1. Introduction

Recent technological development for three-dimensional (3D) television have attracted many people, because the true 3D display provides us a realistic world just as it exists in front of us. Holography invented by Gabor [1] is a unique technique to display the true 3D objects recorded in the media. Conventional holograms are permanently recorded in the media such as silver halides, photopolymers, and dichromated gelatin [2]. After recording in the media and via processing of several steps required, these holograms are reconstructed with coherent light such as a laser or incoherent light sources of LEDs. However, these media lack the capability of image-updating, resulting in the limitation of use. On the contrary, photorefractive polymers [3,4] are the promising media for providing updatable or rewritable holograms [5,6]. Thus, using photorefractive polymers, updatable real-time three-dimensional holograms can be recorded and simultaneously reconstructed or replayed. Spatial resolution of the conventional holograms is more than 5000 lines/mm [7], which is surpassed the resolution of the recently developed digital holographic devices with a CCD or a spatial light modulator (SLM). Peyghambarian’s group had successfully demonstrated that the holographic stereogram was recorded and reconstructed in photorefractive polymer composite for 2 seconds with hogel images under the pulse laser system operation [6]. In our previous paper [8,9], we have demonstrated the real-time holographic display device fabricated by the polymer composite based on 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) (30 wt%) doped transparent poly(methylmethacrylate) (PMMA) and showed the capability of recording and displaying new hologram of object within a few seconds and viewing for a long time at ambient condition.

In this paper, we present fully updatable holographic 3D display system using a holographic stereographic technique with a transparent optical device of PMMA doped organic compound of NACzE. 100 elemental holograms which are a series of pictures of object took from different angles can completely reproduce updatable entire hologram of object.

2. Experimental Sections

Figure 1 shows the schematic diagram of updatable 3D holographic display system. Laser source is a palmtop diode-pumped solid-state (DPSS) CW laser (Samba^TM 1500, linewidth <1 MHz, 1.5 W at 532 nm, Cobolt, Sweden). CW laser beam was expanded using a combination of an object lens and a plano-convex lens. s-Polarized expanded beam split off by a beam splitter work as a reference beam and p-polarized beam transmitted through a beam splitter is reflected on a spatial light modulator (SLM, Holoeye LCR-1080 with 1920 x 1200 pixel resolution and 8.1 μm pixel size). Reflected beam work as an object beam. Polarization of a reflected beam from SLM is changed to s-polarization. Object image from SLM was projected on a diffuser, which was final object image for hologram. The distance between diffuser (holographic plane) and polymer composite (view plane) is 300 mm. Reference beam was further expanded using a combination of two plano-convex lenses with different focus length. Expanded beam was reshaped as stripe form using a combination of two cylinderical lenses. Object and reference beams are interfered on a polymer composite (view plane) and recorded hologram is simultaneously reconstructed by a palmtop s-polarized yellow-orange DPSS laser (Mambo^TM, linewidth <1 MHz, 100 mW at 594 nm, Cobolt, Sweden) or a palmtop s-polarized red probe beam (Omicron-Laserage semiconductor laser PhoxX 642, 140 mW at 642 nm). Typical intensity of an object beam is 22 mW and unit intensity of reference beam is 220 mW cm⁻². Relatively thick sample device with thickness ca. 50 μm sandwiched between two glass plates of 100 mm × 100 mm size is shown in Fig. 2(a). Thin sample device fabricated using a spin coating technique on a glass plate with 100 mm × 100 mm size is also shown in Fig. 2(b).

 

Fig. 1 Schematic diagram of updatable 3D holographic display system. Laser sources are a green CW laser at 532 nm for recording and a yellow-orange laser at 594 nm or a red laser at 642 nm for reading. Image on SLM is projected on a diffuser and it is used as an object image. Collimated reference beam is adjusted as stripe beam using two cylindrical lenses.

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Fig. 2 Pictures of sample device. (a) Relatively thick sample device with thickness ca. 50 μm sandwiched between two glass plates of 100 mm × 100 mm size. (b) Thin sample device using spin coating technique on a glass plate with 100 mm × 100 mm size.

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Holographic stereography is used to reconstruct updatable 3D hologram in a device. Series of images of object on a rotational stage were captured by a CMOS camera (2M pixels, 40 fts) with a Tamron lens using SGVIEW (Sigma Koki, Japan) software operated on a computer. To get a horizontal parallax hologram, simultaneously rotating object, device equipped on a translational stage was moved every 0.6 or 0.8 mm interval and focused object image from SLM and stripe reference beam (60 mm long and 2 mm width) was interfered in a device every 600 ms including the time for moving translational and rotational stages. In this case total recording time for 100 steps is 60 s. Schematics of this procedure is shown in Fig. 1. To obtain holographic stereogram using the series of image of object already taken from different angles, we have developed the software, named holographic stereogram, KIT, to record the elemental hologram of object and to move translational stage. This software can provide minimum time for recording is 1 ms, but because of the limitation of the response time of liquid crystalline display and SLM, actual response time is limited 20 ms and above. Using this software, we can provide minimum holographic recording time for one elemental hologram, 20 ms for recording and 80 ms for stage translation.

