Water-free hygroscopic laser crystals

Non-linear crystals are used for frequency conversion of lasers. Typically, in a non-linear second-order process, a new, higher-energy photon is formed from the sum of two photons. In particular, this technology is concerned with the non-linear family of hygroscopic borates (LBO, BBO, CLBO), which are particularly suitable for the to generate photons in the ultraviolet (UV) spectral region. Utilizing this patent, it was possible to increase the radiation hardness in the UV range while simplifying the laser design by removing the need to heat the crystals permanently.


Coating the optical surfaces does not protect the cladding surfaces. However, in the long run, due to the hygroscopic nature of borates, water penetrates through the cladding surfaces into the crystal interior, which increases the mobility of Li+ ions and thus has a negative impact on the radiation hardness. The usual solution for reliable operation of borates is to provide a dry compartment and optionally additional heating. However, this not only generates additional costs in laser design, but also complicates the production process, as the crystals must be subjected to a closed dry cycle. must be. Attempts to fully seal hygroscopic crystals with directional coating methods usually fail due to a lack of coating homogeneity in the edge region.

Our solution

The invention now makes it possible to simplify borate handling and reduce costs in laser design while increasing the radiation hardness of the crystals. In practice, the optimum bakeout temperature for a special type of crystal is first determined with respect to the radiation hardness. Then the crystal is heated to this temperature in a coating chamber and sequentially hermetically sealed with a thin layer. The resulting crystal is optimized and stabilized in its structure. In a preferred technical embodiment, the optical surfaces of the crystal are first coated with a directional coating method (e.g., IBS) antireflective (AR). Only then are the crystals hermetically sealed on the cladding surfaces, covering the optical layers. The technical solution for a hermetic cladding lies in the application of a non-directional (in contrast to the directional IBS method) coating method, e.g. the ALD (atomic layer depostition) The advantage with this solution is that directional coating methods are more compact and have a higher optical damage threshold.

CPA-1902-LZH-imageSetup for testing of the functionality of a non-linear crystal (source: www.lzh.de)


  • Simplification of borate crystal handling and laser sytems
  • Removes need to heat laser crystals (hygroscopic borates)
  • Reduces operational costs of a these laser systems
  • Higher damage threshold of the crystals
  • Increase of the radiation hardness
  • Long-term stable sealing
  • Can be optimized for different crystals


  • Laser system where laser crystals for frequency conversion are used

Development Status

Process successfully tested and optimized for different laser crystals

Patent Status

Laser Zentrum Hannover e.V.
DE102016101227B4 (granted)




Dr. Markus Muchow
Patent Manager Physics & Technology
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Tel.: +49 551 30724 159
Reference: CPA-1902-LZH

Tags: Laserphysik und Optik, Physik und Technik & Software


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