Invited Paper Thick-layered light-sensitive dichromated gelatin for 3D hologram recording Yuri N. Denisyuk, Nina M. Ganzherli and Irma A. Maurer loffe Physico-Technical Institute of the Academy of Sciences of Russia St.Petersburg 194021, Russia ABSTRACT A new method of 3-D image recording, so called reference-free selectogram is considered. According to this method, the object wave is split into two parts with the help of a diffraction grating. The pattern originated from the interference of these components must be recorded in a thick-layered light-sensitive material that is positioned just behind the grating. Gel-like thick-layered dichromated gelatin has been suggested for the selectogram recording. In this case, the layer represents a gel of dichromated gelatin that is sealed between two glasses. The thickness of layers varied from 1 to 3mm. Holograms are recorded in such a material immediately during the process of exposure. The experimental data on the dependence of the diffraction efficiency on the exposure and the data on the dependence of the diffraction efficiency on the spatial frequency of the recorded grating are presented. The angular selectivity of the holograms recorded in the layers was about 10'. The experiments have shown that the lifetime of the hologram recorded in gel-like gelatin is measured by several hours. The experiments on the recording of the reference-free selectogram were carried out. To compensate the low value of the sensitivity of the material, the selectograms were recorded when concentrating the light on a small area of the layer during the exposure. Keywords: 3D hologram, selectogram, thick-layered materials, dichromated gelatin 1 INTRODUCTION 3-D hologram, i.e., a hologram registered in a volume light-sensitive material exhibits much more useful properties in comparison to a 2-D one.1 Among them it should be mentioned a high spectral and angular selectivity, absence of conjugate image, possibility of multiple recording holograms in one and the same region of a holographic material, associative retrieval of information, possible reconstruction of holograms by white light, etc.1'2 All these properties are intensively exploited at present when developing such modern fields of technology as 3-D imaging, optical memory, optical interconnections and so on. Unfortunately, the application of really deep holograms whose thickness is measured in millimeters is associated with great difficulties caused by the difficulties of the technology of producing thick-layered light-sensitive media. Below we consider a new type of 3-D image recording, a so-called reference-free selectogram that is essentially based on using thick-layered materials and some experiments on the development of such a kind of material on the base of dichromated gelatin. 234 /SP!E Vol. 2849 O-8194-2237-1/96/$6.OO
2 REFERENCE-FREE SELECTOGRAM The general scheme of the recording and reconstruction of the reference-free selectogram is shown in Fig. 1. Rays lo of the object 0 are filtered by a horizontal slit S and then fall onto diffraction grating G. The diffraction grating splits off a part of the rays of the object wave io and thus forms its twin component 1R,the virtual image OR corresponding to this component. The pattern resulted from the interference of the components lo and 1Ris recorded in a thick-layered light-sensitive material V. The reconstruction of a 3-D image is performed by means of a light scattered by a homogeneously illuminated diffusor DR that is placed in the area where the virtual image OR is positioned. The grating G and object 0 are removed at the stage of reconstruction. As it was shown earlier, the reference-free selectogram transforms the rays of the diffusor DR into the initial image of the object 0 y er ILfl :: pr Fig.1. Scheme of reference-free selectogram recording and reconstruction. 0 - object, S - slit, G - diffractjon grating, V - volume light-sensitive material, DR - homogeneously illuminated diffusor. In fact, tile structure of the selectogram recorded in a volume V can be presented as a sum of partial 3-D holograms ofi the object 0, each of them being recorded when using one of the points of the virtual image of the object OR as a point reference source. By its very nature each of these partial 3-D holograms can be reconstructed only by the radiation of that very point of the diffusor DR whose position coincides with the position of the point of the virtual image of the object OR that played the role of the reference source at the stage of the hologram recording. At the process of reconstruction, the points of the diffusor DR interacting with different 3-D holograms reconstruct number of identical images of the object 0. It is obvious that the process of summing up of these images is not able to. change the configuration of the resulting image of the object reconstructed by the selectogram.. The fact that the nature of the reference-free selectogram strongly differs from that of a conventional 3-D hologram becomes evident in case one tries to decrease the thickness of the light-sensitive layer. Indeed, while a conventioial 3-D hologram when being transformed into a 2-D one retains the property of reconstructing the image of th object (losing some other properties, of course), the reference-free selectogram transforms in this case into a 4mple copy of the diffraction grating G that does not contain any information on the object. SPIE Vol. 2849 / 235
