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اسلاید 1 :

Light Amplification by Stimulated Emission of Radiation

Stimulated emission

Spontaneous emission

اسلاید 2 :

Energy level diagram

  • The possible energies which electrons in the atom can have is depicted in an energy level diagram.

اسلاید 3 :

The operation of the Laser

  • In 1958, Charles Townes and Arthur Schawlow theorized about a visible laser, an invention that would use infrared and/or visible spectrum light.
  • Light Amplification by Stimulated Emission of Radiation- (LASER).
  • Properties of Lasers

–Produce monochromatic light of extremely high intensity.

اسلاید 4 :

The operation of the Laser

اسلاید 5 :

The operation of the Laser

Light: Coherent, polarized

The stimulating and emitted photons have the same:

frequency

phase

direction

اسلاید 6 :

  • n1 - the number of electrons of energy  E1
  • n2 - the number of electrons of energy E2

اسلاید 7 :

Einstein’s coefficients

 

Probability of stimulated absorption R1-2

R1-2 = r (n) B1-2                                                          

 

Probability of stimulated and spontaneous emission :

R2-1 = r (n) B2-1 + A2-1                               

 

assumption: n1  atoms of energy e 1  and  n2 atoms of energy e 2 are in thermal equilibrium at temperature T with the  radiation of spectral density  r (n):

 

n1 R1-2 =  n2 R2-1           n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)

 

 

اسلاید 8 :

The probability of spontaneous emission A2-1  /the probability of stimulated emission B2-1r(n ):

 

1.Visible photons, energy: 1.6eV – 3.1eV.

2.kT at 300K ~ 0.025eV.

3.stimulated emission dominates solely when hn /kT <<1!

(for microwaves: hn <0.0015eV)

 

The frequency of emission acts to the absorption:

 

 

 

if hn /kT <<1.

اسلاید 9 :

Condition for the laser operation

If n1 > n2

  • radiation is mostly absorbed
  • spontaneous radiation dominates.

    if  n2 >> n1    -  population inversion

    • most atoms occupy level E2, weak absorption
    • stimulated emission prevails
    • light is amplified

      Necessary condition:
       population inversion

اسلاید 10 :

How to realize the population inversion?

Thermal excitation:

impossible.

The system has to be „pumped”

Optically,
electrically.

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