**4.7.2 Angular multiplexing holograms**

Using the optical setup shown in Fig.28, the angular multiplexing holographic storage is realized. In the reference beam path, lenses L6 and L7 comprise a 4f system. Rotatable mirror M2 and sample are located at the front focal plane of L6 and at the back focal plane of L7. When the mirror M2 rotates within a certain range round its shaft, the exposure position of the reference beam on the sample does not move with the rotation of M2, but only the incident angle will change, so angular multiplexing can be performed. The angle between the object wave and the normal of the sample and the angle between the centre line of

High-density high-capacity volume holographic multiplexing is one of the most attractive aspects of holographic data storage. The study on holographic multiplexing storage application of fulgide materials is very limited. Using the optical setups shown in Fig.28, polarization multiplexing, angular multiplexing and rotational multiplexing holographic

Polarization multiplexing is based on the photo-induced anisotropic property of the material, at the same location of sample different kinds of polarization holograms are recorded. Todorov et al were first to show that two holographic recording could be stored independently inside the same film when using different combinations for the polarization states of the reference and the object beam during recording. Su et al presented a technique for polarization multiplexing in LiNbO3. Koek et al have presented a technique for

Here different kinds of polarization multiplexing holographic storage were realized in fulgide film, including linearly polarization multiplexing and circularly polarization multiplexing, optical setup as shown in Fig.28. In circularly polarization multiplexing experiments, at the same location of the sample parallel circularly polarization hologram and orthogonal circularly polarization hologram were recorded. By adjusting the Q3 and P, the diffraction images of parallel circularly polarization hologram and of orthogonal circularly polarization hologram can be obtained individually and together, like shown in Fig.35. So it can be seen that the storage density can be doubled by using polarization

**4.7 Different kinds of multiplexing holographic storage** 

simultaneous readout polarization multiplexing in bacteriorhodopsin.

(a) (b) (c)

orthogonal circular polarization hologram; (c) of both holograms

**4.7.2 Angular multiplexing holograms** 

Fig. 35. The reconstruction images in polarization multiplexing holographic storage in fulgide film: reconstruction image (a) of parallel circular polarization hologram; (b) of

Using the optical setup shown in Fig.28, the angular multiplexing holographic storage is realized. In the reference beam path, lenses L6 and L7 comprise a 4f system. Rotatable mirror M2 and sample are located at the front focal plane of L6 and at the back focal plane of L7. When the mirror M2 rotates within a certain range round its shaft, the exposure position of the reference beam on the sample does not move with the rotation of M2, but only the incident angle will change, so angular multiplexing can be performed. The angle between the object wave and the normal of the sample and the angle between the centre line of

experiments are carried out initially.

multiplexing.

**4.7.1 Polarization multiplexing holograms** 

reference wave and the normal of the sample are both 45°. In the angular multiplexing, the angle between two reference beams corresponding to two neighboring holograms should be greater than the minimum horizontal selection angle ΔΘ[10]:

$$
\Delta\Theta = \frac{2\sqrt{\pi^2 - \nu^2}\lambda}{\pi nd} \frac{\cos\theta\_s}{|\sin(\theta\_r - \theta\_s)|}\tag{2}
$$

Where /( cos cos ) ν = Δπ λ θθ *nd r s* . In this experiment, the λ=633nm, the thickness of the sample is *d*=10μm, the refractive index of the sample is about *n*≈1.5, and for our sample at 633nm the refractive index difference between E-form and C-form is Δ*n*≈1.7×10-2, θr=45°, θs=-45°, so it can be calculated that ΔΘ≈3.16°. Because limited by the size of lens L6 and L7, the rotation range of M2 is Δαmax=8°, so the corresponding reference beam maximum multiplexing angle range is Δθmax=16°. Therefore, in this experiment, the angle between two reference beams corresponding to two neighboring holograms is chosen as Δθ=4°. Five images were multiplexing recorded respectively with exposure time: 20s, 18s, 16s, 14s and 12s.

