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P. Piot, PHYS 630 – Fall 2008Tailoring of laser output• Polarization is selected using polarizer or Brewster windows (for gaslaser e.g. CO2)• Transverse mode is selected using apertures• Selection of laser line can be done usinga prismPolarizationselectionP. Piot, PHYS 630 – Fall 2008• To date we considered only CW laser• Pulsed laser have a wide range of applications– Photochemistry– Solid state physics– Generation of ultra-short electron beam (photoemission)Pulsed lasersP. Piot, PHYS 630 – Fall 2008• A laser can oscillate on many longitudinal modes with frequencyseparated by νf=c/2d• These modes usually oscillate independently (free running modes)• External means can be used to couple these modes and phase lockthem, this is the concept of modelocking.Modelocking: introductionP. Piot, PHYS 630 – Fall 2008Modelocking: description I• Consider each of the mode to be a plane wave (this is anapproximation). The the total complex amplitude iswhere• So• rewrite! U(z,t) = Aqq"exp 2#i$qt %zc& ' ( ) * + , - . / 0 1 ! "q="0+ q"f! U(z,t) = Aqq"exp 2#iq$ft %zc& ' ( ) * + , - . / 0 1 exp 2#i$0t %zc& ' ( ) * + , - . / 0 1 = a t %zc& ' ( ) * + exp 2#i$0t %zc& ' ( ) * + , - . / 0 1 ! a(t) = Aqq"exp2#iqtTf$ % & ' ( )P. Piot, PHYS 630 – Fall 2008Modelocking: description II• a(t) is a periodic function of t and z• If the phase and amplitude of Aq are properly chosen a(t) may bemade to take the form of periodic narrow pulses• Example: consider all the Aq to be the same for M modes (q=0, ±1,±2,…). Thenand the optical intensity! a t( )= A exp2"iqtTf# $ % & ' ( )= A xq)= AxS +1/ 2* xS*1/ 2x+1/ 2* x*1/ 2= Asin M"t /Tf( )sin"t /Tf( )! I(z,t) = I0sin2M"(t # z /c) /Tf( )sin2"(t # z /c) /Tf( )Separation=Tf , 2dwidth= Tf/M, 2d/MP. Piot, PHYS 630 – Fall 2008ModelockingP. Piot, PHYS 630 – Fall 2008How to implement modelocking I• Active modelocking– Introduce a modulationusing• Acousto-optic modulator• Electro-optic modulatorP. Piot, PHYS 630 – Fall 2008How to implement modelocking II• Passive modelocking– Introduce an absorber– The most populartechnique in short pulselaser oscillatorsP. Piot, PHYS 630 – Fall 2008Q-Switching• Used to obtain energetic laser pulse• Cannot beat modelocking as far as generating ultra-short laser pulse• Keep high resonator losses, so that lasing cannot occur, pumpingcontinuously increase population inversion• Losses are suddenly reduced to a small value• Laser radiation build up (many cavity round trips) until gain saturateP. Piot, PHYS 630 – Fall 2008Pulse compression and stretchingBlue travel alonga longer path than redRed travel alonga longer path than bluet-correlatedbandwidthFrequency-dependentpathlengthωtP. Piot, PHYS 630 – Fall 2008Pulse compression and stretchingP. Piot, PHYS 630 – Fall 2008Chirp pulsed amplification (CPA)Courtesy of Yuelin Li,


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NIU PHYS 630 - LECTURE NOTES

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