Experimental investigation of spectral characteristics of an erbium-doped fibre amplifier operating in the recirculation regime

The spectral profiles of the gain in an erbium-doped fibre amplifier, incorporated into a recirculating fibre ring, are experimentally investigated. The fibre ring was excited by a superfluorescent fibre source. A procedure for measuring the spectral gain profile is developed, and the criterion of gain-profile uniformity evaluation in the case of broadband radiation is proposed. The influence of the shape of the input radiation spectrum, the pump level, and the composition of the active fibre on the uniformity of the spectral gain profile is studied. It is found that, the uniformity reaches its maximum at a certain pump level. It is shown that, to obtain a maximum number of recirculation cycles for a broadband optical signal, one has to optimise parameters of the fibre amplifier together with parameters of the fibre source of radiation in the active recirculation interferometer.