Investigation of Coherent Beam Combination Techniques for Enhanced Combination Efficiency and Scalability

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Name of the Speaker: Sooraj M S (EE17D055)
Name of the Guide: Dr. Balaji Srinivasan

The idea of combining a large number of laser beams to generate a single high power laser beam opens up a wide range of high power laser applications in different areas including medical, defense and space exploration. Fiber lasers have unique advantages compared to other types of laser systems including efficient electrical to optical conversion, excellent beam quality due to single transverse mode of operation and the option for power scaling through master oscillator power amplifier (MOPA) schemes. Beyond kiloWatt power level, an attractive option for power scaling is beam combination of multiple fiber amplifiers. There are different approaches in beam combining such as spectral beam combining, coherent beam combining and polarization beam combining. Of these, coherent beam combining (CBC) offers much promise for scaling to more than 100 beams. CBC may be achieved using tiled aperture configuration or filled aperture configuration. We are primarily focused on exploring different coherent beam combining methods and optimization of those to achieve high efficiency and better beam quality. The phase locking between lasers for CBC has been implemented with Stochastic Parallel Gradient Descent (SPGD) algorithm with the help of on-loop phase modulators. One of the attractive methods in filled aperture based CBC is diffractive element based combining which has an advantage of high beam quality and better efficiency. We have demonstrated a novel phase synchronization scheme based on higher order minimization in 2 x 100 W CBC. Another power scaling approach is polarization beam combining (PBC), in which nearly ideal efficiency (98%) has been demonstrated. The effect of coherence and phase stabilization in PBC with non-ideal PBS has been theoretically analyzed and compared with experimental results. Considering the scaling of the number of beams, the tiled aperture configuration has advantages but its efficiency is limited by loss of power in higher orders in the fairfield pattern of array structure. The future lies in combining some of these methods to overcome some of the limitations in each method. One of them is mixed field beam combining where we combine the advantages of filled and tiled configurations to achieve better efficiency with the help of microlens arrays. Our efforts are also focused on efficiency improvement and optimization of tiled array configuration by modifying input array configurations and power redistributions in the tiled array.