Identification of Near-Field and Far-Field Using Confocal Microscopy Setup for Optical Sensing Applications

Abstract

The presence of near-field indicates the existence of evanescent waves, which is one of the important requirements for the development of an optical sensor. This study was carried out to identify the presence of near-field and far-field as monochromatic light propagated through various structures and sizes of apertures. The confocal microscopy setup consisted of He-Ne laser with 633nm of excitation wavelength, apertures (slits and numerous sizes of pinholes), objective lens, projected screen and silicon photodetector. The near-field and far-field were determined by calculating the Fresnel number, FN as assorted size and design of apertures were added in the setup. The presence of far-field was identified with the usage of slits (single slits and double slits) represented by the value of FN, which was less than 1(FN<<1). As the aperture was replaced with pinholes with diameters within 30μm and 1.2mm, the near-field was resulted (FN>>1). Value of FN became greater as distance x between lens and pinholes increased. We also discovered a significant finding where the field intensities I were decreased with increment of distance x for the near-field, and vice versa (far-field). By using an extrapolating technique, it was found that the near-field can be created by using pinhole with a diameter of a<1.46mm. In conclusion, the discovery of this work proves an excellence role of pinholes in confocal microscopy setup in creating the evanescent waves for optical sensing applications.