Lasers

• Conditions of laser operation

• Classification of lasers

• Continuous wave lasers (He-Ne)

• Pulsed lasers (Nd-YAG, Ti:Sa)

• Ultrashort pulse generation

• Extreme energies

• Applications

Lasers

• Pumping

• Stimulated emission

• Population inversion

• Amplification

Laser operation

Absorption spontaneous emission Stimulated emission

Absorption and emission

LASER = Light amplification by stimulated emission of radiation

Stimulated emission takes place in the laser active medium

The emitted photons are reflected from the end mirrors stumulating more photon

emission

A small part of the photons quit the resonator trough the output coupler

R = 100% R < 100%

I0 I1

I2 I3 Laser active

medium

Laser operation

Population inversion

Why do we need population inversion?

R = 100% R < 100%

I0 I1

I2 I3 Laser active

medium

I

What do we need for population inversion ?

Population inversion

Two level

system

Laser

transition

Pumping

transition

In the best case we

get equal population

Four level

system

Easy laser operation

Laser

transition

Pumping

transition

Fast relaxation

Three level

system

Hitting hard will

result in lasing

Fast relaxation

Two, three and four level systems

Pump

transition

Laser

transition

Fast relaxation

By energy:

• I. class (harmless, laser pointers, CD players etc. )

• II.class(< 1 mW, no danger due to the blink)

• III. class (needs extra attention, < 500 mW),

• IV. class (extremely dangerous, burning dangers, retina

damage etc.)

Classification of lasers

By the laser active medium:

• Gas lasers (He-Ne, Ar-ion, CO2, N2, krypton etc.)

• Solid state lasers (ruby, Nd-YAG, Nd-YLF, Nd-glass,

titanium-sapphire)

• Diode lasers

• Dye lasers

• Chemical lasers (I, HF, HBr, stb.)

Classification of lasers

By operation:

• CW = continuous wave

• Pulsed lasers : Q-switched lasers, mode-locking lasers

Classification of lasers

694,3 nm

Ruby laser

• The first ever operating laser (Theodore Maiman, 1960 )

• The laser active medium is chromium doped syntetic sapphire

He-Ne laser

• 7:1 – 10:1 mixture of He and Ne gas

• Low pressure (3-7 mbar)

• Excited helium atoms collide with neon atoms

• Four level laser

• Q-switched lasers

• Mode-locked lasers

Pulsed lasers

• Repetion frequency: 15 Hz – 100 MHz

Parameters of pulsed lasers

• Pulse length: ns - fs

• Energy in one pulse: nJ-kJ

• (1.85 MJ @ National Ignition Facility)

National Ignition Facility

Rutherford Appleton Laboratory

Central Laser Facility, UK

ASTRA

• Ultra-high intensity 2.6 kJ in sub-picosecond

pulses, 100 TW → PW

• Fusion and Plasma Research

• Extreme UV generation

• Attosecond pulse generation research

• Smaller scale lasers

• Vibrational

spectroscopy

• Imaging; laser tweezers

and microscopy

VULCAN LSF

Extreme Light Infrastructure

Q-switched lasers

• Maximazing the population inversion then switching out a giant

pulse

• Pulse length: 100-200 ns

Mode-locking

Superposition of seven modes in random phase (A)

And locked phase (B). (Forrás: Enoch Small,19912

Mode locking

• The working horse of femtosecond lasers

• Laser active medium: titanium doped sapphire very good termal

properties, wide absorption spectrum)

• Pulse length: 6-150 fs

• Energy: nJ-mJ

• Hungarian input: Femtolasers (Krausz Ferenc), R&D Ultrafast

Lasers (Szipőcs Róbert)

Ultrashort lasers (Ti:sa)

Titanium-sapphire laser

Tranziens abszorpció @ Biofizika

BBO

CaF2

Minta

Foto-

dióda

Ch

Monokromátor

St

F

100 fs, 1 nJ,

100 MHz, 800

nm

100 fs, ~ µJ,

1 kHz, 800

nm

A pumpa és próba

impulzus térben (és

időben) átfed a

mintában!

CCD

Time correlated single photon counting (ns, ps, lasers)

Microscopy : confocal microscopes, super resolution (CW

lasers), two photon microscopy (Ti:sa lasers)

FTIR (Fourier –transformed infrared spectroscopy)

Laser tweezers (CW)

Raman spectroscopy (CW lasers)

Transient absorption

Applications

LIBS analysis

LIBS = Laser Induced Breakdown Spectroscopy

Cleaning the Acropolis

Cleaning the Acropolis

Laser cleaning

Laser cleaning in Pécs

http://www.physik.uni-wuerzburg.de/femto-welt/pulsstart.html

https://www.youtube.com/watch?v=boiQkDon9Fs