## 5 Dual-Polarization Coherent Receiver
The initial wavelength is 1566 nm. The output power reaches beyond 1 mW at \(\sim\)400 mA gain current. Higher output power can be achieved by using InP GCs with prolonged gain region. The ripples in the P-I curve is caused by the mode-hopping and can be compensated by adjusting the phase-electrode included in the laser cavity. The LO can be continuously tuned over 22 nm, as demonstrated in Figure 15b. The side mode suppression ratio is larger than 40 dB. The power imbalances among the eight outputs are evaluated at three selected wavelengths, as summarized in Figure 15c. The power fluctuations among the wavelengths agree well with the 2 \(\times\)4 MMI measurements from the signal path and can be minimized by adjusting the MMI structures. Figure 14: **(a)** Schematic layout of a coherent receiver consisting of a polymer-based 90 \({}^{\circ}\) hybrid with integrated LO, 8 \(\times\)InP PD array, and 4 \(\times\)InP DTIA array. **(b)** Photograph of the assembled polymer-based 90 \({}^{\circ}\)hybrid. On the signal path, the eight outputs are evaluated by injecting a broadband light source with controlled polarization states. The results are displayed in Figure 15d,e. Both the upper and lower Rx appear to have similar performances, with an average insertion loss of \(\sim\)13.3 dB at 1530 nm and 15.5 dB at 1565 nm, including 6 dB MMI intrinsic loss, 1.5 dB fiber coupling loss (both facets), 3 dB waveguide loss (propagation + bending), 1.5 dB polarization controller loss (PS + \(\lambda\)/2 plate), 0.5 dB VOA loss, and 0.8-3 dB MMI excess loss across the C-band (1535 nm-1575 nm). The power imbalance among all eight outputs is below 1 dB and the polarization extinction ratio is larger than 25 dB across the C-band. The characterization results of the on-chip VOAs are shown in Figure 15f. At 12 mW electric heater power, the VOA reaches an attenuation level of 32 dB. In the first attempt to integrate both tunable LO and VOAs on an optical 90 \({}^{\circ}\)hybrid with dual-polarization functionality, the thermo-optic components, MMIs, and polarization controllers are all functioning properly without any noticeable optical and thermal cross-talk. To improve the performance, the tuning range of the LO can be expanded, e.g. by using a dual-grating structure and a Y-branch-based thermo-optic switch [29,33,36], to cover the full C-band. The MMI structure can be adjusted to shift the minimal loss band toward the center of the LO tuning range. To summarize the hybrid integration technology presented in Section 3-5, the fibers and the TFEs can be inserted passively and secured by UV-epoxy in their defined grooves and slots. The attachment of the planar PDs is assisted by a fine-placer with a positioning accuracy of \(\pm 1\) \(\upmu\)m. The process can be carried out passively with the help of the alignment marks on the chip. In the case of critical coupling of high-speed PDs with small active areas, the process can be switched to active alignment with a specially designed holder that provides electrical connections. In both cases, the exact PD location is calculated  
Figure 15: Characterization results of the assembled 90 \({}^{\circ}\)hybrid: (**a**) P-I and V-I curve of the LO; (**b**) overlaid spectra of the LO during tuning; (**c**) LO power comparison at the 8 outputs at three different wavelengths; (**d**) the lower Rx O1–O4 for the TE component of the signal path; (**e**) the upper Rx O5–O8 for the TM component of the signal path; (**f**) the on-chip VOA. from the actual mirror width and the vertical location of the buried waveguide. Fully automated and scalable processes are being tested for the passive fiber and PD attachment. The gain chip, LD and LD array alignment is active at the moment. The optimal position is fixed once the maximal optical power is obtained at the monitored output. An automated assisting tool has been implemented for fine-alignment using six-axis stages and nano-force sensors. The assembly process is reproducible and the hybrid technology is well proven in the demonstrated devices.