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, wherein 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 if there is no movement or the movement is below a first threshold value then a signal is triggered and/or the laser radiation is turned off or its output is reduced, and wherein the turning off of the laser radiation or its reducing is controlled in dependency of at least one signal change and/or a second threshold and/or a signal change relative to the second threshold determined during at least one course of time starting before and including the entry of the light guide into the canal.
