Nonlinear effects may originate from the dependence of the refractive index of the glass to the intensity of the field, in which case they are responsible for the generation of harmonics or for mixing together waves of different frequencies. In other cases, photons are scattered from phonons in the glass, losing some of their energy in the process. As a result, the wavelength of the light is shifted by an amount that corresponds to the phonon energy. The most common nonlinear effect that is associated with this type of scattering is known as Brillouin scattering, where the phonons interacting with the light are acoustic and typically have a characteristic frequency in the order of 10 GHz. Brillouin effects have important implications in fiber optics, since they are ultimately responsible for restricting the maximum amount of power that can be transmitted over a fiber length; this has very profound effects, especially on narrow-linewidth or monochromatic beams (since the power threshold for the observation of Brillouin scattering increases with the spectral linewidth of the beam). On the other hand, the wavelength shift experienced by the beam depends to a certain extent on the temperature and the pressure of the medium, and is therefore used as a useful measuring parameter in sensing applications.
For all these reasons, the investigation of the Brillouin properties of glasses is of special interest. Glasses with reduced Brillouin gain coefficients are of importance in communications, nonlinear optics and lasers, whereas glass compositions that exhibit a reduced dependence on their Brillouin frequency shift on either temperature or pressure are of specific interest in sensing. The paper by P.D. Dragic et al. studies the properties of a lanthano-aluminosilicate fiber, and shows that whereas the Brillouin frequency shift is dependent on the strain, its dependence on temperature is minimal. The glass used is transparent at relevant wavelengths and can be combined with rare earths to result in an active fiber. The paper is important in that through a thorough study, it identifies a glass composition whose tailored characteristics may find a number of useful applications in specialized areas in fiber optics.
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