Study of Full-Color Multiplexed Transmission Holograms of Diffusing Objects Recorded in Photopolymer Bayfol HX
Study of Full-Color Multiplexed Transmission Holograms of Diffusing Objects Recorded in...
Sevilla, Marina;Marín-Sáez, Julia;Chemisana, Daniel;Collados, María-Victoria;Atencia, Jesús
2021-10-22 00:00:00
hv photonics Article Study of Full-Color Multiplexed Transmission Holograms of Diffusing Objects Recorded in Photopolymer Bayfol HX 1 1 2 , 1 1 Marina Sevilla , Julia Marín-Sáez , Daniel Chemisana * , María-Victoria Collados and Jesús Atencia Applied Physics Department, Aragon Institute of Engineering Research (I3A), Faculty of Science, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain; marina_s_d@hotmail.com (M.S.); jmarinsaez@unizar.es (J.M.-S.); vcollado@unizar.es (M.-V.C.); atencia@unizar.es (J.A.) Applied Physics Section of the Environmental Science Department, Polytechnic School, University of Lleida, Jaume II 69, 25001 Lleida, Spain * Correspondence: daniel.chemisana@macs.udl.cat Abstract: A wavelength multiplexing procedure for color transmission volume holograms of diffus- ing objects recorded in Bayfol HX200 photopolymer is proposed. For the recording, three lasers of 442 nm, 532 nm and 633 nm, and a Spectralon diffusing object were used for monochromatic and polychromatic calibration. Monochromatic calibration shows that the maximum index modulation obtained for each wavelength was not enough to give 100% efficiency, although the efficiency values achieved in the case of monochromatic recordings with diffusing objects were high, at around 90% for 633 nm and 532 nm recordings, and 60% for 442 nm. The efficiency values obtained for multiplexed holograms were 19.1% for the 442 nm hologram, 25.9% for the 532 nm hologram and 15.2% for the 633 nm. Keywords: full-color holography; volume hologram; transmission hologram; photopolymer Citation: Sevilla, M.; Marín-Sáez, J.; Chemisana, D.; Collados, M.-V.; 1. Introduction Atencia, J. Study of Full-Color Multiplexed Transmission Holograms It is known that there are different applications of holography; it should be highlighted of Diffusing Objects Recorded in that color reproduction is one of the most important. The applications involve different Photopolymer Bayfol HX. Photonics fields—for instance, art [1], security holography [2], sensors [3,4], displays [5] and LEDs [6]. 2021, 8, 465. https://doi.org/ These examples show the wide range of color-hologram applications [7]. In terms of 10.3390/photonics8110465 the types of panchromatic holograms, it should be noted that there are transmission- type configurations as well as reflection-type options. Reflection-type configurations are Received: 30 September 2021 appropriate in the case of color reproduction when reconstructed with white light and Accepted: 19 October 2021 are the most extended type. However, in the case of transmission-color holograms, it is Published: 22 October 2021 necessary to illuminate these devices with the required wavelengths with directions that are used in the recording stage. Nevertheless, there are some techniques (for example, Publisher’s Note: MDPI stays neutral rainbow color holography [8]) which allow their reconstruction using white light. with regard to jurisdictional claims in Panchromatic materials were not available until recently—this is a major disadvantage published maps and institutional affil- related to recording-color holograms. In 1996, Bjelkhagen et al. [9] presented a study iations. related to this topic. The results showed that it is possible to record color holograms in a single-layer silver halide emulsion by means of wavelength multiplexing. It was found that multiplexing conditions (exposure times, intensities, etc.) depend on the photosensitive material. Nowadays there are materials (Slavich PFG-03 silver halide plates, Copyright: © 2021 by the authors. or photopolymers such as Bayfol HX) that are sensitive to a wide range of wavelengths, Licensee MDPI, Basel, Switzerland. allowing the recording of color holograms. This article is an open access article Although the most widespread color holograms are the reflection type, numerous distributed under the terms and applications that require the use of transmission holograms such as the recording of rainbow conditions of the Creative Commons color holograms from a master [10], augmented reality systems [11] or diffusers [12], can Attribution (CC BY) license (https:// benefit from the new panchromatic recording materials. creativecommons.org/licenses/by/ 4.0/). Photonics 2021, 8, 465. https://doi.org/10.3390/photonics8110465 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 465 2 of 16 One of the main advantages of panchromatic photopolymers over other panchromatic recording materials like silver halides is the grain absence, which avoids diffusion in the blue region. In previous works published about multiplexed holograms in photopolymers, two main recording techniques are described: simultaneous and sequential exposure. In simultaneous exposure, the recording material is illuminated with different wavelengths at a time and during the same time period. The exposure energy for each wavelength depends on the exposure time (the same for all) and the intensity of each laser, which has to be controlled accurately by means of lasers with variable power, continuous density filters or two combined linear polarizers. An inadequate adjustment of the intensity of each wavelength can lead to troubles in color reproduction, as the resulting efficiencies of the wavelengths involved are then difficult to control [13]. In sequential exposure tech- niques, the recording material is exposed to each laser separately, one wavelength after the other, and during a time interval—which can be different for each one. A variation of this technique founded on time-scheduled iterative exposition, and consisting of repeating an il- lumination sequence for a certain number of cycles was used by Piao et al. [11,12], achieving high diffraction efficiencies (over 50%) for transmission-type multiplexed gratings. It should be noted that in the studies described above (by other authors) the calibration has been carried out by recording holographic gratings. In none of them were the recording and calibration conducted by placing a diffusing object in the object beam. Holograms of diffusing objects perform quite differently than gratings or any type of direct beam interference since, in that case, the light is scattered in the object and multiple interferences are produced between scattered waves and with the reference wave. This phenomenon produces speckle on the recording material, increasing the mean refractive index and, thus, decreasing the effective modulation of the refractive index in comparison to plane-wave holograms [14]. Vazquez et al. analyzed this effect in color reflection holograms recorded in Bayfol HX photopolymer and optimized its efficiency [15] using simultaneous exposure with different times. An important aspect in color hologram recording is the number of wavelengths necessary for reliable color reproduction. Bjelkhagen et al. [16] found a tolerable error in color reproduction when three lasers with wavelengths around 466, 545 and 610 nm were used. Increasing the number of wavelengths leads to an improvement in color reproduction, but also increases the cost and complexity of the recording set-up, so a trade-off solution should be considered to balance complexity and accurate color reproduction. Bearing in mind the issues mentioned above, the present investigation analyzes the behavior of volume transmission holograms of color diffusing objects in self-developing photopolymers, deepening the understanding of these kinds of holograms. The technique uses simultaneous exposure with different times for the lasers involved, with the goal of obtaining uniform and high efficiencies for the comprised wavelengths. The procedure regards the polychromatic calibration of the photosensitive material by directly including the diffusing object (Spectralon target). To the best of the authors’ knowledge, there are no previous works by other authors performing a polychromatic calibration of the photosensi- tive material with a diffusing object in transmission volume holograms. The photosensitive recording material utilized is the panchromatic Bayfol HX200 photopolymer, and the number of lasers selected is three. In the Materials and Methods section, the recording material and the experimental setup used are described, together with an explanation of the effect of chromatic selectivity of volume holograms in multiplexed holograms. Besides this, in this section the methodologies of the monochromatic and polychromatic calibrations are explained. The following section includes experimental results and discussion. Finally, the main conclusions are stated. 