Lecture17

# C b collisional fme p m 3k

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Unformatted text preview: d lifeFme !E !t " ! !f !t " 1 Uncertainty in emission is given by the state lifeFme τ. !t " # 1 LifeFme for a typical laser gas: 10 ns !f = \$ 20 MHz 2"# 25 Doppler broadening •  Atoms are the emi^ers, and they are moving ! v\$ f+ = # 1 + & f0 " c% ! v\$ f' = # 1 ' & f0 " c% Emi^ers are moving towards observer Emi^ers are moving away from observer " v% ! f! = f0 \$ 2 ' # c& 3kT A quick esFmate using equiparFFon theorum: v ! ! 2000 m/s m c !f = (1.3 " 10 #5 ) \$ 10 9 Hz i.e. Doppler broadening MUCH more important than natural broadening 26 600 nm EsFmate for broadening is f+ Collisional broadening •  Molecules of laser medium collide with one another. Exchange can be inelasFc. •  Average thermal velocity: v = 3kBT / m 1 1 ! V \$3 ! k B T \$ 2 •  Average inter ­parFcle spacing: d = # & = # & & #& # & &# # 1 3 "k T% ' !c = \$ B ' \$ Collisional Fme: \$ P ' ' \$ # & "m% ' \$ ' \$ \$ 3k T ' ' \$ # & 1 2 #N& "% #P& " % •  B •  Collisional broadening: !vc = 1 / "# c T=300 K, P = 1 atm, m = Neon mass (10 amu): !vc = 0.8 MHz 27 Linewidths Source Broadening Natural 20 MHz Doppler 1 GHz Pressure (collisional) 0.8 MHz These eﬀects occur in both normal light sources and lasers. Why is the laser linewidth so much smaller?? ANSWER: The laser has a cavity or a resonator deﬁned by its end mirrors… 28 Cavity opFcal pump beam laser light Gain medium cavity mirro...
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