Transverse Mode Instability Research Articles

Dynamic modal characteristics of transverse mode instabilities in ytterbium-doped fiber laser oscillator

In recent years, transverse mode instability (TMI) has been widely observed in fiber laser amplifier systems. The transverse mode instability phenomenon in fiber laser oscillators is less studied. Here, we focus on the dynamical output properties, i.e., its temporal signal and modal characteristics in a 30-μm-core-diameter ytterbium (Yb)-doped fiber laser oscillator. The TMI occurs at a pumping power around 310 W. Different from amplifiers, the basic oscillation frequency is quite low, at around 100 Hz, changing with time and pump power. When the fiber laser oscillator operates beyond TMI threshold at 357 W or 377 W for a while, the temporal fluctuation slowly disappears together with a decreased oscillation frequency, and appears again later. Based on the mode decomposition technique, we find that during the period of fluctuation disappearance at 357 W, the power output stays low and the output beam is still a mixture of fundamental mode and higher-order modes. The fundamental mode content is calculated to be averagely higher when temporal fluctuation disappears, increasing from ∼57% to ∼63%. Our results indicate complex interaction between the fiber laser oscillation and the TMI effect, and calls for more attention into understanding TMI in fiber laser oscillators.

Characterization of transverse mode instability with a 4-quadrant photodiode.

Transverse mode instability (TMI) represents the main limitation for the power scaling of fiber laser systems with a diffraction-limited beam quality. In this context, it has become increasingly important to find a cheap and reliable way to monitor and characterize TMI and distinguish this effect from other dynamic perturbations. In this work, with the help of a position-sensitive detector, a novel method is developed to characterize the TMI dynamics even in the presence of power fluctuations. The position information of the fluctuating beam is recorded in the X- and Y-axis of the detector, which are used to track the temporal evolution of the center of gravity of the beam. The trajectories described by the beam within a specific time window contain rich information about TMI, which can be used to gain further insight into this phenomenon.

Confined-doped fiber enabled kilowatt-level all-fiber laser with 1.28 GHz linewidth.

In this manuscript, a narrow linewidth fiber amplifier based on confined-doped fiber is established, and the power scaling and beam quality maintaining capabilities of this amplifier are investigated. Benefitted from the large mode area of the confined-doped fiber and precisely controlling the Yb-doped region in the fiber core, the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects are effectively balanced. As a result, a 1007 W signal laser with just 1.28 GHz linewidth is obtained by combining the advantages of confined-doped fiber, near-rectangular spectral injection, and 915 nm pump manner. As far as we know, this result is the first beyond kilowatt-level demonstration of all-fiber lasers with GHz-level linewidth, which could provide a well reference for simultaneously controlling spectral linewidth, suppressing the SBS and TMI effects in high-power, narrow-linewidth fiber lasers.

Influence of central wavelength of pump source and pump direction on transverse mode instability threshold of fiber laser

In high-power ytterbium ion-doped fiber lasers where the output power is limited only by the transverse mode instability (TMI), the TMI threshold of fiber lasers with counter-propagating pumping (counter-pumping) structure is usually greater than that of co-propagating pumping (co-pumping), and bidirectional pumping (bi-pumping) is in turn greater than that of counter-pumping. In order to increase the TMI threshold, we optimized the central wavelength of the pump source to a bi-pumped fiber laser oscillator. In the experiment, we found that only the central wavelength of the pump source is changed in the same fiber laser, and the TMI threshold of the bi-pumping structure is sometimes greater than the sum of the TMI thresholds at the time of only co-pumping and only counter-pumping, sometimes approximately equal to, sometimes less. In addition, an interesting phenomenon was also reported that the counter-pump injection power of the fiber laser in the TMI threshold of bi-pumping is larger than the TMI threshold of only counter-pumping. Using the fiber laser simulation software, we explained the above phenomenon from the two directions of thermal effect and population reverse ratio in the active fiber. These research results are helpful to further optimize each device parameter of fiber laser and achieve higher power output.

