We demonstrated the waveguide photodetector (WG PD) integrated with spot-size converter (SSC) for coherent receiver of 100Gb/s operation. The WG PD integrated with SSC was designed as diluted WG, dual lateral taper structure, and PIN-photodiode. The epitaxial layers of In<sub>x</sub>Ga<sub>1-x</sub>As<sub>y</sub>P<sub>1-y</sub>/InP, In<sub>x</sub>Ga<sub>1-x</sub>As<sub>y</sub>P<sub>1-y</sub>, and InGaAs were adopted as the diluted WG, dual tapers, and absorber of PD, respectively. The shape and thickness of each structure were determined through the simulation of 3D finite-difference beam propagation method. Although the evanescent coupling was highly sensitive, we optimized the structures with simulated responsivity and polarization dependent loss (PDL) as 0.70 A/W and 0.1 dB, respectively. We successfully obtained the SSC-integrated WG PD through numerous fabrication process including photolithography. The electrical and optical properties were characterized with laser launching. Fabricated PDs had almost similar responsivity and PDL with the simulation results. The responsivity and PDL were measured as 0.7 A/W and less than 0.3 dB respectively. The 3 dB-bandwidth was measured as 34 GHz. We successfully realized low PDL and high responsivity by adopting the lateral taper structure for SSC-integrated WG PD.
A hybrid-integrated coherent receiver module has been achieved using flip-chip bonding technology, consisting of a
silica-based 90°-hybrid planar lightwave circuit (PLC) platform, a spot-size converter integrated waveguide photodiode
(SSC-WG-PD), and a dual-channel transimpedance amplifier (TIA). The receiver module shows error-free operation up
to 40Gb/s and OSNR sensitivity of 11.5 dB for BER = 10<sup>-3</sup> at 25 Gb/s.