Swiss researchers Camille-Sophie Brès and Svyatoslav Kharitonov have created a thulium-doped fiber laser at 2-µm that eliminates costly and bulky isolators. Credit: Alban Kakulya / EPFL
Scientists in Switzerland have reported a novel thulium-doped fiber laser at the 2 µm wavelength, relying on a simple and inexpensive architecture that eliminates the need for costly optical isolators and amplifiers (Light Sci. Appl., doi:10.1038/lsa.2015.113). An elegant theta-cavity design enables redirecting of light headed in the wrong direction inside the fiber, creating a unidirectional laser that is not only inexpensive, it generates higher power than other similar lasers.
The 2-µm wavelength laser is of interest for its application to several critical areas of science. The first vibration overtone of the O–H bond in water absorbs at a wavelength of 1.92 to 1.94 µm, which makes a 2-µm wavelength useful in “bloodless” laser surgery, in which the water molecules in tissue and blood are targeted for smaller incisions and quick coagulation. Multiple absorption lines of atmospheric H2O, CO2, and NO2 also vibrate in this region, which creates potential in meteorology, environmental science and agriculture. Other applications include free-space telecommunications, material processing and spectroscopy.
Other fiber laser designs in this wavelength region are expensive and bulky, requiring an isolator that forces light to propagate in the same direction (unidirectional). Camille-Sophie Brès, professor of electrical engineering, and doctoral student Svyatoslav Kharitonov at the École Polytechnique Fédérale de Lausanne, Photonic Systems Laboratory, Switzerland, exploited a ring fiber cavity called a theta resonator that introduces non-reciprocal losses via an S-shaped feedback in the main ring. The theta resonator works in place of an isolator, which conventionally uses bulky Faraday rotators and 45-degree cross-polarizers to suppress backwards-propagating light.
The design also involves a nonlinear amplifying mirror in the cavity, which maintains a narrow linewidth of the emitted light. The resulting laser provides sub-Watt output power flat-tunable from 1900 to 2050 nm with a linewidth of 0.2 nm. The researchers plan to further refine the laser in the future to improve stability and quality of the emission.
