laser fundamentals - biofizika

Laser Fundamentals

Dr. Yin LI Department of Biophysics , Medical

School, PTE [email protected]

Outline

• What is a laser?

• What unique properties does laser have?

• How to build a laser?

• Types of Laser

What is a laser?

• Light

• Amplification by

• Stimulated

• Emission of

• Radiation

LASER

Laser History

• In 1917, Einstein proposed the process that makes lasers possible, called stimulated emission.

• In 1954, a predecessor of the laser, "Microwave Amplification by Stimulated Emission of Radiation", was developed at Columbia University by Charles Townes.

• In 1960, Theodore Maiman developed the first working laser at Hughes Research Lab.

Examples of laser application

Laser properties

• High monochromaticity (single color)

• Narrow spectral width

Laser properties

• Highly collimated beam.

– Diffraction limited collimation

Collimated beam

Divergence of lamp light

Divergence of LASER

Laser properties

• Diffraction limited focussing

– Very small focussed spot

Focussing of lamp light Focussing of LASER

Laser properties

• High temporal coherence

– Radiation time without phase interruption is long.

– Can predict the phase and amplitude at any time, at a given position.

• CW (Continuous Wave )

• Pulsed

Two kinds of Lasers

Energy of CW laser: • 10-3 W milliwatt • 100 W watt • 103 W kilowatt • 106 W megawatt

Energy of pulsed laser:

• 109 W gigawatt • 1012 W terawatt • 1015 W petawatt • 1018 W exawatt

Tuning range of lasers

How to build a laser?

• Laser is an optical oscillator.


• How to make an oscillator?


• To overcome the losses, the pendulum requires exterior force.


• To make an oscillator

– A resonator

– A means to overcome the loss


• A laser resonator


• To overcome losses, we need a laser amplifier.


• Where does the collimated beam come from?


• Amplifier gives the laser with high power.


• Tuning range of the laser depends on the bandwidth of the amplifier.


• How is the short pulse generated?


• To generate pulsed laser,

– The bandwidth of gain should be large enough to cover several frequencies.

– All the waves with different frequencies should start to oscillate at the same time (in phase). This technique is called phase-locking or mode-locking.

How does an amplifier work?

• Absorption


• Spontaneous emission

• Stimulated emission

Absorption Spontaneous emission Stimulated emission


• To keep the amplifier work,

– we have to let the excited particles lose their energy through stimulated emission instead of spontaneous emission since emitted light by spontaneous emission has random directions.

– The number of excited particles must be larger than the number of ground-state particles. In this case, net gain could be produced. (Population Inversion)

Population Inversion

• Nexcited > Nground

– Two level system • b12 absorption

• b21 spontaneous emission

• a21 stimulated emission

– Fermi-Dirac distribution

𝑁2

𝑁1=

1

𝑒∆𝐸𝑘∙𝑇 + 1

𝑇 → 0, 𝑁2

𝑁1→

1

∞≈ 0, all the molecules stay at 𝐸1. 𝑇 → ∞,

𝑁2

𝑁1→

1

2


• Three-level system

– W31 denotes the probability of transition from level 3 to level 2.

–

Lifetime at Level 2 > Lifetime at Level 3

W32 >> W31 W21 >> W31

When Wp > W21, 𝑁2

𝑁1> 1

very slow

fast

slow fast

At steady state: d𝑁3

d𝑡= 0 &

d𝑁2

d𝑡= 0


• Four-level system

Four-level system is the easiest way to generation of lasers. There are lots of examples involving four-level system: Argon and Krypton ion lasers, dye lasers

Transverse modes of laser

Transverse modes of laser

• How to get rid of transverse modes?

Types of lasers

Classification of lasers depends on the laser active medium.

• Solid-state lasers have lasing material distributed in a solid matrix (such as ruby or neodymium:yttrium-aluminum garnet "YAG"). Flash lamps are the most common power source. The Nd:YAG laser emits infrared light at 1.064 nm.

• Semiconductor lasers, sometimes called diode lasers, are pn junctions. Current is the pump source. Applications: laser printers or CD players.

• Dye lasers use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.

• Gas lasers are pumped by current. Helium-Neon lases in the visible and IR. Argon lases in the visible and UV. CO2 lasers emit light in the far-infrared (10.6 mm), and are used for cutting hard materials.

Pumping mechanisms

• Flash lamp pump

– Examples: Ruby; neodymium:yttrium-aluminum garnet Nd:YAG

Laser active medium

I

Pumping mechanisms

• Electron-collision pump A + e 𝜖2 → A∗ + e(𝜖1)

– Argon, Krypton, Xenon, Nitrogen, Copper…

• Atom-collision pump Bm + A → B + 𝐴∗

B + e 𝜖2 → Bm + e(𝜖1) Bm denotes particle B in metastable state.

He-Ne, CO2-N2, He-Cd

Pumping mechanisms

• Chemical pump A + B2 → AB ∗ + B A + BC → AB ∗ + C

– Examples: HF; DF; Ar-F; Kr-F; Xe-F; Xe-Cl

• Electron current ejection pump (semi-conductor laser)

Or

Problems: 1. A lamp can emit light centered at 1050.5 nm. If the

spectral width of this lamp is equal to 100 GHz, please calculate the wavelength range of emitted light by this lamp.

2. Calculate the gap in frequency between two longitudinal modes in a linear cavity whose optic length L=600 mm. If the amplifier covers a spectral band of 1 GHz, how many longitudinal modes can oscillate in this cavity?

3. Please give one example of laser for medical application. Explain which type of the laser is and which pumping technique it employs.

laser fundamentals - biofizika

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