Accurate Measurement of the Linewidth Enhancement Factor of Semiconductor Lasers by a Simple Technique

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. The linewidth enhancement factor (LEF) is the quantity that describes the coupling of phase fluctuations to intensity fluctuations and has important consequences such as laser linewidth broadening and pulse chirping. A good knowledge of the LEF is essential to predict the behavior of a modulated laser. The measurement of the LEF is tricky and many methods have been proposed, based on optical injection/feedback, on small/large signal modulation, or on spectral measurements[3]. Here we propose a novel method[4] based on the complete optical field reconstruction technique[5]. This method is of rather simple implementation and requires common opto-electronic lab instruments, i.e. a Mach-Zehnder modulator (MZM), RF synthesizers, a RF phase shifter, a direct-modulation laser and an optical spectrum analyzer (OSA), see Fig. 1a. The technique consists in modulating the source and measuring the optical spectrum for various RF delays
$\tau$
, and allows retrieving iteratively the spectral phase.