LASER A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation. The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies. * Interaction of radiation with matter: 1. A material medium is composed of identical atoms or molecules each of which is characterized by set of discreet energy states. 2. An atom can move from one energy state to another energy state when it receives or release an amount of energy difference between these two states. 3. Energy will get absorbed when atom gets excited when atom gets excited from lower energy level to higher energy level ie. E=E2-E1 4. Energy will get emitted when atom jumps or move from higher energy to lower energy ie. E=E2-E1 5. The time for which an atom can remain in the ground state is unlimited but on other hand an Atom can remain in excited state for limited time only. 6. Hence the limited time for which an atom is in excited state is known as life time of the state. *Stimulated Absorption: E2 _______________

E2 _________________

E1________________

E1 __________________

before

after

A photon incident on the atomic system and excited the atom from the lower energy level E1 to the higher energy level E2 . In this case the atom in lower energy after absorbing the incident energy gets stimulated towards the higher energy E2 . This process is known as absorption. We may express the process as A+h γ=A* Where A –is an atom in the lower state and A*- is an atom in the exited state.

*Spontaneous Emission: E2 _______________

E2 _________________

E1________________

E1 __________________

before

after

Let us consider two energy levels 1 & 2 of some given atom, its energy being E1 and E2 . For convenience level 1 is taken as ground level. Let us now assume that the atom is initially in level 2. Since E2≥E1 .An atom cannot stay in the excited state for a longer time. In a time of about 10-8 sec. the atom reverts to the lower energy state by releasing a photon of energy h γ. The emission of photon occurs on its own and without any external impetus given to excited atom shown in fig. Emission of photon without any external impetus is called spontaneous emission. We may write the process as, A* = A+ h γ The no. of spontaneous transition depends only on the no. of atom N2 at the excited state E2. *Stimulated Emission: E2 _______________

E2

_________________

E1________________ E1

__________________

before

after

An atom in the excited state need not wait for spontaneous emission of photon. Well before the atom can make a spontaneous transition it may interact with a photon with energy h γ= E2- E1 and make a downward transition. The photon is said to stimulate or induce the excited atom to emit a photon of energy E2- E1= h γ The passing photon does not disappear and in addition to it. There is a second photon which is emitted by the excited atom. The phenomenon of forced photon emission by an excited atom due to action of an external agency is called stimulated emission or induced emission. The process may be expressed as, A* + h γ = A+2 h γ In stimulated emission each incident photon encounters a previously excited atom, and the optical field of the photon interacts with the electron. The result of the interaction is a kind of resonance effect which induces each atom to

emit a second photon with same frequency, direction, phase and polarization as the incident photon. *Population Inversion: E2 _______________

_________________

E1________________

__________________

Normal equilibrium

Population Inversion

Population inversion is a state of a system in which the population of an excited state is more than that of a ground state. Naturally, we find the number density to be more in the ground state than in the excited one, hence the name. We can achieve population inversion only in those systems which posses a special kind of excited state called metastable state. The electrons will remain only for about 10-8s in the excited state and after that they make transitions to the ground state by emission of a photon. Whereas, the electrons can stay in the metastable state as long as 10-3s. If the excited state happens to be a metastable state, the atoms can stay excited for longer duration resulting in steady increase in the population of the excited or metastable state and one stage we can achieve the population inversion. Once this happens, the number of stimulated emissions overcome number the spontaneous emissions. The photons from stimulated emission will have the same wavelength, phase and direction. Once the intensity of the photons is sufficient to pass through the partially silvered mirror, we get the laser light. Pumping Energy: Pumping is the mechanism of exciting atoms from the lower energy state to a higher energy state by supplying energy from an external source. For achieving and maintain the condition of population inversion, we have to raise continuously the atoms in the lower energy levels to the upper energy levels. It requires energy to be supplied to the system. examples, 1 )Optical Pumping,2) Electrical Pumping,3) Direct Conversions Principal pumping schemes;

