Power Scaling Of Fiber Lasers Research Articles

Mitigating transverse mode instability in fiber laser oscillator by employing direct pump modulation

The transverse mode instability (TMI) is a thermal-optical effect, inducing a sudden deterioration of beam quality and average power. This non-linear effect will greatly limit the power scaling of fiber lasers. We demonstrate a direct pump modulation to mitigate TMI in a 30 μm-core-diameter all-fiber laser oscillator. It only relies on function generator and pump drivers without the need of feedback control units. The groups of pump LDs and pump drivers and the phase difference between the channels of function generator have a great impact on the beam stability. Compared with the TMI threshold 180 W in continuous wave (CW) mode, the maximum average output power of a stabilized beam is increased up to 310 W via a multi-parameter-based direct pump modulation.

694 W sub-GHz polarization-maintained tapered fiber amplifier based on spectral and pump wavelength optimization.

The comprehensive suppression of the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) is a critical issue for the power scaling of fiber laser with sub-GHz spectral linewidth. In this manuscript, a narrow linewidth and polarization-maintained (PM) fiber amplifier based on tapered Yb-doped fiber (T-YDF) is established, and the effects of spectral linewidth, spectral shape and pump wavelength on the SBS and/or TMI thresholds are investigated. Up to 694 W polarization-maintained fiber laser with just ∼790 MHz linewidth is obtained by combining the advantages of tapered Yb-doped fiber, near-rectangular spectral injection and 915 nm pump manner. This work could provide a well reference solution for the realization of high-power ultra-narrow linewidth fiber lasers.

Comparison and Optimization on Transverse Mode Instability of Fiber Laser Amplifier Pumped by Wavelength-Stabilized and Non-Wavelength-Stabilized 976 nm Laser Diode

We present an investigation of the effect of pumping wavelength on the threshold of transverse mode instability (TMI) in high-power ytterbium-doped fiber laser. By using a fiber laser amplifier with a bi-directional pumping structure, we obtain the TMI thresholds under wavelength-stabilized and non-wavelength-stabilized 976 nm laser diodes (LDs) pumping scheme. The results show that the non-wavelength-stabilized LD pump has a higher TMI threshold compared to the wavelength-stabilized LD pump. By optimizing the operating current of the non-wavelength-stabilized 976 nm LD, a ∼4kW output laser with near single-mode beam quality (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ∼1.3) is achieved, with a slope efficiency of ∼84.4%. The analysis yields that when the pumping wavelength deviates from the 976 nm peak, the TMI threshold can be increased. This reveals that optimizing pump source wavelength has great potential for suppressing TMI effect in the power scaling of fiber lasers.

Laser diode directly backward pumped high-beam-quality 10-kW fiber laser

Fiber lasers have been widely used in the industrial and scientific fields due to their advantages of high conversion efficiency, simple thermal management, and consistent stability. High brightness and high-power fiber lasers are affected by stimulated Raman scattering and transverse mode instability, which limits the power scaling of fiber lasers. Therefore, there are only a few researches achieving a 10 kW-level fiber laser system by laser diode direct pumping or tandem pumping. In this work, we demonstrate an all-fiber laser amplifier based on home-made low numerical aperture (NA) fiber pumped by 976 nm laser diodes. When the signal light is input to the gain fiber with a minimum bending diameter of 12 cm, the beam quality factor &lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; is about 1.72. The onset of transverse mode instability (TMI) is observed at 2467 W output power, accompanied by beam quality degradation. In order to suppress the onset of TMI, the minimum bending diameter of the gain fiber is changed from 12 cm to 20 cm. And the signal light is input into the gain fiber with a bending diameter of 28 cm. Benefiting from this operation, the fiber laser amplifier achieves maximum output power of 10.53 kW with an optical-to-optical efficiency of 74.04%, and there is no TMI onset observed. However, increasing bending diameter inevitably leads the beam quality to degrade. At the maximum output power, the beam quality factor &lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; is 2.88. To the best of our knowledge, this is the highest optical-to-optical efficiency and the best beam quality in 10 kW-level laser diodes pumping fiber lasers. Generally, it is believed that reducing bending diameter can suppress TMI by increasing high-order mode loss. However, this rule is not applicable to few-mode fiber lasers. A larger bending diameter leads more high-order modes to be contained in the signal light instead of leaking into the cladding area. Thus, a higher output and poor beam quality are obtained. Also, it is believed that tightly coiled fiber can make mode coupling easier and trigger off TMI, which results in a positive correlation between the TMI threshold and bending diameter. Low NA fibers are very sensitive to bending, and reducing the bend diameter to control the beam quality will result in lower efficiency and a lower TMI threshold. Therefore, although producing a 10 kW-level fiber laser is simple, maintaining good beam quality in the power scaling process is still a challenge. The results of this study will be a valuable reference for high power fiber laser design.

