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We present the results obtained using the new panchromatic ultra-fine grain emulsion BBVPan, manufactured by Colourholographics Ltd., to produce multiple band holographic reflection gratings, multiplexed on a single layer of material. Three different laser systems were used: He-Cd, frequency-doubled Nd-YAG and He-Ne. High diffraction efficiencies, of over 52%, were obtained for each of the three bands, with little wavelength shifting. The holographic response of the multiplexed hologram was compared with the theoretical response to demonstrate that there is negligible contribution of the crosstalk between recordings in the visible range of the spectrum for this specific configuration.
©2003 Optical Society of America
1. Introduction
Multiplexed holograms have a great importance in a variety of applications, such as high density holographic storage, optical computing, multiple beam combining, holographic color displays and photonic crystals. In these applications, two or more gratings with different spatial frequencies and slant exist simultaneously in a single diffractive element. In the special case of multiplexed reflection gratings, the main applications are multi-color combiners for devices such as head-up displays, and multi-color filters. Traditionally, the materials used for recording these holographic gratings have been dichromated gelatin and photopolymers. Dichromated gelatin was and still is used for high performance holographic combiners in the aerospace industry. When projection displays changed from monochrome to color it became obvious that holographic combiners had to change from single band to multiple band. For this specific recording material, with spectral sensitivity in the blue-green zone of the visual spectrum, the red gratings had to be recorded with very special procedures. The blue and the green holograms were recorded individually, not multiplexed, and the final multiple band element had to be built up by laminating the three individual gelatin layers one over the other with complex and costly techniques [1]. In the late 80’s and early 90’s, commercial panchromatic photopolymers [2] emerged as an alternative to DCG for these specific applications, although there is not much information available on applications for holographic reflection gratings [3]. Silver halide sensitised gelatin processes with panchromatic emulsion PFG03-C have been also used, with high diffraction efficiencies for single wavelength recordings, although no information on multiplexed gratings has been given [4].
Silver halide holographic emulsions exhibit a better sensitivity than all the recording materials mentioned above. Nevertheless, although they have been used for recording multiplexed reflection holograms with different laser lines [5, 6], their usage has been limited by their relatively low index modulation capacity, as well as by their spectral sensitivities, since most of them are sensitised to a single spectral band only. Besides, the material is composed of ultra-fine silver halide grains, with an intrinsic absorption band around 400 nm. Therefore, blue recordings do not work properly, since they have low diffraction efficiencies due to high levels of absorption and scattering. This led to the use of techniques involving recordings in more than one plate [7] or more than one recording material [8]. The use of monochromatic emulsions for multiplexing reflection gratings with different swelling factors between recordings has been also reported. In this case, diffraction efficiencies lower than 20% for each band were obtained [9]. All these problems meant that the emulsions were only useful for display and artistic holography. In the mid 90’s, new ultra-fine grain panchromatic emulsions, specially Slavich PFG-03C, with a mean grain size of 10 nm - smaller than that of the emulsions previously available - boosted advances in these two fields [10]. More recently, results obtained with a non commercial ultra fine grain panchromatic emulsion have been reported, with diffraction efficiencies for single exposure diffraction gratings higher than 50%, although no information on multiplexed gratings is given [11]. To our knowledge, no results have been reported regarding applications of ultra fine grain panchromatic emulsions for multiple band holographic reflection gratings in a single plate.
There are several aspects that have to be considered when working with multiplexed reflection holograms with different wavelengths in silver halide materials. The first is the high scattering mentioned above that occurs in the blue part of the spectrum. This scattering can be reduced by working with ultra-fine grain emulsions. In this study we used the new panchromatic ultra-fine grain emulsion BBVPan, based on the known family of BB emulsions, currently manufactured by Colourholographics Ltd, with a mean grain size of 20 nm. The second aspect is related to shrinkage or swelling of the emulsion after the plate is processed, since in reflection holography this is directly related to the wavelength of reconstruction, and this affects the final replay spectrum and colour rendition of the grating. The last aspect is the effect of multiple exposures on a single emulsion, since this is associated with a reduction in the diffraction efficiency. This reduction has been evaluated as inversely proportional to the square of the total number of recordings [12].
2. Experimental
Multiple band holographic reflection gratings were recorded using the new panchromatic ultra-fine grain emulsion BBVPan, batch no. 174. In all tests, plates were pre-sensitised by soaking for 2 min. in a 3% triethanolamine water solution, soaked for 7 min. in deionized water, dried with a photographic roll and warm air and left in the exposure room for half an hour in normal laboratory conditions (20° and 60% RH) [13].
Plates were exposed to single collimated beams in a Denisyuk configuration [14] using a blue He-Cd laser (wavelength 442 nm), a green frequency-doubled Nd-YAG laser (wavelength 532 nm), and a red He-Ne laser (wavelength 632.8 nm). The recording setup consists of an optical sandwich composed of a first surface mirror that reflects the incident beam back into the emulsion. The emulsion side of the plate is in contact with the mirror via an index matching fluid, and the glass side is in contact with an anti-reflection coated glass plate via another thin layer of index matching fluid to prevent internal reflections.
