Extrinsic scattering

Abstract

The minimum losses that can be achieved in infrared fibres will probably be limited ultimately by extrinsic scattering. This loss mechanism is associated with imperfections embedded within the glass which can cause outscattering of light propagating through the glass or along a fibre. These imperfections can take many forms although probably the most common consist of small crystals, gas bubbles, and foreign particles of materials such as platinum or oxide remaining undissolved within the glass. Such imperfections will present any propagating light with an abrupt change in refractive index and hence will cause scattering. They will show up a discrete scattering centres within the glass when illuminated by a high intensity light source such as a HeNe laser. Chapter 2 dealt with intrinsic Rayleigh scattering caused by frozen-in density (and hence refractive index) fluctuations caused by the supercooled nature of a glassy material. Here a distinction is made between intrinsic Rayleigh scattering which is uniform throughout the glass, and extrinsic scattering caused by extrinsic defects, and which manifest themselves as discrete scattering centres.