Depressed-cladding thulium-doped fiber for applications below 1800 nm.

We present a thulium-doped silica fiber, featuring a depressed cladding, for applications at wavelengths below 1800 nm. The depressed cladding is used as a distributed filter suppressing amplified spontaneous emission at longer wavelengths, which helps promote emission at shorter wavelengths. We describe the fiber design process that was carried out by using a combination of numerical methods. The fiber was prepared in-house by a combination of the standard modified chemical vapor deposition method and nanoparticle doping. We demonstrate the effectiveness and tunability of ASE filtering, which is influenced by fiber bend radius and its variation.

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.