Abstract: A device for non contact detection of low concentration substances outside a laboratory environment is disclosed. A probing laser emission is split into two linear orthogonally polarized emission components and one component is delayed in time relative to the other. Both components are directed to a focal region that is proximate to a target medium thought to contain the substance to be detected. Between the arrival of the first and second emission components, an excitation light pulse at a wavelength corresponding to an absorption line in the spectrum of the substance is directed to the focal region. If vapours of the substance are present, they will be heated by the excitation pulse and will change the index of refraction of the focal region before the second emission component passes through it, thus altering the phase of the back-scattered emission returns. The device delays the first returned component by an equal delay and coherently couples the returned emission components.