Frequency Conversion

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:

Sum Frequenby Generation (SFG):

ω1+ω2=ω3

(or 1/λ1+1/λ2=1/λ3 in wavelength)

It combines two low energy (or low frequency) photons into a high energy photon. For example:

1064nm+1064nm→532nm

Differential-Frequency Generation (DFG):

ω1-ω2=ω3

(or 1/λ1+1/λ2=1/λ3 in wavelength)

It combines two high energy photons into a low energy photon. For example:

532nm-810nm→1550nm

Optocal Parametric Generatiom (OPG):

ωp-ωs=ωi

(or 1/λp=1/λs+1/λi in wavelength)

It splits one high energy photon into two low energy photons.

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.The simplest scheme for frequency doubling is extracavity doubling.The laser passes through the nonlinear crystal only once as shown in the Fig.3. 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 lon lasers.

Sum Frequency Generation

Frequency Tripling or Third Harmonic Generation(THG)is an example of Sum Frequency Generation where,for THG of Nd:YAG laser, w1=1064nm, w2=532nm and THG of a Ti:Sapphire laser in BBO crystal, it can generate wavelength as short as 193 nm.

Optical Parametric Oscillation

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 from a cavity as shown in fig. 7, an Optical Parametric Oscillator (OPO) is established. For a fixed pump wavelength, an infinite number of sigal and idler wavelengths can be generated by tilting a crystal. Therefore, OPO is an excellent source for generating wide tunable range coherent radiation. BBO, KTP, LBO and LINbO3 are good crystal for OPO and Optical Parametic Amplifier (OPA)Applications.