Design and performance study of long depth of field based on spatial light modulators

Conventional lenses are constrained by the trade-off between depth of focus (DOF) and focal spot size, making it challenging to simultaneously improve resolution and DOF. Non-diffracting beams such as Bessel beams alleviate this trade-off to a certain extent but suffer from non-uniform intensity within the DOF. To address these issues, a method for generating a long-depth-of-focus (LDOF) beam based on a spatial light modulator (SLM) is proposed. The core idea involves superimposing a focus-shifting phase term onto the focusing phase of a conventional lens to construct a multi-focus distribution along the optical axis. Dynamic adjustment of the spacing between adjacent foci counteracts intensity fluctuations caused by energy spreading among the foci, thus avoiding non-uniform intensity within the DOF. Additionally, it is necessary to suppress focal spot broadening induced by multi-focus superposition to prevent resolution degradation. Simulation results demonstrate that the DOF of the designed optical needle is 12.5 times that of a conventional lens with the same numerical aperture (NA), and the focal spot size approaches the diffraction limit. Experimental validation using an SLM confirms the validity of the simulations. This LDOF beam design shows strong potential in fields such as particle manipulation, high-density optical data storage, laser processing, and bioimaging.