## 3 Receiver Integration for WDM with 10-Gb/s Throughput  
Here, we describe our photoreceiver integrated with the MM-AWG for 10-Gb/s throughput. The reason for applying WDM for such a \"low speed\" is that dividing the bitrate per channel,  
Figure 4: Optical coupling between MM-AWG and photodiode (PD). The design condition is based on a silica PLC for the 1.5-\(\upmu\)m band with an index difference by 0.75%. The diameter of the PD is 80 \(\upmu\)m. (**a**) Calculated relative responsivity of optical coupling for the highest mode. The parameter \(z\) is the distance between the edges of MM-AWG output, and \(dx\) represents displacement from the center. (**b**) Measurement results of spectrum shape by offset \(dx\) at \(z=150\)\(\upmu\)m.  
or reducing the baud rate, provides three merits. First, it enables us to use a PD with a large receiving diameter, which relaxes the assembly tolerance required to maintain minimum coupling loss between higher-mode beams and PD. Second, a lower baud rate is more suitable for longer reaches in terms of the dispersion penalty and noise figure. In addition, the link margin or distance can be further improved by using an avalanche photodiode (APD), which, theoretically, provides more than 100 times the gain of the conventional PD [32]. Third, cost-effective parts are available for electrical circuits and boards, as well as optical components operating in the 1.5-\(\upmu\)m band, or C-band. The use of CWDM is another option to relax the specifications of those components. We chose eight-wavelength CDWM with 20-nm channel spacing with the baud rate of 1.25 Gb/s [14]. Figure 5a is a schematic of the configuration of the integrated 10-Gb/s CWDM photoreceiver. It is composed of an MM-AWG, a PD array attached to a ceramic carrier, a circuit board including transimpedance amplifiers (TIAs), and a sub-mount. A pigtail fiber is connected to the input edge of the AWG. For hybrid integration with the MM-AWG and the PD array, we used mechanical alignment. The sub-mount, which serves as an alignment platform, is made with machining accuracy of better than 10 \(\upmu\)m. On the sub-mount (inset), several grooves are formed and filled with bonding adhesive in assembly. Owing to the tensile strain induced by the adhesive spreading in the grooves while curing, high mounting accuracy for the MM-AWG and PD array of better than 5 \(\upmu\)m is provided. Therefore, the total alignment error in this mechanical alignment is less than 15 \(\upmu\)m.  
A photograph of the fabricated photoreceiver is shown in Figure 5b. The MM-AWG was fixed at the center of the aluminum sub-mount. A circuit board with TIAs was also mounted on the sub-mount. The eight-channel silica-based MM-AWG had the index difference between the core and cladding of 0.75%, and the cores of the input and output waveguides are \(6\times 6\) and \(6\times 30\)\(\upmu\)m in size, respectively. The eight-channel PD array consists of InGaAs pin-PDs on a ceramic carrier. Each PD has a responsivity  
Figure 5: MM-AWG photoreceiver for 10 Gb/s. For integration, mechanical alignment was employed. (**a**) Schematic configuration of photoreceiver. (**b**) Photograph of the fabricated photoreceiver. (**c**) Responsivity spectra of the photoreceiver. (**d**) Bit-error-rate characteristics with experimental setup. PPG: pulse pattern generator. Tx: 10-Gb/s transmitter. \(8\times 1\): \(8\times 1\) optical coupler. SMF: single-mode fiber. VOA: variable optical attenuator. 10G Rx: 10-Gb/s receiver. CDR: clock and data recovery. ED: error detector. of over 1 A/W. The signal output from the PD array was fed into electrical transmission lines on the ceramic carrier followed by the TIAs via bonding wires. The size of the package is \(100\times 50\) mm. The responsivity spectra of the photoreceiver shown in Figure 5c indicate that we obtained a high, flat-top responsivity for all channels. Within each passband of 13 nm, the responsivity ranges from 0.7 to 0.85 A/W. The crosstalk level is better than \(-22\) dB.