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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.
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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 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.
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