Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can
help achieve deeper interferometric nulls. Silver halide fibers of composition AgClxBr1-x(0
Spatial filtering is necessary to achieve deep nulls in optical interferometer and single mode infrared fibers can serve as
spatial filters. The filtering function is based on the ability of these devices to perform the mode-cleaning function: only
the component of the input field that is coupled to the single bound (fundamental) mode of the device propagates to the
output without substantial loss. In practical fiber devices, there are leakage channels that cause light not coupled into
the fundamental mode to propagate to the output. These include propagation through the fiber cladding and by means
of a leaky mode. We propose a technique for measuring the magnitude of this leakage and apply it to infrared fibers
made at the Naval Research Laboratory and at Tel Aviv University. All measurements are performed at 10.5 μm
wavelength.
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.
During the last decade, there has been rapid progress in the development of integrated optical circuits, incorporating thin optical waveguides, for the Visible and Near IR (NIR) spectral ranges. In this work we extended the same concept to the Mid-IR (MIR) spectral range 3-30 microns. Towards this goal we developed diffused and strip buried planar waveguides based on silver halide crystals.
Diffused planar waveguides were created by diffusion of Br- ions into crystalline substrates, forming silver chloro-bromide (AgClBr) layers of higher refractive indices and different thickness of 65μm to 600μm. Strip buried planar waveguides were constructed from press-flattened silver chloro-bromide (AgClBr) fibers with different thickness, which were buried into an almost pure substrate. All the waveguides were in a thickness range of 60μm to 170μm. Using CO2 laser we found that the transmission losses at 10.6mm were about 4 to 12dB/cm for the buried waveguides and about 4 to 16dB/cm for the diffused waveguides. We also formed and characterized curved waveguides on AgCl substrate.
By using the planar waveguides as sensing elements for infrared evanescent wave spectroscopy, we were able to detect 1% of glucose or 1% of alcohol in water. In conclusion, we demonstrate the feasibility of using AgClBr waveguides as integrated optical elements in the MIR.
Laser-induced breakdown (LIB) thresholds in AgClxBr1-x crystals and fibers were studied under CO2 laser pulses and CW excitation. The value of LIB threshold Pc of the bulk crystals is about 7.2 X 108 W/cm2 for AgCl and 4 X 109 W/cm2 for AgBr under 60 ns TEA laser excitation. The LIB threshold in fibers is much smaller; about 2 X 108 W/cm2. The absorption of the crystals at 10.6 micrometers changes from 8 X 10-5 cm-1 for AgCl to 2 X 10-5 cm-1 for AgBr; for fibers with the same composition absorption is much greater. The dependence of the LIB and IR absorption on composition, and mechanical and temperature treatments suggests that the LIB in silver halide crystals and fibers is due to the avalanche electrons in a high electric field. The initial free electrons for this process are supplied by ionization of the cation vacancy - charged dislocation complexes. The relatively lower optical stability of the fibers is due to the increased concentration of defects formed in the hot extrusion of the crystal. A simple thermal annealing method for reduction of the IR absorption is proposed.
The maximal laser power transmitted through polycrystalline silver halide fibers is limited by their IR absorption and optical strength. The mechanisms of the IR absorption and laser-induced breakdown (LIB) thresholds Pc in AgClxBr1-x crystals and polycrystalline fibers were studied. Investigations, including mechanical treatments, heat treatments, and luminescence properties were performed to clarify the reason for the IR absorption and LIB in silver halide crystals and fibers. From these experiments we concluded that one of the reasons for LIB may be the avalanche of electrons in the conduction band of the crystal. These seed electrons are produced by the IR absorption of the cation vacancies in these crystals. A simple annealing procedure for reduction of the IR absorption of fibers is proposed.
We have developed core/clad polycrystalline silver halide optical fibers with a loss of roughly 0.3 dB/m at 10.6 micrometers. Such fibers, with core diameters 0.3 - 0.6 mm and lengths of 1 to 2 meters are capable of continuously delivering output power densities as high as 14 KW/cm2. The fibers were repetitively bent in the plastic and elastic regimes and the optical transmission monitored during bending. The mechanical properties of the core/clad fibers and of the core only fibers are similar. It was also demonstrated that the 'bending' properties of the core/clad fibers are determined by the cladding material. Our investigations suggest that proper design of the core/clad structure may give significant improvement in mechanical properties such as more cycles to optical failure. This will be very important especially for endoscopic laser surgery and other medical applications.
Bundles of silver halide optical fibers have been fabricated by extrusion of preforms containing core-clad fibers. Silver halide fibers are transparent in the mid-IR spectral range (2 - 20 micrometers ), and therefore ordered arrays of such fibers have the potential to deliver thermal images at room temperature. Several kinds of bundles were prepared, consisting of 100 to 2500 fibers, with a fill factor of 0.34. Thermal images were delivered through the bundles and recorded using a IR cooled camera. This paper discusses the optical properties and the imaging characteristics of bundles with several fiber densities of 3 - 160 fibers per mm2 and lengths up to 30 cm. The modulation transfer function of the bundles was measured by the knife edge sampling process, and by `bar-chart' imaging. The maximum spatial frequency that could be resolved after passing through a typical bundle was 3.5 lines per millimeter.
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.
For the first time a multimode silver halide mid-infrared bifurcated (Y) optical fiber coupler was constructed. The optical characteristics of the coupler are given. The coupler was used to irradiate a sample with a CO2 laser and to simultaneously measure the temperature rise of the sample.
Functional endoscopic sinus surgery (FESS) utilizing laser energy delivered by optical fibers shows promising results. Due to high absorbance in tissue, pulsed CO2 laser energy (10.6 micrometers ) has the theoretical advantage of causing minimal damage to the area surrounding the spot of vaporized tissue. However, conventional optical fibers are not capable of transmitting in this mid IR region. Unclad optical fibers made by extrusion of silver halide crystals are capable of transmitting CO2 laser energy but their 0.9 mm diameter and output beam divergence results in less than desirable power density. We recently developed a silver halide optical fiber consisting of core and cladding, which can transmit CO2 laser energy with lower divergence and an increase in the power density delivered to tissue. This preliminary study presents the histological effects on tissue of CO2 laser energy, delivered through a clad silver halide optical fiber. Higher ablation rate and less circumferential damage to tissue have been observed.
The IR transmittance of silver-halide fibers was measured while they were undergoing various flexing procedures leading to mechanical fatigue. The fatigue experiments consisted of repetitive bending, going from small bending radii (plastic regime) to large bending radii (elastic regime). Various types of fibers have been investigated including unclad fibers, fibers with a core-clad structure, fibers of various AgClxBr1-x compositions and fibers of different diameters. The optical measurements which were done during the flexing experiments involved CO2-laser transmission and spectral (FTIR) transmission. The results for the various conditions are reported and discussed in terms of high-cycle and low- cycle fatigue. Such investigations are of practical importance in characterizing fibers useful for endoscopic surgery and IR detection.
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