Gas sensor comprising an enclosure, capable of receiving the gas, the sensor also comprising: • a light source, configured to emit a light wave that is propagated in the enclosure by forming, from the light source, an emission cone (Ω1), called first cone; • a measurement photodetector capable of detecting the light wave emitted by the light source and being propagated through the enclosure; the sensor being such that the enclosure extends between two transverse walls (21, 22), positioned facing one another, the transverse walls being linked to one another by a peripheral wall, extending between the transverse walls, the peripheral wall comprising: • a first reflecting surface, of rank 1, curved, of less than 0.7 eccentricity, and having a first focus (P1-1) and a second focus (P1-2), the first reflecting surface being positioned facing the light source; • at least one reflecting surface (Sn) of rank n, curved, n being an integer number strictly greater than 1, each surface of rank n having: - a first focus (P1- n), coinciding with the second focus of a reflecting surface of preceding rank n-1; - a second focus (P2- n), distinct from the first focus (P1- n), coinciding with the first focus of a surface of next rank n +1; • a last reflecting surface (S N), of rank N, curved, comprising a first focus (P1- N), coinciding with the second focus of a reflecting surface of preceding rank N -1, the last reflecting surface also comprising a second focus (P2- N); such that the light wave emitted by the light source is successively reflected by the reflecting surfaces, according to their respective rank, before converging towards the second focus (P2- N) of the last reflecting surface (S N) by forming a last light cone (Ω8, Ω10), the measurement photodetector being positioned, with respect to the last reflecting surface (S N) so as to extend into the last light cone. • a light source, configured to emit a light wave that is propagated in the enclosure by forming, from the light source, an emission cone (Ω1), called first cone; • a measurement photodetector capable of detecting the light wave emitted by the light source and being propagated through the enclosure; • a first reflecting surface, of rank 1, curved, of less than 0.7 eccentricity, and having a first focus (P1-1) and a second focus (P1-2), the first reflecting surface being positioned facing the light source; • at least one reflecting surface (Sn) of rank n, curved, n being an integer number strictly greater than 1, each surface of rank n having: - a first focus (P1- n), coinciding with the second focus of a reflecting surface of preceding rank n-1; - a second focus (P2- n), distinct from the first focus (P1- n), coinciding with the first focus of a surface of next rank n +1; - a first focus (P1- n), coinciding with the second focus of a reflecting surface of preceding rank n-1; - a second focus (P2- n), distinct from the first focus (P1- n), coinciding with the first focus of a surface of next rank n +1; • a last reflecting surface (S N), of rank N, curved, comprising a first focus (P1- N), coinciding with the second focus of a reflecting surface of preceding rank N -1, the last reflecting surface also comprising a second focus (P2- N);