## 4 Characterization of the Fiber Compound-Ring Lasers
### Fiber Laser Power Tunability and its Switchable Dual-Output Port Operation  
The proposed fiber compound-ring laser has a feature of operating with two adjustable and switchable output ports (i.e., OUT1 and OUT2). The output power from either output port can be tuned by adjusting the gain of the semiconductor amplifiers, SOA\({}_{1}\) and SOA\({}_{2}\), (i.e., by controlling their bias current levels) or by adjusting the reflectivity of the Sagnac loop mirrors, SLM\({}_{1}\) and SLM\({}_{2}\), while keeping the former constant. Figure 5: Output power and combining efficiency spectra of the proposed dual-Sagnac loop mirror SOA-based tunable fiber compound-ring laser system using two SOAs as gain media. Individual SOA output power spectrum: SOA\({}_{1}\) (filled squares), SOA\({}_{2}\) (unfilled circles), output power summation spectrum of both SOAs (unfilled triangles), and actual measured output power (crosses) at the output port, OUT2 with SOA\({}_{1}\) and SOA\({}_{2}\) driven at a 200-mA and 500-mA constant bias current. The PC\({}_{1}\) and PC\({}_{2}\) were maximized for each wavelength. The SOA gain adjustment method was performed by setting the tunable filter at 1550 nm wavelength and adjusting the bias current levels, \(I_{B1}\) and \(I_{B2}\) of both SOAs. Table 2 shows the output power evolution at both output ports, OUT1 and OUT2 as a function of the bias current levels, \(I_{B1}\) and \(I_{B2}\). The reflectivity of the Sagnac loop mirror, SLM\({}_{1}\) and SLM\({}_{2}\) were set at \(\geq\)99.9% and \(\leq\)0.1%, respectively. The achieved maximum dynamic range was 40.75 dB at both SOAs' standard bias current levels of 200 and 500 mA for SOA\({}_{1}\) and SOA\({}_{2}\), respectively. The second approach involves the adjustment of the reflectivity of both Sagnac loop mirrors, SLM\({}_{1}\) and SLM\({}_{2}\) while keeping the gain of both SOAs constant (i.e., \(I_{B1}\) and \(I_{B2}\) set at 200 and 500 mA, respectively). The proposed fiber compound-ring laser can be operated in single or dual-output configuration depending on the reflectivity of the SLM1 and SLM\({}_{2}\). In single output configuration, one of the Sagnac loop mirrors, SLM\({}_{1}\) or SLM\({}_{2}\), should be kept at high reflectivity (i.e., \(\geq\)99.9%) by adjusting its polarization controller, PC\({}_{4}\) or PC\({}_{5}\), respectively, while keeping the other Sagnac loop mirror at its lowest reflectivity of \(\leq\)0.1%. The tunable filter was set at a 1550-nm wavelength in order to characterize the power tunability performance of both output ports of the fiber laser. Then, we initialized the reflectivity settings of the SLM\({}_{1}\) and SLM\({}_{2}\) to \(\leq\)0.1% and \(\geq\)99.9%, respectively. The initial measured output power from both output ports, OUT1 and OUT2, was +11.85 dBm and \(-\)28.9 dBm, respectively. Moreover, we gradually adjusted the reflectivity of the Sagnac loop mirror, SLM\({}_{1}\), by slowly changing the polarization state of the counter-propagating light beam into the SLM\({}_{1}\) by adjusting the polarization controller, PC\({}_{4}\), while recording the power meter readings at both output ports, OUT1 and OUT2. An optical spectrum analyzer (OSA) was used to record the reflectivity dependence of 3 dB-bandwidth of the output signal spectrum at the output port, OUT 1, We were able to control the output power from the output port, OUT1, from +11.85 dBm to \(-\)28.5 dBm while keeping the other output port, OUT2, at \(-\)28.9 dBm by also optimizing the polarization controller, PC\({}_{5}\), of SLM\({}_{2}\). Similarly, we also set the reflectivity of SLM\({}_{1}\) and SLM\({}_{2}\) to \(\geq\)99.9% and \(\leq\)0.1%, respectively, and checked both output ports' performance in the similar manner as stated above, where the measured output power from output port, OUT\({}_{2}\) was adjusted from +11.87 dBm to \(-\)28.9 dBm while keeping the output port, OUT1, at \(-\)28.9 dBm.