2. Materials and Methods 2.1. Photosensitive Material The material used in this work is a self-processing photopolymer, Bayfol HX200 [17,18], which has enough sensitivity to make transmission or reflection volume holograms with Photonics 2021, 8, 465 3 of 16 wavelengths from 440 nm to 680 nm. The photopolymer layer had a thickness of 16 2 m and was deposited on a polycarbonate substrate. The characteristics of the unexposed photopolymer and the performance parameters can be found in [17,18]. The material had peaks of absorption around 655 nm (85%) and 520 nm (55%). Since the absorption curve presented a minimum around 570 nm (30%), a darkroom safelight around this wavelength could be used to manipulate the material before exposition. In the recordings conducted in the frame of this work, a 590 nm LuxeonRebel LT1012 LED light located at least 50 cm away from the unexposed material was used. The hologram formation in this material was related with the gradient of monomer concentration achieved during the recording step. When the material was exposed to light, the polymerization began in the regions where it received enough light, decreasing the monomer concentration and causing a monomer concentration gradient and its diffusion from the high concentration to the low concentration regions. After 5 min, a refractive index variation pattern was obtained and the hologram was already formed in the photopolymer. To fix the hologram (polymerization of the remaining monomer and bleaching of the dye) it was necessary to illuminate it with a white light led lamp of 50 W for 25 min [19]. There were no significant changes in material thickness during the process. This fea- ture makes the Bayfol HX 200 ideal for recording holograms in real color. The maximum index modulation achievable with this material in reflection gratings is Dn 0.033 [17]. However, in the case of transmission gratings, the index modulation depends linearly on the spatial frequency of the recorded hologram (e.g., for a frequency of 1000 lines/mm, Dn = 0.024 [20]). Smaller effective modulation is expected for the recording of diffusing objects due to intermodulation noise produced by the interference between object points. Bayfol HX 200 sheets have a slight curvature that allows the identification of the surface of the photopolymer and the substrate. The convex part corresponds to the pho- topolymer. Before recording, the samples should be directly adhered to a 3.9 mm thick glass thanks to the viscosity of the photopolymer, in order to increase their mechanical stability. Prior to this step, the glass must be carefully cleaned to provide as few dust particles as possible. The presence of these particles when adhering the photopolymer to the glass causes the existence of bubbles that can produce vibrations in that zone of the photopolymer, averaging the interferential figure and so avoiding hologram formation. 2.2. Experimental Setup To obtain color transmission holograms, three lasers with different wavelengths have to be used in the recording. The optimal wavelengths to obtain a color reproduction with tolerable error were stated by Bjelkhagen and Mirlis [16]. In the present work, we chose three lasers with wavelengths close to the optimal ones. Figure 1 shows the main characteristics of the lasers, the reference optimal wavelengths and the wavelengths shown in the CIE diagram. The lines marked in white enclose the colors that can be reproduced with the three wavelengths used in this work. Figure 2 shows a scheme of the set up used in the recording of color transmission holograms. The three beam lasers were combined using dichroic mirrors, designed to reflect only a portion of the visible range. Red and green lasers were combined using a Chroma T556lpxr mirror, with high transmittance for wavelengths above 556 nm. Both lasers were combined with the blue one by means of a Chroma T470lpxr mirror, which has high transmittance for wavelengths above 470 nm. To equalize the diameter of the three lasers, the beam transversal section of the blue laser was increased by using a Galileo telescope. The three beams passed through a 30/70 beam splitter. The reflected beam had less intensity and was used to illuminate the object. In this way, a relation of 1/10 between object and reference beam was obtained. This relation is desirable in diffusing object hologram recordings to minimize the intermodulation terms produced by the interference of the light diffused by each point of the object, while maintaining high contrast in the interference of the object and reference beams. To assure the coherence between the two beams, the optical path travelled was similar for both beams. Photonics 2021, 8, x FOR PEER REVIEW 4 of 17 Power Laser Type λ (nm) Optimal λ (nm) (mW) Kimmon He–Cd 150 442 466 IK4171I-G Photonics 2021, 8, 465 4 of 16 Photonics 2021, 8, x FOR PEER REVIEW 4 of 17 DPSS Solid State 50 532 545 Oxxius LMX (DPSS) Power Laser Type λ (nm) Optimal λ (nm) (mW) Uniphase He–Ne 35 633 610 1145P Kimmon He–Cd 150 442 466 Figure 1. Main characteristics of the lasers used in the recording setup, together with the optimal λ for color reproduction IK4171I-G stated in [16]. CIE diagram with the representation of the color reproduction area obtained with the three lasers. DPSS Solid State Figure 2 shows a scheme of the set up used in the recording of color transmission 50 532 545 Oxxius LMX (DPSS) holograms. The three beam lasers were combined using dichroic mirrors, designed to re- flect only a portion of the visible range. Red and green lasers were combined using a Chroma T556lpxr mirror, with high transmittance for wavelengths above 556 nm. Both Uniphase He–Ne 35 633 610 lasers were combined with the blue one by means of a Chroma T470lpxr mirror, which 1145P has high transmittance for wavelengths above 470 nm. To equalize the diameter of the three lasers, the beam transversal section of the blue laser was increased by using a Gal- Figure 1. Main characteristics of the lasers used in the recording setup, together with the optimal λ for color reproduction Figure 1. Main characteristics of the lasers used in the recording setup, together with the optimal for color reproduction ileo telescope. stated in [16]. CIE diagram with the representation of the color reproduction area obtained with the three lasers. stated in [16]. CIE diagram with the representation of the color reproduction area obtained with the three lasers. Figure 2 shows a scheme of the set up used in the recording of color transmission holograms. The three beam lasers were combined using dichroic mirrors, designed to re- flect only a portion of the visible range. Red and green lasers were combined using a Chroma T556lpxr mirror, with high transmittance for wavelengths above 556 nm. Both lasers were combined with the blue one by means of a Chroma T470lpxr mirror, which has high