Welcome to the Glass Age

84 some challenges for the use of glass, such as its poor thermal conductivity and difficulties in free-form cutting and cleaving, but technology keeps advancing to find proper solutions [7]. Several optical, electronic, and mechanical interfaces can be designed in a way that multiple glass boards can be stacked on each other; different modules can be fabricated, ready for assembly on a main glassy board. Figure 5.4 shows as an example a glass main board with size 100 mm x 50 mm, hosting various sub-assemblies: on the left, a system for frequency modulating and splitting the laser light coming from a fiber, and on the right two smaller glass boards detecting the light coming from an external sensing unit through two separate optical fibers [8]. Some innovative guided-wave devices have also been developed thanks to glass properties: an example is constituted by glass microspherical resonators, which find application in narrow-band and add-drop optical filtering, feedback elements for external lasers, multiwavelength and very low-threshold laser sources, nonlinear photonics, and optical sensing. These miniaturized optical resonators are based on dielectric structures having circular symmetry, like cylinders and spheres, which sustain the so-called Whispering Gallery Modes that can be interpreted as circulating electromagnetic waves that are strongly confined within the structure [8]. The peculiar properties of these microresonators are best exploited in the area of sensing: if scattering losses at the surface and absorption of light in the material are very low, the trapped photons are able to circulate for a very long time, providing a very long optical interaction path. Any minimal change at the resonator’s boundaries induces a change in the quality factor of the resonator or a shift in the resonance frequency. Thus, detection of small forces, either mechanical or optical (optomechanics), or of micro- or nanoscopic objects (biological ones too, e.g., a bacterium or molecule) is possible with very high sensitivity, even better than the frequently used surface plasmon resonance (SPR) sensors. In the last decades, microspherical devices have fully demonstrated their capability of detecting even single molecules, virions, DNA, antibodies, enzymes, and aptamers. Most of these excellent results have been possible thanks to glass. Pure silica represents the preferred material, due to the very high purity and nanometer-scale surface smoothness available. These two characteristics have made possible the achievement of a top quality factor for the resonator of Q ≈ 8 × 10 9 , with a corresponding finesse F ≈ 2.2 × 10 6 . Single spheres with the desired diameter and reproducible high quality are produced very simply by melting the tip of a glass fiber, a very cheap method that allows Telcordia 1209 and 1221 compliant, single-mode, commercially available splitter (Figure 5.3b). Many other processes have been developed to produce glass optical waveguides and photonic devices, e.g., depositing thin glass films with radio frequency (RF) and magnetron sputtering, chemical vapor deposition (CVD, and in particular plasma- enhanced chemical vapor deposition- PECVD), flame hydrolysis deposition (FHD), spray pyrolysis (SP) deposition, pulsed laser deposition (PLD), and sol-gel coating. Besides ion-exchange, the local modification of refractive index may be achieved by ion implantation, UV irradiation, and femtosecond laser writing; the latter two techniques are also suitable for the direct definition of a channel waveguide circuit. The continuous advances in microsystems for communication, computing, sensing, and biomedical applications require a higher integration of micro-electronic, optoelectronic and micro-optical components. Even in this area, glass offers unique properties, and advanced hybrid packaging technologies are often based on glass substrates, where photonic integrated circuits (PICs), laser diodes, modulators, isolators, beam splitters, microlenses, and detectors may be interconnected through electrical stripes (i.e., metallized glass) and optical waveguides (possibly with mode field expansion sections). There are, of course,