Design method for ultimate efficiency in linear-cavity continuous-wave lasers using distributed-feedback
A novel analytical method for the design of linear-cavity continuous-wave laser cavitiesthat guarantees the ultimate efficiency is developed, theoretically studied and experimentally verified. Opposed to the earlier methods, which optimise the parameters of a priori defined cavity, the developed method derives the cavity analytically based on the active medium properties for the chosen pumping scheme. The method combines the general grating design equations, valid for both passive and active media, and the optimum signal power calculations. The idea that lies at the heart of the design method is to sustain the optimum signal power at every single point in the entire cavity byemploying distributed-feedback for the maximum local, as a result, for the maximum overall conversion efficiency. Theoretical study starts with the critical investigation of the previous optimisation approaches. After addressing the limitations of these approaches, it is shown how toimprove the efficiency further than the parametric optimisation using intuitive arguments based on the effective cavity length and optimum reflectivities in DFB lasers. The critical importance of the signal distribution is highlighted, and following this observation the grating design method for arbitrary signal distributions is developed. The concept of optimum signal power is introduced and the spatial unfolding of the optimum values is illustrated. Boundary conditions, grating production limitations and effects of modelling parameters are addressed. Modal stability of the new designs is investigated. A novel approach to the simulation of Er/Yb co-doped fibre lasers is presented with experimental justification. Accurate laser characteristics are predicted for different designs, including the ultimate efficiency designs. Theoretical studies are verified with experimental data in Er/Yb co-doped fibre and discussions are extended to Yb doped fibres, high pump powers and alternative pumping schemes.