# InP-Based Foundry PICs for Optical Interconnects
## 2 InP-Based Generic-Foundry Photonic Integrated Circuits
### InP-Based Integration Technology for Generic-Foundry Service  
A schematic of the technological implementation of the generic-foundry platform of the Fraunhofer HHI is shown in Figure 1. This schematic serves to highlight the epitaxial layer stack of the various BBs, which are the following:  
* Three different passive waveguides with different index contrasts (referred to as E200, E600, and E1700, as shown in Figure 1), to allow PIC designers to design their own passive-waveguide structures, such as bends, multi-mode interferometers (MMIs), arrayed-waveguide gratings (AWGs), etc. (propagation loss = 1.5-2.0 dB/cm)
* Photodetectors and balanced detectors (f\({}_{\text{3 dB}}\)-45 GHz; internal responsivity \(>\) 0.8 A/W)
* Semiconductor optical amplifiers SOAs (gain-4 dB/100 \(\upmu\)m)
* Distributed-feedback- (DFB) and distributed Bragg-reflector (DBR) lasers (output power \(>\) 3 mW)
* Bragg gratings, including uniform-, superstructure-, and sampled gratings (\(\kappa\)-80 cm\({}^{-1}\))
* Electro-absorption modulators (EAMs; \({}_{\text{3 dB}}\)-45 GHz)
* Spot-size converters (SSCs; mode size = 10 \(\times\) 7 \(\upmu\)m\({}^{2}\); \(<\)2 dB coupling to an SMF)
* Polarization splitters and rotators are also available (not shown on the schematic below)  
Our integration platform is based on Butt-Joint integration of the active components with the Q(1.06) Fe-doped passive waveguides [16]. The (InGa)(AsP) material throughout the whole structure  
Figure 1: Schematic showing the epitaxial layers of the generic foundry process along the propagation direction, and the insets show cross-sections of the various waveguides. The purpose of the diluted waveguide structure is to implement I/O spot size converters with a much larger mode size (10 \(\upmu\)m \(\times\) 7 \(\upmu\)m) than that of the other waveguides in the process (PD: photodetector; SOA: semiconductor optical amplifier; EAM: electro-absorption modulator; DFB: distributed feedback laser; SSC: spot-size converter). is grown by metalorganic vapor phase epitaxy (MOVPE) on a 3-inch semi-insulating (Fe doped) InP substrate. The active components (i.e., the SOAs, the DFB lasers, and the electro-absorption modulators EAMs), are based on the same 8 multi-quantum well (MQW) layer with a photoluminescence peak centered around 1540 nm wavelength. Below the MQW, there is a 250-nm Q(1.3) quaternary bulk layer, which acts as the core guiding layer in the regions where the MQWs are removed, i.e., an offset-QW configuration. At places where DFB laser elements are to be positioned, first-order complex-coupled DFB gratings are patterned using electron-beam lithography and reactive ion etching. The designer is free to choose any DFB wavelength within the C-band. In the same way Bragg gratings (e.g., for tunable DBR lasers and also grating couplers based on 2 nd-order gratings for vertical optical in/out-coupler elements) are realized on the 250-nm Q(1.3) quaternary bulk waveguide layer after selectively removing the active MQW material. The Bragg gratings are etched 30 nm into the 250 nm Q(1.3) layer resulting in a coupling coefficient around 80 cm\({}^{-1}\). The photodetector devices are based on evanescent coupling of light from the Q(1.06) passive waveguide to the InGaAs absorbing layer on top. Thanks to the semi-insulating nature of the Q(1.06) waveguide layer and the substrate underneath, true electrical isolation of the integrated active transmit-type (Tx) elements is enabled. On the receiver (Rx) side, this feature also allows for the realization of efficient balanced detectors. There are three thin Q(1.06) layers running deep below the surface and under all the structures, which are used on the spot-size converters (SSCs) to form a so-called diluted waveguide at the facets, as shown on the right most inset on Figure 1. Figure 2a shows a schematic of the SSC illustrating the vertical taper that is used to couple light from the passive Q(1.06) waveguide to the diluted waveguide.