Highly-Integrated Signal and Pump Combiner in Chirally-Coupled-Core Fibers

Integrated fiber components enable compact and robust laser systems but are usually not available with specialty fibers. However, specialty fibers allow power scaling of fiber lasers and amplifiers with all advantages of these fiber designs. In this context, chirally-coupled-core fibers show promising properties for optical fiber components. In this paper, we present the development of a highly-integrated signal and pump combiner in chirally-core-fibers using a side-pumping technology. Combining up to four fiber-coupled pump diodes, a pump-light limited power handling of 600 W can be achieved with an efficiency of 78%. The combiner was tested in a side-pumped single-frequency all-fiber amplifier but can also be implemented in almost any fiber laser or amplifier.

Optimizing the pump wavelength to improve the transverse mode instability threshold of fiber laser by 3.45 times

Transverse mode instability (TMI) in fiber lasers refers to the phenomenon of dynamic coupling between the fundamental mode (FM) and high-order modes (HOMs) when the pumping power exceeds a certain threshold, which seriously affects the power scaling of fiber laser. In order to improve the TMI threshold in the fiber laser, we optimized the pump wavelength both consider the quantum defect and the pump absorption. In the experiment, the pump wavelength is optimized and a wavelength-stabilized laser diode (LD) with a central wavelength of 969 nm is proposed as the pump source to suppress the TMI. A forward-pumped Yb-doped fiber laser oscillator with a core/cladding diameter of 30/400 µm was built experimentally. Experimental results show that the TMI threshold of the laser is 719 W when pumped by 969 nm LDs, which is 3.45 times that value when pumped by commonly used 976 nm LDs.

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.

Characterization of Yb-doped ZBLAN fiber as a platform for radiation-balanced lasers

Recent advances in power scaling of fiber lasers are hindered by the thermal issues, which deteriorate the beam quality. Anti-Stokes fluorescence cooling has been suggested as a viable method to balance the heat generated by the quantum defect and background absorption. Such radiation-balanced configurations rely on the availability of cooling-grade rare-earth-doped gain materials. Herein, we perform a series of tests on an ytterbium-doped ZrF 4 – BaF 2 – LaF 3 – AlF 3 – NaF (ZBLAN) optical fiber to extract its laser-cooling-related parameters and show that it is a viable laser-cooling medium for radiation balancing. In particular, a detailed laser-induced modulation spectrum test is performed to highlight the transition of this fiber to the cooling regime as a function of the pump laser wavelength. Numerical simulations support the feasibility of a radiation-balanced laser, but they highlight that practical radiation-balanced designs are more demanding on the fiber material properties, especially on the background absorption, than solid-state laser-cooling experiments.

Efficient 240W single-mode 1018nm laser from an Ytterbium-doped 50/400µm all-solid photonic bandgap fiber.

Lowering the quantum defect by tandem pumping with fiber lasers at 1018nm was critical for achieving the record 10kW single-mode ytterbium fiber laser. Here we report the demonstration of an efficient directly-diode-pumped single-mode ytterbium fiber laser with 240W at 1018nm. The key for the combination of high efficiency, high power and single-mode at 1018nm is an ytterbium-doped 50μm/400μm all-solid photonic bandgap fiber, which has a practical all-solid design and a pump cladding much larger than those used in previous demonstrations of single-mode 1018nm ytterbium fiber lasers, enabling higher pump powers. Efficient high-power single-mode 1018nm fiber laser is critical for further power scaling of fiber lasers and the all-solid photonic bandgap fiber can potentially be a significant enabling technology.

Quasi-static mode instability in few-mode fiber amplifier

One of the most outstanding limitations in the evolution of the power scaling of fiber laser with near diffraction limited beam quality has been the mode instability since it was found in 2010. For a long time, researchers have focused on the dynamic mode instability (DMI) theoretically and experimentally, and it was not until 2016 that a new analytical model called quasi-static mode instability (QSMI) was proposed. Unlike DMI, because of the one-way energy transfer characteristic on a specific time scale, QSMI will show no apparent fluctuations with respect to the time domain traces. In this paper, based on a counter-pump few-mode fiber amplifier schematic system, the output power, beam quality and time traces of the amplifier under changing seed laser power are measured to investigate its mode instability effect. The ytterbium-doped fiber of the amplifier has a core diameter of 25 μm and inner cladding diameter of 400 μm, which can support 4-5 modes to be transmitted in the amplifier. The experimental results reveal that QSMI happens in the few-mode fiber amplifier. Taking 234 W seed power for example, it is found that when the output power reaches 2030 W, the optical-to-optical efficiency begins to fell from 86% to 32%, and at the same time the M2 value has an abrupt degradation from 2.2 to 2.8, which indicates that MI happens. On the other hand, it can be seen from the time traces of the output laser that there exist no rapid fluctuations, and the Fourier analysis shows no sign of DMI characteristic frequency components either. Quoting the definition of drifting ratio σ, when the output power is 2030 W under 234 W seed power, it is only 4%, and thus verifying that it is QSMI instead of DMI. The experiment also indicates that increasing the seed power has an effective influence on enhancing the mode instability power. When the seed power is raised from 86 W to 528 W, the corresponding threshold power is increased from 1560 W to 3090 W. And for 528 W seed power, when the output laser surpasses 3000 W, the optical-to-optical efficiency does not decline as fast as other relatively low seed power. To sum up, the mode instability effect represents a kind of quasi-static property in these large core diameter few-mode fiber amplifiers, which needs further studying.

