**4.1.2 Results and discussions**

164 Holograms – Recording Materials and Applications

**reconstruction hologram (including transmission type hologram and reflection type** 

The experimental setups for non-collinear holographic storage are shown in Fig.23and Fig.24. Fig.23 is the experimental setup of reference beam reconstruction holography. Fig.24 is the experimental setup of conjugated beam reconstruction holography. In every one of these two optical setups, the transmission type holographic recording and reflection type

**Mask**

**BE L1**

**IR**

**Mask**

**IR**

**BS**

**IC IO A1**

**P4** 

**ID**

**L1**

**L2**

**P3**

**P1** 

**A2**

Fig. 24. Experimental setup of conjugated beam reconstruction holography

**P2**

In Fig.23, the reference beam reconstruction is used. The nonpolarized He-Ne laser(632.8nm, 3mW) is split into three beams by beam splitters BS1 and BS2. The three beams are object beam *I*O and two phase conjugated beams *I*R and *I*C. In transmission type holographic recording, reference beam is *I*R,which is *I*C in reflection type holographic recording. The signal of object beam is loaded by the Mask, which is imaged on the CCD's photosensitive surface by the positive lens L1 and L2. The focused *I*O beam cross with reference beams after the focus of L1, where a recording medium, the fulgide/PMMA film, has been placed for recording the hologram. In reconstruction process, using the reference beam as reconstruction beam, diffractive image is captured by the CCD. The shutter S is used to control the exposure time (the best exposure time is 10s in this experiment). The

**BE**

**A3**

**M3** 

**M3**

**Sampl**

**IC**

**CCD P4 L2**

**Sample**

**P3**

**A3**

**P1** 

**A2 P2**

Fig. 23. Experimental setup of reference beam reconstruction holography

**BS2**

**BS2**

**S A1 IO**

**BS1**

**M2**

**M1**

**BS1**

**M2**

**M1**

**4.1 Reference beam reconstruction hologram and phase conjugated beam** 

**4. Holographic image storage in fulgide film** 

holographic recording both can be realized.

**He-Ne laser**

**He-Ne laser**

**hologram)** 

**4.1.1 Experiment methods** 

#### **4.1.2.1 Transmission type hologram and reflection type hologram**

In Fig.25, the reconstructed images of parallel linearly polarized transmission type hologram and reflection type hologram (recorded in setup shown in Fig.23) are shown.

Fig. 25. The reconstructed images of parallel linear polarized transmission hologram and reflection hologram: (a) reconstructed image of transmission recording hologram; (b) reconstructed image of reflection recording hologram

It can be seen that: compared with transmission-type hologram, reflection-type hologram has higher SNR. This is because that the noise in the reconstructed image of transmissiontype hologram is come from the forward scattering, but that of reflection-type holographic recording hologram is come from the backward scattering. Usually the forward scattering is always larger than backward scattering, so the reflection-type hologram has smaller noises. But reflection-type hologram has lower diffraction efficiency.

#### **4.1.2.2 Reference beam reconstruction hologram and phase conjugated beam reconstruction hologram**

In Fig.26, the reconstructed images of reference beam reconstruction hologram and conjugated beam reconstruction hologram are shown. It can be seen that: compared with reference beam reconstruction, the phase conjugated beam reconstruction can effectively correct the phase aberration caused by the mis-adjustment of optical setup and the real-time detection of the changing progress of the diffraction image can be realized.

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 167

reference, and readout waves. The nonpolarized He–Ne laser is split (1:1) into two beams, a horizontally polarized object wave (O-Beam) and avertically polarized reference wave (R-Beam), by a polarization-sensitive beam splitter (PBS). Lenses L1 and L2 in the object beam path comprise a beam expander. Located in the front focal plane of the Fourier transform lens L3, the spatial light modulator (SLM) was positioned such that the encoded data are loaded from the computer PC1. The polarization state of the object wave becomes vertical after passing through the SLM. Diaphragm D1 is placed in the spatial frequency spectrum plane of the 4f system composed of L3 andL4, which is used to lter high-order diffractive waves, so on the back focal plane of L4 the object image without grid structure to be recorded can be obtained. In the reference beam path, lenses L6 and L7 comprise a 4f system. Rotatable mirror M2 is located at the focal plane of L6, which is used to perform angular multiplexing (detail see the section 4.7.2). The angle between the object wave and the normal of the sample and the angle between the reference wave and the normal of the sample are both 45°. Quarter-wave plates Q1, Q2, and Q3 are used to change the polarization states of the recording, read-out, and diffracted waves. Polarizer P is used to lter scattered light. In the Fourier transform holographic storage experiment, the imaging system is another 4f system composed of lens L5 and L8. The object image to be stored located in the front focal plane of L5 and the CCD is placed in the back focal plane of L8. The fulgide lm is placed on

