| A low-switching-voltage electrooptic directional-coupler modulator for use in analog optical communication systems at the wavelength of 1300 nanometers was investigated. Conventional optical directional couplers have a pair of their constitutive waveguides running in parallel, and thus the coupling between them is constant throughout the length of the structure, and this gives rise to an approximate sinc-squared amplitude transfer function. This transfer or response function is nonlinear, and therefore modulators based on the constant-coupling directional coupler may only have a small modulation depth of about 2%–5% for analog communication systems. In this work, the directional coupler was modified by varying the etch depth between the two constitutive waveguides while keeping the spacing between them constant along the length of the structure, so that the modulator's response function of the desired form was obtained and exhibited high linearity. Consequently, large dynamic range and low switching voltage operation may be realized with this directional-coupler modulator. Based on this concept, the device built in AlGaAs/GaAs semiconductor material was designed, fabricated and tested. The do experimental result shows that the directional-coupler modulator, which is 1.5 cm long, has a switching voltage of about 1.8 volts—one of the lowest values for electrooptic modulators built to date. With further investigation on the RF/microwave characteristics, the directional-coupler modulator based on this design might prove to be promising for use in the optical communication mainstream. |
