# 专业术语解释（一）

Nonlinear Optical Crystals Application

The nonlinear optical (NLO) crystals mentioned in our catalogue are used in frequency conversion for lasers. INNOWIT has a complete line of NLO crystals including KTP, BBO, KDP & DKDP, KTA, RTP, BiBO, CLBO, LiNbO3, MgO:LiNbO3, LiIO3, etc.

Frequency Conversion

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 Frequency 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:1064 nm + 532 nm -> 355 nm 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 scheme for frequency doubling is extra- cavity doubling. The laser passes through the nonlinear crystal only once as shown. However, if the power density of laser is low, focused beam, intra-cavity 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.  Frequency Tripling

Frequency Tripling or Third Harmonic Generation (THG) is an other 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 crystal, it can generate wavelength as short as 193nm.

Differential-Frequency Generation (DFG):

ω1  - ω2  = ω3  (or 1/λ1  - 1/λ2  = 1/λ3  in wavelength)

It mixes one high energy photon with one low energy photon to

get another low energy photon. For example:

532 nm - 810 nm -> 1550 nm 