## 2 Architecture of the Proposed Optical Metro Access Network
### Schematic of the CO  
The schematic diagram of the proposed CO is shown in Figure 2a. It is usually more favorable for the OMAN to have a great number of frequency channels. This can be achieved by either using laser arrays or frequency combs. We have utilized an OMC generation scheme, shown in Figure 2b, to provide several carriers to facilitate over 60 end-users with high data rates.  
Figure 1: The proposed metro access network. CO: central office; RN: remote node; ONU: optical network unit.  
The proposed technique in this study can handle higher data rates with increased frequency spacing and practically double the bandwidth. The proposed OLT, placed at the CO of the metro network, is shown in Figure 2a. It essentially eliminates the need for a single laser diode per single subscriber. The end-face of the multicarrier generator is connected to a 1 xN optical fork (a fork with \(N\) outputs) arrangement, followed by a set of optical filters to separate the frequencies. The filtered carriers are injected into an optical duo-binary modulator \(\sim\)20 Gbps (ODBM). An optical multiplexer combines the modulated signals into one (MUX). A 1 \(\times\) 2 optical splitter (OSp1-co) amplifies and splits the multiplexed signal, which is then followed by two optical circulators (OCircs-co), optical couplers (OCups-co), and optical switches (OSws-co). The OSp1-co's first port is coupled to the OCircl-co's input port1. The OCircl-co's port 2 is linked to the OCoup1-co's port 1. To address the demand for network expansion, the OSp1-co's second port is connected to an optical switch (OSw1-co). Following the switch is OCirc2-co, which has the same structure as OCirc1-co. The OCoup1-co and OCoup2-co end-ports are connected to OSw2-co and OSw3-co, which direct the signal to either the operating mode fiber or the protection fiber. The protection mechanism section explains the needs and working mechanism of OSw2-co and OSw3-co. This arrangement of OCirc-co, OCoup-co, and OSw-co is for normal working conditions, and one is kept reserved for the protection mode for the primary feeder fiber,  
Figure 2: (**a**) Schematic diagram of the proposed central office. OMCG: optical multicarrier generator; ODBM: optical duo-binary modulator; EDFA: erbium doped fiber amplifier; optical switch: optical switch; OC: optical coupler; PD: photodetector; LPBF: low bandpass Bessel filter (**b**) Schematic of the proposed optical multicarrier generator. EA: electrical amplifier; PM: phase modulator; MZM: Mach–Zehnder Modulator; OV and A: optical visualizer and analyzer.  
both for uplink and downlink transmission. The switch manages the uplink and downlink transmissions in normal conditions or when a fault is detected, it switches the transmission into the reserved/protected fiber cable.  
Similarly, the CO contains another section of uplink transmission after receiving the data during the normal operation via OSw2-co or during the protection mode through OSw3-co. The data are passed from the OCoups-co to the OCircs-co via port2 and forwarded to the power combiner via port 3 of the OCircs-co. The outputs of these two OCircs-co are combined to receive the signal from both modes directly. The receiver section is further discussed in the coming sections.