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V. Pramithaa; Rani Josephb; K. Sreekumarc; C. Sudha Karthaa

a Department of Physics, Cochin University of Science and Technology, Cochin 682022, India b

Department of PS & RT, Cochin University of Science and Technology, Cochin 682022, India c

Department of Applied Chemistry, Cochin University of Science and Technology, Cochin 682022, India

First published on: 02 July 2010

Pramitha, V. , Joseph, Rani , Sreekumar, K. and Kartha, C. Sudha(2010) 'Peristrophic multiplexingstudies in silver doped photopolymer film', Journal of Modern Optics, 57: 10, 908 — 913, First published on: 02 July 2010(iFirst)

10.1080/09500340.2010.496538

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Journal of Modern Optics

Vol. 57, No. 10, 10 June 2010, 908–913

Peristrophic multiplexing studies in silver doped photopolymer film

V. Pramithaa

, Rani Josephb

, K. Sreekumarc

and C. Sudha Karthaa

*aDepartment of Physics, Cochin University of Science and Technology, Cochin 682022, India; bDepartment of PS & RT,

Cochin University of Science and Technology, Cochin 682022, India; cDepartment of Applied Chemistry, Cochin University of Science and Technology, Cochin 682022, India

(Received 10 March 2010; final version received 24 May 2010 )

The peristrophic multiplexing technique with rotation of the film in a plane normal to the bisector of the incidentbeams was employed for recording plane-wave transmission gratings at the same location of silver dopedacrylamide photopolymer film. Both constant and variable exposure scheduling methods were adopted forstoring gratings using a 632.8 nm HeNe laser. The diffraction efficiency (DE) and M number (M/#) obtainedfrom both methods were compared to determine which method enabled the greatest number of gratings to berecorded with uniform diffraction efficiencies. By the variable exposure energy scheduling method, 30 nearlyuniform plane wave gratings with M/# equal to 4.7, could be recorded in a 130 mm thick photopolymer layer.

Keywords: photopolymers; peristrophic multiplexing; holography

1. Introduction

Storage requirements all over the world are mounting

day by day, making data storage one of the biggest

challenges in the expanding multimedia market.

Holographic data storage (HDS) with high storage

density, fast data transfer rate and short random access

time is envisioned as one of the promising technologies

that can efficiently meet this challenge. In the past few

years, researchers have experimentally demonstrated

data storage density as high as 500 Gbits/sq.in [1]

and sustained optical data transfer rate as high as10 Gbits/s [2] separately in different optical systems.

These potentialities have been achieved through the

page-oriented nature of the systems and also through

the application of different multiplexing techniques.

Developing suitable recording media with a large

dynamic range is critical in achieving a practical

HDS system and research is now focused on

developing the optimum holographic recording

medium [3]. The dynamic range is the number of

holograms with a diffraction efficiency of 100% that

can be stored in a material with a specific thickness. It

is the storage capacity of a holographic material and

is characterized by the parameter M/#. With thedynamic range, it is possible to know how many

holograms can be stored in the material with specific

diffraction efficiency, or what diffraction efficiency

the holograms would have if a specific number of

holograms were recorded in the material.

Metal ion doped acrylamide photopolymers

with excellent holographic characteristics such as

high refractive index modulation, large dynamic

range (M/#), high diffraction efficiency (DE), good

light sensitivity, real-time image development, high

optical quality and low cost are potential candidates

for recording write-once read many (WORM)

holographic memories [4–10]. To store numerous

pages of data holographically, various multiplexing

techniques such as angle, peristrophic (rotational),

shift and wavelength multiplexing are commonly

used [11–19]. The number of holograms that can be

multiplexed in a given holographic system is primarilya function of two parameters – the system’s bandwidth

(either temporal or spatial frequency) and the

material’s dynamic range [15]. The angular bandwidth

problem can be alleviated by making the film thicker,

but the scattering increases rapidly with thickness

in the recording materials. Peristrophic (Greek word

for ‘Rotation’) multiplexing was introduced as

a solution to the bandwidth limited capacity problem.

With this method, the hologram is physically rotated,

with the axis of rotation being perpendicular to the

film’s surface every time a new hologram is stored.

The rotation shifts the reconstructed image away from

the detector; permitting a new hologram to be storedand viewed without interference, and it can also cause

the stored hologram to become Bragg mismatched.

