The invention relates to a method and an arrangement for the cleaning of a circumferentially closed canal by means of a light guide conducting a laser beam. The entry of the laser beam into the light guide is interrupted when the free end of the light guide is outside of the canal and/or the movement of the light guide within the canal is monitored and that if there is no movement or the movement is below a threshold value then a signal is triggered and/or the laser radiation is turned off or its output is reduced.