1 Three level Pumping 2 Four level Pumping

Characteristics of Laser: 1 Coherence 2 High intensity 3 High directionality 4 High monochromaticity. 5. Less divergence. Types Of Laser 1 Ruby Laser – A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by T.H. Maiman at Hughes Research Laboratories on May 16, 1960. It consists of three main parts, 1 An active working material- a rod of ruby crystal. 2 A resonant cavity made up of fully reflecting plate at the left of ruby crystal and a partially reflecting plate at the right of the ruby crystal. Both the plates are optically plane and exactly parallel to each other. 3 Exciting system- A helical xenon flash tube with power supply source. Diagram:

Construction; Ruby rod is a basically Al2O3 crystal doped with 0.05% of chromium oxide (Cr2O3 ).The Al3+ ions are replaced by Cr 3+ ions. These impurity ions are responsible for pink color of ruby rod.

Ruby rod is taken in the form of a cylindrical rod of about 2-20cm in length and 0.1-2 cm in diameter. Its ended are grounded and polished such that the end faces are exactly parallel and are also perpendicular to axis of rod, and xenon flash tube surround the ruby rod in the form of spiral. Working: The operation sequence starts from the ignition of xenon flash tube, chromium ions in ruby are the active centers which are responsible for laser transition. In normal state most of the cr3+ ions are in the ground state E1 When light from flash tube of wavelength 5500A0 is made to fall upon the ruby rod, these incident photons are absorbed by the chromium ions and they are raised to the excited state E3 & E3- by the green and blue component of white light. These energy levels have a very small life time. Hence excited cr3+ ions rapidly lose some of energy to the crystal lattice and undergo nonradioactive transition. They quickly drop to levels E2. E2 is Meta stable state having a lifetime of 1000 times more than the life time of E3 level. Therefore cr ions accumulate at E2 level. When more than half of the cr ions present, population accumulates at E2 level. A chance photon emitted spontaneously by a cr ion initiates a chain of stimulated emissions by other cr ions in the Meta stable state. Red photons of wavelength 6943A0 travelling along the axis of the ruby rod are repeatedly reflected at the end mirrors and light amplification takes place. A strong beam of red light emerges out of the front end mirror. Drawbacks of ruby laser 1. 2. 3. 4.

It is pulsar beam. As only green component is used efficiency is very low. Cooling system is required. Some inertial sites are presents.

Holography; Holo means whole and graphy means writing i. e. complete writing. We know that our ordinary photography gives 2D picture of 3D object i. e. camera lense can be focused only in particular part and other part is out of focus that means it gives only intensity distribution no phase information is there. But holography gives intensity distribution as well as phase information i. e. it gives 3D picture of the object.

Principle; It can be explained in two steps 1 Recording of hologram or construction of hologram-It is based on interference of coherent light waves. 2 Reconstruction of hologram- It is based on diffraction of light waves. Recording or construction of hologram; The process of making a hologram is referred as recording of a hologram or construction of hologram. This is based on the principle of the interference of light. The light from a laser source is split into two components S and R . one of beam s is directed towards the object while the other beam is i. e. R is directed towards the photographic plate. The wave illuminating the object is called the object wave or signal wave, and the wave directed towards the photographic plate is called the reference wave. Since both the waves are derived from the same source. So they behave as a coherent beam. Thus two beam interferes with each other producing interference pattern on the photographic plate. Thus the record of this interference pattern constitutes a hologram. The developed hologram will look like an ordinary negative. But it contains a complete record of the original object, recorded in the form of an interference fringe pattern.

(a) Construction of hologram hologram

(b) Reconstruction of

Reconstruction of hologram : This is based on the principle of diffraction of light. The developed holograph is exposed to a laser beam of the same

wavelength which was used while constructing the hologram. This laser beam called reconstruction wave interact with the interference pattern on the hologram and gets diffracted to produce two images of the original object. On looking from the far side of the hologram an observer can see the virtual image occupying the same position as the original object. This virtual image V1 is indistinguishable from the object and appears in complete three dimensional forms. One can observe the different perspectives of the object on moving his eye. The other image called the real image R1 is formed between the observer and the hologram. The real image will appear inverted in depth this image is also known as pseudoscopic image which can be recorded.

