3D optical data storage
![]() | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (July 2010) |
Optical discs |
---|
![]() |
3D optical data storage is any form of
This innovation has the potential to provide
No commercial product based on 3D optical data storage has yet arrived on the mass market, although several companies[
Overview
Current optical
3D optical data storage is related to (and competes with) holographic data storage. Traditional examples of holographic storage do not address in the third dimension, and are therefore not strictly "3D", but more recently 3D holographic storage has been realized by the use of microholograms. Layer-selection multilayer technology (where a multilayer disc has layers that can be individually activated e.g. electrically) is also closely related.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/3D_optical_storage_cross-section.svg/400px-3D_optical_storage_cross-section.svg.png)
As an example, a prototypical 3D optical data storage system may use a disc that looks much like a transparent DVD. The disc contains many layers of information, each at a different depth in the media and each consisting of a DVD-like spiral track. In order to record information on the disc a
In order to read the data back (in this example), a similar procedure is used except this time instead of causing a photochemical change in the media the laser causes fluorescence. This is achieved e.g. by using a lower laser power or a different laser wavelength. The intensity or wavelength of the fluorescence is different depending on whether the media has been written at that point, and so by measuring the emitted light the data is read.
The size of individual chromophore
History
The origins of the field date back to the 1950s, when Yehuda Hirshberg developed the
Processes for creating written data
Data recording in a 3D optical storage medium requires that a change take place in the medium upon excitation. This change is generally a photochemical reaction of some sort, although other possibilities exist.
Writing by nonresonant multiphoton absorption
Although there are many nonlinear optical phenomena, only multiphoton absorption is capable of injecting into the media the significant energy required to electronically excite molecular species and cause chemical reactions. Two-photon absorption is the strongest multiphoton absorbance by far, but still it is a very weak phenomenon, leading to low media sensitivity. Therefore, much research has been directed at providing chromophores with high two-photon absorption cross-sections.[6]
Writing by two-photon absorption can be achieved by focusing the writing laser on the point where the photochemical writing process is required. The wavelength of the writing laser is chosen such that it is not linearly absorbed by the medium, and therefore it does not interact with the medium except at the focal point. At the focal point two-photon absorption becomes significant, because it is a nonlinear process dependent on the square of the laser
Writing by two-photon absorption can also be achieved by the action of two lasers in coincidence. This method is typically used to achieve the parallel writing of information at once. One laser passes through the media, defining a line or plane. The second laser is then directed at the points on that line or plane that writing is desired. The coincidence of the lasers at these points excited two-photon absorption, leading to writing photochemistry.
Writing by sequential multiphoton absorption
Another approach to improving media sensitivity has been to employ
Microholography
In micro
Data recording during manufacturing
Data may also be created in the manufacturing of the media, as is the case with most optical disc formats for commercial data distribution. In this case, the user can not write to the disc – it is a ROM format. Data may be written by a nonlinear optical method, but in this case the use of very high power lasers is acceptable so media sensitivity becomes less of an issue.
The fabrication of discs containing data molded or printed into their 3D structure has also been demonstrated. For example, a disc containing data in 3D may be constructed by sandwiching together a large number of wafer-thin discs, each of which is molded or printed with a single layer of information. The resulting ROM disc can then be read using a 3D reading method.
Other approaches to writing
Other techniques for writing data in three-dimensions have also been examined, including:
Persistent spectral hole burning (PSHB), which also allows the possibility of spectral multiplexing to increase data density. However, PSHB media currently requires extremely low temperatures to be maintained in order to avoid data loss.
Void formation, where microscopic bubbles are introduced into a media by high intensity laser irradiation.[7]
Chromophore poling, where the laser-induced reorientation of chromophores in the media structure leads to readable changes.[8]
Processes for reading data
The reading of data from 3D optical memories has been carried out in many different ways. While some of these rely on the nonlinearity of the light-matter interaction to obtain 3D resolution, others use methods that spatially filter the media's linear response. Reading methods include:
Two photon absorption (resulting in either absorption or fluorescence). This method is essentially
Linear excitation of fluorescence with confocal detection. This method is essentially
Measurement of small differences in the refractive index between the two data states. This method usually employs a
Second-harmonic generation has been demonstrated as a method to read data written into a poled polymer matrix.[9]
Optical coherence tomography has also been demonstrated as a parallel reading method.[10]
Media design
The active part of 3D optical storage media is usually an
Media form factor
Media for 3D optical data storage have been suggested in several form factors: disk, card and crystal.
