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How Lasers Work Simulated Emission Population Inversion the Laser Medium

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How Lasers Work Simulated Emission Population Inversion the Laser Medium L 36 Modern Physics [2] How lasers work • First we must understand z How lasers work the difference between z Medical applications of lasers incoherent and coherent radiation z Applications of high power lasers • Ordinary light sources z Medical imaging techniques (light bulbs, fluorescent z CAT scans lights, etc) produce incoherent light z MRI’s • lasers produce coherent lightÆ all atoms radiate in the same manner Spontaneous vs Stimulated Emission Simulated emission • Coherent radiation is Ei (larger energy) • With stimulated emission, one photon produced when an atom comes in and its vibrations cause the undergoes stimulated emission. photon electron to fall to the lower energy level, emitting another photon (the bridge) • Spontaneous emission occurs Ef (smaller energy) when an electron makes an Spontaneous emission • Thus, one photon goes in and 2 come out, unprovoked transition to a moving in step and in the same direction lower energy level • Stimulated emission occurs • To get a system of atoms to undergo lasing when an incoming photon Incoming action, we must arrange to have many induces the electron to photon atoms in the excited state Æ this is called change energy levelsÆ amplification Stimulated emission population inversion. Population Inversion The laser medium • In a normal situation (a) • The atoms are pumped more atoms are in the into an excited state lower state than the upper state • The excited electrons fall into an intermediate • If an external energy state (upper laser state) source is provided to and stay there until a excite electrons into a photon comes along higher energy state, a and causes them to fall population inversion can down to the lower laser be created as in (b) state. • this is called pumping 1 He Ne Laser (633 nm red) A Helium-Neon (HeNe) Laser • A HeNe laser is a gas laser which uses a medium of 15% Helium and 85% Neon. • A high voltage discharge is produced in this gas mixture and this produces the population inversion. • The lasing action starts when one atom emits a photon which then induces another atom to emit and so on. • The partially silvered mirror keeps most of the photons in. • Photons which are not moving horizontally do no become part of the laser beam. Medical Applications of Lasers Applications of High Power Lasers Laser surgery to correct for (a) nearsightedness, and Using lasers to (b) farsightedness Cut metals Laser Fusion pellet Multiple beams of a powerful laser are focused on a tiny pellet containing fusion fuel. The laser energy compresses the pellet producing a mini-hydrogen bomb that produces energy 2 Solid State Laser Diodes Lasers Diodes • Diode lasers use semiconductor materials (tiny chips of silicon) as the lasing media small • When current flows through the silicon chip it emits an intense beam of coherent light. • Diode lasers are used to read the information embedded in the pits in CD’s Come in a variety of and DVD’s, and also to read UPC’s in bar different colors code scanners and in laser pointers! Medical Imaging Techniques X-rays • x-rays • very short wavelength (0.01 – 0.1 nm) • CT and CAT scans (Computerized electromagnetic waves Tomography) • produced when • MRI’s (Magnetic Resonance Imaging) energetic electrons slam into a metal target • able to penetrate soft tissue, but not bone • produces a two dimensional shadow image A pineapple and a bananna CAT Scans • A shadow image can be misleading • two shadows taken from different angles provides a better picture • shadows taken at multiple angles gives a more complete picture X ray images are taken at many different angles passing through the patient. Some of the cuts • this is what a CT or CAT overlap. A full three dimensional image can be scan does reconstructed using computers. Æ this procedure is called tomography. 3 Computerized Tomography Is there a better medical diagnostic? • A computerized tomography • A CAT scan does a good job of imaging or CT scan image is formed bones, but it does not provide as good an by analyzing x-ray shadow image of soft tissue images taken at many different angles and • Also, it requires that the patient receives a positions big dose of x-rays, which can be harmful in • an x-ray source and an array themselves Æ it is an invasive diagnostic of electronic detectors • Magnetic resonance imaging (MRI) is a rotates around the patient as the patient slowly moves better method of imaging soft tissue through the ring. MRI- how does it work? A bar magnet in a magnetic field • MRI works by locating the hydrogen atoms magnetic field inside the body. Since the body is mostly water, there are lots of hydrogen atoms N • the nucleus of a hydrogen atom is a single S N protons. Protons behave like tiny bar S magnets with a north pole at one end and a south pole at the other end. • If you put a bar magnet in a magnetic field, Solenoid for producing a strong it will try to align itself with the field. magnetic field by passing a large current through a set of coils Magnetic Resonance Imaging Magnetic Resonance • The rules of atomic physics (quantum • Protons has a “spin” that can be either mechanics) require that the atomic “up” or “down” relative to the direction of hydrogen bar magnets can only have 2 the magnetic field orientations when placed in a magnetic • If radio waves (FM) hit the protons, it can field Æ either parallel or antiparallel to it, cause it them to flip from one spin state to the other at a frequency that depends on we call this spin-up or spin-down the strength of the magnetic field magnetic • These spin flips result in the absorption or field release of radio wave energy that can be spin-up spin-down detected electronically protons 4 Magnetic Resonance Imaging MRI DEVICE • In effect, the magnetic field makes the protons act like tiny radio transmitters that only broadcast their signal when the value of the magnetic field is just right • By varying the strength of the magnetic field as a function of position in the body, the spin flips can be detected in various Homer parts of the body • A computer is used to combine the signals from various parts of the body to generate detailed cross-sectional images MRI safety considerations • The magnetic field used in MRI are very strong – 30,000 times the strength of the earth’s magnetic field. • Because the magnet coils are cooled to liquid helium temperatures, they are usually kept on all the time • Because the magnetic field is on, all iron and steel objects must not be allowed to enter the room. http://www.mercola.com/2001/aug/15/mri.htm 5.
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