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Laser Sources Laser sources: We can obtain ınformation with laser that is difficult to to obtain with other conventional sources. Principles of laser: When an electromagnetic radiation passes through the medium of atoms or molcules the flux of radiation will change : 1 ni is number of particle at ground state and nj is number of particle at excited state 2 3 The condition that the number of particles at excited state is biger than the number of ground state called population inversion and this condition is obtain by mean of pumping process. Pumping is obtained by : • Optical methods • Chemical methods • Electrical methods 4 Types of lasers: Continous wave : when excited state life time is longer than ground state the output of laser is constant with respect to the time. Pulsed: when the upper level life time is not longer than lower level it causes self-terminating. Q-switched laser: have more efficient population inversion 5 Three level laser: Because lower level is ground state population inversion is somewhat hard. λ of interest for lasing Lased radiation Pumping λ 6 Four level laser: Because lower level λ of interest for lasing Lased radiation isn’t ground state population inversion is somewhat easy. Pumping λ 7 Two level laser: they could not Radiation of interest for lasing pump optically. An example of Lased radiation this type is diode lasers. If we try to pump the two level system with optical Pumping force process the wavelength of interest absorbed for pumping and couldn’t lased because when population of upper level be equals population of lower level absorption and emission exist at same rate . 8 longitudinal modes • Mode locking 9 Solid-state lasers: The first operating laser was a pulsed laser which had as its active medium a ruby 3+ crystal (Cr in Al2O3). The active medium is shaped as rod and is optically pumped. 690nm 5ms pulses 10 Nd YAG laser Neodymium Yttrium, Alminum, Garnet 1.6 m ns pulses, Q switch 4 levels 11 Gas laser: • The helium-neon (He-Ne) laser is the most common laser that works continuously and pumping process is made by electrical discharge . • The lasing occur at neon energy levels and helium works as the component which is improve the pumping efficiency . • All of the noble gases have been made to lase . 12 Ar ion laser 13 CO2 laser Emits radiation at IR region and pumped Excimer lasers by means of collision of electrons . are among newest gas laser have been made. They made with rare gas hlides N2 laser that always exists at excited state and utilizes the vibronic transition for lasing population inversion easily occurred. and it used for pumping source of tuneble dye lasers . 14 Dye laser: The first liquid laser successfully operated was dye laser .dyes have been made to lase IR to UV region . Dye laser pumped with optical methods for example by N2 laser .wavelength tuning is made by prism or interference filters or fabry perot etalons. Output frequency of dye laser can be higher(intermode 15 spacing) or lower(Cavity damping) than pumping source. Ring cavity +Etalon 16 Semiconductor lasers: in the semiconductor or diode lasers population inversion occure between conductive bond and valance bond of a pn- junction diode . transition of electrons from conductive bond to valance bond is responsible for lasing process and radiation is made by electron-hole recombination process. 17 Chemical Lasers exoenergic reaction pumping Example: HF laser Electric discharge H + F HF* (lasing) 18 Laser characterstics: (laser radiation sources have different charactristics from other conventuional sources): • Highly directionality • Spectrally pure (<0.01nm • Coherency Temporal coherency Spatial: constant phase difference • Very high radiance 19 Nonlin optical effect • Eqn.s should be added 20 21 22 In the name of God Next slides in this file should be merged with previous slides in this file. Lesson 12 , page(106-115) n. omidikia Omdk name should be included in the file name 23 Source Characteristic ; a)Continium (wolfram) 1-Spectrum Output b)Line (HCL) 2-Wave length region a) palse 3-Temporal behavior b)continious c)sinouidal 24 4-Stability 5-life time a) Point source (Laser) 6-Area of emission b) Extended Area Source a-Coherent 7-Spatial behavior b-non coherent 25 a- Incondescent Type of sources b- non incondescent Incondescent sources; 1-nernst Glower: 26 2-Globar CR>0 if T increase then R increase. without controlling current Wave length output 1-40 µm 3-wolfram lamp; Wave length output 300-3500 nm Incondenscent lamp; 1-Arc lamp 27 The electric arc in an arc lamp consists of gas which is initially ionized by a high voltage and therefore becomes electrically conductive. To start an arc lamp, a very high voltage is pulsed across the lamp to "ignite" or "strike" the arc across the gas . e’ + Ar Ar+ +e’ Ar +hv 28 Line(low pressure) Continium(molecular emission) 29 1-high collision In high pressure Line broading 2-velosity of atom In high pressure line spectrum convert to continium spectrum. High pressure Low pressure 30 2-HCL( hallow cathod lamp); Typical primary source of radiation: Hollow cathode lamp * Typically one lamp per element * Different intensities for different elements * Multielement lamps for multielement analysis Continuum sources (e.g. Xe arc lamp) only for multielement analysis 31 microwave UV/VIS radiation 3-E.D.L(Electrodeless Discharge Lamp); Mw 1) e’ e’(accelerated) Free electron in EDL are accelerated by microwave field. 2) e’ + Ar Ar++2e’ e’+Ar+ Ar* 32 3- Ar*+Hg Hg*+Ar Hg+hv Excited argon atoms (Ar*) collide with mercury atoms to form exciting states (Hg*) ,which emit energy (hv) as UV/VIS radiation at return to ground state (Hg). 4)L.E.D ( Light Emission Diode); LED are a general source of continuous light with a high luminescence efficiency , and are based on the general properties of a simple twin- element semiconductor diode encased in a clear epoxy dome that acts as a lens. 33 34 5) Laser (Light Amplification by Stimulated Emission of Radiation); 35 3 level laser; 36 4 level laser Because of level 2 is empty laser occuring simpler. Level 2 is unstable. And population inversion occur in level 3 to 2. 37 ? ? ? ? ? 2 level laser, It can be occur or not? 38 Type of laser; There are many ways to define the type of laser. 1-Based on its pumping scheme a laser can be classified as Optically pumped laser Electrically pumped laser 2-On the basis of the operation mode, laser fall into classes of Continuous Wave Lasers Pulsed Lasers. 39 3-According to the materials used to produce laser light, lasers can be divided into three categories Gas Lasers 1-He/Ne laser 2-Gallium Arsenate (GaAs) Laser 3-Co2 laser Solid State Lasers 1-Ruby laser(AL2O3+Cr(VI)) Semiconductor Lasers 40 Ruby laser (Earliest solid-state laser) 4-Dye laser 41 ? ? ? ? ? ? Pomping scheme in a laser can be Thermally or not ?Why? 42 43.
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