Supermode stabilization in FM harmonically mode-locked Ti:Er:LiNbO 3 Fabry-Perot waveguide lasers

Mode locked Ti:Er:LiNbO₃ waveguide lasers have attracted considerable attention as sources of short optical pulses at high repetition frequencies. Due to the long cavity, the typical FSR is about 1 GHz, thus the device must be operated in harmonic mode locking regime. This implies stability problems due to supermode competition: supermode coupling is due to longitudinal spatial hole burning effects: but even in absence of these effects the spontaneous emission noise is sufficient to cause jumps between supermodes, due to the intrinsic instability of the dynamic system and to the large gain bandwidth of the active medium. We theoretically show that, in order to achieve stable long-term operation, it is necessary both to suppress longitudinal spatial effects and to create sufficient gain difference between the supermodes of the cavity, for instance by putting a Fabry-Perot etalon in the laser cavity