# Brief Review of Recent Developments in Fiber Lasers
## 5 Brillouin Fiber Lasers
#### Multiwavelength Fiber Lasers  
SBS is the most attractive technique for the development of multiwavelength fiber lasers, which have the advantages of a narrow linewidth, low-intensity noise, stable operation at room temperature, and potential applications in optical communication, optical sensors, gas spectroscopy, and microwave photonics.  
The cascaded SBS process occurs when the power of the first-order Stokes signal reaches the SBS generation threshold of the second-order Stokes (Figure 8). The first-order anti-Stokes signal can also be generated due to the four-wave mixing process between the pump and the first-order Stokes wave. The typical frequency shift in the Brillouin spectrum in most glass is about \(\sim\)10 GHz. This narrow frequency shift is a challenge for the demultiplexing and signal filtering processes in optical communication applications. Several approaches have been developed to increase the frequency spacing range and to reduce the complexity of demultiplexing. These approaches include widely used double Brillouin frequency spacing of 0.17 nm [91; 92; 93] and a triple Stokes line spacing of 0.25 nm [94; 95; 96; 97]. A quintuple wavelength spacing of 0.4 nm was presented in ref. [98], where four quintuple Brillouin Stokes signals with a high Stokes power of 10 dBm and a signal-to-noise ratio of 50 dB were achieved. The recorded Stokes signals were tuned over a wavelength range of 40 nm (1540-1580 nm).  
Figure 8: Illustration of the cascaded SBS process. It occurs via the interplay of SBS and four-wave mixing.  
The sextuple Stokes signal 0.48 nm has been demonstrated in ref. [99], utilizing a triple unidirectional ring laser cavity. Three sextuple channels are achieved and tuned over a wavelength range of 15 nm (1560-1575 nm). In this work, up to three sextuple Stokes lines with a high signal power of 10 dBm and a large optical signal-to-noise ratio of more than 55 dB have been demonstrated. It has been shown that the generated Stokes lines can be tuned over a wavelength range of 15 nm from 1560 to 1575 nm.  
A switchable multiwavelength Brillouin fiber laser has been proposed and demonstrated with channel spacing from 10 GHz up to even more than 100 GHz at a step of 10 GHz [100]. The structure comprises two nonlinear fiber loops, a feedback path, and a tunable optical bandpass filter for switchable operation. The multiwavelength Brillouin fiber laser is generated in the nonlinear fiber loop, where a dual-pump source produces dual Brillouin lasers that give rise to a wideband multiwavelength Brillouin fiber laser via the four-wave mixing effect. Some achievements of multiwavelength Brillouin fiber lasers across different spectral regions have been discussed in ref. [101].  
Fiber lasers based on backward SBS have been studied and employed for over thirty years. As mentioned, SBS in fibers takes place in the forward direction as well, with amplification bandwidths which are narrower by two orders of magnitude. The first forward Brillouin fiber laser, using a bare off-the-shelf, panda-type PMF, has been reported in ref. [102]. Pump light in one principal axis provides Brillouin amplification for a co-propagating lasing signal of the orthogonal polarization. Feedback is provided by Bragg gratings at both ends of the optical fiber cavity. Single-mode, few-mode, and multimode regimes of operation have been observed. A laser threshold has been observed at a pump power level of 26.5 dBm. The output power of the laser was saturated at 250 uW due to the onset of backward SBS in the fiber cavity. The differential slope efficiency of the laser output power between the threshold and saturation was approximately 0.005. Forward SBS fiber lasers are extremely sensitive to changes in media outside the cladding. They can serve as highly coherent laser sources and ultraprecision forward SBS sensors.