Measurement of Focused Laser Beam by a Finite Aperture Lens

The field distributions of focused beam through a finite aperture lens varies depending upon its incidence conditions and aperture dimension. When the aperture radius of the lens is not large enough compared with the spot size of the incident laser beam, the most focused point cannot be determined uniquely. Two of the most simple definitions of this focused point are, 1) the point where the axial intensity is maximum and 2) the point where the spread of the field is smallest. These points are easily obtained theoretically or numerically. The focused points are measured and compared wih them obtained from above definitions. The results show that the point defined by the point of maximum axial intenisty coincide with theoretical calculations, while the point of the minimum field spread has some deviation from theoretical one