Modal instability induced by stimulated Raman scattering in high-power Yb-doped fiber amplifiers.

Modal instability (MI) and stimulated Raman scattering (SRS) are the main obstacles in the power scaling of fiber lasers and amplifiers. In the power scaling of a high-power ytterbium (Yb)-doped master oscillator power amplifier system, a new type of MI has occurred. Experimentally, it is shown that just at the onset of the SRS effect, MI takes place, and the degradation of the beam quality is observed. By the spectra and beam quality measurements, it is revealed that this type of MI can be mitigated firmly by suppressing the SRS effect in high-power Yb-doped fiber amplifiers.

Mitigate transverse mode instability by linear inner-cladding fiber

Transverse mode instability (TMI) limits power scaling of fiber lasers. A semianalytical model is established to calculate the TMI threshold in high-power fiber laser systems of the multi-kW-class. A linear inner-cladding fiber can mitigate the TMI effect by smoothing the heat profile and decreasing the nonlinear coupling coefficient along the fiber. We investigate strong pump absorption of a linear inner-cladding fiber, which leads to shorter fiber length. Utilizing a 915-nm copumping scheme, the linear inner-cladding fiber can realize 10-kW output power in single-mode regime theoretically.

Experimental demonstration of transverse mode instability enhancement by a counter-pumped scheme in a 2 kW all-fiberized laser

Transverse mode instability (TMI) has become the major limitation for power scaling of fiber lasers with nearly diffraction-limited beam quality. Compared with a co-pumped fiber laser, a counter-pumped fiber laser reveals TMI threshold enhancement through a semi-analytical model calculation. We demonstrated a 2 kW high-power counter-pumped all-fiberized laser without observation of TMI. Compared with the co-pumped scheme, the TMI threshold is enhanced at least 50% in counter-pumped scheme, moreover, stimulated Raman scattering and four-wave mixing are suppressed simultaneously.

High-power fiber lasers based on tandem pumping

Power scaling of fiber lasers is challenged by several factors, such as the brightness of the pump source, the nonlinear effect, modal instability, and so on. Pumping active fibers with high-brightness fiber lasers instead of laser diodes is a promising solution for the brightness limitation and modal instability. In this paper, for the first time to our knowledge, we present a general review on the achievements of various kinds of high-power fiber lasers based on the tandem pumping scheme in the past few years. The requirements for tandem pumping ytterbium (Yb), erbium (Er), thulium (Tm), and holmium (Ho)-doped fibers are analyzed, and corresponding achievements are summarized. Hundreds of watts of fiber lasers at ∼1020, ∼1500, and 1900 nm and hundred-watt-level fiber lasers at ∼1150 and ∼1180 nm have been successfully achieved. Then, these powerful fiber lasers with high brightness can be employed as pump sources for Yb-, Er-, Tm- and Ho-doped fibers. Moreover, a recent experimental result of a 3.5 kW Yb-doped fiber amplifier in an all-fiber format is reported in addition to previous typical achievements. The underlying challenges for further power scaling, including the nonlinear effect suppression and special fiber design, are briefly discussed. Exploring the tandem pumping scheme in novel application fields is discussed as well.

Direct experimental observation of stimulated thermal Rayleigh scattering with polarization modes in a fiber amplifier

Modal interference can lead to intensity modulations in optical fibers, which can produce refractive index gratings under the influence of quantum defect heating in a fiber laser. These gratings are perfectly phased matched for mode couplings, which can lead to transverse mode instabilities at high average powers in fiber lasers. A detailed understanding of this process is critical for further power scaling of fiber lasers. We have directly observed and characterized this quantum-defect-assisted mode coupling for the first time using polarization modes in a PM fiber amplifier, providing solid experimental evidence for this key mechanism for transverse mode instability in fiber lasers.

Large-mode-area fibers operating near single-mode regime.

