טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentNegri Daria
SubjectInstantaneous Decoherence for Generating High Radiance
and Highly Uniform light Sources
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Carmel Rotschild


Abstract

A light source of high uniformity and high radiance is essential in many optical systems. However, thermodynamics sets a trade-off between high radiance and uniformity, which is why laser illumination benefits high radiance, but suffers from poor uniformity after propagating due to speckles. This results in performance degradation of the optical system, for example in imaging, the lack of uniformity damages the resolution of a system, and in excitation systems, such as lithography, the uniformity of the light source limits the resolution and the control over the process. In this work we suggest a novel design for coherence reduction of laser source enabling high radiance with minimal reduction in uniformity. The concept is based on a generation of efficient random walk propagation inside an optical system which introduces an optical path variance that is higher than the coherence length of the incident beam. This leads to effective reduction in the spatial coherence of the beam at each instantaneous moment (as opposed to dynamic methods for coherence reduction that are limited by long exposer time). To minimize the radiance losses through broad angular distribution, we use NA limited system and show that such a constrain defines the limit of coherence reduction and radiance loss. Finite temporal coherence length is achieved by using a broadband supercontinuum laser so that the phase difference accumulated between the random walks will exceed the temporal coherence and reduce speckle contrast. Scattering mediums are introduced within the fiber to enable the random walk realization during the propagation in the multi-mode fiber. With this concept, we can tailor an inner scattering fiber where the output coherence is a free parameter of both the source and the fiber.   

In order to evaluate our solution, we developed a method for effective beam propagation that preserve the physics of the system without the need of exact analytical solution that require extensive computational power. Using this method, we theoretically demonstrate that decoherence of light propagation through a random (pre-defined) scattering medium, benefits the evolution of the standard deviation in the optical path, exceeding the temporal coherence of the source.