The Compact Disc
The compact disc if by far one of the most revolutionary mediums to have been invented in recent times. It has completely changed society in a numbers of ways and has benefited everyone. In this coursework I aim to discover how a CD is produced, written on and how it is read inside a CD-ROM. I will also be looking at the difference between a CD-R and a CD-RW. The average CD-R or RW can hold about 6 billion bits of binary data. This is about 780 megabytes of data, and at 2000 characters per page an average CD can store up to 275,000 pages of text.
A CD can also hold about 74 minutes of audible music as it samples at 44. 1kHz. The first aspect I chose to look at was how a CD is produced at a manufacturing plant and what materials and components go into making a CD. A CD is made up of a number of components. Its base material is polycarbonate; it is what makes the CD strong and provides a surface for other layers to be applied to. A reflective layer is then applied to the surface of the polycarbonate using a process called sputtering. This is a shiny layer that is used to bounce the laser beam back to its original source.
This means that this layer must have a very high integrity so that it remains in shape and not break apart. This reflective layer is usually made up of silver, but on occasions it is made up of gold or platinum. With a CD-R and CD-RW there is an extra photosensitive layer, this layer is very sensitive to different frequencies. This allows for a CD recorder to imprint the bumps and pits onto the CD. Each pit is approximately 0. 5 microns wide and 0. 83 microns to 3. 56 microns long. After this process a clear lacquer coating is applied to prevent oxidation and to seal in the reflective layer.
This thin layer gives no protection form scratches or any other harm that might come to the CD. The picture above shows the layers that are involved in producing a CD. A complete CD is normally 1. 2mm thick. This is very thin as there are six or seven layers that make up the CD. After discovering what a CD was made from, I then wanted to look at how a CD is read in a CD-ROM drive. This was very interesting as there are a number of things that have to happen in order for the CD-ROM to read the CD. The model below shows how the CD-ROM scans a CD.
The laser beam that is sent out from the laser inside the CD-ROM travels through a grating. This diffraction grating separates the colours of incident light. This is done to generate tracking beams, which can be sent through a prism and up onto the CD. The polarizing prism in a CD is known as a Wollaston Prism. The Wollaston prism is made up from two right triangle prisms with perpendicular optic axis. Two beams enter this prism known as the O-ray and the E-ray. At the point where the two prisms meet, the E-ray in the first prism becomes and O-ray in the second and is bent towards the normal. Read if reflection is the bouncing of light rays off of a surface. which situation is not an example of reflection?
However the O-ray becomes an E-ray and is bent away from the normal. This technique gives two different polarized beams. The angle of divergence for the two beams is calculated by the angle that the two prisms are placed together. The picture below shows this happening. You can see the E-ray and the O-ray being produced at different angles in the second prism. This is used to achieve polarized light that is then sent into the 1/4 wave plate. The quarter wave plate consists of a carefully adjusted thickness of a birefringent material such that light with the larger index of refraction is retarded by 90o in phase (i. . a quarter wavelength). The material is cut so that the optic axis is parallel to the front and the back plates. Any linear light, which hits this plate, will be split into two different indices of refraction. This converts linear light into circularly polarized light (to be explained later). This is achieved by adjusting the plane of incident light so that a 450 angle is attained with the optic axis. This will then give the O-rays and the E-rays that are explained above. The O-waves fall behind the E-waves by 900 and this is what gives the circularly polarized light.
The picture above shows how linear light goes into the quarter wave plate and exits it as circularly polarized light. The birefringent material that they use in the Wollaston prism is called Calcite. This mineral is used because its birefringence is so high. Its birefringence is so large then when a crystal of calcite is placed over an arrow on a page it reproduces to arrows in the crystal. The three pictures below show how the calcite produces different images under polarization. The first of these three pictures shows two images of an arrow in the calcite crystal.
The second shows under polarization only the ordinary arrow is transmitted. However under polarization that has been rotated 900 the calcite only shows the extraordinary arrow. This is why calcite is so readily used in a number of different polarizing prisms. The light that has been emitted from the laser then travels into a diffraction grating. This is shown on the picture on the next page. This picture shows how the light is split in the grating into a number of different tracking beams. Once this light has been split it then passes into a polarizing prism.
