High-power Double-clad Fiber Laser Research Articles

Various pumping modes of high-power double clad fiber laser

As one of the first invented laser categories, fiber laser plays a crucial role in many specific fields, e.g., filed-optic communication, sensing, industrial processing, national defense, military. In order to effectively couple the pump light into the single-mode fiber, double-clad fiber laser was invented, i.e., the power and efficiency of fiber lasers have been significantly improved. There are two mainly categories of pumping mode for double clad fiber laser, named as end smoothing the gain curve pumping and side pumping. This paper will focus on the analysis and comparison of these two pumping modes in terms of the power, efficiency, relative population density, thermal load and feasibility. The experimental procedure will be achieved based on the MATLABR2021b and SFTool. The aim of the paper is to investigate the effects on the forward pumping and backward pumping in the process of side pumping and end pumping. According to the analysis, side pumping and bi-direction pumping smooth the gain curve effectively. Choosing appropriate pumping mode can reduce the loss and save the cost of optical transmission to a large extent. These results shed light on selection of pumping mode for high power fiber laser.

Fiber core mode leakage induced by refractive index variation in high-power fiber laser**Project supported by the National Natural Science Foundation of China (Grant Nos. 61307057 and 61675114).

This paper presents an investigation of specific optical fiber core mode leakage behavior that occurs in high-power double-clad fiber lasers as a result of thermally-induced refractive index variations. A model of the power transfer between the core modes and the cladding modes during thermally-induced refractive index variations is established based on the mode coupling theory. The results of numerical simulations based on actual laser parameters are presented. Experimental measurements were also carried out, the results showed good agreement with the corresponding simulation results.

Comparison between threshold and sensitivity of stimulated Brillouin scattering at different pumping configurations in high-power double-clad fiber lasers

Theoretical analysis of stimulated Brillouin scattering (SBS) in high power ytterbium-doped double-clad (YDDC) fiber lasers is presented by solving the steady-state rate equations with regard to SBS at different pumping configurations. The behavior of Brillouin Stokes power growth along the fiber is investigated and the SBS threshold power is quantitatively compared at forward, backward and bi-directional pumping configurations. It is shown that the backward pumping configuration has higher SBS threshold power than two others. Also. The effect of cavity parameters such as cavity length, front mirror reflectivity, dopant concentration and laser linewidth at different pumping configuration have been studied. It can be seen although the backward puming has higher SBS threshold but its sensitivity to cavity parameters is higher than other pumping configuration which is a negative impact for cavity designing.

Mode-field adapter for tapered-fiber-bundle signal and pump combiners.

We report on a novel mode-field adapter that is proposed to be incorporated inside tapered fused-fiber-bundle pump and signal combiners for high-power double-clad fiber lasers. Such an adapter allows optimization of signal-mode-field matching on the input and output fibers. Correspondingly, losses of the combiner signal branch are significantly reduced. The mode-field adapter optimization procedure is demonstrated on a combiner based on commercially available fibers. Signal wavelengths of 1.55 and 2μm are considered. The losses can be further improved by using specially designed intermediate fiber and by dopant diffusion during splicing as confirmed by preliminary experimental results.

Study of the stimulated Brillouin scattering power threshold in high power double-clad fiber lasers

In this paper, the effect of stimulated Brillouin scattering (SBS) on increase in the output power of a fiber laser is investigated by solving the steady-state rate equations with the SBS. The effects of fiber core diameter, cavity length, reflectivity bandwidth of rear and front mirrors, and fiber laser pumping configuration on SBS threshold are illustrated. Numerical results show that the reflectivity bandwidth of the front mirror is more effective in suppression of SBS at the fiber laser resonator. On the other hand, increase in the fiber core diameter, reduction of cavity length and using a backward pumping configuration can increase the SBS threshold. In our 220 W experimental fiber laser oscillator setup no SBS effect was observed.

Three-dimensional simulation of the temperature field in high-power double-clad fiber laser

The 3D temperature distribution in high-power double-clad fiber laser (DCFL) and the evolution of the temperature field in the fiber are analyzed, according to the transient heat conduction equation. The temperature in the fiber reaches the maximum after opening the pump light about 20 s, and cools down to the room temperature after shutting down for also about 20 s. The shape of the inner-clad can only affect the spacial distribution of the temperature outside of the core, but have no effect on the maximum temperature in the core.

An improved shooting algorithm and its application to high-power fiber lasers

An improved shooting algorithm for high-power fiber laser is proposed based on the relation of the two-end boundary laser powers. By the improved shooting algorithm, the evolutions of the pump and laser powers along the fiber position for Yb-doped and Tm-doped high-power double-clad fiber lasers with the laser scattering loss are analyzed. The results prove that the improved shooting algorithm can be efficient even if the pump power is up to kilowatt level, and the initial value can all be fast convergent as long as the initial guessed laser power is smaller than the truth value. So, this improved shooting algorithm can be used in high-power double-clad fiber lasers efficiently.

