Tài liệu Chapter XXV Quantum Generators pptx

Quantum generators are light sources that work by amplification Light D7131 1 Ь Stimulated Emission of Radiation... §1.Principle of light amplification: 1.1 Radiative processes

GENERAL PHYSICS Ill

Optics

fo CQuan†um Physics

Chapter XXV

Quantum Generators

§1 Principle of light amplification

§2 Properties of laser beams

Quantum generators are

light sources that work by

amplification

Light

D7131 1 (Ь

Stimulated Emission of

Radiation

§1.Principle of light amplification:

1.1 Radiative processes:

1.1.1 Absorption:

- Atoms absorb light (photon) and transist

from a lower to higher (exited) energy levels

Number of atoms or

energy level (typical value ~10°" per cm’) Depends on properties

of levels | and 2 and #

of photons

- Note: Typical value of Absorption | proton Flux

Cross-Section | [ini , Hem?-sec

^¬

4 ga ~ 1013 + 1018 cmZ

1.1.2 Spontaneous emission:

- The emission of light that takes place

compleTely

° When, and ¡in which direction, the light will be erni††ed? It cannot be

determined before it actually happens

> We cannot manage this emission

Spontaneous lifetime, depends on properties of levels [Units — Seconds]

The process is spontaneous by nature This gives the effect its

name:

1.1.3 Stimulated emission:

- Imagine an atom with excess energy Without external action,

the atom would, after a period of time, spontaneously emit light,

whose properties (e.g its direction) are random — only the

energy is fixed

- This situation changes when the

atom is shone with a light wave of

the corresponding energy There

is then a higher probability that the atom will also simply copy the remaining properties of the incident wave: It then

- In contrast to spontaneous emission, this effect is known as

This is the effect that causes the light in the laser to be amplified, and also gives the process its name: L ya

- The rate:

Stimulated Emission Cross-Section

- For degenerate levels:

g2 sub-Levels

each with the same

energy Ez

g¡ sub-Levels

- The LASER idea:

# of photons produced per unit volume per unit time

The Inversion

<Q) <> Net Absorption

>0 <> Net Amplification

- Thermodynamical equilibrium:

M4 e

Absorption

- It means that the thermodynamical equiblirium situation corresponds to the absorption

- The possibility of emission requires non-equilibrium (unnatural) situation:

It must be

—- [It is called

- Innumerable copies are produced from a few light waves

However, to achieve this effect the light waves must be reflected

back and past the atoms again and again This is done using mirrors

Mirror with reflectance R1 Mirror with reflectance R2 The mirro rs are used

Gain Medium to capture the light

are exactly superimposed and oscillate in step

equilibrium state - “The Pump" a standing Wave

* Light amplification: If we supply continually with new energy to the

atoms, for example by using a bright light => there are more atoms

with additional energy than without = a population inversion is created

1.2 Pumping schemes:

1.2.1 Two-level laser:

Suppose that we try fo increase

N, with string light hv to create

a population inversion

—› lhis won † work Ì

Fast Decay

- By fast decay processes from level 3

population inversion

2

- Ruby lasers is based on this scheme E,-E

eee,

- It is a better way to create

E.-E, | ° * Example: Nd:YAG laser

®—=e®&——Q

1.2.4 Quasi-three (or quasi-four) level laser:

- The lower lasing level is partially occupied in thermal equilibrium

“Example: Yb: YAG

(Ytterbium)

§2 Properties of laser beams:

- Laser rays have many valuable properties that cannot be found in light

form other sources

- Laser rays have many applications in science, technology, medicine

The distinguished properties of laser rays are

¥ Monochromaticity

¥ Temporal and spacial coherence

V Directionality

2.1 Monochromaticity:

- Lasers amplify light waves that have defini†e frequency

- But there is still a narrow spread for v because of finite upper state and

interaction with surounding environment

- Example: For Laser Nd:YAG

dX = 1.064 um: v = 2.8 x 104 Hz,

the spead is Av ~ 3 kHz

— A laser beam is

2.2 Temporal and spacial coherence:

It means that different points in the laser beam have definite relation in phase We can define wave fronts for a laser beam

2.3 Directionality:

- The property of coherence and the result of the laser cavity make Laser beams ot be high directional

(=P) )) 1

—> Laser beams are

- Because of high directionality we can creat very bright beams and concentrate light energy in an exact locations = a rich supply

of applications !

Laser †ypes

Gas (HeNe, CO2), Liquid (dye), Solid-S†a†e (Nd:YAG, Yb:YA6, Ruby,

Ti:Sapphire, Fiber, Semiconductor), Chemical (HF), X-ray

A scheme of laser

Prisms

7| (Dispersion compensation) Output — >

1i:Sapphire

High

refiector

I8 ee

| nh | FoéusinB-

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