Enhanced Raman gain coefficients and bandwidths of sodium-niobium-phosphate glasses for Raman gain media

Raman amplification was demonstrated in the early 1970s, but the invention of erbium-doped fiber amplifiers (EDFA) overshadowed the interest in it. Today, Raman scattering has again become a competitive approach for optical amplification because it is only restricted by the pump wavelength and Raman active modes of the gain medium. Pure- silica and germanium-doped silica fibres, currently utilized as Raman gain media in telecommunications, have quite low Raman gain coefficients and a limited usable spectral bandwidth of approx. 5 THz. Therefore, the investigation of materials possessing both large Raman gain coefficients and broader spectral bandwidth than fused silica is becoming necessary to satisfy the increasing telecommunications demands. This paper presents the Raman spectroscopy characterization of Erbium- or Samarium- doped sodium-niobium phosphate glasses. Spontaneous Raman scattering measurements allowed the identification of groups formed by glass formers and oxygen and of the type of chains. Moreover, Raman spectra have been used to evaluate the Raman gain of such materials by means of a comparison with pure silica glass. The results reveal the presence of NbO₆ octahedra and Nb-O-P-Nb-O mixed chains. A broadening of bandwidth and a significant enhancement (~24 times) of gain coefficient G with respect to conventional silica glasses are also demonstrated.