Fig. 1. Three optical coupling schemes of Ge-on-Si PD: (a) vertical incidence and waveguide-integrated coupling including (b) butt coupling and (c) evanescent coupling. In evanescent coupling the optical input waveguide can be positioned on top, at the bottom, or lateral to the absorber (germanium). The inside of the orange rectangle is the evanescent-coupling configuration based on double lateral Si3N4 waveguides, which is first proposed and demonstrated in this work.

Fig. 2. (a) Three-dimensional (3D) schematic of Ge-on-Si PD with double lateral Si3N4 waveguides. (b) Top view of Ge-on-Si PD with double lateral Si3N4 waveguides. (c) Cross-sectional view of Ge-on-Si PD with double lateral Si3N4 waveguides. (d) Cross-sectional view of the Si3N4 waveguide optical field with 800 nm and 450 nm width and the side view of the optical field distribution of the Ge absorption region. The light propagates along the x axis.

Fig. 3. (a) Micrograph of the fabricated Ge-on-Si PD with double lateral Si3N4 waveguides. (b) and (c) Schematic of the experimental setup for the measurement of the 3 dB OE bandwidth and eye diagrams. The black and red lines represent the optical and electrical connections, respectively. PD, photodetector; AWG, arbitrary waveform generator; EDFA, erbium-doped fiber amplifier; VOA, variable optical attenuator; WSS, wavelength-selective switch; PC, polarization controller; LN MZM, lithium niobite Mach–Zehnder modulator.

Fig. 4. (a) Current-voltage (I-V) characteristics of Ge-on-Si PD with double lateral Si3N4 waveguides in the dark illuminated state. (b) Measured photocurrent and responsivity as a function of input optical power at −1  V bias voltage.

Fig. 5. (a) and (b) Experimental and fitted magnitude/phase part of the small-signal S11 reflection parameters from 100 MHz to 60 GHz at −3  V bias voltage. The inset plots the extracted equivalent circuit model of our proposed Ge-on-Si PD with lateral Si3N4 waveguides.

Fig. 6. (a) Normalized RF response of the Ge-on-Si PD with lateral Si3N4 waveguides. The bias voltage is fixed at −3  V. (b) Measured 3 dB OE bandwidths with different photocurrent levels. The inset plots the experimental and fitted results of the S21 transmission parameter under 4 mA photocurrent.

Fig. 7. Measured 70, 80, 90, and 100 Gbit/s NRZ eye diagrams under 3 V reverse-bias voltage.

Fig. 8. Measured 100, 120, 140, and 150 Gbit/s PAM-4 eye diagrams under 3 V reverse-bias voltage.

Fig. 9. Measured 60 Gbit/s NRZ eye diagrams with 5, 10, 15, and 20 mA photocurrent at the DC bias voltage of −3  V.