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 , 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 (S1), 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 (S1), 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 ); <EOS>