Multilayer recording in microholographic data storage This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 J. Opt. 14 072401 (http://iopscience.iop.org/2040-8986/14/7/072401) Download details: IP Address: 128.59.62.83 The article was downloaded on 11/04/2013 at 12:06 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Multilayer recording in microholographic data storage
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2012 J. Opt. 14 072401
(http://iopscience.iop.org/2040-8986/14/7/072401)
Download details:
IP Address: 128.59.62.83
The article was downloaded on 11/04/2013 at 12:06
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
Received 19 March 2012, accepted for publication 18 June 2012Published 3 July 2012Online at stacks.iop.org/JOpt/14/072401
AbstractThe potential of multilayer recording in microholographic data storage is investigated.Micrometer-scaled depth localization of resolution-limited microgratings is achieved inphotopolymers sensitized to green and violet light. Confocal readout results in an opticaldepth of approximately 2 µm. The spatial Bragg selectivity of resolution-limited microgratingstructures allows reducing their longitudinal depth spacing down to 4 µm. Multilayerrecording is demonstrated as depth multiplexing of microgratings written in 50 layer locationswithin a 300 µm thick photopolymer.
Keywords: optical data storage, optical recording, volume gratings, holographic and volumememories
(Some figures may appear in colour only in the online journal)
1. Introduction
Multilayer recording is a simple approach to higher densityin optical data storage [1]. When storing the data in multiplelayers, the third dimension of a disk, its depth, becomesusable for optical disk technology and the overall datacapacity linearly grows with the number of layers. A standardoptical drive can easily address different depth positions whileconfocal filtering widely reduces interlayer cross-talk. Thesuccess of a dual-layer DVD-ROM has attracted interest, butin conventional optical systems based on readout from thereflective layer, the multilayer approach has only a moderatepotential to increase the storage capacity. It implies the costlyproduction of multiple-layer disks with rapidly decreasingtilt and flatness tolerances. Also, there is a fundamentalphysical trade-off between the recording layer reflection (togenerate the readout signal generation) and transmission (tooptically access each data layer of a multilayer stack). Opticalrecording in many layers primarily requires a homogeneous,low-absorption recording material while reflection-mode is
favored for confocal implementations of a standard opticalpickup.
An elegant solution to overcome these trade-offs ismicroholographic recording [1–4]. Being fundamentally amatching of the optical disk technology and holographicvolume recording, the microholographic approach concomi-tantly offers reflective bit features and confocal opticalpickup design, as well. In addition, photopolymer materialsused as recording media allow a simple disk design witha monolithic photopolymer layer sandwiched between twosubstrates. Photopolymers developed for holographic storagepossess the characteristics essential for multilayer recordingincluding large thickness of up to 1 mm, low absorptionfor writing, sufficient refractive index change and negligibleabsorption for reading.
The microholographic approach advantageously utilizesthe third dimension while the optical system design remainsvery similar to a standard optical drive. Multilayer recordingrelies on a volumetrically localized index modulation ofmicrogratings while multiple data layers are addressedby simple confocal movement to different depths within
Figure 1. Optical write/read system for multilayer microholographic data storage. Essential features of the optical configuration primarilyare single-beam path, diffraction-limited high-NA focusing, retroreflection and confocal filtering.
the photopolymer layer. As photopolymers are widelytransparent, the number of microholographic layers canbecome quite high. Already with a DVD-like areal datadensity in a single layer, storage capacities far beyond theBlu-Ray specification can be achieved.
In this paper we investigate the potential of multilayerrecording for high-density microholographic data storage.The effective parameters of multilayer microholographicrecording, i.e. the optical depth and minimum depthspacing between adjacent microgratings, are experimentallydetermined. The physical limitations imposed by boththe Bragg interaction and dynamic photoresponse of therecording material are discussed.
2. Write/read system architecture and methodology
The optical write/read system shown in figure 1 isfundamentally a standard optical pickup with the additionof a reflecting unit as needed for reflection-type holographicrecording. The write/read laser beam is focused by ahigh-numerical aperture objective onto a storage location. Thereflecting unit comprising a second identical objective and aretroreflector mirrors the incident write beam into its reflected,counterpropagating part. The two overlapping beams forman interference pattern which is transferred into the mediumthrough the process of polymerization. The recording mediumis a cationic ring-opening photopolymer system developedby Aprilis [5–7]. Two different photopolymers are usedas recording media: a green-sensitized system with peakabsorption at 532 nm and a violet-sensitized system optimizedfor 405 nm. The refractive index change in the testedAprilis CROP photopolymer materials is in the range2–3 × 10−3, while the noise level is on the order of10−5 diffraction efficiency units. The recording process is
finalized by applying a noncoherent uniform illumination. Thephotopolymer does not require any additional processing step.The shrinkage of the material is less than 1% and does notaffect the Bragg-selective readout efficiency.
