A fiber block is configured with a fiber block including a Nd-doped active fiber and a pump-light delivery fiber which has a stretch extending along the active fiber in a side-to-side configuration so as to lunch pump light into the Nd-doped core of the active fiber. The core of the active fiber is surrounded by at least one or more claddings which, like the core, have a double bottleneck cross-section with a relatively large-area central region and relatively small input and output regions. The pump light delivery fiber is structured to have a substantially dumbbell cross-section with a relatively small-area central region coextending with the central region of the active fibers. The active fiber is dimensioned so that the overall length of the active fiber is configured to provide for the maximal amplification of the laser signal in a 900 nm range while limiting amplification in the 1060 nm range to the preset threshold. The fiber block is further configured so as to have the major fraction of the unabsorbe
