EM Radiation Sources 1. Fundamentals of EM Radiation 2. Light Sources
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1 EM Radiation Sources 1. Fundamentals of EM Radiation 2. Light Sources 3. Lasers
2 Light Amplification by Stimulated Emission of Radiation What is a laser?Light Amplification by Stimulated Emission of Radiation
3 OverallIngle and Crouch, Spectrochemical Analysis
4 Stimulated Absorption Einstein Coefficient for Absorption Bij (cm3 J-1 s-1 Hz):with Un: energy density of the field at the appropriate frequency n (J cm-3 Hz-1)Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
6 Stimulated Emission Einstein Coefficient for Stimulated Emission: Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
7 OverallIngle and Crouch, Spectrochemical Analysis
8 BijUnni = Ajinj + BjiUnnj For an ideal black body, the rate of absorption and emission must be balanced:BijUnni = Ajinj + BjiUnnjRearrange:
9 Are you getting the concept? Determine the population ratio for atoms/molecules in two energy states spaced by 1 eV at T = 300 K:njni
10 Spectral Energy Density We know:Set equal and solve for Uv:Looks similar to Planck’s Radiation Law:
11 Population InversionGoal: More atoms or molecules in the upper energy level than the lower energy level.Heating the lasing medium will not work:nj = nie-(Ej-Ei)/kTMust selectively excite atoms/molecules to particular energy levels. Most common approaches:*light*electricity
12 Optical Pumping Intense light source at h (e.g. flash lamp) Excites to a metastable state to achieve population inversionWith fast flashing, initial photons start chain reactionEugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
13 Electrical DischargeAccelerated e- and ions excite atoms/molecules into higher energy statesCommon in gas lasersIngle and Crouch, Spectrochemical Analysis
14 Better for pulsed mode operation Three - Level SystemNo saturationNot very efficientBetter for pulsed mode operationIngle and Crouch, Spectrochemical Analysis
15 The ruby laser is a three – level laser Commercial ruby laseroperates with efficiency ~ 1%Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
16 Four - Level System More efficient than 3-level Laser transition does not involve ground state or most highly excited stateEasier to achieve population inversionIngle and Crouch, Spectrochemical Analysis
17 The He – Ne laser is a four – level laser He* + Ne → He + Ne* + ΔEIngle and Crouch, Spectrochemical Analysis
18 Resonance Cavity and Gain Gain = degree ofamplification basedon positive feedbackIngle and Crouch, Spectrochemical Analysis
19 Gain Gain (G) = es(nj-ni)b s = transition cross-section b = length of active mediumOscillation begins when:gain in medium = losses of systemr1r2G2 = 1Threshold population inversion:Ingle and Crouch, Spectrochemical Analysis
20 Light Amplification in Resonance Cavity Highly collimated beamTypically ~mm beam width, ~mrad divergenceA typical photon travels about 50 times forward and backward within the cavityEugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
22 Are you getting the concept? Knowing that the purpose of the resonance cavity is to direct the majority of the photons back through the active medium, what cavity characteristics will be most important?Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
23 Achieving ResonanceStimulated emission is coherent (all light waves in phase)If the cavity is an integer multiple of the wavelength, each wave will be at the same phase when it reflects from one of the cavity mirrors (recall that a photon make many round trips in a laser cavity before it is emitted).This allows constructive interference between all photons.Want: ml = 2nLOther wavelengths will not be strongly amplified, and thus, will die out.In practice, laser transitions have gain over a range of wavelengths – the gain bandwidth… so that resonance cavity lengths are not impossible to achieve.
24 Achieving Resonance Goal: Laser cavity where L = ml/2n Estimate amplification factor:Amp = (1+Gain)LThis condition is not as strict as it sounds because:Laser transitions have gain over a range of wavelengthsAny integer multiple (longitudinal mode) of l will work
25 Longitudinal ModesActual is the convolution of the transition bandwidth and the of the longitudinal modes.Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
26 Transverse ModesTransverse modes determine the pattern of intensity distribution across the width of the beam.TEM00 has a Gaussian distribution and is the most commonly used.The resonator geometry of many commercial lasers is designed to obtain “single transverse mode” operation.and