Influences of axial position manipulation and misalignments of optical elements on radial trap position manipulation

Optical tweezers in which a tightly focused laser beam is used to trap micron-sized or nanometer-sized particles have become indispensable tools for measuring the forces and displacements associated with molecular biomechanical events in a noninvasive manner. A complete beam manipulation system is composed of a beam expander input lens, beam expander output lens, focusing lens, piezoelectric translator mirror to control the trap position, with the overfilling degree of the objective entrance aperture retained. The accurate manipulations of trap position in three dimensions are the bases for the realization of the position clamp and force clamp. The optical path of optical tweezers based on infinity corrected conjugate microscope is calculated using matrix optics. The influences on radial trap position manipulation caused by axial position adjustment of focusing lens and objective, and by the installation location error of focusing lens and piezoelectric translator mirror are analyzed. The result shows that axial position adjustment of objective introduces a nominal error in radial trap position manipulation. The misalignments of focusing lens and piezoelectric translator mirror have a greater influence on optical tweezers performances. The theory points out the accurate dynamic axial position adjustment range, which is useful to optical tweezers design and experiments.