Power Combining of Raman Fiber Lasers

Abstract

Fiber lasers have become ubiquitous in industry and research for their numerous attractive properties that other lasers such as solid state lasers lack. However, conventional doped fiber lasers, though providing high power, do so only at specific wavelengths, with lots of white spaces in the spectrum. Raman Fiber Lasers are currently the only known mature technology to achieve wide degree of wavelength agility with high powers. Such Raman fiber lasers start off with a single high power pump source at a given wavelength and use the concept of Stimulated Raman Scattering to get to the otherwise inaccessible longer wavelengths through a series of Stokes shifts. Using a single pump has its own drawbacks, as it would primarily overburden the single high power pump source (and also leads to Raman instability). This thesis starts with mitigating these drawbacks to power scaling of Raman lasers by proposing the concept of nonlinear Raman based power combining. The goal of Raman based power combining is to see if one can achieve simultaneous power combining and wavelength conversion of multiple lower power laser modules into a single lasing line at any arbitrarily longer wavelength through the Raman effect. By using multiple lower power modules, one would not stress the system components and failure of one of the components would not lead to a total collapse of the system. A greater bonus would be if all these pump modules were operating at different wavelengths (but in the same band) and yet if one could achieve a wavelength conversion to a single lasing line (with a combined power of the input modules). This is what has been demonstrated in this work, where the first step was to perform a simultaneous power combining of two ~ 100 W class lasers operating at different wavelengths in the Yb emission band (1 and#956;m band) to a single lasing line of ~ 100 W at the 1.5 micron band. An explanation is proposed for why the nonlinear power combining technique works the way it does, viz...