All-optical gating operation in hybrid Si/III–V Mach-Zehnder interferometer

Silicon-on-insulator (SOI) platform allows highly-integrated photonic circuits with CMOS fabrication process. Hybrid integration technologies of Si and III-V materials have been developed in order to realize active functional devices. Intersubband transition (ISBT) in a III-V semiconductor provides ultrafast cross-phase modulation (XPM) between TE probe light and TM pump light. All-optical signal processing have been demonstrated by using Mach-Zehnder interferometer (MZI) switches composed of free-space optics. However, it is difficult to integrate the MZI switch monolithically in the III-V waveguides, because the TM pump light is strongly absorbed by the ISBT excitation at the waveguide edge. In this study, we proposed a hybrid MZI switch of a Si and II-V waveguides for the all-optical switching device. Figure 1(a) shows a schematic of the proposed hybrid MZI switch which has one 3-dB coupler of Si waveguide and the other 3-dB coupler of II-V waveguide. TM pump light is combined with TE probe light in one MZI arm of the Si waveguide, and then coupled to the III-V waveguide. Therefore, the XPM due to the ISBT occurs in one MZI arm. The output signal light related to the XPM is obtained from the end of the III-V waveguide. Figure 1(b)-(d) shows the microscopic image of the fabricated device. The 3-dB coupler, the phase shifters, and the TE/TM beam combiners were fabricated with Si waveguides having a cross section of 450 × 220 nm². The first 3-dB coupler was a variable coupler based on an MZI with thermo-optic phase shift which fairly compensates power imbalance between the two arms. The second 3-dB coupler was a multimode interference (MMI) coupler with III-V waveguides having a width of 1.6 μm fabricated in a core layer with separate confinement hetero-structure of 0.2-μm-thick InGaAs(Si)/AlGaAs/AlAsSb coupled double quantum wells (QWs) sandwithed by 0.175-μm-thick InGaAs/AlAsSb multi-QWs grown on an InP substrate. The Si - - and III-V waveguides were aligned and butt-jointed using a piezo stage. In order to match the mode field diameters, we fabricated tapers at the ends of the waveguides. A calculation result shows that the widths of each mode field are about 2.0 μm when the width of Si waveguide is 180 nm and that of III-V waveguide is 2.5 μm, and then the coupling efficiencies are estimated to be 72% and 79% for TE and TM modes, respectively. First, we characterized the extinction ratio of the hybrid MZI switch by tuning the phase shifter. Figure 2 shows the optical output power for the TE light with a wavelength of 1560 nm modulated by the triangular wave voltage at 1 kHz applied to the phase shifter. The extinction ratio of 25 dB was obtained by adjusting the power balance in the variable coupler. Next, we demonstrated all-optical gating operation. The output of TE probe light was set to be destructive in the MZI switch. A TM pump light with a pulse width of 2 ps, a repetition rate of 10 GHz, and a wavelength of 1545 nm was injected to another input port of the Si waveguide. Figure 3 shows the output TE probe signal. The 10 Gb/s gated signals associated with the TM pump signal were obtained.