The main advantages of the reference free selectogram are low sensitivity to the vibrations of the recording setup and of the object and very low requirements imposed on the coherence of the recording light. One of the main drawbacks of the reference-free selectogram is its low diffraction efficiency. In fact, a part of the reconstructing light is wasted in this case on the interaction with additional gratings that are formed due to the interference between the components of the radiation emitted by an extended reference source OR. Besides that, the other part of the reconstructing light will be inevitably lost due to the difference of light distribution on the surface of the diffuse screen DR and that of the virtual image of the object OR Indeed, as it was mentioned earlier, only those points of the diffuse screen DR whose position coincides with the position of the points of the virtual image OR can participate in the process of reconstruction. As far as we do not a priori know the configuration of the image recorded on the selectogram, we have to illuminate the entire area where the image OR can appear. If we take these processes into account, then it becomes evident that the requirements to the diffraction efficiency of the light-sensitive material are very severe in this case. 3 GEL-LIKE DICHROMATED GELATIN Initially the experimental proof of the validity of the principle of the reference-free selectogram has been done when using the technique of so-called pseudodeep hologram and then when using special thick-layered lightsensitive material reoxan.4'5 Though these experiments were successful, both of these types of recording were not convenient for a further development of the method. Finally, we have decided to develop a special type of a light-sensitive material, which while being unexpensive, will permit us at the same time to record a hologram at a depth of about several millimeters. In its general aspects, the technology of preparing this material is in part like the widely-known techniques of preparing layers of dichromated gelatin used for recording thin-layered reflection holograms.6 The major difference is that the deposited layer of liquid dichromated gelatin after it hardens is not subjected to drying, during which process its thickness decreases by a factor of 10 to 15. The hologram is recorded directly in the moisture-saturated layer, which is protected from drying by a coating of glass or a polymer film. The layer itself is a rather dense gel, in which a hologram can be recorded as readily as in a solid material. In detail, the method of preparing the gel-like dichromated gelatin comes to the following. A 6% solution of gelatin is prepared, in which ammonium dichromate (NH4)2Cr2O7 is dissolved in an amount equal to 5% by weight of the dry gelatin. The ratios among the weights of the gelatin, the ammonium dichromate, and the water was recommended to us by S.B.Soboleva. The liquid gelatin solution was poured onto a glass substrate at 40 C. The layer thus produced, with a thickness of 1 to 3.5mm, was covered with glass or a Mylar film and then was set to harden in a refrigerator. The principal holographic parameters of the layers obtained in this way were determined by recording holograms of two plane waves formed from the radiation of a helium-cadmium laser () = 440nm) with a power of 16 mw. The interfering beams were sent symmetrically about the normal to the layer surface, with the angle a of convergence equal to 14. The diameter of the exposed spot was about 10mm. By covering periodically one of the interfering beams and measuring the intensity of the beam reconstructed by the recorded hologram, it would be possible to measure the dependence of its diffraction efficiency on the exposure. In fact, the reconstructed wave appeared in just the first few seconds of the recording. the exposure time at which the diffraction reached near the maximum value was about 30 mm for a film 2mm thick. Taking into account the parameters of the radiation used in the exposure, we are able to estimate holographic sensitivity of the material at about 10 J/cm2, which is quite close to the sensitivity of reoxan. It should be noted that mere fact that the water-impregnated gel-like layer of dichromated gelatin is light-sensitive was very unexpected, since it is known that the dry layers of this material must be exposed in a very dry atmosphere. 236 / SPIE Vol. 2849
The value of the diffraction efficiency of the recorded holograms depended on the type of gelatin that was used in the experiment and it varied from 7% to 17%. A characteristic feature is the fact that during the first few hours after the exposure the diffraction efficiency of the hologram increases somewhat presumably, because of some structural reorganization within the layer, but later (after 24 hours) such reorganization causes the destruction of the hologram. The typical curve of the dependence of the diffraction efficiency on the exposure time is shown in Fig.2 (curve a). As it is seen from the Figure, the dependence is characterized by two maxima. It can be supposed that two different processes take place inside the layer with an individual maximum of the curve corresponding to each process. Trying to smooth the curve of the dependence of the diffraction efficiency on the time of the exposure, we have subjected the layer to the exposure by homogeneous light just before the hologram recording. The curve b in Fig.2 refers to the case when the layer was illuminated before the exposure for 10 mm by the light of the reference beam only. As it is seen from the Figure, the pre-exposure does eliminate the first maximum but the value of the diffraction efficiency in the second maximum also decreases. A O (.FC 8 6 4 2 #0 0 500 1000 1500 )17A'fl Fig.2 Fig.3 Fig.2. Dependence of the diffraction efficiency of the hologram on the time