Single microgratings are written with pulsed exposureat equidistant depth locations corresponding to data layersof a multilayered disk. The microgratings are recorded instop-and-go mode. The two inch Aprilis coupon mediumwhich consists of a 300 µm thick polymer sample betweentwo antireflection-coated glass substrates is mounted on athree-axis nano-positioning stage. Different depth positionsare addressed by the confocal displacement of the joint writebeam focus. This is realized by translating the photopolymersample along the optical axis.
For readout and detection, the reflector unit is blockedby a shutter and the second beam is reconstructed by theholographic grating written in the polymer. The reflected lightis directed by a polarizing beam splitter to a photodetector.The readout is performed dynamically by a continuousreflection scanning through the entire depth of a photopolymersample. The readout signal is conditioned in a confocalfiltering section to reduce cross-talk and noise detection.Placing the confocal filter in the incident beam path simplifiesits alignment and optimizes the readout resolution.
3. Resolution-limited depth-multiplexed recording
As for the areal data density the volume localization ofmicrogratings recorded in a photopolymer is the physicalpremise for multiplexing of several data layers through thedepth. In contrast to the transversal localization given by theGaussian beam shape, the depth localization of microgratingscreated by two counterpropagating beams requires a relativelyhigh-numerical aperture (NA) of the focusing objectives,
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Figure 2. Intensity distribution in the longitudinal, depth direction resulting from the interference of two focused counterpropagatingbeams at 405 nm. Numerical aperture of the focusing objectives: NA = 0.4 (left), NA = 0.6 (middle), NA = 0.75 (right). Microgratingfringes are assumed to follow the intensity profile in a linear-photoresponse recording material with the normal refractive index of 1.5.
Figure 3. Resolution-limited microholographic recording in depth. Volumetric scan of a micrograting recorded at 532 nm in y–zcross-section (left). Micrograting depth (FWHM) is 2.5 µm at a write/read wavelength of 532 nm (right). Recording parameters are: writeenergy 1 nJ, exposure time 100 µs, NA 0.6.
e.g. 0.6 or higher. The effect of NA onto the grating depthprofile is plotted in figure 2 for the violet line at 405 nm:The grating depth approximately follows 1/NA2 and goesdown to less than 2 µm at NA of 0.75. In this casea micrograting comprises only a few grating fringes thateffectively contribute to diffraction. The effect of NA on thediffraction efficiency (DE) of a micrograting can be estimatedby 1/NA4. Obviously there is a physical trade-off betweenthe depth localization and diffraction efficiency, i.e. thesignal-to-noise ratio needed for error-free detection. Thisinterdependency becomes a growing complexity by takingthe photochemical response of a recording photopolymer intoaccount [8, 9].
Consequently, volumetric localization of microgratingsrequires a material-specific adjustment of the recordingexposure fluence [4]. Applying an optimized exposure weachieve strong depth localization in both green-sensitizedand violet-sensitized Aprilis CROP samples. The longitudinalscans of microgratings yield a FWHM depth of 2.50 µmat write/read wavelength of 532 nm (figure 3), which scalesdown to 2.25 µm on average at 405 nm (figure 4). Theseresults display the Bragg interaction of the read beamwith the recorded gratings. The depth selectivity of thereadout system is improved by confocal filtering of thediffracted light. The optically detected depth of the recordedmicrogratings, however, exceeds the quite rough theoreticalestimate of double Rayleigh length [1], which, however, ishardly achievable in practice [10]. Operating at the nanoscaledoptical resolution limit, both the write/read system and
recording material cause different kinds of mismatch thataffect the optical micrograting depth.
The achievable minimum spacing between microgratingsrecorded along the same longitudinal axis rather than theirphysical depth is decisive for the effectiveness of multilayerrecording. Like their lateral distribution, the minimum depthspacing between adjacent microgratings is limited by theoptical resolution of the write/read system and of therecording material. In figure 4 microgratings are recordedthrough the depth of a violet-sensitive polymer sample inequidistant layers.
During recording the write beam is first focused atthe central position of a photopolymer sample wherethe objectives fully correct for spherical aberration. Forsubsequent recordings at equidistant depth locations thesample is translated by the positioning system. The readoutis performed by a continuous longitudinal scan through theentire sample depth. The average FWHM depth is 2.25 µmonly. At 6 µm spacing all single microgratings are clearlyresolved while at 4 µm spacing they begin to interact,which leads to strong variations in the readout signal. Theinteraction between adjacent microgratings is attributed to anoverlap of their peripheral grating fringes. In this case theBragg-selective readout is disturbed by mismatching gratingstructures resulting in a DE decrease. This effect is stronglyrelated to the local photoreaction of the recording materialwith additional influence of the opto-mechanical systemcomponents.
