Rare earth doped fibers lasers can be robust and efficient sources of high quality light, but are usually limited to the highest gain transitions of the active species. But rare earths typically possess a multitude of potentially useful transitions that might be accessed if the dominant transition can be suppressed. In fiber lasers this suppression is complicated by the very high net gain the dominant transitions exhibit; effective suppression requires some mechanism distributed along the length of the fiber.
Lawrence Livermore researchers have developed a novel waveguide with resonant leakage elements that frustrate guidance at well-defined and selectable wavelengths. Based on this waveguide, the LLNL team has fabricated a Large Mode Area Neodymium doped fiber with suppression of the four-level transition around 1060 nm, and demonstrated lasing on the three-level transition at 930 nm with good efficiency.
LLNL's invention relates to optical waveguides in dielectric materials, specifically optical fibers (and amplifiers), which are typically longitudinally invariant. The critical waveguide properties are the modes they support, the coupling between these modes, and their propagation constants (or effective indices, neff). In particular, LLNL's invention provides means to guide (and in some embodiments, to amplify) light of a given wavelength (λ1) in the main element while suppressing the guidance of light of a different, specific and undesired, wavelength (λ2).
The advantages of LLNL’s novel waveguide design includes:
- allows for scaling the power by increasing the core size
- supports well-defined modes, or transmission paths in the laser—that enables it to operate at a wavelength of 925 nanometers (nm) instead of neodymium's otherwise-strongest characteristic wavelength of about 1060 nm
- useful for conversion to shorter wavelength harmonics (e.g. 463 and 308 nm) that would not be easily accessible in other laser systems
The invention would be widely useful for spectral management of gain in fiber lasers and amplifiers. It can apply to directly emitting species, as well as up-conversion schemes. Examples include but not limited to:
- Neodymium three-level transition around 930 nm, useful for remote sensing (e.g.,water vapor) and underwater communications (by harmonic conversion to the blue-green);
- Neodymium transitions around 1,400 nm useful for fiber telecommunications;
- Erbium/Ytterbium co-doped fibers at 1,550 nm, useful for power scaling by suppression of parasitic lasing around 1 µm;
- Thulium doped up-conversion fibers at 1470 nm, by suppression of the dominant transition around 2 µm; and
- Ribbon fibers (slab waveguides implemented as fibers).
LLNL has filed a patent application for its Novel Waveguide Design for Line Selection in Fiber Lasers and Amplifiers invention that has been reduced to practice.
LLNL has fabricated a Neodymium doped fiber based on this waveguide design, and from that constructed a fiber laser. LLNL has achieved lasing on the three-level 4F3/2 to 4I9/2 transition at 930 nm, with multi-watt output power and good efficiency and beam quality.