Temporal dynamics of mode instabilities in high-power fiber lasers and amplifiers

Novel Bidirectional Output Ytterbium-Doped High Power Fiber Lasers: From Continuous to Quasi-Continuous

Traditional ytterbium-doped high-power fiber lasers generally use a unidirectional output structure. To reduce the cost and improve the efficiency of the fiber laser, we propose a bidirectional output fiber laser (BOFL). The BOFL has many advantages over that of the traditional unidirectional output fiber laser (UOFL) and has a wide application in the industrial field. In theory, the model of the BOFL is established, and a comparison of the nonlinear effect in the traditional UOFL and the BOFL is studied. Experimentally, high-power continuous wave (CW) and quasi-continuous wave (QCW) BOFLs are demonstrated. In the continuous laser, we first combine the BOFL with the oscillating amplifying integrated structure, and a near-single-mode bidirectional 2 × 4 kW output with a total power of above 8 kW is demonstrated. Then, with the simple BOFL, a CW bidirectional 2 × 5 kW output with a total power of above 10 kW is demonstrated. By means of pump source modulation, a QCW BOFL is developed, and the output of a near-single mode QCW laser with a peak output of 2 × 4.5 kW with a total peak power of more than 9 kW is realized. Both CW and QCW output BOFL are the highest powers reported at present.

Polarization-resolved mode evolution in TMI-limited Yb-doped fiber amplifiers using a novel high-speed Stokes polarimeter

In this work we have developed a high-speed Stokes polarimeter method based on simultaneous 4-channel imaging with a high-speed camera. Thus, current speed limitations of imaging polarimeters for wavelengths around 1 µm can be overcome, allowing a sub-ms polarization-resolved characterization of transverse mode instability (TMI). Additionally, the Stokes parameters of each individual mode are calculated by a simultaneous 4-beam mode reconstruction algorithm during post-processing and can be analyzed with unprecedented temporal resolution. We demonstrate the measurement capabilities of this polarimeter setup by characterizing TMI of a large-mode-area Yb-doped polarization maintaining (PM) fiber amplifier with 30 kHz video frame rate. Upon thorough characterization, we have found for the first time that at the onset of TMI in a PM fiber, the modal polarization states begin to oscillate on circular and elliptical trajectories at the same frequencies as the modal energy transfer occurs. The ability to measure the modal polarization states with sub-ms temporal resolution is key to developing a fundamental understanding and subsequently possible mitigation strategies of TMI in PM-fiber lasers.

Mitigation of transverse mode instability by heat-load modulation

We present the first experimental realization of a new mitigation strategy for TMI based on controlling the phase shift between the modal intensity pattern and the thermally induced refractive index grating. If specific modulation parameters are applied while pulsing the seed and/or pump radiation, the direction of energy transfer is forced from the higher-order modes into the fundamental mode. In this way, the fiber amplifier can operate at an average output power significantly higher than the TMI threshold with a diffraction-limited beam profile. A stable beam profile is observed at an average output power that is 83% higher than the TMI threshold of the free-running system, with an intra-burst average power that is 4.15 times higher than the TMI threshold.

TMI and polarization static energy transfer in Yb-doped low-NA PM fibers

In this work, we conduct experimental investigations of transverse mode instabilities (TMI) in a large mode area ultra-low numerical aperture polarization maintaining fiber amplifier. This fiber is few mode in the slow-axis (conventional operation mode), but single mode in the fast-axis. We test the stability of the output beam by changing the input polarization angle and systematically investigate the transverse mode instability threshold in the two principal polarization axes. The lowest TMI threshold at 300 W was found when the input polarization angle was aligned parallel to the slow-axis. Detuning the input polarization angle from the slow-axis led to increased TMI thresholds. For input polarization angle of 90° (parallel to the fast-axis), the output signal was stable up to 475 W and further scaling was limited by the available pump power. However, for fast-axis operation a lower polarization ratio compared to slow-axis operation was observed as well as an unexpected static energy transfer from the fast-axis into the slow-axis above 400 W.

