## 1 Introduction  
Today, Time Division Multiplexing-Passive Optical Network (TDM-PON) standard is used. TDM-PON splits the optical power over a limited number of users based on a synchronized time slot in which all users share the available bandwidth. Due to the increasing demand for advanced multimedia applications, TDM-PON becomes insufficient enough for the high data rate that is required [1]. Wavelength Division Multiplexing-Passive Optical Network (WDM-PON) is the future of the next-generation network, which achieves the full usage of bandwidth by assigning for each user his own wavelength that creates a virtual connection between the Central Office (CO) and the user terminal [2]. To meet the rapid growth in current bandwidth requirements, DWDM-Passive Optical Networks (DWDM-PONs) have become the optimum choice. This type of network is considered to be the future of ultra-capacity networks that require an increasing number of channels by decreasing the channel spacing [3]. DWDM-PON also uses a remarkable low-cost Optical Network Unit (ONU) colorless operation [3]. Finally, the efficient utilization of the wavelength is another important requirement [4]. To meet low-cost ONU colorless operation requirements, several techniques, devices, and architectures are proposed, tested, and evaluated. A famous technique to realize ONU colorless operation is the use of reflective transmitters. These transmitters can be obtained by using(1) injection-locked Fabry-Perot Lasers (FP-Ls) [5; 6], (2) Reflective Semiconductor Optical Amplifiers (RSOAs) [7], and (3) Semiconductor Optical Amplifier (SOAs) with Reflective Electro-Absorption Modulators (R-EAMs) [8]. However, reflective transmitters that utilize FP-L require polarization and temperature control, which add complexity [5]. Those that use RSOAs suffer from a limited bit rate \(\sim\)1.25 Gb/s due to internal noises and nonlinearities [9]. Finally, any colorless ONU that uses SOA with R-EAM (SOA-REAM) suffers from chromatic dispersion and high interference in the signal due to the wide bandwidth of R-EAM [10]. As a result, the reflective transmitter technique generates a high level of Rayleigh Backscattering (RB), which affects the signal received at the CO [11]. Before reviewing the DWDM-PON system's key requirements, it is useful to conduct a review of the origins and effects of RB. Rayleigh Scattering is a dominant intrinsic loss mechanism in the low-absorption window between the ultraviolet and infrared absorption tails. It results from random inhomogeneities occurring on a small scale compared to the operating wavelength. These inhomogeneities act as refractive index fluctuations (or induced dipole moment) within the fiber and arise from density and compositional variations, which are frozen into the glass lattice upon cooling [12; 13]. Optical losses, due to RB in optical fibers, have caused considerable problems. Some examples of these problems include lowering the allowable bit rate, increasing the system's noise levels, and minimizing the transmission distances [14; 15; 16; 17]. Detailed examples and deeper analyses can be found for bi-directional wavelength-reuse fiber systems and WDM-PON Systems in [14; 15; 16; 17]. To the author's best knowledge, this work is one of the rare studies that considers RB for DWDM-PON systems with remarkable operating conditions. Now, methods for satisfying the most critical requirements for DWDM-PON systems (i.e., a low-cost ONU colorless operation with remarkable RB elimination generated by reflective transmitters) will be reviewed. In addition, methods for satisfying the other two DWDM-PON requirements will be highlighted (i.e., the reduction of channel spacing and the efficient utilization of wavelengths). Wavelength Shift (WS) and Optical Carrier-Suppressed Subcarrier-Modulation (OCS-SCM) techniques can efficiently avoid the RB effect and achieve a reasonable level of wavelength utilization by adopting a centralized light source [18; 19].