Reading order of diffracted images is reverse to the recording order. The read out time of each image is 0.2s. The results are shown in Fig.36. It can be seen that, no crosstalk exist between five images and the images' qualities are good. So the storage density can be increased 4 times.

Fig. 36. The experimental results of angular multiplexing holograms

Because each time when a new hologram is recorded, the hologram recorded before will be erased. In multiple holograms recording, the erase to the first image is largest and the last recorded image is not affected. If each hologram is recorded for the same time, diffraction efficiency of the first hologram will be lowest, and the diffraction efficiency of final hologram will be highest. Therefore, in order to obtain same diffraction efficiency for the holograms, the exposure time should be reduced with the increasing of the recording order of holograms. The simple diagram is shown in Fig.37.

Fig. 37. A simple diagram to explain the exposure time of each hologram in holographic multiplexing: (A) condition with equivalent recording time; (b) condition with decreased recording time

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 175

Holographic interferometer is an important aspect of holographic application. At present no reports about the application of fulgide films on holographic interferometer is found. In the section, using the current experimental setups the holographic interferometer application experiment of fulgide film are carried out, including double exposure method and single

The experimental setup shown in Fig.28 is used for holographic interferometer application experiment of fulgide film. The recording medium is placed near the frequency-plane. The process of double exposure experiment is as follows: Two holograms are recorded early or late at the same place of the sample by using same setup. When reconstructed by original reference beam, two object waves will be reconstructed together and interference with each other, whose interference fringe can be captured by the CCD. The process of real-time holographic interferometer (single exposure method) experiment is as follows: A hologram of original object beam (OB1) is recorded first. Then the object wave surface to be tested (OB2, whose intensity is decreased to similar with the diffraction light intensity) and the reference beam irradiate the hologram at the same time. The interference fringe between the diffracted wave of OB1 and transmitted wave of OB2

According to the theoretical analysis and experimental results, it can be understood that the experimental results of double exposure interferometer and real-time interferometer are same, and their processing methods are same too. So here just four experimental results of

Using data processing, the thickness variety values of measured objects at any point on the X-Y surface can be got. In Fig.40 shows the thickness variety of measured objects got by processing the experiment results shown in Fig.39(a,b). And the tilt angles of the optical wedge and of axicon are calculated as 1.83°and 1° respectively. Form Fig.39(c,d) it can be calculated that the rotation angle of optical wedge is 7.91°and the movement of axicon is

(a) (b) (c) (d)

Fig. 39. Experimental results of double exposure interferometer: (a) no object( air ) in the first exposure and the wedge is the object to be tested in the second exposure; (b) no object( air ) in the first exposure and the axicon is the object to be tested in the second exposure; (c) the wedge is the object, which is rotated slightly in two exposure; (d) the axicon is the object,

**4.8 Holographic interferometer based on fulgide film** 

double exposure interferometer are given, like shown in Fig.39.

which is moved slightly toward right-or-left in two exposure

exposure method.

can be captured by the CCD.

1.86 mm [11].

#### **4.7.3 Circumrotation multiplexing holograms**

Circumrotation multiplexing is a special angular multiplexing method [10]. In basic circumrotation multiplexing light path of two recording beams unchanged. Every time when a new hologram is recorded, recording medium is rotated at an angle around an axis perpendicular to sample's surface. Fig.38a is a diagram of circumrotation multiplexing holograms, i.e. a diagram of the grating vector directions. The x-y plane is material's surface, and z axis is the rotational axis. To avoid the merger of gratings, the rotation angle of the medium must meet one of the following two conditions: (1) which is more than vertical selection angle; (2) the unnecessary reconstructed images can be filtered by the aperture of detector. The phase conjugated beam reconstruction hologram storage system shown in Fig.24 was used for the circumrotation multiplexing holograms storage experiment. According to the experimental condition, rotation angle is chosen as 10°, after 180° of rotation, 18 images are recorded at the same position of the sample. In Fig.38b, the reconstructed images of the holograms recorded at 0° and 170°are given. It can be seen that no crosstalk exist between the holograms, but reconstructed images' quality is poor.

