The technology used in blu-ray discs and players. The Blu-ray Disc enables the recording, rewriting and play back of up to 25 gigabytes (GB) of data on a single sided single layer 12cm CD/DVD size disc using a 405nm blue-violet laser.

To obtain this capacity a number of techniques have been developed such as:

employing a blue-violet laser, increasing the numerical aperture of objective lens, making the

optical beam passing substrate thin - 0.1 mm, and evenly thick, using an aberration compensation

method of pickup adapted to the substrate thickness and dual layer discs, improving the

modulation method, enhancing the ability of the error correction circuit without sacrificing the

efficiency, employing the Viterbi decoding method for reading signals and improving the S/N

ratio and the inter symbol interference, developing high speed recording phase change media, etc. In addition, the convenient functions of a recording device have also been realized in the application

format.

 Following the rewritable system, the planning of a read-only system and write-once system has already started. In addition to high picture quality, the introduction of core and new functions is indispensable for the spread of the next generation package media. For example, during the switch from VHS to

DVD, digital recording and interactive functions were newly introduced. Consequently, it is

anticipated that the specifications of BD-ROM will provide a high performance interactiveness

and a connection to broadband services, reflecting the demands of the movie industry

OPTIMIZATION OF THE COVER LAYER THICKNESS

Roots of a 1.2 mm substrate existed in the video disc. One of advantages of laser discs

has been that they are hardly affected by dirt or dust on the disc surface since information is

recorded and read through a cover layer. The first commercial optical disc, which was the

videodisc called VLP or Laser Disc, used a 1.2 mm thick transparent substrate, through which

information was read. This thickness was determined from conditions such as: - Deterioration of

the S/N ratio due to surface contamination was suppressed to a minimum since it used analog

recording,

• A disc of 30 cm in diameter can be molded,

• The disc has sufficient mechanical strength,

• The disc is as thin as possible while satisfying the flatness and optical uniformity.

The last condition is because the thinner the cover layer, the more easily the performance of the

objective lens to converge the laser beam can be improved. This convergence performance of

the objective lens is expressed by what we call NA (Numerical Aperture), and the diameter of a

converging light is inversely proportional to NA . Thus NA is required to be as large 

as possible. However, when the optical axis of the objective lens shifts from the perpendicular to

the disc surface, a deterioration of the convergence performance (aberration) occurs and its

amount grows proportionally to the cube of NA. Since we cannot avoid discs from tilting to

some extent from the optical axis of the objective lens due to the bending of discs or inclination

of the mounting, it has prevented the value of NA from increasing. 

NA- Numerical Aperture is defined as sin(). Where () is half angle of converging light converged by an objective lens. Around 80% of light energy is converged in an area with diameter of / NA

On the other hand, an aberration caused by a disc inclination is proportional to the thickness of

the cover layer. This aberration was originate in a of the refraction angle error at the cover layer

interface resulting from the disc inclination. Further, the amount of blur in the beam spot due to

the refraction angle error is proportional to the distance between the disc surface and the focal

point as shown below.

When the disc tilts refraction angle error, which is deviation from ideal angle to form an ideal

light spot, occurs at the disc surface. This refraction angle error causes aberration at the focal

point. Then the aberration is in proportion to the distance between disc surface and the focal

point, i.e., the aberration is in proportion to thickness of cover layer.

LASER TECHNOLOGY

The technology utilizes a "blue" (actually blue-violet) laser diode operating at a wavelength of 405 nm to read and write data. Conventional DVDs and CDs use red and infrared lasers at 650 nm and 780 nm respectively.

As a color comparison, the visible color of a powered fluorescent black light tube is

dominated by mercury's bluish violet emissions at 435.8 nm. The blue-violet laser diodes used in

Blu-ray Disc drives operate at 405 nm, which is noticeably more violet (closer to the violet end of

the spectrum) than the visible light from a black light. A side effect of the very short wavelength is

that it causes many materials to fluoresce, and the raw beam does appear as whitish-blue if shone on

a white fluorescent surface (such as a piece of paper). While future disc technologies may use

fluorescent media, Blu-ray Disc systems operate in the same manner as CD and DVD systems and

do not make use of fluorescence effects to read out their data.

