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Coherence - PowerPoint PPT Presentation

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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 = l 2 /n Dl.

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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 = l2/nDl



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 nair = 1.00:
  • light bulb emitting from 400-1000 nm
  • semiconductor laser emitting from 799.5 – 800.5 nm
  • He-Ne laser emitting from 632.799 – 632.801 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:

Dn/n = [5.545 kT/Mc2]½

where M is the mass of the atom/molecule


Limiting Emitted ls 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 l depends on:
  • the angle the light travels through the etalon (q)
  • the thickness of the etalon (l)
  • 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)

DEDt ≥ħ/2


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:
  • variations in power level with time
  • efficiency of converting excitation energy into laser energy
  • excitation method
  • 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



Peak Power







Pulsed Laser Power Considerations

Consider a Gaussian beam profile:

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


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




Build up population inversion by preventing lasing while pumping.

Systemis momentarily realigned to allow lasing.

Results in short (~10-200 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: 1996.


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

Method for producing very short pulse widths (~10-12 s).

Synchronize longitudinal modes.

Edward Piepmeier, Analytical Applications of Lasers, John Wiley & Sons, New York, 1986.


Are you getting the concept?

A laser has a bandwidth of 4.4 GHz (4.4 x 109 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, 100-200 nm) and XUV (eXtremeUltraViolet; 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: Aji/Bij = 8  h 3 / c3

diode lasers
Diode LASERs
  • 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.

McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000

stimulated emission
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

  • Nd3+ in yttrium-aluminum-garnet (Y3Al5O12)
  • Four level laser
  • Powerful line @ 1064 nm; often doubled or tripled
  • Pump: Kr/Ar arc lamp or flash lamp
  • CW or pulsed operation

Ingle and Crouch, Spectrochemical Analysis


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.

Cluster of ~10 lines in 454 – 529 nm.

UV: 334, 352, 364 nm (need several W in visible to get ~50 mW in UV)

Deep UV: 275 nm (need 20-30 W in visible to get ~10mW @ 275 nm)


Excimer Lasers

Excimer is a dimer that is only stable in the excited state.

e.g. ArF+, KrF+, XeF+

Pass current through noble gas / F2 mix.

Lasing occurs as excimer returns to the ground state.

Ingle and Crouch, Spectrochemical Analysis


Dye Lasers

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.

Ingle and Crouch, Spectrochemical Analysis


Dye Lasers

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