A gas detector comprising a launcher unit comprising a first set of light sources for emitting light beams in a mid-infrared wavelength range, from 3.5 μm to 20 μm, with one of the light beams having a different wavelength than at least one other of the light beams, a first light transmission network for bundling the light beams from the first set of light sources to produce a first bundled light beam , a second set of light sources for emitting light beams in an infrared wavelength range, from 0.8 μm to 3 μm, with one of the light beams having a different wavelength than at least one other of the light beams, a second light transmission network for bundling the light beams from the second set of light sources to produce a second bundled light beam , and a coupling unit for receiving and coupling the first and second bundled light beams to produce a single coupled light beam and for transmitting the coupled light beam towards a gas flow path ; a first window , arranged such that the coupled light beam is transmitted through the first window towards the gas flow path ; a second window ; a retro-reflector , arranged such that after interacting with the gaseous species in the gas flow path the coupled light beam emerges from the second window , is transmitted towards the retro-reflector , emerges from the retro-reflector after undergoing two or more reflections and travels through the gas flow path, before emerging from the first window ; and a receiver unit for receiving the coupled light beam emerging from the first window . a launcher unit comprising a first set of light sources for emitting light beams in a mid-infrared wavelength range, from 3.5 μm to 20 μm, with one of the light beams having a different wavelength than at least one other of the light beams, a first light transmission network for bundling the light beams from the first set of light sources to produce a first bundled light beam , a second set of light sources for emitting light beams in an infrared wavelength range, from 0.8 μm to 3 μm, with one of the light beams having a different wavelength than at least one other of the light beams, a second light transmission network for bundling the light beams from the second set of light sources to produce a second bundled light beam , and a coupling unit for receiving and coupling the first and second bundled light beams to produce a single coupled light beam and for transmitting the coupled light beam towards a gas flow path ; a first window , arranged such that the coupled light beam is transmitted through the first window towards the gas flow path ; a second window ; a retro-reflector , arranged such that after interacting with the gaseous species in the gas flow path the coupled light beam emerges from the second window , is transmitted towards the retro-reflector , emerges from the retro-reflector after undergoing two or more reflections and travels through the gas flow path, before emerging from the first window ; and a first window , arranged such that the coupled light beam is transmitted through the first window towards the gas flow path ; a second window ; a retro-reflector , arranged such that after interacting with the gaseous species in the gas flow path the coupled light beam emerges from the second window , is transmitted towards the retro-reflector , emerges from the retro-reflector after undergoing two or more reflections and travels through the gas flow path, before emerging from the first window ; and a receiver unit for receiving the coupled light beam emerging from the first window . <EOS>