Measuring the modulation properties of Injection-Locked VCSELs using coherent optical demodulation

Abstract

Vertical Cavity Surface Emitting Lasers (VCSELs) have many desirable properties as directly modulated laser sources in high speed data, short reach communications. The improvements in size, weight and power by avoiding external modulators make VCSELS attractive for an avionics platform. VCSELs are used as directly modulated lasers in two regimes, free-running or injection-locked to a master laser. With free-running VCSELs at 1.55 μm, Amplitude Modulation (AM) and Phase Modulation (PM) were both measured by performing phase retrieval on sonograms [1]. Injection-locking significantly changes the modulation properties of VCSELs, increasing the modulation bandwidths and reducing noise, for example [2]. Previous research has focused on AM; PM characteristics at high frequencies have not been well studied because independently measuring AM and PM at GHz modulation rates is challenging. The PM properties of Injection-Locked (I.L.) VCSELs at GHz rates are of central importance because it is used as a phase modulation element in a linear interferometric modulator [3]. Here we use a coherent optical demodulation technique to measure the complete electric field in real-time from a modulated I.L. VCSEL. Modulation dynamics can be studied at GHz rates, including non-linear phenomena like the theoretically predicted arcsine shape of the PM. Based on balanced photodiodes and oscilloscopes available today, the technique can be applied to measure modulations beyond 25 GHz.