the spectrum plane of the system to record the Fourier transform holograms.

beams can be used as reference beam and reconstruction beam.

**4.3 The holograms with different polarization recording waves** 

and P the scattering noise can be filtered.

individually.

Only a difference of a quadratic phase factor occurs between Fraunhofer diffraction and Fourier transform. So when objects (or images to be stored) placed anywhere around the imaging lens the Fraunhofer hologram can be obtained. Here in the Fraunhofer holographic storage experiment, the image to be stored placed at about two-focal-distance before the imaging lens L5, so the upside down same size image of the object will be formed at about two-focal-distance after the L5, where is also the CCD photosensitive surface. Fulgide films were still near the back focal plane of L5, so the size of the hologram is small, narrow laser

Compared with Fraunhofer hologram, Fourier-transform hologram has small recording spot, higher storage density and the 4f system has better imaging quality than the singlelens. But because of the small recording spot, the diffracted light is weaker, which can be stronger when stronger reconstruction light is used, however the erasing effect will be sharpen up. The experiment results are shown in Fig.29, here will not be given

In Section 3.1 the diffracted wave polarization states (DWPS) and the diffraction efficiencies (DE) of different polarization holograms recorded in a 3-indoly-benzylfulgimide /PMMA film are given. In this section, different polarization holographic image storages are realized in this film. Experimental setup as shown in Fig.28, four kinds of polarization holograms, like the parallel linearly polarization hologram, parallel circularly polarization hologram, orthogonal linearly polarization hologram and orthogonal circularly polarization hologram, are recorded, the Q1, Q2 are used to adjust the polarization states of O-beam and R-beam. The polarization state of scattering noise is similar to that of the original reconstruction light. And in the orthogonal polarization if the reference beam itself is used as the reconstruction light, the DWPS is orthogonal to the polarization state of reconstruction light, so using Q3

Fig. 26. Reconstructed images of (a) reference beam reconstruction hologram (Phase aberration image) and (b) conjugated beam reconstruction hologram (Corrected image)

#### **4.2 Fraunhofer hologram and Fourier transform hologram**

According to the different arrangement of optical setup, holograms can be divided into Fresnel hologram, Fraunhofer hologram, Image plane hologram, Fourier transform hologram and quasi-Fourier transform hologram etc. In which Fraunhofer hologram and Fourier transform hologram are recorded here, the optical setups are shown in Fig.27 and Fig.28.

Fig. 27. Fraunhofer angular multiplexing holographic storage experimental setup

Fig. 28. Fourier transform angular multiplexing holographic storage experimental setup

It can be seen that in two optical setups, there are many common things, difference exist in finally imaging set up in the object beam path. In both setups, a He–Ne laser (633nm, 3mW) is used for recording and read-out beams. A diode laser (405nm, 10mW) is used as an erasing beam. Shutter S1 and S2controls the exposure time and erasing time respectively. The continuously adjustable attenuators A1 andA2 are used to adjust the intensities of the object,

(a) (b)

According to the different arrangement of optical setup, holograms can be divided into Fresnel hologram, Fraunhofer hologram, Image plane hologram, Fourier transform hologram and quasi-Fourier transform hologram etc. In which Fraunhofer hologram and Fourier transform hologram are recorded here, the optical setups are shown in Fig.27 and

