Optical detector of a gas

The invention relates to a gas sensor comprising a planar mirror (2); a spherical mirror (4) facing the concave mirror plane (2), optical axis (6) orthogonal to the plane mirror, the distance (D) between the plane and spherical mirrors being equal to 0.75 times the radius of curvature the spherical mirror, within 10%; a transmitter / receiver (E) of radiation disposed at the intersection of the spherical mirror (4) and the optical axis (6); and a receiver / transmitter (R) of radiation arranged at the plane of mirror intersection point (2) and the optical axis (6).