Spatial mode filtering of mid-infrared (mid-IR) laser beams with hollow core fiber optics

Measurements characterizing spatial mode filtering of mid-infrared (mid-IR) laser beams using hollow core fiber optics are presented. The mode filtering depends strongly on the fiber diameter, with effective mode filtering demonstrated with bore diameters of d = 200 µm and 300 µm. In addition to mode filtering, beam profile measurements also demonstrate the strong dependence of the mode quality on the fiber coupling conditions. As predicted, optimal coupling is achieved using relatively slow optics that produce focused spots that nearly fill the fiber diameter. Examples of the utility of using hollow fibers for mode-filtering to improve molecular spectroscopy experiments are also discussed.