We present comprehensive theoretical study of mode instability with a universal model for the first time, which includes quantitative study on the various influence factors and mitigation methods. With large numerical results, the influence of various fiber and system parameters has been studied to summarize mitigation methods. The mode instability threshold behavior of multiwavelength pumping configurations has been studied for the first time, and the influence of pump wavelength shifting in the tandem pumping technique and amplitude modulation have also been given for the first time. The mitigation methods are discussed with review of the latest development in experimental results. The drawbacks and possible overcome measures are also discussed. By properly combining the mitigation methods, an approach to achieve beyond 10-kW diode pumped fiber laser with near diffraction limited beam quality has been proposed, which provides a creative way to overcome the mode instability limitation on diode pumped fiber lasers, and demonstrates the effectiveness of the mitigation methods.

Comprehensive Theoretical Study of Mode Instability in High-Power Fiber Lasers by Employing a Universal Model and Its Implications

Impact of mode instability on dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers has been studied for the first time, which reveals another characterization of mode instability from the aspect of optical spectrum. It shows that, after the onset of mode instability, the measured light spectrums, especially the Raman light spectrums, are different from those without mode instability, which become burr-like. As mode instability evolves into different stages, the intensity of stimulated Raman scattering effects as laser power increasing also behaves differently. During the transition region, the stimulated Raman scattering effect becomes stronger as the lasing power increases until the mode instability evolves into chaotic regions, where the stimulated Raman scattering effect weakens. The effect of stimulated Raman scattering on mode instability has also been studied. Due to that the stimulated Raman scattering effect is weak and that the fraction of Raman light is only a few percent, the stimulated-Raman-scattering-induced mode instability has not been observed in the experiment, and the observed mode instability is induced by ytterbium ion gain of signal laser. It also revealed that the stimulated Raman scattering has negligible influence on the mode instability induced by ytterbium ion gain.

Dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers in the presence of mode instabilities

https://iopscience.iop.org/article/10.1088/1612-202X/ab2acf/pdf

Comparative study on transverse mode instability of fiber amplifiers based on long tapered fiber and conventional uniform fiber

A gain-tailored Ge-free Yb/Ce codoped aluminosilicate fiber is fabricated by MCVD combined with solution doping technique. Through regulating the temperature in the tube and designing the solution doping process, the refractive index profile of this fiber is close to a step-index without any center dip. The laser performance of this fiber is proved through contrast experiments with conventional fiber in a kW-level MOPA setup. The gain-tailored fiber amplifier presents a beam quality of M2 ~1.43 at 1.2 kW. Its MI threshold is 1.25 kW, about 1.74 times as much as that of the conventional fiber amplifier. The laser slope efficiency of the gain-tailored fiber amplifier is 86.75%. Stabilized at an output power of 1.1 kW for 15 hours, the MI threshold does not decrease after this long-term operation, demonstrating a strong resistance to photodarkening effect. These results have confirmed that MCVD-fabricated gain-tailored Yb/Ce codoped aluminosilicate fibers have great potential in power scaling and output stability of high-power fiber lasers and amplifiers.

Gain-tailored Yb/Ce codoped aluminosilicate fiber for laser stability improvement at high output power

High‐power laser sources are widely used in industrial precision processing and act as a new platform for strong‐field physics research using peak power over petawatt. This review focuses on realizing high‐energy solid‐state disk and slab systems and the nonlinear‐suppression strategies for high‐power fiber systems using the functional fibers. First, the implementations and enabling technologies of the solid‐state lasers for increasing peak power from gigawatt to petawatt are reviewed. Then the mechanisms and suppression strategies of the deterioration effects (including stimulated Raman scattering, stimulated Brillouin scattering, and transverse mode instability) in various fiber amplifiers are analyzed. At the same time, the mechanism and achievements of the current functional fibers are introduced. Finally, the challenges and perspectives of high‐power solid‐state and fiber amplifiers are summarized.

