  We describe the design and implementation of a stable high-power 1064 nm
laser system to generate optical lattices for experiments with ultracold
quantum gases. The system is based on a low-noise laser amplified by an array
of four heavily modified, high-power fiber amplifiers. The beam intensity is
stabilized and controlled with a nonlinear feedback loop. Using real-time
monitoring of the resulting optical lattice, we find the stability of the
lattice site positions to be well below the lattice spacing over the course of
hours. The position of the harmonic trap produced by the Gaussian envelope of
the lattice beams is stable to about one lattice spacing and the long-term
(six-month) relative RMS stability of the lattice spacing itself is 0.5%.