Fig. 38. Circumrotation multiplexing holograms storage experiment: (a) schematic diagram; (b) experimental results

### **4.8 Holographic interferometer based on fulgide film**

174 Holograms – Recording Materials and Applications

Circumrotation multiplexing is a special angular multiplexing method [10]. In basic circumrotation multiplexing light path of two recording beams unchanged. Every time when a new hologram is recorded, recording medium is rotated at an angle around an axis perpendicular to sample's surface. Fig.38a is a diagram of circumrotation multiplexing holograms, i.e. a diagram of the grating vector directions. The x-y plane is material's surface, and z axis is the rotational axis. To avoid the merger of gratings, the rotation angle of the medium must meet one of the following two conditions: (1) which is more than vertical selection angle; (2) the unnecessary reconstructed images can be filtered by the aperture of detector. The phase conjugated beam reconstruction hologram storage system shown in Fig.24 was used for the circumrotation multiplexing holograms storage experiment. According to the experimental condition, rotation angle is chosen as 10°, after 180° of rotation, 18 images are recorded at the same position of the sample. In Fig.38b, the reconstructed images of the holograms recorded at 0° and 170°are given. It can be seen that

no crosstalk exist between the holograms, but reconstructed images' quality is poor.

X

Z

Y

*k*1

(a)

0° 170° (b)

Fig. 38. Circumrotation multiplexing holograms storage experiment: (a) schematic diagram;

(b) experimental results

*k*2 *k* α

**4.7.3 Circumrotation multiplexing holograms** 

Holographic interferometer is an important aspect of holographic application. At present no reports about the application of fulgide films on holographic interferometer is found. In the section, using the current experimental setups the holographic interferometer application experiment of fulgide film are carried out, including double exposure method and single exposure method.

The experimental setup shown in Fig.28 is used for holographic interferometer application experiment of fulgide film. The recording medium is placed near the frequency-plane. The process of double exposure experiment is as follows: Two holograms are recorded early or late at the same place of the sample by using same setup. When reconstructed by original reference beam, two object waves will be reconstructed together and interference with each other, whose interference fringe can be captured by the CCD. The process of real-time holographic interferometer (single exposure method) experiment is as follows: A hologram of original object beam (OB1) is recorded first. Then the object wave surface to be tested (OB2, whose intensity is decreased to similar with the diffraction light intensity) and the reference beam irradiate the hologram at the same time. The interference fringe between the diffracted wave of OB1 and transmitted wave of OB2 can be captured by the CCD.

According to the theoretical analysis and experimental results, it can be understood that the experimental results of double exposure interferometer and real-time interferometer are same, and their processing methods are same too. So here just four experimental results of double exposure interferometer are given, like shown in Fig.39.

Using data processing, the thickness variety values of measured objects at any point on the X-Y surface can be got. In Fig.40 shows the thickness variety of measured objects got by processing the experiment results shown in Fig.39(a,b). And the tilt angles of the optical wedge and of axicon are calculated as 1.83°and 1° respectively. Form Fig.39(c,d) it can be calculated that the rotation angle of optical wedge is 7.91°and the movement of axicon is 1.86 mm [11].