High‐Power Laser Systems

We use a semi-analytical model of stimulated thermal Rayleigh scattering to present theoretical study of seed power on mode instability with the presence of photodarkening. The behavior of mode instabilities as a function of seed power has been investigated. The nonlinear dependence of the threshold with lower seed powers is ascribed to the influence of gain saturation while the reduction behavior of the threshold with higher seed powers is put down to the effects of photodarkening, which agrees well with the experimental results.

Influence of seed power on mode instabilities in high power fiber amplifiers

The invention relates to a method for controlling array light beam co-target collimation of a target in a loop based on light spot centroid calculation. The coordinates of the centroid of a light spot on an imaging plane are obtained through a light spot centroid algorithm, a tilt control quantity which needs to be conducted on each light beam is obtained according to the quantitative relation between tilt control of a tilting mirror of the corresponding light beam and a light spot movement vector, and the centroid of the light spot is moved to a target spot of an image device through control over the tilting mirrors. Co-target collimation of all the light beams of the target in the loop is realized through sequential control cover the array light beams. There is no need to accurately describe or obtain a light path, and a control scheme is simple and convenient and easy to implement. The adopted centroid of the light spot serves as a performance evaluation function which is simple, convenient, high in real-time performance, and remarkable in effect, and consumes a short time. The method for controlling array light beam co-target collimation of the target in the loop based on light spot centroid calculation has extensive application prospects in satellite tracking, the directed energy technology and other fields.

Method for controlling array light beam co-target collimation of target in loop based on light spot centroid calculation

The invention provides a fiber laser coherent combining system based on adaptive precision light path adjustment. In the system, a light beam output by a laser seed source is amplified by a pre-amplification module and then split by a beam splitter module; all the light beams pass through a primary application array, a phase modulator array, a light path rough adjustment device, a primary light path fine adjustment device, a secondary amplification module, a collimator system array and a secondary light path fine adjustment device; then, all the output light beams are combined by a beam combining device; then, the combined light beam enters a synthetic beam processing module, and a part of light is separated out by the synthetic beam processing module and then is fed in a photoelectric detection module; the photoelectric detection module is capable of converting an optical signal into an electric signal which then enters a control system; the control system is capable of processing the electric signal so as to respectively provide a control signal to the phase modulator array, the primary light path fine adjustment device and the secondary light path fine adjustment device, thus locking phases among all the light beams and compensating light paths among all the light beams in an adaptive way. The fiber laser coherent combining system based on adaptive precision light path adjustment can be applied to a multi-laser coherent combining system.

Fiber laser coherent combining system based on adaptive precision light path adjustment

The invention relates to an efficient cooling method for a high-power optical fibre. The efficient cooling method consists of the high-power optical fibre, a hollow cylindrical metal sleeve, a heat sink which contains flowing cooling liquid inside, a heat conductive medium and a waterproof material. The method comprises the following steps that: the high-power optical fibre to be cooled is put in the hollow cylindrical metal sleeve of a proper size; the heat conductive medium with a high heat conductive coefficient is injected into a clearance between the fibre and the metal sleeve; the heat conductive medium transmits heat of the fibre to the metal sleeve; the metal sleeve is directly put in the flowing cooling liquid of the heat sink, so that heat of the metal sleeve is taken away by the flowing cooling liquid; the waterproof material is filled at input and output ports of the metal sleeve and a junction of the heat sink, thereby preventing the cooling liquid from flowing out. The efficient cooling method for the high-power optical fibre disclosed by the invention effectively increases a contact area between the optical fibre and a cooling medium and increases a coefficient of heat conduction, is capable of decreasing temperature of a core of the optical fibre and a clad layer, increasing an output power of a high-power fibre laser, and enhancing stability of a system, performs bending at any radius in a permitted range of the optical fibre by the metal sleeve, and can perform convenient shape control upon the optical fibre.

Si Lei

Papers

Influence of seed power on mode instabilities in high power fiber amplifiers

Method for controlling array light beam co-target collimation of target in loop based on light spot centroid calculation

Fiber laser coherent combining system based on adaptive precision light path adjustment

Efficient cooling method for high-power optical fibre

Novel two-dimensional light beam deflection method and device