In optical tweezers, it can be comprehended that the larger the inclination angle between the condensed laser beam and
the optical axis contributes more to axial trapping force, while the central part of laser beam with smaller inclination
angle contributes more transverse trapping force. Therefore donut-shaped beam is used to improve the problem of lessaxial
trapping for common optical tweezers. Some research reports have shown that the efficiency of a trapping force
can be enhanced by using a donut-shaped beam. In this paper we present the dependence of the axial and the transverse
components of a trapping force on the configuration of a focused donut-shaped beam. The simulation result will provide
a simple and easy guide for optical tweezers users to adjust the configuration of a focused donut-shaped beam for optimal trapping performance.
Sample tracking with a high spatial sensitivity is highly desired in force measurement with optical tweezers. However, the trick that sample tracking via forward scattering pattern detection would provide a higher sensitivity than that via regular image detection has never been investigated. In this paper, we systematically study the influences of the position and the numerical aperture of the condenser on sample tracking via forward scattering pattern detection. In our experiment, a 60X condenser is used to form the forward scattering pattern of a sample bead upon a CCD camera. As the bead is transversely shifted at a step size of 30nm by a PZT XYZ stage, we measure the magnitude of the corresponding shift of the forward scattering pattern when the 60X condenser of different angular apertures is placed at various locations along the optical axis. Our result shows that the most sensitive forward scattering pattern occurs when the condenser collimates the forward scattering light from the sample bead. We also find that the larger the numerical aperture is, the higher the sensitivity of forward scattering pattern detection will be.