Holographic
interferometry
If we combine those two
optical schemes, it is possible to reach direction of sounding, given beforehand, and
convenience of supervision of a picture by not armed eye on a background of diffuser. This
optical scheme is given below. Here L1 is lens, L2 - lens with short focus.
 Such scheme of holographic
interferogram registration allows to project the necessary section of an object on the
plane of the diffuser. It improves binding of interferometric strips to the object and
makes the picture more convenient for further decoding.
It is possible to not
use the lens in this scheme, but the object should placed closely to diffuser.
Figures below show
inerferograms of similar objects received at various adjustments of interferometer: a) -
adjustment on an infinitely wide strip, b) - adjustment on strips of finite width.
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| a) |
b) |
There are many other
variants of the schemes for holographic interferometry of transparent objects.
The note
In the case of strips of finite width all schemes can sometimes contain additional
optical elements regulating adjustment of system of basic strips.
HI of the focused image
Apart from the
described schemes exists HI, constructed on the scheme of holograms of the focused image
(HFI). They also can be applied both with diffusers and without them. But using the
diffuser in this case is not necessary.
The inerferograms, that
were made in the scheme of HFI (see figure: K1, K2 - collimators, L - lens) are convenient
to restore in white light. The speckle-structure which depends only on properties of laser
beam and lens, becomes averaged. As the result, separate speckles are not visible and the
contrast of interferometric strips is slightly increased. Using diffuser allows to cover
already existing speckle-field the with additional field, generated by diffuser. The
situation is worsened, especially at the small sizes of heterogeneity.
It depends on
experiment and available optical elements - what beam (diffused or flat) should be used as
reference. HI by the scheme of HFI allows to investigate thin structure of interferometric
strips, including photo through microscope. For an example, figures below show
interferogram and its enlarged fragment.
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| a) |
b) |
Unique properties and
opportunities of holographic interferometry were already repeatedly mentioned. Holographic
interferograms from figures are clearly demonstrate it. All three interferograms, received
with the different optical schemes, have recorded perturbations, that initiated by the
electrical discharge inside of the solid isolator.
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| a) |
b) |
Classical
interferometry is absolutely powerless in such cases.
The note 1
All optical schemes are over-simplified here. At construction of the real schemes, it is
necessary to take into account a type of beamsplitter and sizes of a zone of registration.
As result, the arrangement specificity of the object of research also can require complete
change of an arrangement of optical elements. And the laser could be placed differently,
than on these schemes.
The note 2
For achievement of the maximal contrast of interference pattern, planes of polarization of
both laser beams should be are perpendicular to that plane, at which the beams of light
converge or the beams should have circular polarization.
Besides, the perpendicular to a the plane of a plate should be the same as bisector of an
angle between the object and reference beams.
The note 3
The modern lasers have large length of coherence. However, differences of a lengths
between object and reference beams should be reduced to a minimum.
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