II.2.1- Recording To a certain extent the recording process depends on

whether the disk is to be replicated in large numbers for

the consumer market or is essentially a one-off for

storage purposes. Most disks, whatever their purpose,

contain information recorded in the form of a height

profile. Because of this, replication of the disk is

relatively simple and therefore inexpensive.

Recording information from, for example, a video tape

into the surface relief pattern is called mastering. In this

process a master disk is produced and this is used to

form stampers, which in turn are used to generate large

numbers of video disks by injection-molding techniques.

In a typical mastering process, the master disk, which

is a flat glass substrate, is coated with a thin layer of

photosensitive material (photo resist) about 0.12 m

thick. The surface relief pattern is then recorded by

exposing the resist to a focused laser beam, the

irradiance of which is modulated in accordance with the

information to be stored using a fast acousto-optic or

electro-optic modulator as illustrated in Fig. (3-a). The

exposed areas of resist can now be dissolved away

leaving holes or pits in the resist. This process is very

similar to the familiar of photolithography used in the

mass production of integrated circuits.

The master disk is rotated at an angular frequency of

25 Hz under the focused laser beam, which is scanned

radially outwards, thereby producing a spiral track of

pits. Using, for example, a 25mW HeNe laser beam with

a lens of NA of 0.65 it is relatively simple to produce

pits which are 0.6-0.8 m wide with a track spacing of

1.6 m. Pits can be formed at the rate of several million

per second, their spacing and length being of the order

of a micron as illustrated in Fig. 1.

Fig. 3 Schematic diagram of laser beam recorder.

The recorded master disk is now inspected and if

satisfactory it is used to form a negative of the surface

relief called a ‘father’. This is fabricated by electroplating

the master with nickel. The nickel father is then separated

from the master and subsequently used to form a family

of stampers. This is done by growing mother positives by

further electroplating with nickel after chemical

modification of the surface of the father. In turn each

mother is used to form several negative sons, which are

used in mass replication. (See Figure 3-b)

Figure (3-b) CD Manufacturing Process

II.2.2- Data readout from optical disks

Figure 4 shows the basic arrangement for readout. A

laser beam, usually from a laser diode because of size

considerations is focused through the substrate onto

the reflective layer of the disk. The focusing lens is a

microscope-type objective lens, and to scan the whole

disk, is mounted together with the laser in the readout

head on a carriage below the disk. Part of the reflected

light, which is modulated by the relief pattern of the disk

, is gathered by the same lens and is directed to the

photo detector.

Light is strongly reflected from the areas where there

are no pits (often called ‘land’) and is largely scattered

by the pits so that the output of the detector varies as

the beam follows the track. In digital storage, for

example, a change in the level of the reflected signal

represents a transition from a pit to land or vice versa.

These transitions are, in fact, used to represent ones,

while the path length between transitions, on either pit or

land, represents a certain number of zeros, as illustrated

in Fig. 5.

The use of reflected rather than transmitted light offers a number of advantages. For example:

1- since the disk is approached from one side only the player construction is simplified and the number of optical components required is thereby reduced.

2- A protective coating needs to be present on only one side of the information layer and the relief structure can be shallower than in transmission; both these points simplify mass replication of the disks.

3- Finally focus control is made much simpler and dirt and scratches on the protective surface are separated from the information layer and are thus out of focus, thereby removing their effect on the playback signal.

Fig. (4). A schematic drawing of a small (30 mm high) optical readout head. The two prisms deviate the light reflected from the disk to the

four photo detectors. In addition to providing the required optical “signal” (by summing the output of all four detectors) the detector outputs can be used or focus control and accurate tracking of the

Spiral track.

Fig. 5 Digital storage. A binary ‘one’ is represented by a land-pit or pit-land transition: the number of ‘zeros’ is defined by the

path length (either pit or lend) between transitions.

III- Laser in Military III.1- Coder-Decoder

An assembly of randomly oriented Fibers can be used

in the field of cryptography for coding and decoding

optical information. In this case a deliberate effort is

made to misalign the component fibers so that when an

object is viewed through a random bundle of fibers, For

example, an unrecognizable picture is received at the

other end as shown in Fig. (1)

Fig. 1. Operation of a fiber optics encoding device.

This picture can be decoded only when it is viewed

through the same bundle or an identical one. It is

possible to make two bundles of fibers that produce

identical coding by randomly winding a bundle of fibers

along part of the periphery of a cylindrical drum and

aligning them perfectly at one position on the periphery

of the drum.

When two cuts are applied to this bundle, one at the

aligned portion of the bundle and another diametrically

opposite to it, one of these bundles can be used for

coding and the other for decoding purposes. Whereas it

is possible by this technique to produce two identical

coding-decoding units, the task of producing a large

number of identical units is a difficult one.

Figure 2 shows photographs of two test objects, the

coded transmitted image and the decoded images,

through two fiber optics coder-decoders. The resolution

through the transmitted image is obviously dependent on

the fiber diameter is well as the degree of registration

between the coded image and the decoder.