The blue-violet laser has a shorter wavelength than CD or DVD systems, and this shrinking

makes it possible to store more information on a 12 cm (CD/DVD size) disc. The minimum "spot

size" that a laser can be focused is limited by diffraction, and depends on the wavelength of the

light and the numerical aperture (NA) of the lens used to focus it. By decreasing the wavelength

(moving toward the violet end of the spectrum), using a higher NA (higher quality) dual-lens

system, and making the disk thinner (to avoid unwanted optical effects), the laser beam can be

focused much tighter at the disk surface. This produces a smaller spot on the disc, and therefore

allows more information to be physically contained in the same area. In addition to optical

movements, Blu-ray Discs feature improvements in data encoding, closer track and pit spacing,

allowing for even more data to be packed in.

DIODE

A laser diode is a laser where the active medium is a semiconductor p-n junction

similar to that found in a light-emitting diode. Laser diodes are sometimes referred to (somewhat

redundantly) as injection laser diodes or by the acronyms LD or ILD.

(a) PRINCIPAL OF OPERATION

When a diode is forward biased, holes from the p-region are injected into the

n-region, and electrons from the n-region are injected into the p-region. If electrons and holes are

present in the same region, they may radiatively recombine—that is, the electron "falls into" the hole

and emits a photon with the energy of the band gap . This is called spontaneous emission, and is the

main source of light in a light-emitting diode.

Under suitable conditions, the electron and the hole may coexist in the same

area for quite some time (on the order of microseconds) before they recombine. If a photon of exactly

the right frequency happens along within this time period, recombination may be stimulated by

the photon. This causes another photon of the same frequency to be emitted, with exactly the same

direction, polarization and phase as the first photon.

In a laser diode, the semiconductor crystal is fashioned into a shape

somewhat like a piece of paper—very thin in one direction and rectangular in the other two. The of

the crystal is n-doped, and the bottom is p-doped, resulting in a large, flat p-n junction .The two

ends of the crystal are cleaved so as to form a perfectly smooth, parallel edges; two reflective

parallel edges are called a Fabry-Perot cavity. Photons emitted in precisely the right direction

will be reflected several times from each end face before they are emitted. Each time they pass

through the cavity, the light is amplified by stimulated emission. Hence, if there is more

amplification than loss, the diode begins to "lase"

(b) TYPES OF LASER DIODES

(i) Double heterostructure lasers

In these devices, a layer of low band gap material is sandwiched between two

high band gap layers. One commonly used pair of materials is GaAs with AlGaAs. Each of the junctions between different band gap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article is referred to as a "homojunction" laser, for contrast with these more popular devices.

The advantage of a DH laser is that the region where free electrons and holes

exist simultaneously—the "active" region—is confined to the thin middle layer. This means that

many more of the electron-hole pairs can contribute to amplification—not so many are left out in the

poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the

light is confined to the region where the amplification takes place.

ii) Quantum well lasers

If the middle layer is made thin enough, it starts acting like a quantum well.

This means that in the vertical direction, electron energy is quantized. The difference between

quantum well energy levels can be used for the laser action instead of the band gap. This is very useful

since the wavelength of light emitted can be tuned simply by altering the thickness of the layer. The

efficiency of a quantum well laser is greater than that of a bulk laser due to a tailoring of the

distribution of electrons and holes that are involved in the stimulated emission (light producing)

process.

The problem with these devices is that the thin layer is simply too small to effectively

confine the light. To compensate, another two layers are added on, outside the first three. These layers

have a lower refractive index than the center layers, and hence confine the light effectively. Such a

design is called a separate confinement heterostructure (SCH) laser diode. Almost all commercial laser

diodes since the 1990s have been SCH quantum well diodes

HARD-COATING TECHNOLOGY

The entry of TDK to the BDF (as it was then), announced on 19 March 2004, was accompanied by a number of indications that could significantly improve the outlook for Blu-ray. TDK was introduced for  hard-coating technologies that would enable bare disk handling, along with higher-speed recording heads and multi-layer recording technology (to increase storage densities).TDK's hard coating technique would give BDs scratch resistance and allow them to be cleaned of fingerprints with only a tissue, a procedure that would leave scratches on current CDs and DVDs.

CONTRIBUTION OF HIGH NA TO THE LARGE CAPACITY

Like the BD-RE system, the pick up head for BD-ROM uses a high numerical aperture

(NA) lens of 0.85 and a 405 nm blue laser. In early BD-RE systems the high NA was realized by

using 2 lenses in combination. Today many single lenses with working distance larger than

0.5mm have been developed, and even lenses which can be used in DVD/BD compatible pick

ups and CD/DVD/BD compatible pick ups have been developed.

