02a LASERs Session 2 ( Nd YAG Laser, He-Ne Laser and Semiconductor Laser).pdf

https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE Module 2a - LASERs Session 2 : Nd:YAG Laser, He-Ne Laser and Semicond...

https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE Module 2a - LASERs Session 2 : Nd:YAG Laser, He-Ne Laser and Semiconductor Laser Nd: YAG LASER Nd: YAG Laser is one of the most popular types of solid state laser. It is a four level laser. Yttrium aluminum garnet, Y3Al5O12, commonly called YAG is an optically isotropic crystal. Some of the Y3+ ions in the crystal replaced by neodymium ions (Nd3+). The crystal atoms do not participate in the lasing action but serve as a host lattice in which the active centers namely Nd 3+ ions reside. Fully reflecting Partially reflecting Mirror ELLIPTICAL REFLECTOR Mirror OUTPUT BEAM Nd:YAG rod Krypton Arc LAmp Trigger Pulse Capacitors R Nd:YAG ROD Construction of ELLIPTICAL REFLECTOR Nd:YAG Laser Krypton LAMP Prepared by Sanjiv Badhe Page 1 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE Construction The figure illustrates a typical design of the Nd:YAG laser. It consists of an elliptical cylindrical reflector. Nd:YAG rod is kept along one of its focus lines and a krypton arc lamp along the other focus line. The light leaving one focus of the ellipse will pass through the other focus after reflection from the silvered surface of the reflector. Thus all the light emitted by the arc lamp gets focused on the Nd:YAG rod. One fully reflecting mirror and one partially reflecting mirror are fixed at the two ends which constitute the optical resonator. Working The energy levels of the Neodymium ion in YAG crystal are shown in the following figure. The energy level structure of the free neodymium atom is preserved to a certain extent because of its relatively low concentration. The pumping of the Nd3+ ions to upper states is done by a krypton arc lamp. The optical pumping excites the ground state Nd3+ ions to higher states. The meta- stable state E3 is the upper laser level, while the E2 forms the lower laser level. The upper laser level E3 will be rapidly populated, as the excited Nd3+ ions quickly make downward transitions from the upper energy levels. The lower laser level E2 is far above the ground level and hence it cannot be populated by Nd3+ ions through thermal transitions from the ground level. Therefore, the population inversion is readily achieved between the E3 level and E2 level. The laser emission occurs in infrared (IR) region at wavelength about 10,600 A (1.06 µm). As the laser is a four level laser emission. Thus Nd: YAG laser can be operated in CW mode. An efficiency of better than 1% is achieved. E6 E5 Spontaneous Emission E4 Energy Level Diagram E3 Metastable State 10600 A0 for Nd 3+ ions (Infrared) E2 E1 Nd: YAG lasers find many industrial applications such as resistor trimming, machining operations like welding, hole drilling etc. They are also used in surgery. Prepared by Sanjiv Badhe Page 2 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE HELIUM NEON LASER (He-Ne laser) Helium-Neon laser was the first successful gas laser. It was build by Ali Javan, W. Bennett and D. Heriot in 1961. Construction Brewster Window Partially Fully reflecting reflecting mirror mirror Plasma Tube He + Ne Gas Discharge Cathode Anode Output R Laser beam Current Power Limitter supply He-Ne Laser The schematic of a typical He-Ne laser is shown if fig. It consists of a long discharge tube of length about 50 cm and diameter 1 cm. The tube is filled with a mixture of Helium and Neon gases in the ratio 10:1. Electrodes are provided to produce a discharge in the gas and they are connected to a high voltage power supply. The tube is hermetically sealed by windows inclined at Brewster’s angle at its two ends. This arrangement serves the purpose of getting polarized beam of light. One fully reflecting mirror and one partially reflecting mirror are fixed at the two ends along the axis of the tube which constitute an optical resonator. The distance between the mirrors is adjusted such that it equals mλ/2 and supports standing wave pattern. Working Helium-neon laser is a four-level laser system. The energy level diagram is shown in the following figure- Prepared by Sanjiv Badhe Page 3 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE Energy Transfer through inelastic collision E3 E6 3.39 m E5 E2 E4 1.15 m 6328 A0 E3 Excitation due to Spontaneous emission collision with E2 electrons deexcitation due to collision with walls E1 E1 Energy Levels of Energy Levels of Helium Neon Energy Level Diagram for He-Ne laser When the power is switched on, the electric field ionizes some of the atoms in the mixture of helium and neon gases. Due to the electric field, the electrons and ions will be accelerated towards the anode and cathodes as shown if fig. Since the electrons have a smaller mass, they acquire a higher velocity. Role of He in He-Ne laser? The helium atoms are more readily excitable than neon atoms because they are lighter. The energetic electrons excite helium atoms through collisions to the excited meta-stable levels F2 and F3 which lie at 19.81 eV and 20.61 eV above the ground state respectively. These Helium atoms can return to the normal state by transferring their energy to Neon atoms through collisions. Such energy transfer can take place when two colliding atoms have identical energy levels. Energy E4 and E6 of Neon atoms nearly coincide with energy levels F2 and F3 of Helium respectively. When Helium atoms in excited energy levels F2 and F3 collide with the Neon atoms in the ground level, Neon atoms are excited to energy levels E 4 and E6 and Helium atoms come back to the ground state. This is pumping mechanism in He-Ne laser. The neon atoms are much heavier