Thulium Doped Fibre Laser based on Theta Cavity Lasing Direction Rectification

The development of thulium-doped fibre lasers (TDFL) is motivated by numerous potential applications in spectroscopy, remote sensing, medicine, material processing, and telecommunications. The all-fibre corepumped ring cavity TDFLs exploiting fiberized grating-based filter [1] or Fabry-Pérot etalon [2] as a wavelength selective element were reported. An optical isolator should be inserted into the fibre ring cavity to ensure unidirectional lasing. Isolator-free unidirectional ring fibre cavity (sometimes referred to “theta” [3] or “yinyang” [4] resonators) represents an attractive and cost-effective alternative solution. In theta cavities, nonreciprocal losses are introduced by providing an S-shape feedback within the main ring. An erbium doped fibre laser (EDFL) with theta cavity, providing close to 20 dB extinction ratio (ER) between output signals, propagating in favoured and suppressed directions, was demonstrated [5]. We report for the first time the demonstration of a unidirectional 2 μm TDFL, exploiting properties of the theta cavity. The theta cavity laser consists of a fibre ring, and two additional couplers (DC1,3) used to introduce nonreciprocal losses (Fig. 1). A counter-clockwise (CCW) lasing direction is favoured, and a clockwise (CW) one in suppressed. The fibre ring includes the gain unit (GU), tunable grating based band-pass filter, and 50% output directional coupler (DC2). The GU consists of 11.5 m of thulium doped fibre (OFS TmDF200) bi-directionally core pumped with a 1600 nm pump obtained from an amplified tunable laser source (TLS).