Laser, the Ruler of Modern Technology
Introduction:
Laser, Laser, Laser- the word is even uttered by children now-a days. The word is already accepted as a real word having verb form "to lase". Actually the word Laser is an acronym for "Light Amplification by Stimulated Emission of Radiation". Each word is carrying a definite information.
The enormous growth of laser technology has stimulated a broad range of scientific and engineering applications that exploit some of the unique properties of laser light. These properties derived from the distinctive way laser light is produced in contrast to the generation of ordinary light. For example, in an ordinary sodium vapour street lamp the atoms spontaneously emit photons at irregular times in random directions. There, wave fronts are produced in 'packets', by randomly born photons with unrelated births. The result is a non-synchronised light emission resulting in isotropic illumination of incoherent light over a broad spectrum.
Lasers are often termed as monochromatic and coherent sources of light. Coherence concerns more than the frequency of the radiation. Not only is laser light monochromatic but the phases of all its constituent photons are also the same. The light produced by an Light Emitting Diode (LED ) equipped with an optical notch filter, may well be monochromatic radiation but not coherent. These coherent characteristics ( i.e., work in unison ) make the light in a laser beam very special and allow lasers to be used in various applications. Our discussion includes short information about laser principle, different types of laser and citation of few of the thousands of applications for lasers. Lastly but not the least , it is appropriate to mention about the hazards of using lasers carelessly.
Laser Principles:
Let us now discuss each letter of the word L A S E R. i.e., "Light Amplification by Stimulated Emission of Radiation":
Light: Light does not mean only visible light - it may be of other form of electromagnetic wave i.e., infrared or ultraviolet light which are invisible.
Amplification : Amplification means gain in intensity. Here light is being amplified by a particular process called stimulated emission of radiation.
Next the question arises about the words "stimulated emission of radiation". we know that electrons exist at specific energy levels or states characteristic of a particular atom or molecule (Fig.1).
Figure 1: Energy level diagram of an atom with various allowed states. The lowest energy state, E1 , is the ground state. All others are excited states.
These energy levels can be imagined as orbits around the nucleus of an atom. Usually the atoms exit in the lowest energy state i.e., ground state. Now, electrons of the atoms from the ground state can be pumped to higher energy levels (Fig.2) i.e., outer orbits by providing energy in different ways. This is known as stimulated absorption process.
Figure 2(a) and 2(b) represent the states before and after the stimulated absorption of a photon by an atom respectively. The dots represent electrons. One electron is transferred from the ground state to the excited state when the atom absorbs a photon whose energy hf= E
In contrast to the absorption process, the emission process can occur in two different ways. Firstly, if an electron spontaneously decays from higher to lower energy states i.e., from outer to inner orbit, it emits a photon having energy equal to the energy difference between the two energy states. This is called spontaneous emission (Fig.3) process.
Figure 3. Diagram representing the spontaneous emission of a photon by an atom that is initially in the excited state E2. When the electron falls to the ground state, the atom emits a photon whose energy hf = E2 - E1 The frequency or wavelength of emitted radiation is precisely related to the amount of energy released. Depending on the material being used, specific wavelengths of light are absorbed to excite the electrons ( stimulated absorption) and specific wavelength or wavelengths are emitted at the time of de-excitation process (spontaneous emission). However, it is possible to force an emission process by means of a photon. That means a photon of definite energy can force an electron to move from higher to lower energy states having the same energy difference, yielding another photon. This second process results in two photons of the same energy i.e., same frequency or wavelength (monochromatic) and furthermore these two photons will be in phase (coherent). This is known as stimulated emission process (Fig.4). Figure 4.: Diagram representing the stimulated emission of a photon by an incoming photon of energy hf. Initially, the atom is in the excited state. The incoming photon stimulates the atom to emit a second photon of energy hf= E2 - E1
Therefore, an excited atom can relax to a stable state by releasing a photon which is identical in energy, direction, and phase with the incident photon. These two photons can in turn interact with other excited atoms. Thus started with one photon, we can have two, four, eight photons and so on. This amplification corresponds to a build up of photons in the system as a result of the chain reaction of events.
