Interaction of transverse modes in a single-frequency few-mode fiber amplifier caused by local gain saturation

Polarization-resolved mode evolution in TMI-limited Yb-doped fiber amplifiers using a novel high-speed Stokes polarimeter

In this work we have developed a high-speed Stokes polarimeter method based on simultaneous 4-channel imaging with a high-speed camera. Thus, current speed limitations of imaging polarimeters for wavelengths around 1 µm can be overcome, allowing a sub-ms polarization-resolved characterization of transverse mode instability (TMI). Additionally, the Stokes parameters of each individual mode are calculated by a simultaneous 4-beam mode reconstruction algorithm during post-processing and can be analyzed with unprecedented temporal resolution. We demonstrate the measurement capabilities of this polarimeter setup by characterizing TMI of a large-mode-area Yb-doped polarization maintaining (PM) fiber amplifier with 30 kHz video frame rate. Upon thorough characterization, we have found for the first time that at the onset of TMI in a PM fiber, the modal polarization states begin to oscillate on circular and elliptical trajectories at the same frequencies as the modal energy transfer occurs. The ability to measure the modal polarization states with sub-ms temporal resolution is key to developing a fundamental understanding and subsequently possible mitigation strategies of TMI in PM-fiber lasers.

Modal decomposition of an incoherent combined laser beam based on the combination of residual networks and a stochastic parallel gradient descent algorithm

With the increase of the superimposed eigenmodes number, the traditional numerical modal decomposition (MD) technique will inevitably suffer from ambiguity and local minima problems and thus is typically unsuitable for conducting modal decomposition of an incoherent combined laser beam. In this paper, we propose a novel, to the best of our knowledge, MD algorithm, named ResNet-SPGD, which combines the advantages of residual networks (ResNet) and stochastic parallel gradient descent (SPGD) algorithm. Via setting the modal mode coefficients obtained from the CNN model as the initial value of the SPGD algorithm, such algorithm shows an attractive solution to mitigate the problem of modal ambiguity. The proposed algorithm is preliminarily applied to the modal decomposition of an incoherent combined laser beam, and the feasibility is demonstrated via numerical simulations. Complete MD is performed with high accuracy, and the only cost is the sacrifice of some real-time capacity.

500 W level MOPA laser with switchable output modes based on active control

We report a high power transverse-mode-switchable fiber laser in a master oscillator power amplifier (MOPA) configuration. The output modes of a few-mode fiber amplifier can be actively controlled by the input polarization state of the fundamental mode seed laser using SPGD algorithm. A fast, stable and safety mode switching between LP01 and LP11 modes is achieved in the amplifier at output power of 500 W level.

Polarization-dependent transverse mode selection in an Yb-doped fiber laser

A simple technique for selectively generating the fundamental LP01 and the next higher order donut-shaped LP11 mode with vortex phase front in a cladding-pumped few-moded ytterbium-doped fiber laser is reported. Our approach exploits the difference in polarization behavior of individual transverse modes due to transverse variation of birefringence in a few-mode fiber allowing robust mode discrimination through the use of an appropriately aligned intracavity polarizing element. The laser yielded ∼36  W of output power for both transverse modes with corresponding slope efficiency of 74% with respect to absorbed pump power and a mode purity of 98% for the donut-shaped LP11 mode.

Emission polarization of random lasers in organic dye solutions

This Letter presents a polarimetric study of the emission of random lasers from organic dyes. Coherent lasing modes from samples with ethanol solvent showed a high degree of polarization and did not influence each other in polarization. The proper choice of a laser dye with asymmetric absorption momenta, a highly viscous solvent, and a linear pump polarization can cause the random lasing emission to be completely linearly polarized for all wavelengths within the amplification range.

General analysis of the mode interaction in multimode active fiber

General analysis of the mode interaction in multimode fiber is presented in this paper. By taking local gain into consideration, the general coupled mode equations in multimode active fiber are deduced in the model and the effect of various factors can be analyzed based on the general coupled mode equations. Analytical expression of the beam quality factor is deduced for the optical field emerging from the multimode active fiber. The evolution of the mode power and M2 factor along the fiber are analyzed by numerical evaluations.

Optically induced long-period fiber gratings for guided mode conversion in few-mode fibers

We propose and demonstrate guided mode conversion by an optically induced long-period fiber grating (OLPG). The beating of two guided modes excited by a nanosecond pulsed laser beam is used to induce a refractive index grating via the Kerr effect. This grating converts a counter-propagating continuous-wave beam from the LP(01) to the LP(11) mode. Numerical simulations using a beam propagation method show that a full conversion is possible. Experimentally, a mode conversion with an efficiency of about 50% is firstly demonstrated.

Modeling of transverse mode interaction in large-mode-area fiber amplifiers

We model the transverse mode interaction in a large-mode-area fiber amplifier by solving the Fresnel wave equation including local gain saturation. In order to calculate the electric field distribution we apply a finite difference beam propagation method, which is followed by the derivation of the modal powers and modal polarization states. A polarization dependent mode amplification is found that is in good agreement with recent experimental results.