  In the last few years, integrated optics (IO) beam combiners have facilitated
the emergence of 4-telescope interferometers such as PIONIER or GRAVITY,
boosting the imaging capabilities of the VLTI. However, the spectral range
beyond 2.2microns is not ideally covered by the conventional silica based IO.
Here, we propose to consider new laser-written IO prototypes made of GLS
glasses, a material that permits access to the mid-infrared spectral regime.
Our goal is to conduct a full characterization of our mid-IR IO 2-telescope
coupler in order to measure the performance levels directly relevant for
long-baseline interferometry. We focus in particular on the exploitation of the
L and M astronomical bands. We use a dedicated Michelson-interferometer setup
to perform Fourier Transform spectroscopy on the coupler and measure its
broadband interferometric performance. We also analyze the polarization
properties of the coupler, the differential dispersion and phase degradation as
well as the modal behavior and the total throughput. We measure broadband
interferometric contrasts of 94.9% and 92.1% for unpolarized light in the L and
M bands. Spectrally integrated splitting ratios are close to 50% but show
chromatic dependence over the considered bandwidths. Additionally, the phase
variation due to the combiner is measured and does not exceed 0.04rad and
0.07rad across the band L and M band, respectively. The total throughput of the
coupler including Fresnel and injection losses from free-space is 25.4%. The
laser-written IO GLS prototype combiners prove to be a reliable technological
solution with promising performance for mid-infrared long-baseline
interferometry. In the next steps, we will consider more advanced optical
functions as well as a fiber-fed input and revise the optical design parameters
in order the further enhance the total throughput and achromatic behavior.
