Fig. 1. (a) Schematic of a waveguide SiGe APD integrated with a DBR. (b) SEM cross-section view of an APD. (c) Top views of FDTD simulated photo-carrier generation profiles for 4  μm wideby10  μm long APDs with no DBR, DBR1, and DBR2. (d) Calculated reflectivity versus number of period for two DBR designs. The inset is the reflectivity spectra for the two types of DBRs with 2, 4, and 6 periods, respectively.

Fig. 2. (a) Dark and photo current versus bias voltage with input optical power of −10, −5, and 0 dBm for a 4  μm×10  μm APD with a 6-period DBR2. (b) Photocurrent versus input optical power for 4  μm×10  μm APDs with no DBR, DBR1, and DBR2, respectively. (c) Responsivity at unity gain and quantum efficiency for three types of APDs, each with the same waveguide width of 4 μm but various waveguide lengths of 10 μm, 25 μm, and 50 μm. (d) Measured impulse responses of a 4  μm×10  μm APD with DBR2 at various multiplication gains. The shortest pulse has an FWHM of 14.5 ps. (e) and (f) Device bandwidth versus multiplication gain for APDs with DBR1 and DBR2, respectively. A 25 GHz bandwidth was achieved for both designs.

Fig. 3. Experimental setup for eye diagram and BER measurement: MZ, Mach–Zehnder; EDFA, erbium-doped fiber amplifier; BPF, bandpass filter; DUT, device under test; Scope, sampling oscilloscope; BERT, bit error rate tester.

Fig. 4. Measured 64 Gb/s PAM4 eye diagrams for 4  μm×10  μm waveguide APDs with and without DBRs at reverse bias of (a) 8 V and (b) 10 V, respectively. The DCA sampling scope was set with 10 ps/div in the x axis and 10 mV/div in the y axis.

Fig. 5. 32 Gb/s NRZ and 64 Gb/s PAM4 BERs versus received average optical power with a 5 dB ER for 4  μm×10  μm waveguide APDs with and without DBRs at a reverse bias of (a) 8 V and (b) 10 V, respectively. These BER curves were measured without a transimpedance amplifier (TIA).

Table 1. Design Parameters for DBR1 and DBR2