Fig. 1. Structure of the waveguide-integrated Ge photodetector. (a) Three-dimensional schematic and (b) cross-section view of the proposed device. (c), (d) Simulated static optical field and electric field distribution in Ge region. The simulated bias voltage is −8  V. (e) Optical microscope image of the waveguide Ge PD with an active length of 150 μm. The inset in (a) is the simulated optical reflection spectrum of the DBR.

Fig. 2. (a) Measured I-V curves without illumination (dark) and with different input optical powers Pin. (b) Measured responsivity with different input optical powers Pin. (c) Measured multiplication gain at different bias voltages as the input power Pin varies. The inset in (b) shows the ratio of the responsivity between the proposed PD with DBR and the reference PD without DBR.

Fig. 3. Experimental setup for (a) small-signal radio frequency (RF) measurements and (b) eye diagram measurements. The red and black lines represent the electrical and optical links, respectively. VNA, vector network analyzer; PC, polarization controller; LN MZM, lithium niobate Mach–Zehnder modulator; TDFA, thulium-doped fiber amplifier; VOA, variable optical attenuator; AWG, arbitrary waveform generator; DUT, device under test; RF Amp, RF amplifier.

Fig. 4. (a) Equivalent circuit model for Ge PD. (b) Measured and simulated reflection coefficients for the proposed Ge PD at −4  V.

Fig. 5. (a) Normalized optic-electro frequency response (S21) of the Ge PD at 2 μm under the bias voltages of −4 to −9  V. The drops at around 46 GHz are introduced by the response of the LN MZM. (b) 3-dB bandwidths and responsivity at 2 μm from −2 to −9  V and (c) corresponding GBP.

Fig. 6. Review of the responsivity and 3-dB bandwidth of high-speed photodiodes operating at 2 μm wavelength reported in recent years.

Fig. 7. (a) Measured 56 and 70 Gbps NRZ eye diagrams at 7.5, 8.0, 8.5, and 9.0 V. (b) Measured 90 and 112 Gbps PAM-4 eye diagrams at 7.5, 8.0, 8.5, and 9.0 V.

Table 1. Extracted Electrical Parameters for the Proposed Ge PD at −4 V

Table 2. Literature Overview of the State-of-the-Art High-Speed Photodetectors Operating at 2 μm Wavelength in Different Material Groups^a