L6

L7

PC2

PC2

Sample

L6

L7

Sample

Q1 Q3 P L8

Q1 Q3 P

M3

E-Beam

Fig. 26. Reconstructed images of (a) reference beam reconstruction hologram (Phase aberration image) and (b) conjugated beam reconstruction hologram (Corrected image)

He-Ne CCD

PC1 LD

D1

Fig. 27. Fraunhofer angular multiplexing holographic storage experimental setup

M1 M2 A2 D2

He-Ne CCD

D1

Fig. 28. Fourier transform angular multiplexing holographic storage experimental setup

It can be seen that in two optical setups, there are many common things, difference exist in finally imaging set up in the object beam path. In both setups, a He–Ne laser (633nm, 3mW) is used for recording and read-out beams. A diode laser (405nm, 10mW) is used as an erasing beam. Shutter S1 and S2controls the exposure time and erasing time respectively. The continuously adjustable attenuators A1 andA2 are used to adjust the intensities of the object,

A1 PBS L4 L5

PC1 LD

SLM

M1 M2 A2 D2

Q2

S2

Q2

E-Beam

S2

M3

A1 PBS L4 L5

SLM

**4.2 Fraunhofer hologram and Fourier transform hologram** 

S1 L1 L2 L3

S1 L1 L2 L3

O-Beam

O-Beam

R-Beam

R-Beam

Fig.28.

reference, and readout waves. The nonpolarized He–Ne laser is split (1:1) into two beams, a horizontally polarized object wave (O-Beam) and avertically polarized reference wave (R-Beam), by a polarization-sensitive beam splitter (PBS). Lenses L1 and L2 in the object beam path comprise a beam expander. Located in the front focal plane of the Fourier transform lens L3, the spatial light modulator (SLM) was positioned such that the encoded data are loaded from the computer PC1. The polarization state of the object wave becomes vertical after passing through the SLM. Diaphragm D1 is placed in the spatial frequency spectrum plane of the 4f system composed of L3 andL4, which is used to lter high-order diffractive waves, so on the back focal plane of L4 the object image without grid structure to be recorded can be obtained. In the reference beam path, lenses L6 and L7 comprise a 4f system. Rotatable mirror M2 is located at the focal plane of L6, which is used to perform angular multiplexing (detail see the section 4.7.2). The angle between the object wave and the normal of the sample and the angle between the reference wave and the normal of the sample are both 45°. Quarter-wave plates Q1, Q2, and Q3 are used to change the polarization states of the recording, read-out, and diffracted waves. Polarizer P is used to lter scattered light.

In the Fourier transform holographic storage experiment, the imaging system is another 4f system composed of lens L5 and L8. The object image to be stored located in the front focal plane of L5 and the CCD is placed in the back focal plane of L8. The fulgide lm is placed on the spectrum plane of the system to record the Fourier transform holograms.

Only a difference of a quadratic phase factor occurs between Fraunhofer diffraction and Fourier transform. So when objects (or images to be stored) placed anywhere around the imaging lens the Fraunhofer hologram can be obtained. Here in the Fraunhofer holographic storage experiment, the image to be stored placed at about two-focal-distance before the imaging lens L5, so the upside down same size image of the object will be formed at about two-focal-distance after the L5, where is also the CCD photosensitive surface. Fulgide films were still near the back focal plane of L5, so the size of the hologram is small, narrow laser beams can be used as reference beam and reconstruction beam.

Compared with Fraunhofer hologram, Fourier-transform hologram has small recording spot, higher storage density and the 4f system has better imaging quality than the singlelens. But because of the small recording spot, the diffracted light is weaker, which can be stronger when stronger reconstruction light is used, however the erasing effect will be sharpen up. The experiment results are shown in Fig.29, here will not be given individually.

#### **4.3 The holograms with different polarization recording waves**

In Section 3.1 the diffracted wave polarization states (DWPS) and the diffraction efficiencies (DE) of different polarization holograms recorded in a 3-indoly-benzylfulgimide /PMMA film are given. In this section, different polarization holographic image storages are realized in this film. Experimental setup as shown in Fig.28, four kinds of polarization holograms, like the parallel linearly polarization hologram, parallel circularly polarization hologram, orthogonal linearly polarization hologram and orthogonal circularly polarization hologram, are recorded, the Q1, Q2 are used to adjust the polarization states of O-beam and R-beam. The polarization state of scattering noise is similar to that of the original reconstruction light. And in the orthogonal polarization if the reference beam itself is used as the reconstruction light, the DWPS is orthogonal to the polarization state of reconstruction light, so using Q3 and P the scattering noise can be filtered.

