## 5 Receiver Integration for WDM with Throughput by 100-Gb/s and beyond  
The third WDM photoreceiver has the baud rate of 25 Gb/s. Here, we mainly describe the receiver with throughput of 100 Gb/s by four-channel WDM. Moreover, our basic design can leverage beyond 100 Gb/s, such as 200 and 400 Gb/s. Using a low-loss MM-AWG with a higher index difference as well as highly accurate assembly with lensed optical coupling, we were able to develop a compact photoreceiver [16]. We referred to IEEE's standard for transceivers for Ethernet 100 GbE [25] and multi-source agreements (MAs), which stipulate that the size of the transceiver should have a size, or form factor, that can be accommodated in the restricted rack space in datacenters. For the transmission reach of 10 and 40 km, a 4-channel WDM should be used with a 25-Gb/s NRZ signal for each channel. The wavelength allocation is called LAN-WDM, which is in the 1.3-\(\mu\)m band, and the grid spacing is 800 GHz with the passband of 360 GHz. This is one of the best allocations for a lower dispersion penalty than in CDWM as well as for more cost-effective components than in DWDM. Figure 7a illustrates the core assembly of 100-Gb/s LAN-WDM photoreceiver, which is composed of an MM-AWG, a collimating lens, a focusing lens array, and a PD array, all bonded as a single assembly block. The four-channel InGaAs PD array is mounted on a ceramic carrier by flip-chip bonding for back illumination. Between the MM-AWG and PD array, the focusing lens array based on a graded-index (GRIN) micro lens is fixed to the edge of the output waveguides. On the input side,  
Figure 6: MM-AWG photoreceiver for 40 Gb/s. Visual alignment was applied. (**a**) Schematic configuration of photoreceiver. (**b**) Photograph of the fabricated photoreceiver. (**c**) Responsivity spectra of the photoreceiver. (**d**) Bit-error rate (BER) characteristics along with the experimental setup. 40 G Tx: 40-Gb/s transmitter. 40 G Rx: 40-Gb/s receiver. the beam-collimating lens is directly attached to the MM-AWG. For all the optical connection in the core assembly, we used an active alignment technique. It provides very high accuracy alignment by way of inputting an optical signal into each component. The assembly configuration between the PLC and PD array is the same as that in our integrated coherent receiver [34], while the main difference is that the multimode beam from the MM-AWG is coupled with the PDs by a dedicated optical design. For both optical coupling to the multimode beam and high-speed operation, we use a racetrack shape in the PD active region. A photograph of the fabricated photoreceiver is shown in Figure 7b along with a photograph of the footprint of the Ethernet transceiver. Viewed from the outside, the receiver consists of a metal hermetically sealed package, a receptacle LC connector with another focusing lens, and flexible printed circuits (FPCs). The core assembly including the AWG and PDs is mounted in the package together with a four-channel TIA on a circuit board. The MM-AWG was fabricated with index difference of 2% with the core width of 5 \(\upmu\)m for single-mode input and 15 \(\upmu\)m for multimode output. The receiver can be applied for a reach of 10 km by using pin-PD. Furthermore, the reach can be extended by replacing the pin-PD with an APD [35]. The size of the package is \(7\times 18\times 6\) mm\({}^{3}\), excluding the receptacle and FPCs, which is small enough to install it in compact transceivers such as QSFP-DD and OSFP [36; 37]. Figure 7c shows the responsivity spectrum of our fabricated photoreceiver. A flat-top spectral shape of over 500 GHz was observed, and the maximum responsivity reached higher than 0.7 A/W for all channels. The total optical loss is about 1.5 dB, which includes 0.8-dB AWG loss, 0.3-dB coupling loss from PLC to PD, and 0.3-dB loss from PLC to receptacle. The crosstalk between adjacent channels is less than 25 dB in the 360-GHz passband.