Raman
amplifier having an
optical fiber made of a
tellurite glass is disclosed. The
tellurite glass has at least two further
metal oxides, the metals of said respective two oxides being selected from a first group of Nb, W, Ti, Tl, Ta, and Mo and from a second group of Nb, W, Ti, Pb, Sb, In, Bi, Tl, Ta, Mo, Zr, Hf, Cd, Gd, La, and Ba. The so obtained
fiber has improved optical (
Raman gain) and / or thermal (
thermal stability index) properties. Alternatively, the tellurite based glass compositions of the
fiber have at least one additional
metal oxide, where the
metal is selected among Nb, Ti, Tl, Ta, and Mo, the glass showing a particularly high
Raman gain. The maximum
Raman gain of these glasses is typically higher than 100 times of the maximum Raman
gain of pure silica and the respective total cross-section of the Raman spectrum is typically greater than 100 times the total cross-section of pure silica, in the frequency measurement range of 200 cm−1 to 1080 cm−1.