Power scaling limits of diffraction-limited fiber amplifiers considering transverse mode instability.

An empirical TMI threshold formula is derived based on a recently developed model and used to analyze the power-scaling performance of ytterbium-doped silica glass and YAG (Y3Al5O12) and lutetia (Lu2O3) single-crystalline fiber amplifiers. Overall, the single-crystalline fiber lasers are found to scale potentially to higher average powers due to their higher thermal conductivities compared to silica glass. This work serves as a useful extension to earlier works and shines significant new light on optimal fiber and amplifier designs for maximum average output power with TMI considered.

High Power Ytterbium-Doped Fiber Lasers Employing Longitudinal Vary Core Diameter Active Fibers

Thanks to the advantage of balancing nonlinear effects and transverse mode instability, vary core diameter active fiber (VCAF) has been widely used in high power ytterbium-doped fiber lasers in recent years. Up to now, VCAF has developed from the basic form of the original tapered fiber to the spindle-shaped and saddle-shaped fiber with different characteristics and has been applied in conventional fiber lasers, oscillating–amplifying integrated fiber lasers, and quasi-continuous wave fiber lasers and successfully improved the performance of these lasers. In the present study, a 6110 W fiber laser amplifier is realized based on a tapered fiber. The maximum output power of a fiber laser amplifier based on spindle-shaped fibers is 6020 W with a beam quality of M2~1.86. In this paper, we first introduce the basic concept of VCAF and summarize its main fabrication methods and advantages in high-power fiber laser applications. Then, we will present the recent research results of high-power fiber laser employing VCAF in our group and clarify the outstanding advantages of VCAF compared with the constant core diameter active fiber (CCAF).

A 3.7-kW Oscillating-Amplifying Integrated Fiber Laser Featuring a Compact Oval-Shaped Cylinder Package.

Combining the advantages of high efficiency, environmental robustness, and anti-reflection behavior, oscillating-amplifying integrated fiber lasers have become popular for use in high-power laser structures in industrial applications, wherein the size of the laser source matters. Here, an oscillating-amplifying integrated fiber laser in an oval-shaped cylinder package has been proposed and demonstrated, the footprint for which only occupies an area of 0.024 m2 apart from the pump diode, which is much smaller than in traditional planar fiber laser packages. Numerical simulations have been carried out, which have revealed that an oval-shaped cylinder package can effectively suppress the high-order mode in large mode area fiber setups and thereby benefit the integration of fusion points and the unpackaged elements at the same time. Over 3.7 kW of transverse mode instability (TMI)-free output power has been obtained, with a slope efficiency higher than 80%. With a custom-made chirped and tilted fiber Bragg grating (CTFBG), the Raman suppression ratio is improved to reach 38 dB at peak output power. The oval-shaped design has been verified to assist with the realization of TMI suppression and improve the integration of high-power fiber lasers. To the best of our knowledge, this fiber laser has among the smallest footprints of the various fiber sources at such high-power operating levels.

Experimental comparison of Yb/Al/Ce and Yb/Al/P co-doped fibers on the suppression of transverse mode instability

We presented an experimental comparison of the core-composition difference on the suppression of the photodarkening and transverse mode instability effects. Two core-composition fibers, entailing Yb/Al/Ce and Yb/Al/P co-doped fibers, were fabricated by MCVD process combined with solution doping technique. The parameters of two fibers were almost the same. The PD-induced loss at equilibrium was 3.94 dB/m at 702 nm in Yb/Al/Ce fiber, while it was 0.99 dB/m in Yb/Al/P fiber. To obtain a deeper understanding of the impact of PD on laser performance, a bidirectional pumping fiber amplifier was constructed. Compared with Yb/Al/Ce co-doped fiber, the TMI thresholds of Yb/Al/P co-doped fiber were enhanced in co-pumped and counter-pumped schemes. Meanwhile, the slope efficiency in bidirectional scheme was promoted by 4%. Moreover, the transmittance at 638 nm confirmed the superior PD resistance of Yb/Al/P co-doped fiber. These experimental results pave the way for the further development of high-power fiber lasers.

