A high-power laser system includes a laser master oscillator, a plurality of fiber laser amplifiers producing intermediate output beamlets, a combiner for combining the intermediate beamlets into a combined output beam, and a piston error controller for minimizing errors related to beam combination that may degrade the quality of the combined output beam. A piston error controller processes a sample of the combined output beam using a Diffractive Optical Element to isolate a signal representing the total piston error of the combined beam. The controller uses amplitude modulation based on Hadamard code words to tag each non-reference intermediate beamlet with a unique code sequence orthogonal to those used for the other beamlets. For each intermediate beamlet, the associated piston error contribution is recovered using a Hadamard decoder. A very small phase dither is also introduced to allow the sign or direction of the piston error to be recovered. The decoded piston error contribution is processed by a cascaded product detector unit to derive a piston error control signal, which is provided to a phase modulator to thereby adjust the phase of a beamlet and minimize the piston error contributed thereby.