Increasing the birefringence of KTP and its isomorphs for type II phase matching

A nonlinear optical device comprising a nonlinear optical crystal and input means for directing electromagnetic radiation into said crystal to generate output waves from said crystal having a selected wavelength different from the wavelength of input waves to said crystal using type II phase matching, characterized by:

• said crystal consisting essentially of doped MTiOXO.sub.4 wherein M is selected from the group consisting of K, Rb, Tl and mixtures thereof and X is selected from the group consisting of P, As and mixtures thereof, said crystal containing at least one dopant selected from the group consisting of Fe, Nb, and Ta in an amount, totalling at least about 100 ppm, effective to increase the n.sub.z -n.sub.x optical birefringence of the doped crystal at least about 0.001 compared to a crystal without said dopant;
• and
• said input means being adapted to provide input waves suitable for generating said output waves at an optical birefringence higher than the highest corresponding optical birefringence of a crystal without said dopant.

A crystal consisting essentially of doped MTiOXO.sub.4 containing at least about 100 ppm Ta, wherein M is selected from the group consisting of K, Rb, Tl and mixtures thereof and X is selected from the group consisting of P, As and mixtures thereof.