# Tunable 60 GHz Multiwavelength Brillouin Erbium Fiber Laser
## 2 Experimental Setup  
Figure 1 shows the experimental setup of the proposed laser configuration. A 3 dB optical coupler in addition to one four-port circulator (Cir2) as well as four three-port circulators (Cir1, Cir3, Cir4, and Cir5) are used in this design. The Brillouin gain medium is presented by three dispersion compensation fiber (DCF1, DCF2, and DCF3) spools and  
one single-mode fiber (SMF) spool. Three erbium-doped fiber amplifiers (EDFA1, EDFA2, and EDFA3) are employed to achieve a high Bp gain and to amplify the generated sextuple Stokes signals. EDFA1 was pumped by 150 mW using a 1480 nm pump unit, and both EDFA2 and EDFA3 were pumped by 300 mW through a 50:50 optical coupler using the same pump wavelength. The Bp signal was achieved via an external tunable source laser (TSL) with a maximum power of 14 dBm and a tuning range of 150 nm (1480-1630 nm). The residual Bp power and the generated Stokes signals were measured at the output side using an optical spectrum analyzer (OSA).  
Firstly, TSL provides the Bp signal that is inserted into the first Brillouin gain medium (DCF) through a 3 dB coupler and Cir1. The first Brillouin frequency shift (10 GHz\(\simeq\)0.08 nm) Stokes signal is generated when the Bp value is greater than the threshold power. The generated first Stokes signal is circulated in the first ring cavity via Cir1 and Cir2 to achieve the second (second Stokes) Brillouin frequency shift (20 GHz\(\simeq\)0.16 nm) within the same Brillouin gain medium (DCF1). A high Bp power of 14 dBm (25 mW) is used to be sufficient enough to generate both the first and second Stokes signals. The second Stokes signal is directed by Cir2 to be amplified by EDFA1 and inserted into the second Brillouin gain medium (DCF2). In this stage, the third (third Stokes signal) Brillouin frequency shift (30 GHz\(\simeq\)0.24 nm) is generated. Pre-amplification is used in the second laser cavity to enhance the subsequent Bp (2 nd Stokes) and to improve the tuning range of the generated Stokes signal (3 rd Stokes). The amplified third Stokes signal is forced to be inserted into DCF2 from the other end via Cir2 and Cir3. When the third Stokes signal is inserted, the fourth Brillouin frequency shift (40 GHz\(\simeq\)0.32 nm) is achieved.  
In the pre-amplification cavity, EDFA2 is used as a pre-amplifier to amplify the generated fourth Stokes signal (the subsequent Bp), which is inserted into DCF3 via Cir4. The generated Stokes signal (fifth Stokes signal (50 GHz:0.4 nm)) is also amplified by EDFA2 for more gain to be sufficient enough to generate the higher-order Stokes, signal since another cavity is used, and the generated Stokes signal cannot be inserted at the other end of DCF3. The amplified fifth Stokes signal is inserted into the SMF spool for generating the sextuple Stokes signal (sixth Stokes, 60 GHz:0.48 nm). Finally, the generated sixth Stokes signal is amplified by EDFA3 for the next sextuple Stokes generation. The generation of the sextuple Stokes signal will continue until the total gain inside the cavities is equal to the total losses. Two open-edge cavities (DCF3 and SMF) are used to enhance the tuning range by extracting the unwanted self-lasing cavity modes that increase competition with the Brillouin gain out of the cavity. In addition, the optical signal-to-noise ratio is improved with this kind of geometrical cavity.  
Figure 1: Experimental setup of the proposed cavity.