Gas sensor comprising an enclosure able to receive a gas (G), the enclosure comprising a sidewall that extends, about a transverse axis (Z), between a first wall and a second wall , the sensor also comprising: - a light source configured to emit a light wave that propagates through the enclosure and forms, from the light source, a first cone of light (Ω 1 ); - a measuring photodetector able to detect a light wave emitted by the light source and propagating through the enclosure; the sensor being such that the first wall and the second wall each comprise at least one reflective surface, forming a portion of an ellipsoid of revolution, with each reflective surface being associated with a rank n, n being an integer higher than or equal to 1, such that the sensor comprises: - a first reflective surface of rank 1 (S 1 ), having a first focal point (P1-1) and a second focal point (P1-2), the first reflective surface being placed facing the light source , the first reflective surface forming a portion of an ellipsoid of revolution associated with an ellipse the eccentricity of which is lower than or equal to 0.7; - at least one reflective surface ( S n ) of rank n strictly higher than 1, each reflective surface of rank n having: • a first focal point (P1- n ), coincident with the second focal point (P2- n - 1) of a reflective surface ( S n-1 ) of preceding rank n -1; • a second focal point (P2- n ), distinct from the first focal point (P1- n) and coincident with the first focal point of a reflective surface (S n + 1 ) of following rank n +1; - a last reflective surface ( S N ) of rank N, comprising a first focal point (P1- N) coincident with the second focal point of a reflective surface of preceding rank ( S N - 1 ), the last reflective surface also comprising a second focal point (P2- N ); such that the light wave emitted by the light source is successively reflected by the N ellipsoidal-of-revolution reflective surfaces, according to their respective rank, before converging towards the second focal point (P2- N ) of the last reflective surface ( S N ) in such a way as to form a last cone of light ( Ω N ), the measuring photodetector being placed so as to lie in the last cone of light. - a light source configured to emit a light wave that propagates through the enclosure and forms, from the light source, a first cone of light (Ω 1 ); - a measuring photodetector able to detect a light wave emitted by the light source and propagating through the enclosure; - a first reflective surface of rank 1 (S 1 ), having a first focal point (P1-1) and a second focal point (P1-2), the first reflective surface being placed facing the light source , the first reflective surface forming a portion of an ellipsoid of revolution associated with an ellipse the eccentricity of which is lower than or equal to 0.7; - at least one reflective surface ( S n ) of rank n strictly higher than 1, each reflective surface of rank n having: • a first focal point (P1- n ), coincident with the second focal point (P2- n - 1) of a reflective surface ( S n-1 ) of preceding rank n -1; • a second focal point (P2- n ), distinct from the first focal point (P1- n) and coincident with the first focal point of a reflective surface (S n + 1 ) of following rank n +1; • a first focal point (P1- n ), coincident with the second focal point (P2- n - 1) of a reflective surface ( S n-1 ) of preceding rank n -1; • a second focal point (P2- n ), distinct from the first focal point (P1- n) and coincident with the first focal point of a reflective surface (S n + 1 ) of following rank n +1; - a last reflective surface ( S N ) of rank N, comprising a first focal point (P1- N) coincident with the second focal point of a reflective surface of preceding rank ( S N - 1 ), the last reflective surface also comprising a second focal point (P2- N ); <EOS>