Laser intensity mapping across a spatial light modulator to generate uniform and aberration-free focal spots

Abstract

We present a novel and direct technique to map the incident laser intensity across the active area of a spatial light modulator (SLM) by using diffraction based setup in a holographic optical tweezers (HOTs) system to generate structured light. The incident laser beam profile on the SLM is used as input to the optical trap generating algorithms such as weighted Gerchberg Saxton algorithm, generalized adaptive additive algorithm etc. The input laser beam profile is generally assumed to be located at the center of SLM in these algorithms. However, it is not straight-forward to ensure that the center of the Gaussian laser beam coincides with the center of SLM. The importance of mapping the laser intensity across the SLM is demonstrated by a study of the effect of mismatch between the input laser beam center and the actual beam center on the trap properties such as uniformity of intensity, and sharpness of the focal spots. We also propose a method of centering the laser beam profile on the SLM which enables the use of known beam profile. This method allows the use of HOTs with any spatial transverse distribution of a laser beam, such as elliptical profile of diode lasers, Laguerre Gaussian beams or with any distorted beam shape deviating from Gaussian profile.