Note: Descriptions are shown in the official language in which they were submitted.
lO9U~30
This invention relates to recording media, and in particular to
recording media suitable for holography at wavelengths comparable to the
10.6 ~m of a CO2 laser.
Numerous applications of holography using wavelengths in the visible
spectrum have been published in recent years. In some cases, the potential
advantages of using longer wavelengths, such as the 10.6-~m beam of a CO2
laser, justify the search for a good recording medium at these wavelengths.
Among applications of holography at a wavelength of 10.6 pm, one can expect
a considerable increase of sensitivity in plasma diagnostic by interferometry,
as compared to the techniques used presently. On the other hand, the use of
such a long wavelength in the interferometric study of solids and their
surfaces will allow the observation of much larger displacements or deforma-
tions in comparison to similar experiments in the visible spectrum.
This search for recording media in the IR region at 10.6 pm has
given rise to many experiments with thermochromic materials, liquid crystals,
bismuth films, wax gelatins, etc.
Further~ certain transparent thin plastic films, in partlcular,
those whose trade ~ark~ are Saran Wrap manufactured by the Dow Chemical
Company and Stretch'n Seal manufactured by the Colgate Palmolive Company,
have been investigated by D.T. Rampton and R.W. Grow, Appl. Opt. 15, 1034
(1976), for use as IR polarizers by vlrtue of their good light transmlssion
at wavelengths of the order of 10.6 um.
According to one aspect of the invention, an improved method for
recording a hologram on a recording medium at a wave length in the IR region
at about 10.6 ~m is contemplated, the improvement comprising employing as
recording medium a transparent plastic medium having high light absorption
at said wavelengthO
According to another aspect of the invention a medium for recording
a hologram at a wavelength in the IR region at about 10.6 ~m, and re-con-
structing the recorded image by reflection, in the IR region, is contemplated,
the medium comprising a transparent plastic medium having hi8h light absorption
at said wavelength and a thin coating of silver deposited on at least one ma~or
surface of said transparent plastic medium.
1090~i30
In the drawings which serve to illustrate embodiments of the
inventlon,
Figure 1 is a schematic illustration of the experimental arrange-
ment for recording a hologram on a recording medium according to the inven-
tion, and
Figure 2 is a graph illustrating the diffraction efficiency
at 632.8-nm of various recording media according to the invention. The
holograms were produced by a 10.6 ~m laser beam on (a) a 603 mm thick
acrylic sheet and ~b) a 8aran Wrap film, and
Figure 3 is a graph illustrating diffraction efficiency with a
632.8-nm reconstructing beam reflected on the surface of an exposed Saran
Wrap film. Circles represent reconstruction from the side opposite to that
of the recording, while squares represent reconstruction from the recording
side.
Referring to the drawings, in Figure 1 the experimental arrangement
is seen to include an IR light source of a wave length of about 10.6 ~ ,
conveniently a stab~lized single mode continuous wave C02 laser, operating
at a power output of 1 watt which produces a polarized laser beam of a
wavelength of about 10.6 ~m and yielding incident intensities up to about
3.5 w/cm20
The incident laser beam was projected on the novel recording medium
1 through a conventional shutter 2 and then through a 50/50 germanium beam
splitter 3. Any incident radiation reflected by the beam splitter 1 is
directed to the recording medium 1 by a mirror 40
The recorded hologram thus comprises straight parallel fringes
typical of the interference of two plane waves.
Simultaneous re-construction of the hologram in the visible spectrum
may be achieved with a light source of a wavelength of about 632.8-nm,
typically, a Helium-Neon laser. The reconstruction of 63208-nm is obtained
by transmission through the hologram, i.eO in the direction opposite to the
incident laser beams, the resulting waves being totally reflected at the
germanium beam splitter toward the photographic plates 5, where visual
- 2 -
lO~t)~30
observation on a white sCreen i8 also possible.
Holographic reconstruction in the IR region i.e. at about 10.6 pm
was also made possible by vacuum-depositing a 50-nm layer of silver on at
least the front major surface of the recordlng medium after recording the
hologramO
In the latter case, the hologram which is pre-recorded on the
recording medium i8 then illuminated with a laser beam of wavelength of
10.6 pm to obtain an image by reflection in the infrared.
In order to measure the intensity of the holographic images
reconstructed in the infrared by reflection using the latter method, the
reflected beam is chopped and its intensity measured with a Molectron P~3
detector (not shown) located at the position of the re-constructed image.
"Chopping" is a technical term meaning that the laser beam is shut periodi-
cally with a shutter. The measured reflectivity of the silver coating is
lO~h at 10.6 pm and more than 9~Z at 63208 nmO
Various transparent plastic materials were investigated including
those known by the trade name Saran Wrap which is a polyvinylidene chloride
resin film and Stretch'n Seal, as well as a thick acrylic resin plate of a
thickness of about 2-7 mmO e.gO a 603 mm (1/4 inch) thick Plexiglas plateO
20 Plexiglas is a trademark Bor thermoplastic poly(methyl methacrylate)-type
polymers. These materials are characterized especially by their low price,
by their ease of use, by the absence of need for development (as with emul-
sions currently used in the visible spectrum), and consequently by their
potential advantages for application to real-time holography.
