Frequently Asked Questions
1. How should I select the right non-linear optical crystal for my applications?
The following factors need to be taken into consideration before ordering a NLO crystal device.
Factors For NLO Crystal Selection
|Laser Parameters||Crystal Parameters|
|Lasing wavelength(s)||Transmittance of Crystal|
|NLO Process||Phase-Matching Type and Angle|
|Power or Energy||Damage Threshold, effective nonlinear optical coefficient|
|Beam Size||Crystal Size, Walk-Off Angle|
|Pulse Width||Group Velocity Mismatching|
|Repetition Rate||Damage Threshold|
|Environment||Temperature Acceptance, Moisture|
2. How should I choose the right cutting angle and the size?
To order a nonlinear optical crystal, the crystal orientation (or crystal cut) and size have to be known. The orientation is solely determined by the nonlinear optical process.
The crystal size is divided into three dimensions noted as W x H x L mm3. The careful design of crystal size is imperative because the price of a device is determined mainly by the crystal size. More importantly, the crystal length can affect the conversion efficiency.
To select the optimum crystal height (H), the laser beam diameter upon the crystal should be taken into account. The optimum crystal height should be slightly (for instance, 1 mm to 2 mm ) larger than the laser beam diameter upon the crystal.
Both the laser beam diameter on the NLO crystal and the tunable wavelength range have to be considered when designing the optimum device width (W). We select W=2H, If it is a wide wavelength tuning. For example, frequency-doubling a Dye laser form 440 NM to 660 NM by using BBO crystal, the crystal should be tuned from q=36° to q=66.6°. The width (W) is set to H + tg((66.6° - 36°)/2)xL. If the crystal height (H)is 4 mm and length (L) is 7mm, the W should be approximately 8mm long.
Very few NLO crystals has a standard length (L) for frequency-doubling lasers with pulse width longer than nanoseconds (ns). For example, the standard crystal lengths for BBO and KTP are 7 mm and 5mm, respectively. However, OPO and OPA need a longer crystal, for example, >12 mm for BBO. On the other hand, ultrashort pulse lasers have to use the extremely thin NLO crystal to take the pulse broadening effect under control, so, the thickness of less than 100 microns is quite common.
Don't forget that the engineers in United Crystals are always standing behind you.
3. How should I handle a NLO crystal device?
Keep Device Clean
Please review the polished or coated device surfaces first. If the surfaces are contaminated, please blow the surfaces with a dust blower. If there is still pollution on the crystal surfaces, please clean the surfaces with cleaning liquid and soft silk. For BBO crystal, a mixture of 50% high purity alcohol and 50% high purity ether is recommended as a cleaning fluid. Please note that the contaminated surfaces are very easy to be damaged in laser systems.
Angle TiltingPlease click here to see the instructions (NLOAdjustment.pdf).
Frequency Doubling or Second Harmonic Generation (SHG) is a special case of sum frequency generation with the same two input wavelengths. There are two kinds of frequency doubling, one is extracavity, and another is intracavity. The simplest setup for frequency doubling is extracavity doubling.