Light from rare earths for optical telecom systems

Rare earths are a group of chemical elements well known for their
important spectroscopic properties and widely used as dopants of glass or
crystalline matrices to constitute active materials. Neodymium is perhaps
the most widely used rare earth in laser physics (when excited, it emits
in the near-infrared region, at 1060 nm): one of the first solid-state
lasers was a Nd:glass, and today Nd:YAG lasers are present both in
research laboratories and in manufacturing companies worldwide. Recently,
however, another rare earth, Erbium, started to attract an increasing
attention, due to its "eye-safe" emission (namely, in the region around
1530 nm, where there is less risk of damage for the human eye), and,
mostly, because this wavelength band corresponds to the main transmission
window of optical silica fibres. Nowadays, optical telecom systems are
actually based on transmission of light signals in the 1.5 micron
wavelength band. The growth of the metropolitan area networks, where it is
necessary to distribute a same signal to a high number of users, has led
to the requirement of integrated optical devices where the signal
attenuation induced by passive branch splitters can be balanced by the
optical amplification of the signal, obtained by the stimulated emission
of Erbium ions excited by a pump laser diode.
The researchers in the Optoelectronic Technologies Laboratory of IFAC have
a great expertise in the development of integrated optical components and
devices, and since a few years - also thanks to the participation in
European ESPRIT and ACTS projects - they have started to investigate rare-earth-doped glasses, and Erbium-doped glasses in particular, to achieve optical amplification.
In the frame of an international collaborative network (including several
CNR and Italian Universities' groups; University of Nottingham; INESC,
Lisbon; Optical Science Center, Tucson; Vavilov Optical Institute, St.
Petersburg) it has been possible to synthesize and characterize a series
of novel glasses, all containing Erbium, but based on different oxide
formers. Thus, silicates, phosphates and tellurites (i.e. glasses based on
tellurium oxide) have been produced, and optical waveguides were
fabricated in these glasses. The main aim was to combine the property of
high transparency (low propagation loss) with intense emission (high
quantum efficiency). The results achieved so far by the research group are
quite promising, and an optical amplifier with a net optical gain of 1,5
dB/cm (i.e. light intensity is doubled after 2 cm propagation length)
using a pump diode at 980 nm was demonstrated. The group is now working,
under an Italian FIRB project, with the aim of developing integrated
optical lasers and amplifiers in tellurite glasses, which exhibit a
bandwidth almost three times wider than silicate glasses.