of the exposure. (a) Case of normal recording; (b) Case when the layer has been pre-exposed for 10 mm by a homogeneous light of a reference beam only. Fig.3. Dependence of the diffraction efficiency of the grating recorded in gel-like gelatin on the spatial frequency of the grating. The experimental data on the dependence of the diffraction efficiency of the grating recorded in the layer on the spatial frequency of the grating are presented in Fig.3. In these experiments, the layer was exposed by two beams of a helium-cadmium laser. The angle of convergence between the interfering beams took the values from 14 to 50. As it is seen from Fig.3, the diffraction efficiency of the recorded grating substantially decreases as its spatial frequency increases. If we take the value of the diffraction efficiency equal to 1% as an acceptable one, the resolution of gel-like gelatin can be roughly appreciated as equal to 1000 lines/mm. Unfortunately, such value of the diffraction efficiency while being acceptable for some cases of the hologram recording, is far from being sufficient in the case of selectogram, because of the losses of energy characteristic of this method. The experiments on the measuring of the angular selectivity of the holograms recorded in gel-like gelatin SPIE Vol. 2849 / 237
layers have been also carried out. The subject of the investigation was the grating recorded in a layer of 2mm thick, the angle of convergence of the recording beams being equal to 14. The experiments have shown that the dependence of the diffraction efficiency on the angle of incidence of the reconstructing beam has maximum. During the first hours after the recording, the width of the maximum at its half-height was about 10'. With time the structural reorganization inside the layer leads to the increase of the width of the maximum and to the decrease of its height. Finally, after 24 hours, the width became equal to 1 and the diffraction efficiency decreased by a factor equal to two. To appreciate this result one should take into account that the width of the maximum equal to 10' corresponds to the case when the thickness of the layer is equal to 0.6mm. Finally, we have carried out an experiment on the reference-free selectogram recording using the layer of gellike dichromated gelatin. The special feature of this experiment was the low level of sensitivity of the material. To compensate this drawback we have taken measures permitting us to concentrate the energy of the exposing light on a small area of the material. This was done by reducing the distance between the slit S and the light-sensitive material V (see Fig.1). The selectogram was recorded in the blue light of a He-Cd laser (,\ = 440nm) with a power of 16 mw. We used as an object a grating that represented a regular system of transparent and non-transparent bars with a spatial period of 1mm, the size of the object being loxlomm. The grating was inclined by 45 relative to the vertical axis. The selectogram recorded in such a way did reconstruct the image of the object, but its brightness and contrast were very low due to the low value of the diffraction efficiency of gel-like gelatin and the high level of the scattered light caused by inhomogeneities of the layer. 4 CONCLUSION The new method of 3-D imaging - a reference-free selectogram has many advantages as compared to a conventional hologram. Low requirements as to the coherence of the recording light and to the vibration of the recording setup are the most important among them. The main drawback of the method are the necessity of using thick-layered light-sensitive materials and the severe requirements as to their diffraction efficiency and to the level of the scattered light. The new type of a thick-layered light-sensitive material - gel-like dichromated gelatin has been suggested. Being very unexpensive and simple in manufacturing, it can be used for the recording of conventional 3-D holograms. However, the low value of the diffraction efficiency of this material and the imhomogeneities of its structure do not permit using this material for the reference-free selectogram recording. The further development of the technology of this type of a thick-layered material and the improvement of its drawbacks will greatly contribute to the problem of 3-D hologram and selectogram recording. 5 ACKNOWLEDGMENTS The financial support of the International Science Foundation (grant NU-4300) and the Russian Foundation for Fundamental Investigation (grant 95-02-03887-A) is greatly acknowledged. 6 REFERENCES [1] Yu.N.Denisyuk, "On the reflection of the optical properties of an object in the wave field of light scattered by it", Dokiady Akademii nauk SSSR, Vol.144, pp.l775-l2'78, 1962. [2] P.J.Van Heerden, "Theory of optical information storage in solids", Appl.Opt., Vol.2, No.4, pp.393-400, 1963. 238 / SPIE Vol. 2849
[3] Yu.N.Denisyuk, "Window effect of 3-D deep hologram as a base of 3-D imaging by means of a selectogram (Reference-free Selectogram)", in: Practical Holography IX, Stephen A,.Benton, Editor, Proc.SPIE 2406, pp.65-73, 1995. [4] N.M.Ganzherli, "Formation of three-dimensional images when using selectograms and diffraction gratings", in: Practical Holography IX, Stephen A.Benton, Editor, Proc.SPIE 2406, pp.357-363, 1995. [5] Yu.N.Denisyuk, N.M.Ganzherli, and N.A.Savostyanenko, "3-D imaging by means of a reference-free selectogram recorded in a thick-layered light-sensitive material", in: Practical Holography X, Stephen A.Benton, Editor, Proc.SPIE 2652, pp.188-195, 1996. [6] T.A.Shankoff, "Phase holograms in dichromated gelatin", Appi. Optics, Vol.7, No.10, pp.2101-2105, 1968. SPIE Vol. 2849 / 239