Transcript Laser and its applications

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 mm
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 mm wide with a track spacing of
1.6 mm. 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 landpit 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
These
are
successfully established
in
surgery
as
a
themselves.
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 10.6mm 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 mm 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.