Now, an incident photon can cause atomic transitions either upward (stimulated absorption) or downward (Stimulated emission). Both processes are equally probable. When light is incident on a system of atoms, there is usually a net absorption of energy because there are many more atoms in the ground state than in excited states when the system is in thermal equilibrium. That is, in a normal situation, there are more atoms in the ground state ready to absorb photons than there are atoms in the excited states, ready to emit photons. However, if one can invert the situation so that there are more atoms in an excited state than in the ground state, a net emission of photons can result. Such a condition is called population inversion. This, in fact, is the fundamental principle involved in the operation of a laser. Thus if we don't have a lot of excited atoms to start with, the chance of having a lot of stimulated emission is not great. Therefore, to get a laser to work, we need to have a population inversion. This is done by providing an initial energy to the atoms either by passing electrical current or by illuminating with a bright light pulse and so on.
Actually Einstein predicted theoretically in 1917, 43 years before the first laser was made that under certain circumstances, an incident photon will generate another one , of exactly the same energy and hence the same frequency. Einstein added that in this type of emission both photons, old and new, will be in phase, will have the same polarisation and will propagate in the same direction. At that time of his prediction, electronics was unheard of, transistors didn't exist and vacuum valves were still a novelty.
The phenomenon of stimulated emission was first used by Townes in 1954 in the construction of a micro wave amplification device called the MASER, an acronym for Microwave Amplification by Stimulated Emission of Radiation. At about the same time a similar device was also proposed by Prochorov and Basov. The maser principle was later extended to the optical frequencies by Schawlow and Townes in 1958, which led to the realisation of the device now known as the laser. The first successful operation of a laser device was demonstrated by Maiman in 1960 using ruby crystal.
Thus laser light originates from atoms, ions, or molecule through a process of stimulated emission of radiation. The active laser medium is contained in an enclosure or cavity which organises the normally random emission process into an intense directional, monochromatic and coherent wave. Any laser device (fig. 5) is having the three main components as follows :
Figure 5. is a schematic diagram of a laser set up. The tube contains atoms which represent the active medium. An external source of energy (optical, electrical, etc.) is needed to pump the atoms to excited energy states. The parallel end mirrors provide the feedback of the stimulating wave.
i). Active medium: The active laser medium consists of a collection of atoms, molecules or ions. The excited state of the active laser medium must be having a meta stable state, which means its lifetime must be long compared with the usually short lifetimes of excited states. When such is the case, stimulated emission will occur before spontaneous emission.
Different Types of lasers :
There are many ways to define the types of laser. Based on its pumping scheme a laser can be classified as
On the basis of the operation mode , laser fall into classes of either
Pulsed laser output is obtained through techniques such as Q- Switching (QS) or Mode Locking (ML) or Transverse Electrical Atmosphere (TEA). According to the materials used to produce laser light, lasers can be divided into three categories :
All these lasers work on the same principle but the lasing material or the mechanism for energy pumping vary. Different types of lasers & their uses are given in table I.
MEDIUM POWER(~) Gas HENE Supermarket Scanners Argon Entertainment, Medical CO2 Cutting and Welding CO2 Heat treating Semiconductor GaAs CD players AlGaAs Laser printers GaInAsP Fibre communications Solid State Ruby Live holography Nd:YAG Semiconductor processing Nd:YAG (QS) Medical applications ND:YAG(ML) Short-pulse studies Nd:Glass Laser fusion Dye Ring dye Spectroscopy Rh6G(ML) Scientific studies Chemical HF Weapons Excimer AlF Materials processing XeCl Medical applications Table I : Different types of Lasers & their Uses
Characteristics of lasers can be vastly different considering different aspects which varies as follows :
For the uniqueness of semiconductor diode laser, it is having numerous applications in today's world for two way video, audio and data transmission, information storage and processing. Uniqueness is for the following factors :
Applications of LASER:
Applications of lasers exist throughout our society, and new uses are discovered almost daily. Following are the few of the thousands of applications for lasers :
Applications in manufacturing: Of course, the beam itself is, invisible but laser tools can cut a variety of materials. Because of laser, it is perfectly possible to weld, cut and drill material in industry. Laser machining / wielding is being performed with efficient high power CO2 laser beam (fig. 6).