and could not be pumped efficiently without Helium atoms. The role of Helium atoms is to excite Neon atoms and cause population inversion. The probability of energy transfer from Helium atoms to Neon atoms is more as there are 10 Helium atoms per 1 Neon atom in a gas mixture. Energy levels E4 and E6 of Neon atoms are meta-stable states. Hence the number of Neon atoms accumulate in these levels and population inversion exist energy levels E6 and E5, E6 and E3 and E4 and E3. and lasing action takes place corresponding to the transition due to stimulated emission of radiation between energy levels E6 and E5, E6 and E3 and E4 and E3. Prepared by Sanjiv Badhe Page 4 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE E6 → E5 transition Generates a laser beam of wavelength 33900 A0 (3.39 m) E6 → E3 transition Generates a laser beam of wavelength 6328 A0 E4 → E3 transition Generates a laser beam of wavelength 11500 A0 (1.15 m) Atoms in energy level E2 loose their energy due to collisions with walls of the discharge tube and come to the ground state. Merits and Demerits : He-Ne laser operates in a continuous wave mode and is widely used as a monochromatic source in interferometer, laser printing, bar code reading etc. They are also used as a reference beam in surveying, for alignment in pipes etc. He-Ne laser is highly stable. No separate cooling is needed. But the output power is very low. SEMICONDUCTOR DIODE LASER A semiconductor diode laser is a specially fabricated p-n junction device that emits coherent light when it is forward biased. Population inversion is required for producing stimulated emission and then amplification of light. Semiconductor is not a two level atomic system, but consists of electrons and holes distributed in the respective energy bands. Therefore, laser action in semiconductor involves energy bands rather than discrete levels. Construction A schematic diagram of a homojuction semiconductor laser is shown in following figure- Ohmic contact Active Region p - region Laser Beam n - region Ohmic contact Semiconductor laser Prepared by Sanjiv Badhe Page 5 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE The diode is extremely small in size with sides of the order of 1mm. The junction lies in a horizontal plane through the center. The top and bottom faces are metallized and ohmic contacts are provided to pass current thorough the diode. The front and rear faces are polished parallel to each other and perpendicular to the plane of the junction. The polished faces constitute the resonant cavity. The other two opposite faces are roughened to prevent lasing action in that direction. The active region consists of a layer of about 1µm thickness. Working A simple way of achieving population inversion in a semiconductor is to use it in the form of a heavily doped pn-junction and to forward bias the junction. The energy band diagram of such a pn-junction diode having highly doped p- and n- regions is shown below- EC EC E EC Fn Recombination h=E2-E1 EV EF EF EV EC EFp EV Population inversion EV p-region n-region p-region n-region Fig (b) Heavily doped p-n junction Fig (a) Energy bands of heavily doped p-n forward biased junction With very high doping on a n-side the donor levels as well as a portion of the conduction band are occupied by electrons and the Fermi level lies within the conduction band. Similarly, on the heavily doped p-side the acceptor levels are unoccupied and holes exist in the valence band and the Fermi level lies within the valence band. At thermal equilibrium, the Fermi level is uniform across junction, as shown if Fig (a). When a forward bias is applied to the junction, the energy levels shift and the new distribution as shown fig (b) will be taken up. Electrons and holes are injected into the depletion region which results decrease in depletion region width. The injected electrons and holes appear in high concentrations in this transition region. At low forward current level, the electron-hole recombination cause spontaneous emission of photons and the junction acts as an LED. The bandwidth of the emitted light will be larger. As the current is increased, the intensity of light increases linearly. When the current reaches a threshold value the carrier concentration in the depletion region will reach very high values. The region contains a large concentration of electrons within the conduction band and a large concentration of holes within the valence band, as indicat4ed in the hatched region of fig (b). The upper levels in the same region are vacant. This is the state of population inversion. The narrow region where the state of population inversion is achieved is called inversion region or active region. Thus the forward bias (current) plays the role of pumping agent in semiconductor diode laser. The photons Prepared by Sanjiv Badhe Page 6 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/eQGVkFBjGzE that propagate in the junction plane induce the conduction electrons to jump into the vacant states of valence band. The stimulated electron-hole recombination causes emission of coherent radiation of very narrow bandwidth. At room temperature, GaAs laser emits light at a wavelength of 9000A in IR region. A GaAsP laser radiates at 6500A in the visible red region. The p-n junction lasers are also called injection lasers since the laser action is generated by minority charge carriers injected across the depletion region of the junction. The semiconductor diode lasers are simple, compact and highly efficient. They require very little power and little auxiliary equipment. In contrast to He-Ne gas laser, diode lasers give more divergent beam having an angular spread of the order 50 to 150. They are less monochromatic and highly temperature sensitive. Prepared by Sanjiv Badhe Page 7 of 7