Welcome to the Glass Age

83 by two dominant approaches, namely a local modification of the bulk glass or the deposition of a thin glass film on a lower-index glass substrate. Indeed, one of the first technologies employed for planar waveguide fabrication, in the early 1970s, was that of ion-exchange in glass, exploiting the same process used for chemical strengthening of glass and already known since the beginning of the 20 th century [5]. In fact, the in-diffusion of ions having different atomic size in the pristine glass matrix (e.g., larger potassium ions K + from a molten salt KNO 3 substituting smaller sodium ions Na + in the glass), while increasing its mechanical strength by preventing or healing over the formation of superficial micro/nano-cracks, at the same time induces an increase of the refractive index at the surface. Ion exchange was also used in 1968 by Nippon Sheet Glass (NSG) and NEC Corporation for an innovative method to vary the central and peripheral refractive indices of a glass fiber in a parabolic profile, with the aim of reducing the spreading of the envelope of a propagating optical pulse and thus increasing the fiber transmission capacity. Ten years later, these fibers were introduced in the market under the product name of SELFOC®. The major advantage of ion-exchange in integrated optics is the simplicity of the technique, requiring only a furnace, a container of the nitrate salt to be melted, and a proper holder of the sample; its applications in creating optical devices are countless (Figure 5.2). Regarding material properties, the only strict requirement for the glass is to contain an alkali ion (K + , Na + , Li + being those most frequently employed). Different glass matrices may be used, depending on the application; soda-lime and borosilicate glasses are among the most common. An example of a glass ion-exchanged integrated optical power splitter device of use in fiber communication systems, is shown in Figure 5.3 [6]; some twenty years were necessary to move from laboratory demonstration (Figure 5.3a: a multi- mode highly scattering sample) to a Figure 5.4. Photo of a glass main board: on the left side the light from a fiber-pigtailed external laser is modulated by separately mounted electrodes; at the center, the modulated light is split and coupled into opposite optical fibers, connected to an external sensing device (not visible in the photo); on the right, two separate glass boards detect the light from the sensing unit coming through the same fibers. Source: Reprinted from Ref. [7] under Creative Commons license.