transmittance for wavelengths above 470 nm. To equalize the diameter of the three lasers, the beam transversal section of the blue laser was increased by using a Gal- ileo telescope. Figure 2. Recording setup diagram. GT is the Galileo telescope, M are the broadband mirrors, S1–S3 Figure 2. Recording setup diagram. GT is the Galileo telescope, M are the broadband mirrors, S1– are the shutters for each laser, DM1 and DM2 are the dichroic mirrors, BS is the beam splitter, SF is S3 are the shutters for each laser, DM1 and DM2 are the dichroic mirrors, BS is the beam splitter, SF the spatial filtering stage (microscope objective lens + pinhole), CL is the collimating lens, HP stands is the spatial filtering stage (microscope objective lens + pinhole), CL is the collimating lens, HP for the photopolymer and DO represents the diffusing object. stands for the photopolymer and DO represents the diffusing object. The object and reference beams were expanded and spatially filtered by using micro- The three beams passed through a 30/70 beam splitter. The reflected beam had less scope objectives (x10 in the case of the reference beam and x4 in the case of the object beam) intensity and was used to illuminate the object. In this way, a relation of 1/10 between and pinholes. A collimating lens was used to obtain a plane reference beam. object and reference beam was obtained. This relation is desirable in diffusing object As a diffusing object, we used a Spectralon sheet, which has highly Lambertian hologram recordings to minimize the intermodulation terms produced by the interfer- behaviour and uniform spectral reflectance. ence of the light diffused by each point of the object, while maintaining high contrast in Figure 2. Recording setup diagram. GT is the Galileo telescope, M are the broadband mirrors, S1– The exposure time of each laser was controlled by three shutters located before the S3 are the shutters for each laser, DM1 and DM2 are the dichroic mirrors, BS is the beam splitter, SF beam splitter. is the spatial filtering stage (microscope objective lens + pinhole), CL is the collimating lens, HP The angle between object and reference beam must assure the recording of three stands for the photopolymer and DO represents the diffusing object. independent gratings, as is explained in the next subsection. In this case, this angle had to be 50 . The three beams passed through a 30/70 beam splitter. The reflected beam had less intensity and was used to illuminate the object. In this way, a relation of 1/10 between 2.3. Chromatic Selectivity in Transmission Volume Holograms object and reference beam was obtained. This relation is desirable in diffusing object When a volume transmission hologram was illuminated in the reconstruction step, hologram recordings to minimize the intermodulation terms produced by the interfer- two diffraction orders appeared at the output: zero order (transmitted wave that is not ence of the light diffused by each point of the object, while maintaining high contrast in Photonics 2021, 8, 465 5 of 16 diffracted) and +1 diffracted wave. In this type of hologram, absolute efficiency (h ) and relative efficiency (h,) can be defined as follows: h = (1) h = (2) I + I +1 0 where I is the intensity of the incident beam; I and I are the intensities of the 0 and +1 i 0 +1 order at the output. In the relative efficiency definition (h ), reflection and absorption losses derived from the recording material are not taken into account. This is the efficiency definition that we used in this paper. Angular and chromatic selectivity types are characteristics of volume holograms: for each wavelength, maximum efficiency is obtained only when the incident direction in the reconstruction fulfils the Bragg condition: 2n L sinq = l (3) 0 0 where n is the average refraction index of the recording material, L is the distance between two planes with the same refraction index and 2q is the angle between object and reference beams into the medium. Relative efficiency values obtained when the illumination is near Bragg condition can be computed from the following expression: 2 2 sin n + x h = (4) 1 + where n and x coefficients are defined by: pn d n = p (5) l c c r s Jd x = (6) 2c d is the holographic material thickness, n is the refraction index modulation obtained in the recording step, c is the cosine of the incidence angle of the object beam, and c is s r the cosine of the incidence angle of the reference beam into the medium. The n value depends on the characteristics of the recording material and can be experimentally adjusted to obtain h = 1 if the material dynamic range is enough. The J coefficient is related to the deviation of the illumination from the Bragg condition. If a transmission volume hologram is illuminated with polychromatic light in a range of wavelengths around the wavelength l that fulfills the Bragg condition, J can be expressed as [21]: K (l l) J = , (7) 4pn 2p where K = . When the Bragg condition is fulfilled, x = 0 and the efficiency obtained for l will be 100% if the index modulation is enough (n = ). Figure 3 shows relative efficiency vs. wavelength curves (chromatic selectivity) of holograms recorded with 50 between the object and reference beams (in air), when the Bragg condition is fulfilled for the three l values. These l values correspond to the 0 0 wavelength values of the lasers used in the recording (Figure 1). In Figure 3, the maximum efficiency value corresponding to one l coincided with low efficiency values of the other two curves, which implies that the three holograms recorded with l and 50 between 0 Photonics 2021, 8, x FOR PEER REVIEW 6 of 17 value depends on the characteristics of the recording material and can be experimentally adjusted to obtain 𝜂 = 1 if the material dynamic range is enough. The 𝜗 coefficient is related to the deviation of the illumination from the Bragg con- dition. If a transmission volume hologram is illuminated with polychromatic light in a range of wavelengths around the wavelength 𝜆 that fulfills the Bragg condition, 𝜗 can be expressed as [21]: ( ) 𝐾 𝜆 − 𝜆 𝜗 = , (7) 4𝜋 𝑛 2𝜋 where 𝐾 = . When the Bragg condition is fulfilled, 𝜉 = 0 and the efficiency obtained for 𝜆 will be 100% if the index modulation is enough (𝜈 = ). Figure 3 shows relative efficiency vs. wavelength curves (chromatic selectivity) of holograms recorded with 50º between the object and reference beams (in air), when the Bragg condition is fulfilled for the three 𝜆 values. These 𝜆 values correspond to the 0 0 wavelength values of the lasers used in the recording (Figure 1). In Figure 3, the maxi- Photonics 2021, 8, 465 6 of 16 mum efficiency value corresponding to one 𝜆 coincided with low efficiency values of the other two curves, which implies that the three holograms recorded with 𝜆 and 50º between the object and reference beams were independent. When the holograms were illuminated with monochromatic light in the reconstruction step, only one hologram was the object and reference beams were independent. When the holograms were illuminated efficient for the illumination wavelength, which allowed the reconstruction with the laser with monochromatic light in the reconstruction step, only one hologram was efficient for light of the transmission color holograms. the illumination wavelength, which allowed the reconstruction with the laser light of the transmission color holograms. Figure 3. Chromatic selectivity curves for three holograms recorded with three different wavelengths Figure 3. Chromatic selectivity curves for three holograms recorded with three different wave- le (633 ngth nm, s (63 532 3 nnm m, 5and 32 n442 m an nm) d 44 with 2 nm 50 ) wbetween ith 50º bet the ween object the and objec refer t anence d refe beams. rence beams. 