# InP-Based Foundry PICs for Optical Interconnects
## 1 Introduction  
Optical Interconnects and photonic integration are currently gaining a lot of attention in the optical communications community. The telecommunications market is gearing up towards a huge increase in human connectivity, but also in human-to-machine- and machine-to-machine connectivity (5G, cloud-computing services, IoT, Industry 4.0, car-to-car communications, video connectivity, etc.) [1]. Datacenters are expected to play an increasingly bigger role, and the associated traffic is expected to increase dramatically for both the traditional intra-datacenter fiber-optic links from server to server (the so-called east-west traffic), but also for the inter-datacenter fiber-optic links from one datacenter to another (the north-south traffic) [2; 3; 4]. The main factors driving deployment of ultra-high-capacity fiber-optic links (such as 400G) in datacenters are cost, size, and power consumption. Fiber-optic links in datacenters today rely on compact- and low-cost direct-detection systems, which typically consist of low-power light sources such as vertical-cavity surface-emitting lasers (VCSELs), surface-illuminated photodiodes, and multi-mode fibers that allow for large fiber-chip alignment tolerances [5]. In order to increase link capacity to meet the expected traffic demand, more complex methods of transmission through single-mode fibers (SMFs) will have to be introduced, such as wavelength multiplexing,polarization multiplexing, multi-core fibers, and coherent methods employing higher-order modulation formats (such as quadrature-phase-shift-keying QPSK, quadrature-amplitude modulation QAM, and pulse-amplitude-modulation PAM formats). However, such complex transmission formats typically result in larger components, higher device costs, and increased power consumption. In order to balance this problem out, researchers are looking into hybrid integration of PICs from different technologies (such as InP-, glass-, polymer-, silicon-nitride- SINx-, and silicon-on-insulator SOI technology) into compact modules, in order to increase link- or channel capacity, while simultaneously tailoring optical specifications, cost, module size, or power consumption. Complex transmission techniques have long been implemented in backbone fiber-optic links that operate at longer wavelengths (O-, C-, and L band operating at 1.3 \(\upmu\)m, 1.55 \(\upmu\)m, and 1.7 \(\upmu\)m, respectively) than the current datacenter systems (around the 1\(\upmu\)m wavelength region; Gallium-Arsenide GaAs technology), which is why datacenters are considering a transition towards these longer wavelengths. At the same time, these wavelengths allow for mono-mode signal transmission, particularly required for the longer north-south traffic links. The Indium-Phosphide (InP) technology becomes indispensable at these longer wavelengths, being the only material technology that can generate light in these bands. In addition, the InP technology is also suitable for realizing all the other required optical functionalities, such as passive waveguiding, detection, amplification, and phase- and amplitude modulation. The Fraunhofer Heinrich-Hertz Institute (HHI) in cooperation with several other partners has been developing a fabrication process for realizing complex InP photonic-integrated circuits (PICs) providing full integration of transmitter- (Tx) and receiver- (Rx) capability, and a variety of passive-optical functions, including polarization control [6]. Access to InP PICs from this fabrication process is provided in the form of generic-foundry runs (so-called multi-project wafer MPW runs) at scheduled intervals. This generic-foundry process has already been used to realize numerous complex InP PICs for various applications [7; 8; 9; 10; 11; 12; 13; 14; 15]. In order to accommodate datacenter applications that would benefit from hybrid integration, foundry-service providers will have to expand their component portfolio to address, in particular, the efficient coupling of light in- and out of the PICs in the most effective way.