Peristrophic multiplexing makes it possible to multi-

plex many holograms in thin films. Thin-film

materials, such as DuPontTM HRF-150 photopolymer,

have been developed with a relatively large dynamic

range [15]. As the number of holograms recorded

*Corresponding author. Email: [email protected]

ISSN 0950–0340 print/ISSN 1362–3044 online

ß 2010 Taylor & Francis

DOI: 10.1080/09500340.2010.496538http://www.informaworld.com



in these films increases, the optimal utilization of

the available dynamic range becomes particularly

important since the diffraction efficiency scales

as 1/M 2, where M is the number of holograms

multiplexed [16]. Peristrophic multiplexing can also

be combined with other multiplexing techniques such

as angle and wavelength multiplexing to increase thestorage density and with spatial multiplexing

to increase the storage capacity of the system.

There are several reports of peristrophic multiplex-

ing studies in photopolymer layers with different

compositions and film thickness. Curtis et al. [15]

recorded 295 holograms with average diffraction

efficiency of $4Â 10À6 in 38mm thick DuPontTM

HRF-150 photopolymer film by combining peristro-

phic multiplexing with angle multiplexing. Peristrophic

multiplexing permitted almost two orders of magnitude

increase in the storage capacity of the DuPontTM

photopolymer and changed the limiting factor from

the angular bandwidth of the optical system tothe dynamic range of the material. Dye-sensitized

acrylamide-based photopolymer systems have recently

attracted a great deal of attention because of their high

diffraction efficiency (DE) and low cost. Sherif et al. [12]

have reported an M/# of 3.6 while recording 30 gratings

in 160mm thick acrylamide-based photopolymer film.

Recording multiple gratings in the same volume of the

film entailed rotating the photopolymer film between

recordings. Ortuno et al. [13] recorded nine holograms

with uniform efficiency having an M/# of 3 using

900mm thick polymer films. Elena Fernandez et al. [19]

have developed 700Æ 10mm thick acrylamide

photopolymer layer and peristrophically multiplexed

90 gratings which gave an M/# of 12. We have

previously reported preliminary studies on peristrophic

multiplexing in which 15 holographic gratings were

stored in a silver doped photopolymer layer [10].

In order to fully exploit the dynamic range of the

material, as many holograms as possible, need to be

stored. Hence, in the present study, efforts were made to

exploit the available dynamic range of the silver-doped

films by recording more number of gratings using a

peristrophic multiplexing technique with rotation of the

sample normal to the recording media. In applications

of multiplexed holography, it is very important to makethe diffraction efficiency of all holograms uniform [16]

and hence much emphasis has been put on equalizing

the diffraction efficiency of the multiplexed gratings

by using a proper exposure scheduling technique.

2. Methodology

The photopolymer material used in the study consists

of acrylamide (AA) as the polymerizable monomer;

methylene blue (MB) as the sensitizer dye; triethano-

lamine (TEA) as the radical generator; silver nitrate as

the crosslinker and a binder of poly (vinyl alcohol)(PVA). The role of various components and details of

film fabrication are discussed in our previous paper

[10]. Table 1 shows the concentrations of various

constituents of the 130mm thick (measured using

Dektak 6 m stylus profiler) photopolymer film.

The optical absorption spectrum of the film was

recorded using a UV-VIS-NIR spectrophotometer

(JASCO-V-570). The film has good spectral sensitivity

in the red region of the spectrum (Figure 1) and

a He-Ne laser (Melles Griot) with emission at 632.8 nm

was used for recording and reconstructing the gratings.

The two-beam holographic recording setup (Figure 2)

was used to record plane wave transmission gratingsin the film. The laser beam was split into two using

a beam-splitter and these beams were directed onto the

film using front-silvered mirrors. These beams were

expanded using spatial filters and collimated. The

collimated laser beams were allowed to interfere in

the polymer film from the same side. Path lengths of

the beams were made equal. The exposure time was

controlled by placing an electronic shutter in front of

the laser. Gratings were recorded using constant and

Figure 1. Optical absorption spectrum of the unexposedfilm.

Table 1. Concentration of the photopolymer constituents.

