On the reflective surface of an optical disc, this represents the binary number 1.

DVDs

Originally, DVD was an acronym for Digital Video Disc and then later Digital Versatile Disc. Today it is generally agreed that DVD is not an acronym for anything. However, although these discs were originally meant to store video, they have become a common method of storing data. In fact, DVD-ROM drives are not only able to copy (rip) or create (burn) data on DVD discs, but also they are backward compatible and can copy and create CDs as well.

DVDs represent an evolutionary growth of CDs, with slight changes. Considering that the development of DVD follows the CD by 14 years, you can see that the CD was truly a revolutionary creation in its time. It is important to understand that both CDs and DVDs are electro optical devices, as opposed to nearly all other computer peripherals which are electromagnetic. No magnetic fields are involved in the reading or recording of these discs; therefore, they are immune to magnetic fields of any strength, unlike hard drives.

Due to their immunity to magnetic fields, CD and DVD media are unaffected by Electromagnetic Pulse (EMP) effects, X-rays, and other sources of electromagnetic radiation. The primary consideration with recordable CD media (and to a lesser extent, manufactured media) is energy transfer. It takes a significant amount of energy to affect the media that the writing laser transfers to the disc. Rewritable discs (which we'll discuss later) require even more energy to erase or rewrite data.

This is in direct contrast to floppy discs and hard drives, which can be affected by electromagnetic devices such as Magnetic Resonance Imaging (MRI) machines, some airport X-ray scanners, and other devices that create a strong magnetic field. CDs and DVDs are also immune to EMPs from nuclear detonations.

It is important to understand that CD and DVD media are read with light and that recordable discs are written with heat. Using an infrared (IR) laser, data is transferred to a CD or DVD onto a small, focused area that places all of the laser energy onto the target for transfer. It should be noted that all CD and DVD media are sensitive to heat (that is, higher than 120°F/49°C), and recordable media are sensitive to IR, ultraviolet (UV), and other potential intense light sources. Some rewritable media are affected by EPROM erasers, which use an intense UV light source. Various forensic alternative light sources can provide sufficient energy to affect optical media, especially if it is focused on a small area. It is not necessarily a question of heat but one of total energy transfer, which can result in heating.

Both CD and DVD media are organized as a single line of data in a spiral pattern. This spiral is more than 3.7 miles (or 6 kilometers [km]) in length on a CD, and 7.8 miles (or 12.5 km) for a DVD. The starting point for the spiral is toward the center of the disc, with the spiral extending outward. This means that the disc is read and written from the inside out, which is the opposite of how hard drives organize data.

With this spiral organization, there are no cylinders or tracks like those on a hard drive. The term track refers to a grouping of data for optical media. The information along the spiral is spaced linearly, thus following a predictable timing. This means that the spiral contains more information at the outer edge of the disc than at the beginning. It also means that if this information is to be read at a constant speed, the rotation of the disc must change between different points along the spiral.

As shown in Figure 4.8, all optical media are constructed of layers of different materials. This is similar to how all optical media discs are constructed. The differences between different types of discs are as follows:

CD-R The dye layer can be written to once.

CD-ROM The reflector has the information manufactured into it and there is no dye layer.

CD-RW The dye is replaced with multiple layers of different metallic alloys. The alloy is bi-stable and can be changed many times between different states.

DVD DVDs are constructed of two half-thickness discs bonded together, even when only one surface contains information. Each half disc contains the information layer 0.6 millimeters (mm) from the surface of the disc.

DVD media consist of two half-thickness polycarbonate discs; each half contains information and is constructed similarly to CD media. DVD write-once recordable media use a dye layer with slightly different dyes than those used for CD-R media, but otherwise they are very similar physically. Manufactured DVD media have the information manufactured into the reflector and no dye layer is present. Rewritable DVD media use bi-stable alloy layers similar to those for CD rewritable media. The differences between manufactured, write-once, and rewritable media are physically similar between CD and DVD media.

The key to all recordable media types is the presence of a reflector with the ability to reflect laser energy. Data is represented by blocking the path to the reflector either by dye or by a bi-stable metallic alloy. The bottom of a CD is made of a relatively thick piece of polycarbonate plastic. Alternatively, the top is protected by a thin coat of lacquer. Scratches on the polycarbonate are out of focus when the disc is read, and minor scratches are ignored completely. It takes a deep scratch in the polycarbonate to affect the readability of a disc. However, even a small scratch in the lacquer can damage the reflector. Scratching the top of a disc can render it unreadable, which is something to consider the next time you place a disc on your desk top-down “to protect it.” A DVD has polycarbonate on both sides; therefore, it is difficult to scratch the reflector.

