We studied self-phasing dynamics in a Q-switched passively coupled two-gain-element fiber laser array. Simultaneous Q-switching and self mode-locking were observed in our experiments. Our data shows that phase-locking in our laser array was stable under perturbations. The phase-locked state was able to react to the phase perturbations in a time comparable to the rise time of the self mode-locked pulse. The establishment of the phase-locked state from the incoherent state was found to be shorter (or perhaps much shorter) than the rise time of the Q-switched pulse.
We studied coherent beam combining in a specific laser cavity architecture in which two Ytterbium-doped fiber
amplifiers are passively coupled using a homemade binary phase Dammann grating. Our experimental results show that
coherent beam combining is robust against phase perturbation in such a laser cavity architecture when the operating
point is sufficiently above the lasing threshold. We observed redistribution of energy within the supermode of this laser
cavity in response to an externally applied path length error. The energy redistribution is accompanied by an internal
differential phase shift between the coherently coupled gain arms. Self-phasing mitigates or even completely neutralizes
the externally applied optical path length error. We identify the physical origin of the observed self-phasing with the
resonant (gain related) nonlinearity in the gain elements under our experimental conditions.
A Yb-doped, dual-core, double-clad, polarization-maintaining fiber is used to demonstrate passive coherent beam combining. A homemade Dammann grating is employed as a passive beam-combining optical element. Self-phasing is observed in this laser system, where we attribute the self-phasing behavior to the Kramers-Kronig effect. We experimentally demonstrate the importance of polarization on coherent beam combining efficiency as well as on Kramers-Kronig induced self-phasing.
A procedure is described to accurately measure the self phase-tuning in a coupled fiber laser. A fiber is designed and fabricated to eliminate the effects of self-tuning from wavelength shifting, thermal expansion, and thermally induced index change, allowing us to study the phase effects produced by Kramers-Kronig phase shifting as a function of various cavity parameters. For sufficient pump power, we observe that the Kramers-Kronig effect is capable of compensating for all path length errors introduced into the cavity, resulting in efficient lasing under all path length conditions. We have directly measured the Kramers-Kronig-induced phase shift and present experimental evidence that this additional phase compensates for the applied phase error and promotes efficient lasing.
External cavity coherent beam combining represents a path forward to higher fiber laser radiance, with several groups demonstrating scalable approaches. In this paper, we review recent advances in coupled laser cavity design. In particular, we compare various designs and describe the pros and cons of each with regard to sensitivity to path length errors. Experimental measurements using a specially designed dual-core fiber demonstrate the modal loss from a superposition architecture. A second area of investigation is concerned with Q-switch suppression in coupled laser cavities. The increased cavity loss that accompanies path length errors in the laser arms can suppress lasing, causing an energy build-up in the laser inversion. When the path length errors are removed and the cavity resumes its low loss state, the stored energy can be released in a manner analogous to Q-switching, creating a giant laser pulse. Since the peak power of this pulse can be many orders of magnitude larger than the cw power, the high instantaneous intensity can cause irreparable damage to optical components. We investigate passive systems that are designed to suppress this unwanted Q-switching by allowing alternative lasing paths to clamp the gain.