Constituent Concentration

PVA 10% w/vAA 0.38 MTEA 0.05 MMB 0.14Â 10À4M

AgNO3 0.5Â 10À4M

Journal of Modern Optics 909



variable exposure energy scheduling schemes. The

recorded gratings were reconstructed using a

632.8nm He-Ne laser. The diffraction efficiency was

calculated as the ratio of the diffracted beam intensityto the intensity of the incident beam. The intensity was

measured by an optical power meter (Ophir PD200).

For peristrophic multiplexing studies, the experi-

mental setup is the same as in Figure 2, except that

a rotation stage was added and the photopolymer film

was mounted on the rotation stage. Figure 3 shows the

geometry for peristrophic multiplexing. After each

exposure, the recording material was rotated in

a direction perpendicular to the plane of incidence.

This rotation causes the reconstruction from the stored

holographic grating to come out in a different direc-

tion, permitting a new grating to be recorded at the

same location and viewed without interference, and

it can cause the stored grating to become Bragg

mismatched.

3. Results and discussion

3.1. Constant exposure scheduling method

In the constant exposure method, gratings were

recorded in the photopolymer film using a 632.8 nm

He-Ne laser with 1 mJ/cm2 incident exposure energy

per grating (2.5 s exposure at 0.4 mW/cm2 total inci-

dent intensity). The recording beams had an incident

angle of 20 with respect to the normal on the

photopolymer surface and the beam intensity ratio

was 1:1. Twenty plane wave gratings were recorded

with an angular separation of 5 in the film by this

method. This angle was chosen so that the first-order

diffracted peak of each grating would not contribute to

or detract from the diffraction efficiency of neighbor-

ing gratings. The stored holographic gratings were

reconstructed using He-Ne laser (2 mW, 632.8 nm).

The diffraction efficiency of 20 peristrophically multi-plexed plane-wave gratings recorded with a uniform

exposure schedule is shown in Figure 4.

Recording with a constant exposure schedule

resulted in non-uniform diffraction efficiency gratings.

The diffraction efficiencies (DE) of the first recorded

gratings were high, while the last gratings had very low

DE values. The efficiency of the first grating was 16%

while that of the 20th one was 2Â 10À3%. The mean

DE was calculated using the expression

DE m ¼1

M

XM

i ¼1

i ð1Þ

where, i represents the maximum DE of each grating

and M , the total number of multiplexed gratings. The

value of DE m was approximately 3%. The maximum

efficiency i was seen to decrease as the number of

recorded gratings increases, which may be due to

the consumption of the dynamic range of the photo-

polymer film as each new grating was recorded [13].

The recording behavior of the photopolymer can be

best characterized by plotting the cumulative grating

Figure 4. Diffraction efficiency of 20 plane wave gratingsrecorded with a constant 1 mJ/cm2 exposure per grating.

Figure 2. Geometry for recording transmission grating.

Figure 3. Peristrophic multiplexing scheme.

910 V. Pramitha et al.



strength as a function of exposure energy. The curve

shown in Figure 5 is obtained by integrating the squareroot of the diffraction efficiency of the peristrophically

multiplexed gratings recorded with a 1 mJ/cm2 con-

stant exposure schedule. From the figure, it can be seen

that the cumulative grating strength grows quasi-

linearly with exposure energy and then saturates. The

dynamic range (M/#) was calculated using the

expression

M=# ¼XM

i ¼1

1=2i ð2Þ

where i is the maximum diffraction efficiency of

each recorded grating and the sum is over the M holographic gratings multiplexed in the same location

of the film [20]. The dynamic range (M/#) used to

record the gratings by constant exposure scheduling

was equal to 2.7. It is the saturation value of the

cumulative grating strength and can also be calculated

from the plot of cumulative grating strength versus

exposure energy (Figure 5). From multiplexing studies

at constant exposure time, it was seen that this type of

multiplexing will not result in uniform gratings, which

is essential for holographic data storage. When the

gratings are recorded in the photopolymer film, the

monomer and dye are being consumed and therefore

the material becomes less sensitive [13,19]. Hence, itis necessary to increase the exposure time for the last

gratings so that they also attain the same diffraction

efficiency as the first recorded gratings.

