# Single-Mode Polymer Ridge Waveguide Integration of Organic Thin-Film Laser
## 1 Introduction  
The light from only a few light sources is able to couple effectively into single-mode waveguides. Lasers are light sources with very high coupling efficiencies. For polymeric waveguides, however, not all laser types are suitable. Especially for LoC devices, single-mode operation is mostly required. The coupling of the laser light into the single-mode waveguide is mainly achieved by prism-coupling, grating couplers with a free-space set-up or butt-coupling with a lensed fiber [1]. An important step towards waveguide integrated lasers can be realized by combining the laser resonator, such as distributed feedback resonator (DFB) with the waveguide structure. The DFB wavelength selectivity and high efficiency as well as the ease of fabrication makes them very suitable for this kind of integration. OLAS are well suited for this application due to their low layer thickness, their flexibility and simple processing [2]. In the last decade, the integration of organic DFB lasers into waveguides and LoC systems has been demonstrated. [PERSON] et al. [3] realized a LoC system with an integrated organic laser source and a photodiode. The laser light source is achieved by the laser dye Rhodamine 6G dissolved in ethanol, which is purged through a microfluidic channel to a DFB grating.  
The integration of the laser source and the multi-mode SU-8 waveguides is realized by butt coupling. [PERSON] et al. [4] reported on active and passive SU-8 waveguides either doped or undoped with the laser dye Rhodamine 6G. First, the active waveguide is constructed by a lithography and imprint process. The following passive waveguide is subsequently built up and also butt coupled to the active waveguide. [PERSON] et al. [5] and [PERSON] et al. [6] report on an integration of an OLAS into PMMA. The resonant DFB structure is formed by the aid of hot-embossing and the waveguides are manufactured by deep UV irradiation of the PMMA bulk material. Up to now, all the integrations are only realized with multi-mode waveguides or slab waveguides. However, it is a great approach for intensity based LoC applications where, for example, fluorescence excitation of an analyte in a microfluidic channel is used. Despite that, pure single-mode operations is required for interferometric based sensors such as optical based LoC-systems. Since higher modes propagate with different speed of light and have different evanescent wave characteristics, this leads to an inefficient interferometric interaction in the sensor as well as signal losses due to modal dispersion [1]. Recently [PERSON] et al. [7] integrated DFB gratings onto a 2.0 m in width and 2.5 m in height few-mode ridge waveguide. This is realized by a straightforward combined nanoimprint and photolithography process (CNP process) using OrmoCore, a silicon-containing hybrid waveguide core material. On it, the OLAS is finally evaporated. In our study, we used a three-step fabrication process to integrate the OLAS into the 1.0 m in width and 1.0 m in height polymeric single-mode waveguide as schematically depicted in Figure 1a. Therefore, we used the photopatternable epoxies EpoClad (refractive index = 1.579 @650 nm) and EpoCore (refractive index = 1.593 @650 nm) for the cladding layer including the grating structure and the waveguide core, respectively. The waveguide materials are commercially available and distributed by Micro Resist Technology GmbH in Germany. The active organic material (Alq\({}_{3}\):DCM2) was then evaporated on top of the waveguide structure with the DFB gratings underneath. The final device contains five DFB gratings in parallel and five single-mode waveguides on top of each grating area, forming 25 different waveguide integrated lasers with five different emission wavelengths.