TRAPPING FORCES IN A MULTIPLE-BEAM FIBEROPTIC TRAP

Transverse and axial trapping forces are calculated in the ray optics regime for a multiple-beam fiberoptic light-force trap for dielectric microspheres located both on and off axis relative to the beam axis. T rap efficiencies are evaluated as functions of the effective index of refraction of the microspheres, normalized sphere radius, and normaliz ed beam waist separation distance. Effects of the linear polarization of the electric field and of beam focusing through microlenses are con sidered. In the case of a counterpropagating two-beam fiber-optic trap , using microlenses at the distal ends of the fiber to focus the beams may somewhat increase the trapping volume and the axial stability if the fiber spacing is sufficiently large but will greatly reduce the st iffness of the transverse force. Trapping forces produced in a counter propagating two-beam fiber-optic trap are compared with those generate d in multiple-beam fiber-optic gradient-force traps. Multiple-beam fib er-optic traps use strong gradient forces to trap a particle; therefor e they stabilize the particles much more firmly than do counterpropaga ting two-beam traps. (C) 1997 Optical Society of America.