## 1 Introduction  
The development of low-cost colorless sources has become a major research topic. They are needed to make the widespread deployment of wavelength division multiplexed passive optical networks (WDM-PONs) practical [1,2]. Relatively new alternatives are self-seeded reflective semiconductor optical amplifier fiber cavity lasers (RSOA-FCLs) [3]. The RSOA-FCLs' colorless feature emanates from the automatic and passive self-tuning of their emission wavelength by only a WDM filter positioned at a remote node (RN). The colorless feature allows for the mass production and installation of identical upstream devices at all optical network units (ONUs) of a WDM-PON. Having only one common source at the ONUs will lower the cost of WDM-PONs, which has been a burden for widespread deployment [4]. In addition, a passive wavelength allocation by the PON infrastructure simplifies the wavelength management process. Thus, the availability of low-cost colorless sources, such as RSOA-FCLs, allows for the implementation of WDM-PONs that offer high bandwidth. These could, for instance, be used for mobile fronthaul networks (MFHs), which will be discussed in the next subsection [1,5]. Quite a few other colorless transmitter schemes are currently under discussion. The two key contenders are:  
* Amplified spontaneous emission (ASE) spectrum-sliced schemes: These are the simplest low-cost solutions available. Directly modulated broadband light sources such as light-emitting diodes (LEDs) [6, 7, 8, 9] or RSOAs [10,11] can be used for upstream (US) transmission. As an example, Figure 1a shows an RSOA-based ASE spectrum-sliced WDM-PON. The upstream data is directly encoded onto the broadband ASE of the RSOAs, which are placed at the ONUs. The waveguide grating router (WGR, WDM MuxDeMux) at the RN slices the modulated ASE spectrum and sends a sliced band upstream. The US spectral band from each ONU is determined by the WGR port to which the individual ONUs are connected. Even if the low cost of LEDs and RSOAs make this scheme attractive, their modulation speeds are limited to 1 Gbit/s over less than 10 km of transmission reach. The ASE amplitude noise and thus the high relative intensity noise (RIN) are limiting factors. There is also a high filtering power loss during spectral slicing. In addition, the broad bandwidth of each spectrum slice makes the scheme highly sensitive to chromatic dispersion. * Externally seeded RSOA schemes: These offer higher reach and speed at a higher investment. The idea is to use a bank of lasers at the central office (CO), one for each ONU, which send continuous wave (cw) light downstream (DS) to the ONUs, as presented in Figure 1c [12, 13, 14, 15, 16]. The WGR splits and distributes the different wavelengths to the respective ONUs. The RSOAs at the ONUs amplify and directly encode data onto the seeding cw light and send them back upstream. The amplification of the DS cw light by the RSOAs compensates the transmission power loss by offering a higher link power budget. The chromatic dispersion penalty for this scheme is also more relaxed than that of the spectrum-sliced scheme. Of course, all the improvement comes at the price of placing relatively expensive lasers at the CO.  
Figure 1: Scenarios showing how the three sources would be integrated in a WDM-PON. (**a**) RSOA ASE spectrum-sliced scheme: An upstream (US) signal can be directly modulated onto the ASE of the RSOAs at the optical network units (ONUs). The waveguide grating router (WGR) slices the ASE into bands and sends them upstream; (**b**) Self-seeded RSOA-FCL: A lasing resonant cavity, shown by the yellow shading, is formed between the RSOA at the ONU and a mirror at the remote node (RN). The US signal is encoded onto this multimode laser signal; (**c**) Externally seeded RSOA: A bank of lasers at the central office (CO) send cw light downstream (DS). The WGR distributes the seeding light to the respective ONUs. The RSOAs at the ONU encode data, amplify the light, and send them upstream.