| Popis: |
Since the early and fundamental development of the low loss silica fiber, most technological advances in fiber optics have been based on new fiber designs (dispersion-shifted, large mode area, microstructured etc.) and new optical technologies (pumping schemes, modulation formats, signal processing etc.). And so far, most applications have primarily been in the near-IR range of wavelengths (1300nm–1700nm), where silica offers its lowest loss (∼1500 nm) and its lowest dispersion (1310 nm). As fibers have become ubiquitous in a number of fields, they have become attractive solutions for an even broader scope of new applications in an extended range of wavelengths, especially now into the mid-infrared (mid-IR >2 µm). These include defense (free space communications and optical guidance), medical (diagnostic and surgical), biological and environmental (detection/imaging and sensing). With the prospect of these new applications, innovation in optical fibers is going back to the fundamentals of the materials, of the glass and of its forming/processing. The candidate materials for new applications have been fluoride, phosphate, chalcogenide and tellurite glasses.1–3 Whereas silica only transmit up to 2 µm, fluoride glasses transmit up to ∼2.7 µm, tellurites up to ∼6 µm and chalcogenide glasses up to ∼20 µm, these characteristics stemming from the material properties of the glass. Without reviewing in details the advantages and disadvantages of each of these glasses, we believe that tellurite fibers offer the best compromise of all for future applications in the mid-infrared (mid-IR). Besides an extended transmission window compared to fluorides, tellurite glasses offer lower intrinsic loss than chalcogenides, little photosensitivity and high optical damage threshold. Most importantly, tellurites are simple oxide glasses and therefore mechanically, thermally and chemically very stable.4,5 In addition, tellurite glasses can be easily modified with a wide variety of modifiers/formers to adjust their dispersion profile, maximize their nonlinear coefficients and their emission cross-sections when doped. As examples, i) tellurite glasses can be doped with much higher concentrations of rare-earth ions (up to ∼50 times) than silica without clustering of the ions or concentration quenching of their emission and ii) their Raman coefficient, which is intrinsically ∼30 times higher than silica can be further increased by the addition of modifiers. |