With this configuration spatial frequencies of 7145 l/mm (blue), 5936 l/mm (green) and 4990 l/mm (red) were recorded (considering a refractive index of 1.579 for the unexposed emulsion). The sandwich was mounted on a computer controlled motorised holder which enabled us to record 9 gratings with different exposure energies on a 2.5″×2.5″ plate.
Exposed plates were developed with AAC developer (Ascorbic Acid 18 g/l+Sodium Carbonate 60 g/l) [15]. After washing they were bleached with fixation-free rehalogenating bleach R-10 (Potassium Dichromate 2 g/l+Sulphuric Acid 10 cc/l+Potassium Bromide 35 g/l). After bleaching, the plates were washed and soaked in deionized water with a few drops of Photoplo and Acetic Acid to prevent printout, and dried in the normal laboratory conditions cited above.
After drying, the plates were analysed using a fibre fed spectroradiometer. With this device we measured the zero order of the grating with a replay angle of 0°, matching the recording geometry. A short arc Xenon lamp was used as the light source, collimated and polarised perpendicular to the plane of incidence to match the recording conditions. Light was collected by an optical fibre that feeds the spectrophotometer and data were transferred to a computer for storage and analysis. Reflection losses were experimentally evaluated and found to have a value of 6.7%.
Since it was clear from the transmission curves of the gratings that the absorption coefficient is a function of the wavelength, the dependence was calculated using the envelope curve of the transmitted spectrum obtained from the experimental data of the diffracted bands [16].We used a semi-empirical saturation curve corrected by a sinusoidal expression given by Eq. (1) as the envelope of the transmittance curve.
where A, B, C, and D are fitting parameters obtained using a non linear fit technique [17]. In most of the calculations, regression coefficients are better than 0.99. Diffraction efficiencies were calculated from the experimental transmission data as the difference, at the wavelength with minimum transmission, between the value of the envelope curve Te and the experimental transmission curve. In the case of multiple bands, local minima are considered. The bandwidths are calculated using the diffraction efficiencies and the minimum transmission wavelengths.
Two different studies were performed: a preliminary spectral sensitivity characterisation of the plates, followed by a study of multiplexed gratings on a single plate.
2.1. Characterisation of the plates
The plates were first tested for single wavelength recordings with each of the laser beams used in order to check their spectral sensitivity and response of the material when recording holographic reflection gratings. Three sets of tests, one for each wavelength, were performed, including pre-sensitising, exposing, processing and analysis as explained above. Exposure energies ranged from 30 through 2400 µJ/cm 2 for the He-Cd laser and from 150 through 2400 µJ/cm2 for the frequency doubled Nd-YAG and He-Ne lasers.
2.2. Multiplexed holograms
In order to obtain a good correspondence between the recording and the reconstructed hologram wavelengths, the pre-sensitising steps described above must be done carefully. Exposure of the reflection gratings was performed sequentially, starting with the blue wavelength, followed by the green and ending with the red. A set of multiplexed reflection gratings with different exposure energy combinations for each wavelength was obtained, ranging from 120 through 225 µJ/cm 2 for the He-Cd laser, 150 through 250 µJ/cm 2 for the frequency doubled Nd-YAG laser and 800 through 1200 µJ/cm 2 for the He-Ne laser.
3. Theoretical
It is widely accepted that multicolour holograms can avoid crosstalk if the bandwidth Δλ of each constituent grating is smaller than the separation between any pair of the reconstructed wavelengths of the hologram [18]. With the thickness (d) of the plates used in this study, around 7 µm, this can be achieved with the laser wavelengths used. According to this idea, we used Kogelnik’s theory [19] to fit the experimental data and check the negligible contribution of cross-talk effects between the different spatial frequencies multiplexed in the recorded grating.
We started with the expression for the amplitude of the direct transmitted waves for the lossy unslanted reflection gratings of Kogelnik’s theory, given by Eq. (2), with the proper γ1, γ2 and ϑ grating parameters for a mixed lossy dielectric unslanted reflection grating with normal incident recording beams [16].
From the envelope curve given by Eq. (1) we can calculate the absorption coefficient α(λ), since this parameter and the transmittance are related according to Eq. (3).
Since we are considering multiple bands in the holographic reflection grating, with no crosstalk, we can apply Kogelnik’s expression to all the bands, so that the final global transmittance will be given by the product of the individual transmission curves of each diffracted band, expressed by Eq. (4). λi are the wavelengths corresponding to each minimum of the transmission curve. Each of these individual is given by Eq. (2).
4. Results
Single sensitivities of BBVPan plates for each of the three recording wavelengths are presented in Fig. 1(a), and the most relevant results are summarised in Table 1. From these results it is clear that the exposure energy for maximum diffraction efficiency (DE) of this emulsion is the highest for the blue wavelength (320 µJ/cm 2), followed by the green (1200 µJ/cm 2) and with the lowest sensitivity for the red (2400 µJ/cm 2). Replay wavelengths match very closely with those used at recording, with an error of less than 2%. This wavelength shift can be modified by changing the soaking bath times in the pre-sensitising process.