5.3.2.1 Introduction to Derivation of NLCM Equations.

Before the invention of lasers, scientists that all optical media were purely linear. Fortunately, the invention of the laser enabled believed scientists to examine the behavior of light in optical materials at higher intensities, making the study of nonlinear optics possible. Since then, it has been extensively investigated by many researchers, and hence the area of nonlinear optics has progressed steadily.

It is obvious that in a nonlinear medium, the presence of an optical field modifies the properties of the medium, which in turn modifies another optical field or the field itself. In other words, we can say that the properties of the medium are dependent on the intensity of the incident applied optical field. Under this circumstance, the refractive index of the medium can be expressed as

(17)n=n0+n2I

We point out that the presence of Kerr nonlinearity may change the width and depth of the PBG described in Eq. (12). Therefore, the transmission of light through a nonlinear periodic structure depends on both wavelength and intensity of the incident light pulse.

In recent times, there has been great interest in considering the nonlinear laser pulse propagation through a medium consisting of alternating oppositely signed (i.e., positive and negative) Kerr coefficients [20, 21]. Let us derive the pulse governing equation in such kind of medium considered above. To do so, it is necessary to have knowledge about the refractive index profile n(z, |E|2) in this periodic structure. Therefore, as a first step, we calculate the refractive index profile as follows.

2.3.1 LIGHT LOSS DUE TO REFLECTION

Light losses due to reflection from the surfaces of the optical media are small and largely wavelength-independent. The largest reflectance for normally incident illumination occurs at the front surface of the cornea, where about 3 percent of the light is reflected. This reflection is large because of the substantial difference in the refractive indices of air and cornea. Reflections from other optical surfaces total less than 0.3 percent of the incident light (Boettner and Wolter, 1962). Although small, the specular reflections or Purkinje images from the front and back surfaces of the cornea and lens can be used to noninvasively track the direction of gaze (Cornsweet and Crane, 1973) and to measure the spectral transmittance of the ocular media in situ (van Norren and Vos, 1974).

4.2.1 Basic principles

The first aspect concerns the stimulated laser emission process. An optical medium like silica glass may convert some energy coming from a first light (pump) to a second light (signal) through a stimulated emission process. This energy process happens with the triple interaction of pump light (higher energy), or signal light (lower energy) and of glass (amplifying medium that accumulates the energy of the pump but also absorbs the energy gap between these two lights, generally as optical phonons corresponding to vibrational molecular states of the glass). The basis of the amplification comes from the population inversion between an upper level populated by the pump and a lower level, the energy gap between both being close to the signal energy. To improve the efficiency of the optical amplification process, it is useful to provide some energy storage capability of the material. This is the case when atoms or ions are excited from their fundamental ground-state levels to other metastable energy levels of higher energy. Depending on the lifetime of such excited state energy levels, the efficiency of amplification may be strongly enhanced. Glass doped with trivalent rare earth such as erbium ions is a good candidate for the laser action in the glass matrix (Figure 4.1). The pump light is then absorbed in a first step by the erbium ions if its center wavelength matches the energy of one relevant excited-state level of erbium. During a second step (during less than 1 μs), the erbium ions decay from this first excited state level (or sublevel) to a metastable excited state, having a significant lifetime (10 ms in the case of erbium). During a third step, other incoming input photons will be duplicated through stimulated emission, resulting in some erbium ions returning back into the fundamental ground-state level (see [4] for in-depth analysis of the principles of amplification).

Optical Memory

The popularity of the optical disk memory as mass storage media has rapidly grown over the past few years. Optical mediums have much higher recording density than their conventional magnetic counterparts. In 1980, Phillips in partnership with Sony developed the compact disc (CD) that quickly replaced the long-playing record. CDs can be prepared by using high-power infrared laser to burn holes (pits) in a master disk. Once a master disk is made, the successive copies can be made very inexpensively (about 2000 copies for $1.00). To read the disk low-power infrared light is shined through the disk. Since the surface of the CD consists of pits and smooth surfaces (lands), the variations of reflected light are used to translate data into a digital signal. Unlike the hard drive in which the disk is rotated at a constant angular velocity (e.g., 5400 rpm), the CD drive needs to achieve constant linear velocity of 120 cm/sec. This means that the angular velocity when reading on the outside of the disk is 330 rpm, and the velocity when reading the inside is 530 rpm. In 1984, the standard for using CDs to store computer data was precisely defined, and the media was referred to as compact disc readonly memory (CD-ROM). The standard states that CD-ROM is to have physical appearance of CDs. Moreover, it must be optically and mechanically compatible with CDs, which means the manufacturing techniques must be compatible. Since then, there have been many advances in CD-ROM technology, such as creation of the CD-recordable (CD-R), the CD-rewritable (CD-RW), the digital versatile disc (DVD), and the DVD-RAM.

