ECE-1466 Modern Optics Course Notes Part 9

1
ECE-1466Modern OpticsCourse NotesPart 9

• Prof. Charles A. DiMarzio
• Northeastern University
• Spring 2002

2
Lecture Overview

• Basics of CW Lasers
• Gain
• Feedback
• Pulsed Lasers
• MOPA
• Gain-Switched
• Q-Switched
• Mode-Locked

3
Some Material Properties
Absorption
Emission
Absorption
Stimulated
Spontaneous
Energy
Emission
4
Laser Gain

• Materials
• Solid
• Insulating Materials
• Semiconductors
• Liquid
• eg. Dyes
• Gas

• Pump Mechanisms
• Electrical Discharge
• Electrical Current
• Light
• Flashlamp
• Laser
• Chemical
• Thermal
• Other

5
Rate Equations for 2 Levels
Photons
Populations
Energy
3
2
Actual Rate Equations Include Other Levels as Well
1
0
6
Typical Laser Materials
4-Level
3-Level
Energy
Energy
3
3
Fast
Fast
2
2
Pump
Laser
Pump
Laser
1
1
Fast
0
7
4-Level Steady State, No Lasing
4-Level
Energy
3
2
Pump
1
0
8
Gain vs. Pump
4-Level
g
3-Level
g
R03
R13
9
Feedback
Gain
Round Trip
f
10
Threshold Gain
Gain
Round Trip
f
Amplitude Equation
11
Laser Frequency
Gain
Round Trip
f
Phase Equation
Cavity Modes
f
12
Steady State
Gain
Round Trip
f
Amplitude Equation
Cavity Modes
f
13
Gain Saturation Mechanism
Energy

• Laser Light Depletes
• Upper-State Population
• Lower Level Has a Fast Decay Time
• Laser Does Not Pump Upper Level
• Populations End Nearly Equal

3
2
1
0
14
Gain Saturation Modes
Inhomogeneously Broadened Line
Homogeneously Broadened Line
f
f
15
Master Oscillator Power Amp
Power Amplifier 30 dB? for kilowatts output
Faraday Isolator Rejects Reflected Light
Master Oscillator (CW Laser) Typically a few Watts
Modulator Typically E/O With Pulsed Input
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Gain Switched Laser
Pump
t
Gain
Power
17
Q-Switched Laser
Pump
t
Gain
Cavity Q
Power
18
Mode-Locked Laser
Gain Medium
Modulator at fFSR
Gain
f
Cavity Modes
f
19
Mode Locking Example
Laser Frequency 10 GHz. (for illustration
only) FSR Modulation Frequency 100
MHz. 11 Modes
Sum
Laser Modes
Irradiance
20
Second Harmonic
a
(Electron as a Mass on a Spring)
a
v
x
v
x
21
Energy Level Diagrams
Fluorescence
2-photon
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Some Lasers (1)

• Helium Neon
• Gas Elect. Discharge
• 633 nm Wavelength
• milliwatts CW
• Argon Ion
• Gas Elect. Discharge
• 514, 488, and others
• Watts CW

• NdYAG
• 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

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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)

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Green Laser Pointer
Laser Diode
NdYAG Laser
Frequency Doubler
Battery
1064 nm
532 nm
780 nm
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Titanium Sapphire Laser
Laser Diode
NdYAG Laser
Frequency Doubler
Power
1064 nm
532 nm
780 nm
Titanium Sapphire
Very Broad Band and Can Be Mode- Locked
Red to NIR