in 1996, they were used in the Atlantic and Pacific fibre optic cables (Otani et al., 1995).

While WDM techniques were mostly used in long-haul systems employing EDFA for online amplification, access networks were using more and more bandwidth. Access network includes the infrastructures used to connect the end users (Optical Network Unit - ONU) to one central office (CO). The CO is connected to the metropolitan or core network. The distance between the two network units is up to 20 km. The evolution of access network was very different from in the core network. High bit-rate transmissions are a recent need. At the beginning, it provided a maximum bandwidth of 3 kHz (digitised at 64 kbit/s) for voice transmission and was based on copper cable. Today, a wide range of services need to be carried by our access network and new technologies are introduced which allow flexible and high bandwidth connection. The access network evolution is obvious in Europe with the rapid growth of xDSL technologies (DSL: Digital Subscriber Loop). They enable a broadband connection over a copper cable and allow maintaining the telephone service for that user. In 2000, the maximum bit rate was around 512 kbit/s while today it is around 12 Mbit/s. However since 2005, new applications as video-on-demand need even more bandwidth and the xDSL technologies have reached their limits. The introduction of broadband access network based on FTTx (Fiber To The x) technology is necessary to answer to the recent explosive growth of the internet. Today, Internet service providers propose 100 Mbit/s using optical fibre. The experience from the core network evolution is a great benefit to access network. The use of WDM mature technology in access and

Next Generation of Optical Access Network Based on Reflective-SOA 3

development of optical communication systems. Then, we detail the physics involved in SOA and RSOA. A multi-section approach is chosen to model the device behaviour under static conditions. The aim of this section is not to propose a complex model but to underline

The first idea of a Reflective SOA (RSOA) was proposed by Olsson in 1988 to reduce the

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

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

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 re-

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

Gain in a semiconductor material results from current injection into the PIN structure. The relationship between the current I and the carrier density (n) is given by the rate-equation. The rate-equation should include the stimulated emission as well as the spontaneous and absorption rate. R�n� is the rate of carrier recombination including the spontaneous emission and excluding the stimulated emission. Electrons can recombine radiatively and non-

When an electron from the Conduction Band (CB) recombines with a Valence Band (VB) hole and this process leads to the emission of a photon, it is called the radiative

���� � ������� � ����������� (1)

R����n� � �� n� (2)

and understand the different mechanisms in RSOA devices.

bit rate was increased to 155 Mbit/s (Buldawoo et al, 1997).

modulation was demonstrated (Prat et al., 2005).

**2.2 Fundamentals of SOA and RSOA** 

radiatively therefore R�n� can be written as:

recombination. The rate of radiative recombination is:

polarization sensitivity via a double pass configuration using classic SOA.

without using an extra amplifier, which simplifies the overall solution.

**2.1 RSOA evolution** 

Kamelian.

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

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

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

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

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 active zone and the operating conditions.

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 high speed RSOA.

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