The invention relates to an apparatus 100 for
machining a workpiece with a
laser beam 101 coupled into a fluid jet. The apparatus 100 comprises a
laser unit 101a for providing the
laser beam 101, a
nozzle unit 102 with an aperture 102a for producing the fluid jet, and an optical unit 103 configured to provide the laser beam 101 from the laser unit 101a onto the
nozzle unit 102. Further, the apparatus 100 comprises a
control unit 104 configured to control 108, 110 the optical unit 103 and / or
nozzle unit 102 to change a point of incidence 109 of the laser beam 101 on the nozzle unit 102. The apparatus 100 also comprises a sensing unit 105 configured to sense
laser light 106 reflected from a surface 102b of the nozzle unit 102 and produce a sensing
signal 107 based on the sensed reflected
laser light 106. The
control unit 104 is particularly configured to evaluate the sensing
signal 107 and to determine a defined sensing pattern in the sensing
signal 107 indicative of the laser beam 101 being fully and / or partially aligned with the aperture 102a.