A disc media offers a progression from CD/DVD, and allows reading and writing to be carried out by the familiar spinning disc method.
A credit card form factor media is attractive from the point of view of portability and convenience, but would be of a lower capacity than a disc.
Several science fiction writers have suggested small solids that store massive amounts of information, and at least in principle this could be achieved with 5D optical data storage.
Media manufacturing
The simplest method of manufacturing – the molding of a disk in one piece – is a possibility for some systems. A more complex method of media manufacturing is for the media to be constructed layer by layer. This is required if the data is to be physically created during manufacture. However, layer-by-layer construction need not mean the sandwiching of many layers together. Another alternative is to create the medium in a form analogous to a roll of adhesive tape.[11]
Drive design
A drive designed to read and write to 3D optical data storage media may have a lot in common with CD/DVD drives, particularly if the form factor and data structure of the media is similar to that of CD or DVD. However, there are a number of notable differences that must be taken into account when designing such a drive.
Laser
Particularly when two-photon absorption is utilized, high-powered lasers may be required that can be bulky, difficult to cool, and pose safety concerns. Existing optical drives utilize
Variable spherical aberration correction
Because the system must address different depths in the medium, and at different depths the spherical aberration induced in the wavefront is different, a method is required to dynamically account for these differences. Many possible methods exist that include optical elements that swap in and out of the optical path, moving elements, adaptive optics, and immersion lenses.
Optical system
In many examples of 3D optical data storage systems, several wavelengths (colors) of light are used (e.g. reading laser, writing laser, signal; sometimes even two lasers are required just for writing). Therefore, as well as coping with the high laser power and variable spherical aberration, the optical system must combine and separate these different colors of light as required.
Detection
In DVD drives, the signal produced from the disc is a reflection of the addressing laser beam, and is therefore very intense. For 3D optical storage however, the signal must be generated within the tiny volume that is addressed, and therefore it is much weaker than the laser light. In addition, fluorescence is radiated in all directions from the addressed point, so special light collection optics must be used to maximize the signal.
Data tracking
Once they are identified along the z-axis, individual layers of DVD-like data may be accessed and tracked in similar ways to DVDs. The possibility of using parallel or page-based addressing has also been demonstrated. This allows much faster
Development issues
Despite the highly attractive nature of 3D optical data storage, the development of commercial products has taken a significant length of time. This results from limited financial backing in the field, as well as technical issues, including:
Destructive reading. Since both the reading and the writing of data are carried out with laser beams, there is a potential for the reading process to cause a small amount of writing. In this case, the repeated reading of data may eventually serve to erase it (this also happens in phase change materials used in some DVDs). This issue has been addressed by many approaches, such as the use of different absorption bands for each process (reading and writing), or the use of a reading method that does not involve the absorption of energy.
Thermodynamic stability. Many chemical reactions that appear not to take place in fact happen very slowly. In addition, many reactions that appear to have happened can slowly reverse themselves. Since most 3D media are based on chemical reactions, there is therefore a risk that either the unwritten points will slowly become written or that the written points will slowly revert to being unwritten. This issue is particularly serious for the spiropyrans, but extensive research was conducted to find more stable chromophores for 3D memories.
Media sensitivity.
Academic development
Much of the development of 3D optical data storage has been carried out in universities. The groups that have provided valuable input include:
- Peter T. Rentzepis was the originator of this field, and has recently developed materials free from destructive readout.
- Watt W. Webb codeveloped the two-photon microscope in Bell Labs, and showed 3D recording on photorefractive media.
- Masahiro Irie developed the diarylethene family of photochromic materials.[12]
- Yoshimasa Kawata, Satoshi Kawata, and Zouheir Sekkat have developed and worked on several optical data manipulation systems, in particular involving poled polymer systems.[13]
- Kevin C Belfield is developing photochemical systems for 3D optical data storage by the use of resonance energy transfer between molecules, and also develops high two–photon cross-section materials.[14]
- Seth Marder performed much of the early work developing logical approaches to the molecular design of high two–photon cross-section chromophores.
- Tom Milster has made many contributions to the theory of 3D optical data storage.[15]
- Robert McLeod has examined the use of microholograms for 3D optical data storage.
