Deisgn and development of coupling procedures for improved coupling efficiency in a LED- single mode fiber unit

• Main Author: Anjali Balakrishnan
• Other Authors: Murukeshan Vadakke Matham
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/55143
• Institution: Nanyang Technological University

Optical fiber based devices have evolved themselves as an indispensable tool in various domains of optical imaging and sensing with prominent contributions towards fluorescence imaging and sensing. Although laser based excitation sources are widely used for performing fluorescence studies, increased efforts have been also devoted towards developing fiber-optic imaging and sensing devices based on cheaper and miniaturized excitation sources. Recent progress in semiconductor technology have enabled the development of highly efficient and stable light emitting diodes (LED) that are cost effective, reliable and are capable of operating at a multitude of wavelengths. However, the highly divergent nature of LED demands the usage of an appropriate lens system so as to facilitate adequate collimation and coupling for various imaging and sensing applications. A variety of lensing schemes have been used to increase the coupling efficiencies between small area emitters and optical fibers. In this context, this dissertation details the various optical modelling and experimental based investigations performed to determine the best lens system for the collimation and focusing of LED source for developing fiber optic based imaging and sensing systems. Three classes of lens systems were optically modelled, namely: bi-convex lens, ball lens and aspheric lens. By computing the number of rays reaching the target surface using Tracepro, it was concluded that ball lens or aspheric lens based collimation systems are to be favoured. However, the lower optical aberrations exhibited by aspheric lens makes them a preferred choice for collimating a broadband source like LED and this particular lens system was chosen for further experimental studies. The experimental studies initially involved laser to single mode fiber coupling and the experimental results demonstrated the significance of tuning the focal spot diameter of the beam for achieving good levels of coupling efficiencies. Further, LED based fiber optic imaging probe was developed using the proposed aspheric lens based coupling system and the experimental results show that the system is capable of imaging micron sized fluorescent signatures. The proposed system therefore offers low cost, portable and multi-wavelength imaging capabilities to image and illuminate inaccessible biological samples. This can have potential applications especially in simultaneous remote light delivery and imaging for various biomedical imaging and sensing applications.