The advantages of Blu-ray disc

The advantages of Blu-ray disc Dayu Chen, Dianyong Chen, Bei Wang* Philips Research East Asia, 38F, 218 Tian Mu Xi Road, Shanghai 200070, China ABSTRA...
Author: Lesley Butler
40 downloads 0 Views 390KB Size
The advantages of Blu-ray disc Dayu Chen, Dianyong Chen, Bei Wang* Philips Research East Asia, 38F, 218 Tian Mu Xi Road, Shanghai 200070, China ABSTRACT Blu-ray Disc format is required by the forthcoming of High Definition TV era which calls for a brand new generation of optical storage after DVD. It is such a technology with wavelength short as 405nm and numerical aperture high as 0.85. The format is designed to have an even wider disc tolerances than those of DVD. Thanks for those more innovative concepts Blu-ray Disc is the most economical storage solution in terms of cost per giga byte. And it is expected to be a long-term optical storage standard rather than an interim solution. Keywords: Blu-ray Disc; BD; CD; DVD; HD-DVD; HDTV

1. INTRODUCTION Blu-ray Disc format is required by the forthcoming of High Definition TV era, which calls for a new generation of optical storage after DVD. The storage demanding MPEG2 format HDTV normally has a data rate between 20 and 30 Mbps. For example, a 135 minutes recording of 25Mbps HDTV program requires a storage capacity about 24GB. Bluray Disc is specifically defined for such a capacity. To have the capacity five times that of DVD, BD employs the wavelength short as 405nm and numerical aperture high as 0.85. It is regarded the blue-violet wavelength will be the shortest wavelength in optical disc and 0.85 will also be the highest numerical aperture used for far-field optical storage. To ensure a quick learning curve not only for the drive manufacturing but also for the disc production, Blu-ray Disc is designed to have even wider disc tolerances than those of DVD. Thanks for those more innovative concepts Blu-ray Disc is the most economical storage solution in terms of cost per giga byte. We can expect an even better result at a mass production level. To introduce a new generation of optical storage, a long term standard is much more preferred rather than an interim solution. It is just like what happened in the history when the DVD format was defined. At that time, an interim solution of less capacity was proposed. Finally this solution was not introduced to the market.

2. THREE GENERATIONS OF OPTICAL STORAGE The development of Fourier Optics in the middle and late 20th century provided an essential theoretical basis for the advent of optical storage technology 1. The development during the same period in motion control technology, audio processing technology, error-correction coding technology, etc, enabled the standardization of disc storage to be first realized in the audio recording field, and commercialized at a low cost. In the diffraction limit the laser beam spot size is in proportion to ( λ / NA )2 . The beam spot size of BD approximates 1/5 of that of DVD allowing the capacity of a BD disc to be about 5 times the DVD capacity. The current BD specification provides three storage capacities for single layer storage: 23.3GB, 25GB and 27GB. With such capacity, over 2 hours of high definition TV programs or about a 40-episode TV series of VCD quality can be stored onto one single layer BD. As a matter of fact, the capacity can be increased further to provide 35GB on a single layer applying new signal processing technology 2. On the creation of CD specification, the capacity requirement is to be able to record on one single disc a complete 9th symphony of Beethoven. This tradition also reflected in the requirement for video recordings, the studios expect a disc to hold 135 minutes of program. Mainstream high definition video compression methods usually have a code rate of 20 – 30 Mbps. Multiply the 135 minutes with the code rate and we get the requirement for disc capacity, which is within the capacity of a BD. *

[email protected]; phone 86-21-63541088 Ext. 5939; fax 86-21-63544954 Seventh International Symposium on Optical Storage, edited by Fuxi Gan, Lisong Hou, Proceedings of SPIE Vol. 5966 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.649801

Proc. of SPIE Vol. 5966 59661R-1

BD is referred to as the 3rd generation optical storage standard, and has technology improvements in various aspects. First, 405 nanometer blue-violet semi-conductor laser technology is now a proven technology and has been commercialized. Secondly, high NA value objectives (NA=0.85) 3 read disc data from the 0.1mm cover layer, ensuring a better tolerance of the whole system. In addition, the improvement in digital signal processing technology and encoding/decoding technology brings the data readout and error correcting to a new level, as a result, the reliability for data storage has significantly improved. 4,

,rI,

____________________

Infrared laser

Red laser

Blue laser

BD 25GB(Single) 50GB(Dual)

NAO.45 DVD 4.7GB(Single) 8.5 GB Dual)

,— ,-

?

