sample pulse is injected this second injected pulse also causes a transfer of light to the other polarization and since it is precisely timed this additional signal is of the same phase than the signal caused by the first pulse and therefore strengthens the signal in the other polarization if the birefringent optical waveguide is sufficiently long a third fourth fifth etc pulses can all cause corresponding polarization coupling and the detected signal at the end of the birefringent optical waveguide includes the sum of each individual timed sample pulse signal contribution all being in phase henceforth one can detect the collective signal sum stemming from each individual pulse where any given one of the pulses did not necessarily yield a signal of a strength sufficient to be detected the sensitivity of the sensor is thus enhanced more particularly the injection can consists of a series of pulses each having at the entrance in the passage a concentration distribution given by f o z each pulse moves at a velocity v and diffuses taking the form f z t determined by the diffusion equation the so called mass balance equation at some stage the number of pulses simultaneously present in the passage can reach a maximum value m l vt l being the fiber length and the overall concentration distribution can read as where t is the time delay between each impulse that is the inverse of the injection frequency for the sake of simplicity we now consider the specific case of a gaussian initial pulse shape this makes the model analytically tractable while having no impact on the main conclusions the diffusion equation admits such a gaussian solution this means that as each of the injected pulse moves and diffuses it maintains its gaussian shape the distribution.