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Figure 4. Depth recording of microgratings at write/read wavelength of 405 nm and different interlayer spacing: center-to-center spacing is6 µm (left) and 4 µm (right), average FWHM depth 2 µm. Recording parameters are: write energy 25 nJ, exposure time 1 ms, NA 0.75.
Figure 5. Depth multiplexing in a 300 µm thick, green-sensitizedAprilis photopolymer. Microgratings are recorded at 532 nm in 50layer locations spaced by 6 µm. Write energy varies in the range of0.25–1 nJ, NA is 0.75.
Similar effects are also observed when many micrograt-ings are written in the longitudinal direction through the entiredepth of a sample. In figure 5 50 layer locations are addressedfor depth multiplexing of single microgratings in a 300 µmthick Aprilis photopolymer. At the layer spacing of 6 µmall 50 microgratings are clearly resolved with acceptable DEvariations. Coherent scattering originating from both multiplelayers and photopolymer noise is effectively eliminated byconfocal filtering. The bright reflection peaks at the marginsoriginate from the glass–photopolymer interfaces.
Both objectives are aberration-corrected for the centraldepth position within the photopolymer. Apparently, theimpact of spherical aberration (SA) is significantly weakerthan one can expect for the full recording sample thicknessof 300 µm. The observed readout signal variations areprimarily a result of the applied exposure scheduling andpartly attributable to the grating distortion caused by sphericalaberration. This becomes obvious from figure 6 where anarrangement of microgratings stored in six layers locatedupon each other with 6 µm depth spacing is displayed.The longitudinal grating depth is 2.2 µm only. In spite ofthis extremely localized Bragg-interaction range, the artifactsoccurring in the interspace between the layers indicate anextended grating structure. However, in the readout signalgeneration, interlayer cross-talk resulting from overlapping
Figure 6. Microgratings in six depth layers. Lateral spacing is1 µm, interlayer spacing is 6 µm. Write/read wavelength is 532 nm,NA = 0.75.
aberration-distorted grating structures is effectively reducedby confocal filtering of the diffracted light.
Experiments on full-depth recording with smallerinterlayer spacing reflect these observations. Reducing thespacing to 4 µm microgratings are recorded at 75 layerlocations in a 300 µm thick photopolymer (figure 7).Although single microgratings are strongly confined andclearly resolvable, the readout signal is partly contaminated bythe aberration-caused overlap of grating fringes. In addition,manifold, complex pre- and post-exposure effects play arole when many gratings are recorded close to each other.The related signal variations can be reduced by applyinga dynamic exposure adjustment. Exposure scheduling takesinto account the grating interaction and share of the polymerdynamics and consequently adjusts the recording energy toequalize the diffraction efficiency. It prevents a prematureexhaustion of the dynamic range and allows the utilization ofthe entire storage capacity of the medium.
4. Conclusion
The multilayer recording technique is expected to make astrong impact on the performance of microholographic datastorage. The physical premises are strong depth localization
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Figure 7. Depth multiplexing in a 300 µm thick, green-sensitizedAprilis photopolymer. Microgratings are recorded at 532 nm in 75layer locations spaced by 4 µm. Write energy varies in the range of0.25–1 nJ, NA is 0.75.
of individual microholographic grating structures and opticalseparability of numerous gratings written along the samedepth axis of a recording medium. Using high-NA objectivesresolution-limited microgratings are written through thedepth of few hundred micron thick photopolymer samples.The spatial Bragg selectivity of the recorded microgratingstructure combined with confocal readout result in anoptically detected depth of approximately 2 µm. Severalmicrogratings are stored along the same depth axis withinterlayer spacing of 4 µm only.
The CROP photopolymer media developed by Aprilisallow both resolution-limited recording and depth mul-tiplexing of microgratings in a large number of layerlocations. The combined impact of spherical aberrationand photopolymer dynamic response limits the achievablemultiplex rate when numerous microgratings are denselyrecorded. In contrast to the majority of holographic recordingphotopolymers, photosensitive materials with a nonlinearphotoresponse characteristic would allow higher multiplexfactors and fundamentally advance the multilayer technique.While the nonlinearity of recording media is still a subject ofresearch and development, spherical aberration can easily beeliminated by using variable-thickness correction plates or byspecifically designed objective lenses with axially adjustablecompensation elements.
In the present work, we demonstrate depth multiplexingof microgratings recorded in 50 layers spaced by 6 µm. Theserecordings achieved without any SA compensation evidencethe potential of microholographic data storage to capitalize onmultilayer recording for data capacities far beyond the state ofthe art.
Acknowledgments
We acknowledge Dr David A Waldman, Aprilis co-founderand CTO, now founder of HolFocus, for developing andsupplying the photopolymer media for microholographicrecording, and the European Commission for financial support(IST-511437).
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