Lecture 38

Lasers

Final Exam next week

LASER

Light

Amplification by

Stimulated

Emission of

Radiation

Electronic Energy Levels

0

1

2

Ene

rgy

e-

e-

Fluorescence or ‘Spontaneous Emission’

e-

e-

Stimulated Emission

0

1

2

Ene

rgy

e- e-

The stimulated emission has the same frequency and phase as the incoming photon

3 level system

0

1

2

e-

‘Short’ lifetime

‘long’ lifetime

Stimulated Emission Light Amplifier

Pump (does not need to be light)

Gain medium can be gas, liquid, or solid

All light is in phase and has the same frequency

Laser Cavity

Once the amount of gain is equal to the cavity losses,‘lasing’ happens

Losses can include output coupler, reflectivity, other optics in cavity

Cavity modes

The mirrors force the amplitude of the wave to zero.

Only integral number of half wavelengths can exist.Every wave exits in phase

2c

vL

Allowed frequencies

0

1

It is a combination to the energy spread and the cavity modes

0.00 0.05 0.10 0.15 0.20 0.25 0.30

X Axis

0

2

4

6

8

10

12

14

YAxis

Laser pulse generation

PC

“Q-switching”, ns pulses

“Mode-locking”,fs pulses

Cavity length ~1.5m

Gain medium

Q=2*Energy stored

Energy Lost per cycle

Transverse modes

Transverse Modes

The 00 mode is the smallest….

Types of lasers

Gas: HeNe

Solid-state: Semi-conductor Diode, Crystals (sapphire), nano-particles

Liquid: Dyes,

Chemical laser: CO2, Excimer