Relative intensity noise and modulation distortion characteristics of vertical-cavity surface-emitting laser diodes

Vertical-Cavity Surface-Emitting Laser (VCSEL) diodes are seen as effective light sources for future optical-fiber communication and optical signal processing applications. Essential to these applications, however, is the device's ability to yield a low-distortion and adequately broad band intensity modulation response. Our recent experimental investigations conducted on a number of different circular index-guided VCSELs have indicated that the presence of higher-order cavity modes, even for devices exhibiting reasonably good side-mode suppression ratios (for example, 30 dB below the main longitudinal mode), can modify shape of the modulation response and often leads to harmonic distortion even at relatively low modulation signal levels. WE attribute these effects to the cavity mode interaction that is associated with the optical gain non linearity. This mode interaction results in signal (or noise) beating effects. This is also referred to as the spectral down-or up-conversion process or mixing. In order to investigate the influence of these beat effects on the intrinsic modulation response, 'parasitic-free' relative intensity noise (RIN) characteristics for different medium-power VCSELs (6-9μm diameter range) have been measured. An analysis of the VCSEL intensity modulation response intended to explain the observed signal (noise) beating effects has been performed using multimode rate equations which include the Langevin noise sources. Both the models developed to analyze the above results will be discussed in the presentation.