Origin of SRS-induced beam quality distortion under TMI threshold

In high power fiber lasers, the degradation of beam quality caused by Raman effect has attracted more and more attention in recent years, but its physical mechanism is still unclear. We're going to differentiate between heat effect and nonlinear effect by duty cycle operation. The evolution of beam quality at different pump duty cycles has been studied based on a quasi-continuous wave (QCW) fiber laser. It is found that even if the Stokes intensity is only -6 dB (energy proportion: 26%) lower than that of the signal light intensity, the beam quality has no obvious change with the duty cycle of 5%; on the contrary, when the duty cycle gradually approaches 100% (CW-pumped scheme), the beam quality distortion changes faster and faster with the increase of Stokes intensity. The experimental results are contrary to core-pumped Raman effect theory [IEEE Photon. Technol. Lett.34, 215 (2022)10.1109/LPT.2022.3148999], and further analysis confirms that the heat accumulation in the process of Stokes frequency shift should be responsible for this phenomenon. That is the first time, to the best of our knowledge, for intuitive reveal of the origin of stimulated Raman scattering (SRS)-induced beam quality distortion under transverse mode instability (TMI) threshold in an experiment.

Suppressing transverse mode instability through multimode excitation in a fiber amplifier

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, science, and defense. The power scaling for fiber amplifiers is currently limited by transverse mode instability. Most techniques for suppressing the instability are based on single- or few-mode fibers in order to output a clean collimated beam. Here, we study theoretically using a highly multimode fiber amplifier with many-mode excitation for efficient suppression of thermo-optical nonlinearity and instability. We find that the mismatch of characteristic length scales between temperature and optical intensity variations across the fiber generically leads to weaker thermo-optical coupling between fiber modes. Consequently, the transverse mode instability (TMI) threshold power increases linearly with the number of equally excited modes. When the frequency bandwidth of a coherent seed laser is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either the input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are required for fiber amplifiers in various applications.

SBS suppression using PRBS phase modulation with different orders

The Brillouin instability (BI) caused by stimulated Brillouin scattering (SBS) can limit the output power of high-energy laser amplifiers. Pseudo-random bitstream (PRBS) phase modulation is an effective modulation technique to suppress BI. In this paper, we study the impact of the PRBS order and modulation frequency on the BI threshold for different Brillouin linewidths. PRBS phase modulation with a higher order will break the power into a larger number of frequency tones with a lower maximum power in each tone, leading to a higher BI threshold and a smaller tone spacing. However, the BI threshold may saturate when the tone spacing in the power spectra approaches the Brillouin linewidth. For a given Brillouin linewidth, our results allow us to determine the order of PRBS beyond which there is no further improvement in the threshold. When a specific threshold power is desired, the minimum PRBS order required decreases as the Brillouin linewidth increases. When the PRBS order is too large, the BI threshold deteriorates, and this deterioration occurs at smaller PRBS orders as the Brillouin linewidth increases. We investigate the dependence of the optimal PRBS order on the averaging time and fiber length, and we did not find a significant dependence. We also derive a simple equation that relates the BI threshold for different PRBS orders. Hence, the increase in BI threshold using an arbitrary order PRBS phase modulation may be predicted using the BI threshold from a lower PRBS order, which is computationally less time-consuming to compute.

Influence of Pump Current Waveform on The Mitigation of Transverse Mode Instability in Fiber Laser Oscillator

We carry out a detailed investigation of TMI mitigation by pump modulation based on multiple current waveforms in a fiber laser oscillator. Compared with continuous wave (CW), the modulation of various waveforms, including sinusoidal wave, triangular wave, and pulse wave with a duty cycle of 50% and 60%, can increase the TMI threshold. The average output power of a stabilized beam is boosted via the adjustment of phase difference between the signal channels. The TMI threshold is increased to 270 W under a modulation of pulse wave (duty cycle: 60%) with a phase difference of 440 μs, where the beam quality is 1.45. This threshold can be further improved by adding groups of pump LDs and drivers, which is a promising approach for beam stabilization of high-power fiber lasers.

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.

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.

Low-numerical aperture confined-doped long-tapered Yb-doped silica fiber for a single-mode high-power fiber amplifier

A low-numerical aperture (NA) confined-doped long-tapered (LCT) Yb-doped fiber is proposed and fabricated by modified chemical vapor deposition combined with solution doping technique. The LCT fiber owns the core NA of ∼0.05 and the gain dopant doping diameter ratio of ∼77%, with a core/cladding diameter of 25/400 µm at both ends and 37.5/600 µm in the middle. The laser performance is demonstrated by a bidirectional pumping all-fiber amplifier, of which a 4.18-kW single-mode (M² factor ∼1.3) laser output is achieved with a slope efficiency of ∼82.8%. Compared with the conventional fiber, the co-pumped and counter-pumped transverse mode instability thresholds and beam quality of the LCT fiber are remarkably enhanced. Throughout the continuous operation, the LCT fiber amplifier presents high power stability with fluctuation of < 1%. These results indicate that LCT fiber has great potential in power scaling remaining excellent beam quality.