2.4. Monochromatic Calibration 2.4. Monochromatic Calibration Monochromatic calibration was used to obtain the exposure energy value that gave Monochromatic calibration was used to obtain the exposure energy value that gave the maximum efficiency in the recording with each wavelength. With each laser sepa- the maximum efficiency in the recording with each wavelength. With each laser sepa- rately, several recordings were carried out with different exposure times. The dependence rately, several recordings were carried out with different exposure times. The depend- curve of h with exposure could be different, depending on the dynamic range of the ence curve of 𝜂 with exposure could be different, depending on the dynamic range of recording material. the recording material. As can be seen in the previous section (Equations (4) and (5)), the relative efficiency in As can be seen in the previous section (Equations (4) and (5)), the relative efficiency Bragg conditions (x = 0) depends on the index modulation n : ( ) in Bragg conditions 𝜉 = 0 depends on the index modulation 𝑛 : pn d h = sin p (8) l c c r s Index modulation can be controlled by exposure time. Usually, in photopolymer materials, index modulation is linear, with exposure only for a range of exposure values. Above this range, index modulation saturates, reaching a maximum value n . Taking 1,max this into account, the following theoretical model for the index modulation–exposure dependence can be proposed [15]: (E E ) n (E) = n 1 e (9) 1 1, max where E is the minimum exposure value needed to get a value of n different from zero, 0 1 and refers to the slope of the linear region. Considering Equations (8) and (9), theoretical curves of h vs. exposure present different shapes, depending on the n value. If the n value is not enough to reach 1, max 1, max 100% efficiency for reconstruction with the recording wavelength, the curve is as shown in Figure 4a. If the material can reach a maximum index modulation value n above 1, max the value that would give 100% efficiency for the recording wavelength, the efficiency vs. exposure curve is as presented in Figure 4b. In this case, the hologram was over modulated, Photonics 2021, 8, x FOR PEER REVIEW 7 of 17 π𝑛 d 𝜂 = sin ( ) 𝑟 (8) λ c c r s Index modulation can be controlled by exposure time. Usually, in photopolymer materials, index modulation is linear, with exposure only for a range of exposure values. Above this range, index modulation saturates, reaching a maximum value 𝑛 . Taking 1, this into account, the following theoretical model for the index modulation–exposure dependence can be proposed [15]: −α(E−E ) ( ) n E = 𝑛 (1 − e ) (9) 1 1, where E is the minimum exposure value needed to get a value of n different from 0 1 zero, and α refers to the slope of the linear region. Considering Equations (8) and (9), theoretical curves of 𝜂 vs. exposure present different shapes, depending on the 𝑛 value. If the 𝑛 value is not enough to 1, 𝑥 1, reach 100% efficiency for reconstruction with the recording wavelength, the curve is as Photonics 2021, 8, 465 7 of 16 shown in Figure 4a. If the material can reach a maximum index modulation value 𝑛 1, above the value that would give 100% efficiency for the recording wavelength, the effi- ciency vs. exposure curve is as presented in Figure 4b. In this case, the hologram was over modulated, and 100% efficiency could be reached for reconstruction with wavelengths and 100% efficiency could be reached for reconstruction with wavelengths higher than the higher than the recording one. recording one. (a) (b) Figure 4. Efficiency vs. exposure curves for different values of 𝑛 . Black dots represent theo- Figure 4. Efficiency vs. exposure curves for different values of n . Black dots represent theoretical 1,𝑚𝑎𝑥 1,max retical dependence of Equation (8), assuming linear dependence of index modulation with expo- dependence of Equation (8), assuming linear dependence of index modulation with exposure. The sure. The blue curve represents the dependence when the index modulation reaches a saturation blue curve represents the dependence when the index modulation reaches a saturation value n : 1, max value 𝑛 : (a) n is not enough to reach 100% efficiency and (b) 𝑛 is higher than the 1,𝑚𝑎𝑥 1,max 1,𝑚𝑎𝑥 (a) n is not enough to reach 100% efficiency and (b) n is higher than the index modulation 1, max 1, max index modulation value needed to obtain 100% efficiency (overmodulation). value needed to obtain 100% efficiency (overmodulation). To test if the recorded holograms in Bayfol ® HX200 with efficiency lower than 100% To test if the recorded holograms in Bayfol HX200 with efficiency lower than 100% were overmodulated, the efficiency as a function of wavelength was measured with a were overmodulated, the efficiency as a function of wavelength was measured with a spectrophotometer (OCEAN OPTICS USB2000). If a maximum efficiency is obtained for a spectrophotometer (OCEAN OPTICS USB2000). If a maximum efficiency is obtained for a reconstruction wavelength higher than the recording one, the hologram is overmodu- reconstruction wavelength higher than the recording one, the hologram is overmodulated. lated. If the maximum efficiency is obtained for a reconstruction wavelength similar to If the maximum efficiency is obtained for a reconstruction wavelength similar to the the recording one, the efficiency vs. exposure curve would be similar to Figure 4a. The recording one, the efficiency vs. exposure curve would be similar to Figure 4a. The maximum efficiency value in this case depended theoretically on the dynamic range of maximum efficiency value in this case depended theoretically on the dynamic range of the the material (𝑛 ). 1, material (n ). 1, max If the spectral efficiency of the holograms recorded with each wavelength were not If the spectral efficiency of the holograms recorded with each wavelength were not overlapping—as suggested in Figure 3—if we illuminate a hologram recorded with one overlapping—as suggested in Figure 3—if we illuminate a hologram recorded with one of the three wavelengths with the laser light of the other two wavelengths, the hologram of the three wavelengths with the laser light of the other two wavelengths, the hologram would not be efficient. To test the performance of each hologram when the reconstruction would not be efficient. To test the performance of each hologram when the reconstruction was carried out with several wavelengths, photographs of the reconstructed diffusing was carried out with several wavelengths, photographs of the reconstructed diffusing object were taken. object were taken. 2.5. Polychromatic Calibration In a previous work [15], we studied different methods to record three reflection holograms with three different wavelengths in the same material. It was found that the method of simultaneous exposition with different exposure times for each wavelength provided the best results (similar efficiencies for every hologram, keeping them at the highest possible value). In the present work, we applied this method to transmission holograms; neutral filters were added when necessary to obtain exposure times of the same order of magnitude in each hologram and independent shutters allowed the operation of all the beams simultaneously, but the closing of each one at the desirable time for achieving the adequate exposure energy. To perform the polychromatic calibration, different exposure times for every laser were considered, keeping proportionality with the exposure energy values previously found in the monochromatic calibration for reaching 99% of the maximum theoretical efficiency. When three holograms were multiplexed in the same material, the efficiency of each hologram was lower than that of the one achieved with monochromatic exposure, because the maximum index modulation was distributed among the three wavelengths [15]. Due to this, the expected efficiency in polychromatic calibration was lower than the efficiency obtained for each hologram in monochromatic calibration. 