Low Capacity: 15G

CD 0.7GB

Figure 1: comparison of the three generations of optical storage technology. At present, BD employs single object lens, with a work distance of 0.5mm or even larger, there is no risk of disc scratching, and it is fully backward compatible with CD and DVD in both reading and writing.

Low Capacity: 3GB

Figure 2: Evolution of optical disc

The single layer capacity of BD is approaching the capacity limit of far-field recording. There are two indicators for this. The laser wavelength employed by BD is now in the blue-violet area, which is near the absorption limit of polycarbonate material, shorter wavelength will result in disc decrepitude and deteriorated light path efficiency; moreover, due to the forbidden band gap limit of semiconductor, shorter wavelength semiconductor laser is difficult to be attained. In addition, the numerical aperture has been increased from 0.6 to 0.85. The NA limit of far-field recording is 1.0, and increase numerical aperture further from 0.85 won’t contribute much to the storage capacity. Furthermore, increasing the NA above 0.85 will lead to improportional cost increase of the optics involved. The compatibility is also concerned by the optical storage industry. Current BD employs single objective light path, with the adjustment of the field distribution at the pupil, the optical pickup unit that can read and write CD, DVD and BD is now available. On reading and writing BD discs, the working distance of an objective lens can be 0.5mm or larger, and no disc scratching would arise. We can learn an important lesson from history. In fact, the physical principles of optical recording show that for an optimal system design (i.e. optimal from point of view of system tolerances and ability to manufacture the various components) there is a certain relation between laser wavelength, cover layer thickness and numerical aperture. Many years ago, at the time of transition from CD to DVD, there were two proposals: (1) was to change only laser wavelength, and to maintain cover layer thickness and numerical aperture. The result was a low capacity disc (called MMCD) (2) was a proposal with optimal system design, reducing cover layer thickness and Numerical Aperture in line with the reduced laser wavelength. Needless to say that DVD was finally based on the better system design and it turned out that, even if disc manufacturing process was different from CD, the better system design made it easy to overcome potential cost problems. The current discussion about next generation optical system follows the same path: there is a low capacity proposal, based upon just changing the laser wavelength, and there is BD, based upon optimal system design. There must be more innovations in disc manufacturing, but the wider tolerances will make it possible to make an easy transition.

3. 0.1 mm COVER LAYER When employing the 0.85 NA objective lens, considering the system tolerance to disc tilt, disc processing, antismudginess, data recording and system complexity 4,5, etc, the cover layer thickness has decreased to 0.1mm in the new specification. As a result, the physical structure of BDs is in sheer contrast to that of CDs, information is read from the thinner side. In optical read/write systems, the optical aberration is in linear relationship with the thickness of the cover

Proc. of SPIE Vol. 5966 59661R-2

layer, thinner cover layer has effectively less optical aberrations 6,7. BD system has better tolerance to tilt angle than DVD especially in coma forming, avoiding the use of complex real-time tilt servo system. In spherical aberration elimination, spherical aberration compensation is introduced in BD system, but since this compensation needs no real time servo control, the optical head realization is much simpler. The following table shows comparison of BD, HD-DVD and red laser DVD in two major optical aberrations as coma and spherical aberration. As we can see, all relative tolerances of BD optical characters exceed two times that of HD-DVD. Table 1: Tolerance comparison of BD, HD-DVD and red laser DVD. Disc smoothness affects beam spot coma, cover layer thickness affects beam spot spherical aberration. Disc smoothness tolerance (bigger is better) Formula

Cover layer thickness tolerance (bigger is better)

λ

λ

d ⋅ NA

3

d ⋅ NA4

Red laser DVD λ=650nm; NA=0.60;

1

1

1.32

0.93

0.49

0.45

d= 0.6mm Blu-ray Disc λ=405nm; NA=0.85; d= 0.1mm HD- DVD λ=405nm; NA=0.65; d= 0.6mm

