There are very few optical fibers that are highly transparent in the mid-IR. Silver halide crystals are non-toxic and non-hygroscope and biocompatible. The crystals are relatively soft and they can be extruded through dies to form long and flexible fibers. These fibers are among the best candidates for transmission in the mid - IR. We are growing large crystals of AgCl_xBr_1-x of extremely high purity. The crystals are extruded through dies, to form unclad fibers of diameters 0.5 - 1.0mm and lengths up to 20 meters. The attenuation coefficient of the fibers at 10.6 microns is 0.2 dB/meter. Fibers of length of 1-2 meters can deliver CO₂ laser energy of 30-40 W continuously or long pulses of up to 0.5 Joule. The fibers could be bent to a radius of 10 mm with practically no additional loss. We are also using the "rod in tube" method for fabricating core/clad fibers. These include AgCl_xBr_1-x core and AgCl_yBr_1-y cladding, with y>x. The attenution coefficient in these fibers is higher than 1dB/meter.

For many applications like single mode fibers developing we need to know the index of refraction of silver halide crystals with a very good accuracy. We are interested to measure the refractive index as a function of wavelength and as a function of crystal composition. We use two techniques to measure the above: with Michelson interferometer and with FTIR (Fourier Transform Infrared Spectroscopy). The disadvantage of inferometric measurements is that we can measure it only in a few wavelengths, restricted by the abilities of the existing lasers. Furthermore, FTIR measurements also allow the dependence between the refractive index and the wavelength to be measured. In both techniques we measure thin parallel samples with thickness of about 300μm. For the better precision in evaluation of the refractive index we also need to measure sample’s thickness with a good accuracy

Single mode fibers (SMFs) made of silica have been widely used in many applications. These fibers are highly transparent in the visible and the near-IR but are opaque in the middle infrared (mid- IR), i.e. in the spectral range 3-30µm. SMFs with a broad transmission in the mid- IR are needed for the fabrication of thermal imaging fiber bundles, IR heterodyne detection systems, spectroscopy, interferometry, fiber lasers, mid-IR sensors and as spatial filter elements for the terrestrial planet finder (TPF) project of NASA-JPL or of the Darwin project of the European Space Agency ( ESA).

In standard infrared radiometry, one measures the total emission from a warm surface, over a very broad spectral range. This "single band" radiometry is used to determine its temperature T. Such a method cannot be reliably used if the emissivity is not known. This is particularly difficult if the emissivity changes during a process. We have measured the emission in several spectral bands. "Multi band" radiometry is being used to determine the emissivity and the temperature, independently. We have developed several types of radiometers that are being used for thermometry. Some are based on our own design and others make use of commercial radiometers The applications for IR radiometers are: Measurements inside a MRI system, Measurements in Otolaryngology, Measurements of Heated Tissues and more.

Laser heating can be used to bond tissues. The exact mechanism of such laser welding is not fully understood, but it has been found that it is critically dependent on the temperature. We have developed a welding system based on two optical fibers. One fiber is used to carry laser energy to heat a spot on tissue. The other fiber is part of a fiberoptic radiometer that is being used to determine the exact temperature of the heated spot. A computerized system makes use of the signal obtained from the radiometer to control the temperature. The temperature of a spot on a living tissue can be controlled to within 5 deg. We have conducted theoretical and experimental studied of the heating of tissues by CO₂. We found the optimal laser welding conditions for cuts in tissues. Histological studies showed better results than standard suturing of cuts. Research is being continued now for other lasers (e.g. GaAs or Nd:YAG) , whose radiation penetrates deeper into tissues. We also study the tensile strength of the cut, in comparison to that of standard sutures.

Mid IR radiation from a tunable source is transmitted through an AgClBr fiber. Most of the transmitted radiation is localized inside fiber, but there is an evanescent field that extends to a distance of few wavelengths outside the fiber. A sample, which has some characteristic absorption lines, can be placed in contact with the fiber. At these wavelengths the evanescent field will be absorbed and therefore the total transmission of the fiber will be decreased at these characteristic absorption lines. This is the basis of a very useful method for measuring the absorption spectrum of samples in contact with fibers that transmit in the MIR. It is called Fiberoptic Evanescent Wave Spectroscopy (FEWS). Measurements on such samples could be carried out in real time and in a remote location. We have carried out detailed theoretical and experimental studies of the FEWS method. We evaluated various sources of tunable IR sources (FTIR, tunable diode lasers and tunable gas lasers), fibers of different diameters, different launching conditions etc. We also evaluated the effects of various coating materials on the fibers.

Single crystals doped with rare earth ions, such as Nd:YAG or Er:YAG have been used as solid state amplifiers and lasers. These ions can also be incorporated in glass and used for similar purposes. Moreover fibers pulled from such glass have been used as fiber lasers. These solid state lasers emit in the visible and the near IR. All the crystals and glasses that have been studied so far are opaque in the mid-IR at wavelengths longer than 3-4 microns. There are no solid state lasers or fiber lasers in the longer wavelength range. We have studied the optical properties of silver halide crystals and fibers doped with the following rare earth ions: Pr³⁺, Nd³⁺, Tb³⁺ and Dy³⁺. The emission, excitation, and absorption spectra, as well as the kinetic parameters, were measured. Several optical emission lines were found in the spectral range 3-6μm. Further investigations of the optical properties of doped silver halide crystals will bring us closer to developing a silver halide based laser.

During the last decade a new frontier has emerged, which deals with the control of the optical properties of materials by periodic structures implemented in the materials. One can engineer a material that prohibits propagation of light, or allows it only in certain directions at certain frequencies, or localize light in specified cavities. In the visible and near infrared ranges, photonic crystal fibers (PCFs) have shown great potential in overcoming the limits of conventional fibers and achieving remarkable abilities that regular fibers cannot accomplish. In the mid-IR range, PCFs have not yet been implemented due to the lack of suitable materials and techniques. Design and fabrication of PCFs for the mid-IR can be extremely useful, not only as large core area endlessly single-mode fibers but also for other applications like polarization maintaining with low losses and high power transmittance. Our research involves the design, fabrication and optical testing of photonic crystal fibers for the mid-IR, based on silver halide polycrystalline materials. The work includes numerical simulations and calculations, design of equipment and optical setups, fabrication of fibers and optical measurements.