𝑚𝑎𝑥 𝑚𝑎𝑥 𝑚𝑎𝑥 𝑚𝑎 𝑚𝑎𝑥 𝑚𝑎𝑥 Photonics 2021, 8, x FOR PEER REVIEW 8 of 17 2.5. Polychromatic Calibration In a previous work [15], we studied different methods to record three reflection holograms with three different wavelengths in the same material. It was found that the method of simultaneous exposition with different exposure times for each wavelength provided the best results (similar efficiencies for every hologram, keeping them at the highest possible value). In the present work, we applied this method to transmission holograms; neutral filters were added when necessary to obtain exposure times of the same order of magnitude in each hologram and independent shutters allowed the oper- ation of all the beams simultaneously, but the closing of each one at the desirable time for achieving the adequate exposure energy. To perform the polychromatic calibration, different exposure times for every laser were considered, keeping proportionality with the exposure energy values previously found in the monochromatic calibration for reaching 99% of the maximum theoretical efficiency. When three holograms were multiplexed in the same material, the efficiency of each hologram was lower than that of the one achieved with monochromatic exposure, be- cause the maximum index modulation was distributed among the three wavelengths [15]. Due to this, the expected efficiency in polychromatic calibration was lower than the Photonics 2021, 8, 465 8 of 16 efficiency obtained for each hologram in monochromatic calibration. 3. Results and Discussion 3. Results and Discussion 3.1. Monochromatic Calibration 3.1. Monochromatic Calibration Figure 5 shows the results of the calibration for the three wavelengths together with Figure 5 shows the results of the calibration for the three wavelengths together with the theoretical curve (Equation (8)) and the adjustment parameters. the theoretical curve (Equation (8)) and the adjustment parameters. (a) (b) (c) Figure 5. Efficiencies versus exposure and theoretical curve in the recording with each of the three wavelengths (a) 633 Figure 5. Efficiencies versus exposure and theoretical curve in the recording with each of the three wavelengths (a) 633 nm, nm, (b) 532 nm and (c) 442 nm. (b) 532 nm and (c) 442 nm. All three graphs present a threshold exposure below which the monomer did not polymerize and the hologram did not form. To carry out the theoretical adjustment, it was taken into account that the efficiency curve followed the dependence of Figure 4a, since overmodulation in each of the holograms had been ruled out with the help of the spectrophotometer. Figure 5 charts show that the maximum efficiency reached (both with the wave- length of 633 nm and with 532 nm) was 90%, while for 442 nm it only reached 60%. The efficiency did not reach 100% due to instabilities during the recording that caused the inter- ference figure to average, decreasing the contrast and, therefore, decreasing the registered index modulation. In the first tests carried out to obtain the calibration curve for the wavelength of 442 nm, very low efficiencies were obtained. This was due to the powerful fan of the He–Cd laser, which produced vibrations and air currents that caused the interference figure created in the recording step to average, significantly lowering efficiency. To avoid these inconveniences, the blue laser was isolated, placing it on a separate table. This partially solved the stability problems and higher efficiencies were achieved, although with a greater dispersion in the efficiency values obtained with this wavelength, as can be seen in Figure 5c. This dispersion is also due to the fact that the exposure times were longer, as the photopolymer had lower absorption on this wavelength. Photonics 2021, 8, 465 9 of 16 For long exposure times, the beams were more likely to experience some perturbation, decreasing the contrast of the interference in the recording and, therefore, decreasing the efficiency of the recorded hologram. For this reason, optimal samples were not made for exposures greater than 160 mJ/cm . However, the trend of the curve in Figure 5c shows a saturation at 60% efficiency. In the three calibrations, the maximum efficiency was achieved with an exposure energy that tended asymptotically to infinity (E ! ¥), due to refractive index modulation saturation; therefore, the exposure value was calculated for 99% of the maximum efficiency. Table 1 shows this value, as well as the maximum value of the refractive index modulation obtained with the theoretical setting. Table 1. Maximum relative efficiency, exposure energy at which 99% of the maximum efficiency was achieved and modulation of the maximum refractive index for the three wavelengths. l (nm) h (%) E for 0.99h (mJ/cm ) n max max 1,max 442 60 147.2 0.0062 532 90 24.2 0.0104 633 90 14.1 0.0125 At each wavelength, a maximum modulation of the refractive index was reached. Figure 6 shows the dependence of the index modulation with the exposure for each of the wavelengths, calculated from the experimental efficiency values with Equation (8), as well as its theoretical curve calculated with Equation (9). The ideal value of the index modulation would give an efficiency of 100%. However, as the maximum efficiency reached experimentally was lower, it also decreased with respect to the expected value. Table 2 shows both values, experimental maximum and expected maximum if 100% efficiency were achieved, for each of the three wavelengths. Figure 7 shows the spectrum of the diffracted beam for each of the three samples, calculated from the measured transmitted spectrum assuming that, as the holograms are volume holograms, there are only order 0 and order +1 of diffraction at the output of each hologram. Figure 7 illustrates how the three maxima do not overlap, which means that if the three holograms are recorded with the three wavelengths involved in the same material, they will behave as three independent holograms. Table 3 shows the width at half height of the experimental maxima compared to that theoretically calculated (Figure 3). The heights of the maxima in the experimental graph (Figure 7) are lower than the ones theoretically calculated (Figure 3) because (for the theoretical model) an efficiency of 100% was assumed, and the efficiency experimentally achieved was lower. To verify that the efficiency values were minimal when reconstructing the hologram with a different wavelength than the construction one, each of the three samples used to obtain the spectra of Figure 7 was illuminated with the different lasers, and + 1 and 0 orders were measured to calculate the relative efficiency for each wavelength according to Equation (2). These values are included in Table 4. Thus, when reconstructing the 633 nm hologram based on a wavelength other than that of the recording, the efficiency was lower, decreasing by 90% when reconstructing with 532 nm and 93.7% when reconstructing with 442 nm. The efficiency of the 532 nm hologram decreased by 94% when reconstructing with 633 nm and 89.9% when reconstructing with 442 nm. Finally, when reconstructing the 442 nm hologram based on the wavelength 633 nm, the efficiency decreased by 99.8%, and when reconstructing with 532 nm there was a reduction of 98.5%. The fact that the efficiency for other wavelengths (other than the construction one) was not totally zero can affect the reconstruction and indicates that the holograms were not totally independent. Photonics 2021, 8, x FOR PEER REVIEW 10 of 17 Photonics 2021, 8, 465 10 of 16 (a) (b) (c) Figure 6. Index modulation with respect to the exposure in the recording with each of the three wavelengths (a) 633 nm Figure 6. Index modulation with respect to the exposure in the recording with each of the three wavelengths (a) 633 nm (red) (b) 