Figure shows that the high NA lens increases the areal density by 2 times while the blue

laser contributes an additional factor of 2.6 times compared to the areal density of DVD. In total,

the Blu-ray spot size is less than 1/5 that of DVD, resulting in more than 5 times the capacity of

DVD. Figure2-3 shows the optical beam degradation due to the disc tilt. This degradation is

proportional to NA3 and the thickness of the cover layer. We selected 0.1 mm as the thickness

of the cover layer, achieving more than +- 1.60 deg for the radial tilt margin for BD-ROM,

which is similar to that of DVD-ROM.

Figure shows that the high NA lens increases the areal density by 2 times while the blue

laser contributes an additional factor of 2.6 times compared to the areal density of DVD. In total,

the Blu-ray spot size is less than 1/5 that of DVD, resulting in more than 5 times the capacity of

DVD. Figure2-3 shows the optical beam degradation due to the disc tilt. This degradation is

proportional to NA3 and the thickness of the cover layer. We selected 0.1 mm as the thickness

of the cover layer, achieving more than +- 1.60 deg for the radial tilt margin for BD-ROM,

which is similar to that of DVD-ROM

DISC STRUCTURE

Configuration of SL and DL Discs

Figure shows the outline of a Single Layer BD Read-Only disc and Figure shows the

outline of a Dual Layer BD Read-Only disc. To improve scratch resistance, the cover layer can

optionally be protected with an additional hard coat layer. One of the features that differentiate

Blu-ray Disc from DVD recording systems is the position of the recording layer within the disc.

For DVD, the recording layer is sandwiched between two 0.6-mm thick layers of plastic - typically

polycarbonate. The purpose of this is to shift surface scratches, fingerprints and dust particles to a position in the optical pathway where they have negligible effect - i.e. well away from the point of focus of the laser. However, burying the recording layer 0.6 mm below the surface of the disc also has

disadvantages.

Due to the injection molding process used to produce them, disc substrates suffer from

stress-induced birefringence, which means that they split the single incident laser light into two

separate beams. If this splitting is excessive, the drive cannot read data reliably from the disc.

Consequently, the injection molding process has always been a very critical part of CD and

DVD production. Another critical manufacturing tolerance, particularly for DVDs, is the flatness

of the disc, because the laser beam becomes distorted if the disc surface is not perpendicular to

the beam axis - a condition referred to as disc tilt. This distortion increases as the thickness of

the cover layer increases and also increases for higher numerical. To overcome these

disadvantages, the recording layer in a Blu-ray Disc sits on the surface of a 1.1-mm thick plastic

substrate, protected by a 0.1-mm thick cover layer.

With the substrate material no longer in the optical pathway, birefringence problems are

eliminated. In addition, the closer proximity of the recording layer to the drive's objective lens reduces disc tilt sensitivity. This only leaves the problem of surface scratching and fingerprints

Dual Layer Disc

Figure shows the outline of a Dual Layer BD Read-Only disc. To improve scratch

resistance, the cover layer can optionally be protected with an additional hard coat layer. The

different layers are shown. A spacing layer is used to separate the two information discs. Also

The different transmission stack are shown

SPECIFICATIONS OF BLU-RAY

TECHNICAL DETAILS

The table below shows the technical specification of Blu-Ray

FORMATS

Unlike DVDs and CDs, which started with read-only formats and only later added recordable and

re-writable formats, Blu-ray was initially designed in several different formats:

• BD-ROM (read-only) - for pre-recorded content

• BD-R (recordable) - for PC data storage

• BD-RW (rewritable) - for PC data storage

• BD-RE (rewritable) - for HDTV recording

DATA RATE

                              For high-definition movies a much higher data rate is needed than for standard definition. With the BD format’s choices for both NA and wavelength we have been able to realize a format with 5X higher data rate while only doubling the rotation rate of DVD-ROM discs.

The following numbers offer a comparison: Data bit length: 111.75 nm (25GB) (267

nm for DVD) Linear velocity: 7.367 m/s (Movie application) (3.49 m/s for DVD). User data

transfer rate: 53.948 Mbit/s (Movie application) (10.08 Mbps for DVD)

The BD system has the potential for future higher speed drives.

                              The BD-RE (rewritable) standard is now available; to be followed by the BD-R

(recordable) and BD-ROM formats in mid-2004, as part of version 2.0 of the Blu-ray specifications.

