Square-wave emission in vertical-cavity surface-emitting lasers

This contribution numerically and experimentally illustrates how regular and robust square wave emission can be obtained in Vertical Cavity Surface Emitting Lasers (VCSELs) using the polarisation degrees of freedom of these devices. The scheme propose is based on crossed-polarization re-injection (XPR). In such scheme, the emission from the laser is split into its two linearly polarized components, LP-x and LP-y, and only one of these components (LP-x, say) is fed back into the laser after being rotated into the orthogonal polarization direction (LP-y). In this way, cross-saturation of the polarization states modifies the photon density in each LP component and may eventually lead to square-wave modulation of the power emitted in each of the LP components with a period close to 2τr. When the dichroism in the device is strong enough and the emission occurs on a single polarization mode, XPR above a critical level leads to square-wave modulation which is stable over large ranges of bias currents. The emission dynamics for different levels and delays of XPR and PSF (polarization-selective optical feedback) were experimentally characterized. An asymmetry in the effects of PSF for the two polarizations is found. The experimental observations are satisfactorily explained within the Spin-Flip Model supplemented with appropriate terms describing XPR and PSF.