## 4 Receiver Integration for WDM with 40-Gb/s Throughput  
The second photoreceiver we developed operates at higher speeds and is more compact than the previous one. One option to scale up from 10 to 40 Gb/s is just to increase the number of wavelengths; however, a configuration with a few tens of wavelengths for DWDM is not appropriate in terms of compactness and cost-effectiveness. To decrease the number of wavelengths, or increase baud rate per channel, we necessarily consider not only higher-speed operation but also the dispersion penalty and electrical crosstalk. Regarding dispersion, we choose the 1.3-\(\upmu\)m wavelength band, the so-called O-band, which mitigates the penalty compared with that in the C-band. As for electrical crosstalk, we applied a fan-out design in a PLC circuit to enhance electrical isolation as much as possible. Here, we referred to a specification from the Very Short Reach (VSR) interface for 40-Gb/s, allocated in the Optical Internetworking Forum, which is called VSR-5 [33]. The channel number is four, with wavelength spacing of 24.5 nm, which is slightly wider than the 20-nm spacing of CWDM with the baud rate of around 10 Gb/s [15].  
Figure 6a is a schematic view of our integrated photoreceiver. It comprises a MM-AWG PLC, a fan-out PLC, and an electrical circuit including TIAs. The PLC of the AWG and the fan-out PLC are separated chips bonded by adhesive with high optical transparency. Such a dual-PLC structure enables us to optimize the two PLCs separately in terms of yield and size. On the fan-out PLC, edge-illuminating PDs are flip-chip bonded. For assembly with higher accuracy than that with 10-Gb/s photoreceiver, we used visual alignment. The inset illustrates the hybrid integration of the PDs on the fan-out PLC. The alignment is performed visually using markers on the PD and PLC, and the assembly error is less than 1 \(\upmu\)m [4]. Designing the fan-out so that the channels are separated can suppress electrical inter-channel crosstalk. According to our estimation from RF simulation, separation over 1.5 mm provides electrical isolation of more than 20 dB at the frequency of 10 GHz.  
Photographs of the fabricated photoreceiver are shown in Figure 6b. The MM-AWG bonded with the fan-out PLC and circuit board are mounted in a package. The enlarged photograph shows the InGaAs PD, which is hybridly integrated on the fan-out PLC, connected to TIA via electrical wire. The index difference between the core and cladding was 1.5%. The measured 3-dB-down bandwidth of the receiver was larger than 9 GHz, and the adjacent crosstalk was better than 21 dB, as we estimated. The size of the small and compact package is 64 \(\times\) 25 mm.  
Figure 6c shows the responsivity spectrum of the receiver. For all channels, a high responsivity of about 0.4 A/W was observed. While there is some degradation of flatness because of the loss of higher modes in the optical coupling, the responsivity shows that high assembly accuracy was achieved.  
The BER characteristics of the 4 \(\times\) 10-Gb/s receiver are shown in Figure 6d. The experimental setup is shown on the left. Each 10-Gb/s signal was input into a 40-Gb/s transmitter, which we also fabricated. The transmission length of the fiber was 5 km. When all the channels were driven simultaneously, error-free transmission was successfully achieved with a receiver sensitivity of less than \(-\)12 dBm. The sensitivity degradation was less than 1.5 dB compared with a single-channel drive. The results indicate that the inter-channel crosstalk was sufficiently suppressed by using the fan-out PLC.