It was found that absorption at 1006 ~m is nearly 1007o for the
acrylic material~ Saran Wrap and Stretch'n Seal have a measured transmission
of respectively, 7~h and 6~/oo After a few experiments, Stretch'n Seal films
showed a surface optically improper for the purpose at hand, and this material
was thus eliminated from our study.
- 3 -
lU90~;3U
Table I. Effect of Exposure Time on Clear Acrylic
Incident Exposure
IR time
lnten~ity (sec)
(W/cm ) Remarks
3.5 2.2 Holographic recording threshold
3.5 2.2-402 Increase in diffraction efficiency; the
recording is permanent after the shutter
is closed
305 4.2 Deterioration of the recording begins
3.5 . 6.5 Holographic reconstruction disappears: the
deterioration is complete
Table II. Effect of Exposure Time on Saran Wrap
Incident IR Exposure
intensity time
(W/cm2) (sec) Remarks
1.75 0.9 Holographic recording threshold
1.75 0.9-5 Diffraction efficiency increases up to a
constant level
1.75 5-300 Diffraction efficiency remains constant
The observations made are presented in three group~ of equal
importance to the end-usersO These groups are the recording energy density,
the linearity of recording, and the diffraction effeciency at reconstruction.
Energy Density at Recording i~
,
The energy density is defined as the product of the exposure
time by the power incident on themmedium per unit surface. Tables I and II
summarize the observations on the time behavion of t~e medium when exposed
to a continuous IR laser beam interference in terms of the behavior of the
real-time holographic reconstruction at 632.8 nm.
Table 1 indicates that for acrylic the initial exposure of 202 sec
can be interpreted as the time required to provide the necessary energy to
soften the surface material enough for a permanent deformation to occur. With
more energy (exposure time greater than 4.2 sec), melting and/or heat conduc-
tion probably is the agent that destroys the recordingO It is interesting
to note that an exposure t~me between 2.2 sec and 4.2 sec, that is, an
energy density between 8 ~/cm2 and 14 J/cm2, will allow a permanent recording
105~0~;30
of the hologram on the scrylic surface.
The case of thin plastic films, as reported in Table 1l, is some-
what different: even with lower absorption at 10~6 ~m, recording is made
with much lower incident energy. The threshold energy for holographic recor-
ding on these films is about four times lower than on acrylic. Moreover,
no deterioration of the recording was observed even for a very long exposure
time. This behavior suggests that after a period of 5 sec, the film temper-
ature stabilizes as a result of an equilibrium between the energy input and
the heat exchange by conduction through the plastic and to the surrounding
atmosphere.
Linearity of the Recording
We have chosen to characterize a good linearity of the recording by
a high ratio of the intensity of the reconstructed first order image to that
of the higher orders.
A typical reconstruction at 632~8 nm showed that the recording on
acrylic cannot be characterized as linear since the intensity of the second
order reconstruction is appreciable. Experimental measurements of the
intensity in both orders give l~/o for the first order and 27/o for the second
order. However, it was found that the recording on saran films has a better
20 linearity since there are no apparent high order images: less than Q.0570
of the intensity is diffracted in thessedond order as compared to 4% in the
first order.
Diffraction Efficiency
The diffraction efficiency at reconstruction is defined as the
ratio of the intensity of reconstructed first order image to that of the
incident reconstructing beam. Measurements of the diffraction efficiency
transmission with a wavelength of 632~8 nm are shown in Figo 2~ We observe
a maximum efficiency of ~2~h for acrylic LFig. 2(a~ and of~ 4.5% for plastic
films rFig. 2(b~ : these values are compared tentatively to the value
30 of 33~/o suggested in the literature for a sinusoidal phase grating. The
maxim~m diffraction efficiency measured when using a wavelength of 10.6 ~rn
reflected from the silver-coated suri!ace of the hologram is ~0.5% for acrylic
and lower than our detection means (~0.17~ for the saran filmsO The important
lO90~j30
discrepancy between the two wavelength measurements is attributed to the
lower phase shift induced by the same hologram on longer wavelengths, this
phase shift depending on the ratio of the depth of the recording to the
wavelength.
After having recorded a hologram on a sample of Saran Wrap film
which was subsequently coated on both sides with silicon, it is observed
that holographic reconstruction at 63208 nm occurs on each surface, as
shown by the values of diffraction efficiencies in Figure 30 This observation
indicates that the deformation of the plastic film does not seem to occur
on the surface only but through its whole thicknessO
The low diffraction efficiencies observed at 1006 pm, both for
silver coated acrylic and for silver coated saran films, are attributed to
thermal conduction and/or melting which prevent obtaining a deep enough
surface modulation pattern. It is thus proposed that a pulsed C02 laser
instead of the cw laser be used to eliminate the conduction effects. The
number of pulses (at low repetition rates) may then become the determining `~`
factor in obtaining the deformation magnitude that corresponds to the maximum
diffraction efficiency.
It will be appreciated by those skilled in the art that the inven-
tion has been described in terms of specific embodiments thereof and that -;
modifications and variations will be apparent, without departing from the
spirit or central characteristics of the invention, and that such embodiments
are to be considered as illustrative and by no means restrictiveO