CD-ROMs are the most widely used optical storage. With the capacity of 650 MB, CD-ROMs are ideal for distribution of text, images, and programs in an electronically readable form. Since the mid-1990s, CD-ROMs have become the standard on PC systems.

CD-recordable (CD-R) disks at first glance, appear to be similar to regular CD-ROMs except CD-Rs are either gold or green instead of silver. These green and yellow colors are dye, and they are used to simulate lands and pits. In the initial state, the dye is transparent and allows the laser light to pass through and reflect the inner layer (made out of gold). During the writing stage, the laser power is increased to heat up the dye, which results in a dark spot. During the reading stage, the differences between a dark spot and a transparent spot are interpreted as the differences between pits and lands. Kodak is one of the first manufacturers to produce CD-R disks. One of the first uses of CD-Rs was for Kodak's PhotoCD. CD-Rs are being used for backing up hard disks, and they also make it possible for individuals and small companies to manufacture a small number of CD-ROMs. Unfortunately, this technology also allows many individuals and companies to duplicate CD-ROMs and CDs without any regard for copyright violations.

CD-rewritable (CD-RW) technology allows individuals and companies to write, erase, and rewrite CD-ROMs. Unlike CD-R, CD-RW uses a reflecting alloy that possesses two stable states, crystalline and amorphous. These two states reflect light differently. Inside CD-RW drive, three levels of laser power can be used. At high power, the alloy melts and loses some of its reflectivity (amorphous state). This in effect simulates pits. At medium power, the alloy melts again, but this time returns to its crystalline state and regains its reflectivity (returns lands). At low power, the disk is read but there is no state change. Because CD-RWs still cost considerably more than CD-Rs, they are not as widely used as CD-Rs.

Digital versatile disc-ROM (DVD-ROM) technology is similar to CD technology with only three exceptions. First, the pit size is half of CD's pit size (0.4 mc versus 0.8 mc). Second, the spiral grove for recording is 54% tighter. Last, the laser beam is 17% smaller. With these refinements, the capacity is improved 7-fold. A typical CD-ROM can store 650 MB of data, whereas a DVD-ROM can store 4.7 GB. This translates to the ability to hold up to 133 minutes of high-resolution video, with soundtracks for eight languages and subtitles for additional thirty-two languages. The packaging of DVD-ROM comes in four different formats. The first is single-sided with a single layer, which translates to 4.7 GB. The second is single-sided with a dual layer, which translates to 8.5 GB of storage capacity. The third is double-sided with a single layer, which has the capacity of 9.4 GB. Finally, a double-sided formate with a dual layer has the capacity of 17 GB.

DVD-recordable (DVD-R) is a write-once system. The original capacity of a single-sided DVD-R disk that has a single-layer was 3.95 GB, which is slightly lower than a single-sided, single-layer DVD-ROM. The capacity has increased to 4.7 GB.

DVD-rewritable (DVD-RAM) is new type of rewritable DVD that provides much greater data storage than today's CD-RW systems. A single-sided, single-layer DVD-RAM has a capacity of 2.6 GB, and a single-sided, single-layer DVD-RAM has a capacity of 5.2 GB. This allows an hour of MPEG-2 video, which is not very practical for home entertainment use when videotapes can have a capacity of up to 5 hr (standard play). This DVD-RAM specification is therefore aimed at the computer market where it can be used to store large amounts of data.

ISO Images

A common theme for all of these methods of creating a LiveCD is the use of an image at the end to write to the optical media. This image is typically an ISO image and is a standardized method of taking all of the data which will be extracted to a CD or DVD and archiving it into a single file. Instead of a directory structure with a bunch of different files, you have a single file which can be extracted to a hard disk or extracted and written simultaneously to optical media in real time using a number of tools.

In Windows 7 and later, the ability exists natively within the operating system to burn an ISO image to an optical disk. In prior releases, the ISO Recorder “power toy” was required to perform this function or a variety of freeware or commercial tools could be used. In Linux, the cdrecord utility (part of the cdrtools collection) is typically used for this purpose. An example command line for this tool is

cdrecord myimage.iso

This will burn the ISO to the first identified optical drive at the highest rate of speed and will default to building a data CD.

(1) Thick magnetic garnet films

Magnetic garnet materials such as yttrium iron garnet (Y3Fe5O12; YIG), known as transparent ferromagnetic materials, are useful and key magneto-optical (MO) media in optical isolator devices for optical communication and spatial light modulators for holographic data storage.

What represents a 1 in an optical storage device quizlet?

What best describes an optical disc?

What is the shape of hard disk?

Is a term used for tiny depressions on the reflective layer of an optical disc?