# InP-Based Foundry PICs for Optical Interconnects
## 2 InP-Based Generic-Foundry Photonic Integrated Circuits
### Balanced Waveguide Avalanche Photodetector Chip  
Higher-order modulation formats can efficiently increase channel capacity, but they typically require coherent receivers to detect the signal. Heterodyne coherent receivers require integration of an optical local oscillator, which increases the power consumption of the receiver. In contrast, homodyne coherent receivers (as shown on Figure 5a) include a 1-bit delay line, such that the previously received bit signal acts as the local-oscillator (LO) signal, thereby avoiding the need for a local oscillator. However, homodyne receivers suffer from a higher power penalty compared to heterodyne setups. This power-penalty problem can be overcome by using avalanche PDs (APDs) instead of pin-PDs, due to their higher sensitivity. In addition, when using balanced APDs instead of separate APDs, the differential input stage of the trans-impedance amplifier (TIA) can be eliminated to further reduce the power consumption. Thus, a homodyne receiver PIC with balanced APDs would be a step towards deployment of multilevel formats in datacenters.  
Figure 4: Schematic of the measurement setup used to characterize the DPEML (**a**), and eye diagrams of the TE- and TM polarized signal from the DPEML at 20 Gb/s (**b**).  
Figure 5b shows a cross-section of the layer stack of the current photodiodes on the generic-foundry platform, showing that the absorbing region consists of intrinsic InGaAs. In order to introduce APDs into the generic-foundry process, a multiplication and charge layer consisting of InAlAs was added, to create a so-called separate charge absorption multiplication (SCAM) structure with a partially p-doped absorber region (see Figure 5c) [18,19]. This change in the layer stack of the photodiodes introduced almost no additional process steps to the existing generic-foundry process.  
Figure 6a displays the measured dark current, photocurrent and resulting gain characteristics for a single waveguide APD. Up to a gain of M = 14, the reverse bias voltage remains below 16 V. At unity gain voltage of 8.5 V, the responsivity is higher than 0.60 A/W over the entire C-Band. The dark current is lower than 10 nA (Figure 6b). Additionally, the APD shows low polarization dependency where the PDL is below 0.5 dB. The f\({}_{\text{3 dB}}\) bandwidth, as well as the linearity, was measured separately on chip level while applying different bias voltages U corresponding to different gains. The bandwidth (Figure 6c) is up to f\({}_{\text{3 dB}}\) = 24 GHz for low gain at U = 12 V and decreases with increasing bias voltage. For a gain up to M = 5.5 at an operation voltage of U = 15 V, the resulting bandwidth is still f\({}_{\text{3 dB}}\) = 20 GHz for both APDs of a balanced-detector configuration, being sufficient for 28 Gb/s. At a higher gain up to M = 14, a f\({}_{\text{3 dB}}\) bandwidth of 17 GHz is obtained, resulting in a maximum gain-bandwidth-product of GBP = 238 GHz and a responsivity-bandwidth product of 147 A/W GHz. The linearity of the APD was determined by varying the input power and measuring the electrical output power at 20 GHz (Figure 6d). A good linear behavior for input powers in the range of \(-\)10 to 5 dBm was observed.  
Figure 5: Schematic of a homodyne receiver for multi-level format employing balanced avalanche photodetectors (**a**). Epitaxial layer stack of the waveguide photodiodes in the generic-foundry process (**b**), compared to the epitaxial layer stack of the waveguide avalanche photodiodes (**c**).