Lower NA in large-mode-area fibers enables better single-mode operation and larger core diameters. Fiber NA has traditionally been limited to 0.06, mostly due to the control tolerance in the fabrication process. It has been recognized recently that transverse mode instability is a major limit to average power scaling in fiber lasers. One effective method to mitigate this limit is to operate nearer to the single-mode regime. Lower fiber NA is critical in this since it allows relatively larger core diameters which is the key to mitigate the limits imposed by nonlinear effects. We have developed a fabrication process of ytterbium-doped silica glass which is capable of highly accurate refractive index control and sufficient uniformity for LMA fibers. This process is also capable of large-volume production. It is based on a significant amount of post-processing once the fiber preforms are made. We have demonstrated 30/400 and 40/400 LMA fibers with a NA of ~0.028 operating very close to the single-mode regime. The second-order mode cuts off at ~1.2μm and ~1.55µm respectively. We have also studied issues related to bend losses due to the low NA and further optimization of LMA fibers.

Large-Mode-Area All-Solid Photonic Bandgap Fibers for the Mitigation of Optical Nonlinearities

There is still significant need for power scaling of fiber lasers. Large-mode-area fibers are a key for the mitigation of optical nonlinearities. In recent years, mode instability has shown itself to be an additional significant limiting factor for single-mode power scaling in the regime of a few hundred watts to kilowatts. It is better appreciated now that further power scaling requires significant high-order-mode suppression in addition to a large effective mode area in a fiber. In recent years, we have shown that all-solid photonic bandgap fibers are a superior approach due to their unsurpassed higher-order-mode suppression in large-mode-area designs, making them well suited for applications at high average powers. We will review of some of the recent progress, challenges, and prospects of all-solid photonic bandgap fibers in this invited paper.

고출력 광섬유 레이저 기술의 현황 및 전망

Over the past two decades, fiber-based lasers have made remarkable progress, now having reached power levels exceeding kilowatts and drawing a huge amount of attention from academy and industry as a replacement technology for bulk lasers. In this paper we review the significant factors that have led to the progress of fiber lasers, such as gain-fiber regimes based on ytterbium-doped silica, optical pumping schemes through the combination of laser diodes and double-clad fiber geometries, and tandem schemes for minimizing quantum defects. Furthermore, we discuss various power-limitation issues that are expected to incur with respect to the ultimate power scaling of fiber lasers, such as efficiency degradation, thermal hazard, and system-instability growth in fiber lasers, and various relevant methods to alleviate the aforementioned issues. This discussion includes fiber nonlinear effects, fiber damage, and modal-instability issues, which become more significant as the power level is scaled up. In addition, we also review beam-combining techniques, which are currently receiving a lot of attention as an alternative solution to the power-scaling limitation of high-power fiber lasers. In particular, we focus more on the discussion of the schematics of a spectral beam-combining system and their individual requirements. Finally, we discuss prospects for the future development of fiber laser technologies, for them to leap forward from where they are now, and to continue to advance in terms of their power scalability.

Decreasing effective reflectivity of the output coupler in the power scaling of fiber lasers

Increasing power leads to the spectral broadening of fiber laser beams. In this effect the spectral line width of the laser beam becomes broader than that of the output coupler (OC) due to power scaling. Therefore, the effective reflectivity of the OC mirror decreases. In this paper, this phenomenon is investigated theoretically and experimentally. A Liekki Application Designer V3.3 was used for the theoretical investigation and the experiments were conducted during the fabrication of a 630W fiber laser. Given the excellent agreement between the simulation and the experimental results, a decrease in the effective reflectivity of the OC mirror was proved. The decrease in the effective reflectivity of the OC mirror changes some of the laser parameters, such as optical efficiency, longitudinal distribution of population inversion and the backward power inside the resonator. This may also lead to power instability. The effects of a decrease in the reflectivity of the OC on the mentioned parameters of the fiber laser are investigated in the present paper. Finally, recommendations for choosing appropriate OC mirror parameters in high-power (kW class) fiber lasers are suggested.

Numerical analysis of lasing characteristics in highly bend-compensated large-mode-area ytterbium-doped double-clad leakage channel fibers.

The bend-induced mode-area reduction and thermal effects are vital factors that affect the power scaling of fiber lasers. Recently, bend-compensated large-mode-area double-clad modified hybrid leakage channel fiber (M-HLCF) has been reported with a mode area greater than 1000 μm, while sustaining the single-mode behavior at 1064 nm for high-temperature environments. In this work, the lasing characteristics of a newly designed ytterbium-doped double-clad M-HLCF (YDMHLCF) have been numerically investigated for strongly pumped conditions. The doped region size is optimally found through simulations, equivalent to the size of core diameter ∼38 μm in order to achieve maximum conversion efficiency for the bent and straight cases. Numerical simulations further confirm that a 2 m long YDMHLCF exhibits slope efficiency of 78% and conversion efficiency of 79% for the straight case and also almost the same for the practical bending radius of 7.5 cm when pumped with a 975 nm laser source.