Few-cycle Yb laser source at 20 kHz using multidimensional solitary states in hollow-core fibers

We demonstrate ultrashort pulse compression from 300 fs down to 17 fs at a repetition rate of 20 kHz and 160-µJ output pulse energy (3.2 W of average power) using multidimensional solitary states (MDSS) in a 1-meter hollow-core fiber (HCF) filled with N₂O. Under static pressure, thermal limitations at this repetition rate annihilate the MDSS with suppression of spectral broadening. The results obtained in differential pressure configuration mitigate thermal effects and significantly increase the range of repetition rate over which MDSS can be used to compress sub-picosecond laser pulses.

Demonstration of constant-cladding tapered-core Yb-doped fiber for mitigating thermally-induced mode instability in high-power monolithic fiber amplifiers

In this work, a large-mode-area (LMA) step-index constant-cladding tapered-core (CCTC) Yb-doped fiber with a cladding diameter of ∼600 µm is successfully fabricated. The CCTC fiber has a small-core region (diameter of ∼20 µm) at both ends and a large-core region (diameter of ∼36 µm) in the middle. To prove the laser performance of the CCTC fiber, a detailed comparison experiment with conventional uniform fiber with the same effective core diameter is carried out in a multi-kW all-fiber MOPA configuration. The experimental results show that employing the CCTC fiber can effectively mitigate the thermally-induced transverse mode instability (TMI) in both co-pump and counter-pump schemes, and realize high slope efficiency and single-mode beam quality (M²∼1.30). Under the counter-pump scheme, the TMI threshold of the CCTC fiber is observed at ∼2.49 kW with a slope efficiency of 86.2%, while the uniform fiber amplifier exhibits a TMI threshold of ∼2.05 kW. The theoretical analysis based on a semi-analytical model indicates this CCTC fiber can effectively improve the TMI threshold owing to a stronger gain saturation. Our results verify the great potential of such an LMA CCTC fiber to mitigate thermal-induced TMI effect and achieve single-mode operation without sacrifice of laser efficiency in high power monolithic fiber lasers, and the further power scaling is expected by optimizing the fiber design.

2 × 2 kW near-single-mode bidirectional high-power output from a single-cavity monolithic fiber laser

Traditional monolithic fiber lasers can only achieve unidirectional high-power laser output. In this Letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high-power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high-reflectivity fiber Bragg grating with a low-reflectivity fiber Bragg grating to realize bidirectional laser output. In our experiment, the laser cavity was composed of two fiber Bragg gratings with a reflectivity of about 10%. The pump power provided by the 976 nm laser diodes was injected into a double-clad Yb-doped fiber with core/cladding diameters of 20/400 µm. At the maximum pump power, the bidirectional output powers were 2025 W and 1948 W, respectively, and the output laser beam quality (M² factor) at both ends was about 1.5. For the first time, to the best of our knowledge, the feasibility of a bidirectional output fiber laser that can achieve double high (2-kW-level) power was verified. Compared with a traditional unidirectional output laser, this type of bidirectional output laser can achieve a double high-power laser by employing a laser resonant cavity. Thus, the average cost and structure size can be further reduced in mass production.

High-peak-power pump-modulated quasi-CW fiber laser

Quasi-continuous wave (CW) laser output with high peak power and high energy is preferred in some industrial applications. Due to the non-linear effects and transverse mode instability, such high-peak-power laser output is difficult to achieve via monolithic fiber lasers in CW mode. For diode-pumped monolithic fiber lasers, by applying overshoot pulse modulation to the pumping diodes, we obtain a pulse laser output with a peak power much higher than that in CW mode. In this paper, it has been theoretically studied that stable pulses with the width of µs level can be generated with quasi-CW operation without distortion according to our simulation. We also experimentally demonstrate a bi-directional pumped quasi-CW monolithic fiber laser operating in CW and pulse modes. In quasi-CW mode, by applying overshoot pulse modulation to the diodes, with a frequency of 1 kHz and a pulse width of 100 µs, the peak power of the output laser reached 9713 W with an average power of 898 W and M² of 2.4 and 2.3 in the two orthogonal directions, respectively. To the best of our knowledge, this is the very first quasi-CW fiber laser of a 10-kW level with the M² level of 2.

