In this paper, a technique based on the use of a Mach–Zehnder (MZ) interfero-meter is proposed to evaluate chirp properties, as well as the linewidth enhancement factor (alpha-H factor) of optoelectronic devices. When the device is modulated, this experimental setup allows the extraction of the component’s response of amplitude modulation (AM) and frequency modulation (FM) that can be used to obtain the value of the alpha-H factor. As compared with other techniques, the proposed method gives also the sign of the alpha-H factor without requiring any fitting parameters and, thus, is a reliable tool, which can be used for the characterization of high-speed properties of semiconductor diode lasers and electro-absorption modulators. A comparison with the widely accepted fiber transfer function method is also performed with very good agreement.