Traveling-wave lithium niobium trioxide optical modulator using Y-branch directional coupler

High-speed optical modulators are important components of wide-band single-mode optical communication systems. Such systems are very attractive for providing high capacity data links due to their small group velocity dispersion and low loss at 1.3 {dollar}mu{dollar}m. Electrooptic external modulators using LiNbO{dollar}sb3{dollar} waveguides are potentially useful devices for high bit rate transmissions and signal processing.; Most intensity modulators are based on the theory of the uniform directional coupler. The traveling-wave electrodes are required to achieve the high-speed switching operation. There are a number of design parameters that determine the ultimate performance of Ti-indiffused LiNbO{dollar}sb3{dollar} modulators: namely, a good frequency response, a low switching voltage and a high extinction ratio. A trade-off is required depending on the parameter we emphasize.; We have developed several tools of theoretical analysis. These include the evaluation of the effective index method for step-index waveguides, new eigenvalue equations for the asymmetric directional coupler with graded-index profile using the WKB approach, nonuniform finite difference method for modeling LiNbO{dollar}sb3{dollar} waveguides, and electrode analysis by conformal mapping in the case of an anisotropic substrate. Additionally, we have analyzed the frequency response of the traveling-wave electrode.; Based on the theoretical results obtained above, we designed a traveling-wave LiNbO{dollar}sb3{dollar} optical modulator using a Y-branch directional coupler. By employing the Y-branch directional coupler, we have reduced the device length by 30% as in comparison with the conventional uniform directional coupler. This enables us to achieve a device with large bandwidth. We have also designed a traveling-wave electrode of semi-infinite coplanar strip structure in order to achieve high-speed operation.; The dc test showed that the coupling length of the Y-directional coupler l{dollar}sb{lcub}rm y{rcub}{dollar} was about 3.25 mm. The switching voltage of the device was nearly 12.5 V and the extinction ratio was 17 dB. We have fabricated two samples with coupling lengths of l{dollar}sb{lcub}rm y{rcub}{dollar} and 0.75 l{dollar}sb{lcub}rm y{rcub}{dollar}. The switching voltage and extinction ratio of the device of 0.75 l{dollar}sb{lcub}rm y{rcub}{dollar} were 20 V and 13 dB, respectively. Since we emphasized the large band-width, the device of 0.75 l{dollar}sb{lcub}rm y{rcub}{dollar} was chosen for the RF test at the cost of having a high switching voltage and a low extinction ratio. The projected band-width is around 23 GHz. The microwave test will be conducted in the near future.