The frequency conversion processes include frequency doubling (which is a
special case of sum frequency generation), sum frequency generation (SFG), differential-frequency
generation (DFG) and optical parametric generation (OPG) which are
demonstrated in the following equations:
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Sum Frequency Generation (SFG):
w1 + w2 = w3 (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:1064 nm + 532 nm -----> 355nm
Sum Frequency Generation
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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: 2w1 = w2
(or l1 = 2l2 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. If you meet any problems in
scheme selection, please contact SUWTECH for a solution
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Extracavity
SHG
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SHG
With Focused Beam
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Intracavity
SHG
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External
Resonant cavity SHG
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Frequency Tripling
Frequency Tripling or Third Harmonic Generation (THG) is an other example of
Sum Frequency Generation where, for THG of Nd:YAG laser, l1=1064nm, l2=532nm
and generated wavelength l3=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.
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Differential-Frequency Generation (DFG):
wp = ws + wi (or 1/lp = 1/ls + 1/li in wavelength) It splits one high-energy
photon into two low-energy photons. For example:355 nm ---->532 nm + 1064
nm
Differential Frequency Generation
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Optical Parametric Generation (OPG):
wp = ws + wi (or 1/lp = 1/ls + 1/li in wavelength) It splits one high-energy
photon into two low-energy photons. For example:355 nm ---->532 nm + 1064
nm
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Optical Parametric Generation (OPG) is an
inverse process of Sum Frequency Generation. It splits one high-frequency
photon (pumping wavelength, lp) into two low-frequency photons (signal, ls,
and idler wavelength, li). If two mirrors are added to form a cavity as shown
in following Figure, an Optical Parametric Oscillator (OPO) is established.
For a fixed pump wavelength, tilting a crystal can generate an infinite
number of signal and idler wavelengths. Therefore, OPO is an excellent source
for generating wide tunable range coherent radiation. BBO, KTP, LBO and
LiNbO3 are good crystals for OPO and Optical Parametric Amplifier (OPA) applications.
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