## 3 Principal of Operation  
When the pump level (i.e., bias current threshold level) of either SOA is more than the total fiber compound-ring cavity losses, amplified spontaneous emission (ASE) emitted from SOAs either propagates in the forward and backward directions. For instance, when a bias current \(I_{B}\) of around 75 mA) is injected into the SOA\({}_{1}\) (branch I), the ASE (Amplified Spontaneous Emission) emitted by the SOA\({}_{1}\) (branch I) circulates in clockwise (cw) direction by propagating through a tunable optical filter, which selects a passband of certain wavelengths. The selected wavelengths reach a 3-dB fiber coupler C\({}_{2}\) after propagating through a polarization controller, PC\({}_{1}\). Then, the selected light beam that arrives at port 1 of the 3-dB fiber coupler C\({}_{2}\), is equally split into two branches, II and III at port 2 and port 3, respectively. The light beam that circulates into branch II propagates through a polarization controller, PC\({}_{2}\), before it is amplified by SOA\({}_{2}\) when its bias current level, \(I_{B}\), is around 180 mA. Then, the amplified light beam arrives at port 2 of the 3-dB fiber coupler C\({}_{1}\) where it is equally divided between port 1 and port 4. Similarly, light beam from Branch III reaches port 3 after passing through a polarization controller, PC\({}_{3}\). Half of the light beam coupled into port 1 of the 3-dB fiber coupler C\({}_{1}\) is further amplified by SOA\({}_{1}\). Thus, a round-trip is completed in the fiber compound-ring structure and allows lasing to occur. Furthermore, the remaining 50% of the light beam is coupled into the output port 4 of the 3-dB fiber coupler C\({}_{1}\) is injected into the input port 4 (i.e., I\({}_{\rm in}\)) of the Sagnac loop mirror, SLM\({}_{1}\). The polarization controller, PC\({}_{4}\), controls the reflectivity of the SLM\({}_{1}\) and it is achieved by adjusting the state of polarization of the light beams propagating through the Sagnac loop mirror. For a single output port configuration, the polarization controller PC\({}_{4}\) of SLM\({}_{1}\) is adjusted for minimum power at the output port 1 (OUT1). The counterclockwise and clockwise light beams interfere destructively at the output port 1 while they interfere constructively at port 4 of the 3-dB fiber coupler, C\({}_{3}\), and thus it channels all the power back to the compound-ring cavity.  
As there is no optical isolator placed in any of the three branches of the fiber compound-ring resonator, the two counter-propagating light beams circulate in the nested ring cavities as shown in Figure 1. The counter-clockwise (ccw) propagating beam from the SOA\({}_{1}\) reaches the port 1 of the 3-dB coupler C\({}_{1}\), splits into two equal light beams (i.e., 50%) and is transmitted into the ports 2 and 3. The light beam that propagates into branch II undergoes amplification by SOA\({}_{2}\). The amplified light beam that takes the path of branch II passes through a polarization controller PC\({}_{2}\) before it reaches port 2 of the 3-dB fiber coupler C\({}_{2}\), while the light beam that propagates through the branch III passes through the polarization controller PC\({}_{3}\) before it reaches port 3 of the 3-dB fiber coupler C\({}_{2}\). Half of the light beam at the 3-dB fiber coupler C\({}_{2}\) is coupled into port 1 where it propagates back into branch I to complete one round trip, while the other half of the beam is channeled into SLM\({}_{2}\). Similarly, the light beam that is fed into the SLM\({}_{2}\) exits at the output port 1 of the 3-dB fiber coupler C\({}_{4}\) (OUT2). The output power can be controlled by the polarization controller, PC5. An optical spectrum analyzer (OSA), variable optical attenuator (VOA) and optical power meter (PM) were used to characterize the proposed fiber compound-ring laser. Note that the path lengths of both loops are the same since all branches have identical length and all fiber connections are done by using FC/APC (Fiber channel/Angled polished Connector) connectors.