Characteristics of laser

SARDAR VALLABHBHAI PATEL INSTITUTE OF TECHNOLOGY

ElectricalDepartment

TOPIC: LASER,FIBER OPTICS & ULTRASONIC PREPARED BY :SMIT SHAH 112BHAVIK PANCHAL 124RISHABH ZALA 113HARSHIL BHATT 115JAGRUT BRAMBHATT 125DHAVAL SONI 127

BATCH C

Characteristics of laser Coherence:The wave trains which are identical in phase and direction are called coherent waves High intensity:Cue to the coherent nature of laser,it has the ability to focus over a small area of 10-6 cm2 High directionality: an ordinary light source emits light in all possible direction. but, since laser travels as a parallel beam it can travel over a long distance without spreading. high monochromaticity:The ligh from a normal monochromatic source spreads over a range of wavelength of the order. But, the spread of the order of 1nm for laser.

Induced absorptionLet the atom initially in the lower state E1. if a photon of energy hv is incident on the atom in the lower state, the atom absorbs the incident photon and gets excited to the higher energy state E2. this process is called induced absorption.

Spontaneous emissionIt is a process in which there is an emission of a photon whenever an atom transits from a higher energy state to a lower energy state without the aid of any external agency. For this process to take place , the atom has to be in the excited state . Since, the higher energy level is an unstable one, the excited atom in the higher energy level E2 spontaneously returns to the lower energy level E1 with the emission of a photon of energy hv=E2-E1 .

Stimulated Emission for this process also, the atom should be already in the excited states. Let a photon having energy hv=E2-E1Interact with an atom in the excited states. Under such interaction, the incident photon stimulates the excited atom in the level E2 to transmit to the lower energy level E1, resulting in the emission of a photon of energy.

Nd: YAG LASERActive medium: This is a four- level solid state laser system. Yttrium aluminium garnet(Y3 Al5 O12),commonly known as YAG, doped with neodymium ions Nd3+ is the active medium. The active medium is taken in the form of a crystal and is drawn into a rod. The neodymium ions Nd3+ are the active center.Resonator cavity: The end faces of the Nd: YAG rod are ground polished and silvered to act as the optical resonator mirrors, or the optical cavity can be formed by using two external reflecting mirrors M1 and M2.

Optical pumping A xenon flash lamp or a krypton flash lamp is used as a pumping source.Construction: The schematic diagram of a Nd: YAG laser is shown in figure . A Nd: YAG rod and a krypton flash lamp are enclosed inside an ellipsoidal reflector. In order to make the entire flash radiation to focus on the laser rod, the Nd: YAG rod is placed at one focal axis and the flash lamp at the other focal axis of the ellipsoidal reflector.

WorkingAny of the spontaneously emitted photon will make the excited Nd3+ ions to undergo a transition between E2-E1 state. Thus,during this transition the stimulated photon is generated.The photons travelling parallel to the resonator axis experience multiple reflections at the mirrors. As a result, the transition E2-E1 yields an intense and coherent laser beam of wavelength 1.064mum. These lasers give beam continuously. The Nd3+ ions then make a transition beteeen E1-Eo, which is a non-radiative transition.

Only a part of the energy emitted by the flash lamp is used to excite the Nd3+ ions,while the rest heats up the crystal. Thus,the system can be cooled by either air or water circulation.

APPLICATION OF LASER(A) In industries, laser are applied to a large extent for the following processes:(1) for welding and melting.(2) for cutting and drilling holes.(3) to test the quality of the materials.(4) for the heat treatment of metallic and non-metallic materials.

(B) In medicine(1) used for the treatment of detached retinas.(2) used in performing micro and bloodless surgery.(3) used for the treatment of human and animal cancer and skin tumours

(C) Military applications(1) the laser beam can serve as a war weapon,i.e., a powerful laser beam can be used to destroy in a few seconds, the big size objects like aeroplanes, missiles etc.,by pointing the laser beam on to them. For this reason, it can be even called as death ray.(2) the laser beam can be used to determine precisely the distance, velocity and direction as well as the size and from of distant objects by means of the reflected signal.it is known as LIDAR.

(D) science and engineering applications(1)it is used in fiber optic communication.(2) communication between planets is possible with laser.(3) it is used in holography.(4) it is used in underwater communication vetween submarines, as they are not easily absorbed by water.(5) it is used to accelerate some chemical reactions.(6) it is used to create new chemical compound by destroying atomic bonds between molecules.(7)it is uses to drill minute holes in cell walls without damaging the cell itself.

Introduction about fiber opticsYou hear about fiber-optic cables whenever people talk about the telephone system, the cable TV system or the Internet. Fiber-optic lines are strands of optically pure glass as thin as a human hair that carry digital information over long distances. They are also used in medical imaging and mechanical engineering inspection.

