Shaul Shalem

Mechanical and optical properties of silver-halide infrared transmitting fibers

Abstract 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 AgclxBr1-x 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.

Modal filtering for midinfrared nulling interferometry using single mode silver halide fibers

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.

Single-mode mid-infrared silver halide planar waveguides

A single-mode silver halide symmetric step-index planar waveguide for 10.6 microm is described. We fabricate the waveguide from extruded multimode silver halide fiber by pressing the fiber between metal plates while heating. The device is therefore automatically pigtailed to a fiber, which could ease the difficulties of coupling light into it. We achieved single-mode confinement by suitably adjusting the refractive index of the core and the cladding and by reducing the core thickness to 30 microm. The output radiation from the planar waveguide was measured to confirm single-mode confinement. Silver halide single-mode waveguides for the mid-infrared can be used to enhance sensitivity in fiber-optic evanescent-wave infrared spectroscopy measurements or for the development of mid-infrared single-mode lasers and interferometers.

Silver halide midinfrared optical fiber Y coupler

A description of a novel silver halide midinfrared optical fiber Y coupler including the construction method and the optical characteristics of the coupler is given. The coupler was used to irradiate a sample with a CO2 laser and to measure simultaneously the temperature rise of the sample.

Effects of thermal treatment on the infrared transmission of polycrystalline silver halide fibers

Polycrystalline silver halide fibers were thermally treated by a variety of heating and quenching procedures. For each procedure, the recrystallization process and the grain-size distribution were investigated. The absorption and scattering coefficients at 10.6 mum and the infrared transmittance spectra in the 3-20 mum wavelength range were also measured. Treatment at temperatures above 170 degrees C and long time intervals generally lead to an increase in grain size, with a dependent increase in absorption and scattering coefficients at 10.6 mum. Heating only to temperatures below 170 degrees C reduced the absorptive and scattering losses. The Rayleigh-Gans scattering model was utilized to describe the scattering behavior. A model involving cation vacancies localized at charged dislocations or casual divalent impurities is suggested to explain the infrared absorption of the fibers.

Core-clad silver halide fibers with few modes and a broad transmission in the mid-infrared

There is interest in single-mode fibers that are highly transparent in the mid-infrared. Such fibers would be valuable for spectroscopy, interferometry, and heterodyne detection. We have developed core-clad fibers made of crystalline silver halides with an external diameter of 900 microm and core diameters as small as 60 microm. These fibers exhibit homogeneous core and cladding regions, round cores, and smooth core-cladding interfaces. The 60 microm fibers support roughly 70 modes and have low losses over a broad spectral range in the mid-infrared. This will pave the road for the development of single-mode mid-infrared fibers. ie ty of America

Hollow glass waveguides and silver halide fibers as scanning elements for CO2 laser marking systems

We have designed and constructed a CO2 laser marking system based on a scanned flexible waveguide (silver halide infrared optical fiber or a hollow glass waveguide). CO2 laser power was introduced into the proximal end of the waveguide and the distal tip was placed near the surface of a sample. A permanent magnet was attached to the distal tip, which was displaced in the X and Y directions, using a varying electro magnetic field. An on-line computerized system was used to control the laser power, the position of the distal tip and the scanning speed. The system is capable of cutting, heating or marking on various surfaces. The output profile distribution, spot size and resolution of hollow waveguides and silver halide fibers were investigated and compared.

Optical properties of silver-halide core/clad IR fibers

Optical fibers with low transmission losses are very useful in medical endoscopic laser surgery. In the past we developed unclad silver halide IR fibers for the transmission of carbon dioxide laser energy. We have recently developed core/clad polycrystalline silver halide optical fibers with a loss of roughly 0.3 dB/m at 10.6 micrometers. Such fibers, with a core diameter 0.35 - 0.6 mm and length of 1 to 2 meters are capable of continuously delivering output power densities as high as 14 KW/cm2. We have studied the transmission properties of these fibers for different launching conditions such as the acceptance angle at the input end and the near and far field distributions at the output end. We have also investigated the effects of bending on the optical transmission. We show that by properly designing the core/clad structure we obtain significant improvements with respect to unclad IR fibers.

Silver halide infrared transmitting core/clad fibers with small cores

Polycrystalline silver halide AgClBr fibers are highly transparent in the mid-IR. They are flexible, insoluble in water non-toxic and biocompatible. These fibers are potentially useful for many applications, such as laser surgery, fiberoptic thermometry and infrared spectroscopy. Typical core/clad fibers consist of Br rich core and Cl rich cladding, and they normally have relatively large cores (diameters larger than 350μm) and low transmission losses. There is a wide interest in the development of core/clad fibers with core diameters smaller than 200μm, yet with low transmission losses. Such small core fibers would be useful for all the applications mentioned above. We have developed core/clad fibers with core diameters 140μm and with transmission losses of about 1dB/m at a wavelength of 10.6 microns. The properties of the fibers and some of their applications will be discussed.

Fiber optic thermal imaging system based on hollow glass waveguides or silver halide fibers as scanning elements

A simple fiber optic thermal imaging system based on a thin and flexible IR waveguide is constructed. Two types of waveguides are used: silver halide fiber and hollow glass waveguide. The thermal image of a warm object is formed at the focal plane of an IR transmitting lens. The proximal end of the waveguide is fixed and attached to a pyroelectric IR detector. The distal end of the waveguide scans the thermal image in two directions. The IR radiation is transmitted through the waveguide to the detector. The signals from the detector are coupled into a suitable monitor, which produces a representation of the thermal image. In preliminary experiments, we attach a small magnet to the distal tip of the waveguide and move the tip using an electromagnetic field. For a target of spatial frequency 1.25 cycles/mm the modulation transfer function (MTF) of the system is 0.2 and for a target of spatial frequency of 0.1 cycles/mm the minimum resolvable temperature difference (MRTD) is 0.5°C. The system could be applied in industry or in medicine where imaging in the mid-IR and in a restricted space is required.