The polarizing prism is actually made up of two prisms. The angle at which these prisms is cut is very accurate and means that the plane of polarization parallel to the surface undergoes total internal reflection, however any light which is perpendicular to the surface passes straight through the prism. This processes is used to get the quarter wave plate, which is at 450 degrees. This is done so that when the light travels up and hits the bumps and pits the beam returning from them will be polarized parallel to the surface and will be reflected 900 degrees towards the photodiode detector.
This idea is used so that the light that strikes the land travels 1/4 + 1/4 = 1/2 of a wavelength further than light striking the top of the pit. The light that then returns from the land is then delayed by 1/2 a wavelength and is therefore exactly out of phase with the light reflected from the pit. These two waves then interfere with each other destructively, which effectively means that not light has been reflected at all. The picture below shows a wave hitting a pit and hitting the surface of the CD.
The photodiode then interprets the light into binary code as either 0’s or 1’s. This is easy for the diode to do as any light that has hit the surface of the CD comes back at the quarter wave, and is encoded into 0’s, whilst any light that has reflected off one of the pits will be out of phase and therefore will be coded as a 1. The pits are different lengths and widths and this helps the computer to interpret the information. This method produces a series of 0’s and 1’s that can be turned into a set of information by the computer
A positioning coil has to be used to detect if there is an error in the reading of the CD. It checks to see that the laser is scanning the CD at the right angle and that the bumps and pits are not being scanned incorrectly. The picture below shows how this is done. From this picture above it is easy to see how the positioning coil uses the photodiode segments to respond to an error signal, so that the tracking of the disc remains constant and correct. This picture above shows the different lengths of the pits and also how thick the actual layer of the read only part of the CD is.
The hardest part of this reading system is centreing the laser on the data track. The tracking system, which does this job, has to move the laser outward as the CD spins. As the laser moves outwards from the centre of the disc, the pits travel past it faster. This is because the speed of the bumps is equal to the radius times the speed at which the disc is revolving (rpm). Therefore as the laser moves outwards the spindle motor must slow the CD down so that the laser passes over the pits at a constant speed, and the data is read at a constant rate.
If this did not happen the laser would be scanning the CD faster and faster and would be more likely to make an error in reading it. This tracking system must be very precise. In the “three beam system”, a grating is used to produce a first order diffraction maximum to either side of the main beam. These two beams therefore overlap and the reflected light from these two beams should be equal if the main beam is centred on the track correctly. These three beams then travel through a polarizing beam splitter, which only transmits polarizations parallel to the page.
This light then goes through the 1/4 wave plate, which converts it into circularly polarized light. The light is then focused onto the disc and if it strikes the surface of the disk it is reflected back into the laser emitter. However if the light hits a pit it is reflected back into a 1/4 wave plate. As the light is now going in reverse it will be polarized perpendicular to the original beam. This will mean that it can then be sent through the focusing lens and into the photodetector array. The difference between a normal CD and a CD-R is that the later or the two has an extra layer of dye.
On a blank CD-R disc, the dye layer is completely translucent, so all light reflects. The write laser darkens the spots where the bumps would be in a conventional CD, forming non-reflecting areas. These areas are the same as the pits and bumps that are written into a normal CD. By selectively choosing parts of the disc to darken and leaving other areas translucent, the CD-WRITER creates a digital pattern that can be read by a CD-ROM. The light from the laser only reflects off the areas of the CD that are left completely translucent.
However, the light that hits the areas that have been darkened does not reflect back to the photodiode. So even though the CD does not actually have any bumps and pits pressed into it, it still works in exactly the same way as a normal CD. The laser beam that is used to burn onto the CD can heat up to a very high intensity. It has enough energy to reach Curie temperature instantly. (Curie temperature is 300 degrees Celsius, the level at which the magnetic domain loses its characteristic as a magnet). This is much higher than just the read only laser.