Process and device for laser working with remote control
FIELD OF THE INVENTION 
     The present invention relates to laser welding in tubes of small diameter such as the steam generator tubes of pressurized water nuclear reactors. 
     BACKGROUND OF THE INVENTION 
     In certain laser workings, on such as welding within steam generator tubes, it is not possible to observe the working zone directly. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a technique permitting the monitoring of the correct progress of the laser welding, in spite of this difficulty. 
     To this end, the subject of the invention is a process for laser welding in which a laser beam is carried to the vicinity of a working point, on which it is focused, and spectral analysis of the light emitted at the working point during the laser welding is carried out. 
     The analysis may be carried out continuously and/or in real time. 
     The subject of the invention is also a device for laser welding, which is intended for carrying out such a process. This device, of the type comprising a laser, means for conducting the laser beam and means for focusing the beam on a working point, comprises a dichroic mirror interposed in the path of the laser beam, which mirror is transparent to the laser beam and reflects other wavelengths, and a spectrometer receiving the light reflected by the mirror. 
     When said conducting means comprise an optical fiber, the dichroic mirror is mounted upstream of such optical fiber. 
     The spectrometer is adapted to filter the light rays emanating from the flash lamp of the laser; 
     The spectrometer is of the interferometer type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative embodiment of the invention will now be described with reference to the accompanying drawing, the single figure of which shows schematically a device for laser welding according to the invention. 
     DETAILED DESCRIPTION 
     The device shown in the drawing essentially comprises a power laser 1 of the YAG type, emitting a laser beam 2 of wavelength 1.06 micrometer, a working head 3 of cylindrical general form, and a spectrometer 4. This device is intended to carry out a circular weld within a tube of small diameter (not shown), for example for the purpose of fixing thereon an internal repair sleeve (not shown). 
     The laser beam passes through a dichroic mirror 5, inclined at 45°, which is transparent to he wavelength of the beam and reflecting for the other wavelength. It is then focused by a lens 6 onto the upstream end of an optical fiber 7, which conducts it into the head 3. 
     At the exit of the optical fiber, the beam is collimated by a lens 8, and then focused by a lens 9 onto the working point 10 (forming part of the wall of the tube in this example) after reflection on a mirror 11 inclined at 45° and passage through a lateral opening 12 of the working head. 
     In operation, the working point 10 emits a light during the welding. This light follows the path of the laser beam in the opposite direction, as far as the mirror 5, which reflects it towards the spectrometer 4, where it undergoes an analysis continuously and in real time. 
     If the welding is effective at the point 10, the spectrometer detects the ionization lines of the protective welding gas as well as the lines corresponding to the metal vapors of the base metal vaporized during welding, with certain amplitude ratios between these lines. By comparison with a standard spectrum corresponding to a satisfactory weld, it is thus possible to monitor in real time the correct progress of the welding. 
     It should be noted that, in certain cases the spectrometer may additionally receive a light spectrum originating from the flash lamps of the laser 1. In this case, this parasitic spectrum, which is stable and well reproducible, is stored in memory in the spectrometer, and is subtracted from the measured spectrum. 
     The mirror 5 must, of course, be chosen in such a manner as to reflect the entire light spectrum emitted by the point 10. 
     Preferably, the spectrometer employed is of the interferometer type. 
     The process described hereinabove may be employed for monitoring, in real time, any process of laser working including heat treatment, cutting, machining, cleaning, etc.