A distributed substance detection device, comprising: a distributed optical excitation source comprising: - a first optical fiber provided with a plurality of extraction regions , each extraction region being capable of extracting from the first optical fiber part of the light conveyed by the fiber; - a first light source capable of injecting into the first optical fiber a first light signal at an absorption wavelength of the substance to be detected; and - a control circuit capable of modulating the first light signal in intensity at an acoustic frequency; a distributed acoustic detector comprising: - a second optical fiber ; - a second light source capable of injecting a second light signal into the second optical fiber ; and - an optical detector capable of measuring a third backscattered light signal in the second optical fiber to detect and locate an acoustic vibration at the modulation frequency of the first light signal; and a support having the first and second optical fibers arranged therein, the support defining a tubular cavity extending along substantially the entire length of the first and second optical fibers, said cavity being capable of confining a sample of a fluid where the possible presence of said substance is desired to be detected, wherein said cavity comprises a plurality of nip or throat areas distributed all along its length. a distributed optical excitation source comprising: - a first optical fiber provided with a plurality of extraction regions , each extraction region being capable of extracting from the first optical fiber part of the light conveyed by the fiber; - a first light source capable of injecting into the first optical fiber a first light signal at an absorption wavelength of the substance to be detected; and - a control circuit capable of modulating the first light signal in intensity at an acoustic frequency; - a first optical fiber provided with a plurality of extraction regions , each extraction region being capable of extracting from the first optical fiber part of the light conveyed by the fiber; - a first light source capable of injecting into the first optical fiber a first light signal at an absorption wavelength of the substance to be detected; and - a control circuit capable of modulating the first light signal in intensity at an acoustic frequency; a distributed acoustic detector comprising: - a second optical fiber ; - a second light source capable of injecting a second light signal into the second optical fiber ; and - an optical detector capable of measuring a third backscattered light signal in the second optical fiber to detect and locate an acoustic vibration at the modulation frequency of the first light signal; and - a second optical fiber ; - a second light source capable of injecting a second light signal into the second optical fiber ; and - an optical detector capable of measuring a third backscattered light signal in the second optical fiber to detect and locate an acoustic vibration at the modulation frequency of the first light signal; and a support having the first and second optical fibers arranged therein, the support defining a tubular cavity extending along substantially the entire length of the first and second optical fibers, said cavity being capable of confining a sample of a fluid where the possible presence of said substance is desired to be detected, wherein said cavity comprises a plurality of nip or throat areas distributed all along its length. <EOS>