Phase Unwrapping Problems in Digital Holographic Microscopy of Biological Cells - Biomedical Optical Phase Microscopy and Nanoscopy - page 125

technique generates an extended range “beat” wavelength phase image. The beat
wavelength phase image is then used to subsequently adjust one of the original single
wavelength phase images in order to reduce the measurement uncertainty.

Finally, by comparing the two phase images directly, the dual-wavelength linear regression
method accurately unwraps phase images via pixel-by-pixel comparison. The latter
unwrapping method is computationally fast, can process complex topologies, and also
significantly relaxes the limitations on the total optical height, typically associated with a
“traditional” dual-wavelength phase unwrapping.

7.2 Experimental Techniques

7.2.1 Dual-Wavelength Digital Holography Apparatus

Figure 7.1 shows the typical experimental apparatus for simultaneous dual-wavelength
digital holographic microscopy [2,3] . It is based on two overlapping Michelson
interferometers, which enables the adjustment of location of the first-order components
produced by each wavelength in the Fourier space (see later). Here, He Ne ( λ 5 633 nm)

1

and diode-pumped solid-state (

5 532 nm) lasers are used as coherent light sources. Both
beams are attenuated by neutral density filters and then pass through the microscope

λ

2

ND

L11 P1 A

Mirror OBJ11

He-Ne laser

L12 P2 A

ND

Mirror

OBJ12

DPSS laser

L21

L22

Camera

OBJ1

Object

BS2

BS1

Filter

Mirror

Mirror

Figure 7.1
Dual-wavelength digital holography apparatus. The beams are collimated between L11 and L21
and between L12 and L22 and again are collimated after 20 OBJ1 microscope objective.

3