Focusing And Imaging With Light Field Modulation On Bessel Beams

Since the invention of the first laser in 1960,scientific research,technology development and industrial application supported by laser have developed rapidly,and optics has entered the laser era.The laser beam with simple Gaussian mode is difficult to meet the needs of scientific research and application.Scientists have invented various modulation techniques to control the multi-parameter of the light field,including amplitude,polarization,phase and time,generating a series of new spatial structure light field,and bringing a series of novel physical phenomena and effects.The Bessel beam is one of the classical and important non-diffracting beams,and its remarkable characteristics are long focusing depth and self-reconstruction,it can be completely reconstructed after encountering partial obstacles.Therefore,using Bessel beam can increase the depth of field of laser scanning microscopy,enhance the ability of light sheet microscopy,and even achieve multi-channel confocal microscope with large depth of field.Light field modulation based on Bessel beams is promising.This paper presented a composite technique to modulate a Bessel beam for subdiffraction limit focusing.As reported in literature that a Bessel beam propagates through a lens experienced the process of focusing,splitting,and self-recovery.The split region appears as a hollow light field(named as bottle beam)distribution with a central bright spot with extremely low intensity.In this paper,a composite light field modulation scheme based on Bessel beam was proposed,using sharp edge diffraction and the high-pass filtering effect of an annular aperture,the central focal spot of the bottle beam is improving with sharper focus and higher intensity.Both numerical simulations and experimental results show that the modulated Bessel beam retains propagation process of focusing,splitting,and finally self-recovery.The difference is that the intensity of the central bright spot in the split region increases significantly,while the sideband intensity is suppressed.Moreover,the central bright spot achieves sub-diffraction focusing at long distances.Especially when the propagation distance is z=760mm,the focal spot size is only 0.35λ/NA,breaking through the super-oscillation criterion(0.38λ/NA).This local focused light field is far from the sideband and is expected to be used in far-field super-resolution imaging and optical tweezers.The Bessel beam is used to improve the imaging depth of field and resolution of confocal microsystems,while the multi-channel Bessel beams can increase the scanning speed of the confocal microscopy.However,the nowadays multiple Bessel beam using in the confocal imaging system were generated through pinhole array,whose size,alignment and uniformity of light source will affect the imaging quality.We presented a scheme to replace light source(including the pinhole array)with micro-LED.By simulating the total point spread function of the confocal microscopy system with different sizes(10um,50 um and 100um)micro-LED and laser as the light source,it is found that the resolution decline is negligible when using a micro-LED below 10 um as the light source and a high-speed confocal microscopy can be achieved.