Implementation of free-space Fourier Ptychography with near maximum system numerical aperture

Abstract

Over the past decade, the research field of Fourier Ptychographic Microscopy (FPM) has seen numerous innovative developments that significantly expands its utility. Here, we report a high numerical aperture (NA) FPM implementation that incorporates some of these innovations to achieve a synthetic NA of 1.9 – close to the maximum possible synthetic NA of 2 for a free space FPM system. At this high synthetic NA, we experimentally found that it is vital to homogenize the illumination field in order to achieve the best resolution. Our FPM implementation has a full pitch resolution of 266 nm for 465 nm light, and depth of field of 3.6 µm. In comparison, a standard transmission microscope (incoherent) with close to maximum possible NA of 0.95 has a full pitch resolution of 318 nm for 465 nm light, and depth of field of 0.65 µm. While it is generally assumed that a free-space coherent imaging system and a free-space incoherent imaging system operating at their respective maximum NA should give comparable resolution, we experimentally find that an FPM system significantly outperforms its incoherent standard microscopy counterpart in resolution by a factor of 20%. Coupled with FPM’s substantially longer effective depth of field (5.5 times longer), our work indicates that, in the near-maximum NA operation regime, the FPM has significant resolution and depth of field advantages over incoherent standard microscopy.

Spatially‐Coded Fourier Ptychography: Flexible and Detachable Coded Thin Films for Quantitative Phase Imaging with Uniform Phase Transfer Characteristics

“…This simple thought experiment suggests that FP cannot recover the phase information for any specific spatial frequency, a rather unexpected finding given the technique's previous demonstrations on phase imaging. [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] We also note that the phase imaging challenge discussed above is not unique to FP but also affects other common microscopy techniques, such as transport-of-intensity equation, [47] supportconstraint phase retrieval, [48,49] digital in-line holography, [50][51][52] multi-height and multi-wavelength lensless imaging, [53,54] blind ptychography, [55][56][57] and differential phase contrast imaging. [58,59] In FP and these microscopy modalities, despite claims of quantitative phase imaging capabilities, the captured intensity images only contain constant values for any given linear phase ramp, making it difficult to recover the correct phase ramp from intensity measurements.…”

“…This simple thought experiment suggests that FP cannot recover the phase information for any specific spatial frequency, a rather unexpected finding given the technique's previous demonstrations on phase imaging. [ 31–46 ]…”