A self mode locking laser and corresponding method is described. The laser comprises a resonator (2) terminated by first (3) and second (4) mirrors and folded by a third mirror (5). The third mirror comprises a reflector (15) surmounted by a multilayer semiconductor gain medium (16) that includes at least one quantum well layer and an optical Kerr lensing layer (20). A perturbator is also included that provides a means to induce a perturbation on an intensity of one or more cavity modes of the resonator. The pertubator is employed to induce a small perturbation on the intensity of the cavity modes of the resonator which is sufficient for the optical Kerr lensing layer to induce mode locking on the output field. The second mirror (4) comprises an intensity saturable mirror that provides a means for reducing the pulse widths of the generated output field e.g. to around 100 fs. A diamond heat spreader (20) is attached to the top of the half VCSEL gain medium (13) for improved cooling as well as representing the Kerr medium. Further folding mirrors (6-8) may be inserted to adjust the degree of astigmatism to be compensated for by the Kerr effect used for mode-locking.