High-power fiber laser combination technology

Research on fiber laser combination is discussed briefly in this paper. High-power double clad-fiber laser beam combination technology is introduced and different kinds of fiber laser beam combination and coherent combination technologies are evaluated. Tapered fused bundle (TFB) couplers are used in laser combine for higher power. In this paper, the theory and progress in TFB coupling are introduced. The experiment on our self-fabricated TFB is presented. The efficiency of the fiber coupler exceeded 70% and increased as the input power went up. A maximum total output power of 689W was obtained, with an efficiency of 74%. The fiber coupler displayed stability during the course of the experiment, without any cooling provided.

Influence of stimulated Brillouin scattering on the temporal dynamics of an actively Q-switched double-clad fiber laser

The role of stimulated Brillouin scattering (SBS) during the active Q-switching of a typical high power double-clad fiber laser is experimentally and theoretically investigated. Our numerical model reproduces multiples Brillouin Stokes generation and the fast temporal dynamics which were experimentally observed. It clearly demonstrates that SBS is the cause of peak power enhancement effects such as giant pulse generation. The pulse formation process during the active Q-switching and the precise role of SBS in the pulse narrowing is detailed.

Numerical analysis of stimulated Brillouin scattering in high-power double-clad fiber lasers

A theoretical analysis of stimulated Brillouin scattering (SBS) in linear cavity Yb 3+-doped double-clad fiber lasers is presented by solving the steady-state rate equations with the SBS. The effects of cavity length, fiber core diameter, input mirror reflectivity at Stokes wavelength, Yb 3+ concentration and laser linewidth on the SBS are discussed. Numerical results show that the SBS threshold power can be improved by shortening the cavity length, using large mode area fiber, reducing the input mirror reflectivity at Stokes wavelength, lowering the Yb 3+ concentration and broadening the laser linewidth, and the influence of the laser linewidth on the SBS threshold power is more noticeable than other system parameters.

Stimulated Raman scattering in high-power double-clad fiber lasers and power amplifiers

Theoretical analyses of stimulated Raman scattering (SRS) in high-power ytterbium-doped double-clad fiber lasers and power amplifiers with bidirectional end pump and distributed pump are comprehensively presented. The Raman effects are quantitatively compared in these schemes with different fiber lengths, core diameters, signal wavelengths, pump absorption, facet reflections, etc., and the related restrictions are also illustrated. Fiber power amplifiers, characteristically with low facet reflections, have the advantage of significantly higher SRS thresholds than fiber laser resonators, which makes master oscillator power amplifiers better candidates for kilowatt single-transverse-mode fiber laser sources.

High-power cladding-pumped Tm-doped silica fiber laser with wavelength tuning from 1860 to 2090 nm

A high-power double-clad Tm-doped silica fiber laser, pumped by two beam-shaped and polarization-coupled diode bars at 787 nm, was wavelength tuned by use of an external cavity containing a diffraction grating. The Tm fiber laser produced a maximum output power of 7 W at 1940 nm for 40 W of incident diode power and was tuned over a wavelength range of 230 nm from 1860 to 2090 nm, with >5-W output power over the range 1870-2040 nm. The prospects for further improvement in performance and extension of the tuning range are discussed.

Numerical analysis and experimental results of output performance for Nd-doped double-clad fiber lasers

Numerical analysis is investigated for the high-power double-clad fiber lasers and experimental results using different microscope objectives for focusing into a Nd-doped rectangular double-clad fiber also performed. The numerical analysis includes dependence of output power on output mirror reflectivity, absorbed pump power, loss, and fiber length and pump power distribution for the cases of one-end and two-end pumps with 20 dB/km loss. Calculated conversion efficiencies are 76.36%, 69.73%, and 63.84% for lossless, two-end pump, and one-end pump fiber lasers, respectively. Slope efficiencies from absorbed pump power/output powers measured using microscope objectives are 16.8%/182 mW, 53.8%/351 mW, 24.9%/1240 mW, and 13.9%/649 mW for magnifications of 5×, 10×, 20×, and 40×, respectively.

High-power double-clad fiber lasers

Rare-earth doped, laser-diode-pumped, single-mode, double-clad fiber lasers are studied in detail, experimentally and theoretically, with particular attention to properties of scaling to higher output powers. Gain saturation, spontaneous emission, fiber propagation loss, and the optimum output coupler are considered for oscillator design and laser performance. Thermal properties are investigated, including the thermal fracture limit, the decrease of quantum efficiency with increasing pump power through the temperature dependence of multiphonon decay and ion-ion energy transfer, and thermal lensing. Self-focusing and Raman effects are briefly considered. It is shown that output powers in the tens of watts range are readily feasible with available double-clad fiber lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>