532 nm (green) and (c) 422 nm (blue). (red) (b) 532 nm (green) and (c) 422 nm (blue). The ideal value of the index modulation would give an efficiency of 100%. However, Table 2. Maximum modulation values of the refractive index for an ideal efficiency of 100% and that as the maximum efficiency reached experimentally was lower, it also decreased with obtained experimentally. respect to the expected value. Table 2 shows both values, experimental maximum and Laser n (h =1) n (h ) expected maximum if 100% efficiency wer 1 e a r chieved, for each of the 1 three wavelengths. r,experimental Blue (442 nm) 0.0111 0.0063 Table 2. Maximum modulation values of the refractive index for an ideal efficiency of 100% and Photonics 2021, 8, x FOR PEER REVIEW 11 of 17 Green (532 nm) 0.0133 0.0106 that obtained experimentally. Red (633 nm) 0.0159 0.0126 Laser 𝒏 (𝜼 = 𝟏 ) 𝒏 (𝜼 ) 𝟏 𝒓 𝟏 𝒓 ,𝒑𝒆𝒓𝒊𝒆𝒙𝒎𝒆𝒏𝒂𝒍𝒕 Blue (442 nm) 0.0111 0.0063 Green (532 nm) 0.0133 0.0106 Red (633 nm) 0.0159 0.0126 Figure 7 shows the spectrum of the diffracted beam for each of the three samples, calculated from the measured transmitted spectrum assuming that, as the holograms are volume holograms, there are only order 0 and order +1 of diffraction at the output of each hologram. Figure 7. Experimentally measured chromatic selectivity curves for the three maximum efficiency Figure 7. Experimentally measured chromatic selectivity curves for the three maximum efficiency sa samples mples o obtained btained in in the the monochr monoch omatic romatic calibration calibratio of n each of ea of ch the of wavelengths the wavelen (50 gthbetween s (50º bet beams). ween beams). Figure 7 illustrates how the three maxima do not overlap, which means that if the three holograms are recorded with the three wavelengths involved in the same material, they will behave as three independent holograms. Table 3 shows the width at half height of the experimental maxima compared to that theoretically calculated (Figure 3). Table 3. Full width at half maximum (FWHM) for each laser, both experimental (Figure 7) and theoretical (Figure 3) measurements. 𝐋𝐚𝐞𝐫𝐬 𝐅𝐖𝐇𝐌 ( ) 𝐅𝐖𝐇𝐌 ( ) 𝐓𝐑𝐎𝐇𝐄𝐄𝐓𝐈𝐂𝐀𝐋 𝐗𝐏𝐄𝐄𝐈𝐑𝐍𝐌𝐄𝐓𝐀𝐋 Blue (442 nm) 46.1 48.7 Green (532 nm) 66.5 68.6 Red (633 nm) 93.0 94.2 The heights of the maxima in the experimental graph (Figure 7) are lower than the ones theoretically calculated (Figure 3) because (for the theoretical model) an efficiency of 100% was assumed, and the efficiency experimentally achieved was lower. To verify that the efficiency values were minimal when reconstructing the hologram with a different wavelength than the construction one, each of the three samples used to obtain the spectra of Figure 7 was illuminated with the different lasers, and + 1 and 0 orders were measured to calculate the relative efficiency for each wavelength according to Equation (2). These values are included in Table 4. Table 4. Efficiency measured when each sample, recorded with a different wavelength, was illuminated with each wavelength. 𝝀 (nm) 𝟔𝟑𝟑 𝟓𝟑𝟐 𝟒𝟒𝟐 𝒓𝒐𝒊𝒓𝒆𝒄𝒅𝒏𝒈 𝝀 (nm) 633 532 442 633 532 442 633 532 442 𝒊𝒏𝒊𝒆𝒏𝒄𝒅𝒕 𝑣𝑎𝑡𝑖𝑅𝑒𝑙𝑒 𝑒𝑒𝑓𝑓𝑐𝑖𝑛𝑖𝑐𝑦 (%) 88.3 8.8 5.6 5.2 87 8.8 0.1 0.9 58 Thus, when reconstructing the 633 nm hologram based on a wavelength other than that of the recording, the efficiency was lower, decreasing by 90% when reconstructing with 532 nm and 93.7% when reconstructing with 442 nm. The efficiency of the 532 nm hologram decreased by 94% when reconstructing with 633 nm and 89.9% when recon- structing with 442 nm. Finally, when reconstructing the 442 nm hologram based on the wavelength 633 nm, the efficiency decreased by 99.8%, and when reconstructing with 532 nm there was a reduction of 98.5%. The fact that the efficiency for other wavelengths 𝐧𝐦 𝐧𝐦 Photonics 2021, 8, 465 11 of 16 Table 3. Full width at half maximum (FWHM) for each laser, both experimental (Figure 7) and theoretical (Figure 3) measurements. Laser FWHM (nm) FWHM (nm) THEORETICAL EXPERIMENTAL Blue (442 nm) 46.1 48.7 Green 532 nm 66.5 68.6 ( ) Red (633 nm) 93.0 94.2 Table 4. Efficiency measured when each sample, recorded with a different wavelength, was illumi- nated with each wavelength. l (nm) 633 532 442 recording l (nm) 633 532 442 633 532 442 633 532 442 incident Relative efficiency (%) 88.3 8.8 5.6 5.2 87 8.8 0.1 0.9 58 The images of the diffusing object obtained when the hologram recorded for each wave- length was illuminated with the recording wavelength (Figure 8a–c) have been photographed. Figure 8d–f show the images of the diffusing object obtained with the same holograms in Figure 8a–c respectively, simultaneously illuminated in the reconstruction with the three wavelengths. The appearance of a poorly efficient image can be clearly seen for wavelengths other than the construction wavelength. 3.2. Polychromatic Calibration Three holograms, each one with a different laser, were recorded in the same material (multiplexed hologram). Several multiplexed holograms were made, in which the exposure energy used for each wavelength was calculated, keeping proportionality with the expo- sure energy values previously found in the monochromatic calibration for reaching 99% of the maximum theoretical efficiency (Table 1). After several tests with neutral density filters in the red and the green lasers, the best results were achieved by including a neutral density filter of 0.8 in the optical path of the green laser (532 nm) to obtain similar efficiencies in the recording with the three wavelengths. The results obtained for three multiplexed holograms are shown in Figure 9. Each marker type in Figure 9 corresponds to the same multiplexed hologram and the color of the marker represents the wavelength of each individual hologram in the multiplexed hologram. The square markers correspond to a multiplexed hologram in which the proportionality value of the exposure energy used for each wavelength is ~75% of the exposure values shown in Table 1 (Multiplexed holo- gram, MH A). The circle markers correspond to a percentage value of ~50% (Multiplexed hologram, MH B) and the triangles correspond to ~25% (Multiplexed hologram, MH C). Figure 9 includes the mean efficiency values (h) and the standard deviation (s) for each multiplexed hologram. The standard deviation tended to decrease for the three cases studied and the mean efficiency had a peak in the case of MH-B and decreased strongly for MH-C. Therefore, the selected multiplexed hologram is MH-B, since its efficiency was the highest and the deviation was considered satisfactory. For this particular case, the opti- mum configuration could be obtained by considering the (h s) product to be maximum. However, this indicator could fail in some extreme cases (e.g., when the standard deviation is very small) and this limitation should be taken into account. Photonics 2021, 8, x FOR PEER REVIEW 12 of 17 (other than the construction one) was not totally zero can affect the reconstruction and indicates that the holograms were not totally independent. The images of the diffusing object obtained when the hologram recorded for each Photonics 2021, 8, 465 12 of 16 wavelength was illuminated with the recording wavelength (Figure 8a–c) have been photographed. (a) (b) (c) Photonics 2021, 8, x FOR PEER REVIEW 13 of 17 MH-B and decreased strongly for MH-C. Therefore, the selected multiplexed hologram is MH-B, since its efficiency was the highest and the deviation was considered satisfactory. For this particular case, the optimum configuration could be obtained by considering the (d) (e) (f) (𝜂 ̅ · σ) product to be maximum. However, this indicator could fail in some extreme cases Figure 8. Image of the diffusing object obtained when holograms recorded with each wavelength were illuminated with the Figure 8. Image of the diffusing object obtained when holograms recorded with each wavelength (e.g., when the standard deviation is very small) and this limitation should be taken into recording werwavelength: e illuminated (a )w 633 ithnm, the (r bec ) 532 ordi nm ngand wav (cel ) 442 engnm, th: (and a) 63 when 3 nmeach , (b) hologram 532 nm ais nd reconstr (c) 442 uc nted m, using and w thr hen ee lasers account. simultaneously: each holog (d r– af m ). is reconstructed using three lasers simultaneously: (d–f). Figure 8d–f show the images of the diffusing object obtained with the same holo- grams in Figure 8a–c respectively, simultaneously illuminated in the reconstruction with the three wavelengths. The appearance of a poorly efficient image can be clearly seen for wavelengths other than the construction wavelength. 