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)