Degree: Ph.D.

Topic: A Study on the Generation of Custom Designed Beams and their Propagation Characteristics for Adaptive Optics

Supervisor: Dr. Arun Kumar Gupta, Ex. Director, Scientist 'H', IRDE, Dehradun, India.

Co-Supervisor: Dr. Sanjay Kumar Mishra, Scientist ‘F’, DRDO-IRDE, Dehradun, India.

Work Centre: DRDO - Instruments Research & Development Establishment (IRDE), [A premier Electro-optical Laboratory of Defence Research & Development Organisation], Dehradun, India

University: Uttarakhand Technical University, Dehradun, India

Abstract: Laser beam with phase singularities, due to its unique feature of helical wavefront, creates interest in the field of free space laser propagation. These helical wavefront retain their coherence in the turbulent condition while propagating over long distances. Atmospheric turbulence and its effects on the propagation of the laser beam are studied. In numerical simulation, the Zernike polynomial based phase screen model is realised to simulate the turbulence phase screen of known strength whereas, in experimentation, turbulence of various strengths are generated and characterised in the laboratory by the various type of turbulence generators. Besides, evolution of phase singularities is demonstrated in the propagating wavefront under severe turbulence condition, numerically; also an interferometric optical setup is rigged up for demonstrating the evolution of phase singularities in the wavefront using the turbulence cell. The new types of beams are designed and generated in which singular phases are encoded at multiple locations and are termed as Custom Designed Beams (CDB). Generation and their propagation have been studied numerically as well as experimentally in free space and also in the presence of turbulence. These Custom Designed beams are generated by displaying the simulated CGH on to a phase-only Spatial Light Modulator. The effect of atmospheric turbulence on the propagation of five groups of CDB has been investigated with the help of two types of turbulence simulators (such as ATS and the turbulence cell) along with the actual propagation effects of turbulence in the open field up to 200m. The effectiveness of these beams is demonstrated by analysing the PSF spreading and the scintillation index.

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