LASER
PEAK
WAVELENGTH
Uses
1 mW
633 nm
10 W
488 nm
200 W
10.6 mm
5 mW
10.6 mm
5 mW
840 nm
50 mW
760 nm
20 mW
1.3 mm
100 MW
694nm
50 W
1.06 mm
50 MW
1.06 mm
2 kW
1.06 mm
100 TW
1.06 mm
100 mW
Tunable
10 kW
600 nm
50 MW
3 mm
10 MW
193 nm
50 kW
375 nm
Figure 6. : Schematic diagram for beam focusing head design for laser welding when using a shielding gas
Advantages are like low noise, dust, fume and vibration levels, the ease of starting a cut in the middle of a work piece and so on. It also eliminates the need for a wide range of cutting tools. Application of laser leads to higher yields with superior product quality.
Applications in medicine and surgery : The advantages of the laser are the ability to reach inaccessible place, aiming accuracy as laser light can be concentrated into spots and therefore the laser has found applications not only in diagnosis but also in treatment, surgery (fig. 7) and advancement of medical science.
Figure 7. :Schematic diagram of articulated arm laser beam delivery system used in surgery with CO2 lasers
Laser surgery has been known since the mid-1960s, when the first retinal lesions were being successfully repaired. Today, laser surgery is a vast field of activity. The various application areas cover gynaecology, tonsils removal, drilling and cutting bone tissues , stopping of gastric bleeding, removal of birth marks and dermatology. The laser scalpel attacks fewer cells than a steel knife and evaporates them quickly. Since laser beam can be sent down readily through optical fibres and fibres can be introduced into arteries using catheters and it becomes possible to treat coronary artery blockages using lasers (fig. 8).
Figure 8. :Schematic diagram for surgical removal of arterial plague using laser radiation. Laser is carried down an optical fibre inserted into the artery. In the system a viewing fibre bundle is also incorporated.
The optical fibres transmitting the laser beam can remove the plaque, a fatty material, build up on the arterial wall and blocking the blood flow. Again the laser beam induces changes in cells, as opposed to destroying them and so this is applicable in genetic engineering also. Besides this, in laser acupuncture, the thousand-year-old silver and gold needles are replaced by fine, micromanipulator-oriented laser beams. Again excimer laser because of its high level of precision, can change the shape of the cornea to change its refractive power to the desired state and thus correcting the refractive error of eyes with minimal thermal damage to the surrounding tissues.
Applications in communications: Great interest in communicating at optical frequencies was created in the 1960s with the advent of lasers which made available highly coherent optical sources. In the field of communications, laser offer two unusual advantages. These are bandwidth and information transmission rate. Since the optical frequencies are of the order of 5x1014Hz, lasers have a theoretical information capacity exceeding that of microwave systems by five orders of magnitude, which is approximately equal to 10 million TV channels. The use of optical fibres is reliable and versatile media to transmit laser light over long distances. Unlike traditional copper cable that sends information in the form of electrons, fibre optic technology requires the electrons to be converted to photons (light). Owing to successful reduction in optical fibre losses (0.16 dB/km at 1.55nm wavelength) several million km of fibre has been installed world-wide in various optical networks to connect cities , towns and continents. Fibre optic cable is made of glass and carries laser-generated light impulses. This light contains digitized data (video, audio, and text information) that can be rapidly transmitted hundreds of miles. For digital communication system ,a bit , an analogy to "yes" or a "no"/ "on" or "off"/ "0" or "1" defines the unit of information size for storage and transmission. 1 byte is equal to 8 bits. Today, bit rates for long-haul fibre links is far beyond the reach of traditional copper cable communication. Fibre optic cable is much smaller and lighter than copper wire. Two hair-thin strands of optical fibre can transmit the equivalent of 24,000 telephone calls all at the same time. In other words, the smaller and lighter-weight fibre strand can carry 150 times the capacity of the bulkier bundle of copper cable. A optical fibre and laser based communication link consists of three main elements: the emitter which converts the original electrical signal to an optical signal, the fibre itself , transmission medium and the detector which reconverts the optical signal back into an electrical one. Therefore, the information that is sent on fibre optic cable must be coded (placed) onto the light pulses. At the site where the fibre optic information is being sent, a decoder converts the light information into a form (pictures, audio, or written material) that we can understand. A block diagram for conventional optical telecommunication system is depicted in figure 9. Besides this, for communication network, we need in addition repeaters, connectors, couplers, multiplexers, and demultiplexers to increase bandwidth for transmission further.
