A high-resolution displacement detection can be achieved by analyzing the
scattered light of the trapping beams from the particle in optical tweezers. In
some applications where trapping and displacement detection need to be
separated, a detection beam can be introduced for independent displacement
detection. However, the detection beam focus possibly deviates from the centre
of the particle, which will affect the performance of the displacement
detection. In this paper, we detect the radial displacement of the particle by
utilizing the forward scattered light of the detection beam from the particle.
The effects of the lateral and axial offsets between the detection beam focus
and the particle centre on the displacement detection are analyzed by the
simulation and experiment. The results show that the lateral offsets will
decrease the detection sensitivity and linear range and aggravate the crosstalk
between the x-direction signal and y-direction signal of QPD. The axial offsets
also affect the detection sensitivity, an optimal axial offset can improve the
sensitivity of the displacement detection substantially. In addition, the
influence of system parameters, such as particle radius a, numerical aperture
of the condenser NAc and numerical aperture of the objective NAo on the optimal
axial offset are discussed. A combination of conventional optical tweezers
instrument and a detection beam provides a more flexible working point,
allowing for the active modulation of the sensitivity and linear range of the
displacement detection. This work would be of great interest for improving the
accuracy of the displacement and force detection performed by the optical
tweezers.Comment: 10 pages,11 figure