3.2. Polychromatic Calibration Three holograms, each one with a different laser, were recorded in the same material (multiplexed hologram). Several multiplexed holograms were made, in which the expo- sure energy used for each wavelength was calculated, keeping proportionality with the exposure energy values previously found in the monochromatic calibration for reaching 99% of the maximum theoretical efficiency (Table 1). After several tests with neutral density filters in the red and the green lasers, the best results were achieved by including a neutral density filter of 0.8 in the optical path of the green laser (532 nm) to obtain sim- ilar efficiencies in the recording with the three wavelengths. The results obtained for three multiplexed holograms are shown in Figure 9. Each marker type in Figure 9 cor- responds to the same multiplexed hologram and the color of the marker represents the Figure 9. Efficiency as a function of exposure in polychromatic calibration for three multiplexed wavelength of each individual hologram in the multiplexed hologram. The square Figure 9. Efficiency as a function of exposure in polychromatic calibration for three multiplexed holograms. Each marker corresponds to a multiplexed hologram and each color of the same markers correspond to a multiplexed hologram in which the proportionality value of the holograms. Each marker corresponds to a multiplexed hologram and each color of the same marker marker corresponds to the efficiency obtained for each of the three holograms that compose one exposure energy used for each wavelength is ~75% of the exposure values shown in Ta- corresponds to the efficiency obtained for each of the three holograms that compose one multi- multiplexed hologram. ble 1 (Multiplexed plex hed olo h gr olo am gr,a m M . H A). The circle markers correspond to a percentage value of ~50% (Multiplexed hologram, MH B) and the triangles correspond to ~25% Table 5 shows that the best efficiency ratio between the holograms recorded with each Table 5 shows that the best efficiency ratio between the holograms recorded with (Multiplexed hologram, MH C). Figure 9 includes the mean efficiency values (𝜂 ̅) and the wavelength was obtained with an exposure corresponding to half of that necessary for ea achieving ch wavel 99% engt efh ficiency was oin btthe ained monochr with omatic an expo calibration. sure correspo This n result ding corr to h esponds alf of th to at the necessary standard deviation (σ) for each multiplexed hologram. The standard deviation tended to multiplexed hologram marked with circles in Figure 9 (MH-B). for achieving 99% efficiency in the monochromatic calibration. This result corresponds to decrease for the three cases studied and the mean efficiency had a peak in the case of the multiplexed hologram marked with circles in Figure 9 (MH-B). Table 5. Laser intensities, optical density of the filters used, intensity after filtering, exposure for which higher and similar efficiency were obtained for each wavelength and maximum efficiency obtained for each of the three wavelengths (MH-B). 𝐎𝐩𝐭𝐜𝐚𝐥𝐢 ( ) ( ) 𝝀 𝜼 % 𝑰 ( ) 𝑰 ( ) 𝑬 ( ) 𝒎𝒂𝒙 𝒊 𝒕 𝟐 𝟐 𝟐 𝐬𝐃𝐭𝐞𝐧𝐢𝐲 𝐜 𝐦 𝐜 𝐦 𝐜 𝐦 442 0.466 − 0.466 70 19.1% 532 0.438 0.8 0.076 12 25.9% 633 0.079 − 0.079 7 15.2% The efficiency reduction with respect to the monochromatic recording was as ex- pected, because the refractive index modulation was distributed among the three multi- plexed holograms. The difference between the maximum efficiencies obtained for each wavelength in the multiplexed hologram can lead to the color reproduction of the object being altered in the reconstruction when illuminating with white light. Depending on the application, it may be necessary to carry out more precise adjustment of efficiencies. Figure 10 shows the image of the diffusing object obtained by reconstructing the sample of Table 5 with 633 nm in Figure 10a, with 532 nm in Figure 10b, with 442 nm in Figure 10c and with all three simultaneously (Figure 10d). It was observed that, although when reconstructing with red light (Figure 10a) only the hologram recorded with red laser acted, when reconstructing with green light (Figure 10b) both green hologram 𝐧𝐦 𝐦𝐉 𝐦𝐖 𝐦𝐖 Photonics 2021, 8, 465 13 of 16 Table 5. Laser intensities, optical density of the filters used, intensity after filtering, exposure for which higher and similar efficiency were obtained for each wavelength and maximum efficiency obtained for each of the three wavelengths (MH-B). mW mW mJ l (nm) Optical Density h (%) I ( ) I ( ) E ( ) 2 t 2 max i 2 cm cm cm 442 0.466 0.466 70 19.1% 532 0.438 0.8 0.076 12 25.9% 633 0.079 0.079 7 15.2% The efficiency reduction with respect to the monochromatic recording was as expected, because the refractive index modulation was distributed among the three multiplexed holograms. The difference between the maximum efficiencies obtained for each wavelength in the multiplexed hologram can lead to the color reproduction of the object being altered in the reconstruction when illuminating with white light. Depending on the application, it may be necessary to carry out more precise adjustment of efficiencies. Figure 10 shows the image of the diffusing object obtained by reconstructing the sample of Table 5 with 633 nm in Figure 10a, with 532 nm in Figure 10b, with 442 nm in Figure 10c and with all three simultaneously (Figure 10d). It was observed that, although Photonics 2021, 8, x FOR PEER REVIEW 14 of 17 when reconstructing with red light (Figure 10a) only the hologram recorded with red laser acted, when reconstructing with green light (Figure 10b) both green hologram (larger circle at right) and red hologram (smaller circle at left) were reconstructed, and when (larger circle at right) and red hologram (smaller circle at left) were reconstructed, and reconstructing with blue light (Figure 10c) the three holograms acted. This indicates that when reconstructing with blue light (Figure 10c) the three holograms acted. This indi- the three holograms are not totally independent, producing a cross coupling effect between cates that the three holograms are not totally independent, producing a cross coupling them. As the images appeared separated in space, it is possible to suppress the unwanted effect between them. As the images appeared separated in space, it is possible to sup- images by modifying the recording angles or using different shading systems. Figure 10d press the unwanted images by modifying the recording angles or using different shading shows that the superposition of the images of the three holograms illuminated with the systems. Figure 10d shows that the superposition of the images of the three holograms illuminated with the three wavelengths gives a white-color image. three wavelengths gives a white-color image. (a) (b) (c) (d) Figure 10. Reconstruction of the multiplexed hologram MH-B of Table 5 with 633 nm (14 (a)), 532 Figure 10. Reconstruction of the multiplexed hologram MH-B of Table 5 with 633 nm (14 (a)), 532 nm nm (14 (b)), 442 nm (14 (c)) and with the three wavelengths (14 (d)). (14 (b)), 442 nm (14 (c)) and with the three wavelengths (14 (d)). In order to check the behavior of the multiplexed hologram, it was