Simplified expression for transverse mode instability threshold in high power fiber lasers.

In this work, we propose an analytical expression for calculating the transverse mode instability (TMI) threshold power, which clearly shows the role of various fiber parameters and system parameters. The TMI threshold expression is obtained by solving the heat conduction equation and the nonlinear coupling equation using the fundamental mode fitted by Gaussian functions. The calculation results of the proposed TMI threshold expression are consistent with the experimental phenomena and simulation results from the well-recognized theoretical model. The influence of some special parameters on the TMI threshold and the power scaling is also investigated. This work will be helpful for fiber design and TMI mitigation of high-power fiber lasers.

10 kW tandem pumping fiber amplifier with good beam quality based on tapered ytterbium-doped fiber.

In this paper, we established a high power tandem pumped fiber amplifier based on tapered ytterbium-doped fiber (TYDF). The TYDF is developed in-house with a core/inner cladding diameter of 30/250 µm at the small-core region and 48/400 µm at the large-core region. The key parameters of the amplifier in a co-pumped and counter-pumped configuration are experimentally investigated, such as slope efficiency, stimulated Raman scattering (SRS) threshold, and beam quality evolution. Up to 10.28 kW laser free of SRS or transverse mode instability is obtained from the counter-pumped amplifier, and the beam quality factor M2 is 2.29, which is significantly improved compared with the 48/400 µm uniform YDF. To the best of our knowledge, this is the highest average output power achieved so far based on the TYDF. This work could provide a solution for balancing the SRS suppression and high order modes control in high power tandem pumped YDF lasers.

Broadband tunable Raman fiber laser with monochromatic pump.

Raman fiber laser (RFL) has been widely adopted in astronomy, optical sensing, imaging, and communication due to its unique advantages of flexible wavelength and broadband gain spectrum. Conventional RFLs are generally based on silica fiber. Here, we demonstrate that the phosphosilicate fiber has a broader Raman gain spectrum as compared to the common silica fiber, making it a better choice for broadband Raman conversion. By using the phosphosilicate fiber as gain medium, we propose and build a tunable RFL, and compare its operation bandwidth with a silica fiber-based RFL. The silica fiber-based RFL can operate within the Raman shift range of 4.9 THz (9.8-14.7 THz), whereas in the phosphosilicate fiber-based RFL, efficient lasing is achieved over the Raman shift range of 13.7 THz (3.5-17.2 THz). The operation bandwidths of the two RFLs are also calculated theoretically. The simulation results agree well with experimental data, where the operation bandwidth of the phosphosilicate fiber-based RFL is more than twice of that of the silica fiber-based RFL. This work reveals the phosphosilicate fiber's unique advantage in broadband Raman conversion, which has great potential in increasing the reach and capacity of optical communication systems.

Mitigation of TMI in an 8†kW tandem pumped fiber amplifier enabled by inter-mode gain competition mechanism through bending control.

In this work, the impact of fiber bending and mode content on transverse mode instability (TMI) is investigated. Based on a modified stimulated thermal Rayleigh scattering (STRS) model considering the gain competition between transverse modes, we theoretically detailed the TMI threshold under various mode content and bending conditions in few-mode fibers. Our theoretical calculations demonstrate that larger bending diameters increase the high order mode (HOM) components in the amplifier, which in turn reduces the frequency-shifted Stokes LP11o mode due to the inter-mode gain competition mechanism, thus improving the TMI threshold of few-mode amplifiers. The experimental results agree with the simulation. Finally, by optimizing the bending, an 8.38 kW output tandem pumped fiber amplifier is obtained with a beam quality M2 of 1.8. Both TMI and stimulated Raman scattering (SRS) are well suppressed at the maximum power. This work provides a comprehensive analysis of the TMI in few-mode amplifiers and offers a practical method to realize high-power high-brightness fiber lasers.

Seeing the strong suppression of higher order modes in single trench fiber using the S2 technique.

The single trench fiber (STF) is a promising fiber design for mode area scaling and higher order mode (HOM) suppression. In this Letter, we experimentally demonstrate the strong HOM-suppression in a homemade STF using the spatially and spectrally resolved imaging (S2) technique. This STF has a 20-µm core and its performance is compared to a conventional step-index fiber with almost the same parameter. Results show that the bending loss of the HOM in STF is 8-times larger than conventional fiber at a bend radius of 7 cm. In addition, when severe coupling mismatch is introduced at the input end of the fiber, the STF can keep the fundamental-mode output while the conventional fiber cannot. To the best of our knowledge, this is the first time to experimentally analyze the HOM content in an STF and compare its performance with that of a conventional fiber. Our results indicate the great potential of the STF for filtering the HOM in fiber laser applications.

