Sum Frequency Generation
DifferentialFrequency
Generation
Optical
Parametric Generation
Phase
Matching
Conversion
Efficiency
Angle
Tilting
Optimum
Crystal Cut
Crystal
Size
Brewster's
angle
Handle
A NLO Crystal

Sum
Frequency Generation (SFG)

w_{1}
+ w_{2}
= w_{3}
(or 1/l_{1}
+ 1/l_{2}
= 1/l_{3}
in wavelength) It combines two lowenergy (or low frequency)
photons into a highenergy photon. For example:
กก1064nm
+ 532nm > 355nm

Frequency
Doubling
Frequency
Doubling or Second Harmonic Generation (SHG) is a special
case of sum frequency generation if the two input wavelengths
are the same: 2w_{1}
= w_{2}
(or l_{1}
= 2l_{2}
in wavelength). The simplest scheme for frequency doubling
is extracavity doubling. The laser passes through the nonlinear
crystal only once as shown. However, if the power density
of laser is low, focused beam, intracavity doubling and external
resonant cavity are normally used to increase the power density
on the crystals, for example, for doubling of cw Nd:YAG laser
and Argon Ion lasers.



Extracavity
SHG

SHG
With Focused Beam



Intracavity
SHG

External
Resonant cavity SHG

Frequency Tripling
Frequency
Tripling or Third Harmonic Generation (THG) is an other example
of Sum Frequency Generation where, for THG of Nd:YAG laser,
l_{1}
= 1064nm, l_{2}
= 532nm and generated wavelength l_{3}
= 355nm. By sum frequency of fundamental wavelength and THG
of a Ti:Sapphire laser in BBO crystal, it can generate wavelength
as short as 193nm.