Fig. 1. (a) Dependence of the diffraction efficiency of BBVPan plates on exposure energy for each of the recording wavelengths. (b) Maximum diffraction efficiency values for each of the recording spatial frequencies.
Table 1. Relevant results obtained for single wavelength recording of holographic reflection gratings on BBVPan plates.
One of the key characteristics of this new material is its even response to all the wavelengths used in this study. Former western emulsions suffered from high absorption in the blue region of the spectrum that limited their use for recording multiplexed color reflection holograms on a single plate. With the experimental setup described above, we worked not only with three well separated wavelengths, but with the highest spatial frequencies holographically achievable in each case. The results obtained show that the maximum diffraction efficiency with each of these spatial frequencies is almost constant, with a small drop in the case of the blue wavelength due to the proximity of the absorption band of the silver halide grains, located at about 400 nm, and the absorption band of the supporting glass plate and the gelatin emulsion. For the red wavelength there is another small drop in diffraction efficiency, but this time due only to the low sensitivity of the plate in this region of the spectrum. Nevertheless, all maximum DE values are well above 70%. Figure 1(b) shows the response of the material as a function of the spatial frequency of the recordings. This figure, which may be considered a measurement of the resolving power of BBVPan plates for very high recording frequencies, shows an almost flat response, a result that has not being obtained with other silver halide materials [20].
The transmission spectra of the recordings corresponding to the the three diffraction efficiency maxima are shown in Fig. 2. The blue band is affected by the absorption of the ultra-fine grain emulsion referred to above, reducing the diffraction efficiency of this recording, although the zero order is of the same magnitude as that obtained with the green wavelength.
Fig. 2. Transmission spectra corresponding to the three single wavelength holographic reflection gratings with maximum DE recorded on three BBVPan plates.
With all the information obtained after this preliminary study, we multiplexed three reflection gratings, each with a different wavelength, onto a single plate, following the procedure described in Section 2. The best result was obtained with a sequence of exposures with energies of 150 (442 nm)+250 (532 nm)+1200 (632.8 nm) µJ/cm 2, at which the diffraction efficiencies of each band are balanced, as shown in Table 2, with the corresponding spectral transmission curve shown in Fig. 3. DE for all the recordings is higher than 52%, which indicates a high index modulation capacity of this material. Other exposure energy sets were tested, and small changes in one of the exposures were seen to substantially affect the DE of all three bands.
In order to check the negligible effect of the crosstalk between diffraction bands we applied the model based on Kogelnik’s theory with the three bands described in Section 3. With this model we obtained the index modulation and effective thickness of the multiple band recordings with great accuracy. Experimental data were fitted and a good match was obtained, as can be seen in Fig. 3, in which the dashed line corresponds to the theoretical approach. The best result was obtained for an effective thickness d of 7.3 µm. Approaches with three different values of the index modulation n 1, one for each wavelength, were tried, but the best result was obtained when each one had the same value of 0.027. Table 3 summarise these results. The parameters that characterise the envelope curve of the transmittance spectrum have been included.
Table 2. Relevant results obtained for multiplexed holographic reflection gratings recorded with three wavelengths on BBVPan plates.
Table 3. Parameters obtained after applying the model described in the theoretical section to the three band multiplexed reflection holographic grating.
Fig. 3. Transmission spectrum of the multiplexed holographic reflection grating recorded on a single BBVPan plate. The dashed line shows the result obtained with the the theoretical simulation.
After reviewing the results obtained in this study, there are several points raised in the introduction that may be discussed. The high diffraction efficiencies obtained with the multiplexed holograms contradicts what has been said about a reduction in diffraction efficiency of multiplexed holograms (although those studies were done with angular multiplexing). In fact, results obtained with this material, show that its modulation capacity is greater than that needed to record a unique holographic grating, since if we consider such a case, we can obtain a maximum index modulation of 0.054, while if we use the sum of the three individual index modulation as the storage capacity of the material, a value of 0.081 is obtained, which is much higher than any other that has been reported for a silver halide emulsion. Therefore, the total DE is not reduced by multiplexing several gratings, but is increased to another value that corresponds to the real storage capacity of this emulsion.
5. Conclusions
A new panchromatic holographic material with high holographic storage capacity is presented. The material was previously evaluated using single recordings with three different wavelengths, and a DE higher than 72% was reached in all cases. After characterisation, the plates were used to record a three band holographic reflection grating with spatial frequencies from 5000 l/mm to more than 7000 l/mm with high diffraction efficiencies, of over 52% for each of the three bands. Additionally, the cross-talk between recordings was evaluated using a theoretical approach based on Kogelnik’s theory, and it was seen to have a negligible effect on the recordings. With these properties, this material can be used to manufacture cheap, easy to make, low level holographic combiners for projection display systems, as well as color display holograms and photonic crystals.
Acknowledgments
The authors thank gratefully Michael Medora of Colourholographics Ltd. for providing the holographic plates used in this work. This work has received financial support from the Comision Interministerial de Ciencia y Tecnologia (CICYT) of Spain (Project No. MAT2000-1361-C04-03).
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