A system comprises a working laser beam, a sensing laser beam, first and second optical elements, an optical sensor, an aperture and a controller. The first optical element generates a coaxial beam from the working laser beam and the sensing laser beam. The second optical element focuses the coaxial beam onto a workpiece, such that the working laser beam machines the workpiece and the sensing laser beam reflects from the workpiece. The optical sensor senses an intensity of the reflected sensing beam. The aperture determines a focus position by translating along the reflected sensing beam, such that the reflected intensity is maximized. The controller determining a machining parameter of the working laser beam, based on the focus position.