A method for optical and electrical signal processing of a multi-heterodyne signal generated by a multi-mode semi-conductor laser, for a system comprising two laser sources and an sample interaction unit. At least the beam of one of the laser passes through said sample interaction unit before being combined on a detector. The first laser is tuned (40=>42) by an amount keeping the tuning result within the available detector bandwidth (55). Then the second laser is roughly tuned by the same amount as the tuning of the first laser to bring back the signal to the vicinity (48) of the original place in the RF-domain and within the bandwidth (55) of the detector. The tuning steps are repeated with different value of mode spacing for reconstructing the sample spectrum and provide a high resolution image of the dip (41) absorption line (40).