# Single-Mode Polymer Ridge Waveguide Integration of Organic Thin-Film Laser
## 5 Results and Discussion  
The samples were measured and characterized by the optical set-up schematically shown in Figure 7.  
The waveguide integrated OLAS was excited with the third harmonic (\(\lambda_{3}=355\) nm) of a passively Q-switched Nd:YAG laser. The excitation energy was controlled by a neutral density filter. For an efficient excitation, the Gaussian beam out of the Nd:YAG laser was formed into a laser stripe by the cylindrical lenses L1, L2, and L3. Additional apertures form a flat-top profile out of the Gaussian beam. Thus, an excitation stripe of 0.15 mm in width and 3.0 mm in length was created. The dimensions of the laser stripe were measured with a Thorlabs BP209-VIS/M scanning-slit optical beam profiler. The energy out of the pump laser was measured with the Coherent LabMax-Top system. The waveguide integrated laser probes were stored and measured in a N\({}_{2}\)-flushed chamber. Instead of this method, to avoid unnecessary degradation and oxidation under atmosphere conditions, a thin-film encapsulation can be applied on top of the waveguide integrated lasers [20]. The excited laser emission  
Figure 6: (**a**) SEM overview image of the whole waveguide and grating structure and a detailed view on the waveguides on top of the DFB grating; and (**b**) the waveguide end facet. out of the end facet of the waveguide was measured by using the lens pair L4 and L5. An additional aperture at the N\({}_{2}\)-flushed chamber in front of the Lens L4 was included to suppress the scattered light. The waveguide integrated lasers were analyzed by a precise monochromator (Triax 320, HORIBA Scientific) and a liquid nitrogen cooled CCD detection system (Symphony, HORIBA Scientific). The measurement of the OLAS out of the waveguide facet is depicted in Figure 8a, where lasing occurred with all grating periods. For the sake of simplicity, we took a closer look at just one laser line. Figure 8b shows the laser line at \(\lambda_{\text{Bragg}}=640.24\,\text{nm}\). Using the Bragg condition in Equation (1) for the first order laser emission (\(m=1\)) at \(\Lambda=205\,\text{nm}\), an effective refractive index \(n_{\text{eff}}\) of about 1.562 can be determined. This result is comparable with the result of the mode propagation simulation. The effective refractive index \(n_{\text{eff}}\) of the fundamental mode for this waveguide was calculated to be approximately \(1.561\,@640\,\text{nm}\). The spectral width of the emission peak at the full width at half maximum (FWHM) was measured to be 200 pm. Furthermore, by increasing the excitation energy density up to \(274\,\text{J}/\text{cm}^{2}\) (cf. Figure 8c), no further modes except the fundamental mode were found. In previous measurements on few-mode waveguides (waveguide facet dimension \(2\times 2\) m\({}^{2}\)), up to five modes could be found with increased pump energy. Deviating from the DFB theory [21; 22], where two laser modes near the Bragg wavelength are generated due to the symmetry of the gratings in index-coupled DFB-lasers [23], we could only measure one laser line out of our waveguide integrated lasers. We suppose that this effect can be addressed to the replication process of the Bragg grating structure into the polymeric cladding layer. Possible slight thickness variations of the polymer layers due to the spin-coating technique as well as potential variations in the doping concentration of the active laser material can have an effect on the effective refractive index change in the structure leading to an asymmetrical behavior and therefore to single-mode operation of the waveguide integrated laser. Figure 7: Optical set-up for laser performance measurements of the integrated OLAS. The laser emission was measured orthogonal to the pump beam. Figure 9a shows the emitted intensity of the waveguide integrated lasers in dependence of the excitation energy density. An increased laser emission could be found by exceeding the lasers above their laser thresholds.