Lect. 20: Lasers

Optical Amplifier

E2 Pin Pout = G Pin

E1

Light source based on stimulated emission?

- Use photons produced by spontaneous emission as initial seeds

- Recycle output photons as seeds for further stimulated emission

- Use mirror for recycling output photons

 LASER: Light Amplification by Stimulated Emission Radiation

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

LASER: Optical Amplifier + Mirror

Pump

Gain Medium R R L

Optical property of gain medium: n, g

Imaginary part for k ? g g is due to absorption and stimulated emission knk0 j 2 g depends on material property, , amount of pumping factor of 2 because g is often defined for power

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

Pump g Gain Medium knkj 0 2 R R Z=L Assume initially there is one photon moving in z-direction inside gain medium What is the condition that this this photon can be maintained within?  No loss after one round trip.

jkL jkL Ee00  re  r E 11 re22  jkL1 e jkL2   rR2 1 1 ee jnkL2 0 gL   eegL and jnkL2 0 1 R R

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

1 Pump eegL  and jnkL2 0  1 R Gain Medium gL 111 From eg, th ln R LR R R L ==> Sufficient gain to compensate mirror loss

 2L  Frome jnkL2 0  1, 22nk L m  or Lm 0 nm 2n cavity length should be multiples of half wavelength: mode

 Photons are in phase after one round trip

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

11 2L Two conditions for lasing: (1) g  ln and (2) th L Rnm

In real lasers, gain is function of wavelength

gth

λ λ Lasing spectrum Lasing modes has non-zero linewidth λ Laser length determines mode wavelength

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

Any optical gain material with mirrors can form a laser

First working laser by Maiman in 1960 at Hughes Aircraft Company Ted Maiman (1927-2007) Optical Gain Material: Cr in Al2O3 Pump: Xenon flash lamp

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

1964 Nobel Prize in Physics for invention of laser

Nikolay Basov (1922-2001) (1/4)

Gordon Gould Aleksandr Prokhorov (1920-2005) Charles Townes (1916-2002) (1/4) 30 year battle (1915-2015) for laser patent (1/2)

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

Gas Laser (HeNe)

He Ne 5 1 Flat mirror (Reflectivity = 0.999) (1s12s1) Collisions (2p 5s ) Concave mirror (Reflectivity = 0.98 20.61 eV 20.66 eV 632.8 nm Lasing emission Very thin tube (2p53p1) Fast spontaneous decay Laser beam ~600 nm He-Ne gas mixture (2p53s1) Electron impact

Collisions with the walls Current regulated HV power supply A schematic illustration of the He-Ne laser 2 6 0 (1s ) (2p ) Ground states

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

Fiber Laser

Er3+-doped fiber (10 - 20 m) Wavelength-selective Gratings coupler Gratigns Splice Splice

Pump laser diode Termination  = 980 nm 1.27 eV E3 Non-radiative decay 980 nm Pump 0.80 eV E2 1550 nm 1550 nm Out In 0 E1

Another popular laser material: semiconductors

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

Homework

Optoelectronics (16/2) W.-Y. Choi Lect. 20: Lasers

Homework (Continued)

Optoelectronics (16/2) W.-Y. Choi