Coherent Microscopy and Optical Coherence Tomography for Biomedical Applications - Bioanalysis Advanced Materials Methods and Devices - page 111

Fig. 8.4 Backscatter
Michelson DHM
configuration

CCD
Detector

Laser

Reference
Mirror

Microscope
Objective

Object

by introducing a similar lens into the reference arm of the interferometer. The most
important aspect of this configuration is the flexibility to add a tilt (or offset) to the
reference wave which introduces a carrier frequency to the interference pattern that
simplifies numerical reconstruction considerably.
Off-axis DHM can also be implemented in a reflection geometry using a
Michelson interferometer configuration as shown in Fig. 8.4 . In this arrangement
the laser is focused in the back focal plane of the objective such that the object is
illuminated by a plane wave. The light scattered from the object passes back
through the objective and forms an image at the CCD. As before, the scattered
light is mixed with a coherent reference wave of appropriate divergence and tilt
such that plane carrier fringes are observed in the intensity recorded by the CCD.
In general terms, DHM and all the other methods of coherent 3D imaging,
measure the scattered field at a boundary surface that is far beyond the reach of
the evanescent waves that constitute the near-field that surrounds the object. As a
consequence, only plane wave components that have the capability to propagate to
the boundary (i.e., the far-field) can be measured in this way. Further to this, it is
only the plane wave components that propagate within the NA of the objective,
that are collected as shown in Fig. 8.5 .
Mathematically, the process of reconstructing the scattered field from this
limited information can be written as a linear filtering operation. If in the spatial
frequency domain the scattered and reconstructed fields are described by the
functions,

E s E m

ð k Þ and ð k Þ , the DHM reconstruction process can be written as

the product [ 21 ],

E m

Þ¼E s G ~ NA ð ð k ð k Þ k Þ (8.6)