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 169

(a) (b) (c)

(d) (e) (f)

measurement of size of hologram (one grid of the scale corresponds to 10μm).

**4.5 Application of the auxiliary light effect in holographic image storage** 

LD laser is used as the eraser and auxiliary light source.

requirement on the recording exposure is decreased.

Fig. 30. Results of orthogonal circular polarization holographic optical data storage on BR-D96N film by Fourier transformation holographic method. (a) stored file; (b) encoded binary monochromic image; (c) retrieval diffractive image; (d) decoded result; (e) retrieved file; (f)

The effect of auxiliary light is applied in real image holographic storage. An orthogonal linearly polarization transformation type holographic storage experiment setup was used, which is shown in Fig.31, where the reference light is also used as the reconstruction light and the diffuse reflection objects used as the target. A 633nm, 35mW vertically polarized He-Ne laser is used as the recording and reading light source, which has turned to the elliptically polarized light after a λ/4 plate, and then divided into two orthogonal polarized object light and reference light through the polarization beam splitter PBS. The polarizer P in front of the CCD was used to filter the scattered noises of reconstruction light. The 405nm

The images of the self-diffracted signal at three different instants are presented in Fig.32. The first image (Fig.32a) is taken at *E*≈*E*opt, whereas the second one (Fig.32b) corresponds to the case *E*>>*E*opt (auxiliary light is absent in both cases). As one can see, the recorded image is essentially lost through saturation, but it is restored at a significant level when the auxiliary control beam is turned on (Fig.32c), which proved that the diffraction efficiency can be increased and stabled when irradiated by an auxiliary light, and the rigorous

Fig.29 shows the comparison of retrieval diffractive images with different polarization recording waves on the fulgide film, when reconstruction beams has same polarization state with reference beams. It can be seen that these results are same with the measured results shown in Section 3.1. When the intensities of recording wave and readout wave are certain, the parallel linear polarization hologram has the highest DE, the parallel circular polarization hologram followed and the orthogonal circularly polarization hologram has the lowest DE. But in parallel polarization holograms, the scattering noises cannot be filtered, so their signal-to-noise-ratios (SNR) are lower. In orthogonal polarization holograms, the scattering noise can be filtered, so their SNRs are high.

Fig. 29. Results of holographic storage experiments with different polarization recording waves on Fulgide film: (a) parallel linear polarization recording; (b) parallel circular polarization recording; (c) orthogonal linear polarization recording; (d) orthogonal circular polarization recording

#### **4.4 Fourier transformation orthogonal circular polarization holographic optical data storage**

Orthogonal circular polarization hologram has higher DE and high SNR. Fourier transformation hologram has high storage density. So Fourier transformation orthogonal circular polarization recording is chosen as the method of high density holographic data storage in the fulgide film. Optical setup as shown in Fig.28, the intensities of O-beam and R-beam are both about 14mW/cm2, optimum exposure time is 10s, and erasing time is smaller than 5s. The encoded binary data images loaded on the SLM are translated by our developed software in PC1 by reading the data stream of the computer le. The black pixel represents "0" and the white pixel represents "1". The marginal periodically distributed black–white pixels are used as a reference for locating the pixels in the data-decoding process. The diffracted images will be captured by CCD and send to PC2, which can be successfully decoded and recovered to the original le without any errors. The holograms can be restored after erased by the violet light.

In Fig.30 shows the experimental results. The images are separately stored file, encoded binary monochromic image, retrieval diffractive image, decoded result, retrieved file, measurement of size of holographic image. The retrieval diffractive image that is clear is processed by decoding procedure, and the obtained retrieved file is same as the stored file. In the experiment, data size of each stored holographic page is 81×61 bits, and the size of hologram is 60μm×42μm. So the storage area density of 2×108 bits/cm2 is obtained. The nonhomogeneity and flaws of the surface of sample, misalignmen of optical elements or uncertainty of adjustment brings out some distortion of the diffractive image and error codes that can be reduced to the minimum by designing reasonable encoding and decoding procedures.

