We present the experimental characterization results of a 15-to-1 wavelength multiplexer for a Distributed Feedback Quantum Cascade Laser (DFB QCL) array operating in the 7-8.5 μm (mid-long) infrared (IR) range. This design is customized for its use to combine the output from a DFB QCL array with a 0.1 μm wavelength channel spacing for spectroscopy applications, and it is proposed in order to achieve a continuous tuning range overcoming the limited tunability of a single QCLs, required for multi-gas or complex molecule detection. This multiplexer is based on an Echelle diffraction mirror grating scheme, in which multiple output waveguides are deliberately implemented in the design to de-risk for wavelength deviations in the fabrication process. We optimized the location of the input and output guides in order to allow for monolithic integration of the DFB QCL arrays, which would provide for a number of advantages such as a higher stability, less complexity and lower cost over other technologies such as external cavities. We discuss the effects over the device performance of the design, such as the diffraction effects, input channel width overlapping/crosstalk and input channel profile, which are very important to address in order to avoid unaccounted transmission losses. Other parameters such as the profile of the input and output waveguides and fabrication limitations are also discussed as their effect on the device is observed. A series of characterization tests are presented in order to compare the simulation results to the experimental data, which suggests that these multiplexers are a suitable option compared to other IR multiplexer schemes in terms of size and power transmission.