Experimental study on the impact of signal bandwidth on the transverse mode instability threshold of fiber amplifiers

In this work, we conduct a detailed experimental study on the impact of signal bandwidth on the TMI threshold of fiber amplifiers. Both the filtered superfluorescent fiber sources and the phase-modulated single-frequency lasers are employed to construct seed lasers with different 3 dB spectral linewidths ranging from 0.19 nm to 7.97 nm. The TMI threshold of the fiber amplifier employing those seed lasers are estimated through the intensity evolution of the signal laser, and different criteria have been utilized to characterize the spectral linewidth of the seed lasers. Notably, the experimental results reveal that the TMI threshold of fiber amplifiers grows, keeps constant, and further grows as a function of spectral linewidth of seed lasers. Our experimental results could provide a well reference to understand the mechanism of the TMI effect and optimize the TMI effect in high-power fiber amplifiers.

6 kW single mode monolithic fiber laser enabled by effective mitigation of the transverse mode instability

The transverse mode instability (TMI) has been one of the main limitations for the power scaling of single mode fiber lasers. In this work, we report a 6 kW single mode monolithic fiber laser enabled by effective mitigation of the TMI. The fiber laser employs a custom-made wavelength-stabilized 981 nm pump source, which remarkably enhanced the TMI threshold compared with the wavelength of 976 nm. With appropriately distributing bidirectional pump power, the monolithic fiber laser is scaled to 6 kW with single mode beam quality (M²<1.3). The stability is verified in a continuous operation for over 2 hours with power fluctuation below 1%.

Accurate and efficient modeling of the transverse mode instability in high energy laser amplifiers

We study the transverse mode instability (TMI) in the limit where a single higher-order mode (HOM) is present. We demonstrate that when the beat length between the fundamental mode and the HOM is small compared to the length scales on which the pump amplitude and the optical mode amplitudes vary, TMI is a three-wave mixing process in which the two optical modes beat with the phase-matched component of the index of refraction that is induced by the thermal grating. This limit is the usual limit in applications, and in this limit TMI is identified as a stimulated thermal Rayleigh scattering (STRS) process. We demonstrate that a phase-matched model that is based on the three-wave mixing equations can have a large computational advantage over current coupled mode methods that must use longitudinal step sizes that are small compared to the beat length.

Experimental analysis of Raman-induced transverse mode instability in a core-pumped Raman fiber amplifier

The effect of transverse mode instability is a limitation for the power scaling of fiber laser systems, that can originate due to heat caused by stimulated Raman scattering. In this contribution, we experimentally investigate the threshold of transverse mode instability caused by stimulated Raman scattering in a passive fiber. Both, the Stokes seed power and the fiber length of a core-pumped Raman fiber amplifier are varied to systematically study this effect. Mode resolved measurements reveal that the threshold occurs at approximately the same Stokes output power for all tested configurations, independent of the total Raman conversion efficiency. These results increase the understanding of this type of mode instability and show which parameters are important for a further power scaling of high-power Raman fiber amplifiers.

Experimental investigation of quasi-static mode degradation in a high power large mode area fiber amplifier

In this work, quasi-static mode degradation in high power fiber amplifiers has been investigated experimentally. An increase of M² from 1.3 to 2.6 with distortion of the beam profile is observed, which results in the signal spectra and backward light characterization departing from the traditional phenomena. The amplifier has been operated at the same input pump power of 705 W for nearly 2.2 hours to investigate the relationship between quasi-static mode degradation and photodarkening. The evolution of M² factor/beam profile, mode correlation coefficient and output laser power at different working times indicate that the quasi-static mode degradation in the high power fiber amplifiers is dependent on photodarkening and evolves on the scale of tens of minutes. A visible green light has been injected to photobleach the gain fiber for 19 hours, which reveals that the quasi-static mode degradation has been suppressed simultaneously. To the best of our knowledge, this is the first detail report of photodarkening-induced quasi-static degradation in high power fiber amplifiers.

Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries

Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics.

Influence of pedestal diameter on mode instabilities in Yb/Ce/Al-doped fibers

In this paper we present numerical and experimental results revealing that the mode instability threshold of highly Yb-doped, Ce/Al co-doped pedestal fibers is affected by the size of the index-increased pedestal structure surrounding the core. An alternative preparation technology for the realization of large mode area fibers with very large Al-doped silica pedestals is introduced. Three different pedestal fiber design iterations characterized by low photodarkening were manufactured and tested in counter-pumped amplifier setups. Up to 1.9 kW continuous-wave output power of near-diffraction-limited beam quality (M² = 1.26) was achieved with an 18/200/420 µm fiber of very low NA = 0.042, limited only by the occurrence of mode instabilities.

