| PhD Viva

Name of the Speaker: Mr. Bagath Chandraprasad T (EE16D033)
Guide: Dr. Shanti Bhattacharya
Online meeting link: https://meet.google.com/ewa-irjt-nhi
Date/Time: 16th May 2025 ( Friday), 3pm
Title: Methods for optical phase retrieval from a single off-axis interference pattern

Abstract :

The optical phase profile of light is used in many diverse fields like optical metrology, optical topography, imaging applications, etc. While the transverse phase profile of a beam contains important information, it cannot be detected directly using devices like a camera. Interference is used to convert the hidden phase information of a beam to visible intensity information. The most widely used interference method involves the interference of the unknown object beam with a known tilted reference beam. Several algorithms retrieve phase information from such a single off-axis interference pattern. The choice of an algorithm for Optical Phase Retrieval (OPR) depends on the type of sample and noise present, which may be different for various applications. To choose the right algorithm, it, therefore, becomes important to understand how each algorithm works in different circumstances. In this thesis, three transform-based methods are studied in detail, which are the Fourier Transform (FT) method, the Continuous Wavelet Transform (CWT) method, and the Hilbert Transform (HT) method, which can be used for OPR applications from a single off-axis interference pattern without multiple iterations. The mathematical background of these algorithms is explained, and the comparison is done using a simulation study.

The different aspects of choosing one method for OPR applications are explained. Factors such as the tilt angle, visibility of fringes, noise, etc., are considered. The main parameters that will change the performance of the methods are the spatial frequency content of the object beam, the fringe frequency introduced by the tilt angle between the beams and noise. An integral part of the OPR method is the phase unwrapping step, which also plays a vital role in the quality of the retrieved phase profile. The performance of the combination of the OPR method and standard phase unwrapping methods is evaluated using average Root Mean Square (RMS) error and Structural Similarity Index Measure (SSIM). Experimental results of Quantitative Phase Imaging (QPI) of human blood samples to study the cell structure will be presented, and the performance of the methods will be discussed.

We have noticed that the methods can give good-quality retrieved phase profiles for lower spatial frequency applications. However, when the spatial frequency is higher, the CWT method has some disadvantages over the other two methods. The HT method is much more prone to noise but works with higher spatial frequency components compared to the CWT method. The FT method is easy to implement and gives good results, provided the interference pattern's spectral requirements are satisfied. Later, the strategies of choosing a suitable method for a particular application are implemented in a specific case study for characterizing the vibration movement of a Lamellar grating MEMS mirror. The thesis will end with a detailed summary of how to choose the suitable algorithm, as well as discussions on future work.