Opto-Electronics and
Photonics
Woo-Young Choi
Dept. of Electrical and Electronic EngineeringYonsei University
Lecture 22 : Lasers
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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Light source based on stimulated emission?
Laser
Optical Amplifier
è Optical oscillator
(Light Amplification by Stimulated Emission of Radiation)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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( )1 ( )
Y H sX H s=
+
|H(jω) | = 1 and ∠H(jω) = 180∘
In Electronic circuits
Amplifier
With FeedbackOutput without input
è Oscillation
Y =
( )1 ( ) ( )Y H s XH s+ =
( ) ( )X Y H s-
If H(s) = -1,
(Barkhausen oscillation condition)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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|H(jω) | = 1 and ∠H(jω) = 180∘
Oscillator: Amplifier with feedback
In-phase and the same magnitude after one round trip
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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rrLASER:
0 2gk nk j= +In gain medium:
Initially E0
0jkL jkLE e r e r- -× × × ×After one round trip
2 2 1j kLr e-× = 02 1j nk L gLe eR
- =
1 gLeR
\ =
22
1 1j kLer R
- = =
0E=
02 1j nk Le- =
Gain Medium
Pump
L
What provides initial E0 ?Spontaneous emission(Noise)
Optical Amplifier + Mirrors
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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02From 1,j nk Le- =
021 and 1j nk LgLe eR
-= =Gain Medium
Pump
LRR
02 2nk L mp= 2Ln ml
Þ = or 2
L mnl
=
1 1lngL R
= (Threshold gain, gth)
è Minimum gain required for lasing
Cavity length should be integer multiples of half wavelength è lasing mode
(mirror loss compensation)
22 2n L mp pl
=
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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th1 1 2Two conditions for lasing: (1) ln and (2) LgL R n m
l= =
λ
λ
gth
λ
Lasing spectrum
Lasing modes has non-zero linewidth due tovarious linewidth-broadening effects
Gain is function of pumping and l
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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Any optical gain material with mirrors can be a laser
First laser demonstrated by Maimanin 1960 at Hughes Aircraft Company
Optical Gain Material: Cr in Al2O3
Ted Maiman(1927-2007)
Pump: Xenon flash lamp
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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Charles Townes(1915-2015)
(1/2)
Nikolay Basov(1922-2001)
(1/4)
Aleksandr Prokhorov(1916-2002)
(1/4)
1964 Nobel Prize in Physics for invention of laser
Gordon Gould(1920-2005)
30-year battle for laser patent
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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Gas Laser (HeNe)
(1s2)
(1s12s1)
0
20.61 eV
He
(2p6)Ground states
(2p55s1)Ne
(2p53p1)
(2p53s1)
Collisions
Lasing emission632.8 nm
~600 nm
Collisions with the walls
Fast spontaneous decay
20.66 eV
Electron impact
The principle of operation of the He-Ne laser. He-Ne laser energy levels(for 632.8 nm emission).
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Current regulated HV power supply
Flat mirror (Reflectivity = 0.999) Concave mirror (Reflectivity = 0.985)
He-Ne gas mixtureLaser beam
Very thin tube
A schematic illustration of the He-Ne laser
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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E10
1.27 eV
0.80 eV E2
E3
1550 nm 1550 nm
InOut
980 nm
Non-radiative decay
Pump
Fiber Laser with EDF
Most popular laser material: semiconductors
Various fiber lasers
Yb (Ytterbium, atomic No. 70): 1030~1100nm
Tm (Thulium, atomic No. 69): 1750~2100nm
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 22: Lasers
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Homework (Due on 11/30)