|
ω1 + ω2 = ω3 (or 1/l1 + 1/l2 = 1/l3 in
wavelength) It combines two low-energy (or low-frequency) photons into a
high-energy photon. For example: 1064nm + 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: 2ω1 = ω2 (or λ1 =
2λ2 in wavelength). The simplest method of 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, doubling of a 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 another
example of Sum Frequency Generation where, for THG of Nd:YAG laser,
λ1 = 1064nm, λ2 = 532nm and generated
wavelength λ3 = 355nm. By sum frequency of fundamental
wavelength and THG of a Ti:Sapphire laser in BBO crystals, it can generate
wavelength as short as 193nm. |
