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
Irradianceat the distan ce r from the center
Er  E0e 2 r
2
/ r02
E0 : irradienceat the centerof beam(W/m2 )
r0 : dis tan ce at thebeamedge(m)
Gaussian Beam (Cont’)
Laser Longitudinal Mode
Lm
Optical resonance:
Lm

2
(m : integer)   

2
(m : integer)   
2L
m
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 ?
2  300106
m

 750 wavelengths

800109
2L
2L
m
Multi-(Longitudinal) Mode Laser
v
c
n
v: velocity of the wave in cavity
c : speed of light
n : refractive index of cavity material
Cavity resonance frequency
mc
f 
2 nL
Spacing between the adjacent frequency ( frequency spacing )
f 
c
2nL
Wavelength spacing
20 f
20
 
(
)
c
2nL
0 : free space value of the wavelength
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
c
3 108
9
frequency spacing: f 


139

10
Hz
6
2nL 2  3.6  30010
2
9 2
9
wavelength spacing:    0 f  ( 80010 ) 13910  0.275nm
8
c
mode number: m 
3 10
2
 5.51 (at most 6 modes)
0.275
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