Figure 9. : Block Diagram of Optical Telecommunication
a) Subscriber's telephone converts sound signal to electrical signal, b) Wire pairs carry analogue electrical signal, c) Encoder convert analogue electrical to digital signal, d) Optical transmitter which transmits digital optical signal from digital electrical signal, e) Optical fibre carries the optical signal, f) Optical receiver receives the optical digital signal and convert its to digital electrical signal, g) Decoder decodes the digital electrical signal to analogue signal h) Wire pairs again carry the electrical signal to receiving telephone and i) the Subscriber's telephone which ultimately converts the electrical analogue signal to sound signal.
Applications in Three-dimensional imaging by Holography : A conventional photograph is only a flat record of a real image projected onto a photographic film. Information about the three dimensional character of the object is almost entirely lost during the photographic recording process. A hologram on the other hand is a special three-dimensional photography, a 'photograph' of an object that retains information about the phase of waves coming from the object using laser. Holography was invented by Dennis Gabor in 1947 but it was applicable after lasers became available. Today, it is used in a multitude of ways including three-dimensional representation of objects, fingerprint identification and laser beam diffraction scanning. Credit cards often have reflection holograms printed on them. They make the cards very difficult for forgers to copy. Nowadays hologram is even used in book by publisher to establish its genuineness.
Applications in Entertainment Industry (Audio , Video Compact Disc) : The entertainment industry too uses lasers in the form of audio , video compact disc and laser show. All information, whether pictorial, verbal, alphabetical or numerical, is reduced to strings of binary "zeros" and "ones". Compact disc audio , video & data storage uses a GaAlAs laser for writing very high density digital data onto a fine layer of a metallic bismuth compound, recording medium. For reading, the same laser is used in combination with a PIN photo diode (fig. 10).
Figure 10.: The diagram depicts the basis of readout from an optical disk. The laser beam is focused on the surface of CD containing information in forms of pits. The reflected laser beam is focused on the photodiode which extract the information to electrical digital pulses. Even dust particles on the protective layer do not affect the readout .
Writing the data onto the fine layer of a bismuth compound is done by means of ablation. A tiny circular matt area obtained by burning out a hole in the high-shine bismuth layer represents "one" as digital storing while the unburnt and therefore highly reflective digital location will denote a "zero". The data are recorded on a continuous, tightly wound spiral, pre-grooved in the polymer backing prior to the deposition of the bismuth layer. On the top of both surfaces glass layer of 1.1mm are placed for strengthening / protective purpose. The data holes have a diameter of 0.6 mm and the between tracks distance is 1.6 mm. This corresponds to a bit density of 3x1010 bits/mm2. A compact disk is having capacity of 640 Million Byes (1 million Byte = 1 MB = 106 Byte ). It can even store information contained in an encyclopaedia, and that is why now encyclopaedia is available in compact disk (CD). To explain this fact , we can do calculation roughly like this : for storing one letter or character for 8 bit system, one byte is required. Therefore, for information contains in one page of having ~20 lines and each line containing of ~10 words and each word of approximately 5 characters, we need at least (1 x 5 x 10 x 20) bytes = 1000 bytes i.e., 1 kilo bytes ( 1kB ) storage capacity. From the following table II, it is clear that one compact disk having capacity of 640 MB ( 1 million Byte = 106 Byte )., can hold the information contain in 1280 books, each of 500 pages. That means a CD can hold information of approximately 1280 books which can be considered as a small library.
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Table II : Capacity of Compact Disk
Therefore, first microfilm, then magnetic disks and now the optical compact disk (CD) have drastically reduced the volume required for information storage.
Applications in Supermarket's bar code, the Librarian's magic wand : A patch of black- and - white stripes - (fig. 11) which would look like a miniature zebra crossing, appeared on some packages of consumer goods from the beginning of the decade of 1980 and by now, its presence on the supermarket goods is almost universal.
