Nd3+-doped phosphate glass waveguides for short length optical amplifier applications

Phosphate glasses have demonstrated in last years to be an alternative host material to silica glass, especially for laser and optical amplification applications. In fact, they can be doped with large amounts of rare earth (RE) ions (up to 1021 ions/cm3) without clustering, therefore enabling the realization of compact active devices with high gain per unit length. Moreover, phosphate glasses possess a large glass forming range, good thermo-mechanical and chemical properties, low nonlinear refractive index and no evidence of photodarkening even at high population inversion. The aim of this research was the realization of different configurations of Nd3+-doped phosphate glass waveguides, in particular a double cladding (DC) optical fibre and circular and D-shaped cladding rods, to be used in lasers and optical amplifiers, in a CW or pulsed regime. Three different phosphate glass compositions (for the core, inner cladding and outer cladding) were designed in order to obtain an adequate numerical aperture between different elements while maintaining similar thermo-mechanical properties in view of the drawing process. The glasses were prepared by conventional melt-quenching technique in a controlled atmosphere furnace at 1400 °C and then cast on pre-heated brass molds. Physical, thermal and spectroscopic properties of the manufactured glasses were measured. The waveguides realized for this research were manufactured by preform drawing with the preforms being obtained by rod-in-tube technique. The fabrication of each preform required one rod of core glass fabricated by melt quenching and one or two tubes shaped by rotational casting, using an in-house built equipment. Fiber drawing was then carried out using a drawing tower developed in-house. The preform and fibre fabrication parameters were set to generate a DC fibre with the following diameters: 8 μm, 35 μm and 105 μm for the core, inner and outer cladding, respectively. Concerning the rods, two distinct sets of samples with different core/cladding dimensions, were manufactured: 310/850 μm and 260/700 μm respectively. Fig 1 shows the cross section of the manufactured waveguides. Laser action at 1.054 μm was demonstrated by cladding pumping 9 cm of the fabricated optical fibre using a semiconductor pigtailed laser diode emitting at 793 nm. With the aim to realize a diode pumped power amplifier for femtosecond sources, preliminary measurements on the manufactured rods were accomplished.