The high-frequency modulation of semiconductor laser's pump current is used in several important applications, such as high-speed data telecommunications[1], direct-modulation optoelectronic oscillators[2], optical sampling with gain-switched regimes and so forth. linewidth enhancement factor (LEF) the quantity that describes coupling phase fluctuations to intensity has consequences laser broadening pulse chirping. A good knowledge LEF essential predict behavior a modulated laser. measurement tricky many methods have been proposed, based on injection/feedback, small/large signal modulation, or spectral measurements[3]. Here we propose novel method[4] complete field reconstruction technique[5]. This method rather simple implementation requires common opto-electronic lab instruments, i.e. Mach-Zehnder modulator (MZM), RF synthesizers, shifter, an spectrum analyzer (OSA), see Fig. 1a. technique consists modulating source measuring for various delays <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\tau$</tex> , allows retrieving iteratively phase.