Fig.29 shows the comparison of retrieval diffractive images with different polarization recording waves on the fulgide film, when reconstruction beams has same polarization state with reference beams. It can be seen that these results are same with the measured results shown in Section 3.1. When the intensities of recording wave and readout wave are certain, the parallel linear polarization hologram has the highest DE, the parallel circular polarization hologram followed and the orthogonal circularly polarization hologram has the lowest DE. But in parallel polarization holograms, the scattering noises cannot be filtered, so their signal-to-noise-ratios (SNR) are lower. In orthogonal polarization holograms, the

 (a) (b) (c) (d) Fig. 29. Results of holographic storage experiments with different polarization recording waves on Fulgide film: (a) parallel linear polarization recording; (b) parallel circular polarization recording; (c) orthogonal linear polarization recording; (d) orthogonal circular

**4.4 Fourier transformation orthogonal circular polarization holographic optical data** 

Orthogonal circular polarization hologram has higher DE and high SNR. Fourier transformation hologram has high storage density. So Fourier transformation orthogonal circular polarization recording is chosen as the method of high density holographic data storage in the fulgide film. Optical setup as shown in Fig.28, the intensities of O-beam and R-beam are both about 14mW/cm2, optimum exposure time is 10s, and erasing time is smaller than 5s. The encoded binary data images loaded on the SLM are translated by our developed software in PC1 by reading the data stream of the computer le. The black pixel represents "0" and the white pixel represents "1". The marginal periodically distributed black–white pixels are used as a reference for locating the pixels in the data-decoding process. The diffracted images will be captured by CCD and send to PC2, which can be successfully decoded and recovered to the original le without any errors. The holograms

In Fig.30 shows the experimental results. The images are separately stored file, encoded binary monochromic image, retrieval diffractive image, decoded result, retrieved file, measurement of size of holographic image. The retrieval diffractive image that is clear is processed by decoding procedure, and the obtained retrieved file is same as the stored file. In the experiment, data size of each stored holographic page is 81×61 bits, and the size of hologram is 60μm×42μm. So the storage area density of 2×108 bits/cm2 is obtained. The nonhomogeneity and flaws of the surface of sample, misalignmen of optical elements or uncertainty of adjustment brings out some distortion of the diffractive image and error codes that can be reduced to the minimum

scattering noise can be filtered, so their SNRs are high.

can be restored after erased by the violet light.

by designing reasonable encoding and decoding procedures.

polarization recording

**storage** 

Fig. 30. Results of orthogonal circular polarization holographic optical data storage on BR-D96N film by Fourier transformation holographic method. (a) stored file; (b) encoded binary monochromic image; (c) retrieval diffractive image; (d) decoded result; (e) retrieved file; (f) measurement of size of hologram (one grid of the scale corresponds to 10μm).

#### **4.5 Application of the auxiliary light effect in holographic image storage**

The effect of auxiliary light is applied in real image holographic storage. An orthogonal linearly polarization transformation type holographic storage experiment setup was used, which is shown in Fig.31, where the reference light is also used as the reconstruction light and the diffuse reflection objects used as the target. A 633nm, 35mW vertically polarized He-Ne laser is used as the recording and reading light source, which has turned to the elliptically polarized light after a λ/4 plate, and then divided into two orthogonal polarized object light and reference light through the polarization beam splitter PBS. The polarizer P in front of the CCD was used to filter the scattered noises of reconstruction light. The 405nm LD laser is used as the eraser and auxiliary light source.

The images of the self-diffracted signal at three different instants are presented in Fig.32. The first image (Fig.32a) is taken at *E*≈*E*opt, whereas the second one (Fig.32b) corresponds to the case *E*>>*E*opt (auxiliary light is absent in both cases). As one can see, the recorded image is essentially lost through saturation, but it is restored at a significant level when the auxiliary control beam is turned on (Fig.32c), which proved that the diffraction efficiency can be increased and stabled when irradiated by an auxiliary light, and the rigorous requirement on the recording exposure is decreased.

