## 1 Introduction  
Maximizing the bit-rate of colorless, inexpensive and \"poorly\" performing transmitters has become a research topic on its own. This is of particular importance in wavelength division multiplexed (WDM) passive optical networks (PONs) that are considered a possible solution to cope with the capacity surge in access networks [1; 2]. WDM-PONs are being considered for applications requiring bidirectional high capacity and secure links. WDM-PONs are therefore ideal solutions to serve fiber to the home and building (FTTH/B) users with high capacity demands and also ideal to cover the needs of mobile fronthaul (fiber to the antenna, FTTA) [3; 4]. The latter is characterized by small cell sizes with dense base stations. If WDM-PON are to be mass deployed, large quantity of wavelength specific optical network units (ONUs) would be required, which highly complicates the inventory. Thus, the challenge is to develop ONU transmitters that allow the installation of identical devices across all subscribers. The solution are colorless transmitters that operate across a broad wavelength range, while their specific operation wavelength is controlled by an external factor [5; 6; 7; 8]. Installing identical colorless transmitters across all ONUs consequently, reduces the number of specialized components--thus simplifying the inventory. In addition, relatively low-quality mass producible colorless sources will make WDM-PON more economical. For low-cost colorless transmitters, the specific operating wavelength can be controlled by two main methods. A filter at a remote location can automatically choose the transmission wavelength of an ONU [9; 10]. Alternatively, colorless operation can also be implemented by using an external seed laser placed at the central office (CO) to determine the upstream wavelength [11; 12; 13; 14; 15; 16]. One promising colorless source is based on reflective semiconductor optical amplifiers (RSOAs). RSOAs are ideal sources as they offer a broad optical bandwidth at a low cost. There are three main architectures to integrate RSOAs in a WDM-PON as upstream (US) transmitters at the ONU. A theoretical and experimental comparison of the three architectures has been presented in [17]. * The simplest and most economical scheme is using RSOAs as amplified spontaneous emitters (ASE) source in a spectrum-sliced scheme, as shown in Figure 1a. The US signal is directly modulated onto the ASE of the RSOA. At the remote node (RN), a WDM filter slices the modulated ASE, passing only the assigned sub bands from each ONU to the central office (CO) [18; 19]. The scheme is low-cost and simple, but so far only bit rates below 1 Gbit/s have been achieved for each wavelength, with this scheme. The limitations of this scheme are high filtering losses and susceptibility to chromatic dispersion due to a broad optical spectrum. * Another cost-efficient source--however with improved performance--is the self-seeded RSOA fiber cavity laser (RSOA-FCLs) as suggested in [20; 21] and depicted in Figure 1b. In this case, a mirror is placed at the RN behind the WDM filter. This establishes a feedback mechanism to the RSOA to form a Fabry-Perot laser resonator. The operating wavelength of each ONU's RSOA-FCL is determined by the WDM filter port it is connected to. This passive assignment of the emission wavelength simplifies the wavelength control. Thus far, RSOA-FCL have shown to transmit up to 10 Gbit/s [22; 23]. * The highest performance--however, a relatively expensive scheme--is the externally seeded RSOA, shown in Figure 1c. Here, a bank of lasers, one for each ONU, is installed at the CO. The WDM filter at the RN redistributes the individual cw (continuous wave) laser lines to each ONU. At the ONUs the RSOAs amplify the seeding light and encode the US signal. Externally seeded RSOAs have already shown transmission beyond 20 Gbit/s with pulse amplitude modulation (PAM) [24] and more than 25 Gbit/s using discrete multi-tone (DMT) [14; 25].