## 2 RoF and PoF Receiving Analog RF Link  
Fronthaul links can be used to transmit both analog and digital signals from a receiving antenna or to a transmitting antenna, connecting a remote station with a central station [20]. The hybrid architecture proposed in this work for the link shown in Figure 1 works as a receiving system.  
In the central station, the HPLS provides optical power, and the tunable laser source (TLS) provides the RoF optical carrier. An isolator (ISO) is used to protect the HPLS from back-reflected signals. The optical circulator (OC2), located in the central station, allows for the transmission of the RoF optical carrier to the remote station in the forward direction. The two optical signals from HPLS and TLS are combined in the wavelength division multiplexer (WDM1) and transmitted over a single mode SMF-28. Both the HPLS and TLS optical signals are received by the WDM2. Simultaneously, the optical coupler OC2 receives the backward RoF signal from the remote station, coupling it to the photodetector (PD).  
In the remote station, the Mach-Zehnder modulator (MZM) modulates the optical carrier emitted in the central station with the RF signal received by the antenna after being preamplified by the LNA1. The modulated signal is then transmitted back in the backward direction via an optical circulator (OC1). Tunable optical filters are used to minimize optical noise effects at the PD input (TOF2) in the central station as well as at the MZM input (TOF1) in the remote station. A polarization controller (PC) is used to remotely control the optical polarization at the input of the MZM.  
The HPLS signal received by the WDM2 is directed to the 1 \(\times\) 8 optical splitter with a photovoltaic power converter (PPC) module with eight PPCs. The optical energy is then converted into electrical energy to supply electronic devices such as LNA1, the DC/DC converter, and the MZM's Vbias Controller. The Vbias controller is responsible for keeping the MZM at its quadrature operating point and is essential to the operation of the system.  
As a bidirectional system, the Raman gain provided by stimulated Raman scattering (SRS) can be observed in both the forward and backward propagation directions. However, the SRS not only amplifies the power of the transmitted signal but also generates and amplifies noise, which can degrade the overall system performance [5]. For experimental purposes, two optical couplers are employed to estimate the optical gains in both the forward transmission direction and the backward transmission direction of the system. In the forward direction, the couplers are positioned at the input (F1) and output (F2) of the optical fiber, whereas in the backward direction, they are placed at the input (B1) and output (B2).  
The first PoF system uses a 1480 nm Raman-based HPLS. In this case, the TLS wavelength is set at 1550 nm, corresponding to an optimal wavelength spacing of 70 nm to benefit from a good compromise between gain and noise [20]. The second PoF system uses a 1550 nm HPLS based on an EDFA. In this case, the TLS wavelength is set at 1610 nm, corresponding to a wavelength spacing of 60 nm, which is not optimal due to limitations in our setup. This results in a lower amplification capacity for the EDFA-based system [9].