## 3 Experimental Results and Discussion
Then, the obtained results show a dual-wavelength laser emission with preference to generate the shorter laser wavelength which is balanced with a wavelength shift of the FOLM spectrum toward longer wavelength by decreasing the temperature of the Hi-Bi fiber loop [20]. As it is shown, the intra-cavity losses adjustment is more pronounced for  
Figure 2: The transmission spectrum of the Hi-Bi fiber optical loop mirror (FOLM) output signal. shorter wavelengths separations of the generated laser lines. The intra-cavity losses adjustment is less significant for wavelengths separations of 5, 6 and 7 nm. Figure 5 shows the EYDCF laser output power for dual-wavelength emission with equal output powers as a function of the applied pump power. The output power measurements were obtained at the output 1 with a power meter. The wavelength separation was tuned to 5 nm with a Hi-Bi loop temperature of 21.68 \({}^{\circ}\)C with simultaneous laser emission with equal output powers at 1544 and  
Figure 4: The temperature dependence of the Hi-Bi fiber loop on the wavelength separation of the generated laser lines to obtain dual-wavelength laser emission with equal output powers. Figure 3: Continuous wave (CW) operation of the laser: (**a**) dual-wavelength laser emission with equal output powers for different wavelengths separations; (**b**) cavity losses adjustment by Hi-Bi fiber loop temperature for dual-wavelength laser generation with equal output powers and separation of 7 nm. 1549 nm. Initially, the pump power was set to 322 mW, slightly above where laser emission is reached and the Hi-Bi fiber loop temperature was adjusted for dual-wavelength laser operation with equal output powers. With the pump power variation, equal output powers of the two simultaneously generated laser lines remain stable. Therefore, a Hi-Bi loop temperature change for intra-cavity losses adjustment as the pump power is varied is not required. The laser output power as a function of the pump power can be linearly fitted with a slope efficiency of 36.4%. With the decrease of the separation between the generated laser lines, the efficiency increases because of the EYDCF ASE gain profile. For the maximal pump power applied of 10 W (limited to avoid damage of the cavity elements), the output power was of 3.6 W. As it can be observed, two different operation regions are indicated. From 322 mW (the minimal pump power in which laser emission is reached) to 890 mW, the EYDCF laser exhibits SQS laser operation. Then, CW laser operation is reached from the maximum limit for SQS operation. Figure 6 shows the SQS pulses trains evolution as a function of the pump power. The experimental results were performed with the required settings for the dual-wavelength laser operation presented in Figure 5. The measurements were obtained over a pump power range from 322 mW to 1 W. The pulses were detected with a photo-detector and observed with an oscilloscope at the output 2. In the SQS operation region of the laser, when low pump power level is applied, the weakly pumped section of the EYDCF is acting as saturable absorber due to the formation of erbium ion pairs (on highly doped fibers, which is the case), regardless of the EYDCF length. With the increase of the pump power, the population inversion is replenished to a faster rate contributing to generate high energy pulses at increased frequency due to the amplification of the relaxation oscillations at lower pump level, depending on the EYDCF length [21]. Therefore, laser pulse trains with typical characteristics of passive Q-switched pulses were detected in a range from 322 to 890 mW in which SQS laser operation is obtained. As it can be observed, the repetition rate of the SQS pulses increases as the pump power is increased.