Mode locking and temporal cavity solitons

The study of semiconductor lasers operating in the mode-locked regime has undergone considerable developement during the last years, both in extended cavity systems and in monolithic systems. Most of the time these systems (which have phase symmetry) are based on the presence of a saturable absorber and the pulses resulting from mode locking are then described in terms of dissipative solitons.

In parallel, the study of spatial localized structures in optics also called "cavity solitons" has reached some degree of maturity, switching in very few years from model systems (sodium vapor, liquid crystal light valves) to fast and micron-scale systems such as semiconductor microcavities with optical injection. Thus, it is nowadays possible to nucleate these "dissipative spatial solitons" just as pixels (independent of each other) on a nanosecond time scale or to move them in space at speeds reaching several micrometers per nanosecond. However, in spite of these achievements, these spatial solitons remain confined only in the transverse plane, orthogonal to the light propagation, and are most of the time described in the uniform field limit along the propagation direction. These structures are therefore localized essentially with respect to the diffraction phenomenon.

In a remarkably complementary way, temporal cavity solitons have recently been observed in an optical fiber ring cavity under the application of a coherent beam. Just like in the transverse case, the structures which have been reported are bistable pulses independent of each other, individually controllable. Each pulse propagating inside a ring cavity, it is of course repeated after a period corresponding to the time it needs to travel the whole cavity. The analogy with mode locked lasers is striking, but one has to underline that contrary to passively mode locked laser this system does not have phase symmetry, and incidentally does not have any kind of "laser" gain. These temporal cavity solitons are then detected as trains of pulses all identical to each other and separated by arbitrary time intervals, each train of pulses being repeated at the period given by the optical cavity length. In the temporal case cavity solitons are therefore localized with respect to the dispersion phenomenon.

In this project, we will demonstrate the existence of temporal cavity solitons in a semiconductor ring cavity and relate them to the mode locking phenomenon in lasers. We will make use of the advances realized in fiber systems and we will deploy them in semiconductor systems, which have already shown their potential in the "transverse" domain and whose robustness is an obvious advantage for eventual practical applications. One of the benefits of the system which is envisioned for this project is to use (just as succesfully as it was done for transverse cavity soliton) the capability of semiconductor material to amplify light via stimulated emission of radiation, strongly reducing the optical input power required for nonlinear localization to take place.