A device for focusing a laser light beam, comprising: - an emission source of a laser light beam along an optical emission path; - first focusing means of said laser light beam in a focusing point located at a focusing distance; - means for adjusting the position of said focusing point relative to said device; - means for detecting said focusing distance active in feedback on said adjustment means, wherein said detection means detects a parameter characteristic of the light beam leaving said first focusing means and representative of said focusing distance; wherein said characteristic parameter is the wavefront radius of curvature of the light beam leaving said first focusing means, and wherein said detection means comprises: - a glass sheet of predetermined thickness and refraction index, arranged in said optical emission path downstream of said first focusing means and adapted to generate a reflected light beam that propagates along a reflected optical path inclined relative to said optical emission path, said reflected light beam being defined by a pair of light beams (12, 13) partly overlapped and defining an interference region comprising a plurality of interference fringes; - photodetector means arranged in said reflected optical path and adapted to generate a signal representative of the spatial frequency of the interference fringes defined by at least one portion of said interference region on an observation plane located in said reflected optical path, wherein said at least one portion of said interference region has a dimension, along a direction (x) of succession of the fringes, such that, for a predetermined range of the spatial frequency of the interference fringes, the integral function of the light intensity of said reflected light beam at said at least one portion of interference region is monotonic, wherein said photodetector means comprises two photodiodes (32, 33); - means for processing said signal so as to generate a signal indicative of said wavefront radius of curvature of the light beam. - an emission source of a laser light beam along an optical emission path; - first focusing means of said laser light beam in a focusing point located at a focusing distance; - means for adjusting the position of said focusing point relative to said device; - means for detecting said focusing distance active in feedback on said adjustment means, wherein said detection means detects a parameter characteristic of the light beam leaving said first focusing means and representative of said focusing distance; wherein said characteristic parameter is the wavefront radius of curvature of the light beam leaving said first focusing means, and wherein said detection means comprises: - a glass sheet of predetermined thickness and refraction index, arranged in said optical emission path downstream of said first focusing means and adapted to generate a reflected light beam that propagates along a reflected optical path inclined relative to said optical emission path, said reflected light beam being defined by a pair of light beams (12, 13) partly overlapped and defining an interference region comprising a plurality of interference fringes; - photodetector means arranged in said reflected optical path and adapted to generate a signal representative of the spatial frequency of the interference fringes defined by at least one portion of said interference region on an observation plane located in said reflected optical path, wherein said at least one portion of said interference region has a dimension, along a direction (x) of succession of the fringes, such that, for a predetermined range of the spatial frequency of the interference fringes, the integral function of the light intensity of said reflected light beam at said at least one portion of interference region is monotonic, wherein said photodetector means comprises two photodiodes (32, 33); - means for processing said signal so as to generate a signal indicative of said wavefront radius of curvature of the light beam.