What are Fiber Optics? If you look closely at a single optical fiber, you will see that it has the following parts: Core - Thin glass center of the fiber where the light travels Cladding - Outer optical material surrounding the core that reflects the light back into the core Buffer coating - Plastic coating that protects the fiber from damage and moisture

Fiber Optics.

plastic jacket

glass or plasticcladdingfiber coreTOTAL INTERNAL REFLECTION

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How Does an Optical Fiber Transmit Light? The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light

How Does an Optical Fiber Transmit Light?

Types of Fiber Optics.* Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers).* Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).

Advantages of Fiber Optics..Less expensive . Thinner Higher carrying capacity Less signal degradation Light signals Low power Digital signals Non-flammable Lightweight Flexible Medical imaging

Properties of ultrasonic waves(i) The frequency of ultrasonic wave is greater than 20 kHz (ii) Their wavelengths are small. As a result, their penetrating power is high (iii) They can travel over long distances as a highly directional beam (iv) They have high energy content(v) Their speed of propagation increases with increase in frequency

Production of ultrasonic waves Based on frequency range and power output, the ultrasonic wave generators are divided into two groupsMechanical Generator Electrical Generator In the following section, the electrical generator methods of producing ultrasonic waves are explainedMagnetostriction generator or oscillator Piezoelectric generator or oscillator The above two methods are widely used nowadays for producing ultrasonic waves

Magnetostriction method When a ferromagnetic material in the form of rod is subjected to an alternating magnetic field parallel to its length, the rod undergoes alternate contractions and expansions at a frequency equal to the frequency of the applied magnetic field. This phenomenon is known as magnetostriction effect.

Circuit diagram for magnetostriction oscillator method

meritsdemerits1) The design of this oscillator is very simple and production cost is low.2) At low ultrasonic frequences, large power output is possible without the risk of damage to the oscillatory circuit.3) Frequencies ranging from 100 Hz to 3000 kHz can be produced.1) It can not generate ultrasonic of frequency above 3000 kHz.2) The frequency of oscillations depends greatly on temperature.3) There will be losses of energy due to hysteresis and eddy current.

PH0101 UNIT 1 LECTURE 628Piezo Electric Generator or Oscillator Principle : Inverse piezo electric effect If mechanical pressure is applied to one pair of opposite faces of certain crystals like quartz, equal and opposite electrical charges appear across its other faces.This is called as piezo-electric effect.The converse of piezo electric effect is also true.If an electric field is applied to one pair of faces, the corresponding changes in the dimensions of the other pair of faces of the crystal are produced.This is known as inverse piezo electric effect or electrostriction.

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PH0101 UNIT 1 LECTURE 629

ConstructionThe circuit diagram is shown in Figure

Piezo electric oscillator

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PH0101 UNIT 1 LECTURE 630The quartz crystal is placed between two metal plates A and B.The plates are connected to the primary (L3) of a transformer which is inductively coupled to the electronics oscillator.The electronic oscillator circuit is a base tuned oscillator circuit.The coils L1 and L2 of oscillator circuit are taken from the secondary of a transformer T. The collector coil L2 is inductively coupled to base coil L1. The coil L1 and variable capacitor C1 form the tank circuit of the oscillator.

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PH0101 UNIT 1 LECTURE 631Working..When H.T. battery is switched on, the oscillator produces high frequency alternating voltages with a frequency.

Due to the transformer action, an oscillatory e.m.f. is induced in the coil L3. This high frequency alternating voltages are fed on the plates A and B.Inverse piezo-electric effect takes place and the crystal contracts and expands alternatively.The crystal is set into mechanical vibrations.The frequency of the vibration is given by n =

where P = 1,2,3,4 etc. for fundamental, first over tone, second over tone etc.,Y = Youngs modulus of the crystal and = density of the crystal.

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PH0101 UNIT 1 LECTURE 632The variable condenser C1 is adjusted such that the frequency of the applied AC voltage is equal to the natural frequency of the quartz crystal, and thus resonance takes place.

The vibrating crystal produces longitudinal ultrasonic waves of large amplitude.

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PH0101 UNIT 1 LECTURE 633

Advantages Ultrasonic frequencies as high as 5 x 108Hz or 500 MHz can be obtained with this arrangement.The output of this oscillator is very high.It is not affected by temperature and humidity. Disadvantages

The cost of piezo electric quartz is very highThe cutting and shaping of quartz crystal are very complex.

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PH0101 UNIT 1 LECTURE 634(7) SONAR SONAR is a technique which stands for Sound Navigation and Ranging.It uses ultrasonics for the detection and identification of under water objects.The method consists of sending a powerful beam of ultrasonics in the suspected direction in water.By noting the time interval between the emission and receipt of beam after reflection, the distance of the object can be easily calculated.The change in frequency of the echo signal due to the Dopper effect helps to determine the velocity of the body and its direction.

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PH0101 UNIT 1 LECTURE 636

Measuring the time interval (t) between the transmitted pulses and the received pulse, the distance between the transmitter and the remote object is determined using the formula., where v is the velocity of sound in sea water.The same principle is used to find the depth of the sea.

Applications of SONARSonar is used in the location of shipwrecks and submarines on the bottom of the sea.It is used for fish-finding application .It is used for seismic survey.

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