## 3 Experimental Results and Discussion  
The Hi-Bi FOLM output reflection/transmission exhibits sinusoidal spectrum which depends of the input wavelength. With the Hi-Bi fiber characteristics, the calculated wavelength period is 11.2 nm [20]. Figure 2 shows the Hi-Bi FOLM transmission spectrum measured using the EYDCF amplified spontaneous emission (ASE) input signal at the pump power below the lasing threshold of 322 mW. The measurements were obtained at the Output 2 with the OSA. The Hi-Bi FOLM transmission spectrum was obtained in a wavelength range from 1540 to 1570 nm at 22.87 \({}^{\circ}\)C of Hi-Bi fiber loop temperature. By adjusting the rotation stages, the Hi-Bi FOLM was set for maximum contrast between maximum and minimum transmission. As it can be observed, the measured Hi-Bi FOLM transmission spectrum wavelength period is of \(\sim\)11.13 nm for the wavelength period in which laser emission is expected because of the TFBGs central wavelengths. The output signal ratio between the Hi-Bi FOLM reflection values at the two TFBG wavelengths varies with the tuned wavelengths by the TFBGs. Then, a proper wavelength displacement of the Hi-Bi FOLM spectrum by Hi-Bi fiber loop temperature  
Figure 1: Experimental setup of the tunable dual-wavelength Er/Yb double-clad fiber (EYDCF) laser. variations is performed to balance the internal cavity losses in order to obtain dual wavelength laser operation with equal powers. Figure 3a shows the measured output spectra of dual-wavelength fiber laser operation for different separations of the generated laser wavelengths. The measurements were obtained at the output 2 with the OSA with pump power of 2 W. As it can be observed, dual-wavelength laser operation is achieved with maximum wavelengths separation of 7 nm. Initially, TFBG1 and TFBG2 were tuned at 1546 and 1547 nm, respectively, in order to obtain a separation between the generated laser lines of 1 nm. Wavelength tuning interval of 0.5 nm s applied to both TFBGs (toward longer wavelengths for FBG1 and toward shorter wavelengths for FBG2) each measurement in order to increase the wavelengths separation for 1 nm until reaching a maximal separation of 7 nm. The full width at half maximum (FWHM) spectral bandwidth of the generated laser lines is of \(\sim\)0.2 nm. The temperature of the Hi-Bi fiber loop required for dual-wavelength operation with equal output powers, adjusted for each wavelength tuning, is also shown. Figure 3b shows the output spectrum for dual-wavelength laser generation with equal pump powers in logarithmic scale. The measurement was obtained for the case of wavelengths separation of 5 nm were the required Hi-Bi fiber temperature for the intra-cavity losses adjustment is of 21.56 \({}^{\circ}\)C. As it can be observed, the amplitude of both generated laser lines is determined by the Hi-Bi FOLM transmission spectrum gain. The 3-dB linewidth of both laser lines is of \(\sim\)0.19 nm. Dual-wavelength operation with equal output powers is achieved by adjusting the intra-cavity losses for both generated laser lines with temperature variation on the FOLM Hi-Bi fiber loop for each wavelength tuning. Figure 4 shows the FOLM Hi-Bi loop temperature adjustment required to achieve dual wavelength operation with equal output powers for different wavelengths separations (\(\Delta\lambda\)) from experimental results obtained in Figure 3. As it is shown, dual-wavelength laser operation with equal output powers is obtained at temperatures of 23.50 and 21.56 \({}^{\circ}\)C of the Hi-Bi fiber loop for the minimal separation of 1 nm and the maximal separation of 7 nm, respectively. As it can be observed, the Hi-Bi loop temperature required to obtain dual-wavelength operation decreases with the increase of the tuned wavelengths separation. By decreasing the temperature of the Hi-Bi fiber loop, the modulation spectrum of the FOLM shifts toward longer wavelengths.