Hung Sheng Chiang

Direct observation of Kramers-Kronig self-phasing in coherently combined fiber lasers

A highly stable coherent beam-combining system has been designed to measure self-phasing in fiber lasers due to nonlinear effects. Whereas self-phasing in previous coherent combination experiments has been principally attributed to wavelength shifting, these wavelength effects have been efficiently suppressed in our experiment by using a dual-core fiber with closely balanced optical path lengths. The self-phasing from nonlinear effects could then be measured independently and directly by common-path interferometry with a probe laser. The Kramers-Kronig effect in the fiber gain media was observed to induce a phase shift that effectively canceled the applied path length errors, resulting in efficient lasing under all phase conditions. This process was demonstrated to result in robust lasing over a large range of pump conditions.

Experimental measurements of the origin of self-phasing in passively coupled fiber lasers.

We have directly measured the intensity distribution, gain, and induced phase shift between two fiber lasers that are coherently combined by a Dammann grating. The induced phase shift between the lasers has been shown to approximately cancel out any applied phase error introduced into the cavity, allowing the combined resonator to operate at an efficient low-loss state. We show that the origin of this self-phasing stems from a redistribution of power between the two lasers. The resulting difference in circulating intensity produces a differential change in saturated gain, which in turn produces a differential Kramers-Kronig phase shift that effectively cancels the applied phase error.

Nonlinear effects in coherently coupled laser resonators

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.

Investigation of self-phasing dynamics in a Q-switched passively coupled two-gain-element fiber laser array

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.

The physical origin of Kramers-Kronig self-phasing in coherent laser beam combination

The Kramers-Kronig self-phasing observed in coherently coupled fiber laser arrays is experimentally shown to originate from a change in the supermode intensity distribution. The conditions that lead to accurate self-phasing are modeled and experimentally confirmed.

Uncovering the physical origin of self-phasing in coupled fiber lasers

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.

The effect of polarization in passive coherent beam combining of fiber lasers

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.

Passive coherent beam combining of fiber lasers: Accurate measurements of phase error tolerance

A cladding-pumped dual core ytterbium fiber laser is designed and fabricated to measure phase error tolerance in passive coherent beam combining. We show quantitative measurements on the effects of longitudinal modes on phase error tolerance.

Recent advances in coupled laser cavity design

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.

Experimental Measurements of Self-Phasing Due to Nonlinear Effects in Passively Coupled Fiber Lasers

Nonlinear effects are measured in passively coherently combined fiber lasers. We show that these effects can completely compensate for random fiber path length errors and promote robust lasing under many conditions.