Phase Unwrapping Problems in Digital Holographic Microscopy of Biological Cells - Biomedical Optical Phase Microscopy and Nanoscopy - page 126

objectives (OBJ11/OBJ12), which together with the apertures A and collimating lenses L11/
L12, produce plane waves. Their intensity is further adjusted by the polarizing filters P1
and P2. Beam splitters BS1 and BS2 divide the incoming light into the reference and the
object beams. Two separate reference arms are used to fine-tune the location of the
first-order diffraction peaks and separate them from each other in the Fourier domain.
Lenses L21 and L22 and 20 3 microscope objective OBJ1 again collimate the beams in the
object arm. The wave fronts in both reference arms remain spherical and the resulting
curvature mismatch is removed numerically. An interference filter is placed into the
reference arm of the diode-pumped solid-state ( λ 5 532 nm) laser to allow only this

wavelength to pass and block the inverse reflection of the other laser. The interference
pattern between the reflected reference and the object waves is recorded by the CCD
(Charge-coupled device) camera. A relative angle can be introduced between the object and
the reference beams for each wavelength by slightly tilting the reference arm's mirrors. By
introducing different tilts in two orthogonal directions for two reference beams, we can
separate each spectral component in Fourier space ( Figure 7.2B ), which allows us to
capture both the wavelengths simultaneously.

(A)

532 nm

633 nm

(B)

Figure 7.2
Two wavelength hologram of a USAF resolution target. (A) Digital hologram and (B) its Fourier
spectrum with the red and the green wavelengths first-order components shown.