An unbalanced Mach-Zehnder interferometer (UMZ) is used to convert microwave (MW) frequency modulation (FM) of a laser diode into intensity modulation (IM). In coherent interference regime, it is shown that parasite FM, induced by laser-phase noise and the MW-phase noise of the modulation source , causes negligible intensity noise but significant phase noise in the photodetected intensity at the output of the UMZ. Theoretical study is realized in linear- and nonlinearinterference regimes. Results are validated experimentally.