Theoretical Analysis of the Mode Distortion Induced by Stimulated Raman Scattering Effect in Large-Mode Area Passive Fibers

By employing the mode decomposition method to study the relative mode evolution characteristics during the stimulated Raman scattering (SRS) process, a numerical model of the core-pumped Raman effect in few-mode fibers has been proposed, which provides an accurate tool to reveal the underlying physical mechanism of SRS-induced mode distortion. Mode distortion induced by the SRS effect in large-mode area passive fibers has been investigated numerically. It reveals that the few-mode SRS effect can result in an irreversible power transfer from the signal fundamental mode (FM) to signal high order modes (HOMs), which is a static phenomenon, different from the SRS-induced transverse mode instability. To the best of our knowledge, it is the first time that the numerical results coincide with the experimental phenomenon. Simulation results also illustrate that the inter-mode wave-mixing effect among the signal FM and signal HOMs plays a key role in the signal power transfer. With the help of the numerical model, mitigation strategy by designing and using the Raman-gain-tailored fiber has also been proposed, which shows a clear advantage in suppressing SRS-induced mode distortion.

Over 10-kW fiber laser spectral beam combination based on dichromatic mirrors

Owing to the advantages of good beam quality, high conversion efficiency, convenient thermal management and compact structure, high power fiber lasers have been widely desired in industrial processing. So far, the output power has been strictly limited by the nonlinear effects and transverse mode instability. In order to break through the power limitation, here we experimentally demonstrate a power boosting technology called spectral beam combination based on dichromatic mirrors. Firstly, high power fiber amplifiers with different central wavelengths are established for the spectral combination. Secondly, utilizing a dual-beam combined system and two homemade high power fiber amplifiers, an output power of 10 kW is achieved, with a remarkable combination efficiency of 98.3% and a beam quality of <i>M</i> <sup>2</sup>～1.33. Secondly, using a three-beam combined system, an output power of 13.5 kW is obtained with a combination efficiency of 96.8% and beam quality of <i>M</i> <sup>2</sup>～1.61. By increasing the number of input beams and their output power as well, we believe that a higher output power can be achieved based on dichromatic mirror spectral beam combination.

Optimization and demonstration of 6 kW oscillating-amplifying integrated fiber laser employing spindle-shaped fiber to suppress SRS and TMI

Oscillating-amplifying integrated fiber laser is a novel configuration with both advantages of fiber laser amplifiers and oscillators. In this work, a specially designed spindle-shaped fiber is used to suppress Stimulated Raman Scattering (SRS) and transverse mode instability (TMI) for the first time in an oscillating-amplifying integrated fiber laser. This spindle-shaped Ytterbium-doped fiber has a constant pump absorption coefficient of 0.471 dB/m@915 nm with a core/cladding diameter of 25/400 µm at both ends and 37.5/600 µm in the middle. Based on this fiber, we succeeded in suppressing the TMI and raising the TMI threshold of the laser under counter pump to above 1.62 times of that in a reported oscillating-amplifying integrated fiber laser. Combined with the theoretical analysis, we further suppressed SRS by truncating the active fiber of the oscillating section, and achieved a 6 kW laser output with an efficiency of 84.1% and a beam quality of M2 ∼ 1.78. To the best of our knowledge, this is the highest output power of the oscillating-amplifying integrated fiber laser.

