## 5 Dual-Polarization Coherent Receiver  
The exponential growth of the internet traffic is pushing the optical fiber technology from the current 112 Gb/s links based on the DP-QPSK coherent transmission scheme to 400 Gb/s and further beyond 1 Tb/s using dual-polarization 16-QAM up to 128-QAM [30]. On the receiver side, an optical 90 \({}^{\circ}\)hybrid is required as the frontend to convert the phase modulated signal into amplitude variations detectable by the balanced PDs [31,32]. The in-phase and quadrature components of the optical signals are then extracted as photo current variations. Further on, with the help of trans-impedance amplifiers (TIAs) and analog-to-digital converters (ADCs), the electronic signals are amplified and digitalized. Complex digital signal processing (DSP) algorithms are applied to compensate, synchronize, recover and separate the densely encoded signals. The phase-mixing function of the optical 90 \({}^{\circ}\)hybrid requires a laser source as local oscillator (LO). A colorless optical 90 \({}^{\circ}\)hybrid must be able to handle a broad wavelength range, e.g., the C-band, and accordingly the LO should be tunable over that range. To bring down the device footprint, power consumption, packaging efforts and costs, an integrated, low-cost and wavelength tunable LO is highly desired. The schematic layout of the hybrid coherent receiver is displayed in Figure 14a. The device relies heavily on the hybrid integration technology: the polyboard provides low-loss waveguide components including bends, tapers and multi-mode interference devices (MMIs), as well as thermo-optic components including tunable Bragg gratings and variable optical attenuators (VOAs) [33,34]; the thin-film elements, based on the mixed layers of polymers and dielectrics, are used to split and rotate the polarization states; InP photonics platform provides the light source (GC) and the high-speed (100G) waveguide PD array: InP electronics platform offers high-speed differential trans-impedance amplifier (DTIA) and DMX circuits. The optical and electronic layouts from these components need to be arranged collectively to ensure a conflict-free assembly process. For this reason, the polymer output waveguides are extended to reach the same pitch (1 mm) as the waveguide-based PD array. The space is also reserved  
Figure 13: Responsivity measurement of the signal PD at different wavelengths. for a matched wire-bonding process to connect the 4 \(\times\) InP-based DTIA array for the electronic signal processing. On the polyboard, the Bragg grating is coupled to the InP GC to form the tunable LO. Three VOAs are added, both in the signal path and the LO path, to provide extra flexibility in handling signals with potentially large time- and polarization-dependent amplitude variations. The contact pads for the thermal electrodes are arranged on the top- and bottom-left side of the chip so that the electrical connections (DC) can be made in an orderly manner to the pins on the housing. Two 2 \(\times\)4 MMIs are used for the phase mixing and decoding. The thin-film-based polarization splitter (PS) separates the TE (transmitting) and TM (reflecting) components, and the half-wave (\(\lambda\)/2) plate rotates the TM light back to TE polarization. In this way, the MMIs and the PDs only need to be optimized for the TE polarization. It is worth noting that the PS usually causes higher loss for the transmitted TE path (\(\sim\)1.3 dB) than the reflected TM path (\(\sim\)0.5 dB). With waveguide tapers, the \(\lambda\)/2 plate introduces a loss of \(\sim\)1 dB on the TM path, which balances out the loss imbalance and helps to generate more uniformly distributed output powers among the two mixer MMIs. The assembled colorless dual-polarization 90 \({}^{\circ}\) hybrid as the coherent receiver frontend has been demonstrated [35]. A photo of the assembly is shown in Figure 14b. The characterization results are summarized in Figure 15. The P-I and V-I characteristics of the LO are shown in Figure 15a, measured in the fiber at O1.