ABSTRACT
During the last three decades, optical fibers have experienced an extraordinary and rapid development. They are being used, as passive and active mediums, in various industrial applications, in fields as diverse as telecommunication, medicine, aerospace, defense, spectroscopy, sensing, laser power delivery, fiber lasers, etc. Optical fibers are now critical parts in many high-tech devices. Of course fiber parameters have to be tailored to each application needs. Unfortunately, there is no single fiber material that can fulfill all types of application needs. Engineers and researchers have to make some compromises when choosing the right fiber material for the right application. There are some important criteria that one has to consider when choosing an optical fiber to be used in a specific application. The first criterion is technical. The glass should have the required properties, such as transmission window, glass
characteristic temperatures, and durability. The second criterion is economical. The technology should be mature enough to provide high-quality optical fibers at an effective cost with a reasonable yield. Silica fiber technology is the most establish technology so far. Billions of dollars have been spent to develop this technology and bring it to the current level. This was mainly motivated by the huge telecommunication market. Standard silica optical fibers, used in the telecommunication field, are produced in thousands of kilometers, with high mechanical strength and ultralow loss. The fiber loss is approaching the theoretical value, which means that the material has ultrahigh purity and the production process is well under control. However, these fibers cannot be used for application above 2 microns since the material is opaque. More recently, silica photonic crystal fibers (PCFs), with some special design, negative curvature fiber [1], have been shown to transmit at few wavelengths over 2 microns. But these fibers are still at the R&D phase. Of course, applying the same design to materials that are already transmitting in the midinfrared region will enlarge the transmission spectrum to longer wavelengths for infrared materials, too. For applications requesting fibers with either transmission above 2 microns or continuous transmission from the ultraviolet to the midinfrared region, there are few infrared materials that can be considered, such as heavy metal fluorides [2], chalcogenides [3], heavy oxides such as tellurite [4], phosphates [5], single crystals such as sapphire [6], and polycrystalline materials. These fibers are commonly called exotic fibers. Again, one has to choose among these materials according to the application requirements. However, few of these materials can be drawn into high-quality optical fibers that can meet industrial application needs. Amorphous materials have definitely some advantages when compared to crystalline materials. Fibers made from amorphous materials can have a core cladding structure and have low loss. They are also more robust and very flexible. They can also be drawn into long-length fibers. Furthermore, fiber parameters can be tailored to application needs by adjusting core and cladding compositions. Among all exotic materials, fluoride glass fiber technology is the more advanced technology so far [7]. In fact, in the last two decades, significant progress has been achieved and the technology is now
mature enough to meet many industrial application requirements. Fluoride fibers are now being used in some industrial applications, including the most demanding ones, such as defense and aerospace. Almost all different types of fibers have been produced with fluoride glasses, including doped and undoped multimode and single-mode fibers. Some exotic shapes such as hexagonal, square, and D-shaped fibers have been reported as well. Beside the state of the technology, fluoride glasses show also outstanding optical properties that make them the material of choice for many infrared and even multispectral applications. They have a low refractive index and a low and negative dn/dt. They can be heavily doped and co-doped with any rare earth ions for active applications, such as fiber lasers and amplifiers. Stable fluoride glass composition can contain up to 100,000 ppm of rare earth ions. This concentration is ranging from only a few hundreds to a few thousands of ppm in silica fibers. In fact, lanthanum fluoride is one of the elements entering some fluoride glass compositions, and it can be substituted by any other rare earth ion without compromising glass quality. Consequently, compact fiber laser can be made using a short length, a few centimeters, of doped fluoride glass fiber.
