A Tunable Extended Depth of Focus Microscope Through Beam Shape Modulation into an Axicon Lens

Abstract

I propose and demonstrate the groundwork for developing a fluorescence microscopy imaging system by which a Digital Micromirror Device (DMD) and an axicon can be used in combination for remote refocusing of the focal plane while providing high resolution depth profiles, or Axial scans (A-scans), comparable to Optical Coherence Tomography (OCT). Although OCT imaging systems can capture large depth A-scans to buildup high-resolution volumetric images, the information detected by these systems is entirely dependent on the coherent back-scattered light from a sample’s structure. Hence, OCT systems are unable to determine a structure’s molecular information; however, such information can be acquired from fluorescence systems.

Furthermore, unlike conventional spherical lenses, which have a short depth of focus, the length in which the focal spot begins to diverge to a size greater than √2 its spot size, conical lenses like the axicon create a long and narrow depth of focus surrounded by annular rings of lower intensity. Essentially, this focal line, or Extended Depth of Focus (EDF), achieves the same axial resolution as a spherical lens, but has a longer axial length for acquiring A-scans. As a result, a DMD is used to manipulate the size and shape of an illuminated beam pattern, a thin annulus, into the axicon in order to acquire the sample’s entire depth profile point-by-point within the EDF. This proposed technique is supported by the point spread function (PSF) measurements from multiple 10 um diameter-sized polystyrene microspheres (fluorescent 540/560 nm), which agree with the mathematical model used in determining the system’s axial resolution.