 myself, Dr. Mrs. Preeti Sunil Soshi, Assistant Professor, Department of Humanities and Sciences from Valshan Institute of Technology, Solapur. I welcome you in the session related to laser. The learning outcomes of this session are, by the end of this session, the students will be able to understand the construction of helium neon laser, its working features advantages and disadvantages and then applications of helium neon laser. These are the contents of today's session, helium neon laser. Lasers are classified into different types, based on different considerations. Usually, we divide them here on the basis of the material used. Some of the important types of lasers are given here. Solid state laser, which includes ruby laser and NDAG laser. Helium neon laser and carbon dioxide laser are the examples of gas laser. And gallerium arsenide and indium phosphorous are semiconductor diode lasers. Most lasers emit light in the red or infrared regions and they work in either continuous mode or in a pulsed mode. As in this session, we are going to study helium neon laser, which is a type of gas laser. So, let us first know how the gas lasers work. Gas lasers are the most widely used lasers. They operate with rarefied gases as active media and they are excited by electric discharge. In gases, the energy levels of the atoms involved in the lasing process are narrow and as such require the sources with sharp wavelength to excite atoms. The most common method of exciting gas laser medium is by passing an electric discharge through the gas. Electrons present in the discharge transfer energy to atoms in the laser gas by collision. The helium neon laser was the first continuous wave laser. It was built in 1961 by Ali Javan, Bennet and Harriet at Bell Telephone Laboratories. The construction of helium neon laser includes a long discharge tube filled with a mixture of helium and neon gases in the proportion 10 to 1. The electrodes are provided in the discharge tube to produce the discharge in the gas. They are connected to a high voltage power supply. In the diagram, we can see that the tube is sealed by inclined glass windows which are arranged at its two ends and the two mirrors are arranged on the axis of the tube externally which form the fabric-parrot optical resonator. When the power is switched on, a high voltage of about 10 kV is applied across the gas mixture. The electrons and ions produced in the process of discharge are accelerated towards the anode and cathode respectively. They collide with helium and neon atoms which are present on the way. Due to this collision, the electrons excite helium atoms more readily as they are light in weight. One of the excited levels of helium, F2 is at 20.61 electron volt above ground level. It is a metastable state and the excited helium atom cannot return to the ground level through spontaneous emission. However, the excited helium atoms can return to ground level by transferring its excess energy to a neon atom through collision. And such an energy transfer can take place when the two colliding atoms are having identical energy levels. Such an energy transfer is known as resonant energy transfer. Neon energy level E4 is at 20.66 electron volt which is very close to the excited energy level of F2 of helium atom. Therefore, the resonant energy transfer occurs between the excited helium atom and ground level neon atom. At the same time, the kinetic energy of helium provides the additional 0.05 electron volt which is required for excitation of neon atoms. Suddenly after this energy transfer, helium atoms come to the ground level and this is the main pumping mechanism in helium-neon laser. Now let us see the transition of neon. The upper state of neon atom E4 is a metastable state hence neon atoms accumulate in this upper state. The E3 level is partially populated at ordinary temperatures as the population at the higher energy level E4 is greater than the population at lower energy level E3. A state of population inversion is established between E4 and E3. Thus the transition between E4 and E3 generates a laser beam of wavelength 6,328 and stominate. From the level E3, the neon atoms drop to the level E2 spontaneously. E2 is a metastable state. Consequently, neon atoms tend to accumulate at this level. Neon atoms now return to ground level even through frequent collisions with the walls of the glass tube holding the helium-neon gas mixture. The neon atoms are once again available for excitation to higher state and participate in the lasing action. So in this way, helium-neon laser operates in a continuous wave mode. So now the whole lasing action occurring in the helium-neon laser which can be seen after applying the supply and due to the collision of highly energetic electrons and lightweight neon helium atoms. So what is the role of helium atoms in the laser that is to excite the neon atoms and to cause population inversion? Then what is the necessity of narrow glass tube? During the operation of laser, it is necessary that the atoms accumulating at the metastable state E2 are brought to the ground level even quickly. Otherwise, the number of atoms at the ground level will go on diminishing and the laser ceases to function. So the only way of bringing the atoms to the ground state is through collision. Therefore, to increase the probability of atomic collisions with the tube walls, the discharged tube is made narrow. Students, now please pause the video and try to write down the features of helium-neon laser. So now in summary, the features of helium-neon laser can be. The active medium here used is a mixture of helium and neon in the proportion 10 is to 1. Neon atoms are the active centers. The carriers are pumped by electrical discharge. A four-level pumping scheme is used in helium-neon laser. In this, we are getting a red-galer laser beam of wavelength 6,328 and straw minute. And helium-neon laser operates in continuous wave mode. Let us see the advantages of helium-neon laser. It emits light in the visible portion of the spectrum. It is having high stability with less cost and they operate without damage at higher temperatures. The disadvantages includes low efficiency, they are having low gain and they are limited to low power tasks. The applications of helium-neon laser lie in the field of holography for three-dimensional image production from object. They are used in supermarkets to read barcodes and QR codes. They are used in guns for targeting and also they are used in laboratory for demonstration in many colleges or universities. These are the references. Thank you.