Haibin Lü

Real-time mode decomposition for few-mode fiber based on numerical method

Today a specific attention has been paid to look into the modal characteristics of the high-power laser beam. And the instantaneous monitoring and analyzing on modal content via the mode decomposition technique will provide a novel route. We implement the first-ever experimental investigation on the real-time mode decomposition technique for few-mode laser beam based on stochastic parallel gradient descent algorithm. It will reduce the cost and the complexity of the mode decomposition system. We have succeeded to decompose the mode spectra as well as calculating the beam quality factor at about 9 Hz monitoring rate, while the high agreement between the measured and reconstructed intensity profiles in each frame indicating the high accuracy and stability during the process. By employing a fiber-squeezing-based polarization controller, the modal content under test can be time-varying automatically.

Fast and accurate modal decomposition of multimode fiber based on stochastic parallel gradient descent algorithm

A new approach for the complete modal decomposition of the optical fields emerging from the multimode fiber is presented in this paper. Based on the stochastic parallel gradient descent algorithm, mode coefficients for all the bound modes in the multimode fiber can be exactly calculated by utilizing one intensity profile of the beam. Numerical simulation validates the feasibility, and the reconstructed error is below 0.1%. In the case of six modes within the fiber, the running time is about 2 s.

Adaptive mode control of a few-mode fiber by real-time mode decomposition

A novel approach to adaptively control the beam profile in a few-mode fiber is experimentally demonstrated. We stress the fiber through an electric-controlled polarization controller, whose driven voltage depends on the current and target modal content difference obtained with the real-time mode decomposition. We have achieved selective excitations of LP01 and LP11 modes, as well as significant improvement of the beam quality factor, which may play crucial roles for high-power fiber lasers, fiber based telecommunication systems and other fundamental researches and applications.

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.

Modal Analysis of Fiber Laser Beam by Using Stochastic Parallel Gradient Descent Algorithm

The ability of modal decomposition based on the analysis of the near-field intensity profile using the stochastic parallel gradient descent algorithm is experimentally characterized in detail for the first time. The results show that the decomposition method proposed in this letter turns to be highly accurate and rapid. Further, this method is applied to analyze the mode behavior in dependence on the pump power in a high-power fiber amplifier with different initial mode compositions.

Space-propagation model of Tm-doped fiber laser.

In this paper, we propose the space-propagation model for the Tm-doped fiber laser. This model builds the space-propagation equations for the population densities at different energy levels as well as the pump and laser powers. Compared to the conventional models, this model has significant advantage in reducing the computing time significantly when the steady-state population density rate equations cannot be solved analytically. On the basis of the model, the power characteristic and optimization for the Tm-doped fiber laser are investigated. Excellent agreements are achieved between the numerical simulation and experimental results.

Theoretical and Numerical Study of the Threshold of Stimulated Brillouin Scattering in Multimode fibers

In this paper, we have studied theoretically and numerically the thresholds of stimulated Brillouin scattering in multimode fibers. With both the coupling among all optical modes and the effect of optical and acoustic mode field profiles on the Brillouin gain spectrums considered for the first time, we propose the corresponding transcendental equations for the Brillouin thresholds of continuous-wave signal under different conditions. Based on the theoretical results, it is concluded that Brillouin threshold in multimode fiber is determined by the constitution of the transverse modes in the signal. Moreover, the theoretical predictions are compared with the results obtained by numerically integrating the signal-Stokes power equations, and good agreements are achieved between them.

Hybrid Ytterbium/Brillouin Gain Assisted Partial Mode Locking in Yb-Doped Fiber Laser

We have demonstrated experimentally and theoretically that the hybrid ytterbium/Brillouin gain can contribute to partial mode locking in the high-loss ytterbium-doped fiber laser. With only few cavity modes contained within the Brillouin gain curve, the dynamics of self-pulsing was not observed for the stimulated Brillouin scattering signal generated by each pump mode in the experiment. The results from two different experimental schemes show that both the Brillouin and ytterbium gains are required to support enough Brillouin pump and stokes cavity modes for partial mode locking. We also show that it is the coupling between the stimulated Brillouin scattering of the neighboring Brillouin pump modes that plays a direct role in the partial phase synchronization between different stokes modes. Furthermore, we proposed a theoretical model about the partially mode-locked laser dynamics. Based on the model, the numerical results agree quantitatively with the experimental ones. It is also found numerically that the extent of the phase synchronization can be enhanced by the cascaded stimulated Brillouin scattering according to the same physical mechanism.

Nanosecond pulse pumped, narrow linewidth all-fiber Raman amplifier with stimulated Brillouin scattering suppression

We report on a narrow linewidth nanosecond all-fiber Raman amplifier core pumped by a pulsed laser at approximately 1030 nm. The Raman amplifier was based on a standard single-mode fiber with a length of ~1 km, and stimulated Brillouin scattering (SBS) was suppressed by employing pulses with a short pulse width. 1083 nm pulses with an average power of 32.6 mW, a repetition rate of 2 MHz, and pulse widths of ~7.2 ns were achieved. A maximum slope efficiency of 46.1% and a gain of 31 dB were obtained. The output Raman power can be scaled further by using fiber with shorter lengths and pump pulses with a higher power.

Modal decomposition for large-mode-area fibers using stochastic parallel gradient descent algorithm

We have experimentally demonstrated an intensity-based mode decomposition method by using stochastic parallel gradient descent algorithm. Fast and accurate decomposition is performed on a large-mode-area fiber, which can be potentially used in fiber laser.