Transcript Lecture 38 - University of Delaware

Lecture 38
Lasers
Final Exam next week
LASER
Light
Amplification by
Stimulated
Emission of
Radiation
Electronic Energy Levels
2
Energy
1
e-
e-
0
Fluorescence or ‘Spontaneous Emission’
ee-
Stimulated Emission
2
Energy
1
e-
e-
0
The stimulated emission has the same frequency
and phase as the incoming photon
3 level system
2
1
e-
0
‘Short’ lifetime
‘long’ lifetime
Stimulated Emission Light Amplifier
Pump (does not need to be light)
Gain medium can be gas, liquid, or solid
All light is in phase and has the same frequency
Laser Cavity
Once the amount of gain is equal to the cavity losses,
‘lasing’ happens
Losses can include output coupler, reflectivity, other optics in cavity
Cavity modes
The mirrors force the amplitude of the wave to zero.
Only integral number of half wavelengths can exist.
Every wave exits in phase
c
v 
2L
Allowed frequencies
It is a combination to the energy spread and the cavity modes
14
12
1
Y Axis
10
8
6
4
2
0
0.00
0.05
0.10
0.15
X Axis
0
0.20
0.25
0.30
Laser pulse generation
Gain medium
PC
Q=
2p*Energy stored
Energy Lost per cycle
“Q-switching”, ns pulses
“Mode-locking”,
fs pulses
Cavity length ~1.5m
Transverse modes
Transverse Modes
The 00 mode is the smallest….
Types of lasers
Gas: HeNe
Chemical laser: CO2, Excimer
Solid-state: Semi-conductor Diode, Crystals (sapphire), nano-particles
Liquid: Dyes,