There is an interest in single-mode fibers that are highly transparent in the middle infrared. Such fibers would be valuable for spectroscopy, interferometry, fiber lasers, and heterodyne detection. We developed core-clad fibers made of crystalline silver halides, with external diameter 900 μm, small core diameters (50–60 μm) and an extremely small difference (∼0.004) between the indices of refraction of the core and the clad. These fibers behaved as single-mode fibers at the wavelength 10.6 μm.

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We demonstrate the modal filtering properties of newly developed single mode silver halide fibers for use at midinfrared wavelengths, centered at 10.5 microm. The goal was to achieve a suppression of nonfundamental modes greater than a factor of 300 to enable the detection and characterization of Earthlike exoplanets with a space-based nulling interferometer. Fiber segments of 4.5 cm, 10.5 cm, 15 cm, and 20 cm lengths were tested. We find that the performance of the fiber was limited not by the modal filtering properties of the core but by the unsuppressed cladding modes present at the output of the fiber. In 10.5 cm and longer sections, this effect can be alleviated by properly aperturing the output. Exclusive of coupling losses, the fiber segments of 10.5-20 cm length can provide power suppression of undesirable components of the input field by a factor of 15,000 at least. The demonstrated performance thus far surpasses our requirements, such that even very short sections of fiber provide adequate modal filtering for exoplanet characterization.

In this letter, we report the design and development of improved step-index silver halide (AgClBr) single mode fibers. These fibers exhibited round and symmetrical field mode distribution while efficiently suppressing the higher order modes. The losses of the fundamental mode were 10–20dB∕m at λ=10.6μm, and the minimum fiber length needed for single mode operation was ∼50cm. These fibers will be useful as spatial filters for middle infrared interferometry.

This article presents a review of the optical and mechanical properties of infrared transmitting fibers extruded from single crystals of silver-halides at the Applied Physics Group in Tel-Aviv University during the last decade. The optical properties of AgCl, Br, ~ crystals and fibers include the spectral transmission window, laser power transmission, the change of the power distribution traveling along the fiber, and the laser-induced breakdown. The mechanical properties include the investigation of the ultimate tensile strength (UTS), hardness, and the elastic strain limits of these fibers and their composition dependence. The mechanical properties that involve single and multiple bending of fibers in the plastic and the elastic strain limits are also described.Optical fibers for the transmission of visible and near-infrared radiation 0.7 p m to 3 p m have become standard components in many optical systems. Their optical quality has reached the theoretical limit of transparency of 0.15 dB/km at 1.55 pm. These fibers are composed of silica-based glasses or other materials (e.g., organic plastics) that are opaque in the middle infrared at wavelengths longer than about 3 pm.The demand for further reduction of the fibers' transmission losses have led to a search for new materials. In 1978 it was suggested that an ultralow loss of less than lo-* dB/km may be achieved with fibers made of infrared transmitting materials [l, 21. This possibility motivated a research effort on the development and The authors thank A. Yekuel and A. S. Shalem et al.the properties of infrared transmitting optical fibers that are not based on silica glasses. Furthermore, there has been an increasing demand for laser power transmission through flexible optical fibers in the fields of laser surgery and laser machining. CO, laser power transmission is particularly useful in these fields; therefore, studies on low-loss infrared fibers at the 10.6-pm wavelength range have been extensively performed.Applications of infrared transmitting fibers can be classified into two categories: long-distance communications and short-haul light transmissions. The former requires infrared fibers that have ultra-low losses and low dispersion. The latter requires a wide-band transparency and/or high-power transmission.Optical materials studied to date for long fibers are glasses made of heavy-metal oxides or chalcogenides. The most promising candidates for the ultra-low loss fibers in the long-distance optical communications are the ZrF,-based fluoride glasses. The loss value predicted is less than 0.01 dB/km at the 2-pm to 4-pm wavelength range. The lowest loss value measured until now is 0.45 dB/km at 2.35 pm for ZBLAN(P) fibers [3], which are fibers made of AlF3-NaF-LaF,-BaF,-ZrF, doped with Pr3+ as the core composition and AlF3-NaF-LaF,-BaF,-ZrF, as the cladding composition.On the other hand, promising candidates for short-haul transmission are polycrystalline and single-crystal halide materials such as TlBr-TI1 mixed crystals (called KRS-5), KCl, CsBr, and A...