Figure 16.13: DIC microscopy converts gradients in optical path length
into intensity diff erences at the image plane, allowing for visualization of
laser-induced damage that would be otherwise hard to detect
Condenser Lens Objective Wollaston Prism
Light
Source Wollaston
www.edmundoptics.co.uk/LO 57
The signal is either detected by a photodetector or a spectrometer. Photodetectors
integrate the signals of diff erent wavelengths over time, and
applying a Fourier transform algorithm to the captured interferograms
reveals the wavelength-dependent GDD and chromatic dispersion.1 Using
a spectrometer instead of a photodetector eliminates the need for a
Fourier transfer of the captured data.
The sensitivity of photodetector-based white light interferometers is dependent
on the step sizes of the stage used to translate the reference
optic, but this is not an issue with spectrometer-based systems.
Section 16.8: Differential
Interference Contrast Microscopy
Diff erential interference contrast (DIC) microscopy is used for highlysensitive
defect detection in transmissive materials, particularly for identifying
laser damage in optical coatings and surfaces (Figure 16.13). It is
diffi cult to observe these features using traditional brightfi eld microscopy
because the sample is transmissive, but DIC microscopy improves
contrast by converting gradients in the optical path length from variations
in refractive index, surface slope, or thickness into intensity diff erences
at the image plane. Slopes, valleys and surface discontinuities are
imaged with improved contrast to reveal the profi le of the surface. DIC
images give the appearance of a 3D relief corresponding to the variation
of optical path length of the sample. However, this appearance of 3D relief
should not be interpreted as the actual 3D topography of the sample.
DIC microscopy uses polarizers and a birefringent Wollaston or Nomarski
prism to separate a light source into two orthogonally polarized
rays (Figure 16.14). An objective lens focuses the two components onto
the sample surface displaced by a distance equal to the resolution limit
of the microscope. After being collimated by a condenser lens, the two
components are then recombined using another Wollaston prism. The
combined components then pass through a second polarizer known as
an analyzer, which is oriented perpendicular to the fi rst polarizer. The
interference from the diff erence in the two component’s optical path
length leads to visible brightness variations.
One limitation of DIC microscopy is increased cost compared to other
microscopy techniques. The Wollaston prisms used to separate and recombine
the diff erent polarization states are more expensive than the
components needed for microscopy techniques such as phase contrast
or Hoff man modulation contrast microscopy.9
Specimen
Image Plane
Polarizing
Filter
Prism
Polarizing Filter
Figure 16.14: Typical DIC microscopy setup where a Wollaston prism
splits the input beam into 2 separately polarized states
References
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resource/primer/techniques/dic/dicconfi guration/
/LO