Broadband thulium fiber amplifier for spectral region located beyond the L-band

We present the development of a pair of silica-based thulium-doped fiber amplifiers
working together in a broad spectral range from 1.65 µm to 2.02 µm. For the one optimized for
shorter wavelengths, we designed and prepared optical fiber with a depressed cladding. We show
the performance of the amplifiers achieving small-signal gain of at least 10 dB over 350 nm range
from 1670 nm to 2020 nm, maximum gain of 40.7 dB with a noise figure as low as 6.45 dB and
an optical signal-to-noise ratio of up to 50 dB. To the best of our knowledge, it is the first time
that thulium fiber amplifiers of straightforward design without using redundant spectral filters
operating efficiently in such a wide spectral region are demonstrated.