Fig. 39. Experimental results of double exposure interferometer: (a) no object( air ) in the first exposure and the wedge is the object to be tested in the second exposure; (b) no object( air ) in the first exposure and the axicon is the object to be tested in the second exposure; (c) the wedge is the object, which is rotated slightly in two exposure; (d) the axicon is the object, which is moved slightly toward right-or-left in two exposure

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 177

**4.8.1 Application of polarization multiplexing technique in holographic interferometer**  The object to be measured is moving particle. The parallel linearly polarization hologram and orthogonal linear polarization hologram are recorded respectively, and then the two images can be reconstructed individually owing to polarization multiplexing technique, in addition the recorded order of the two holograms is known, so the direction and velocity of the object's movement can be got. But we cannot distinguish the recorded order of the two holograms if they are both recorded by parallel linearly polarization holographic technology. Fig.41 shows the experimental results (because for real moving particles the dispersion of light is too large, the transparent film is used as the measured

(a) (b) (c)

obtained when the polarization P is rotated to 30°

Fig. 41. The double exposure experimental results to record movement track of the particles at different times (a) 1st image recorded as parallel linearly polarization hologram; (b) 2nd image recorded as orthogonal linearly polarization hologram; (c) reconstructed image

The holographic storage applications of 3-indoly-benzylfulgimide/PMMA film were studied in detail including the ordinary holography and polarization holography, which are respectively based on the photochromic and photoinduced anisotropy properties. The properties of holographic recording such as diffraction efficiency, spatial resolution and optimal exposure were measured; especially the diffraction efficiency spectra and dynamic curves of different kinds of polarization holographic recording were theoretically analyzed

The holographic optical image storage was realized in the fulgide films by using different kinds of holographic storage techniques. The experimental results show that: compared with transmission-type holographic recording, reflection-type holographic recording hologram has lower diffraction efficiency and higher SNR; compared with reference beam reconstruction,the phase conjugated beam reconstruction can effectively correct the phase aberration caused by the mis-adjustment of optical setup; compared with Fraunhofer holograms, Fourier-transform holograms have lower diffraction efficiency and higher storage density; compared with parallel polarization holograms, in orthogonal polarization holograms the scattering noise can be filtered, so one can obtain high SNR, in which the orthogonal circularly polarization hologram also has high diffraction efficiency, so it is the best polarization recording method; compared with traditional non-collinear holography, the collinear holographic storage system has simpler optical setup and smaller volume,

object.)

**5. Conclusion** 

and experimentally measured.

Fig. 40. (a) thick variety of optical wedge, (b) thick variety of axicon

#### **4.8.1 Application of polarization multiplexing technique in holographic interferometer**

The object to be measured is moving particle. The parallel linearly polarization hologram and orthogonal linear polarization hologram are recorded respectively, and then the two images can be reconstructed individually owing to polarization multiplexing technique, in addition the recorded order of the two holograms is known, so the direction and velocity of the object's movement can be got. But we cannot distinguish the recorded order of the two holograms if they are both recorded by parallel linearly polarization holographic technology. Fig.41 shows the experimental results (because for real moving particles the dispersion of light is too large, the transparent film is used as the measured object.)

Fig. 41. The double exposure experimental results to record movement track of the particles at different times (a) 1st image recorded as parallel linearly polarization hologram; (b) 2nd image recorded as orthogonal linearly polarization hologram; (c) reconstructed image obtained when the polarization P is rotated to 30°

### **5. Conclusion**

176 Holograms – Recording Materials and Applications

0

x / mm y / mm

(a)

0 1 2 3 4 0 0.05 0.1 0.15 0.2

0

y / mm x / mm

(b)

2

Fig. 40. (a) thick variety of optical wedge, (b) thick variety of axicon

4 0 0.01 0.02 0.03 0.04 0.05 0.06

d / mm

d / mm

<sup>1</sup> <sup>2</sup>

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup>

3 4 5

> The holographic storage applications of 3-indoly-benzylfulgimide/PMMA film were studied in detail including the ordinary holography and polarization holography, which are respectively based on the photochromic and photoinduced anisotropy properties. The properties of holographic recording such as diffraction efficiency, spatial resolution and optimal exposure were measured; especially the diffraction efficiency spectra and dynamic curves of different kinds of polarization holographic recording were theoretically analyzed and experimentally measured.

