Simple 2 μm thulium fiber laser needs no optical isolator
Theta-cavity resonator takes the place of an expensive isolator.
|Camille Brès and Svyatoslav Kharitonov describe a cost-effective way to generate 2 μm lasers, using only thulium-doped optical fibers instead of amplifiers and isolators. (Image: Alban Kakulya / EPFL)|
Lasers that emit at around 2 μm are not only eye-safe, but offer significant advantages compared to traditional, shorter-wavelength lasers in the areas of surgery and molecular detection. Recently, 2 μm fiber lasers have come to the fore as small and rugged 2 μm coherent sources. However, 2 μm fiber lasers can be further simplified, as two researchers at the Photonics Systems Laboratory (PHOSL) at Ecole Polytechnique Fédérale de Lausanne (EPFL; Lausanne, Switzerland) have determined.1
Camille-Sophie Brès and Svyatoslav Kharitonov at PHOSL have created the first unidirectional, isolator-free 2 μm fiber laser by using a ring resonator with an S-shaped fiber element that produces feedback in one direction only (called a theta or yin-yang cavity).
To create a 2 μm fiber laser, light is usually injected into an optical-fiber ring containing a gain region that amplifies 2 μm light. The light circulates in the ring, passing through the gain region many times. For optimal operation, these systems include a costly optical isolator, which forces the light to circulate in a single direction. The simpler, lower-cost theta cavity in the new laser replaces the isolator, producing an extinction ratio of 18 to 25 dB between the desired and suppressed lasing directions. The laser has a slope efficiency of 25%, a a 2 dB flat tuning range of 1900 to 2050 nm, and a 0.2 nm linewidth.
The 2 μm spectral domain has applications in medicine, environmental sciences, and industry. At these wavelengths, the laser light is easily absorbed by water molecules, which are the main constituents of human tissue. In the realm of high precision surgery, they can be used to target water molecules during an operation and make incisions in very small areas of tissue without penetrating deeply; the energy from the laser causes the blood to coagulate, preventing bleeding. 2 μm lasers are also useful for detecting meteorological data over long distances through the air, as well as for processing of certain industrial materials.
1. Svyatoslav Kharitonov and Camille-Sophie Brès, Light: Science & Applications (2015); doi:10.1038/lsa.2015.113