Comparison of backward-scattered detection and forward-scattered detection for measuring optical force in optical tweezers

Optical tweezers have become an important tool to measure forces in biology. The trapped particle displacements acquired from the position detection system are applied to calibrate trapping stiffness using power spectrum method. The near infrared light is typically used as a laser source to reduce the damage to a cell or cellular organelles and the biological objects can be held and moved by exerting piconewton forces. In force measurement, optical force strength is calculated by multiplying trapping stiffness and trapped bead displacement. Optical tweezers perform a wider range of experiments through the integration of a quadrant photodiode for position detection. Both forward-scattered detection and backward-scattered detection are the typical position detection. This study discussed both backward-scattered detection and forward-scattered detection that add a probing beam and their linear detection ranges that describe the precise position of the trapped bead. This work also discussed their linear detection ranges related to the distance between the two laser system focuses, confirming the optimum positions of the two focuses. The result indicated that the linear detection range of backward-scattered detection is longer than the forward-scattered detection. Hence, backwardscattered detection measures the longer displacement of the trapped bead in optical force measurement.