Synthetic Aperture Lensless Digital Holographic Microscopy (SALDHM) for Superresolved Biological Imaging - Biomedical Optical Phase Microscopy and Nanoscopy - page 183

9.4 Conclusions

In this chapter, we have described and experimentally validated two different approaches
capable of improving resolution in biological LDHM by SA generation (SALDHM). Both
approaches are based on the combination of angular- and time-multiplexing incoming from
tilted beam illumination (angular-) sequentially performed (time-) in order to multiplex
different object's spatial regions. Since holography rules the recording of the images, the
proposed multiplexing allows the recovery of different elementary apertures containing
different spatial-frequency content of the sample's spectrum. Those elementary apertures
are coherently added into the expanded SA and a superresolved image is obtained as FT of
the information contained in the SA. As a consequence of the temporal multiplexing, the
main drawback of both approaches is limitation to samples that must be static or having a
slow variation in time (a few seconds).

In the first presented approach, weak diffractive samples are considered. The experimental
setup becomes extremely simple since we are inside the Gabor's regime and verifies the
definition of an SNA above 0.6 in every direction. The full procedure can be realized in a
few seconds by automating the displacement of the source to the off-axis positions and
synchronizing the capture of the holograms. We have calibrated and quantified the method
using standard 1951 USAF resolution test target achieving a submicron resolution limit.
Later on, we have reported on the application of the proposed approach to imaging of a
sperm cell to imaging of a sperm cell demonstrating demonstrating its feasibility for the
analysis of biological samples. The proposed approach can be applied for submicron
resolution lensless imaging purposes when considering essentially transparent (low
confluence) samples.

On the contrary, any type of samples can be analyzed using the second presented approach
since the reference beam is externally introduced assuring the holographic recording at the
CCD plane. The experimental setup can be easily adapted to any system's requirements,
which means a versatile, simple, and customized method to improve the resolution in
digital lensless Fourier holographic microscopy imaging. We have presented experimental
results using a single or two illumination rings, allowing 0.45, 1.41 m, and 5 as best values

for SNA, superresolution limit, and resolution gain factor, respectively.

μ

References

[1] D. Gabor, A new microscopic principle, Nature 161 (1948) 777 778.

[2] D. Gabor, Microscopy by reconstructed wavefronts, Proc. R. Soc. Lond. A197 (1949) 454 487.
[3] D. Gabor, Microscopy by reconstructed wavefronts II, Proc. Phys. Soc. Lond. B64 (1951) 449 469.
[4] G.L. Rogers, Experiments in diffraction microscopy, Proc. R. Soc. Edinb. 63A (1952) 193 221.
[5] A.L. Schawlow, C.H. Townes, Infrared and optical masers, Phys. Rev. 112 (1958) 1940 1949.
[6] E.N. Leith, J. Upatnieks, Reconstructed wavefronts and communication theory, J. Opt. Soc. Am. 52 (1962)
1123 1130.