An interferometric device for characterising a medium (24), comprising means (2, 6, 8, 12, 14) for generating a radiation pulse train at a repetition rate higher than 10 Hz, means for aiming a pump beam (28) at the medium to be measured which responds by producing a signal beam (32), sensing means (26) for receiving the signal beam, means (14) for aiming a probe beam (30) at the sensing means (26), means (34) for sampling an incident signal on the sensor (26) resulting from the interference between the signal beam (32) from the medium and the probe beam (30), and means (44-1, 44-2, ...) for varying the time at which the sampling means are actuated.