Abstract :
Summary form only given. Recent progress in the development of velocity matched broadband LiNb03 modulators has opened new opportunities for the demonstration of low drive voltage devices. Velocity matched traveling wave devices in LiNb03 require the reduction of the microwave phase velocity so that it equals the optical phase velocity. Generally this is done by manipulating the geometric structure and layer thicknesses of the electrodes and buffer layers making up the electrode structure. Early velocity matched devices\´" tended to achieve velocity matching at the expense of impedance matching which in turn results in some degradation in performance. The difficulty in achieving simultaneous velocity and impedance matching arises from the high dielectric constant of LiNb03. Progress in overcoming this problem and achieving improved device operation has come fiom two developments: First, the use of ridge waveguide strutures4 which use physical etching to remove the high dielectric constant LiNb03 material, and thus add an additional degree of fieedom in the design problem. Second, the realization that increasing the electrode gaps can actually improve device performance even though the field across the waveguides is r edu~ed~T?h~i.s occurs because the improvement in electrode loss at the larger gap gives a larger benefit than the negative effect of the simultaneous increase in voltage-length product.
