5. Lasers

Introduction to Lasers

Laser Acupuncture, Laser Scalpel and Laser W/R Head

Laser Scanner

Lasing Conditions

Theory of Laser

General Optical Resonator and Laser Transverse Mode

Gaussian Beam

(m)

)(W/m

0

20

20

20

2

edgebeamce at thetandis:r

beamofthe centeratirradience:E

eEE

the centerce r from tanthe disatIrradiancer/r

r

Gaussian Beam (Cont’)

Laser Longitudinal Mode

m

LmmL

2integer):(

2

hs wavelengt75010800

10300229

6

L

m

Optical resonance:

where λ: wavelength of the resonating frequency, L: length of the cavity, m : an integral

Eg. laser diode : cavity length = 300 m; AlGaAs : gap wavelength 800nm How many wavelength will fit into the cavity ?

m

LmmL

2integer):(

2

Multi-(Longitudinal) Mode Laser

Cavity resonance frequency

Spacing between the adjacent frequency ( frequency spacing )

Wavelength spacing

0 : free space value of the wavelength

nL

mcf

2

nL

cf

2

)nL

(c

f

2

20

20

n

cv v: velocity of the wave in cavity

c : speed of light

n : refractive index of cavity material

Eg. Laser diode : cavity length = 300 m

refractive index = 3.6 ; line width of the cavity = 2 nm

peak radiating wavelength 0 = 800 nm

Find output spectrum from the laser

frequency spacing:

wavelength spacing:

mode number:

Hz.nL

cf 9

6

8

1013910300632

103

2

nm.)(

c

f2750

103

10139108008

92920

modes) 6most (at 51.5275.0

2m

Typical Planar Laser diode (LD)

Structure

Characteristics of Laser Diode

Modulation of Laser Diode

Radiation of Laser Diode

Radiation pattern

Output spectrum

Typical Characteristics of Diode Light Sources

Laser Safety Rules

1) Wear special goggles

2) Never looking the primary laser beam or into the specular

reflection of a beam

3) Higher power beams should be terminated

4) Skin protection should be worn at higher power levels

( e.q. heavy white cloth )

5) Special precaution for invisible laser beam