Unit 4 LASER and Fiber Optics PDF

Chapter 3: Laser and Fiber optics LASERS AND FIBER OPTICS Part a: LASER I. Basic properties of Laser II. Spontaneous and stimulated emission III. Population inversion, Pumping, Lasing action IV. Types of LASER: Ruby Laser, He-Ne Laser, CO2 Laser, Semiconductor Laser V. A...

Chapter 3: Laser and Fiber optics LASERS AND FIBER OPTICS Part a: LASER I. Basic properties of Laser II. Spontaneous and stimulated emission III. Population inversion, Pumping, Lasing action IV. Types of LASER: Ruby Laser, He-Ne Laser, CO2 Laser, Semiconductor Laser V. Applications Part b: Fiber Optics VI. Total internal reflection, principle and working of optical fibers VII. Different types of optical fibers: step index single mode, step index multimode and graded index multimode fiber VIII. V-number, dispersion and losses in optical fiber IX. Applications Part a: LASER LASER is the acronym for Light Amplification by Stimulated Emission of Radiation. I. Basic Properties of Laser A laser is a device that produces a light beam with some remarkable properties, viz. 1. The light is nearly monochromatic. 2. The light is coherent (temporal as well as spatial), with the waves all exactly in phase with one another. 1 Chapter 3: Laser and Fiber optics 3. A laser beam hardly diverges. Such a beam sent from the earth to a mirror left on the moon by the Apollo 11 expedition remained narrow enough to be detected on its return to the earth (total distance covered 1/3 of a million kilometers). A light beam produced by other means would have spread out too much for this to be done. 4. The beam is extremely intense (large energy density), more than the light from any other source. 5. Highly collimated beam. II. Mechanism of Light Emission For atomic systems in thermal equilibrium with their surroundings, the emission of light is the result of Absorption, and, subsequently, Spontaneous Emission of energy. There is another process whereby the atom in an upper energy level can be triggered or stimulated to emit in phase with an incoming photon. This process is called Stimulated Emission. It is the most important process for laser action.  Absorption Every atom, according to the quantum theory, can reside only in certain discrete energy states or energy levels. Normally, the atoms are in the lowest energy state or ground state. When light from a powerful source like a flash lamp or a mercury arc with a photon of energy h = E2-E1 falls on a substance, the atoms in the ground state (E1, say) can be excited to go to one of the higher levels (E2, say). This process is called absorption. 2 Chapter 3: Laser and Fiber optics  Spontaneous emission Consider an atom (or molecule) of the material staying initially in an excited state E2. Since E2>E1, the atom will tend to spontaneously decay to the ground state E1 to attain the lowest energy state, and a photon of energy h = E2-E1 is released in a random direction as shown above. No external radiation is required to initiate the emission. This process is called “spontaneous emission”. Note that when the released [equal to the energy difference E2-E1] is delivered in the form of an electromagnetic (E.M.) wave, the process called "radiative emission" which is one of the two possible ways. “Non-radiative” decay is occurred when the energy difference (E2-E1) is delivered in some form other than electromagnetic radiation (e.g. it may be transferred to the kinetic energy of the surrounding).  Stimulated emission Stimulated Emission requires the presence of external radiation when an incident photon of energy h = E2-E1 passes by an atom in an excited state E2, it stimulates the atom to drop or decay to the lower state E1. In this process, the atom releases a photon of the same energy, direction, phase and polarization as that of the photon passing by, the net effect is two identical 3  Chapter 3: Laser and Fiber optics photons (i.e. energy = 2h) in the place of one, or an increase in the intensity of the incident beam. It is precisely this process of stimulated emission that makes possible the amplification of light in lasers. The reason that the atom is stimulated to drop is that the incoming photon is an electromagnetic wave and its EM field will exert an oscillating force on the excited atom. If the incoming photon is of the correct frequency, this oscillating force will cause the excitedelectron to drop and both photons will exit with the same frequency, phase and direction. Theory of Lasing Atoms exist most of the time in one of a number of certain characteristic energy levels. In any group of atoms, thermal motion or agitation causes a constant motion of the atoms between low and high energy levels. In the absence of any applied electromagnetic radiation the distribution of the atoms in their various allowed states is governed by Boltzman’s law which states that if an assemblage of atoms is in state of thermal equilibrium at an absolute temp. T, the number of atoms N2 in one energy level E2 is related to the number N1 in another energy level E1 by the equation 𝑁2 = 𝑁1𝑒−(𝐸2−𝐸1)/𝐾𝑇 (1) Where E2>E1, thus clearly N2