Concentration effects in Ho-doped fiber lasers

This paper studies the lasing efficiency and dynamic properties of Ho-doped lasers based on the active fibers with different Ho-ions concentration. 5 lasers based on Ho-doped fibers with different active ions concentration: from 1.6-10¹⁹ cm^-3 up to 1.9-10²⁰ cm^-3 were studied. Laser cavity was closed by high-reflective Bragg grating at 2.05 μm and cleaved fiber end. Ho-doped fiber laser was pumped into the core by Yb-doped fiber laser emitting up to 12 W at the wavelength of 1.125 μm. For all holmium lasers the active fiber length was optimized to obtain the maximum output power. Results show that the decrease of the efficiency is caused by the increase of the cluster concentration. In this case a concentration of the ions relaxed to the ground state increases too. That leads to a growth of the signal reabsorption. For the laser with smallest ions concentration a decrease of the efficiency can be caused by the fiber lengthening and influence of the optical losses. Other concentration effect consists in the self pulsing of the laser when ions in the clusters play role of a saturable absorber.. For the laser based on the fiber with concentration of 5.4-10¹⁹ cm^-3 oscillation character depended on the pump power. Self-pulsed oscillation up to pump power of 4 W was observed. At the further increase of a pump power generation came nearer to a continuous mode. CW-oscillation for all levels of the pump power was detected for the laser based on the fiber with smallest active ions.