Non-linear Optical Crystal Overview

Non-linear Optical Crystals (NLO Crystals) have extended the laser spectrum from deep UV to far IR by various harmonic generations (like SHG, THG, 4HG, 5HG), sum frequency generations, difference frequency generations, optical parametric oscillators, etc.

The most common non-linear optical crystals include KTP, beta-BBO, LBO, CLBO, DKDP, ADP, KDP, LiIO3, KNbO3, LiNbO3, AgGaS2, AgGaSe2 etc. Each one has its own pros and cons.

KTP is the most common NLO crystal for SHG or YAG lasers. The high conversion efficiency, non-hygroscopicity, and cost-efficiency make KTP the first choice for middle or low power level 1064nm systems. The relatively low damage threshold makes KTP unsuitable for high power systems. KTP is also used in 1064nm pumped OPO to generate IR lasers.

Beta-BBO was invented in the 1980s, its wide phase matching range, high conversion efficiency, high damage threshold and high transmission at the deep UV band make BBO the most common NLO crystal. BBO is widely used in various harmonic generations of 800nm and 1064nm. BBO is a little bit hygroscopic, extra caution should be taken to store and use it, especially in a humid environment.

Invented almost in the same time as BBO, LBO has the highest damage threshold, and better inner crystal quality than BBO. LBO can achieve non-critical phase matching for SHG1064nm about 150C, which along with its high damage theshold, makes LBO the best SHG crystal for high power 1064nm systems.

DKDP (deuterated KDP), KDP, ADP were the first commercially available NLO crystals . The merits of DKDP or KDP include cost-efficiency and large size availability. For large apertures, such as those over 50mm, DKDP, KDP and ADP are the only options.

KNbO3's conversion efficiency is excellent. And direct SHG of diode lasers has already been reported. But the multi-domain-issue is its major drawback.

LiIO3 has a high conversion efficiency, and it is easy to get in big sizes. But the hygroscopicity and low damage threshold limit its applications.

AgGaS2 and AgGaSe2 are the most important NLO crystals in IR ranges, since most of the above NLO crystals are only transparent in visible or near-IR range. The typical usage of these two crystals are as SHG of CO2 lasers, as well as OPO crystals to generate IR or far IR radiations.

How can you choose the right NLO crystals? Here is the list of questions you have to answer:

1. Is the crystal transparent at the target wavelength(s)?
2. Can the crystal achieve phase-matching (including Type I /II phase matching)?
3. Is the damage threshold suitable for your applications?
4. Is the conversion efficiency high enough?
5. Is the size of the finished device feasible?
6. Is the crystal suitable for your experiment environment?
7. Is it cost-efficient?

In most cases, not all answers to the above questions are "Yes", so you have to stike a balance among these factors to find the best solution for your systems. For example, to get a SHG crystal for Ti:Sapphire laser at 800nm, with the pulse width of 40fs, and the expected pulse broadening effect less than 20fs. KTP, AgGaSe2 or AgGaS2 can not achieve phase matching of SHG800nm; LiIO3's damage threshold is not high enough; Both BBO and KDP are suitable, however, BBO's conversion effieicy is higher than KDP, and 100micron BBO SHG800nm can only create pulse broadening effect of 19fs. So, a good choice is a BBO with 100 micron thickness as this second harmonic generator.

For more information, please feel free to contact the United Crystals' experienced technical support team.

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