Abstract

Mach-Zehnder interferometers have previously been demonstrated for electromagneticfield sensing.¹ Linear modulation is achieved passively, without use of a DC voltage bias, by means of a difference in interferometer arm lengths, corresponding to an intrinsic phase bias ϕ_o = 90°. In practical devices imprecise fabrication results in deviations from the desired 90° angle, because of differences in phase velocity along the two arms.² Methods to tune optical devices by phase-velocity adjustment have been reported^3,4 but are relatively slow, involving lithography of an oxide cladding layer and of limited accuracy. Tuning of an annealed proton-exchanged LiNbO₃ directional coupler by using a CO₂ laser beam focused to ~50 μm spot size was recently reported.⁵ Here we report a fast, accurate method for tuning interferometers to 90° after fabrication. Tuning was performed on a packaged modulator, with the phase bias measured in situ during tuning and correction.

© 1991 Optical Society of America