Coherence characterization of multimode VCSELs via speckle contrast under optical feedback and bias variation

Optical feedback effects on the nonlinear dynamics of vertical-cavity surface-emitting lasers (VCSELs) have been extensively investigated; however, the effects of feedback on the spatial coherence properties of VCSEL emission have received less attention. In this work, we carry out an experimental quantitative analysis of the coherece characteristics of a multimode VCSEL using the speckle technique. Measurements are independently performed for each linear polarization component of the emitted beam, as well as for the total output. We find that optical feedback does not significantly reduce speckle contrast compared to the solitary laser case, but on the contrary, it can cause a pronounced increase. This increase is interpreted in terms of a feedback-induced coupling mechanism that can phase-lock transverse modes that are, in the solitary laser, only partially locked. This effectively reduces mode competition and increases spatial coherence, thereby enhancing the speckle contrast.