Abstract: A monostatic laser rangefinder device including: a laser light source; a photodetector; a double-clad optical fiber coupled to a collimating and focusing device; and an optical fiber diplexer. The diplexer includes a first optical fiber forming an input port and a second, multimode optical fiber forming a first output port. The first fiber is coupled to the laser source, the second fiber is coupled to the photodetector, and the double-clad fiber forms a second output port. A numerical aperture of a light guide formed of an inner and outer cladding of the double-clad fiber is less than or equal to 0.5, and greater than 0.1. The end of the double-clad fiber facing the collimating and focusing device has a face inclined with respect to an axis perpendicular to the axis of the fiber by an angle greater than or equal to half of the arcsine of the numerical aperture.