Power Spectrum Analysis for Optical Tweezers, II: Laser Wavelength Dependence of Parasitic Filtering, and how to Achieve High Band-Width

Abstract

In a typical optical tweezers detection system, the position of a trapped object is determined from laser light impinging on a quadrant photodiode. When the laser is infrared and the photodiode is of silicon, they can act together as an unintended low-pass filter. This parasitic effect is due to the high transparency of silicon to near-infrared light. A simple model that accounts for this phenomenon (Berg-Sorensen et al., J. Appl. Phys., 93, 3167-3176 (2003)) is here solved for frequencies up to 100 kHz, and for laser wavelengths between 750 and 1064 nm. The solution is applied to experimental data in the same range, and is demonstrated to give this detection system of optical tweezers a bandwidth, accuracy, and precision that is limited only by the data acquisition board's band-width and bandpass ripples, here 96.7 kHz, resp. 0.005 dB.