BD-ROM pre-recorded media are to be available by late 2005. Looking further ahead in time, Blu-ray

Discs with capacities of 100GB and 200GB are currently being researched, with these capacities

achieved by using four and eight layers respectively.

CODECS

The BD-ROM format  includes 3 codecs: MPEG-2 (the standard used for

DVDs), MPEG-4's H.264/AVC codec, and VC-1 based on Microsoft's Windows Media 9 codec.

The first codec only allows for about two hours of storage on a single layer Blu-ray Disc, but with the

addition of the latter two more advanced codecs, a single-layer disc can hold almost four hours. High definition MPEG-2 has a data rate of about 25Mbps, while the latter two have data rates of about

I5Mbps for video and 3Mbps for audio.

BD-RE (and by extension BD-R) does not currently support any advanced codecs beyond MPEG-2. Because MPEG-2 is currently used to broadcast HDTV, recorders write

this HD stream directly to a disc. Since there are no consumer level recorders capable of real-time

transcoding from the MPEG-2 used for broadcasting and any other codec that might be used for BDRE,

MPEG-2 is the only format supported by BD-RE.

Encoding methods for the audio stream include Linear PCM, Dolby Digital, DTS and dts++ (loss less compression). The Blu-ray Disc Association is known to be looking into other codecs superior to those supported by the DVD specification.

VARIATIONS

An 8 cm BD specification has been finalized and approved. A one-sided, single layer 8 cm BD can hold 15 GB, giving it the capacity of one and a half regular sized (12 cm) singlesided double layer DVDs. This would be an ideal format for small, portable devices, such as portable movie players and digital video cameras. A new hybrid Blu-ray / DVD combo disc has been developed by JVC and is awaiting acceptance by the Blu-ray Disc Association. This would allow both normal DVD players and Blu-ray players to utilize the disc .Users would be able to purchase a single disc that can play at either high definition or standard DVD quality, depending on the hardware utilized. Users that do not have a Blu-ray disc player can view the video content at standard definition using their current DVD

player, and enjoy the same content at high definition resolution when upgrading to a Blu-ray disc player in the future.

COMPATIBILITY

The BDA announced that, while it was not compulsory for manufacturers, Blu-ray lasers and

drives are capable of reading the various DVD formats, ensuring backward compatibility. This

makes the upgrade more attractive to consumers as it does not require replacing their collections of

DVDs.

CURRENT TECHNOLOGY

CURRENT STORAGE DEVICES

Some of the popular storage devices that are available in the market include:

Analog Storage Technology

• VHS

Digital Storage Technology

• Floppy Disc

• Compact Disc (CD)

• Digital Versatile Disc (DVD)

BLU-RAY Vs VHS

The Blu-ray Disc recorder represents a major leap forward in video recording technology as it enables recording of high-definition television (HDTV). It also offers a lot of new innovative features not possible with a traditional VCR:

• Random access, instantly jump to any spot on the disc

• Searching, quickly browse and preview recorded programs in real-time

• Create play lists, change the order of recorded programs and edit recorded video

• Automatically find an empty space to avoid recording over programs

• Simultaneous recording and playback of video (enables Time slip/Chasing

playback)

• Enhanced interactivity, enables more advanced programs and games

• Broadband enabled, access web content, download subtitles and extras

• Improved picture, ability to record high-definition television (HDTV)

• Improved sound, ability to record surround sound (Dolby Digital, DTS, etc)

BLU-RAY Vs OTHER STORAGE DEVICES

The storage capacity of different digital storage technology varies a lot. A usually used version of floppy disc has a capacity of 1.44MB while that of a CD is 700 MB & for DVD it is 4.7 GB. Also they have varying shell lives out of these DVD has the maximum. A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD does. This huge capacity means that a DVD has enough room to store a full-length, MPEG-2-encoded movie, as well as a lot of other information. DVD can also be used to store almost eight hours of CD-quality music per side. DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic.

COMPARISON OF BD AND DVD

A disc in the DVD format can currently hold 4.7 gigabytes of data. Unlike DVD technology, which uses red lasers to etch data onto the disc, the Blu-ray disc technology uses a blue-violet laser to record information.

The blue-violet laser has a shorter wavelength than the red lasers do, and with its smaller area of focus, it can etch more data into the disc . The digital information is etched on the discs in the form of microscopic pits. These pits are arranged in a continuous spiral track from the inside to the outside.

