Unit-3a Lasers (Introduction to Laser Physics) PDF

Introduction to Laser Physics Light Amplification by Stimulated Emission of Radiation (LASER) Lasers are devices that generate intense, coherent beams of light through the process of optical amplification. This presentation will explore the fundamental principles underlying laser operation, includi...

Introduction to Laser Physics Light Amplification by Stimulated Emission of Radiation (LASER) Lasers are devices that generate intense, coherent beams of light through the process of optical amplification. This presentation will explore the fundamental principles underlying laser operation, including the mechanisms of light absorption, spontaneous and stimulated emission, and population inversion. by Dr. Malay Udeshi Laser Physics: Topics Interaction of radiation with matter Principle and Working of LASER Time-line 1917 – A. Einstein postulates photons Types of LASERS and stimulated emission Application of LASERS 1954 – First microwave laser (MASER), Townes, Shaw low, Prokhorov 1960 – First optical laser (Maiman) 1964 – Nobel Prize in Physics: Townes, Prokhorov, Basov Absorption of Light 1 Light Absorption 2 Excitation When a photon of the The absorbed energy appropriate energy excites the electron, interacts with an atom or temporarily storing it in an molecule, it can be unstable, higher-energy absorbed, causing an state. electron to transition to a higher energy level. 3 Relaxation The excited electron will eventually relax back to its ground state, releasing the stored energy in the form of a new photon. Interaction of Radiation with Spontaneous Emission Photon Emission As the electron transitions to a lower energy state, it Excited State releases the excess energy in the form of a photon. When an electron is in an excited state, it is unstable and will eventually decay back to a lower energy level. Randomness The direction and timing of the spontaneous emission i random, resulting in incoherent light. Stimulated Emission 1 Incoming Photon When a photon with the correct energy interacts with an excited electron, it can stimulate the electron to transition to a lower energy state. 2 Photon Emission The excited electron releases a new photon that is identical in energy, direction, and phase to the incoming photon. 3 Amplification The stimulated emission can lead to a chain reaction, where multiple identical photons are produced, resulting in optical amplification. Population Inversion  The number of atoms present in the excited (or higher) state is greater than the number of atoms present in the ground state (or lower) state is called population inversion. Population Inversion Population Inversion  A mechanism where N2 > N1  This is called POPULATION INVERSION  Population inversion can be created by introducing a so call metastable centre where electrons can piled up to achieve a situation where more N2 than N1  It is not possible to achieve population inversion with a 2-state system.  To create population inversion, a 3-state system is required. Pumping Phenomena of achieving Population inversion is called pumping. Energy into the lasing medium, is pumped external energy source such as a flash lamp or another laser. Population Inversion Metastable States The excited electrons must have a sufficiently long lifetime in the metastable state to allow for the buildup of a population inversion. Amplification Once a population inversion is achieved, stimulated emission can occur, leading to the amplification of light and the creation of a laser beam. Fast Fast decay decay slow decay Laser Cavity Lasing Medium Mirrors Feedback The lasing medium is the The laser cavity is formed The light bounces back material in which the by two highly reflective and forth between the population inversion is mirrors, which trap the mirrors, creating a created and where the light within the medium feedback loop that selects stimulated emission and allow for multiple and amplifies the desired occurs, such as a gas, passes, increasing the laser wavelength. solid, or liquid. amplification.  Ruby Laser Active Medium - Crystalline Substance - Al2O3 Three Main Parts of Ruby Laser 1) Ruby Rod - Al2O3 with 0.05% Chromium doping 2) Resonating Cavity - Ruby Rod 4 cm length and 0.5 cm diameter. Silver 3) Xenon Flash Pump - Pumping Source Laser Beam Properties Collimation Monochromaticity Laser beams are highly Lasers produce light of a single, collimated, meaning the light very narrow wavelength, making waves are parallel and do not the light highly monochromatic. diverge significantly over long distances. Coherence High Intensity Laser light is also highly Lasers can produce very intense, coherent, with the light waves in focused beams of light, with high phase and synchronized. power densities. Applications of Lasers Medical Applications Industrial Applications Scientific Applications Lasers are used in various medical Lasers are used for precision cutting, Lasers are essential tools in scientific procedures, such as eye surgery, welding, and engraving in research, enabling techniques like tumor removal, and hair removal. manufacturing and industrial spectroscopy, interferometry, and processes. laser cooling. Ruby Laser 1) Ruby Rod 1) Al2O3 + 0.05 wt% Cr 2) Mirrors 3) Xenon Flash Tube – optical pumping Working of Ruby Laser Energy level ‘E’ is a metastable Xe gas pumps the Cr atoms at the ground state Cr atoms will be excited to level E1 and E2 state. Atoms will stay The excited atoms will undergo a non-radiative decay to energy level E for finite time at E Thus a population inversion will be there between ‘E’ and ‘E0’ Due to E2 atoms – Blue laser is generated Due to E1 He-Ne Laser The main advantage of gas lasers is that they can be operated continuously. The gas lasers show exceptionally high monochromaticity and high stability of frequency. The output of the laser can be tuned to a certain available wavelength. Hence, the gas lasers are widely used in industries Helium–Neon Laser The first gas laser constructed by Javan in the laboratory is the helium–neon laser as shown in Fig. 11.11. The two important parts of the laser are as follows: Active Medium A mixture of Helium and Neon gases is used. The He:Ne ratio is 10:1. The mixture of gases is filled within the discharge tube Gas Discharge Tube: Made up of Fused quartz tube with diameter of 11.5 cm and a length of 80.6 cm A fully reflecting concave mirror is placed at one end of the discharge tube and a partially reflecting concave mirror is placed at the other end. The population inversion for laser action is achieved by inelastic atom–atom collisions. The collisions of the atoms are made using any one of the following methods given below. (i) Direct current discharge (ii) Alternate current discharge (iii) Electrodeless high frequency discharge, and (iv) High voltage pulses Working of He-Ne Laser The population inversion for laser action is achieved by inelastic atom–atom collisions an energetic electron interact with the ground state helium atom As a result, helium atoms are excited to higher levels 1S0 and 3S1, known as metastable state. The lifetime of these levels are relatively low. The collision of the first kind is represented as Population inversion between a) 3s and 2p – Laser – 6328 A b) 3s and 3p – Laser – 3.39um c) 2s and 2p – laser – 1.15 um Two infrared lasers and 1 visible light laser Is generated using He-Ne laser Fiber Optics Fiber optics is an overlap of science and technology which deals with transmission of light waves into optical fibers, their emission and detection Optical cables are small in size and weight and are more efficient for communication than metal cables The principle behind the transmission of light waves in an optical fiber is total internal reflection. Two parts of the cable are important Core Cladding it is the maximum angle of a ray hitting the fiber core which allows the incident light to be guided by the core since total internal reflection can occur at the core–cladding boundary. For larger incidence angles, there is no total internal reflection, and therefore there are significant power losses at each reflection point. Relation between Numerical aperture and Incidence Angle