3.2. Variable exposure scheduling method

Efforts were made to equalize the diffraction efficiency

of the multiplexed gratings by adopting an exposure

scheduling method designed to share all or part of

the available dynamic range of the recording material

among the gratings to be multiplexed. In this case,

gratings were also recorded at an angular separation of

5 and with a spatial frequency of 1080 lines/mm. The

total intensity at the recording plate was maintained as

0.4 mW/cm2 throughout the recording process and the

beam intensity ratio was 1:1. Exposure energy was

increased in steps by increasing the exposure time while

recording gratings. Sets of 20–30 gratings were

recorded in the film by adopting variable exposure

scheduling methods. The diffraction efficiencies of the

recorded gratings were determined by reconstructingthe gratings by a 632.8 nm He-Ne laser.

The variation of diffraction efficiency with grating

number for 20 multiplexed gratings is shown

in Figure 6. The exposure scheduling scheme used

is shown in the inset. The range of DE was 2.9 to 6%

and the average DE was 4.5%. The plot of cumulative

grating strength as a function of exposure energy for 20

multiplexed gratings is shown in Figure 7. The

dynamic range (M/#) used to record the gratings was

obtained as 4.2 from Figure 7.

The diffraction efficiency of 30 peristrophically

multiplexed plane wave gratings recorded with a

variable exposure schedule is shown in Figure 8.

Recording with a variable exposure schedule resulted

in nearly uniform gratings. The range of DE was 0.9 to

5.5% and the average DE was 2.7%. The curve

in Figure 9 shows the variation of cumulative grating

strength with exposure energy for 30 peristrophically

multiplexed gratings recorded with a variable exposure

schedule method. In this case also, the cumulative

grating strength increased quasi-linearly with exposure

energy and then saturated. This saturation value is

Figure 6. Diffraction efficiency as a function of gratingnumber for 20 gratings.

Figure 5. Cumulative grating strength as a functionof exposure energy for 1 mJ/cm2 constant exposure.




the dynamic range (M/#) and was equal to 4.7. The

range of diffraction efficiency and M/# values calcu-

lated from the corresponding plots is summarized in

Table 2.

In the case of a set of 20 gratings, the recorded

gratings were found to be more uniform than the set

of 30 multiplexed gratings. But the M/# value is larger

for the set of 30 gratings because, as the number

of recorded gratings increases, there will be maximum

utilization of the available dynamic range. The variable

exposure method resulted in a larger M/# value

and more uniform gratings than constant exposure

scheduling, which clearly indicates that the variable

exposure energy scheduling makes better use

of the dynamic range of the photopolymer material.

The M/# obtained while recording 30 gratings in our

130mm thick photopolymer material is assumed

to be larger than the earlier reported values for

acrylamide-based compositions with similar layer

thickness. Sherif et al. [12] has achieved M/# of 3.6while recording 30 holograms in a 160 mm thick

acrylamide-based photopolymer film with slightly

different composition. From Table 2, it can be seen

that the multiplexed gratings have high diffraction

efficiency values. Since, typically, one can work with

holographic diffraction efficiencies of the order of

10À6, we have sufficient dynamic range to record more

gratings. It is expected that more uniform gratings can

be multiplexed in the material by making use of the

variable exposure method developed by Allen Pu et al.

[16] and research work on this is in progress.

4. Conclusions

Holographic transmission gratings could be

peristrophically multiplexed in silver doped photopoly-

mer media using a 632.8 nm He-Ne laser. Variable

exposure scheduling resulted in a larger M/# value of

4.7 while recording 30 gratings in the 130mm thick

photopolymer layer. This clearly indicates that variable

exposure energy scheduling makes better use of the

Figure 8. Diffraction efficiency as a function of gratingnumber for 30 gratings.

Figure 9. Cumulative grating strength as a function of exposure energy for 30 gratings.

Figure 7. Cumulative grating strength as a function of exposure energy for 20 gratings.

Table 2. Comparison of DE and M/#.

No. of multiplexedgratings Range of DE (%) M/#

20 2.9–6 4.230 0.9–5.5 4.7

912 V. Pramitha et al.



available dynamic range of the photopolymer material.

The M/# value obtained for 30 gratings in this 130 mm

thick acrylamide-based photopolymer layer is assumed

to be larger than the reported values for multiplexing

an equal number of gratings in acrylamide-based

photopolymer materials with similar composition and

layer thickness.

Acknowledgement

The authors are grateful to Defence Research andDevelopment Organization (DRDO) for providing financialsupport.

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