Experimental investigations of seeding mechanisms of TMI in rod fiber amplifier using spatially and temporally resolved imaging

In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST). We show that inherent pump intensity noise from the power supply can define significant peaks in the resulting TMI spectrum. ST measurements show that the TMI in the transition region consists of different orientations of LP₁₁. This finding indicates that the simple picture of TMI being seeded by the combination of a static initial fraction of LP₁₁ and pump or signal intensity noise is not valid for our measurements. Furthermore we present seeding of TMI by perturbing the input coupling dynamically. ST measurements of the resulting TMI as a function of perturbation frequency provides quantitative information regarding the frequency response of the non-linear coupling coefficient. Finally, ST measurements of the resulting TMI as a function of signal power shows that the TMI experiences an exponential gain long before visible beam fluctuations appear.

Transverse mode instability in a passive fiber induced by stimulated Raman scattering

Transverse mode instabilities are a major limitation for power scaling of fiber lasers but have so far only been observed in laser-active fibers. In this contribution we present experimental observations of transverse mode instabilities in a passive fiber. In this fiber, stimulated Raman scattering acted as heat source. To demonstrate the effect, a kW-level ytterbium-doped fiber laser was used as pump for a Raman amplifier. Transverse mode instabilities were only observed in the case with high Raman amplification. Frequency resolved stability measurements at various fiber positions as well as spectral and mode resolved measurements pin their origin to the passive fiber. This observation might help to gain further understanding of transverse mode instabilities and shows limitations of high-power Raman amplifiers.

Tapered Yb-doped fiber enabled monolithic high-power linearly polarized single-frequency laser

The all-fiber high-power linearly polarized single-frequency fiber laser based on the polarization-maintaining tapered Yb-doped fiber (T-YDF) is systematically studied. As a result, a 300 W-level stable output with linear polarization and nearly diffraction-limited beam quality is demonstrated. In particular, the overall properties of the transverse mode instability (MI) effect in such a single-frequency laser system are discussed in detail for the first time, to the best of our knowledge, including temporal, frequency, polarization, and spatial domains. Furthermore, the beam pointing error taking the MI effect into account is investigated. Theoretical analyses covering both stimulated Brillouin scattering and the MI effects reveal the great potential of the T-YDF for further power scaling as well.

550 W single frequency fiber amplifiers emitting at 1030 nm based on a tapered Yb-doped fiber

In this paper, we report a high power single frequency 1030 nm fiber laser with near-diffraction-limited beam quality based on a polarization-maintaining tapered Yb-doped fiber (T-YDF). The T-YDF has advantages of effectively suppressing stimulated Brillouin scattering (SBS) while maintaining good beam quality. As a result, a record output power of 379 W single frequency, linearly polarized, nearly single-mode fiber amplifier operating at 1030 nm is demonstrated. The polarization extinction ratio is as high as 16.3 dB, and the M² is measured to be 1.12. Further, the dependence of the thermal-induced mode instability (TMI) threshold on the polarization state of an input signal laser is investigated for the first time. By changing the polarization state of the injected seed laser, the output power can increase to 550 W while the beam quality can be maintained well (M²=1.47). The slope efficiency of the whole amplifier is about 80%. No sign of SBS appears even at the highest output power and the further brightness scaling of both situations is limited by the TMI effect. To the best of our knowledge, this result is the highest output power of all-fiberized single frequency fiber amplifiers.

Ultra-low intensity noise, all fiber 365 W linearly polarized single frequency laser at 1064 nm

We demonstrate a robust linearly polarized 365 W, very low amplitude noise, single frequency master oscillator power amplifier at 1064 nm. Power scaling was done through a custom large mode area fiber with a mode field diameter of 30 µm. No evidence of stimulated Brillouin scattering or modal instabilities are observed. The relative intensity noise is reduced down to -160 dBc/Hz between 2 kHz and 10 kHz via a wide band servo loop (1 MHz bandwidth). We achieve 350 W of isolated power, with a power stability < 0.7% RMS over 1100 hours of continuous operation and a near diffraction limited beam (M² < 1.1).

Extremely robust femtosecond written fiber Bragg gratings for an ytterbium-doped fiber oscillator with 5 kW output power

We present highly robust fiber Bragg gratings (FBGs) in passive large-mode-area fibers for kilowatt fiber laser systems. The gratings were inscribed directly through the fiber coating using near-infrared femtosecond laser pulses and then implemented in an all-fiber ytterbium-doped single-mode oscillator setup reaching up to 5 kW signal output power. The untreated cooled FBGs showed thermal coefficients as low as ${1}\;{\rm K}\;{{\rm kW}^{ - 1}}$1KkW^-1, proving excellent qualification for the implementation into robust high-power fiber laser setups.