Applications of Laser There are many scientific, industrial, military, medical and commercial laser applications. The coherency, high Monochromaticity, focus ability, directionality and ability to reach extremely high powers are all properties which allow for these specialized applications. We discuss a few of them below. 1) Industrial Applications: • Laser cutting: In Industry lasers are used for the precise cutting of flat materials. Lasers have the advantage that there is no physical contact with the material so there is no chance of contamination, also there is less chance of the material warping as the laser energy can be focused on a very small area so the whole material is not heated. Even a three dimensional profile can be cut using lasers. Laser cutting is also employed in tailoring industry. • Laser welding: In laser welding, a beam of laser is focused on to the spot to be welded. Due to the heat generated, the material melts over a tiny area and upon cooling the material becomes homogeneous solid structure. Laser welding is a contact-less process and thus no outside material gets into the welded region. Since the heat affected zones are very small, laser welding is ideal for many microelectronic devices. • Laser drilling: Laser drilling of holes is achieved by subjecting the material to powerful laser pulses of about millisecond duration. The intense heat generated over a short duration by the pulses evaporates the material locally leaving a hole. Very fine holes of the dimensions one tenth of millimeter can be drilled. Since there is mechanical stress involved in laser drilling, even the brittle materials can be drilled. • Measurement of pollutants in the atmosphere: There are various types of pollutants in the atmosphere. In the measurement of pollutants, laser is used in the way a radar system is used. Hence it is called LIDAR meaning Light Detection and Ranging. This can evaluate the distance,

altitude and angular coordinates of the object. 2) Medical Application: 1) blood less, pain less surgery: the laser beam is foused at point of defect in the body and that part sets evaporated due to intense beam of laser without affecting part of body due to focusing properties of light. This action is completed in short duration before feeling the pain. The scalpel which cut the blood vessels in usual surgery, but here, no part of body is not cut, leading to blood less surgery. 2) laser is used in dental field 3) laser is used for destruction of malignant tumer. 4) laser is used in operation of welding of retina on the fundus oculi. 4)Laser is used in ophthalmology. 3) Other Applications of laser: 1) Laser weapons 2) laser printers 3)laser scanner 4)reading printed bar codes 5)playing video disks 6)optical computing and signal processing 7) optical memory cards 4) fiber optical communication Laser has widened the range of information capacity of optical communication channel with the help of fiber optics. Due larege band width information carrying capacity is increased. This communication system provides high security, privacy ,cheep and without cross talk. Fibre Optics: Principle: As the glass has higher R.I. then that of air, thus for some angle of incidence ϕ1 in glass the angle of refraction ϕ2 in air will be more than ϕ1 (fig. (a)). As the angle of incidence ϕ1 is gradually increased; angle of refraction ϕ2 will also be increased gradually. For one particular angle of incidence; the angle of refraction becomes 90˚. Means the reflected ray becomes parallel to the glass surface (fig. (b)). This particular angle of incidence in optically denser medium for which the angle of refraction in air medium becomes 90˚ is called as critical angle ϕc. If the angle of incidence in denser medium is increased beyond ϕc, the ray will be totally reflected back into the same medium. This phenomenon is called as total internal reflection. Optical fibre is based on the same phenomenon.

Structure of Optical Fibre: The fibre optics used for optical communication is waveguide made of transparent dielectric material. Its function is to guide visible/ infrared light over long distance. In optical fibre consists of an inner cylinder which is made of glass, called as core. The core carries light. The core is surrounded by another cylindrical shell of lower R.I. called the cladding. The cladding helps to keep the light within the core through the phenomenon of total internal reflection. The core diameter can vary from about 2 μm to about 100 μm. The cladding diameter is usually 125 μm and above the cladding there is soft plastic coating is done known as sheath for greater strength and protection of fibre. Optical fibre can be constructed either as a single fibre or as a bundle of cable. A fibre bundle consists of number of fibre in single jacket as shown in diagram.

