Improved detection of particle displacement in optical traps using back focal plane interferometry and image processing

Optical tweezers play a crucial role in particle manipulation and force measurement. Therefore, it is essential to calibrate the force measurement parameters, such as the optical trap stiffness and the conversion factor of the sensor deflection signal to the actual displacement of the trapped particle. This calibration is necessary to achieve accurate and efficient parameter calibration. In this study, we employed a suitable image processing method, based on a low-frequency CMOS camera, to calibrate the Displacement Conversion Factor (DCF) of the optical tweezers system. The high accuracy of this image processing method makes it ideal for analyzing the actual displacement of optically trapped microspheres. Interestingly, we discovered that the normalized deflection signal captured by the quadrant photodiode (QPD) through back focal plane interferometry is primarily correlated with the axial height of the trapped microspheres, and is relatively unaffected by laser power variations. Building on this finding, we also examined the linear detection range of the trapped microspheres at different heights. These results address certain limitations associated with the use of optical tweezer systems and provide valuable guidance for utilizing this tool effectively.