Probing the focused laser spot of the apodized light source

This paper deals with both theoretical and experimental studies of near-field imaging of the focused laser spot. In theoretical simulation, we calculate the intensity distribution in the focal region of the apodized light source. Size reduction of the focusing spot of an annular light beam is found to be related with various epsilon, which is the ratio of the obstruction radius to the radius of lens aperture. The experimental setup is based on a near-field scanning optical microscope (NSOM) system. An opaque disk is used to make an annular beam. A high numerical aperture objective lens (NA = 0.8) is used to focus incident lights of different annular light beams, and a tapered near-field optical fiber probe of the NSOM system is used to collect the intensity of the focal field. Results demonstrate that a better transverse resolution can be obtained from the reduction of the focusing spot. The intensity profile of an annular beam may have a significant influence on the size reduction of the focusing spot.