Erbium–ytterbium codoped phosphate core/double silica clad composite optical fibres for compact amplifiers

We report a study of composite fibres as gain media of all-fibre amplifiers. A distinctive feature of the fibres is an erbium–ytterbium doped phosphate core in a double silica cladding. The high Yb³⁺ concentration (above 10²¹ cm^-3) ensures effective pump absorption from the cladding at fibre lengths under 40 cm and effective erbium ion sensitisation. The silica cladding facilitates fusion splicing to standard fibres. The composite fibres offer a large gain coefficient: about 0.4–0.5 dB cm^-1 at a signal wavelength of 1550 nm. The maximum gain at a fibre length of just 36 cm and an input signal power of 0.37 mW at λ = 1550 nm is about 13.5 dB. Under such conditions, the amplified spontaneous emission level is lower than the input signal level by 10 dB.