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May 02002 Chuck DiMarzio, Northeastern University 10100-9-1
ECE-1466Modern OpticsCourse Notes
Part 9
Prof. Charles A. DiMarzio
Northeastern University
Spring 2002
May 02002 Chuck DiMarzio, Northeastern University 10100-9-2
Lecture Overview
• Basics of CW Lasers– Gain– Feedback
• Pulsed Lasers– MOPA– Gain-Switched– Q-Switched– Mode-Locked
May 02002 Chuck DiMarzio, Northeastern University 10100-9-3
Some Material PropertiesAbsorption
Energy
Emission Stimulated Spontaneous
Absorption
Emission
1AN
22 BNAN
May 02002 Chuck DiMarzio, Northeastern University 10100-9-4
Laser Gain
• Materials– Solid
• Insulating Materials
• Semiconductors
– Liquid• eg. Dyes
– Gas
• Pump Mechanisms– Electrical Discharge
– Electrical Current
– Light• Flashlamp
• Laser
– Chemical
– Thermal
– Other
May 02002 Chuck DiMarzio, Northeastern University 10100-9-5
Rate Equations for 2 Levels
Energy
01
2
3
2212 BNANAN
dt
dN
2211 3BNANAN
dt
dN
PopulationsPhotons
212 BNNNAdt
dn
Actual Rate Equations IncludeOther Levels as Well
May 02002 Chuck DiMarzio, Northeastern University 10100-9-6
Typical Laser Materials
Energy
01
2
3
4-Level
Pump
Fast
Fast
Laser
Energy
1
2
3
3-Level
Pump
Fast
Laser
May 02002 Chuck DiMarzio, Northeastern University 10100-9-7
4-Level Steady State, No Lasing
Energy
01
2
3
4-Level
Pump
00333 NRNB
33222 NBNB
May 02002 Chuck DiMarzio, Northeastern University 10100-9-8
Gain vs. Pump
R03
g
R13
g4-Level 3-Level
May 02002 Chuck DiMarzio, Northeastern University 10100-9-9
Feedback
f
GainRound Trip
122
12 ggeEE ik
May 02002 Chuck DiMarzio, Northeastern University 10100-9-10
Threshold Gain
f
GainRound Trip
122
12 ggeEE ikAmplitude Equation
12 EE 112
2 gge ik
12thg
May 02002 Chuck DiMarzio, Northeastern University 10100-9-11
Laser Frequency
f
Gain
f
Cavity Modes
Round Trip
122
12 ggeEE ikPhase Equation
12 EE 12 ike
2
cqf
May 02002 Chuck DiMarzio, Northeastern University 10100-9-12
Steady State
f
Gain
f
Cavity Modes
Round Trip
122
12 ggeEE ikAmplitude Equation
12 EE
112 gg
thgg
May 02002 Chuck DiMarzio, Northeastern University 10100-9-13
Gain Saturation Mechanism
• Laser Light Depletes
Upper-State Population• Lower Level Has a
Fast Decay Time– Laser Does Not Pump
Upper Level
• Populations End Nearly Equal
Energy
01
2
3
May 02002 Chuck DiMarzio, Northeastern University 10100-9-14
Gain Saturation Modes
f f
HomogeneouslyBroadened Line
InhomogeneouslyBroadened Line
May 02002 Chuck DiMarzio, Northeastern University 10100-9-15
Master Oscillator & Power Amp
Master Oscillator(CW Laser)Typically a fewWatts
ModulatorTypically E/OWith PulsedInput
PowerAmplifier30 dB? forkilowattsoutput
FaradayIsolatorRejectsReflectedLight
May 02002 Chuck DiMarzio, Northeastern University 10100-9-16
Gain Switched Laser
t
Pump
Gain
Power
May 02002 Chuck DiMarzio, Northeastern University 10100-9-17
Q-Switched Laser
t
Pump
Gain
Power
Cavity Q
May 02002 Chuck DiMarzio, Northeastern University 10100-9-18
Mode-Locked Laser
Modulator atf=FSR
Gain Medium
f
Gain
f
Cavity Modes
May 02002 Chuck DiMarzio, Northeastern University 10100-9-19
Mode Locking Example
0 2 4 6 8 10 12 14 16 18 200
20
40
60
80
100
120
140
t, time, ns.
Irra
dian
ce
0 2 4 6 8 10 12 14 16 18 20-15
-10
-5
0
5
10
15
t, time, ns.
Opt
ical
Fie
ld
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
t, time, ns.
f, f
requ
ency
, G
Hz.
0 5 10 15 20
9.5
9.6
9.7
9.8
9.9
10
10.1
10.2
10.3
10.4
10.5
“Laser” Frequency 10 GHz. (for illustration only)FSR = Modulation Frequency = 100 MHz.11 Modes
LaserModes
Sum
Irradiance
May 02002 Chuck DiMarzio, Northeastern University 10100-9-20
Second Harmonic
a
vx
(Electron as a Mass on a Spring)
dtav
dtvx
a
vx
dtav
dtvx
mkxa /
mkxa /2
1xk
May 02002 Chuck DiMarzio, Northeastern University 10100-9-21
Energy Level DiagramsFluorescence
2-photon
May 02002 Chuck DiMarzio, Northeastern University 10100-9-22
Some Lasers (1)
• Helium Neon– Gas; Elect. Discharge
– 633 nm Wavelength
– milliwatts CW
• Argon Ion– Gas; Elect. Discharge
– 514, 488, and others
– Watts CW
• Nd:YAG– Glass; Flashlamp or
Laser Pumped– 1064 nm– Watts Average
• Carbon Dioxide– Gas; Elect. Discharge– Around 10.6 m– Watts to kWatts, either
CW or pulsed
May 02002 Chuck DiMarzio, Northeastern University 10100-9-23
Some Lasers (2)
• Diode– Elect. Current
• Low Voltage
– Red to NIR
– mW and up
– Pulsed, Modulated to GHz, and CW
– Small non-circular beam output
• Dyes– Usually Pumped by
Another Laser
– Typically Visible Wavelengths
• Usually Quite Widely Tunable (eg. Grating)
– nJ or more
– Limited Lifetime (often requires flow)
May 02002 Chuck DiMarzio, Northeastern University 10100-9-24
Green “Laser” Pointer
LaserDiode
Nd:YAG Laser
Frequency Doubler
Battery
780 nm 1064 nm 532 nm
May 02002 Chuck DiMarzio, Northeastern University 10100-9-25
Titanium Sapphire Laser
LaserDiode
Nd:YAG Laser
Frequency Doubler
Power
780 nm 1064 nm 532 nm
TitaniumSapphire
Red to NIRVery Broad Bandand Can Be Mode-Locked
