Abstract

An integrated-optical frequency shifter for λ= 1.5µm and a frequency shift of f_mod= 6.5GHz has been designed in Ti:LiNbO₃ technology. An optical input wave (carrier) at frequency f_in is electro-optically converted to an output wave (sideband) at f_out = f_in ± f_mod. Both waves propagate at orthogonal polarizations in the same stripe waveguide. Conversion is effected by an electrical wave of frequency f_mod, counter-propagating to the optical waves on a coplanar transmission line. The 3-wave interaction is phase-matched if where Δβ=β_c-β_s is the waveguide birefringence (β_c, β_s, propagation constant of carrier and sideband), N_gr is the mean index of optical group velocity, and n_el is the ‘effective index’ of the electrical wave. Phase matching is practically achieved by adjustment of the dimensions of the transmission line and of Δβ. In the x-cut, z-propagating LiNbO₃ substrate, Δβ could be controlled by adjustment of the width w of the Ti stripe. The birefringence was measured by scanning a polarization coupling perturbation along the waveguide while analyzing the polarization of the output signal [1]. Polarization controllers are applied to the pigtails so that the incident signal can be adjusted to either TE or TM polarization. In this way TE/TM or TM/TE conversion can be selected. Due to the reversal of tire sign of Δβ between these two cases the device operates either as frequency up- or downshifter.

© 1996 IEEE