## 2 Amplifier Configuration  
In [8], we proposed and demonstrated the operation of the HROPA configuration for amplification of the top half of the CWDM band (e.g., 1450 nm to 1610 nm). We also described and implemented a method to overcome crosstalk issues related to the generation of idlers due to the parametric process. Since DS and US traffic in a CWDM PON counter propagate, we can separately amplify the two bands [2,3]. The same concept used for the implementation of the HROPA design in [8] can be implemented to cover the lower half of the CWDM band (e.g., 1270 nm to 1430 nm), as shown in Figure 1. However, as we will discuss later, there are some significant differences in the design that affect the HROPA performance and applicability of the previous design. The HROPA consists of a Raman amplification stage followed by a parametric amplification stage (note that in Figure 1, the signal propagates from right to left). The Raman and parametric processes are carried out separately in two different highly nonlinear fibers (HNLFs). The reason for this separation of the two processes is based on the fact that, on the one hand, the OPA process requires a short fiber length to achieve a wide gain bandwidth and, on the other hand, the Raman process requires a long fiber to achieve high gain. Hence decoupling the two processes allows for better optimization of the associated fiber lengths and pump optical powers and wavelengths. The Raman pump is placedsuch that the Raman peak gain is at the same wavelength range as the valley of the parametric gain curve. This allows us to compensate for the large gain ripple from the parametric process. Since our ultimate goal is to use the proposed HROPA in a CWDM-based PON, we are interested in simultaneous transmission of signals at the CWDM wavelengths (e.g., US traffic) as defined by the standards [1]. Therefore, we study the amplification of the nine nominal CWDM channels in the 1270 nm-1430 nm band as they propagate simultaneously through the proposed amplifier. A very important issue in optical parametric amplification schemes is the generation of idlers, which contribute to crosstalk in a WDM network. To overcome this limitation, we have proposed and demonstrated an optical parametric amplifier (OPA) stage within the HROPA, as shown in Figure 1, which sufficiently suppresses the idlers [8]. Similarly, for the O- and E-bands, we can split them into two wavelength sets, which are amplified separately (in two parallel paths). In particular, the US CWDM channels (e.g., 1270 nm-1430 nm), after the Raman amplification, are separated into two sub-bands with the use of a standard WDM demultiplexer (DEMUX). Band 1 consists of the first four CWDM channels (_i.e._, 1270 nm, 1290 nm, 1310 nm and 1330 nm), which pass through the lower arm of the OPA section. Band 2 consists of the remaining five CWDM (_i.e._, 1350 nm, 1370 nm, 1390 nm, 1410 nm and 1430 nm) channels, which pass through the upper arm. Each signal wavelength, through the OPA process, gives rise to an idler. Each idler is generated at a wavelength that is separated from the associated signal wavelength by an amount almost double the difference of the signal wavelength from the pump wavelength. In reality, the relation is defined in the frequency space by _o_i_-_o_s_=2(_o_p_-_o_s_), where _o_i_ is the frequency of the idler, _o_s_ is the frequency of the signal and _o_P_ is the frequency of the parametric pump. Hence, the first half of the CWDM signal wavelengths within Sub-band 1 (e.g., 1270 nm to 1330 nm) generate idlers that are in the wavelength range of 1344 nm-1418 nm (e.g., within Sub-band 2) or, in other words, at wavelengths higher than the parametric pump wavelength (e.g.,  
Figure 1: Design of the hybrid Raman-optical parametric amplifier (HROPA) for upstream traffic. CWDM, coarse wavelength division multiplexer; DEMUX, demultiplexer; HNLF, highly nonlinear fiber. 1340.2 nm).