Modes of Propagation: Light propagates as an electromagnetic wave through an optical fibre. It is true that all waves have ray direction above critical angle will trapped within the fibre due to total internal reflection. But it is not true all such wave propagates along the fibre. In reality only certain ray directions are allowed to propagate. The allowed directions corresponds to the modes of the fibre. In simple terms modes can be visualized as the possible number of paths of light in an optical fibre. The paths are all zigzag path excepting the axial directions. According to the paths taken by the light ray while propagating through fibres are classified 1. Axial path 2. Zigzag path. The number of modes that a fibre will support depends on the ratio d/λ where, d is the diameter of the core and λ is wavelength of wave being transmitted. Modes are designated by an ‘order’ number ‘m’. In a fibre of fixed thickness, the higher order modes propagate at smaller angles than the lower order modes. The zero order travels along the axis and known as an axial ray.

Classification of fibre: Optical fibres ae in general of two types 1) Single mode fibre has a smaller core diameter and can support only one mode of propagation. 2) Multimode fibre has a large core diameter and can support number of modes of propagation. Multimode fibres are further distinguished on the basis of index profile. Index profile is a plot of R.I. drawn on horizontal axis versus the distance from the core axis drawn on the vertical axis. a) Single mode step index fibre: It has very fine thin core of uniform refractive index of a higher value which is surrounded by a cladding of lower refractive index. The R.I. changes abruptly at core-cladding boundary, as in fig. because of which it is known as a step index fibre. The fibre is surrounded by an opaque protective sheath. A typical S.M.F. has core diameter of 2μm.

R.I.

b) Multimode step index fibre: It is very much similar to the single mode step fibre except that its core diameter. A typical M.M.F. has core diameter of 100μm which is very large compared to wavelength of light being transmitted. Light follows zigzag path inside the fibre. Many such zigzag paths of propagations are permitted in a M.M.F.

R.I.

c) Graded index fibre: It is multimode fibre with a core consisting of concentric layers of different R.I. Therefore R.I. of the core varies with distance from the fibre axis. It has high value at the centre and falls off with increasing radial distance from the axis. Such profile causes a periodic focusing of light propagation through the fibre.

R.I.

Acceptance Angle: It is defined as the maximum angle that a light ray can have relative to the axis of the fibre and propagate down the fibre and it is given by  n1  n2  n0 

 0  sin 1 

   

n1 = R.I. of core and n2 = R.I. of cladding

NUMERICAL APERTURE: It is defined as the sine of acceptance angle. It refers to the light gathering capacity. N.A. = Sinθ0

Air medium (n0)

θ1

Light ray incidents on fibre core at an angle θ0 to the fibre axis which is less than acceptance angle. For the fibre the ray enters the fibre from a medium of R.I. n0 and fibre core R.I. is n1, which is slightly greater than the cladding R.I. n2. Assuming the entrance face at the fibre core to be normal to axis, then considering the refraction at air core interface and Snell’s law, n0 sin  0  n1 sin 1 -------- (1) At right angle ΔABC;


 1 2 n0 sin 1  n1 sin90  1   n1 cos 1

--------- (2)

Using trigonometric rule; sin 2 1  cos 2 1  1

 cos 1  1  sin 2 1

--------- (3)

Hence equation (1) becomes, n0 sin 1  n1 1  sin 2 1

--------- (4)

Also at core-cladding interface using snell’s law, n1 sin 1  n2 sin 2

--------- (5)

When ϕ1 = ϕc; ϕ2 = 90˚ hence, sin (ϕ2)= 1  sin 1 

n2 n1

Equation (4) becomes, n  n0 sin  0  n1 1   2   n1 

2

For air medium n0 = 1 sin  0  n12  n22  N . A.

Numerical aperture refers to the light gathering capacity of optical fibre. Optical fibre communication system: Transmitter

Information Channel

Receiver

I/P I/P Transducer Transducer

O/P Transducer

Modulator Modulator

Signal Processor Demodulator

Carrier Carrier I/P I/P Channel Channel Coupler Coupler

Information Information Channel Channel

O/P O/P Channel Channel Coupler Coupler

The most important application of optical fibres occurs in the field of communication. A basic optical fibre communication system consists of a transmitter which transforms an electrical signals or an information signal to be transmitted into optical system. A receiver which converts the optical signal back to the original electrical form and an information channel conducts the optical signal from transmitter to receiver. The function of different parts as follows: 1. I/P transducer: It converts a non electrical message into an electrical signal. e.g. Microphone is used for converting sound waves into electric current and video camera for converting image into electric current.

