Dynamics of colliding pulse passively semiconductor mode-locked ring lasers with an intra-cavity Mach-Zehnder modulator

The large number of applications in areas as optical communications, metrology and high-resolution imaging requires short stable and tunable optical pulses which can be generated by passively mode-locked lasers (PMLL). The pulse characteristics of PMLLs depend on the ratio of the saturation energies of the gain and of the saturable absorber (SA), of the available gain bandwidth and on the total cavity losses [1]. In addition, the pulse width can usually only be tuned by changing the bias current which does not leave the pulse energy invariant. In order to improve the performance of PMLLs several techniques were investigated which aim at engineering one of the crucial parameters controlling the pulse width. For instance, it was shown that the pulse width can be reduced by the increasing of field intensity profile in the SA [2] thereby modifying the effective saturation energies. On the other hand, the pulse duration could also in principle be reduced by extending the gain bandwidth as it would allow more modes to lase simultaneously. In [3] it was demonstrated that the gain bandwidth of a ring mode-locked laser can be significantly flattened by putting intra-cavity integrated frequency dependent filter. This filter was based on an asymmetrical Mach-Zehnder interferometer (MZI).