# Tunable 60 GHz Multiwavelength Brillouin Erbium Fiber Laser
## 3 Results and Discussions  
Figure 2 shows the free-running modes of the proposed laser cavity. These modes were recorded in the absence of the Bp signal. An optical span of 120 nm (1500-1620 nm) and a maximum pump power value of 450 mW were used. An erbium gain peak bandwidth of 10 nm from 1570 to 1580 nm was obtained. Measuring these modes is important to select the suitable Bp signal wavelength that can obtain a maximum erbium gain level. The threshold power for the generated sextuple Stokes signals is illustrated in Figure 3. At a fixed Bp power of 14 dBm (25 mW) and a wavelength of 1565 nm, the threshold values of the 6 th, 12 th, and 18 th Stokes signals were recorded. The wavelength of the Bp signal was selected at the maximum gain of the self-lasing cavity modes, as shown in Figure 2. The sixth Stokes signal (first sextuple Stokes) was generated at a 130 mW pump power (the power was divided equally to all EDFAs). The generated sixth Stokes signal is represented as the subsequent Bp power to generate the second sextuple Stokes signal (12 th Stokes), and it became sufficient enough with increasing the pump power further. As the pump power reached 130 mW, the 12 th Stokes signals appeared. To generate the third-order sextuple Stokes signal (18 th Stokes), the pump power was increased to 410 mW. At such a high pump power, the second-order sextuple Stokes signal reached the threshold value of the higher-order sextuple Stokes signal. At the central wavelength of the self-lasing cavity modes (1570 nm) and at a high Bp power of 25 mW, the generated sextuple Stokes signal's spectrum was as illustrated in Figure 4. Referring to Figure 1, a high Bp power is needed to be forceful enough for generating the first  
Figure 3: Threshold values of the sextuple Stokes signals. Figure 2: Free-running modes of the proposed laser cavity at the maximum pump power of 450 mW. and the second Stokes signals in the absence of erbium gain. The erbium amplifiers (EDFA1, EDFA2, and EDFA3) were chosen to be of the same type, so that the generated Stokes signals (third, fourth, and fifth) can experience the same gain within the laser cavity. At the optimum Br power and wavelength and at the optimum pump power of 450 mW, four sextuple Stokes signals (Bp, 6 th, 12 th, and 18 th) were obtained. For all generated Stokes signals, a fixed Brillouin frequency shift of 0.48 nm (60 GHz), high signal power values, and a high optical signal-to-noise ratio (OSNR) of more than 56 dB were recorded. Figure 5 shows the laser tunability of the generated sextuple Stokes signals at the maximum powers of Br and a pump power of 25 and 450 mW, respectively. The tuning range was measured in the absence of the free-running modes at the output laser comb. The recorded tuning range was recorded within the range of 30 nm from 1560 to 1590 nm. Three parameters were behind such a wide tuning range: a high Br power, the use of pre-amplification techniques for the generated Stokes signals, and a reduction in the competition between the Brillouin Stokes signals and the free-running mode by extracting this mode out of the cavity in DCF3 and SMF loops. Figure 6 shows the effect of the pump power on the number of generated Stokes signals and the tuning range of the laser cavity. It is clear that with the increase in the pump power  
Figure 4: The output spectrum of the sextuple Stokes signals at the optimal input parameters. Figure 5: Tuning range of the generated sextuple Stokes signals. value, the number of generated Stokes signals increased linearly with different slopes and was saturated at a pump power value of 540 mW at four Stokes signals. At 150 mW, the first sextuple Stokes signal appeared (sixth Stokes signal). With the increment in the pump power, no higher-order sextuple Stokes signal (12 th Stokes signal) was generated until a pump power value of 350 mW. In other words, there was no change in the number of generated Stokes within the range of 150 to 350 mW.