3. Results and discussion

In previous reports [5,6] holographic display was demonstrated using holographic stereogram with computer calculated holographic pixel known as “hogel” and photorefractive polymeric material applying high electric voltage of 4 kV. Their system is epoch-making for demonstrating the updatable 3D holographic images. However, in their system, data processing requires computer algorism for calculating “hogel” and it has high risk of dielectric breakdown due to applying high voltage. On the other hands, our present holographic system requires no external electric power and no data processing to calculate elemental hologram. These are superior merit for building-up updatable 3D holographic display system.

Pictures of holographic stereogram of object consisted of 100 elemental hologram recorded in 50 μm thickness device is shown in Fig. 3. Each picture shows simultaneously reconstructed hologram when the elemental hologram is recorded. The number i picture is the reconstructed hologram when the first elemental hologram is recorded, and number ii picture is that when the eleventh elemental hologram is recorded. The picture of numbering iii – x was the reconstructed hologram further every eleventh elemental holograms was recorded. Quick over-recording or replacement by new hologram is important factor for updatable holographic display system. Using software we have developed, fast recording of elemental hologram and quick update hologram was demonstrated. Recording elemental hologram and simultaneously reconstructing holographic stereogram from series of elemental holograms is shown in Fig. 4. Pictures numbering i – vi are the reconstructed holograms for the recorded elemental hologram of first object and the following pictures vii – xii is those for the next object. Picture was taken every twentieth elemental hologram. Immediately after recording one holographic stereogram, another holographic stereogram can be over-recorded without erasing. Media 1 shows these dynamic recording, simultaneous replaying hologram and over-recording. Further it can be seen that camera movement demonstrates the 3D nature of the recorded hologram. It took 300 ms for recording each elemental hologram and 80 ms stage translation. Total time for the recording 100 elemental holograms is 38s. Recorded updatable 3D hologram can be viewable for up to a couple of hours directly on a device without any eye glasses and other tools to magnify images. Hologram can be easily refreshed by overwriting without erasing process.

 

Fig. 3 Pictures of holographic stereogram of object consisted of 100 elemental hologram recorded in 50 μm thickness device. The number i picture is the reconstructed hologram when first elemental hologram is recorded, and number ii picture is that when eleventh elemental hologram is recorded. The picture of numbering iii – x was the reconstructed hologram further every eleven elemental holograms was recorded.

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Fig. 4 Pictures of holographic stereogram of two objects continuously recorded. Pictures numbering i – vi are the reconstructed holograms for the recorded elemental hologram of the first object and the following pictures vii – xii is those for the next object. Pictures were taken every twenty elemental hologram (Media 1).

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To apply the present technique and device to commercial product, easy fabrication of devices is demanded. Spin coating technique and roll to roll fabrication technique are preferred to make the large size devices. The possibility of spin coated device to the holographic display application is investigated. As shown in Fig. 2(b), we prepared spin coated film device for holographic display. Device thickness is 2 – 3 μm. Normal spin coating technique is used: 14 wt % of DMF solution was spin coated at 1000 rpm for 7 s followed by at 2000 rpm for 7 s. After spun, device was dried at 100 °C for 12 h. Sample device has enough absorbance of 1.1 at 532 nm. The same procedure shown in Fig. 4 is applied using spin coated holographic device. We could confirm that spin coated devices also work as an updatable holographic devices. These dynamic recording and simultaneous replaying hologram and over-recording are shown in Media 2. In the case of spin coated device, 200 ms is enough for recording each elemental hologram and total recording time for 100 elemental holograms is 28 s. A longer recording time leads to brighter hologram.

It is noted that NACzE/PMMA holographic device has superior feature that updatable 3D hologram can be recorded every 200 - 300 ms/step (20 s - 30 s for total 100 steps) with only interference beams. Conventional organic photorefractive polymer device requires applied electric field up to 40 - 60 V μm⁻¹ to achieve effective diffraction efficiency and fast response time. In that case, fast response time is a merit for holographic reconstruction of object images on a SLM with a video refresh rate [10]. However, fast response time usually loses the memory effect of hologram imaging. Namely quick response of hologram imaging means quick change of hologram imaging. For the holographic imaging of signage applications, memory effect of hologram is required. The present material is suitable for such holographic display. Furthermore, the performance of the device has no change even though one and half years had passed after fabrication. Our system works using palmtop CW lasers, it allows us compact design of system.

4. Conclusions

Our successful demonstration of real-time holographic display using quickly updatable holographic medium will open a door for the large size optical devices targeted on 3D digital signage and true real-time 3D holographic display without any special eye-glasses.