Silver halide crystals, such as AgCl_xBr_1-x (0<x<1) are highly transparent in the middle infrared (mid-IR) in the spectral range 3-30µm and their losses at 10.6µm are ≈ 0.05 dB/m. Some of the best fibers for the Mid-IR have been made by extruding single crystals of diameters 8-10mm through dies to form fibers of diameters 0.7-0.9mm. These fibers are flexible, non toxic and insoluble in water and they have many potential applications in medicine, science and industry. The theoretical attenuation of these fibers should be extremely low, but it was found experimentally that the attenuation was orders of magnitude higher (e.g.  0.2 dB/meter at 10.6µm) than anticipated, most probably due to excess absorption and scattering. The high attenuation in AgClBr fibers has limited their usefulness. In this work we extruded from rods of diameters 8mm rods of smaller diameters (6-2mm) and fibers of diameter 0.9mm.  We investigated the optical and physical properties of samples of different diameters, in an attempt to gain a better understanding of fiber attenuation mechanisms in the mid-IR.

We measured the absorption and scattering at 10.6µm in single crystals, rods and fibers of different diameters made of AgClBr. It was shown that the absorption and scattering increased for samples with smaller diameter. We suggested that pores are the main reason for scattering in the samples and we used Mie theory to estimate that there are 10⁹ – 10¹⁰ cm^-3 pores of average diameter 0.5 µm. The absorption increase was consistent with previous work and with a theoretical model ascribing absorption to cation vacancies localized at charged dislocations.  It is hoped that this may help improve the extrusion conditions, thereby improving the properties of AgClBr fibers in the future.