## 2 Experimental Setup and Results  
The schematic structure of the proposed self-start multi-wavelength laser source for wireless networks is shown in Figure 1. The demultiplexer selects two wavelengths from the multi-wavelength laser source for each channel [17]. The desirable data streams are modulated onto the selected wavelengths by the RSOA. Then, the multiplexer combines all the wavelengths before the SMF transmission. After fiber transmission, the demultiplexer splits the received optical signal into different base stations (BS) and a photodetector (PD) is employed to convert the optical signal into an electrical signal before being emitted by the antenna for wireless transmission. The detailed configuration of the proposed self-start multi-wavelength laser source is depicted in Figure 2. The lightwave emitted from the DFB laser is launched to port 1 of the optical circulator. Then, the lightwave output from port 2 of the optical circulator (OC) is routed to a tunable delay interferometer and reflected back to the OC by the optical fiber reflector. Subsequently, port 3 of the OC passes the lightwave to a \(1\times 2\) optical coupler with a 90/10 ratio. The branch with a 10% power ratio is connected to the optical spectrum analyzer for the monitor, while the other branch sends the lightwave to a PD (PP-10G) for optical-to-electrical conversion. The electrical signal is amplified by an electrical  
Figure 1: Schematic illustration of the self-start multi-wavelength laser for RoF transport systems for wireless applications (RSOA: reflective semiconductor optical amplifier, BS: base station, and SMF: single-mode fiber). amplifier (EA), filtered by a high-Q bandpass filter (BPF), and then inserted into the DFB laser for direct modulation after being amplified again by the second EA. After several rounds of the above-mentioned operation, the self-start multiwavelength laser based on the OEO structure is generated. In the optical path, TDLI is employed to reshape the multi-wavelength, and the OFR reflects the lightwave back to the TDLI for the second reshaping to boost the tone-to-noise ratio (TNR). In the electrical path, the first EA is utilized to compensate for the loss caused by the optical-to-electrical conversion, and the second EA is used to boost the electrical signal before being fed into the DFB. The 3-dB bandwidth of the high-Q BPF is 10 MHz, and the Q factor is calculated to be 995.3. The radio-frequency (RF) tone generated by the OEO structure is measured by the electrical analyzer as illustrated in Figure 3. By adjusting the gain of the second EA, the output power of the RF tone can be tuned to modulate the DFB laser for manipulating the output number of the wavelength of the self-start multi-wavelength laser source. As demonstrated in the inset of Figure 3, the numbers of multiwavelength are 3, 5, and 10 when the power of the inserted RF tone is 6 dBm, 10 dBm, and 20 dBm, respectively. Figure 3: A generated RF signal’s output spectrum and laser output vs. RF power. Figure 2: Experimental setup of OEO-based multi-wavelength generation (DFB: distributed feedback laser, TDLI: tunable delay interferometer, OFR: optical fiber reflector, PD: photodetector, EA: electrical amplifier, High Q BPF: High-Q bandpass filter). When the power of RF tone inserted into the DFB laser increases, the number of the generated multi-wavelength increases. In contrast to the number of wavelengths, the tone-to-noise ratio (TNR) decreases. Therefore, to enhance the TNR, the TDLI is deployed to reshape the multi-wavelength. Figure 4 demonstrates the measured spectra and the measuring setup of two different structures of TDLI. For the first structure, OSA captures the optical spectrum of the broadband light source after passing the TDLI for single filtering, as the red line illustrates in Figure 4. The signal-to-noise ratio (SNR) of single filtering is only 10 dB. To enhance these SNR results, a second structure with double filtering is proposed.