Centre de recherche sur les Ions, les MAtériaux et la Photonique (UMR 6252)

Absorption and emission spectra of a Nd-dopded fiber

The main objective is the power scaling of Nd-doped fiber lasers in continuous and pulsed regime. The major problem to achieve high laser efficiency on the 4F3/2-4I9/2 transition of Nd3+ is related to the strong parasitic laser or amplified stimulated emission (ASE) at 1060 nm that normally prevents laser emission near 910 nm. However, we have shown that efficient laser operation near 910 nm may be achieved by increasing the core/clad surface ratio. Indeed, the high pump fluence results in a strong increase of the population inversion and gain difference between the two transitions can be small enough to allow laser emission at 910 nm. The large overlap of pump modes with the doped large mode area (LMA) core also permits to reduce the Nd3+ concentration and to avoid ions clustering effect which may strongly decrease the laser efficiency. A numerical modeling of the competition between the two laser transitions was developed and it was anticipated that cladding-to-core area ratio lower than 16 was necessary to obtain efficient laser operation at 910 nm. As a matter of fact, 25 W of output power was achieved at 910 nm using a Nd-doped fiber with a 20 um core diameter and a 80 um clad diameter.

New fibers with an inner cladding reduced to 60 um were recently developed to optimize the laser performance below 900nm and improve the spatial quality of the output beam. We developed a fiber laser source which was tunable in wavelength by means of a volume Bragg grating (VBG). A high-power laser tunable over the spectral range 872 – 936 nm was demonstrated for the first time in a silica optical fiber. The laser source delivered up to 22 W of output power with a spectral width of 0.035 nm and a laser conversion efficiency close to 47%.