Ph.D.

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

Outline:

PhD work is focused on the generation and propagation of custom designed beams through atmospheric turbulence. The work involves the numerical simulation with experimental validation. The main findings of my research work are as follows:

  • The study involves effects of atmospheric turbulence on the propagation of the laser beam. The Zernike polynomial and Fourier transform based turbulence phase screen model is realised to simulate the turbulence phase screen of known strength, also, turbulence of various strengths are generated and characterized, experimentally, in the laboratory by the various type of turbulence generators.
  • Evolution of phase singularities is demonstrated in the propagating wavefront under severe turbulence condition, numerically; also a novel interferometric optical setup is demonstrated for the affirmation of phase singularities generation in the wavefront using the turbulence cell.
  • Generation and propagation of Gaussian beam array are studied in numerical simulation followed by the experimental validation.
  • 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 (CDBs). Generation and their propagation have been studied numerically as well as experimentally in free space and also in the presence of turbulence.
  • The effect of atmospheric turbulence on the propagation of five groups of CDBs 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 field up to 200m. The effectiveness of these beams is demonstrated by analysing the PSF spreading and the scintillation index.


1. Generation and Characterization of Atmospheric Turbulence

Atmospheric-like-turbulence is generated and characterized by two types of turbulence generators such as turbulence cell and atmospheric turbulence simulator. For the characterization, a MATLAB based offline video processing method is developed for estimating the various turbulence parameters. The Turbulence Cell (TC) is based on the concept of hot air chamber turbulence simulator; it consists a couple of heating rods and a fan placed at predefined configuration. In this, the fan creates different wind velocities perpendicular to the path of the laser beam propagation, and the heating rods increase the temperature of surroundings. This gives rise to the convective currents (turbulence eddies) inside the cell. Consequently, the effect of turbulence is simulated in the laboratory nearly the same as in actual open atmosphere. Moreover, Atmospheric turbulence simulator (ATS) consists of random phase coded glass plate (made by a near-index-matching approach) with a motorized mount. By selecting the different aperture sizes of the ATS, different strengths of turbulence are generated in the laboratory.

2. Experimental Generation of Phase Singularities in Strong Turbulence

An optical setup is rigged up for demonstrating the evolution of phase singularity in the wavefront using turbulence cell, interferometrically. The Turbulence Cell (TC), which is used for generating the phase singularity in the beam, is based on the concept of hot air chamber turbulence simulator. By analysing interferograms, an evolution of phase singularity has been confirmed.

3. Generation and propagation of Gaussian Beam Array

Generation and propagation of Gaussian beam array are studied in numerical simulation followed by the experimental validation.Array of Gaussian beam is generated by displaying the simulated CGH on to a phase-only SLM. Propagation characteristics of Gaussian beam array are studied by their controlled combining at the focal plane in order to get maximum energy. It is found that by combining the array of five beams, the peak intensities in simulation and experimentation are increased by 17.22 and 12.23 times in comparison to the single Gaussian beam.