2. Modulator: It performs two main functions. First, it converts the electrical message into proper formats and second it impresses this signal onto the wave generated by the carrier source. 3. Carrier: Its function is to generate the wave on which information is to be transmitted. 4. I/P information coupler: It feeds power into information channel i.e. optical fibre. 5. Information channel: It provides path between transmitter and receiver through glass/plastic optical fibre. 6. O/P information coupler: It directs the light emerging from fibre onto the light detector. For this purpose a simple butt connection is used. 7. Detector: It separates the information from carrier wave. Here optical wave is converted into an electrical current by photo detector. 8. Signal processor: It includes amplification and filtering of undesired frequencies along with transmissions. In digital the processor may include decision circuit in addition to amplification & filtration. 9. O/P transducer: It converts electrical message into required form. e.g. for hearing, loud speaker is used while for visual image C.R.O. tube is used. ADVANTAGES OF OPTICAL FIBRES: 1. Extremely high carrier frequency (1014 to 1015 Hz) provides large bandwidth, which can accommodate millions of channel per carrier. 2. As material used here is dielectric in nature hence fibre optics cable does not generated or receive any electromagnetic and RF interference crosstalk between the two adjacent fibres virtually eliminated. 3. Provides low loss and as glass is mainly used which is cheaper than copper hence cost has low. 4. Being an insulator (glass, plastic) it can provide a good isolation between the input and output parts. 5. Not affected by moisture or corrosion. 6. Can be operated over high temperature range. 7. Fibre is light in weight and small in size. Applications of optical fiber: 1. In Medical Field: Endoscopic applications: The optical fibres can be used see the inaccessible part/ organ of human body. For this purpose a bunch of optical fibres is used to illuminate the part and another bunch used to collect the reflected light from the part. Forming image from reflected light; one can see the inaccessible part of the body otherwise. It is also used to kill the cells affected by cancer. The medicine can send through fibres to the specific tissue. In Ophthalmology: It can be used to attach the detached retina of eye. Laser Angioplasty: Use of optical fibres may replace balloon angioplasty and bypass surgeries by laser angioplasty. If heart cannot work properly due to blockages in veins attached to heart; balloon angioplasty is done. If the vein is

not curable; the whole vein is changed by bypass vein. For laser angioplasty; a special catheter is developed which having three channels- one for viewing; one for sending the laser to remove blockages and third for sucking or flushing out the blockages. 2. Military: The aircrafts or ships containing communication system winded with tons of copper wire. Hence, the weight increases. Also, due to leakage current, it may be accessible to the enemy. Instead of Cu wires; if optical fibers are used due to light weight reduces much weight of communication system and provide the necessary silence to enemy. 3. Entertainment applications: A coherent optical fiber bundle is used to enlarge the image displayed on T.V. screen. Conventional optical projection system is bulky and expensive. 4. Optical fiber sensors: A smoke detector, pollution detector can be built using fibers. A beam of light radiating from one end of fiber can be collected by another fiber. If foreign particles are present, they scattered light and variation in intensity of collected light reveals their presence. If fiber is subjected to heating, the temperature causes a change in the refractive index of fiber. As temperature increases, the difference between the R.I. of core and cladding reduces, leading to the leakage of light to the cladding. A simple thermometer can built by using L.E.D. as light source, a coil of fiber as heat sensing element and photo detector to measure the intensity of light. 5. Communication applications: In many organizations, LAN distributes the information to several stations within the organization. Example: Industry or bank, hospitals, Institutes etc. 6. Optical fiber pollution detector: Fiber optics are used to measure pollution detector and foreign suspended particles in the air. A beam of light is sent from one end of fiber and in case foreign particles are present, they scatter the light and measurement of variation of intensity of light will account of the extent of the presence of foreign particles. 7. Optical fiber level detector: Fiber optics may also be used to measure the level of a liquid in a container. A part of fiber is suspended in the liquid and light is directed to pass through it and its intensity is measured. A bare core lose more core light when it immersed in liquid than when it is placed in air. A sudden change in intensity of light will indicate the liquid level. A LED source , a photodetector and a MMf is used to monitor the liquid level.