Low Vπ high-speed GaAs travelling-wave electrooptic phase modulators using an n-i-p-n structure

High speed semiconductor optical modulators are very important components for optical communication systems [1]. Characteristics such as low drive voltage, wide bandwidth, low insertion loss, low wavelength and temperature sensitivity, and low cost are essential for these optical modulators. Recently 40 GHz InP modulators with a drive voltage of only 2.3 V have been reported which use an n-i-n waveguide structure instead of the conventional p-i-n structure to reduce both the microwave and optical losses [2]. However InP-based modulators have the drawback of the wavelength dependence of the drive voltage. GaAs based modulators do not have this problem due to its larger bandgap energy. To date GaAs modulator has been demonstrated with a 35 GHz optical bandwidth and 5 Vcm
⁻¹
V
π
[3]. In this paper, we report a new design for GaAs modulators based on an n-i-p-n structure. The travelling-wave coplanar waveguide (CPW) electrodes are employed to realize high speed operation. By optimization, an electrical 3-dB bandwidth of nearly 40 GHz (optical 3-dB bandwidth of 80GHz) and a V
π
around 6.6 V are predicted for a 5 mm long phase modulator.