In addition, increased NA aperture results in smaller beam spot size, which means a stronger focused energy intensity, thus the laser diode emission power in disc writing can be greatly lowered. Especially in multi-layer disc application, laser light must pass through the front recorder layer to write on underlying layers. If the first layer of dual layer disc has 50% transparency 8, and the laser beam energy intensity is too low, the request for laser emission power may be excessive. The development trend indicates an expectable four-layer storage, for that reason the increase of the NA to 0.85 to increase the spot energy intensity appears to be vital. In addition, increased laser spot power intensity makes high speed burning easier. When using a thick cover layer, when the disc is smudged, the error caused by the smudge will be distributed to various digital bits, but data recorded on the disc can still be read out; this resembles the design of the error correction code: when data are parity protected, the error correction code distributes original information to a larger length scope by adding checksum data, error of individual bytes won’t cause the loss of original information. Therefore, reduced cover layer thickness can be compensated by an enhanced error correcting code in a certain range. BD specification introduced a unique burst error-correcting scheme, which enhances the error correcting ability greatly. However, as the ReedSolomon error correction code is based on byte, the bit error will be amplified to byte error, blindly reducing the thickness of cover layer may result in the contrary. The 0.1mm thickness is an optimal physical size with comprehensive considerations. By the way, technology for hard coating on disc surface is improving 9, and cost will be reduced with the mass production. As we can see in Figure , after applying advanced hard coating, the tolerance against fingerprint for BD increased dramatically, even surpassing that of DVD±RW. BD disc can be cartridge free, thus there’s no reason to linger on the 0.6mm thickness DVD specification, which may sacrifice disc capacity and unit capacity cost, and all the benefits of a thinner cover layer.

4. BD DISC TOLERANCE AND PROCESSING

Proc. of SPIE Vol. 5966 59661R-3

The disc tolerance range is a symbol of processing difficulty. In considering disc processing, due to the 0.1mm cover layer, people may take for granted that the processing for BD is formidably complex. However, according to the major tolerance listed in table 2, we can find BD has less strict technical requirements than other specifications. On the introduction of new products, there is a so-called “learning effect”, bigger design tolerance will provide a faster learning curve for the manufacturers; hence making the introduction of new products easier.

Fingerprints Heavy

Medium

Blu-ray Disc with AFP Hard-Coat

Light

OK

Blu-ray Disc without

NG

OK

DVD±R

NG

OK

DVD±RW

NG

OK

DVD-RAM

NG

OK

OK: Both recording and playback completed

NG: neither recording nor playback possible

Figure 3: After applying advanced hard coating, the anti-smudge consistency of BD increased drastically, even surpassing that of DVD±RW. Table 2 Key tolerance comparison of two blue laser storage schemes. Both are in rewritable form. Tolerance comparison

Blu-ray Disc

HD-DVD

Thickness (n=1.6)/ Relative value

100±5µm ±5%

600±13µm ±2%

Thickness evenness

±3µm ±3%

±13µm ±2%

Tangential disc tilt

±0.7°

±0.7° Additional requirement: maximum warping change per cycle ±0.25°

Radial disc tilt

±0.3°

±0.2°

Birefringence

∆n//≤1.5×10-4 ∆n⊥≤1.2×10-3

40nm equal to requiring ∆n//≤0.33×10-4

Relative value

The first line in Table 2 is the specification for the cover layer. The tolerance of BD seems to be smaller in absolute value. However, such comparison in absolute value is inappropriate. For example, a 10 meter absolute tolerance can be easily controlled at the length of 100 meters, but in a length of 100 kilometers, it would be much more difficult. A wider range of tolerance enables cheaper equipment in processing. From the comarism of Table 2 we will come to the conclusion that the relative tolerance of BD is larger. That’s why spin coating can process the cover layer of BD, which is technically very convenient. The substrate of BD has no requirement on optical parameters, which allows significant freedom in the selection of material. As an environment friendly solution even paper can be used for the disc substrate10. In the procudtion of BD only one piece of substrate is needed while HD-DVD requires two pieces of substrates, which means two times injection moulding would be required to prepare for the material of one disc.

