*2.1. Arrayed Waveguide Grating Fabrication*

The AWG is the central sensor component of the experiments carried out in this work. The main goal, therefore, is to realize a reproducible and long-term stable status monitoring with an all-polymer AWG, which also achieves the required measurement accuracy for this purpose. Since initially only one optical measuring point is to be evaluated, in the first development step, an AWG with one input and three output channels is utilized. The latter have their intensity maximum at the wavelengths 850.6, 851.6 and 852.6 nm, respectively.

Before production, an optical simulation is performed with a commercially available software program (Epipprop, Photon Design), taking into account the refractive index data and attenuation losses of the materials used as well as the resolution capability of the production machine applied. In order to minimize possible stray light e ffects, input and output waveguides should have an angular o ffset of 90◦. The simulation result represents a compromise of high transmission e fficiency, high channel sensitivity and low modal dispersion. The finalized design consists of 50 single arrayed waveguides with a grating order of 40 and a radius of 1.5 mm. At the input and output of this arrayed waveguides, the two free propagation zones are arranged, which each are assigned a length of 4.0 mm by the simulation. For the best possible light wave guidance at the relevant wavelength, a layer height of the entire polymeric structure of 3.2 μm is determined. The AWG is calculated with a waveguide width of 5.2 μm causing all waveguides to guide a second mode over the fundamental one. Due to the manufacturing process, a minimum distance of 1.0 μm must be maintained between the individual waveguides, which correlates with the minimum pattern size of the applied sensor structuring direct laser lithography machine (μPG 101, Heidelberg Instruments). This resolution also dictates the non-single-mode waveguides—however, the negative e ffects of this are satisfactorily compensated by the relatively low grating order.

In the next step, a graphic production template for the fabrication is created from the simulation result. As substrate material Cyclo-olefin polymer (COP, Zeonex ® flexible foil, Microfluidic ChipShop, Jena, Germany) with a thickness of 188 μm is used. Its surface is treated with oxygen plasma for 1 min (Plasma Prep, Gala Instruments, Bad Schwalbach, Germany) to e ffect favorable adhesion with the photoconductive polymer, which consists of an inorganic–organic copolymer system (EpoCore/EpoClad, microresist technology). At first, EpoClad as the lower cladding with a refractive index of 1.5708 is spin-coated to a height of 2 μm. This layer is then pre-baked for 5 min at 120 ◦C, subsequently cured by 365 nm UV flood exposure and lastly hard-baked for 60 min at 120 ◦C. After another surface plasma treatment for 1 min, EpoCore as the light-guiding patterned material with a refractive index of 1.5836 is spin-coated to a height of 3.2 μm. According to the graphic template, the sensor element structure is created by laser direct patterning with the lithography machine at a wavelength of 375 nm and a

power of 6 mW. This is followed by a post-bake phase for 5 min at 90 ◦C, before the non-cured areas are removed with the developer (mrDev600, microresist technology). The remaining cured AWG structure is subsequently hard-baked for 60 min at 120 ◦C. In the last step, EpoClad as upper cladding is again applied with a height of 20 μm by spin coating, hereafter pre-baked, UV flood exposed and finally post-baked. In Figure 1, a total-view microscope image of the AWG structure and associated height profile measurements are shown.

**Figure 1.** Laser scanning microscope images of the arrayed waveguide grating (AWG) manufactured with the described parameters. Between the end of the first free propagation zone (a) and the beginning of the second free propagation zone (b), the 50 arrayed waveguides are arranged (c). At the entrance to the first free propagation zone is the input waveguide (d) and at the exit of the second free propagation zone are the three output waveguides (e). The height of the entire structure is homogeneously 3.2 μm.

In order to have low insertion losses, the input waveguide is vertically cut and then polished. The output waveguides are treated in the same way. In the next section, the integration of the polymer AWG into the interrogator is described.