Thermally-induced transverse mode instability: Hopf bifurcation in high-power fiber laser

The thermally-induced transverse mode instability (TMI) has become a major challenge for further enhancing the output power of single-mode fiber laser. Even though much work has been done over the past decades, the spontaneous generation process of output spot oscillation has not been well understood yet. Here, we establish a new theoretical model of the thermally-induced TMI in a high-power fiber oscillator and provide a new mechanism explanation from the point of view of nonlinear dynamics. Theoretical analysis shows that the spontaneous oscillation (∼kHz) of the output spot caused by the thermally-induced TMI is a limit-cycle oscillation via Hopf bifurcation, and the oscillation frequency is proportional to square root of the relaxation rate γ1 of thermally-induced refractive index grating (RIG) (νTMI∝γ1) when the thermally-induced TMI is initially triggered. Furthermore, the relationship between the threshold power, together with oscillation frequency and several parameters of fiber laser system was also discussed in detail. This work help us better understand the underlying dynamics of the thermally-induced TMI.

Modeling of the thermal-lens induced mode coupling in fiber amplifiers

In this paper, a novel model for describing the thermal-lens induced mode coupling (TLMC) effect in the high power fiber amplifier is proposed firstly, to the best of our knowledge. Based on the rate equation and couple-mode equation, the model is built with both consideration of the thermal-lens induced mode field variation and the gain effect. With this model, the mode coupling behaviors with various pump schemes are numerically investigated and the impact of the TLMC is quantitatively discussed in a finite-length single-frequency fiber amplifier for paradigm. It is found that the higher-order mode (HOM) can be induced by the TLMC even with pure single-mode input signal and this effect is most severe with the co-pumped scheme while weakest with the counter-pumped scheme. It is also revealed that the TLMC will enhance the average HOM content and will keep predominant until the incident HOM is larger than a certain content. Such an enhancement of HOM content can play an important role as the seed triggering the transverse mode instability (TMI) and the pertinent study is helpful on investigating the relationship between the thermal-lens effect and the TMI.

Tradeoff between the Brillouin and transverse mode instabilities in Yb-doped fiber amplifiers.

The Brillouin instability (BI) due to stimulated Brillouin scattering (SBS) and the transverse (thermal) mode instability (TMI) due to stimulated thermal Rayleigh scattering (STRS) limit the achievable power in high-power lasers and amplifiers. The pump power threshold for BI increases as the core diameter increases, but the threshold for TMI may decrease as the core diameter increases. In this paper, we use a multi-time-scale approach to simultaneously model BI and TMI, which gives us the ability to find the fiber diameter with the highest power threshold. We formulate the equations to compare the thresholds of the combined and individual TMI and BI models. At the pump power threshold and below, there is a negligible difference between the full and individual models, as BI and TMI are not strong enough to interact with each other. The highest pump threshold occurs at the optimal core size of 43 µm for the simple double-clad geometry that we considered. We found that both effects contribute equally to the threshold, and the full BI and TMI model yields a similar threshold as the BI or TMI model alone. However, once the reflectivity is sufficiently large, we find in the full BI and TMI model that BI may trigger TMI and reduce the TMI threshold to a value lower than is predicted in simulations with TMI alone. This result cannot be predicted by models that consider BI and TMI separately. Our approach can be extended to more complex geometries and used for their optimization.

First Demonstration and Comparison of 5 kW Monolithic Fiber Laser Oscillator Pumped by 915 nm and 981 nm LDs

Fiber laser performances including transverse mode instability (TMI), stimulated Raman scattering (SRS) and optical-to-optical efficiency are in connection with the pump wavelength. Here we studied the output characteristics of a 5-kW ytterbium-doped fiber laser oscillator pumped with two different pump sources, i.e., 915 nm and 981 nm laser diodes (LDs). The output characteristics of fiber laser oscillators pumped by 915 nm and 981 nm have been compared strictly and directly with the same structure in a high-power situation. Experimental results show that both pump wavelengths can scale the power up to more than 5 kW by suppressing the TMI effect. While in the case of pumping by the 981 nm LDs, the laser oscillator has an optical-to-optical efficiency of 87%, which is 13% higher than that of the 915 nm pumped scheme. In addition, due to the higher backward pumping ratio and lower total pump power, the laser oscillator has a better SRS suppression ratio when pumped at 981 nm. Thus, it reveals a great potential to balance the limitations of TMI and SRS for scaling up to an even higher output while pumping at 981 nm. All the devices of the oscillator are commercial, and it will be helpful for the commercialization of high-power fiber laser oscillators.

