# Tunable 60 GHz Multiwavelength Brillouin Erbium Fiber Laser
## 1 Introduction  
Due to its advantages in many applications, such as waveguiding [1], spectroscopy [2; 3], optical sensors [4; 5; 6; 7], quantum computing, and spectroscopy [8], multiwavelength fiber laser sources have received much attention from many researchers. Different techniques have been proposed for producing multiwavelength fiber laser sources, such as four wave mixing (FWM) [9], nonlinear optical mirrors [10], and stimulated Brillouin scattering (SBS) [11; 12]. Among these methods, SBS in optical fiber is considered the best option due to the ultraslim linewidth [13]. In a multiwavelength Brillouin erbium fiber laser (MBEFL) cavity, a higher-order Brillouin Stokes signal is generated by an amplified lower-order Brillouin Stokes line [14; 15]. However, the main drawback of the MBEFL's source is the narrow frequency shift of the Brillouin Stokes lines (0.08 nm) [16; 17], which makes the laser comb inapplicable in optical communication system applications, because it cannot be distinguished within the de-multiplexer in the receiver side. Many researchers have suggested double Brillouin frequency shifting of 0.17 nm [18; 19; 20; 21; 22; 23]. To increase the Brillouin frequency shift, fewer researchers have suggested a triple Brillouin frequency shift of 0.25 nm to increase the possibility of using multiple laser sources in the de-multiplexing side [24; 25; 26; 27; 28; 29; 30]. A switchable Brillouin frequency shift was achieved via three single-mode fiber (SMF) spools [24]. Only two Brillouin Stokes lines with a low power of \(-3.5\) dBm that could be tuned over 22 nm were recorded. A long SMF length of 50 km was utilized in two ring laser cavities to produce up to 10 Stokes lines with a triple frequency shift [25]. A wide tuning range of 35 nm was reported. However, a high pump power of 600 mW was utilized atthe expense of a low Stokes power of \(-4\) dBm. Switchable Brillouin frequency shift lines were produced using serial SMF spools in the MBEFL cavity [26]. However, to provide switchable frequency, these spools were manually added and removed within the cavity. In addition, a high pump power of 1.2 W was utilized. A MBEFL cavity with a triple Brillouin frequency shift was presented by [27]. Up to six Stokes lines were generated using two erbium amplifiers and 20 km of two dispersion compensation fiber (DCF) spools. A MBEFL cavity that utilizes two erbium-ytterbium amplifier sections was implemented by [28]. Up to nine Stokes lines with a triple frequency shift with a low signal power of \(-15\) dBm were recorded at the central frequency of the self-lasing cavity modes (1565 nm). However, the laser tunability was limited to the free-running mode's central frequency, and these modes emerged at the output laser comb once the frequency was shifted beyond 1565 nm. Previous literature [24; 25; 26; 27; 28] has shown self-lasing cavity modes' competition, which affects the tunability of the laser cavity. Four triple Brillouin frequency lines that were tuned over 40 nm (1530-1570 nm) were produced by [29]. The tunability was limited only to the emission bandwidth of the erbium. However, a DCF spool with 12 km length was used, and for such a long length, both Brillouin gain saturation and high transmission losses occurred. Recently, a simple and widely tunable MBEFL with a triple frequency shift was presented by [30]. Six Brillouin Stokes lines that had a tunability of 50 nm (1530-1580 nm) were recorded at a relatively low pump power of 220 mW. Enlarging the Brillouin frequency spacing beyond 33 GHz is required in the demultiplexing process at the receiver side. However, only few studies have proposed a quadruple and quintuple Stokes channel (40 and 50 GHz) [26; 31; 32; 33]. Four quadruple Brillouin Stokes lines with a low Stokes power of \(-2\) dBm were produced by [26]. However, a long SMF length of 112 km and a high pump power of 1.1 W were utilized. Using such high values, the proposed setup may not be practical. Only two quadruple Stokes lines were generated [31].