RESUMO
We compare the filtering capabilities of two infrared fibers developed to achieve a high rejection ratio of the higher order modes in order to obtain compact modal filters devoted to stellar interferometry. Two types of double-clad fibers are studied: a fiber with a second thin absorbing cladding and a fiber with a second thick absorbing cladding closer to the fiber core; both are single mode around the CO(2) band (10.6 µm). We present the single-mode spectral domain and the nulling capabilities of both fibers for different fiber lengths, comparing simulations with experimental results. We show that the filtering capabilities are improved when the absorbing clad is closer to the fiber core, as the propagation distance needed to filter out these modes is shorter. Thus, to obtain high rejection ratios in compact devices, an absorbing cladding close to the core of the fiber is compulsory in order to suppress cladding modes that could eventually recouple into the waveguide. We present an empirical model that allows determining the minimum filter length, considering only one effective leaky mode with low attenuation, which considerably simplifies the theoretical studies.
RESUMO
A flat waveguide for the middle infrared was made by co-extrusion of two silver halide crystals of different chemical compositions. The transmission of the waveguide and its modal behavior was studied using a Fourier Transform Spectrometer and a dedicated optical bench. Analyzing this spectrum, we were able to obtain the cut-off wavelength of the waveguide. We observed a single mode behavior for wavelengths longer than 8.83mum, in good agreement with the theoretically expected values. This novel procedure is ideal for tailoring the properties of the waveguide for specific applications, in particular the spectral range where it exhibits a single-mode behavior. It can thus be applied to achieve modal filtering for mid-IR astronomical interferometers (e.g. beam combiners, nullers, etc.).
RESUMO
This paper reports the characterization of hollow metallic waveguides (HMW) to be used as single-mode wavefront filters for nulling interferometry in the 6-20microm range. The measurements presented here were performed using both single-mode and multimode conductive waveguides at 10.6microm. We found propagation losses of about 16dB/mm, which are mainly due to the theoretical skin effect absorption in addition to the roughness of the waveguide's metallic walls. The input and output coupling efficiency of our samples has been improved by adding tapers to minimize the impedance mismatch. A proper distinction between propagation losses and coupling losses is presented. Despite their elevate propagation losses, HMW show excellent spatial filtering capabilities in a spectral range where photonics technologies are only emerging.
