The guided-to-leaky hybrid mode transition in slab optical waveguides made on uniaxial crystals such as LiNbO/sub 3/, or LiTaO/sub 3/, is analyzed. Two different guided-to-leaky transitions have been identified, namely the ordinary cutoff and the extraordinary cutoff, which occur when considering negative and positive birefringent materials, respectively. Analytical but transcendental expressions have been obtained, yielding the critical optical axis orientation, relative to the waveguide axis, above which the totally guided hybrid modes become leaky. The results indicate that the value of the critical orientation strongly depends on the waveguide parameters. The possibility of tuning this critical orientation to a desired value through the waveguide parameters is discussed.

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