Using a red laser, with 650 nm wavelength, we can only store 4.7 GB on a single sided DVD. TV recording time is only one hour in best quality mode, and two, three or four hours with compromised pictures. Data capacity is inadequate for non-stop backup of a PC hard drive. The data transfer rate, around 10 Mbps, is not fast enough for high quality video.

NEXT GENERATION TECHNOLOGIES

BLU-RAY Vs HD-DVD

Next generation optical disc format developed by Toshiba and NEC. The format is quite different from Blu-ray, but also relies heavily on blue-laser technology to achieve a higher storage capacity. The read-only discs (HD DVD-ROM)  hold 15GB and 30GB, the rewritable discs(HD DVD-RW) hold 20GB and 32GB, while the recordable discs (HD DVD-R) don't support dual-layer discs, so they are limited to 15GB. The format is being developed within the DVD Forum as a possible successor to the current DVD technology.

UPCOMING OF RIVALS

The technology is proven, but that's no guarantee of a smooth migration. Already, a standards war much like those that have broken out over every major medium since the videocassette is threatening this latest optical innovation. The nine electronics companies, led by Sony, Pioneer, and Matsushita Electric Industrial, unveiled a standard format dubbed the Blu-ray Disc, which incorporates blue-violet laser technology and sets the recording capacity of the disks between 23 and 25 gigabytes per side. Within the coalition, Sony, Matsushita, and Hitachi have demonstrated prototypes of lasers that meet the requirements.

HD DVD AS A CONTESTEE

The group (BDF), however, faces competition on several fronts. On one side stands Toshiba Corp, which has refused to endorse the Blue-ray Disc. That's troubling because in the early 1990s, Toshiba led the alliance of electronics and film companies that produced the standard for today's DVD systems, trouncing a competing effort by Sony and Royal Philips Electronics of the Netherlands. Toshiba, which continues to head the DVD Forum, demonstrated its own rewritable optical disk, boasting a capacity of 30 GB per side. And Toshiba is not the only holdout: Mitsubishi Electric and AOL Time Warner, both important members of the DVD Forum, have yet to join the Blue-ray Disc group.

Toshiba is developing another kind of disc using the BLUE LASER Technology under name AOD (Advanced Optical Disc) more popularly known as HD DVD (High Definition DVD).And this technology is also backed up by the DVD Forum similar to the BDF.

There are three versions in development.

1. HD DVD-ROM discs are pre-recorded and offer a capacity of 15 GB per layer per

side. These can be used for distributing HD movies.

2. HD DVD-RW discs are re-writable and can be used to record 20 GB per side for

re- writable versions.

3. HD DVD-R discs are write-once recordable format discs with a capacity of

15 GB per side.

Like Blu-ray discs they need a blue laser of 405 nm wavelength, but are physically similar to DVD discs, as they use a cover layer of 0.6 mm. Therefore HD DVD discs can be manufactured using existing DVD lines, and existing UV mastering equipment.

COMPARISON OF FORMATS

The following table provides a comparison of the two formats .

CONCLUSION

The growing popularity of high-definition television (HDTV) has fostered a new wave of recording technology, soon to supplant the VCR, and possibly even DVD.

The Blu-ray Disc Founders is a consortium of 13 leading electronics firms. It has developed a

superior optical disc known as the Blu-ray Disc (BD). As opposed to the red lasers currently

used to produce DVDs, blue beams have a shorter wavelength, allowing for enhanced precision and more tightly compressed data. While a typical DVD holds 4.7 GB of information, a BD contains 25 GB - enough storage for two hours of HDTV or 13 hours of standard television. Dual-layer discs under development will hold an astounding 54 GB. Aside from greater storage capacity, Blu-ray discs will also contain more interactive features.

The world's two foremost computer manufacturers, Hewlett-Packard and Motley Fool Stock Advisor holding Dell Computer, were formally added to the Blu-ray alliance, virtually ensuring the future adoption of BD technology for PC data storage.

The competing format, known as HD-DVD, is simultaneously under joint development by

Toshiba and NEC. Though HD-DVD technology appears to be an underdog at this point, it

has recently gained notoriety by winning the support of the DVD forum, a confederation of

DVD-related companies.

Further, with consumers clamoring for faster transfer speeds and storage capacity (two of the more notable advantages of BD technology), it's possible that the industry is headed to a

point where BD sales will one day outstrip DVDs. It's too early to call the game just yet, but

this will be an interesting technological development to follow.

REFERENCES

Websites

www.bluray.com

www.blu-raytalk.com

www.opticaldisc-systems.com

www.gizmodo.com