Laser processing head and procedures to avoid damage to one optical fiber

Laser machining head (10) for machining a workpiece (16) by means of a laser beam (18), comprising: - an optical system (22, 28) for focusing the emerging from an optical fiber (14) laser beam (18) onto the workpiece (16), to determine a back reflection sensor device (12) having a radiation detector (36) which is adapted to the intensity of one of the workpiece (16) back-reflected laser beam (18), runs back which in the direction of the beam exit portion (26) of the optical fiber (14) and - this is true, and - receives a control device (44) which is connected to the back reflection sensor device (12) and an intensity signal from the back reflection sensor device (12), wherein the control device (44) is adapted to switch off the laser beam (18) when the (from the back reflection sensor device 12) exceeds certain and time-integrated intensity of a predetermined threshold laser energy, and wherein the back reflection sensor device (12) comprises a sensor optics (40) which is disposed (before the radiation detector 36) so that a surface of the workpiece (16) in the same manner the beam exit area (26) of the optical fiber (14) is projected as the sensor surface (38) of the radiation detector (36).