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.

Mode instabilities in Yb:YAG crystalline fiber amplifiers

Mode instabilities (MI) threshold in the Yb:YAG crystalline fiber amplifier is simulated by a full numerical model. The propagation of signal fields is simulated by the finite-difference beam-propagation method combined with the rate equations, and the time-dependent heat equation is solved by the alternating-direction-implicit method. Considering the strong temperature-dependent laser performance of Yb:YAG, an iterative method is applied to reach the steady state of Yb:YAG, the crystalline fiber amplifier, before the simulation of MI behavior. The simulated MI thresholds in Yb:YAG crystalline fiber amplifiers are found to be at least 28 times of those in Yb-doped silica-glass fiber amplifiers, up to tens of kilowatts. Simulation results show that, in addition to the expected higher thermal conductivity and lower thermo-optic coefficient, strong gain saturation also plays an important role in the high MI threshold of the Yb:YAG crystalline fiber.

Instability transverse mode phase transition of fiber oscillator for extreme power lasers

High-power fiber lasers have been widely explored in engineering and science, and improving the beam quality restricted by transverse mode instability (TMI) is waiting to be solved for extra high-power applications. Here, we theoretically propose a phase transition model for understanding TMI in a fiber oscillator. A general dynamics model describing TMI is established by modifying the heat equation, and then a special dynamics model of a two-mode fiber (TMF) oscillator is obtained by applying this general dynamics model to TMF oscillator case. Theoretical analysis shows that there is a reversible phase transition point in this TMF oscillator model, which can well explain the sudden and reversible change of TMI. Based on linear stability analysis near the phase transition point, an analytical threshold formula of TMI is given to calculate the TMI threshold in the TMF oscillator. The calculated results are consistent with the reported experimental results. Furthermore, the relationship between the TMI threshold and several parameters was also discussed in detail such as laser wavelength, pump wavelength, core radius, cladding radius, etc. This theoretical model will be useful to understand and suppress the TMI in fiber oscillators.

Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier

Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.

High power monolithic tapered ytterbium-doped fiber laser oscillator

In the power scaling of monolithic fiber lasers, the fiber nonlinear effects and transverse mode instability are main limitations. The tapered gain fiber has a longitudinally varying mode area, which has the advantage of mitigating fiber nonlinear effects. However, the transverse mode instability (TMI) was seldom reported in the tapered fiber lasers at high average power levels. In this work, we have constructed a monolithic tapered ytterbium-doped fiber laser oscillator and investigated the laser oscillator performance with respective 976 nm and 915 nm pump, especially on the aspects of the TMI. The double cladding tapered ytterbium-doped fiber has a narrow end of ~20/400 μm and a wide end of ~30/600 μm. Fiber Bragg gratings (FBG) are respectively inscribed on double cladding fibers with core/inner cladding diameter of 20/400 μm and 30/400 μm to match with the narrow and wide end of the tapered ytterbium-doped fiber. When 915 nm pump is employed, the TMI occurs at the output power of ~1350 W. The output power is further scaled to a maximum of 1720 W. The M² factor of the output laser is ~2.1 and the full width at half maximum (FWHM) of the signal laser is ~3.6 nm. To the best of our knowledge, this is the highest average power for the tapered ytterbium-doped fiber lasers.

Modal instabilities in high power fiber laser oscillators

Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.

Ultrafast thulium fiber laser system emitting more than 1 kW of average power

In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.

Static and dynamic mode coupling in a double-pass rod-type fiber amplifier

This Letter describes an experimental realization of a double-pass amplifier using rod-type fiber. In this device, the gain reaches 26 dB amplifying a 300 mW, 20 ps, 20 MHz seed up to 120 W, with an optical-to-optical efficiency of 50% and excellent beam quality. In addition, by design the output of the amplifier has a polarization extinction ratio of 33 dB. Besides these good performances, we report a marginal degradation of mode quality and degree of polarization followed by the so-called transverse mode instability which occurs at 120 W signal power. The first degradation is static, and by analyzing its two polarizations, we conclude it is caused by a coupling between modes due to the formation of a static thermal long-period grating, which in turn initiates the dynamic instability.

Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers

Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.

