EFONGA - European Forum on New Glass Applications    

Introduction to Glasses for Optoelectronics

Current developments in optoelectronics and photonics need materials and devices with increasing optical functionality and more and more complexity. Novel or advanced glasses can respond to these demands: application areas range from special coatings for optical components (including transparent conductive or electrochromic films) to optical telecommunications, from displays to optical storage, and to biomedical and sensing.

In the optical communications area, the extension of optical functionality within networks, that are already optical, and the implementation of optics at lower levels, particularly the access network, have been major goals in recent years. Both these development areas will impose new requirements for optical components, demands for which a number of different technologies are already competing. Implementation of all-optical functionality (e.g. all-optical switching) by exploiting nonlinear properties of special glasses and low-cost manufacturing are two key objectives. A specific attention is being devoted to glasses for waveguides, optical amplifiers and lasers

In the wider optoelectronics area, researches on flat panel displays and on luminescent materials that may be efficiently excited by low-cost light sources, such as LEDs, constitute a main concern.

In this scenario, composite and nanostructured glasses have great relevance. For instance, there is an increasing interest in the development of nanostructured materials for innovative photonic devices. Since the discovery of the quantum size effect in semiconductor nanoparticles, academic and professional researchers have been interested in the potential applications of similar effects in metallic nanoparticles. Noble-metal elements are particularly attractive in such an area, due to the well-known surface plasmon resonance, which originates from a confinement effect on the electronic properties in metal systems of finite size. This phenomenon corresponds to the collective excitation of the free electrons, classically described as the oscillation of the electronic cloud with respect to the ionic background of the nanoparticle, and results in a potentially large local field enhancement around the excited nanoparticle. It has to be noted that, in addition to the optical applications, metal nanocluster composite glasses synthesised with transition elements are important for their magnetic properties as well.

Hybrid materials, where inorganic glasses may be mixed, for instance, with organic materials or with inorganic crystalline materials, may well exhibit peculiar properties and/or extended functionality. Their study will extend further the range of optical properties available.

Laser micro-machining and nano-structuring of glasses by interaction with high power ultra-short pulse lasers are also attracting much attention. Femtosecond laser writing of optical structures and self-organised nano-gratings formation probably due to laser-induced birefringence are two examples of new phenomena, which can be exploited for integrated optics and information storage, respectively.

Finally, the possibility of implementing periodic structures in glasses acting as photonic crystals (that has already been very successfully demonstrated in optical fibres) by using low-cost processes will be another matter of investigation in the next few years.

Optical fibres are now almost universally used in telecommunication systems for point-to-point high bandwidth links. The more recent developments of fibre communication systems often rely on the availability of integrated optical circuits, namely of devices involving waveguides on planar substrates. Related manufacturing technologies include CVD, flame-hydrolysis, RF-sputtering and sol-gel deposition processes, or diffusion, ion-exchange and ion-implantation processes.

Direct uv-laser or fs-pulse laser writing is now challenging conventional photolithographic patterning of the circuits.

A key requirement of materials and integrated optical components is that they must be optimised for 1.55 mm operation, because this is the wavelength of minimum attenuation in silica fibres and is currently the standard operational wavelength of most telecom systems. In future, however, other wavelengths, in combination with non-silicate fibres and planar waveguides, may gain a foothold.

Erbium-doped fibre amplifiers are now the commercially available choice for optical amplification in telecom systems operating near 1.55 mm, using laser diode pumping at 980 nm or 1480 nm. The use of integrated optical amplifiers, on the other hand, permits significant savings in space occupied and the integration of other optical components such as switches, splitters, and wavelength-division-multiplexers together with the amplifier on the same glass chip, with minimal losses. The shorter interaction length, however, requires higher erbium doping levels to achieve an acceptable gain, with the disadvantage of increased ion-to-ion interactions. Materials issues to be addressed include the reduction of multiphonon quenching of the lasing excited-state lifetime, the minimising of the effects of amplified spontaneous emission and of up-conversion processes, and therefore the identification of optimal glass matrices and the corresponding optimal rare-earth doping concentration.

In the area of glass optical amplifiers the key goals are:

-         To achieve a broader emission band, that could allow the system designers to fully exploit wavelength division multiplexing in order to increase the transmission capacity, and a higher net optical gain.

-         Similar considerations apply to the identification of the most appropriate matrix-dopant combination for the manufacturing of integrated optical Er:glass lasers.

-         Having in mind future developments towards longer wavelength bands, non-oxide glass matrices and rare earth elements other than erbium will also be carefully investigated.  

References to the optical glasses which are commercially available and to their specifications are provided in the data page.

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Website established on 31-01-2006; updated on 10-01-07.