Proc. of SPIE Vol. 5966 59661R-4

Upgrading old DVD production lines to produce HD-DVD is not necessarily an economical process. A HD-DVD has very strict mechanical requirement on disc tilt and high optical requirements on birefringence, thus the injection moulding process will need a relatively longer cooling time for lower internal stress, so that discs will not distort excessively over time. Besides, when a disc leaves the bonding process station, it will have to undergo printing, long time storage, etc. Prolonged cooling time in injection moulding alone will result in less disc output in unit time, thus increasing disc processing cost. If new material is to be used, then processing time may be able to be reduced, but this would be shifting time cost on to material cost. To achieve acceptable yield for a tightened disc specification, injection moulders, substrate materials, recording materials and various processing parameters all have to be updated simultaneously. As for some old production lines, low cost upgrading may be just impossible. Furthermore, the new DVD format employs land/groove recording, requiring the disc processing to take care of the thickness both on the land track and on the groove track, which will increase the complexity of disc processing. BD uses only one substrate, skipping the substrate bonding process of DVDs. Dye recording layer, 0.1mm cover layer and hard coating layers can all be processed with spin coating. Current UV curable resin spin coated cover layer has achieved desirable thickness consistency, and the thickness variation of the cover layer has been controlled to be less than ±1.5µm. The spin coating time for a disc is less than 15 seconds. Spin coating processing is simple and easy; multiple spin coating machines can work in parallel. Another way to produce the 0.1mm cover layer is PSA bonding. This method utilizes pressure-sensitive adhesive to laminate prepared coating on the substrate. Disc produced with this method can keep the thickness evenness error within ±1.0µm.

5. CONCLUSION The advantages of BD are the capacity of reaching the far-field recording limit, large tolerance in processing, the capability of the drive to be fully compatibility with CDs and DVDs in reading/writing, lowest storage cost per gigabyte. All these advantages make BD the first choice of storage in the high definition television era. The Blu-ray Disc Association has included almost all major companies in the optical storage industry, covering fields from content providing, disc processing, equipment manufacturing, product integration, to sale, and is forming a complete value chain.

REFERENCES 1.

H. H. Hopkins, “Diffraction theory of laser read-out systems for optical video discs”, J. Opt. Soc. Am. Vol. 69, pp. 4-24, 1979

2.

Alexander Padiy, Bin Yin, Coen Verschuren, Juil Lee, Ruud Vlutters and Theo Jansen, “Signal processing for 35GB on a singlelayer Blu-ray Disc”, SPIE Vol. 5380, pp. 56-69, 2004

3.

Makoto Itonaga, Fumihiko Ito, Kunihisa Matsuzaki, Shuichiro Chaen, Kenji Oishi, Tomonori Ueno and Akira Nishizawa, “Single objective lens having numerical aperture of 0.85 for a high density optical system”, Jpn. J. Appl. Phys. Vol. 41, pp. 1798-1803, 2002

4.

Yourii V. Martynov, Benno H. W. Hendriks, Ferry Zijp, Jan Aarts, Jan-Peter Baartman, Gerard van Rosmalen, Jean J. H. B. Schleipen and Henk van Houten, “High numerical Aperture Optical Recording: active tilt correction or thin cover layer?”, Jpn. J. Appl. Phys. Vol. 38, pp. 1786-1792 1999

5.

Kenji Yamamoto, Kiyoshi Osato, Isao Ichimura, Fumisada Maeda and Toshio Watanabe, “0.8-Numerical-Aperture TwoElement Objective Lens for the Optical Disk”, Jpn. J. Appl. Phys. Vol. 36, pp. 456-459, 1997

6.

Joseph Braat, “Influence of substrate thickness on optical disk readout”, Appl. Opt., Vol. 36, No. 30, pp. 8056-8062, 1997

7.

Joseph Braat, “Analytical expressions for the wave-front aberration coefficients of a tilted plane-parallel plate”, Appl. Opt., Vol. 36, No. 32, pp. 8459-8467, 1997

8.

Hartmut Richter, Holger Hofmann, Jachim Knittel, Osamu Kawakubo, Toshiyuki Kashiwagi, Andrei Mijiritskii and Jochen Hekkmig, “System aspects of dual-layer phase-change recording with high numerical aperture optics and blue laser”, Jpn. J. Appl. Phys. Vol. 42, pp. 956-960, 2003

9.

Naoki Hayashida, Hideki Hirata, Tsuyoshi Komaki, Mamoru Usami, Tomoki Ushida, Hidetake Itoh, Kenji Yoneyama and Hajime Utsunomiya, “High-performance hard coat for catridge-free Blu-ray Disc”, Jpn. J. Appl. Phys. Vol. 42, pp. 750-753, 2003

10.

Announced by Sony on ODS conference 2004

Proc. of SPIE Vol. 5966 59661R-5