Impact of gamma-ray radiation-induced photodarkening on mode instability degradation of an ytterbium-doped fiber amplifier

The laser performance of a high-power ytterbium-doped fiber amplifier is mainly hindered by the onset of mode instability. In this work, the slope efficiency and mode instability threshold of the ytterbium-doped fiber under various gamma-ray radiation doses have been measured. Experimental results reveal that gamma-ray radiation-induced photodarkening degrades mode instability severely, and gamma-ray radiation-induced mode instability degradation can be partly bleached by hours of pump-light injection. It is shown that gamma-ray radiation-induced photodarkening results in a steep reduction of slope efficiency and mode instability threshold; moreover, the entire irradiated fiber can be partly bleached by hours of pump-light injection and exhibits both time and gamma-ray radiation-dose saturation properties. The experimental results indicate that mode instability mitigation can be partly realized by pump-light injection and implies photodarkening suppression is beneficial for TMI mitigation, which is very promising for the advancement of high-power fiber lasers.

Phase-shift evolution of the thermally-induced refractive index grating in high-power fiber laser systems induced by pump-power variations

A phase shift between the modal interference pattern and the thermally-induced refractive index grating is most likely the ultimate trigger for the damaging effect of transverse mode instabilities (TMI) in high-power fiber laser systems. By using comprehensive simulations, the creation and evolution of a thermally-induced phase shift is explained and illustrated in detail. It is shown that such a phase shift can be induced by a variation of the pump power. The gained knowledge about the generation and evolution of the phase shift will allow for the development of new mitigation strategies for TMI.

Transverse mode instability threshold enhancement in Yb-doped fiber lasers by cavity modification

To enhance the transverse mode instability (TMI) threshold of a fiber oscillator, a novel configuration is presented. In this configuration the oscillator cavity length is considerably reduced and the remaining active fiber is released out of the cavity to absorb the rest of the pump power and amplify the output signal of the cavity. In fiber oscillators, the index gratings generated by both forward and backward propagating signals can interact with light propagating in the backward and forward directions and degrade the output beam quality. In the proposed modified configuration, due to lower intra-cavity forward and backward signal power the index grating is smoothed and the TMI threshold is increased. Experimental results indicate that this modified configuration has a higher TMI threshold than a conventional fiber oscillator. Finally a higher TMI threshold is achieved in the bidirectional pumping scheme of the modified configuration.

Finite element steady periodic beam propagation analysis of mode instability in high power fiber amplifiers

A finite element steady periodic scalar beam propagation method suited to treating mode instability in amplifiers incorporating fibers with complicated design features is presented and demonstrated on test cases. These incorporate large pitch photonic crystal fibers with confined and down-doped cores in co-pumped and counter-pumped configurations. Seed power fluctuations and slight launch misalignment are proposed and implemented as a noise source to seed the instability. Down doping the confined core by -7.0E-5 was found to increase the instability threshold by 25% in the co-pumped case and 9% in the counter-pumped case. The increase in threshold is correlated with the suppression of higher frequency components of intensity gratings associated with the instability.

All-fiber, single-frequency, and single-mode Er3+:Yb3+ fiber amplifier at 1556 nm core-pumped at 1018 nm

Emerging applications, such as gravitational wave astronomy, demand single-frequency lasers with diffraction-limited emission at 1.5 μm. Fiber amplifiers have greatly evolved to fulfill these requirements. Hundreds of watts are feasible using large-mode-area and specialty fibers. However, their application in a few watts to tens of watts in monolithic systems is unnecessarily complex due to the poor commercial availability of fiber components and standard integration procedures. In this Letter we propose and experimentally demonstrate a novel and simple method to amplify single-frequency signals at 1.5 μm up to tens of watts by core-pumping single-mode Er³⁺:Yb³⁺ fiber amplifiers at 1018 nm. The proof-of-principle system is tested with different active fibers, lengths, and seed power levels. Over 11 W with an efficiency of more than 48% versus launched power is achieved. Additionally, performance degradation during operation was observed for which photodarkening due to P1 defects might be an explanation.

Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold

A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of ~600W, which is a factor of 2 above the mode instability threshold.

Experimental study of the mode instability onset threshold in high-power FA-LPF lasers

We report here on an experimental investigation of the temporal behavior of transverse mode instabilities into "fully aperiodic large-pitch fibers" (FA-LPFs) operated in high-power continuous-wave laser configuration. To ensure an effective transverse single-mode emission into FA-LPFs, a perfect index matching between the active core and the background cladding materials (Δn=0) is required. The original design of such fibers enables an effective transverse single-mode emission by strengthening the higher-order mode delocalization out of the gain region, even for high heat load levels, consequently leading to the improvement of the beam spatial quality. The study was conducted over fibers of various gain region diameters, from 58 to 100 μm, for a refractive index mismatch Δn of about +8×10^-5. The emitted beam is characterized using both M² measurements and time traces to study the changeover of a stable temporal behavior to an unstable one.