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 171

diameter of hologram stored in the media is about 0.2mm, as shown in Fig.34(a). In this system the shutter S is used to control the exposure time (the best exposure time is 13 second in this experiment). The continuously adjustable attenuator A is used to adjust the intensities of *I*O and *I*C (stronger for recording and weaker for readout). The diagram D, placed in front of the CCD, is used to filter the transmission light of *I*C (zero-order diffraction), and to pass only the diffraction light *I*D (+1 order diffraction). After erased by

Fig. 33. Optical setup and the patterns displayed on the SLM in rewritable collinear holographic image storage; (a) Optical setup; (b) Recording process pattern; (c) Reading

(a) (b) (c)

Fig. 34. Size measurement, objective image and reconstructed image of a collinear hologram stored in the media: (a) Size measurement (the scale is 100mm/div); (a) Original objective

In Fig.34(b,c), the results of parallel linear polarized collinear holographic storage experiment on a BR-D96N film are shown. Since the collinear hologram is recorded on the focus of lens, the size of the recording point is very small. So the diffracted light is very weak, which makes the scattering light of the reconstruction light entering into CCD (noises) look too much. In that case, the signal noise ratio (SNR) is very low. But this kind of noise can be filtered by using orthogonal polarization hologram recording technology. And high storage density can be realized in collinear holography because of its small recording area. Comparing with traditional "two-beam interference recording method", the collinear holographic storage system has simpler optical setup and smaller volume. And in this system the demands for environment are lower, because the objective light and reference light pass through the same one path, in which there are same interferences from the vibration of environment, the change of temperature and the variety of airflow, those effects

(a) (b) (c)

ultraviolet light, the holograms can be recorded repeatedly.

process pattern

image; (b) Reconstructed image

can be cut down.

Fig. 31. Diffuse reflection object orthogonal linearly polarization transformation type holographic storage experiment setup

Fig. 32. The diffracted images of different holograms: (a) at *E*≈*E*opt, without auxiliary light; (b) at *E*>>*E*opt, without auxiliary light; (c) exposed long enough time to be stable after the auxiliary light is turned on

#### **4.6 Collinear hologram**

In collinear holographic storage system, the experimental setup for rewritable collinear holographic image storage is shown in Fig.33(a). A He-Ne laser is used as the light source for recording and readout, and an ultraviolet laser diode is used as the light source for erasing. The non-polarized laser beam (632.8nm, 2mW) turns to be vertical polarized light by passing through the polarizer P, after being expanded and collimated by the lens L1 and L2. Then the beam is projected to transparent mask SLM (special light modulator). The pattern on the SLM used in recording process is shown in Fig.33(b), in which the center cross is the objective information pattern and the outer semi circles are the reference pattern. So the modulated laser beams by the SLM include both objective light (*I*O) and reference light (*I*R), whose intensities are respectively 318mW/cm2 and 382mW/cm2 (*I*O:*I*R≈1:1.2). The patterns on the SLM are imaged on the CCD's photosensitive surface by the positive lens L3 with focal length of 70mm. The *I*O and *I*C beams are focused and interfere with themselves at the focus of L3, where a record medium, the BR film, has been placed for recording the hologram. In reconstruction process, only the outer semi circle patterns, as shown in Fig.33(c), is displayed on the SLM, where the object light pattern is covered. So the reconstruction light is same with the original reference light *I*C, whose intensity is about 50mW/cm2. And the retrieved diffractive image is captured by the CCD sensor. The

**IR**

**P**

**Sample**

**IO**

(a) (b) (c)

Fig. 32. The diffracted images of different holograms: (a) at *E*≈*E*opt, without auxiliary light; (b) at *E*>>*E*opt, without auxiliary light; (c) exposed long enough time to be stable after the

