Abstract

Lithium Niobate (LN), a key crystal for integrated optics and electrooptics, has gained interest since the first demonstration of efficient optical frequency conversion based on quasi-phase matching in periodically poled (PP) ferroelectric domain structures. The possibility of quasi-phase matching (QPM) provides capabilities that exceed those of birefringcnt phase matching. One of the early techniques to produce periodically poled structures is the use Czochralski in an off-centred configuration Whilst other techniques like electric field poling can be used to create the alternating domain structure, this in-growth technique provides large cross sectional areas. When using this technique, impurities need to be added to the melt to allow the formation of the domain structure. Addition of rare-earth oxides to the melt has been shown to create alternating domain structure in crystals which in addition incorporate useful optical ions like Nd³⁺,Yb³⁺ and Er³⁺. Crystals exploiting this combination have been successfully grown [1] and operated as self-frequency converted lasers [2]. One of the main limitations encountered when adding these oxides alone to the melt is the short length -one or two millimetres- so far obtained. Because non-linear frequency conversion efficiency improves with increasing length of the domain structure, ways of increasing such a length are of great interest Although this present limitation may not be of a fundamental character, we have started exploring the combination of more than one impurity added to the melt during crystal growth. Our early results show that addition of this extra impurity improves the formation of the domain structure. In order to avoid interference with useful optical processes in the laser active ion, we combine rare-earth doping with impurities lack optical transitions interfering with those of the active

© 2000 IEEE