Measuring thermal load in fiber amplifiers in the presence of transversal mode instabilities

We report on detailed in situ distributed temperature measurements inside a high power fiber amplifier. The deducted thermal load and the transversal mode instability (TMI) threshold of a commercial large mode area fiber with 25 μm core and 400 μm cladding were measured at various seed wavelengths. By matching these results with detailed simulations we show that photodarkening has a negligible impact on the thermal load and, therefore, on the TMI threshold in this fiber.

Mode instability dynamics in high-power low-numerical-aperture step-index fiber amplifier

The study on mode instability (MI) in the large-mode-area fiber is generating great interest regarding the high-power applications of fiber lasers. To the best of our knowledge, we have investigated for the first time the dynamics of the output beam from a kilowatt-level all-fiber amplifier based on the low-numerical-aperture (<0.04) step-index (SI) fiber before and after the onset of the MI, including the temporal dynamics and mode evolution. The temporal power fluctuations indicate three evolution stages apart from the onset threshold of the MI, defined as stable, transition, and chaotic regions. In addition, the mode decomposition technique is utilized to accurately observe and investigate the mode evolution and relevant modal content corresponding to the transition and chaotic regions in the SI fiber laser for the first time. According to the mode decomposition results, the reduction of the extracted power can be explained by the high bending loss of the high-order mode excited in the MI process. Finally, the difference of MI dynamics between the fiber lasers based on the SI fiber and rod-type photonic crystal fiber is discussed.

Single mode 4.3 kW output power from a diode-pumped Yb-doped fiber amplifier

We investigate the average power scaling of two diode-pumped Yb-doped fiber amplifiers emitting a diffraction-limited beam. The first fiber under investigation with a core diameter of 30 µm was able to amplify a 10 W narrow linewidth seed laser up to 2.8 kW average output power before the onset of transverse mode instabilities (TMI). A further power scaling was achieved using a second fiber with a smaller core size (23µm), which allowed for a narrow linewidth output power of 3.5 kW limited by stimulated Brillouin scattering (SBS). We mitigated SBS using a spectral broadening mechanism, which allowed us to further increase the output power to 4.3 kW only limited by the available pump power. Up to this power level, a high slope efficiency of 90% with diffraction-limited beam quality and without any sign of TMI or stimulated Raman scattering for a spectral dynamic range of higher than -80 dB was obtained.

414 W near-diffraction-limited all-fiberized single-frequency polarization-maintained fiber amplifier

A high-power 1064 nm single-frequency polarization-maintained fiber amplifier based on an all-fiber master oscillator power amplifier configuration is demonstrated. To mitigate the stimulated Brillouin scattering (SBS) and the mode instability (MI) effect, a polarization-maintained Yb-doped fiber with a high dopant concentration and a 25 μm core diameter is adopted in the main amplifier stage; in addition, step-distributed longitudinal strain is imposed on the active fiber to broaden its effective SBS gain spectrum and further increase the SBS threshold. As a result, a pump-limited 414 W single-frequency fiber laser is obtained without signs of SBS and MI. Experimental results show that the SBS threshold is increased by at least two times. The slope efficiency of the main amplifier is about 80%. The polarization degree is higher than 98% at all the power levels. The beam quality is measured with a M² of 1.34. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on an all-fiber structure.

Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity

Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-and-dump scheme with a 30-m-long cavity. Using an acousto-optic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes.

Mitigating transverse mode instability in all-fiber laser oscillator and scaling power up to 2.5 kW employing bidirectional-pump scheme

Transverse mode instability (TMI) is one of the main limiting factors in kW-level fiber lasers. Unlike fiber amplifiers, TMI in fiber laser oscillators attracts less attention from researchers. In this work, we construct an all-fiber ytterbium-doped laser oscillator and investigate the performance in co-pumping and bidirectional-pumping configurations, respectively. In the co-pumping scheme, TMI occurs at ~1.6kW and restricts further output power scaling. Different from the characteristic of dynamic TMI in fiber amplifiers, quasi-static TMI is observed in the laser oscillator. Details of the temporal characteristic around the TMI threshold are provided. In the bidirectional-pumping scheme, experimental results validate that the TMI is mitigated notably by employing bidirectional-pumping instead of co-pumping. The output laser power is further scaled to 2.5kW with a slope efficiency of 74.5% and good beam quality (M²~1.3). At the maximum power, the FWHM bandwidth of optical spectra is 5.2nm, and the Raman stokes light is ~20dB below the signal.