175 W average power from a single-core rod fiber-based chirped-pulse-amplification system.

We report on a fiber-based chirped-pulse-amplification laser system with bulk transmission grating compression to a pulse duration of 357 fs, average power of 175 W, and pulse energy of 233μ J. The compressed pulse train has a beam quality factor M2 of 1.21. The power amplifier is based on a state-of-the-art single-mode photonic crystal rod-type ytterbium-doped fiber operating at 248 W of average power and a repetition rate of 750 kHz. The long-term stability of the laser system has been tested continuously for more than 4000 hours and shows no sign of transverse mode instability.

Investigation of the confined-doped fiber on single-mode operating and power scaling in all-fiber single-frequency amplifiers

In this paper, we proposed a strategy for achieving all-fiber single-frequency amplifiers with near-diffraction-limited beam quality by using confined-doped fiber. Benefiting from the large mode area (LMA) and mode selection properties of the confine-doped fiber, the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects were comprehensively suppressed. Based on this confined-doped fiber assisted amplifier, a 322 W SBS-limited single-frequency laser was obtained with M2 factor of 1.25/1.33 for the x/y direction. Comparing with the full-doped fiber assisted one, the TMI threshold of the confined-doped one is improved more than 1.6 times. Overall, the technique of confined-doped in the fiber core provides a promising approach for the power scaling and single-mode operation of all-fiber single-frequency lasers.

2.5 kW TMI-free co-pump Yb-doped fiber oscillator by 971.5 nm pumping wavelength

Pump wavelength shift from the peak absorption wavelength of 976 nm is one of the strategies for mitigation of transverse mode instability (TMI) in high-power Yb-doped fiber lasers. In the present contribution, 971.5 nm is introduced as an effective pumping wavelength for the increment of TMI threshold. By using this pumping wavelength, a 2580 W co-pumped fiber oscillator is built by 20/400 µm active fiber, with more than a twofold increase in the TMI threshold. Our analytical and experimental investigations indicate that due to the lower heat load of the pumping wavelength of 971.5 nm in the gain medium, it can properly mitigate TMI in fiber oscillators. Power scaling in here presented fiber oscillator is just limited to the occurrence of stimulated Raman scattering (SRS) induced mode instability and to the best of authors’ knowledge, this is the first evidence of SRS-induced mode instability in high-power fiber oscillators and TMI is not the restricting factor in power scaling anymore.

700 W single-frequency all-fiber amplifier at 1064 nm with kHz-level spectral linewidth

In this work, a single-frequency fiber amplifier with output power of 703 W was demonstrated at 1,064.4 nm in an all-fiber configuration. Cascaded Yb3+-doped fiber structure with different dopant concentration and hybrid 915/976 nm pump scheme were employed in power scaling stage to improve the gain saturation for higher transverse mode instability threshold. An overall optical efficiency of 67.5% was achieved at the maximum output power and the M2 was measured to be ∼1.4. A spectral linewidth of 2 kHz was obtained from the 703-W single-frequency laser. To the best of our knowledge, this is the first time that a single-frequency all-fiber amplifier with kHz-level spectral linewidth is achieved at such high output power.

Threshold of transverse mode instability considering four-wave mixing.

In this work, the influence of four-wave mixing (FWM) effects on the transverse mode instability (TMI) is incorporated into the TMI model based on stimulated thermal Rayleigh scattering. The model is capable of analyzing the gain characteristics of different high-power fiber amplifiers, based on which the physical mechanism and functioning boundary of FWM are theoretically investigated. Consequently, a new TMI threshold formula is defined to resolve the inconsistencies in the previous TMI models. It is revealed that it is extremely necessary to consider the influence of FWM on TMI in ultra-large mode field laser systems.