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NLO Crystals (Non-linear Optical Crystals) are used for extending the laser spectrum from deep UV to far IR by various harmonic generations (SHG, THG), sum frequency generations (SFG), difference frequency generations (DFG) and optical parametric oscillators (OPO).
The most common used NLO crystals include BBO, KTP, LBO, KD*P(DKDP), KTA, LiNbO3, AgGaS2, KDP, ADP, LiIO3, KNbO3, CLBO, KBBF, BiBO AgGaSe2, etc.
- BBO is the most widely used NLO crystal. BBO's wide transparent and phase-matchable wavelength range, high conversion efficiency and high damage threshold make it an excellent choice for applications from UV to NIR lasers include Nd:YAG, Ti:Sapphire, Dye laser, etc. BBO is a little bit hygroscopic.
- KTP features high conversion efficiency, non-hygroscopicity and cost-efficiency which make it the mostly common NLO crystal for SHG of middle or low power Nd:lasers. KTP is also used in 1064nm pumped OPO to generate IR lasers. KTP has relatively low damage threshold.
- LBO can achieve non-critical phase matching(NCPM) for SHG@1064nm, along with its highr damage threshold, makes LBO the best SHG crystal for high power 1064nm systems.
- KDP DKDP (deuterated KDP, KD*P), ADP are available in large size and cost-efficiently. KD*P are common used in Pockles cells.
- AgGaS2 and AgGaSe2 are few of NLO crystals which can be used in IR ranges. The typical usage of AgGaS2 is OPO and DFG to generate IR radiations.
1) First, is the crystal transparent at the involved wavelengths and phase-matchable?
2) If the answer is yes, then what's the damage threshold, effective nonlinear coefficient and group velocity mismatch (for ultra fast laser)?
3) Chose suitable crystal sizes according to your laser power and pulse width, then calculate conversion efficiency.
4) Finally, balance the cost, conversion efficiency and environment condition.
For example, SHG@800nm Ti:Sapphire 80fs and the expected pulse broadening effect less than 20fs. KTP is not phase matchable for SHG@800nm. BBO, LBO, KDP and LiIO3 are compared following:
|NLO crystal||deff(pm/V)||Lmax(µm)||Imax*(GW/cm^2)||Relative Efficiency|
*Damage threshold value are not accurate. If someone can correct the data or add BiBO for comparing, that will be appreciated.
So LBO and BBO crystal are chose for this application. Ask for a quote, chose the best supplier, then you are set.
- How to handle a NLO crystal? When receiving a crystal, only qualified personnel are allowed to open crystal boxes at clean environment.
OptoCity always ship clean, ready for use crystals. However, if the surfaces are contaminated, blow the surfaces with air ball. If there is still pollution on the crystal surfaces, you may need use 1:1 high purity alcohol and ether on soft silk to clean the crystal.
Blowing and cleaning should be avoid on very thin crystal.
- What is the Mark on Surface: There is a dot mark on the NLO crystal which is the axis for angle tilting to achieve phase matching. Laser polarization should be parallel to the doted axis for type I or 45deg. for type II phase matching. If you are not sure of your laser polarization, try it with different angles.
Some crystal has an arrow mark if the coating is different on the input and output surface.
- Angle Tilting: Put NLO crystal on a mount with fine angle adjusting bar. Slowly tilting the crystal while watching the output laser beam. You may need a dispersive prism to separate the SHG from fundamental laser or a power meter for monitoring. Don't put full power when adjusting to avoid crystal damage. Some crystal has very small phase matching acceptance angle. In that case, mount with higher angle accuracy (about 5 arc second) should be used.
Some NLO crystals have a low susceptibility to moisture, it's advised to provide dry atmosphere conditions for both use and storage. When polished surfaces are fogged or damaged, please ask for re-polishing service before it's totally gone.
- Lifetime of Nonlinear Crystals:
In many cases, a nonlinear crystal used for nonlinear frequency conversion has a very long lifetime, which is longer than
that of the whole laser system. The crystal material is essentially not modified during operation. However, a reduced crystal
lifetime can occur under various circumstances:
- Excessive optical intensities during operation may instantly damage a crystal. Unfortunately, nonlinear crystals often need to be operated not far from their optical damage threshold in order to achieve a sufficiently high conversion efficiency. This implies a trade-off between conversion efficiency and crystal lifetime. Note that even if the nominal intensity is below the nominal damage threshold, there may be problems due to fluctuations of the beam power or local intensity (e.g., if a beam profile has “hot spots”), or due to isolated defects in a crystal, which are more sensitive than the regular crystal material.
- Even well below the threshold for instant damage, some crystal materials exhibit a continuous degradation within the used volume, e.g. in the form of “gray tracking”. Such phenomena are particularly common for operation with ultraviolet light. Note that a gradual degradation can also lead into instant catastrophic damage via excessive heat generation.
- Hygroscopic crystal materials deteriorate when they are not always kept in sufficiently dry air (or a dry purge gas). This applies e.g. to KDP and BBO, and in a lesser extent to LBO. It can be helpful to keep such a crystal at a somewhat elevated temperature, which makes it easier to keep it dry.
- Operation of nonlinear crystals at temperatures below room temperature (in order to achieve phase matching) is generally problematic, as it may lead to condensation of water on the crystal surfaces if the surrounding air is not very dry. Even if the crystal material or coating is not sensitive to water, small water droplets may focus laser radiation more tightly than under normal operation, and thus damage the crystal material.
- Crystals which are non-critically phase-matched in a crystal oven may exhibit problems when the crystal temperature is changed too rapidly or too often. In particular, anti-reflection coatings may be damaged due to different thermal expansion coefficients of the involved materials.
- Crystal lifetime can also be strongly dependent on the material quality, although certain degradation phenomena appear to be intrinsic limitations of the material.
- For high-power UV generation, nonlinear crystals may become consumables: they need to be replaced quite often within the lifetime of the whole laser system (e.g., every few hundred hours of operation). Often, several problematic aspects come together in the regime UV generation: crystal materials are generally more sensitive to ultraviolet light (having high photon energies), exhibit a higher absorption in that regime, and in case of ultrashort pulses the high group velocity mismatch enforces the use of a shorter crystal, which requires high optical intensities for a given conversion efficiency.
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