Back-focal-plane displacement detection using side-scattered light in dual-beam fiber-optic traps.

In optical traps the position of a trapped bead is usually determined by measuring the intensity distribution of the forward-scattered light and the back-scattered light. In this paper we demonstrate that this position can be determined using the side-scattered light. A quadrant photodiode is used to monitor the position of an optically trapped object in a dual-beam fiber-optic trap by measurement of intensity shifts in the back focal plane of the objective that is perpendicular to the propagating beam. An approximated model based on ray optics is presented with numerical results that describe the use of the side-scattered light for position detection. The influences of system parameters, including fiber separations, the numerical apertures (NA), and the radii of microspheres, are discussed in details. We find out that the displacement sensitivity of the detector is null for some critical radii and numerical apertures. In addition, the noises in laser powers are analyzed, and one power difference regime is proposed to weaken the influences.