How to Handle A NLO Crystal
How to select a NLO crystal for a frequency convensionprocess with a certain laser?
|Laser Parameters||Crystal Parameters|
|NLO Process||Phase-Matching Type and Angle|
|Power or Energy||Damage Threshold, Deff|
|Beam Size||Crystal Size, Walk-off Angle|
|Pulse Width||Group Velocity Mismatching|
|Repetition Rate||Damage Threshold|
|Environment||Temperature Acceptance, Moisture|
Keep crystal clean
When you receive the NLO crystals, pleselook at the polished or coated crystal surface first. If the surface are contaminated, please blow the surface with air ball. If there is still pollution on the crystal surfaces, please clean the surfaces with cleaning liquid and soft silk. For BBO crystal, the mixingliqiudof 50% high purity alcohol and 50% high purity ether is recommended as cleaning liquid. Please note that the contaminated surfaces are very easy to be damaged.
In order to obtain maximum conversion efficiency, angle tilting is normally used to reach phase-matching direction. There are two axis for tilting crystal angles as show. BecausetheNLO crystals are normally cut in principal crystal plane, conversion efficiency is not sensitive to the angle tilting around b-axis. Customers have to pay attention when rotating the crystal around a-axis. A crystal mount with angle accuracy of about 5 arc second is recommended.
Optimum Crystal Size and Cut
When ordering a nonlinear optical crystal, crystal orientationandsize have to be known. The orientation is solely determined by the nonlinear optical process. The crystal size is divided into three dimensions noted as WxHXLmm3. The careful design of crystal size is important because the conversion effciencyhas direct relation to crystal length.
To select the optimum crystal height(H), the laser beam diameter upon the crystal should be taken into accout. The optimum crystal height should slightly(1-2mm) larger than the laser beam diameter upon the crystal.Bothof laser beam Diameter upon NLO crystal and tunable wavelength range have to be considered when designing the optimum crystal width(W). The crystal length is decided by the application. The different crystals and different application will be required the different length's crystals.
The most important thing is to obtain a high conversion efficiency. The conversion effciencyhas the following relationship with effective nonlinear coefficient (Deff), crystal length(L), input power density (P) and phase mismatching (ΔK):
IN general, higher power density, longer crystal length, larger nonlinear coefficients and smaller phase mismatching will result higher conversion efficiency. However, there is always some limitation coming from nonlinear crystals and lasers. For example, the Deffis determined by the nonlinear crystal itself and the input power density has to be lower than the damage threshold of crystal. Therefore, it is important to select a right crystal for your applications. In the following Table, we list the laser and crystal parameters for selecting right crystals.