Spatial mode filtering of mid-infrared (mid-IR) laser beams with hollow core fiber optics Jason M. Kriesel ∗a , Gina M. Hagglund a , Nahum Gat a , Vincenzo Spagnolo b , and Pietro Patimisco b a Opto-Knowledge Systems, Inc. (OKSI), 19805 Hamilton Ave., Torrance CA, USA 90502-1341; b Dipartimento Interateneo di Fisica, University and Politecnico of Bari, Italy. ABSTRACT 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. Keywords: mid-infrared, single-mode, fiber optic, waveguide, quantum cascade laser (QCL), interband cascade laser (ICL), CO 2 laser, molecular spectroscopy 1 INTRODUCTION Mid-IR lasers operating in the wavelength range from Ȝ = 3 µm to 14 µm, such as quantum cascade, interband cascade, and CO 2 lasers, are extremely useful for a range of applications including (but not limited to) molecular spectroscopy, remote sensing, IR counter measures, medicine, and industrial cutting. For many applications it is desirable to have a single spatial mode (i.e., Gaussian beam profile), which can enable such benefits as higher signal-to-noise, lower beam divergence, and diffraction limited performance. In prior studies, hollow core fibers (i.e., hollow waveguides) with bore sizes as large as 30 times the wavelength have been shown to provide a convenient, relatively low-loss means of delivering mid-IR beams with a single spatial mode [1]. The single mode behavior is essentially accomplished by the preferential damping of higher order modes, which is a property of hollow fibers that can also be utilized to effectively filter multi-mode laser beams, leading to improved performance of systems that utilize mid-IR lasers. 2 HOLLOW CORE FIBER OPTIC WAVEGUIDES Figure 1 shows the basic structure of hollow fiber optics used in the studies presented here. Fabrication of the fibers is accomplished using a wet chemistry process developed by Harrington et. al. at Rutgers University, which consists of depositing a reflective silver (Ag) layer followed by a dielectric silver iodide (AgI) layer inside a hollow glass capillary tube [2]. Note: the glass tubing simply provides a smooth surface on which the coatings are applied and does not contribute to the light guiding properties of the fiber; in addition, a protective buffer on the outside of the capillary tube helps to shield the glass from damage, but also does not contribute to the optical properties. Figure 1. Cross section view of hollow fiber optic with internal diameter, d (layer thickness is not to scale). ∗ jason@oksi.com ; phone: 310-756-0520; www.oksi.com AgI Ag Glass capillary Protective buffer d Hollow core Quantum Sensing and Nanophotonic Devices XI, edited by Manijeh Razeghi, Eric Tournié, Gail J. Brown, Proc. of SPIE Vol. 8993, 89930V · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2040018 Proc. of SPIE Vol. 8993 89930V-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/12/2014 Terms of Use: http://spiedl.org/terms