Passive Mode-Locking of Lasers induced by Cross Polarization Gain Modulation

We report on a novel approach for inducing passive mode-locking of
lasers without using any saturable absorber but exploiting the polarization
degree of freedom of light. In our scheme, passive mode-locking is
achieved by crossed-polarization gain modulation caused by the reinjection
of a polarization-rotated replica of the laser output after a time
delay. In this situation, the laser amplifier plays a twofold role,
i.e., to provide amplification and the nonlinearities for pulse shortening.
It is shown that the reinjection time delay defines resonance tongues
that correspond to mode-locking operation. Numerical continuation
reveals that the cw solution is destabilized through a Hopf bifurcation
that defines the onset of multimode operation which evolves sharply
into a mode-locked solution. The method is general and can be applied
to a large variety of laser systems. For vertical-cavity surface-emitting
lasers, in the frame work of a generic model, we demonstrate stable
mode-locked pulses at repetition rates in the GHz range and pulsewidths
of few tens of picoseconds. We also show that VCSELs specific features
like anisotropies and spin-flip processes do not affect significantly
the mode locking regime.