Technique for velocity-matched traveling-wave electrooptic modulator in AlGaAs/GaAs

A design for a velocity-matched traveling-wave directional-coupler intensity modulator in AlGaAs/GaAs is proposed. The structure utilizes a thin coating of Ta₂O₅ on the top of the modulator/electrode structure in order to achieve velocity matching between the optical wave and microwave signal. The film does not significantly affect the optical properties or voltage requirements of the modulator. The optical and RF characteristics of the modulator are analyzed using the effective-index and finite-difference methods. The optical bandwidth is calculated numerically, taking into account both the anticipated velocity mismatches due to fabrication tolerances and the calculated frequency-dependent microwave losses. The predicted small-signal bandwidth of a 3 mm long direction coupler biased at a null point is greater than 45 GHz, and exceeds 100 GHz (~50 GHz electrical bandwidth) when the coupler is biased in the linear region. This device is designed to operate at 830 nm with a maximum modulation voltage of 5 V. The figure of merit of the proposed device is therefore at least 10 GHz/V when an electrical bandwidth of 50 GHz is used