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There are substantial white spaces in output wavelengths of rare-earth doped fiber lasers in the 1000 - 2000 nm range. Cascaded Raman fiber lasers (CRFLs) can fill these gaps by using stimulated Raman scattering (SRS) to achieve gain in intermediate wavelengths. CRFLs transfer power from a pump to a Stokes at a longer wavelength. This process can be cascaded, where Stokes can act as a pump and transfer power to higher Stokes at longer wavelengths.
Figure 1: Schematic of generating power in any wavelength by using cascaded Raman shifts along with a wavelength tunable pump.
The gain provided by SRS is dependent only on the underlying fiber medium. Using a tunable pump in CRFLs allows the SRS gain wavelength to be tuned. While FBGs provide feedback at a fixed wavelength, the backscattered part of Rayleigh scattering (RS), which occurs due to random inhomogeneities in the fiber, provides wavelength-independent random distributed feedback (RDFB) over a long fiber length. Broad feedback at one end reduces the generation threshold of Raman Stokes. A wavelength-tunable pump and broad feedback are reported to provide power at any wavelength in the 1000 - 2000 nm range.
References:
V. R. Supradeepa, Y. Feng, and J. W. Nicholson, "Raman fiber lasers," J. Opt. 19(2), 023001 (2017).
Y. Feng, ed., Raman Fiber Lasers, Springer Series in Optical Sciences (Springer International Publishing, 2017).
S. K. Turitsyn, S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castañón, V. Karalekas, and E. V. Podivilov, "Random distributed feedback fibre laser," Nature Photonics 4(4), 231–235 (2010).
L. Zhang, H. Jiang, X. Yang, W. Pan, S. Cui, and Y. Feng, "Nearly-octave wavelength tuning of a continuous wave fiber laser," Scientific Reports 7(1), 42611 (2017).
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