M2 factor estimation in few-mode fibers based on a shallow neural network.

A high-accuracy, high-speed, and low-cost M2 factor estimation method for few-mode fibers based on a shallow neural network is presented in this work. Benefiting from the dimensionality reduction technique, which transforms the two-dimension near-field image into a one-dimension vector, a neural network with only two hidden layers can estimate the M2 factor directly. In the simulation, the mean estimation error is smaller than 3% even when the mode number increases to 10. The estimation time of 10000 simulation test samples is around 0.16s, which indicates a high potential for real-time applications. The experiment results of 50 samples from the 3-mode fiber have a mean estimation error of 0.86%. The strategies involved in this method can be easily extended to other applications related to laser characterization.

Bidirectional tandem-pumped high-brightness 6†kW level narrow-linewidth confined-doped fiber amplifier exploiting the side-coupled technique.

In this work, a bidirectional tandem-pumped high-power narrow-linewidth confined-doped ytterbium fiber amplifier is demonstrated based on side-coupled combiners. Benefiting from the large-mode-area design of the confined-doped fiber, the nonlinear effects, including stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), are effectively suppressed. While the transverse mode instability (TMI) effect is also mitigated through the combination of confined-doped fiber design and the bidirectional tandem pumping scheme. As a result, narrow-linewidth fiber laser with 5.96 kW output power is obtained, the slope efficiency and the 3-dB linewidth of which are ∼81.7% and 0.42 nm, respectively. The beam quality is well maintained during the power scaling process, being around M2 = 1.6 before the TMI occurs, and is well kept (M2 = 2.0 at 5.96 kW) even after the onset of TMI. No SRS or SBS is observed at the maximum output power, and the signal-to-noise ratio reaches as high as ∼61.4 dB. To the best of our knowledge, this is the record power ever reported in narrow-linewidth fiber lasers. This work could provide a good reference for realizing high-power high-brightness narrow-linewidth fiber lasers.

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.

More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme.

We demonstrate the first all-fiber monolithic bidirectional tandem pumping amplifier, to the best of our knowledge, based on a 30/250 µm conventional ytterbium-doped double-clad fiber. By optimizing the bidirectional pumping power distribution, an output power of 6.22 kW is obtained with near single-mode beam quality (M2=1.53), and no transverse mode instability is observed. This work could provide an excellent reference for high-power, higher-brightness fiber lasers.

Confined-doped active fiber enabled all-fiber high-power single-frequency laser.

In this paper, we investigate the performances of an in-house fabricated confined-doped active fiber in the applications of all-fiber high-power single-frequency amplifiers. A 210-W single-frequency single-mode fiber laser is obtained directly, which confirms the excellent performance of the confined-doped active fiber for high-power single-mode operation. To further demonstrate the power scalability of the fiber amplifier, the strategy of applying a temperature gradient along the active fiber is investigated numerically and experimentally, and an up to ∼75% enhancement of the stimulated Brillouin scattering (SBS) threshold is achieved. As a result, a 368-W single-frequency fiber laser is obtained with the beam quality factor of Mx2 = 1.19, My2 = 1.26. Overall, the technique of the confined-doped active fiber provides a promising approach to scale the output power of single-frequency single-mode fiber lasers.

High-power tandem-pumped fiber amplifier with beam quality maintenance enabled by the confined-doped fiber.

The high absorption confined-doped ytterbium fiber with 40/250 µm core/inner-cladding diameter is proposed and fabricated, where the relative doping ratio of 0.75 is selected according to the simulation analysis. By employing this fiber in a tandem-pumped fiber amplifier, an output power of 6.2 kW with an optical-to-optical efficiency of ∼82.22% is realized. Benefiting from the large-mode-area confined-doped fiber design, the beam quality of the output laser is well maintained during the power scaling process with the beam quality factor of ∼1.7 of the seed laser to ∼ 1.89 at the output power of 5.07 kW, and the signal-to-noise ratio of the output spectrum reaches ∼40 dB under the maximum output power. In the fiber amplifier based on the 40/250 µm fully-doped ytterbium fiber, the beam quality factor constantly degrades with the increasing output power, reaching 2.56 at 2.45 kW. Moreover, the transverse mode instability threshold of the confined-doped fiber amplifier is ∼4.74 kW, which is improved by ∼170% compared with its fully-doped fiber amplifier counterpart.