illuminated with In order to check the behavior of the multiplexed hologram, it was illuminated with continuous spectrum white light under Bragg’s condition for the three recording wave- continuous spectrum white light under Bragg’s condition for the three recording wave- lengths, and the transmitted beam (zero order) was measured by means of a spectro- lengths, and the transmitted beam (zero order) was measured by means of a spectropho- photometer. From this measurement, the diffracted order +1, which is shown in Figure tometer. From this measurement, the diffracted order +1, which is shown in Figure 11, 11, was calculated. The three peaks located at the recording wavelengths, 633 nm, 532 nm and 442 nm could be clearly observed. The efficiencies that can be seen in Figure 11 were slightly different from those indicated in Table 5 (which are measured using mono- chromatic light of each wavelength), which may be due to the superposition of the dif- ferent wavelengths and the cross-coupling effect. Photonics 2021, 8, 465 14 of 16 was calculated. The three peaks located at the recording wavelengths, 633 nm, 532 nm and 442 nm could be clearly observed. The efficiencies that can be seen in Figure 11 were Photonics 2021, 8, x FOR PEER REVIEW 15 of 17 slightly different from those indicated in Table 5 (which are measured using monochro- matic light of each wavelength), which may be due to the superposition of the different wavelengths and the cross-coupling effect. Fig Figure ure 1 11. 1. Diffr Diffracted acted spect spectr rum um o of f t the he sa sample mple w with ith t the he da data ta sh show own n in in Ta Table ble 5 5 when when the the illumination illumination was performed with continuous spectrum white light under Bragg’s condition. was performed with continuous spectrum white light under Bragg’s condition. 4. Conclusions 4. Conclusions In the present work, the Bayfol ®HX200 photopolymer has been characterized, for the In the present work, the Bayfol HX200 photopolymer has been characterized, for recording of full-color transmission volume holograms of diffusing objects. the recording of full-color transmission volume holograms of diffusing objects. In the monochromatic calibration of the material, efficiencies of 60% for the 442 nm In the monochromatic calibration of the material, efficiencies of 60% for the 442 nm holograms and 90% for the 532 nm and 633 nm holograms were achieved. The lower holograms and 90% for the 532 nm and 633 nm holograms were achieved. The lower maximum efficiency value for 442 nm holograms was due to stability problems caused by maximum efficiency value for 442 nm holograms was due to stability problems caused longer exposure times of the 442 nm laser. by longer exposure times of the 442 nm laser. The maximum modulation of the refractive index was calculated for each hologram, The maximum modulation of the refractive index was calculated for each hologram, obtaining the values of 0.0063 for 442 nm, 0.0106 for 532 nm and 0.0126 for 633 nm. These obtaining the values of 0.0063 for 442 nm, 0.0106 for 532 nm and 0.0126 for 633 nm. These values are less than the theoretical maximum value, since the maximum efficiency achieved values are less than the theoretical maximum value, since the maximum efficiency did not reach 100% due to intermodulation noise produced by the diffusing object. achieved did not reach 100% due to intermodulation noise produced by the diffusing The polychromatic calibration was carried out with the simultaneous exposure method object. with different exposure times. The maximum efficiencies achieved with this method were The polychromatic calibration was carried out with the simultaneous exposure obtained for exposure values corresponding to half of the exposure required to achieve method with different exposure times. The maximum efficiencies achieved with this maximum efficiency in monochromatic calibration, and a 0.8 neutral optical density filter method were obtained for exposure values corresponding to half of the exposure re- was included in the green laser path to equalize efficiencies. In this case, the multiplexed quired to achieve maximum efficiency in monochromatic calibration, and a 0.8 neutral hologram had efficiency values of 19.1% for the 442 nm hologram, 25.9% for the 532 nm optical density filter was included in the green laser path to equalize efficiencies. In this hologram and 15.2% for the 633 nm hologram. case, the multiplexed hologram had efficiency values of 19.1% for the 442 nm hologram, Despite the fact that both the calculations and the experimental measurements of the 25.9% for the 532 nm hologram and 15.2% for the 633 nm hologram. chromatic selectivity in monochromatic calibration indicated that the holograms recorded Despite the fact that both the calculations and the experimental measurements of the with the three lasers simultaneously in the same material could be considered independent, chromatic selectivity in monochromatic calibration indicated that the holograms rec- when reconstructing the multiplexed hologram with the three wavelengths, it was observed orded with the three lasers simultaneously in the same material could be considered in- that the holograms were not totally independent. dependent, when reconstructing the multiplexed hologram with the three wavelengths, To record multiplexed holograms in which the three holograms are independent, it it was observed that the holograms were not totally independent. would be necessary to obtain narrower color selectivity curves, which could be achieved by To record multiplexed holograms in which the three holograms are independent, it using a thicker material or increasing the angle between beams at the recording to narrow would be necessary to obtain narrower color selectivity curves, which could be achieved the chromatic selectivity curves, or separating the peak wavelengths as much as possible by using a thicker material or increasing the angle between beams at the recording to within the RGB (diode lasers). In order to obtain higher efficiencies, a thicker material or a narrow the chromatic selectivity curves, or separating the peak wavelengths as much as higher index modulation would be desirable to maximize n d product. possible within the RGB (diode lasers). In order to obtain higher efficiencies, a thicker material or a higher index modulation would be desirable to maximize n1· d product. Author Contributions: Conceptualization, M.S., J.M.-S., D.C., M.-V.C., J.A.; methodology, M.S., J.M.-S., D.C., M.-V.C., J.A.; experimental set up and results: M.S., J.M.-S., D.C., M.-V.C., J.A.; writ- ing—original draft preparation, writing—review and editing, M.S., J.M.-S., D.C., M.-V.C., J.A. All authors have read and agreed to the published version of the manuscript. Photonics 2021, 8, 465 15 of 16 Author Contributions: Conceptualization, M.S., J.M.-S., D.C., M.-V.C., J.A.; methodology, M.S., J.M.-S., D.C., M.-V.C., J.A.; experimental set up and results: M.S., J.M.-S., D.C., M.-V.C., J.A.; writing—original draft preparation, writing—review and editing, M.S., J.M.-S., D.C., M.-V.C., J.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the “Diputación General de Aragón-Fondo Social Europeo” (TOL research group, E44_17R), the “Generalitat de Catalunya” (2017FI_B2_00127 and 2017 SGR 1276) and the “Ministerio de Ciencia e Innovación” (PID2019-108598GB-I00, PID2019-111536RB-I00 and PID2021-114311RA-I00). Acknowledgments: The authors would like to thank Covestro Deutschland AG for supplying the recording photopolymer material. Daniel Chemisana thanks ICREA for the ICREA Academia award. Conflicts of Interest: The authors declare no conflict of interest. References TM 1. 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Photonics
Multidisciplinary Digital Publishing Institute
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