Thermally annealed single-mode proton-exchanged channel-waveguide cutoff modulator

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Abstract
We describe the first reported electro-optic cutoff modulator that utilizes a thermally annealed single-mode proton-exchanged channel waveguide in a X-cut Y-propagating LiNbO3 substrate. The large changes in extraordinary refractive index obtainable by the proton-exchange process enabled us readily to form single-mode waveguides with a channel width as small as 2 μm. As a result, the voltage required for electro-optically controlled propagation cutoff was significantly reduced. Thermal annealing was used to provide fine tuning on the refractive-index changes that was in turn used to bring the waveguide to the very edge of cutoff and thus permitted further reduction in the voltage requirement. Thermal annealing was also found greatly to improve the linearity of modulation. For example, we have measured a modulation depth of 97% at a total voltage swing of only 7 V and a high linearity. In addition, no optical damage has been observed after a 2-h continuous exposure of 632.8-nm He–Ne laser light at an intensity as high as 104 W/cm2.