Seeing the beam cleanup effect in a high-power graded-index-fiber Raman amplifier based on mode decomposition.

Due to the beam cleanup effect, brightness enhancement (BE) can be achieved in a Raman fiber amplifier (RFA) based on multimode (MM) graded-index (GRIN) fiber. In this Letter, a novel, to the best of our knowledge, diagnostic tool of mode decomposition (MD) based on a stochastic parallel gradient descent algorithm is demonstrated to observe the beam cleanup effect in a GRIN-fiber-based RFA for the first time, to our knowledge. During output power boosting up to 405 W at 1130 nm, the output beam quality factor M2 improves from 3.45 to 2.88, with a BE factor of 10.5. The MD results based on the near-field beam profiles from RFA indicate that the modal weight of the fundamental mode increases from 74.5% to 87%, confirming that the fundamental mode dominates with higher Raman gain. Moreover, the beam quality is found to be limited by the existence of a higher-order (Laguerre-Gaussian) LG10 mode, which is insensitive to the beam cleanup effect. The correlation coefficient reaches over 0.98 for all MD results. Thus, the accuracy of the MD method is high enough to provide further valuable insight into the physics of spatiotemporal beam dynamics in MM GRIN fiber.

Leakage channels enabled multi-resonant all-solid photonic bandgap fiber for effective single-mode propagation.

All-solid photonic bandgap fiber (AS-PBGF) has been fully demonstrated to be a promising candidate of large-mode-area fiber for its mode-dependent selectivity and spectral filtering mechanism. In the present work, the concepts of multiple-resonant coupling and leakage channels are taken into consideration simultaneously for mode area scaling of AS-PBGF. The single-mode performance and bending resistance of a modified structure, called leakage channels enabled multi-resonant AS-PBGF (LC-PBGF), are evaluated numerically. Robust single-mode transmission is guaranteed by a specially designed microstructure cladding with only four layers of germanium-doped rods. Multi-resonant cores in the inner layers and leakage channels in the outermost layer, resulting from missing rods in the microstructure cladding, are employed to generate modal dissipation of high-order modes under bent configuration. The missing germanium-doped rods in each layer are properly designed to eliminate the dependence on bending direction, leading to differential bending loss between fundamental mode and high-order-modes with high loss ratio. In addition, some typical derivative structures based on the LC-PBGF concept have also been proved to have great potential for effective single-mode operation.

Hundred-watt-level phosphosilicate Raman fiber laser with less than 1% quantum defect.

Quantum defect (QD)-induced high thermal load in high-power fiber lasers can largely affect the conversion efficiency, pose a threat to the system security, and even prohibit the further power scaling. In this Letter, we investigate evolutions and influences of the reflectivity of the output coupler, the length of phosphosilicate fiber, and the pump bandwidth, and demonstrate a hundred-watt-level low-QD Raman fiber laser (RFL). The RFL enabled by the boson peak of phosphosilicate fiber achieves a maximum power of 100.9 W with a reduced QD down to 0.97%; the corresponding conversion efficiency reaches 69.8%. This Letter may offer not only an alternative scheme for a high-power, high-efficiency fiber laser, but also great potential on the suppression of thermal-induced effects such as thermal mode instability and the thermal lens effect.

5 kW monolithic fiber amplifier employing homemade spindle-shaped ytterbium-doped fiber.

We have demonstrated a 5 kW high-power monolithic fiber amplifier employing a homemade spindle-shaped ytterbium-doped fiber (YDF) based on the main oscillator power amplifier configuration. The YDF consists of a spindle-shaped core and cladding along the fiber length, with a core/cladding diameter of 27/410 µm at both ends and 39.5/600 µm in the middle. An output power of over 5 kW and beam quality of about 1.9 and an optical-to-optical conversion efficiency of 66.6% were achieved in the amplifier under a bidirectional pump scheme. While operating at the maximum power, the laser performance was evaluated, and the transverse mode instability and stimulated Raman scattering effects were well mitigated. To the best of our knowledge, this is the highest power demonstration in a continuous-wave fiber laser employing a tapered fiber. Further power scaling is promising by optimizing the structure of the YDF.

