Abstract

Linear modulators are useful for electromagnetic field sensing¹ and as external modulators in communication and cable television systems². Theoretically, a passively-biased modulator can be achieved with a difference in interferometer arm lengths, to give an intrinsic 90° phase bias angle. Fabrication tolerances result in error from 90° but can be corrected by using excimer laser ablation to phase-tune the angle³. Here we describe the thermal stability of modulator phase angle ϕ₀ for dielectrically packaged devices, and we compare stability before and after ablation. The effect of humidity on the magnitude and stability of ϕ₀ is investigated. Beaumont et al.⁴ showed that humidity significantly affects modulator stability under conditions of constant DC bias voltage. However, the interferometers studied were in Z-cut LiNbO₃ with a silicon dioxide buffer layer. Many effects were thought to be due to a surface phenomenon associated with the SiO₂. In our work we are concerned with X-cut LiNbO₃ devices where no buffer layer is required. Such devices show no longterm drift, as opposed to those with SiO₂ buffers⁵.

© 1995 Optical Society of America