Although intuitively it may appear that the tolerances

to registration for decoding are severe, in practice it has

been found that this registration is simple to achieve by

mechanical means. As would be expected. the

resolution hunt corresponds to approximately twice the

fiber diameter. considering the cascading effect of

compounded error in photography and registration. It is

clear that any optical, electronic, or film shrinkage

distortion in the intermediate processes could cause

significant image deterioration.

Fig. 2. Two different messages—coded images and decoded images

A large number of identical coder-decoders can be

Fabricated by using a coded aver of fibers which is

used to scan an optical image in one direction. The

coded picture is viewed by scanning through a similar

layer of fibers inversely. Obviously the technological

problem of making coded single layers o fibers is

considerably simpler than that of Fabricating large

bundles. Devices based on this technique have been

made and successfully used.

IV- LASERS IN MEDICINE

In medicine there are three main areas in which

lasers have successfully established themselves.

These are in surgery as a cutting tool, in

ophthalmology and in dermatology. As far as surgery

is concerned, the CO2 laser has proved the most

successful all-rounder, although Nd: YAG lasers can

also be used. The 10.6m output of the CO2 laser is

strongly absorbed by the water molecules present in

tissue and the subsequent evaporation of the water

leads to the physical removal of the tissue.

There are several advantages over mechanical cutting:

The laser beam can be positioned and controlled with a

high accuracy, relatively inaccessible regions can be

reached, limited damage is caused to adjacent tissue and

the laser beam has a cauterizing effect on nearby blood

vessels, which reduces bleeding. Obviously an essential

requirement is an easily maneuverable beam delivery

system. The ideal solution to this would seem to be some

type of optical fiber. For the Nd: YAG laser this is no

problem; however, suitable fibers do not as yet exist for

10.6 m radiation and in CO2 systems the beam is usually

passed down the center of a series of articulated metal

tubes with a mirror at each junction (Fig. 7).

In ophthalmology detached retinas have been

successfully treated by lasers for many years now.

Although ruby lasers were used initially in such

operations, the green output from argon ion lasers is

now more popular. The radiation is strongly absorbed

by red blood cells and the resulting thermal effects lead

to a re-attachment of the retina. Ophthalmology is one

area where treatment is sometimes needed at some

point within a uniform transparent optical medium.

Fig (7) Schematic diagram of articulated arm beam delivery system for use with CO2 lasers in surgery.

Normal ‘thermal’ techniques rely on the absorption of

laser radiation and are not suitable here, since areas

other than those needing treatment will also suffer

heating effects. However, it is possible to use a laser

beam to break down a medium which is transparent to the

beam by using the phenomenon of dielectric breakdown.

This only occurs at very high light irradiances when the

electric field exceeds a critical value (in the region of

108Vm-1) Thus it can easily be arranged that the critical

electric fields are exceeded only within a small volume

surrounding, say, the focal point of a lens. In the

breakdown region the high electric fields cause electrons

to be stripped from the atoms present and a plasma is

formed. This in turn generates a local high-pressure

shock wave which expands outwards like a miniature

explosion and vaporizes the surrounding medium.

Some disfiguring skin conditions can be successfully

treated with lasers. Portwine marks, for example, are

often difficult to treat using conventional surgery

because of the extensive areas that can be involved.

Uniform exposure of such areas to an argon ion laser

beam can cause a bleaching of the affected areas which

appears to be permanent. Similar treatment can be used

in the removal of tattoos.

A method of cancer treatment called phototadiation

therapy can also used in conjunction with lasers. Patients

are injected with a dye substance called HpD. After a few

days the dye accumulates in the cancerous tissue

(normal tissue excretes the dye).

Fig. (8) Removal of arterial plaque using laser radiation carried down an optical fiber inserted into the artery. A

viewing fiber bundle is also incorporated.

When exposed to light of about 630 nm wavelength

HpD undergoes a series of photochemical reactions

resulting in the formation of a chemical that kills the

cancer tissue. The radiation needed may be obtained

from a dye laser pumped with an argon ion laser.

Finally in this section we may mention one possible

future application. Since laser beams are readily sent

down optical fibers and since fibers can be introduced

into arteries using catheters, it becomes possible to

contemplate the treatment of coronary artery blockages

using lasers.

The coronary arteries becomes blocked when deposits

of plaque, a fatty material, build up on the arterial wall

and reduce the space available for blood flow. Provided

the optical fibers transmitting the laser beam could be

accurately positioned then the plaque could, in theory, be

removed by being vaporized with the laser beam (Fig.8).

The main danger is of accidentally burning a hole in the

arterial wall. However, since plaque and arterial wall differ

in a number of their optical properties it may be possible

to sense the type of tissue being aimed at or being

vaporized by using an auxiliary sensing fiber. Similar

techniques could also possibly be used to remove other

types of obstruction from veins and arteries such as

blood clots.