Fast modal analysis for Hermite-Gaussian beams via deep learning.

The eigenmodes of Hermite-Gaussian (HG) beams emitting from solid-state lasers make up a complete and orthonormal basis, and they have gained increasing interest in recent years. Here, we demonstrate a deep learning-based mode decomposition (MD) scheme of HG beams for the first time, to the best of our knowledge. We utilize large amounts of simulated samples to train a convolutional neural network (CNN) and then use this trained CNN to perform MD. The results of simulated testing samples have shown that our scheme can achieve an averaged prediction error of 0.013 when six eigenmodes are involved. The scheme takes only about 23 ms to perform MD for one beam pattern, indicating promising real-time MD ability. When larger numbers of eigenmodes are involved, the method can also succeed with slightly larger prediction error. The robustness of the scheme is also investigated by adding noise to the input beam patterns, and the prediction error is smaller than 0.037 for heavily noisy patterns. This method offers a fast, economic, and robust way to acquire both the mode amplitude and phase information through a single-shot intensity image of HG beams, which will be beneficial to the beam shaping, beam quality evaluation, studies of resonator perturbations, and adaptive optics for resonators of solid-state lasers.

2†kW high-efficiency Raman fiber amplifier based on passive fiber with dynamic analysis on beam cleanup and fluctuation.

In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M2 improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.

All-fiberized cascaded random Raman fiber laser with high spectral purity based on filtering feedback.

Cascaded random Raman fiber lasers (CRRFLs) with simple configuration and high spectral purity have become a great candidate for power scaling over the 1.1 µm-2 µm spectral band. Recently, CRRFLs with high spectral purity over 90% have been proposed by applying a highly temporal-stable pump source or a free-space short-pass filter, at the cost of increased system complexity. In this work, pumped directly by a Yb-doped fiber oscillator at 1080 nm, an all-fiberized and simplified CRRFL with a short-pass optical filter based on bending fiber and a thin-film wavelength division multiplexer is demonstrated. The transmission loss of the filter for 5th Stokes order at 1440 nm is up to 70 dB. Spectral purity over 92% for all the first four Stokes orders is achieved. The highest output power is 15 W for the 4th Stokes order at 1341 nm.

Deep learning enabled superfast and accurate M2 evaluation for fiber beams.

We introduce deep learning technique to predict the beam propagation factor M2 of the laser beams emitting from few-mode fiber for the first time, to the best of our knowledge. The deep convolutional neural network (CNN) is trained with paired data of simulated near-field beam patterns and their calculated M2 value, aiming at learning a fast and accurate mapping from the former to the latter. The trained deep CNN can then be utilized to evaluate M2 of the fiber beams from single beam patterns. The results of simulated testing samples have shown that our scheme can achieve an averaged prediction error smaller than 2% even when up to 10 eigenmodes are involved in the fiber. The error becomes slightly larger when heavy noises are added into the input beam patterns but still smaller than 2.5%, which further proves the accuracy and robustness of our method. Furthermore, the M2 estimation takes only about 5 ms for a prepared beam pattern with one forward pass, which can be adopted for real-time M2 determination with only one supporting Charge-Coupled Device (CCD). The experimental results further prove the feasibility of our scheme. Moreover, the method we proposed can be confidently extended to other kinds of beams provided that adequate training samples are accessible. Deep learning paves the way to superfast and accurate M2 evaluation with very low experimental efforts.

Adaptive phase correction of dynamic multimode beam based on modal decomposition.

We propose and demonstrate a method for the adaptive phase correction of dynamic multimode fiber beams. The phase of incident beam is reconstructed in real-time based on the complete modal information, which obtained by using the modal decomposition of correlation filter method. For the proof of principle, both of the modal decomposition and the phase correction are implemented using the same computer-generated hologram, which was encoded into a phase-only spatial light modulator. We demonstrate the phase correction of dynamic multimode beam at a rate of 5 Hz and achieve a 1.73-fold improvement on the average power-in-the-bucket. The experimental results indicate the feasibility of the real-time phase correction for the large mode area fiber laser by adaptive optics.