Transcript ECE-1341 Introduction to Electronics Course Notes Part 9

ECE-1466
Modern Optics
Course Notes
Part 9
Prof. Charles A. DiMarzio
Northeastern University
Spring 2002
May 02002
Chuck DiMarzio, Northeastern University
10100-9-1
Lecture Overview
• Basics of CW Lasers
– Gain
– Feedback
• Pulsed Lasers
–
–
–
–
MOPA
Gain-Switched
Q-Switched
Mode-Locked
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Chuck DiMarzio, Northeastern University
10100-9-2
Some Material Properties
Absorption
Emission
Stimulated Spontaneous
Absorption
AN1
Energy
Emission
AN2  BN2
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Chuck DiMarzio, Northeastern University
10100-9-3
Laser Gain
• Materials
• Pump Mechanisms
– Solid
• Insulating Materials
• Semiconductors
– Liquid
• eg. Dyes
– Gas
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– Electrical Discharge
– Electrical Current
– Light
• Flashlamp
• Laser
– Chemical
– Thermal
– Other
Chuck DiMarzio, Northeastern University
10100-9-4
Rate Equations for 2 Levels
Photons
dn
 AN 2  N1   BN 2
dt
Energy
3
2
Populations
dN 2
 AN 1  AN 2  BN 2
dt
dN1
  AN 1  AN 2  3BN 2
dt
Actual Rate Equations Include
Other Levels as Well
1
0
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Chuck DiMarzio, Northeastern University
10100-9-5
Typical Laser Materials
4-Level
3-Level
Energy
Energy
3
3
Fast
Fast
2
2
Pump
1
0
Laser
Pump
Fast
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Laser
1
Chuck DiMarzio, Northeastern University
10100-9-6
4-Level Steady State, No Lasing
4-Level
B3 N3  R03 N0
Energy
3
2
Pump
B2 N2  B32 N3
1
0
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Chuck DiMarzio, Northeastern University
10100-9-7
Gain vs. Pump
4-Level
g
g
R03
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Chuck DiMarzio, Northeastern University
3-Level
R13
10100-9-8
Feedback
Gain
Round Trip
E2  E1e
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2ik
g2 g1
Chuck DiMarzio, Northeastern University
f
10100-9-9
Threshold Gain
Gain
Round Trip
E2  E1e
2ik
g2 g1
f
Amplitude Equation
E2  E1
e2ik g2 g1  1
gth  2 1
May 02002
Chuck DiMarzio, Northeastern University
10100-9-10
Laser Frequency
Gain
Round Trip
E2  E1e
2ik
g2 g1
Phase Equation
f
Cavity Modes
E2  E1
2ik
e 1
c
f q
2
May 02002
f
Chuck DiMarzio, Northeastern University
10100-9-11
Steady State
Gain
Round Trip
E2  E1e
2ik
g2 g1
Amplitude Equation
f
Cavity Modes
E2  E1
f
g2 g1  1
g  gth
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Chuck DiMarzio, Northeastern University
10100-9-12
Gain Saturation Mechanism
Energy
3
2
• Laser Light Depletes
Upper-State Population
• Lower Level Has a
Fast Decay Time
– Laser Does Not Pump
Upper Level
1
0
• Populations End
Nearly Equal
May 02002
Chuck DiMarzio, Northeastern University
10100-9-13
Gain Saturation Modes
Inhomogeneously
Broadened Line
Homogeneously
Broadened Line
f
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Chuck DiMarzio, Northeastern University
f
10100-9-14
Master Oscillator & Power Amp
Master Oscillator
(CW Laser)
Typically a few
Watts
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Modulator
Typically E/O
With Pulsed
Input
Power
Amplifier
30 dB? for
kilowatts
output
Chuck DiMarzio, Northeastern University
Faraday
Isolator
Rejects
Reflected
Light
10100-9-15
Gain Switched Laser
Pump
t
Gain
Power
May 02002
Chuck DiMarzio, Northeastern University
10100-9-16
Q-Switched Laser
Pump
t
Gain
Cavity Q
Power
May 02002
Chuck DiMarzio, Northeastern University
10100-9-17
Mode-Locked Laser
Gain Medium
Modulator at
f=FSR
Gain
f
Cavity Modes
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Chuck DiMarzio, Northeastern University
10100-9-18
f
Mode Locking Example
15
Sum
10
5
Optical Field
“Laser” Frequency 10 GHz.
(for illustration only)
FSR = Modulation Frequency
= 100 MHz.
11 Modes
0
-5
-10
1
9.5
0.8
9.6
0.6
9.7
-15
0
140
0.2
9.9
4
6
8
10
12
t, time, ns.
14
16
18
20
100
0
10.1
-0.2
10.2
-0.4
10.3
Irradiance
Irradiance
10
2
120
0.4
9.8
f, frequency, GHz.
Laser
Modes
80
60
-0.6
10.4
40
-0.8
10.5
0
5
10
t, time, ns.
15
20
-1
20
0
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Chuck DiMarzio, Northeastern University
0
2
4
6
8
10
12
t, time, ns.
14
16
10100-9-19
18
20
Second Harmonic
(Electron as a Mass on a Spring)
a  kx / m
a
a
a  2 kx / m v   a dt
  k1 x
x
x   v dt
v
x   v dt
x
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v   a dt
Chuck DiMarzio, Northeastern University
v
10100-9-20
Energy Level Diagrams
Fluorescence
2-photon
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Chuck DiMarzio, Northeastern University
10100-9-21
Some Lasers (1)
• Nd:YAG
• Helium Neon
– Gas; Elect. Discharge
– 633 nm Wavelength
– milliwatts CW
• Argon Ion
– Gas; Elect. Discharge
– 514, 488, and others
– Watts CW
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– Glass; Flashlamp or
Laser Pumped
– 1064 nm
– Watts Average
• Carbon Dioxide
– Gas; Elect. Discharge
– Around 10.6 mm
– Watts to kWatts, either
CW or pulsed
Chuck DiMarzio, Northeastern University
10100-9-22
Some Lasers (2)
• Diode
• Dyes
– Elect. Current
• Low Voltage
– Red to NIR
– mW and up
– Pulsed, Modulated to
GHz, and CW
– Small non-circular
beam output
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– Usually Pumped by
Another Laser
– Typically Visible
Wavelengths
• Usually Quite Widely
Tunable (eg. Grating)
– nJ or more
– Limited Lifetime
(often requires flow)
Chuck DiMarzio, Northeastern University
10100-9-23
Green “Laser” Pointer
Battery
Laser
Diode
Nd:YAG
Laser
780 nm
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Frequency
Doubler
1064 nm
Chuck DiMarzio, Northeastern University
532 nm
10100-9-24
Titanium Sapphire Laser
Power
Laser
Diode
Nd:YAG
Laser
780 nm
Very Broad Band
and Can Be ModeLocked
May 02002
Frequency
Doubler
1064 nm
Red to NIR
Chuck DiMarzio, Northeastern University
532 nm
Titanium
Sapphire
10100-9-25