Generation of longitudinally polarized optical chain by 4π focusing system

A longitudinally polarized optical chain is generated in a proposed 4π focusing system. The radially polarized Bessel-Gaussian incident beam is modulated by a complex amplitude filter. Their focusing performance is investigated using the Debye integral. Longitudinally polarized focusing spots with uniform intensity, high axial resolution and small spot volume are achieved in the proposed scheme. The number of the focusing spots in the optical chain is controllable by adjusting the complex amplitude filter. As an example, a longitudinally polarized optical chain with seven uniform intensity focusing spots is obtained. The volume of each focusing spot is reduced by about seven times and the axial resolution is only one-fourth of that in a single lens system with the same parameters. The influence of optical axes mismatching and foci shifting on the focusing performance is also investigated. The focusing system with the complex amplitude filter permits a large foci shifting because of the elongated depth of focus. It is expected that the research results can be used in the field of confocal microscopy, optical trapping and manipulating.