O-Band Bismuth-Doped Fiber Amplifier with 67 nm Bandwidth. - Aleksandr Khegai; Yan Ososkov; Sergei Firstov; Konstantin Riumkin; Sergey Alyshev; Alexander Kharakhordin; Elena Firstova; Fedor Afanasiev; Vladimir Khopin; Alexey N. Guryanov; Mikhail Melkumov (OFC, 2020)