In collinear holographic storage system, the experimental setup for rewritable collinear holographic image storage is shown in Fig.33(a). A He-Ne laser is used as the light source for recording and readout, and an ultraviolet laser diode is used as the light source for erasing. The non-polarized laser beam (632.8nm, 2mW) turns to be vertical polarized light by passing through the polarizer P, after being expanded and collimated by the lens L1 and L2. Then the beam is projected to transparent mask SLM (special light modulator). The pattern on the SLM used in recording process is shown in Fig.33(b), in which the center cross is the objective information pattern and the outer semi circles are the reference pattern. So the modulated laser beams by the SLM include both objective light (*I*O) and reference light (*I*R), whose intensities are respectively 318mW/cm2 and 382mW/cm2 (*I*O:*I*R≈1:1.2). The patterns on the SLM are imaged on the CCD's photosensitive surface by the positive lens L3 with focal length of 70mm. The *I*O and *I*C beams are focused and interfere with themselves at the focus of L3, where a record medium, the BR film, has been placed for recording the hologram. In reconstruction process, only the outer semi circle patterns, as shown in Fig.33(c), is displayed on the SLM, where the object light pattern is covered. So the reconstruction light is same with the original reference light *I*C, whose intensity is about 50mW/cm2. And the retrieved diffractive image is captured by the CCD sensor. The

Fig. 31. Diffuse reflection object orthogonal linearly polarization transformation type

**Object**

**PBS He-Ne CCD**

λ**/4**

holographic storage experiment setup

auxiliary light is turned on

**4.6 Collinear hologram** 

diameter of hologram stored in the media is about 0.2mm, as shown in Fig.34(a). In this system the shutter S is used to control the exposure time (the best exposure time is 13 second in this experiment). The continuously adjustable attenuator A is used to adjust the intensities of *I*O and *I*C (stronger for recording and weaker for readout). The diagram D, placed in front of the CCD, is used to filter the transmission light of *I*C (zero-order diffraction), and to pass only the diffraction light *I*D (+1 order diffraction). After erased by ultraviolet light, the holograms can be recorded repeatedly.

Fig. 33. Optical setup and the patterns displayed on the SLM in rewritable collinear holographic image storage; (a) Optical setup; (b) Recording process pattern; (c) Reading process pattern

Fig. 34. Size measurement, objective image and reconstructed image of a collinear hologram stored in the media: (a) Size measurement (the scale is 100mm/div); (a) Original objective image; (b) Reconstructed image

In Fig.34(b,c), the results of parallel linear polarized collinear holographic storage experiment on a BR-D96N film are shown. Since the collinear hologram is recorded on the focus of lens, the size of the recording point is very small. So the diffracted light is very weak, which makes the scattering light of the reconstruction light entering into CCD (noises) look too much. In that case, the signal noise ratio (SNR) is very low. But this kind of noise can be filtered by using orthogonal polarization hologram recording technology. And high storage density can be realized in collinear holography because of its small recording area.

Comparing with traditional "two-beam interference recording method", the collinear holographic storage system has simpler optical setup and smaller volume. And in this system the demands for environment are lower, because the objective light and reference light pass through the same one path, in which there are same interferences from the vibration of environment, the change of temperature and the variety of airflow, those effects can be cut down.

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 173

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

2 2 2 cos

s=-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

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

1#(20s) 2#(18s) 3#(16s) 4#(14s) 5#(12s)

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

*t*

η1

(a) (b)

η

*t*<sup>1</sup> *t*<sup>2</sup>

η3 η2

*t*3

<sup>0</sup>△*<sup>t</sup>* <sup>2</sup>△*<sup>t</sup>* <sup>3</sup>△*<sup>t</sup> t*

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

Fig. 36. The experimental results of angular multiplexing holograms

η1

η2

η3

of holograms. The simple diagram is shown in Fig.37.

π νλ

*nd r s* . In this experiment, the

π

633nm the refractive index difference between E-form and C-form is Δ*n*≈1.7×10-2,


m, the refractive index of the sample is about *n*≈1.5, and for our sample at

 θθ

θ

<sup>−</sup> ΔΘ = − (2)

λ

=633nm, the thickness of the

θr=45°,

*nd r s*

greater than the minimum horizontal selection angle ΔΘ[10]:

Where /( cos cos )

μ

 λ  θθ

ν = Δπ

increased 4 times.

η

recording time

sample is *d*=10

θ

12s.
