Isolator-free unidirectional thulium-doped fiber laser

We report the first demonstration of a unidirectional, isolator-free 2-μm thulium-doped fiber (TDF) laser, relying on the properties of the theta cavity (ring resonator with S-shaped feedback). The core pumped theta cavity TDF laser provides sub-Watt output power with a slope efficiency of 25%, a 2 dB flat tuning range of 1900–2050 nm, and a linewidth of 0.2 nm, and achieves the extinction ratio of 18–25 dB (depending on the feedback value) between the favored and suppressed lasing directions. It is shown that these characteristics are competitive with, if not superior to, those of conventional ring cavities. The simulation results of the linear and Kerr-nonlinear theta cavities are also presented, explaining certain unexpected features of the laser behavior and establishing the importance of the doped fiber nonlinearity on the spectral shaping of the emitted signal. A thulium-doped fibre laser that uses a theta cavity instead of an isolator to achieve unidirectional lasing is demonstrated by a Swiss team. Tunable laser sources with narrow linewidths in the 2-μm wavelength region are highly desired for various applications, and fibre lasers are promising candidates. Optical isolators are generally used to achieve unidirectional lasing in such lasers, but now Svyatoslav Kharitonov and Camille-Sophie Brès of Ecole Polytechnique Fédérale de Lausanne have experimentally shown that a unidirectional ring fibre cavity (also known as a theta cavity) can be equally, if not more, effective. They obtained emission whose power, linewidth, and optical signal-to-noise ratio were competitive or superior to those of conventional ring cavities. They also found evidence that suggests the nonlinearity of the thulium-doped fibre significantly affects the shaping of the theta cavity lasing output.