Chalcogenide Glasses

Abstract

Observing nonlinear phenomena in periodic structures has been mostly confined to fiber Bragg gratings (FBG’s) in silica fiber because they are straightforward to produce and use. However, the Kerr nonlinearity of silica is extremely small (n ₂ = 2.6 × 10^{− 16}cm²/W) and many experimental regions are not accessible due to the impractically long gratings that are required. Combining the ease of use of fiber or waveguide gratings with a large Kerr nonlinearity is therefore very attractive. Attempts to increase the Kerr non-linearity by adding Germanium have been successful in increasing the Kerr coefficient by factors of three to five, but achieving factors of hundreds or even thousands requires non-oxide glasses, as will be discussed below. In this chapter we will review recent work, theoretical as well as experimental, on the Kerr nonlinearity in chalcogenide glasses. These are typically sulfide, selenide or telluride based glasses (in contrast to the oxide based silica glass), that can be engineered to have a smaller “bandgap” and hence a larger nonlinearity than silica. Both chalcogenide fiber gratings and chalcogenide waveguide gratings have been successfully fabricated and measured. In fact, the measured Kerr nonlinearity at a wavelength of λ = 1.55 μm in bulk As₂Se₃ is about 500 times larger than in silica [1]. This means that in a 10 cm long waveguide with an effective mode area of 1 μm², an optical pulse with only 1 W of peak power will experience a nonlinear phase shift of about π/2. This sort of performance enables the observation of a host of nonlinear phenomena and even some practical devices, such as nonlinear pulse compressors, that will be discussed later in the chapter.