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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,考试时间：.1.4上学时间和教室,第1页,第1页,Chapter 7.,Lasers,Light Amplification by Stimulated Emission of Radiation,(acronym),第2页,第2页,The History,1916,Einstein,predicted the,stimulated emission,.,1954,Townes,and co-workers developed a Microwave Amplifier by Stimulated Emission of Radiation,(maser,)using ammonia,NH,3,.,1958,Schawlow,and,Townes,showed that the maser principle could be extended into the,visible region,.,1960,Maiman,built the first laser using,ruby,as the active medium.,From then on,laser development was nothing short of miraculous,giving optics new impetus and wide publicity.,第3页,第3页,第4页,第4页,7.1,Stimulated Emission of Radiation,Boltzmann Distribution,the transitions that occur between different energy states,absorption,:the upward transition from a lower energy state to a higher state,E,1,E,2,Emission,:the downward transition,E,2,E,1,population,N,:,the number of atoms,per unit volume,that exist in a given state.,given by,Boltzmanns equation,E,:energy lever of the system,:Boltzmanns constant,T,:absolute temperature.,第5页,第5页,7.1 Stimulated Emission of Radiation,Boltzmanns ra,tio,or,relative population,:,the ratio of the populations in the two states,N,2,/N,1,.,Or,plot the energy in the higher state relative to that in the lower state,versus the population in these states(E versus N),the result is an exponential curve known as a,Boltzmann distribution,.,When the,Boltzmann distribution,is,normal,it means that the system is in,thermal equilibrium,having more atoms in the lower state than in the higher state.,第6页,第6页,7.1 Stimulated Emission of Radiation,2.Einsteins Prediction,Assume first,:an ensemble of atoms is in thermal equilibrium and not subject to an external radiation field.,At higher temperatures,a certain number of atoms is in the excited state;on return to the lower state,these atoms will emit radiation,in the form of quanta,h,.-,spontaneous emission,rate of the transition,:the number of atoms in the higher state that make the transition to the lower state,per second.,lifetime,of the transition:the reciprocal of the rate of transition.,rate of the spontaneous transition,:,A,21,:constant of proportionality,N,2,:number of atoms(per unit volume)in the higher state,第7页,第7页,7.1 Stimulated Emission of Radiation,Assume next,:the system is subject to some external radiation field.,one of two processes may occur,depending on:,the direction(the,phase,)of the field with respect to the phase of the oscillator.,第8页,第8页,7.1 Stimulated Emission of Radiation,the two phases,coincide,:a quantum of the field may cause the emission of another quantum.-,stimulated emission,.,Its rate is,B,21,:constant of proportionality,u(,):energy density(J m,-3,),function of frequency,.,the two phase is,opposite,:the impulse transferred counteracts the oscillation,energy is consumed,and the system is raised to a higher state-,absorption,.,Its rate is,B,12,:constant of proportionality.,第9页,第9页,7.1 Stimulated Emission of Radiation,Transitions between energy states,第10页,第10页,7.1 Stimulated Emission of Radiation,Einsteins coefficients,:A,21,B,21,B,12,Einsteins relations,B21=B12,(1)the coefficients for both stimulated emission and absorption are numerically equal,(2)the ratio of the coefficients of spontaneous versus stimulated emission is proportional to the third power of the frequency of the transition radiation,explains why it is so difficult to achieve laser emission in the X-ray range,where,is rather high,第11页,第11页,7.1 Stimulated Emission of Radiation,3.Population Inversion,thermal equilibrium system,absorption and spontaneous emission take place side by side N,2,N,1,on return to the ground state,the system will probably,lase,.,第12页,第12页,Incandescent vs.Laser Light,Light from bulbs are due to spontaneous emission,Many wavelengths,Multidirectional,Incoherent,Monochromatic,Directional,Coherent,第13页,第13页,Coherence,Coherent,:If the phase of a light wave is well defined at all times (oscillates in a simple pattern with time and varies in a smooth wave in space at any instant).,Example:a laser produces highly coherent light.In a laser,all of the atoms radiate,in phase,.,Incoherent,:the phase of a light wave varies randomly from point to point,or from moment to moment.,Example:An incandescent or fluorescent light bulb produces incoherent light.All of the atoms in the phosphor of the bulb radiate with,random phase,.,第14页,第14页,Stimulated vs Spontaneous Emission,Stimulated emission requires the presence of a photon.An“incoming”photon stimulates a molecule in an excited state to decay to the ground state by emitting a photon.,The stimulated photons travel in the same direction as the incoming photon.,Spontaneous emission does not require the presence of a photon.Instead a molecule in the excited state can relax to the ground state by spontaneously emitting a photon.,Spontaneously emitted photons are emitted in all directions.,第15页,第15页,two-level system,(ex.,ammonia