Coherence Factors that compromise coherence: 1. thermal fluctuations 2. vibrational fluctuations 3. emission of multiple wavelengths 4. multiple longitudinal modes Temporal Coherence – How long do the light waves remain in phase as they travel? Coherence Length = 2 /n 

Coherence Spatial Coherence – Over what area does the light remain in phase?

Are you getting the concept Calculate the coherence length for the sources below using n air = 1.00: (a)light bulb emitting from nm (b)semiconductor laser emitting from – nm (c)He-Ne laser emitting from – nm

Laser Wavelengths Factors influencing monochromaticity of laser light: 1. transitions responsible for emission 2. nature of transition determines bandwidth 3. resonance cavity characteristics Doppler bandwidth:  = [5.545 kT/Mc 2 ] ½ where M is the mass of the atom/molecule

Limiting Emitted s with a Fabry-Perot Etalon Insert a pair of reflective surfaces that form a resonant cavity tilted at an angle to the axis of the laser medium. Transmitted  depends on: 1.the angle the light travels through the etalon (  ) 2.the thickness of the etalon (l) 3.the refractive index of the material between the 2 surfaces (n)

Emission Mode Lasers can emit light in continuous wave (cw) mode or they can produce pulses. Heisenberg’s Uncertainty Principle places the limitations: Bandwidth (Hz) = 0.441/Pulse Length (s) Bandwidth (Hz) = 0.441/Pulse Length (s)  E  t ≥ ħ/2 Consequences: Long pulse – narrow bandwidth Short pulse – broad bandwidth Long pulse – high resolution Short pulse – low resolution

Are you getting the concept? Calculate the minimum pulse length for a laser with a 1-nm emission bandwidth at a center wavelength of 500 nm.

Are you getting the concept? Calculate the best spectral resolution (in cm -1 ) that can be achieved with a pulse length of 368 fsec.

Output Power Output power will depend on: 1.variations in power level with time 2.efficiency of converting excitation energy into laser energy 3.excitation method 4.laser size What is wall-plug efficiency? A practical measurement of how much energy put into the laser system (from the wall plug) comes out in the laser beam. Active Medium power supply

Pulsed Laser Power Considerations Consider a Gaussian beam profile: Peak Power FWHM Rise Time Fall Time Power Time If power was constant: E = Pt In this case, E = ∫P(t)dt Average Power = ΣE/t or Peak Power x Duty Cycle Duty cycle = Pulse Length x Repetition Rate

Controlling Laser Pulse Characteristics There are 3 primary methods to control laser pulse time: Q Switched Lasers – cavity mirrors are temporarily unavailable so the laser medium stores energy rather than releasing it. When the mirror is made available, a high energy pulse is released. Cavity dumped lasers – an extra cavity mirror momentarily diverts photons from a fully reflective cavity after photon energy has accumulated for awhile Modelocked lasers – “lock” together multiple longitudinal modes so that a laser simultaneously oscillates on all of them to emit very short pulses

Q-Switching Build up population inversion by preventing lasing while pumping. Systemis momentarily realigned to allow lasing. System is momentarily realigned to allow lasing. Results in short (~ nsec), high-intensity (up to MW) pulse. Only possible if the laser can store energy in the excited state longer than the Q-switched pulse. Demtröder, W. Laser Spectroscopy, Springer, Berlin: switch

Cavity Dumping Laser cavity has two “fully” reflective mirrors. A steady power grows inside the cavity during normal operation. Momentarily, a third mirror enters the light path and directs the beam out of the cavity. All energy is dumped in one pulse lasting as long as it takes the light to make a round trip in the laser cavity. Demtröder, W. Laser Spectroscopy, Springer, Berlin: 1996.

Mode - Locking Edward Piepmeier, Analytical Applications of Lasers, John Wiley & Sons, New York, Method for producing very short pulse widths (~ s). Synchronize longitudinal modes.

Are you getting the concept? A laser has a bandwidth of 4.4 GHz (4.4 x 10 9 Hz). What is the shortest modelocked pulse it can generate according to the transform limit?

Accessible Wavelengths Lasers have also been prepared for the vacuum UV (VUV, nm) and XUV (eXtreme UltraViolet; also called the ultrasoft X-ray region; <100 nm). The shortest wavelength laser produced so far emits at 3.5 nm. Projects to extend this range to 0.1 nm by 2011 are in progress. Why x-ray lasers are so difficult to build: A ji /B ij = 8  h 3 / c 3

Diode LASERs McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000 Conversion of electrical to optical power up to 30%. Conversion of electrical to optical power up to 30%. Polished faces of semiconductor act as mirrors and reflect ≈95% of photons from leaving resonance cavity. Polished faces of semiconductor act as mirrors and reflect ≈95% of photons from leaving resonance cavity.

Stimulated Emission Agrawal, G.P.; Dutta, N.K. Semiconductor Lasers, Van Nostrand Reinhold, New York: 1993.

Semiconductor (Diode) Laser Used in telecommunications, CD players, laser pointers etc. Blue and UV (375 – 400 nm) diode lasers have recently been developed. Eli Kapon, Semiconductor Lasers I, Academic Press, San Diego, 1999.

Semiconductor (Diode) Laser Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.

Neodymium:YAG Laser Ingle and Crouch, Spectrochemical Analysis Nd 3+ in yttrium-aluminum- garnet (Y 3 Al 5 O 12 ) Nd 3+ in yttrium-aluminum- garnet (Y 3 Al 5 O 12 ) Four level laser Four level laser Powerful 1064 nm; often doubled or tripled Powerful 1064 nm; often doubled or tripled Pump: Kr/Ar arc lamp or flash lamp Pump: Kr/Ar arc lamp or flash lamp CW or pulsed operation CW or pulsed operation

Ion Lasers (Ar + and Kr + ) CW – pumped using an electrical discharge. Very reliable. Inefficient because energy is required to ionize gas. Power up to ~40 W (distributed over many lines). Argon ion is most common. 488 nm and 514 nm are most powerful lines. 488 nm and 514 nm are most powerful lines. Cluster of ~10 lines in 454 – 529 nm. Cluster of ~10 lines in 454 – 529 nm. UV: 334, 352, 364 nm (need several W in visible to get ~50 mW in UV) UV: 334, 352, 364 nm (need several W in visible to get ~50 mW in UV) Deep UV: 275 nm (need W in visible to get 275 nm) Deep UV: 275 nm (need W in visible to get 275 nm)

Excimer Lasers Excimer is a dimer that is only stable in the excited state. e.g. ArF +, KrF +, XeF + e.g. ArF +, KrF +, XeF + Pass current through noble gas / F 2 mix. Lasing occurs as excimer returns to the ground state. Ingle and Crouch, Spectrochemical Analysis

Dye Lasers Ingle and Crouch, Spectrochemical Analysis Molecular transitions in the solution phase. Active species is an organic dye (e.g. rhodamines, coumarins, fluoresceins). To prevent overheating, a jet of the dye solution is pumped through focal point of optical system. Broad transitions. Can be tuned over ~50 nm. Lases in UV-Vis-IR Difficult and expensive to operate. Optically pumped with flashlamp or another laser.

Dye Lasers Demtröder, W. Laser Spectroscopy, Springer, Berlin: 1996.