> The holographic optical image storage was realized in the fulgide films by using different kinds of holographic storage techniques. The experimental results show that: compared with transmission-type holographic recording, reflection-type holographic recording hologram has lower diffraction efficiency and higher SNR; compared with reference beam reconstruction,the phase conjugated beam reconstruction can effectively correct the phase aberration caused by the mis-adjustment of optical setup; compared with Fraunhofer holograms, Fourier-transform holograms have lower diffraction efficiency and higher storage density; compared with parallel polarization holograms, in orthogonal polarization holograms the scattering noise can be filtered, so one can obtain high SNR, in which the orthogonal circularly polarization hologram also has high diffraction efficiency, so it is the best polarization recording method; compared with traditional non-collinear holography, the collinear holographic storage system has simpler optical setup and smaller volume,

**8** 

**Three-Dimensional Vector Holograms in** 

Osamu Hanaizumi1, Nobuhiro Kawatsuki3 and Hiroshi Ono2

*2Department of Electric Engineering, Nagaoka University of Technology,* 

*3Department of Materials Science and Chemistry, Graduate School of Engineering,* 

Polarization gratings, in which optical anisotropy is periodically modulated, are very attractive from the point of view of their interesting optical properties, including polarization selectivity of the diffraction efficiency and polarization conversion in the diffraction process (Nikolova et al., 1984). These properties make polarization gratings useful for numerous optical applications related to polarization discrimination (Davis et al., 2001; Asatryan et al., 2004), control (Nikolova et al., 1997; Hasman et al., 2002), and measurement (Gori, 1999; Provenzano et al., 2006). Polarization gratings can be fabricated by holographic exposure with polarized interference light in photoanisotropic media such as azobenzene-containing polymer (azopolymer) films (Todorov et al., 1984; Ebralidze et al., 1992; Huang and Wagner, 1993; Samui, 2008). Since azobenzene molecules reorient in accordance with polarization of light resulting from trans-cis-trans photoisomerization reactions, periodic structures are formed as spatial distribution of the molecular orientation by holographic recording using vectrial light (i.e., by vector holography). In addition, surface relief gratings are obtained by holographic recording in azopolymers (Rochon et al., 1995; Kim et al., 1995; Ramanujam et al., 1996; Naydenova et al., 1998; Labarthet et al., 1998, 1999). Nikolova et al. have investigated the diffraction properties of various types of vector holograms recorded in side-chain azobenzene polyesters (Nikolova et al., 1996). Birabassov and Galstian have analyzed the relationship of polarization states between holographic recording and reconstruction beams with the use of azopolymers (Birabassov et al., 2001). It has been also demonstrated that vector holograms realize recording and reconstruction of polarized optical images (Ono et al., 2009a, 2009b). In a previous paper, we presented three-dimensional (3D) vector holography, which is a novel concept for holographic recording using vectrial light (Sasaki et al., 2008). Common vector holograms are recorded in initially isotropic media such as amorphous azopolymers. In contrast, 3D vector holograms are recoded in initially anisotropic media. Since the propagation velocity of light in anisotropic media varies in accordance with the polarization state, standing waves with a multidimensionally varying state of polarization were obtained in the region of overlap. We recorded anisotropic gratings in a liquid crystalline medium by

**1. Introduction** 

**Photoreactive Anisotropic Media** 

Tomoyuki Sasaki1, Akira Emoto2, Kenta Miura1,

*Graduate School of Engineering, Gunma University,* 

*1Department of Electronic Engineering,* 

*University of Hyogo,* 

*Japan* 

lower environmental demand and higher storage density. The storage density of 2×108 bits/cm2 was obtained in the Fourier-transform holographic data storage by using orthogonal polarization holographic recording, which had a greatly improved signal-tonoise ratio of the diffraction image.

Different kinds of multiplexing holographic storage, like polarization multiplexing, circumrotation multiplexing and angle multiplexing, were also realized in fulgide film, where 2 images, 5 images and 18 images were stored at the same position of the film, and diffracted without crosstalk with each other. And the application of the fulgide films in holographic interferometry was also studied initially.

#### **6. References**