maser),E,n,N,n,E,m,N,m,E,n,N,n,E,m,N,m,Even with very a intense pump source,the best one can achieve with a two-level system is,excited state population=ground state population,第16页,第16页,three-level system,in equilibrium,normal Boltzmann distribution,absorptive rather than emissive,excited,population inversion,第17页,第17页,7.2,Practical Realization,1.General Construction,Pumping:,an,energy source,to supply the energy needed for raising the system to the excited state.,active medium:,in,which,reaches population inversion and lases when excited.,may be a solid,liquid,or gas,thousands of materials that have been found to lase,cavity,:,optional,laser,amplifiers,:,no cavity,laser,oscillators,:,medium enclosed in a cavity,provides feedback and additional amplification,cavity formed by two mirrors:one full reflectance,the other partially transparent,第18页,第18页,第19页,第19页,7.2 Practical Realization,Energy source,Medium,Full reflectance mirrors,Partially transparent mirrors,Radiation,Basic components of a laser oscillator,第20页,第20页,Common Components of all Lasers,Active Medium,The active medium may be solid crystals such as ruby or Nd:YAG,liquid dyes,gases like CO2 or Helium/Neon,or semiconductors such as GaAs.Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source.,Excitation Mechanism,Excitation mechanisms pump energy into the active medium by one or more of three basic methods;optical,electrical or chemical.,High Reflectance Mirror,A mirror which reflects essentially 100%of the laser light.,Partially Transmissive Mirror,A mirror which reflects less than 100%of the laser light and transmits the remainder.,第21页,第21页,2.Excitation,optical pumping,:ruby laser,a light source,another laser.,electron excitation:,argon laser,helium-neon laser,direct conversion,of electric energy into radiation:,light-emitting diodes(LEDs),semiconductor lasers,thermal excitation,:CO,2,laser.,chemical,pumping,:chemical laser,H2+F2,2HF,7.2 Practical Realization,第22页,第22页,7.2 Practical Realization,3.Cavity Configurations,Plane-parallel cavity,:very efficient(good,filling,),difficult alignment(low stability),confocal cavity,:,poor filling,easier to align,concentric cavity(spherical cavity):,poor filling,easier to align,hemispherical cavity,:poor filling,much easy to align,long-radius cavity,:good compromise between the plane-parallel and the confocal variety,type of cavity used most often in todays commercial lasers.,第23页,第23页,7.2 Practical Realization,Cavity configurations,L,:distance between mirrors,R,:radius of curvature,第24页,第24页,7.2 Practical Realization,4.Mode Structure,Assume,:the cavity is limited by two plane-parallel mirrors.,the,wavelength,possible of the standing-wave pattern inside the cavity is:,L:length of the cavity,q:number of half-wavelengths,or,axial modes,the,resonance condition,for axial modes:,n:index of medium contained in a laser cavity,axial modes,第25页,第25页,7.2 Practical Realization,different frequencies are closely,and evenly,spaced,lie within the width of a single,emission line,.,the output of the laser consists of a number of lines separated by c/2S,two consecutive modes(which differ by q=1),are separated by a,frequency difference,第26页,第26页,Mode-locking,第27页,第27页,Active mode-locking,第28页,第28页,7.2 Practical Realization,TEM:transverse electromagnetic,modes,few in number,easy to see.,Aim the laser at a distant screen,spread the beam out by a negative lens.:,bright patches,separated from one another by intervals called,nodal lines,.,Within each patch,the phase of the light is the same,but between patches the phase is reversed.,transverse modes,第29页,第29页,7.2 Practical Realization,lowest possible axial mode:,laser oscillates in one frequency,highest possible temporal coherence,TEM modes,TEM,00,:,lowest possible transverse mode,no phase reversal across the beam,the beam is uniphase,highest possible spatial coherence,can be focused to the smallest spot size and reach the highest power density.,第30页,第30页,第31页,第31页,5.Gain,Gain of a laser depends on several factors.Foremost among them is the,separation of the energy levels,that provide laser transition.,The two levels are father apart,the gain is higher because then the laser transition contains a larger fraction of the energy compared to the energy in the pump transition,7.2 Practical Realization,第32页,第32页,Gain is the opposite of absorption,-definition,:initial power in the cavity,:power of exit light,absorptivity,positive,:for thermal equilibrium where N,2,N,1,laser emission could be considered,negative absorption,gain coefficient,:the negative of the absorption coefficient“,7.2 Practical Realization,第33页,第33页,As the wave is reflected back and forth between the mirrors,it will lose some of its energy,mainly because of the limited reflectivity of one of mirrors.,If the two mirrors have reflectivities r,1,and r,2,-each round trip,:loss per round trip,-For the system,:system will lase,threshold condition,necessary to sustain laser emission.,7.2 Practical Realization,第34页,第34页,7.3,Types of Lasers,Solid-state