Karine Le Foulgoc

Infrared glass fibers for evanescent wave spectroscopy

IR glass optical fibers have been developed in order to optimize their response when they are used as evanescent wave chemical sensors. The diameter of the sensitive part of the fiber can be reduced by tapering the fiber during the drawing process or by chemical polishing. In using an FTIR spectrometer associated with a MCT detector, it was possible to evaluate the influence of the fiber diameter on the polymer coating IR signature as well as the sensitivity of a such sensor. The high flexibility of thin fibers allows the achievement of a detection probe which has been introduced in a microwave oven in order to follow a chemical reaction. It is verified that the chalcogen-based fiber is not sensitive to microwave radiation and gives excellent on line IR fingerprints to check kinetics and reaction mechanisms.

Analysis of biological and chemical compounds by remote spectroscopy using IR TeX glass fibers

The TeX glasses are attracting much attention as materials for low loss mid-IR optical fibers and are consequently good candidates for thermal imaging, laser power delivery, and more recently remote sensing. The TeX glass fiber, transmitting in a wide optical window, has a minimum attenuation in the 9-10 micrometers region. Fibers with an attenuation of less than 0.5 dB/m have been repeatly obtained. These fibers are coated with a UV curable or thermal plastic, in order to improve their mechanical properites. The IR remote spectroscopy using TeX fibers is one of the most promising applications. This technology allows to perform in situ, real-time, and on-line analysis of chemical and biological compounds. The study of industrial processes such as fermentations has been performed by this method, based on the use of these IR TeX fibers.

Tellurium halide IR fibers for remote spectroscopy

The new family of IR transmitting glasses, the TeX glasses, based on the association of tellurium and halide (Cl, Br, or I) are characterized by a wide optical window extending from 2 to 18 micrometers and a strong stability towards devitrification. Optical fibers drawn from these glasses exhibit low losses in the 7 - 10 micrometers range (less than 1 dB/m for single index fibers, 1 - 2 dB/m for fibers having a core-clad structure). The TeX glass fibers have been used in a remote analysis set-up which is mainly composed of a FTIR spectrometer coupled with a HgCdTe detector. This prototype system permits qualitative and quantitative analysis in a wide wavelength region lying from 3 to 13 micrometers , covering the fundamental absorption of more organic species. The evolution of a lactic and an alcoholic fermentation has been monitored by means of this set-up.

Low-temperature measurement by using IR TeX glass fibers

The TeX glass optical fibers have been developed for their broad transparency in the 3 to 13 micrometer region and their good thermal, mechanical and chemical properties. The minimum losses of these fibers are approximately 0.5 dB/m in the mid-IR domain of 7 - 9 micrometer. Owing to these properties, these fibers are useful in a wide range of applications, requiring a relatively low power level, such as temperature sensing. Temperature measurements using a TeX glass fiber have been investigated. The set-up was mainly composed of a polymer-coated fiber which transmitted the signal of a black body to a HgCdTe detector. Fibers with different lengths and different diameters have been used for this experiment and the temperature sensing has been performed in the region of minus 30 degrees Celsius up to 400 degrees Celsius. It has been found that the signal transmitted to the detector increases very rapidly when the temperature is higher than 60 degrees Celsius. However, the sensitivity of the used set-up is still high, even at temperatures as low as minus 30 degrees Celsius. The resolution of the sensor is estimated to be better than 1 degree Celsius in the region of room temperature. These fibers provide a possibility of non-contact low temperature sensing.

Recent improvements in the development of IR TeX glass fibers

Tellurium Iodide based glass fiber preparation has been optimized in order to respond to several IR waveguide technological demands. Three directions have been examined: 1) realization of a two fiber systems for radiometry in the room temperature range, 2) chemical polishing of a core-clad fiber for evanescent wave remote IR spectroscopy, 3) attempts for single mode fiber operating in the 10 μm region for planetary spectroscopy.

IR glass optical fibers for CO2 laser welding

The TeX glass fibers, with high flexibility and relatively low losses, have been developed for many applications especially carbon-dioxide laser power transmission and radiometry. The use of TeX glass fibers to transmit thermal radiation of an object to a remote detector allows temperature measurements, without contact, in inaccessible and hostile environments. The TeX glass fiber sensor can detect temperatures in a wide range [minus 20, 200 degrees Celsius] with a resolution estimated better than 0.2 degrees Celsius at high temperature (200 degrees Celsius) and close to 1 degree Celsius at room temperature. The transmission of carbon-dioxide laser beam through a TeX glass fiber has been performed. More than 2.6 W have been obtained through a 1 meter long fiber by injecting the maximum input power of 6 W at the wavelength of 9.3 micrometer. TeX glass fibers are very promising for biomedical applications such as welding which require an energy transfer through the fiber and a temperature monitoring by another fiber.

TeX glass fibers with a core-cladding structure

TeX glass fibers with a core-cladding structure are prepared by one of three methods: modified crucible method, preform method, or double crucible method. The raw elements are purified in order to eliminate some oxide impurities. They are then all distilled. The Te-Se-As- I system was chosen for the core and cladding glasses because of its stability against crystallization. The numerical aperture (N.A.) of the fiber is typically between 0.15 and 0.4. The diameter ratio of the core and cladding can be varied in the range of 0.15 - 0.9. These fibers are covered with a thermal plastic, to improve their mechanical properties. The optical losses of the fibers are measured between 2 and 13 micrometers by the cut-back method. The modified crucible method was the best to reduce the loss due to structural imperfections at the interface of the core and cladding. The lowest loss of 0.5 dB/m was achieved in the 7 - 9 micrometer region. Many applications of TeX glass fibers are actually tested in our laboratory such as thermal imaging, laser power delivery and remote spectroscopy. This last technology allows in-situ detection and quantification of several chemical compounds which have their characteristic absorptions in the 3 - 13 micrometer region.

Infrared fibers for medium power delivery in the 10-μm region and remote spectroscopy

A new generation of infrared fibers, the TeX glass fibers, operating from 3 to 13 micrometers , exhibits a minimum attenuation of about 0.5 dB/m in the 7 - 9.5 micrometers range. A polymer coating on these fibers increases the mechanical properties and fibers with high flexibility have been obtained. CO2 laser power delivery has been performed using a TeX glass fiber. More than 2.6 W has been transmitted through a 1 meter long fiber by injecting the maximum output power of the laser. One of the most promising applications of these glass fibers is the remote spectroscopy using either evanescent wave or direct absorption analysis. These technologies provide an opportunity to realize in situ and on-line control of chemical and biological processes. Our prototype system using a FTIR spectrometer allows quantitative and qualitative detection of organic species such as alcohol, cosmetic products which have their fingerprints in the spectral region from 3 to 13 micrometers . The detection efficiency using evanescent wave absorption has been studied as a function of the fibers diameter. It has been found that this efficiency increases very rapidly when the fibers diameter decreases.