Figure 11.: A diagram of UPC version of Bar code. User readable numerical are also present for convenience
It takes much less time and greater accuracy to scan the code than to read and key a price into a cash register. Item price can be checked against the computer- held, daily updated value . Advantages of using bar codes in other fields include increased patient safety through label checks on medicine bottles. The bar code is also widely used by lending libraries. Several codes are at present in use. The most frequently encountered is the Universal Product Code (UPC). This bar can be read in both the left - to right and right - to - left directions. In addition the beauty of the system lies in the method of decoding the information. The laser bar-code scanner has two major parts : a laser (LED chip) and a detector (a photo diode + transistor chip). The check-out person puts the bar code in front of the laser beam. The light of the laser is deflected across the scanning window. The light is absorbed by the black lines of the bar code or is reflected by the white lines of the bar code.. The reflected light is sent to the detector which transforms it into an electrical signal made of low and high states which is translated into 0's and 1's and fed into the computer.
Applications of laser as a sensor device i.e., Fibre optic sensing:Light is having wave character, therefore it can demonstrate the physical properties like interference and wave modulation. Laser light has a well-defined phase, permitting a wide variety of applications based on interference or wave modulation. The coherent light produced by lasers leads itself admirably to sensing and measuring of various all kinds of physical parameters. Distance (within the submicrometre to multikilometre range), velocity (from micrometers per second to kilometres per second), temperature, pressure, frequency and electrical current intensity are but a few of these. The methods employed also in metrology where sensing rely often on interference and heterodyne frequency shifting.
Application in Laser Printing: Laser is also used in printer to get high quality printing. Data Information from the computer system can also be impressed on a light beam by modulating the laser itself. The laser printer (fig. 12) uses a modulated semiconductor laser and the principle of xerography.
Figure 12.: Schematic diagram of a photoconductive drum assembly used in a Laser Printer
The laser light is focused and scanned across a selenium drum where it photo activates electrostatic charges, which hold the carbon particles of the toner. Rotating paper over this drum under heat causes the toner to stick to the paper, forming the printing.
Hazards with LASER:
It seems appropriate to mention at this point about the hazards of using lasers carelessly. Burns of lesser consequence may be caused even by unfocused beams from very powerful lasers where as severe local burns can be caused when the laser beam is focuses on any part of the body. The primary danger is , of course, to the eye because it automatically focuses the incident radiation on the retina. Obviously one must under all circumstances avoid looking directly into the laser. The pulsed laser is most dangerous because all its energy is delivered in a much shorter time than it takes to close the eyelid. Since a minute fraction of the normal pulse energy is sufficient to damage the eye. Therefore, one should avoid the possibilities that spectral reflection from a metal or glass surface reaches an observer's eye. The beam may be observed on a matte white screen. The primary danger with a continuously operating laser is from the infrared coherent radiation and from the incoherent ultraviolet radiation which may be emitted by the discharge used for excitation. Besides this, as some laser source uses high electrical voltage, a careless experimenter may be electrocuted by the power supply.
Since the energy output of pulsed lasers varies a lot and the collimation of the output beams is also highly variable, it not difficult to state quantitative safety rules applicable in every case . For example, the optical power emitted by an inexpensive lasers (HeNe, GaAs ) are a few milliwatt (10-3W ) while peak power of pulsed laser may be multi gigawatt (109 W). Therefore, a pulse ruby laser (peak power of 108 W ) focused to a peak intensity of 1016 W/cm2 , can blast holes in razor blades and ionize the air. Even an unfocused 1 mW HeNe laser has a brightness equal to sunlight on a clear day (0.1 W/cm2 ) and it is dangerous to stare at the beam. Usually, laser products are classified in the increasing order of hazardous level and unsafe as class I , class II, class III, class IV. It is better to remember while entering a laser laboratory as "EYE HAZARD AREA" and to obey the rules for laser safety.
LASER , the name is very common nowadays. Past scientific friction become today's reality because of the invention of Laser in 1960. We have discussed briefly the qualitative nature of laser and a few of its various applications. The laser has provided magic solution to numerous problems
Conclusion:
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- Information through INTERNET
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