Lasers,ruby laser,Ruby is synthetic aluminum oxide,Al,2,O,3,with 0.03 to 0.05%of chromium oxide,Cr,2,O,3,added to it.The Cr,3+,ions are the active ingredient;the aluminum and oxygen atoms are inert.,The ruby crystal is made into a cylindrical rod,several centimeters long and several millimeters in diameter,with the ends polished flat to act as cavity mirrors.,Pumping is by light from a xenon flash tube.,第35页,第35页,7.3 Types of Lasers,E,3,:fairly wide and has a short lifetime;the excited Cr,3+,ions rapidly relax and drop to the next lower state,E,2,.This transition is nonradiative.,E,2,:,metastable,and has a lifetime longer than that of E,3,and the Cr,3+,ions remain that much longer in E,2,before they drop to the ground state,E,1,.,The E,2,E,1,transition is radiative;it produces the spontaneous,incoherent red fluorescence typical of ruby,with a peak near 694 nm.,As the pumping energy is increased above a critical threshold,population inversion occurs in E,2,with respect to E,1,and the system lases,with a sharp peak at 694.3 nm.,Three-level energy diagram typical of ruby,第36页,第36页,Lasing Action Diagram,Energy Introduction,Ground State,Excited State,Metastable State,Spontaneous Energy Emission,Stimulated Emission of Radiation,第37页,第37页,efficient pumping,slow relaxation,Metastable state,fast,slow,Population inversion,Fast relaxation,Requirements for Laser Action,第38页,第38页,neodymium:YAG laser,The active ingredient is trivalent neodymium,Nd,3+,added to an yttrium aluminum garnet,YAG,Y,3,Al,5,O,12,.,It has four energy levels.The laser transition begins at the metastable state and ends at an additional level somewhat above the ground state.,7.3 Types of Lasers,第39页,第39页,7.3 Types of Lasers,2.Gas Lasers,Gas lasers consist of a gas filled tube placed in the laser cavity.A voltage(the external pump source)is applied to the tube to excite the atoms in the gas to a population inversion.The light emitted from this type of laser is normally continuous wave(CW).,helium-neon laser,Typically,it consists of a tube about 30 cm long and 2 mm in diameter,with two electrodes on the side and fused silica windows at both ends.The tube contains a mixture of 5 parts helium and 1 part neon,kept at a pressure of 133 Pa.,第40页,第40页,7.3 Types of Lasers,argon laser,It generates a strong turquoise-blue line at 488 nm and a green line at 514.5 nm,in either pulsed or c.w.operation.,helium-cadmium,It emits a brilliant blue at 441.6 nm.,第41页,第41页,7.3 Types of Lasers,carbon dioxide,laser,high power,:,the first CO,2,lasers had a continuous output of a few milliwatts.Today we have powers of some 200 kW,more than enough to cut through steel plates several centimeters thick in a matter of seconds.,High efficient,:the efficiency in converting electrical energy into radiation is better(more than 10%)than that of any other laser.(,TEA CO,2,laser,),Relatively simple,in construction and operation are.,Tunable,in a small range,Emission is at 10.6,m.,第42页,第42页,7.3 Types of Lasers,Excimer lasers,contain rare-gas halides such as XeCl,KrF,or others.These molecules are unstable in the ground state but bound in the excited state.,exceedingly powerful,with outputs as high as several GW.,emit in the ultraviolet.,第43页,第43页,7.3 Types of Lasers,3.Semiconductor Lasers,LED,:,light-emitting diode,emit almost anywhere in the spectrum,from the UV to the IR,an efficiency much higher than with optical pumping(around 40%versus 3%).,small,less than 1 mm in diameter,main application,:,waveguides,integrated optics,第44页,第44页,7.3 Types of Lasers,4.Tunable Lasers,dye lasers:,first tunable lasers,parametric oscillator:,more compact,less expensive,easier to operate,tuning range much wider,Color center lasers,:tuned over wide bands in the UV,the visible,and the IR.,free-electron laser:,high powers of the order of megawatts,very efficient,tuned through a wide range of wavelengths.,Tunable lasers are most welcome to spectroscopists,第45页,第45页,Argon fluoride(Excimer-UV)Krypton chloride(Excimer-UV)Krypton fluoride(Excimer-UV)Xenon chloride(Excimer-UV)Xenon fluoride(Excimer-UV)Helium cadmium(UV)Nitrogen(UV)Helium cadmium(violet)Krypton(blue)Argon(blue)Copper vapor(green)Argon(green)Krypton(green)Frequency doubled Nd YAG(green)Helium neon(green)Krypton(yellow)Copper vapor(yellow),0.1930.2220.2480.3080.3510.3250.3370.4410.4760.4880.5100.5140.5280.5320.5430.5680.570,Helium neon(yellow)Helium neon(orange)Gold vapor(red)Helium neon(red)Krypton(red)Rohodamine 6G dye(tunable)Ruby(CrAlO,3,)(red)Gallium arsenide(diode-NIR)Nd:YAG(NIR)Helium neon(NIR)Erbium(NIR)Helium neon(NIR)Hydrogen fluoride(NIR)Carbon dioxide(FIR)Carbon dioxide(FIR),0.5940.6100.6270.6330.6470.570-0.6500.6940.8401.0641.15 1.5043.392.709.6 10.6,Key:UV =ultraviolet(0.200-0.400,m)VIS =visible(0.400-0.700,m)NIR =near infrared(0.700-1.400,m),WAVELENGTHS OF MOST COMMON LASERS,Wavelength,(,m,m),Laser Type,第46页,第46页,Laser Output,Continuous Output(CW),Pulsed Output(P),watt(W)-,Unit of power or radiant fl
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