LASER Dr. Darshan G P PDF

LASER Dr. Darshan G P Dept. of Physics, RUAS...

LASER Dr. Darshan G P Dept. of Physics, RUAS 1 Faculty of Mathematical and Physical Sciences © Ramaiah University of Applied Sciences  Outline Brief introduction – Importance and History of LASER Properties of LASER Interaction of radiation with matter – Explain stimulated absorption, spontaneous emission and stimulated emission 2 Faculty of Mathematical and Physical Sciences © Ramaiah University of Applied Sciences  Objectives At the end of the session students should Distinguish between laser light and ordinary light List the properties of laser light Understand and explain – stimulated absorption, spontaneous emission and stimulated emission 3 Faculty of Mathematical and Physical Sciences © Ramaiah University of Applied Sciences  Introduction LASER - Light Amplification by Stimulated Emission of Radiation It is a device that emit light having unique properties compared to the light being emitted by conventional sources of light such as incandescent bulb, Mercury vapor lamp, etc., Laser has found innumerable applications in various fields because of their unique properties of the light being emitted by them. The fundamental principles of laser operation are largely based on quantum mechanics 4 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Introduction LASER - Light Amplification by Stimulated Emission of Radiation The concept of Stimulated Emission was introduced by Albert Einstein in 1917 in his paper entitled ‘The quantum theory of Radiation’ Amplification by Stimulated Emission – Can light amplify light? MASER – built by Charles H Townes in 1953 LASER – built by Theodore H Maiman in 1960 C H Townes T H Maiman 5 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Distinction between ordinary light and laser beam Ordinary light Polychromatic – Constitute more than one wavelength Incoherent – Photons will not be in same phase with each other Non-directional – Photons travels in different direction and exhibit high divergence Non-Polarized and Less intense Laser light Monochromatic – Constitute single wavelength Coherent – All photons will have same phase and frequency Directional – All photons travels in same direction and exhibit least divergence Polarized and High intense 6 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Coherence Temporal or longitudinal The phase difference of the waves crossing the two points lying on a plane parallel to the direction of the propagation of beam is independent of time. t = 0s t = 6s a a b b Spatial or transverse The phase difference of the waves crossing the two points on a plane perpendicular to the direction of propagation of the beam is time independent. t = 0s t = 8s a a b b 7 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Interaction of Radiation With Matter Stimulated absorption Spontaneous Emission Stimulated Emission Quantum Theory of Light Max Planck (In 1900) proposed that the electromagnetic energy is absorbed or emitted by the matter in discrete packets, or quanta named as Photons. Energy of a Photon is given by E = hυ, where h is Planck’s constant 11 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Interaction of Radiation With Matter Stimulated Absorption Atom absorbs a photon of right frequency (Δ𝐸 = ℎ𝜈) and get excited to the higher energy level. A* hυ + A → A* A The rate of stimulated absorption Where, B12 is the Einstein’s coefficient for stimulated absorption 12 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Interaction of Radiation With Matter Spontaneous Emission Atom in the excited energy level comes back to the lower energy level after spending relaxation time (≈10-8 s) by emitting a photon of energy, Δ𝐸 = ℎ𝜈. A* m j A*→A + hυ A The rate of spontaneous emission Where, A12 is the Einstein’s coefficient for spontaneous emission 13 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Interaction of Radiation With Matter Stimulated Emission Atom in the excited energy level is forced by a photon of right frequency to comes back to the lower energy level by emitting two photons of energy, Δ𝐸 = ℎ𝜈. A* m j hυ + A*→ A + 2hυ A The rate of stimulated emission Where, B21 is the Einstein’s coefficient for stimulated emission 14 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  hυ + A → A* A*→A + hυ hυ + A*→ A + 2hυ 15 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Relation between Einstein’s coefficients and energy density of radiation 16 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Matter in thermal equilibrium – Boltzmann’s ratio In a system consisting of large number of atoms, the distribution of atoms in different energy levels is given by Maxwell-Boltzmann distribution function. At thermal equilibrium the relative population of the excited state with respect to the ground state is given by the Boltzmann’s factor Ni – Number of atoms in ith state with energy Ei N0 - Number of atoms in ground state The atoms densities in the lower energy state (N0) will be more compared to that of higher energy state (Ni), i.e., N0>>Ni 17 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Relation between Einstein’s coefficients and energy density of radiation Rate of Stimulated Absorption ----- (1) Rate of Spontaneous Emission ----- (2) Rate of Stimulated Emission ----- (3) At thermal equilibrium condition for the system, Rate of Absorption = Rate of Emission From equations (1), (2), & (3), 19 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Relation between Einstein’s coefficients and energy density of radiation From Boltzmann’s law, ----- (5) Substituting equation (5) in (4), ----- (6) From Planck’s formula for energy density of radiation ----- (7) Comparing equation (6) and (7) ----- (4) 20 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Relation between Einstein’s coefficients and energy density of radiation Relation between Einstein coefficients From equation (6), i.e., Expression for energy density In terms of Einstein’s coefficients 21 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences  Ratio of rate to stimulated emission to spontaneous emission As N1/N2 >> 1 , the number of stimulated emission is insignificant compared to that of the spontaneous emission Einstein ‘s coefficient A21 = 1/τ where, τ is the relaxation time of the excited state 22 FacultyofofMathematical Faculty Mathematicaland and Physical Physical Sciences Sciences ©M. © Ramaiah S. Ramaiah University University of Applied of Applied Sciences Sciences