- Min Gu has examined confocal readout and methods for its enhancement.[16][17]
Commercial development
In addition to the academic research, several companies have been set up to commercialize 3D optical data storage and some large corporations have also shown an interest in the technology. However, it is not yet clear whether the technology will succeed in the market in the presence of competition from other quarters such as
![](http://upload.wikimedia.org/wikipedia/en/8/84/3D_Discs.jpg)
- Call/Recall was founded in 1987 on the basis of Peter Rentzepis' research. Using two–photon recording (at 25 Mbit/s with 6.5 ps, 7 nJ, 532 nm pulses), one–photon readout (with 635 nm), and a high NA (1.0) immersion lens, they have stored 1 TB as 200 layers in a 1.2 mm thick disk.[18]They aim to improve capacity to >5 TB and data rates to up to 250 Mbit/s within a year, by developing new materials as well as high-powered pulsed blue laser diodes.
- Mempile are developing a commercial system with the name
- Digital Multilayer Disk(DMD).
- Storex Technologies has been set up to develop 3D media based on fluorescent photosensitive glasses and glass-ceramic materials. The technology derives from the patents of the Romanian scientist Eugen Pavel, who is also the founder and CEO of the company. At ODS2010 conference were presented results regarding readout by two non-fluorescence methods of a Petabyte Optical Disc.
- Landauer Inc. are developing a media based on resonant two-photon absorption in a sapphire single crystal substrate. In May 2007, they showed the recording of 20 layers of data using 2 nJ of laser energy (405 nm) for each mark. The reading rate is limited to 10 Mbit/s because of the fluorescence lifetime.[22]
- Colossal Storage aim to develop a 3D holographic optical storage technology based on photon-induced electric field poling using a far UV laser to obtain large improvements over current data capacity and transfer rates, but as yet they have not presented any experimental research or feasibility study.
- Microholas operates out of the University of Berlin, under the leadership of Prof Susanna Orlic, and has achieved the recording of up to 75 layers of microholographic data, separated by 4.5 micrometres, and suggesting a data density of 10 GB per layer.[23][24]
- 3DCD Technology Pty. Ltd. is a university spin-off set up to develop 3D optical storage technology based on materials identified by Daniel Day and Min Gu.[25]
- Several large technology companies such as Matsushitahave applied for patents on two–photon-responsive materials for applications including 3D optical data storage, however they have not given any indication that they are developing full data storage solutions.
See also
References
- PMID 11777420.
- ^ Burr, G.W. (2003). Three-Dimensional Optical Storage (PDF). SPIE Conference on Nano-and Micro-Optics for Information Systems. pp. 5225–16. Archived from the original (PDF) on March 8, 2008.
- .
- .
- S2CID 7494304.
- PMID 9733507.
- hdl:1959.3/1948.
- PMID 19529382.
- ^ Fort, A. F.; Barsella, A.; Boeglin, A. J.; Mager, L.; Gindre, D.; Dorkenoo, K. D. (29 August 2007). Optical storage through second harmonic signals in organic films. SPIE Optics+Photonics. San Diego, US. pp. 6653–10.
- S2CID 119092748.
- ^ US patent 6386458, Leiber, Jörn; Noehte, Steffen & Gerspach, Matthias, "Optical data storage", issued 2002-05-14, assigned to Tesa SE
- PMID 11777416.
- ISBN 0-12-635490-1.
- .
- ^ Milster, T. D.; Zhang, Y.; Choi, T. Y.; Park, S. K.; Butz, J.; Bletscher, W. "Potential for Volumetric Bit-Wise Optical Data Storage in Space Applications" (PDF). Archived from the original (PDF) on 4 October 2006.
- S2CID 121467147.
- hdl:1959.3/1798.
- .
- S2CID 59161795.
- ^ Genuth, Iddo (27 August 2007). "Mempile - Terabyte on a CD". TFOT. Archived from the original on 15 September 2007.
- ^ "Digital Multilayer Disk - More Cost Effective than Blue Laser". May 28, 2004. Archived from the original on 2004-05-28.
- .
- ^ Criante, L.; Vita, F.; Castagna, R.; Lucchetta, D. E.; Frohmann, S.; Feid, T.; Simoni, F. F.; Orlic, S. (28 August 2007). New composite blue sensitive materials for high resolution optical data storage. SPIE Optics+Photonics. San Diego, US: SPIE. pp. 6657–03.
- ^ Orlic, S.; Markötter, H.; Mueller, C.; Rauch, C.; Schloesser, A. (28 August 2007). 3D nano and micro structurization of polymer nanocomposites for optical sensing and image processing. SPIE Optics+Photonics. San Diego, US: SPIE. pp. 6657–14.
- ^ "Swinburne Ventures". Swinburne University of Technology. Archived from the original on 5 August 2012.