## 1 Introduction  
Photonic integration endeavors to bring multiple optical functions into a compact device with a small footprint, low power consumption and reliable performance, analogous to electronic integration. However, contrary to the situation in the electronic industry where silicon CMOS dominates, photonic devices nowadays do not tend to converge to one major material system. Light sources are mostly realized using III-V semiconductor-related materials. Modulators have been dominated by lithium niobate (LiNbO\({}_{3}\)), but have been gradually taken over by InP and silicon-on-insulator (SOI) platforms. Detectors are fabricated using III-V semiconductors and Si-Ge materials. Waveguide networks, filters, and other passive optical functions can be realized in low-cost platforms such as polymer, silica and silicon nitride planar lightwave circuits (PLCs). Hybrid photonic integration allows for selecting individual components from the respective best-suited material platform and for assembling them in a common motherboard. These components can be tested prior to the assembly. In this way, high freedom of device performance and yield optimization can be realized, leading to a relatively high cost-efficiency. With the help of the automatic alignment tools and machine-assisted assembly equipment, hybrid integration can offer the benefits of micro-optical packaging at reduced costs, potentially higher overall module performance and higher reliability. Its versatility makes it particularly useful for low to medium production volumes.  
In the last decade, polymer-based photonic devices and the hybrid integration platform have witnessed significant advancement [1; 2; 3]. Multi-core polymer waveguides and multi-step 45 \({}^{\circ}\) mirror arrays have been fabricated as a bridging devices toward spatial-division-multiplexed (SDM) transmission systems [4; 5; 6]. Silicon nitride (SiN\({}_{\text{x}}\)) has been introduced as high-index core material and related thermo-optic devices have been reported [7; 8; 9; 10]. Polymer arrayed waveguide gratings (AWGs) coupled with 40-channel laser diodes (LDs) and photo detectors (PDs) have been implemented as optical line terminals (OLTs) in the wavelength-division-multiplexed passive optical network (WDM-PON) [11; 12]. Clever arrangement of thermally tunable Bragg gratings, vertically inserted thin-filmelement (TFE), on-chip U-groove and 45 \({}^{\circ}\)mirror has led to the assembly of a compact, bi-directional optical network unit (ONU) [13]. Ultrafast electro-optic polymer modulators, integrated with InP-based light sources and driver electronics, have been packaged into transmitter modules, providing serial 100 Gb/s connectivity in data centers and metro-area optical networks [14; 15; 16; 17; 18; 19].  
In this work, a review is performed summarizing the latest development of the polyboard technology. From waveguides to optical assemblies and finally to packaged modules, details of the heterogeneous/hybrid integration technology are revealed, uncovering the integration flexibility, scalability, and cost-effectiveness this platform offers. Moreover, functionalities such as wavelength tunability and polarization control can be realized in a simple and clean manner, making this platform highly attractive in applications where colorless and polarization-multiplexed operations are required.