Polarization Dragging in Injected Lasers

The vectorial injection locking of a slave laser by a linearly polarized master laser is theoretically and experimentally investigated, taking the nature and the stability of the eigenstates of the slave laser into account. It is proved that the behavior of the polarization, intensity, and frequency of the slave laser can be described by four nonlinear coupled differential equations, for lasers in which population inversion remains quite constant. In particular, it is shown that the stability of the eigenstates of the slave laser plays a dramatic role in the response of this laser to injection. Isotropic slave lasers are shown to follow adiabatically the polarization of the master laser in the frequency locking range. Loss anisotropic slave lasers exhibit a specific Adler tongue behavior and can support the transfer of the polarization of the master laser only along their eigenstates. Phase anisotropic slave lasers are shown to exhibit two bistable or simultaneous Adler curves and to offer new possibilities of all-ojtlical command. in all of these cases, a good agreement is obtained between theory and experiment and the study of polarization throws light on the physics of injection locking.