# Brief Review of Recent Developments in Fiber Lasers
## 4 Raman Fiber Lasers
### Heat Mitigation in Raman Fiber Lasers  
In 1963, [PERSON] introduced the concept of stimulated Raman scattering (SRS) [74]. In ref. [74], it has been shown that since the laser light incident on a medium is coherent, the resulting emission occurs through a parametric process due to the interaction between the coherent molecular oscillations (optical phonons) and the coherent laser light. The lasing mechanism of Raman lasers is based on stimulated Stokes Raman scattering (SSRS) in a Raman medium (Figure 6a). An incoming pump photon with the energy \(\hbar\omega_{P}\) scatters into a lower-energy Stokes photon \(\hbar\omega_{S}\) and a phonon with the energy \(\hbar\Omega_{op}\), which serves as a source of heat generation. As can be seen in Figure 6a, Raman lasers, like RE-doped lasers, suffer from heat dissipation inside the active medium caused by the quantum defect between the pump and lasing (Stokes) photons. Like in the case of RE-doped lasers, heat generation in Raman lasers can degrade their performance. Heat mitigation in Raman lasers can be achieved in different ways. One way to mitigate heat dissipation in Raman lasers has been proposed in [75; 76; 77]. It relies on three different Raman processes that can occur inside the medium of an optically pumped Raman laser illustrated in Figure 6. Besides SSRS, stimulated anti-Stokes Raman scattering (SARS) and coherent anti-Stokes Raman scattering (CARS) can take place in the  
Figure 6: Processes that take place in a Raman laser: (**a**) stimulated Stokes Raman scattering (SSRS), (**b**) stimulated anti-Stokes Raman scattering (SARS), (**c**) coherent anti-Stokes Raman scattering (CARS) when it converts a Stokes photon and a pump photon to an anti-Stokes photon and a pump photon, annihilating two phonons, and (**d**) CARS when it converts an anti-Stokes photon and a pump photon to a Stokes photon and a pump photon, creating two phonons. \(\omega_{P}\), \(\omega_{S}\), \(\omega_{aS}\), and \(\Omega_{op}\) are the pump, Stokes, anti-Stokes, and optical phonon frequencies, respectively. Raman medium. The maximum Raman gain point is considered in Figure 6, that is, all scattering processes are exactly at the Raman resonance. As stated above, the SSRS, which is responsible for lasing, is a source of heat generation. In common with SSRS, the SARS process generates quantum-defect heating too. In this process, an anti-Stokes photon is converted into a pump photon and a phonon (Figure 6b). CARS at the Raman resonance presented in Figure 6c is a four-wave mixing process. It converts a pump photon and a Stokes photon into a pump photon and an anti-Stokes photon, annihilating two phonons responsible for heating [78]. This process reduces the quantum-defect heating in the Raman medium. Phase matching is very important in this scheme. If it is not satisfied, the induced phase variation along the propagation direction of the amplified signal will cause the CARS process to alternate, reversing periodically at the coherence length (Figure 6c,d). This approach to Raman processes, which was developed by [PERSON] and [PERSON] in ref. [78], is different from the traditional one, where the CARS is seen as a mechanism that converts two pump photons into Stokes and anti-Stokes photons without exchanging energy with the Raman medium. By this means, the intrinsic heat-mitigation technique in Raman lasers relies on CARS (Figure 6c) instead of anti-Stokes fluorescence. The harnessing of frequency-selective dissipative coupling to reduce the heat generated by CARS reversed cycles in active Raman media with phase mismatching was proposed in ref. [79] for waveguide systems. This approach can be applied to a fiber Raman system too. Heat mitigation in this approach is accomplished using a coupled waveguide structure, which includes the active Raman waveguide (RW), where the Stokes signal undergoes amplification via SSRS, and a dissipative waveguide (DW), which is tuned to the anti-Stokes wavelength so as to evacuate the corresponding anti-Stokes photons from the RW by coupling.