Experimental study of speckle generated by semiconductor light sources: application in double pass imaging

Author: Donatus Halpaap
Title: Experimental study of speckle generated by semiconductor light sources: application in double pass imaging
Director: Cristina Masoller and Meritxell Vilaseca
Presentation date: December 09, 2019
Link to text: https://www.tdx.cat/handle/10803/668261#page=1

Abstract: With the double pass (DP) technique it is possible to quantify the optical quality of a patient’s eyeby measuring its point spread function. Due to the low reflectivity of the retina a high-intensity,point-like illumination source is required. Usually laser diodes (LDs) are used, but the coherentlaser light produces speckle, an interference phenomenon that deteriorates image quality and canmake DP images difficult or impossible to interpret. A low-cost solution to reduce speckle in DPimaging is to include a vibrating mirror in the optical path of the system and average over thedifferent speckle realizations. However, vibrating mechanical parts are undesired because theylimit longevity and reliability of medical equipment.

The goal of this thesis is to find an inexpensive non-mechanical solution for speckle reductionin DP imaging based on low-coherence semiconductor light sources. We compare an LD, a light-emitting diode (LED) and a superluminescent diode (SLED) in terms of speckle formation, cost andusability in DP systems. We find that the SLED is a good alternative to LD illumination, as theamount of speckle in the image is almost as low as that obtained with an LD and a vibrating mirrorin the beam path. However, the SLED is not a low-cost solution. In order to identify a cost-efficientall-optical solution, we analyze the speckle generated by an LD as a function of its pump current.

Our experiments suggest that driving the LD below the lasing threshold can be an inexpensivesolution for speckle reduction. While undesired in many imaging applications, speckle can alsocontain useful information that is exploited, e.g., in blood flow analysis or for reconstruction of theobject that generates the speckle pattern. We find that adjusting the pump current of an LD andthe exposure time of the image acquisition system can be a simple and effective way to increase orreduce the amount of speckle by tailoring the coherence of light used for imaging. In particular,we identify conditions that allow to record images with similar average intensity, but with speckle contrast values as low as 0.16, or as high as 0.99.