Power Combining of Raman Fiber Lasers
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 μm 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., why it leads to a single lasing line rather than a dual wavelength output. The next step is to see the limits of power combining. Does the proposed method work for any arbitrarily spaced wavelength pumps? How close do the pump lasers have to be before the technique works no more? How far apart can they be? Specifically, it is shown that the technique of nonlinear Raman based power combining works even in the impressive scenario where the pumps are separated by as close as 5 nm (for pump input wavelengths of 1074 nm and 1079 nm). And it works when they are separated as large as 29 nm, which is almost half a Stokes shift away (for pump input wavelengths of 1088 nm and 1117 nm). This proves the versatility of the technique for two pumps. The versatility of the nonlinear power combining technique is further strengthened by demonstrating the technique with 3 input pump lasers all operating at different wavelengths (pump wavelengths of 1074 nm, 1088 nm and 1117 nm). Even in this scenario, simultaneous power combining and wavelength conversion to a single lasing line in the 1.5 micron band is demonstrated, with more than 90% of the power residing in the final band! A serendipitous discovery in the process of achieving Raman based power combining was to observe the Raman fiber spools glowing an iridescent rainbow of hues – all this when the input pump lasers and the Stokes wavelengths were in the Near Infrared (NIR). This led to the fascinating study and understanding of visible light generation in Raman fibers. Specifically in this thesis, it is theorised that the visible rainbow of hues is formed due to the harmonic conversion of the propagating NIR Stokes in the Raman fiber and mediated through a Cherenkov phase matching process which enables the light to escape from the cladding of the fiber. What follows is an extremely rigorous mathematical analysis of the entire phenomenon. The fruit of such rigor led to the development of a robust image processing algorithm that takes as input static DSLR images of the glowing Raman fiber spools and produces as the output a length-resolved plot of the effective NIR Stokes wavelengths propagating in the core of the fiber. This is extremely beneficial as a non-contact diagnostic tool for spectral analysis of fiber lasers. Further verification of the mathematical theory developed is provided where the same theory is applied to analyse the visible light generation in a fiber supercontinuum source developed in-house. The mathematical model was remarkably successful in predicting the visible spectrum which was verified with a visible spectrometer. In summary, this research thesis aims to advance the field of Raman fiber lasers by delving into more niche yet extremely fruitful aspects, beyond the typical view that they are just a source to achieve wavelength tunability. Innovative power combining approaches and a solid analysis of visible light generation in such Raman fiber lasers, along with an elegant image processing algorithm, all goes to show that with a bit of math, Raman fiber lasers can be the next generation laser systems in every field.