Disclosed is a VCSEL (128) comprising a first and a second reflecting element separated by an optical path length to form an optical resonator cavity, a first actuator means mechanically coupled with one or both of said first and second reflecting elements, said first actuator means being configured to modulate the optical path length between said first and second reflecting elements by a modulation amplitude, and a waveguide, in particular an optical fiber, coupled with one of the reflecting elements for coupling light out of the resonator cavity, wherein the surface of at least one of the first and second reflecting elements has a concave recess (118) formed therein.