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 (M 2 <1.3). The stability is verified in a continuous operation for over 2 hours with power fluctuation below 1%.
Transverse mode instability (TMI) is one of the main limitations for the power scaling of high-power fiber lasers. In order to suppress TMI and increase the output power of the fiber laser, we ...demonstrated a novel strategy by shifting the center wavelength of the pump source, i.e., laser diodes (LDs), from conventional 976 nm to the optimized 981 nm. A fiber oscillator is built to investigate the TMI effect in different pump wavelengths. Compared with 976 nm LDs, the TMI threshold increases from 208 W to 448 W when the laser is pumped by 981 nm LDs, indicating a 2.2 times enhancement of the TMI threshold. We also simulated and analyzed that the main heat source leading to TMI effect is the thermal effect caused by pump absorption, followed by quantum defect. We believe the optimized pump source has potential applications for suppressing TMI and in the power scaling of both broadband and narrow linewidth fiber lasers.
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 M
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.
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 (M
~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.
Since the concept of distributed feedback fiber random laser was put forward, random Raman fiber laser (RRFL) has made great progress in high power operation. For RRFL with a full-open cavity, the ...simplest cavity structure, further power scaling was restricted by the rapid increase of high-order Stokes wave. In this paper, we demonstrate that the output power of the RRFL can be further improved by optimizing the fiber length. The relationship between the RRFL output power and fiber length is researched theoretically and experimentally. Results show that to optimize the RRFL with a full-open cavity in output power to the best, the fiber length should be as short as possible, under the premise of avoiding causing strong four-wave mixing (FWM) and ensuring the sufficient absorption of signal light.
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 (M 2 ∼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.
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.
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.
High-power fiber lasers have been widely used in various industrial manufacturing and military defense applications. During the development of fiber lasers in the past decades, the thermal effect has ...always been one of the biggest obstacles. It is crucial to study the temperature characteristics and overcome the thermal restrictions for a better output performance. With the systematic design and optimization in this article, optical frequency domain reflectometry (OFDR) can achieve in situ distributed temperature measurement of the fiber core in high-power fiber lasers. This allows a better study of the temperature characteristics and thermal effects. The fiber-core distributed temperature of a kilowatt-level fiber oscillator is first demonstrated based on this method here. The splicing point between the high reflectivity fiber Bragg grating (HR-FBG) and gain fiber withstands the highest temperature, reaching 101.6 °C at a 1.47 kW output. In addition, the temperature of the gain fiber gradually decreases from 92 °C to 30 °C along the pumping direction. The internal temperatures of the combiner and HR-FBG are also measured to evaluate their performances in the high-power regime. The temperature distributions in the experiment agree well with the theoretical simulation.
•First application of a spindle-shaped gain fiber in a QCW fiber oscillator system.•Achieving the highest output peak power of near-single-mode QCW fiber lasers ever reported (peak power 6.4 kW, M2 ...factor 1.38).•Analyzed the causes of relatively low optical conversion efficiency and waveform deformation of QCW fiber laser.
High peak power quasi-continuous wave (QCW) fiber lasers with good beam quality are widely used in industry. But it’s difficult to balance the peak power and the good beam quality for common lasers. We proposed a novel QCW fiber laser oscillator, employing a 24 m long spindle-shaped ytterbium-doped fiber (YDF), which has a core/cladding diameter of 25/400–37.5/600–25/400 μm. The large core diameter section in the middle and tapered sections are 20 m long in total for suppressing the nonlinear effects in the laser cavity, and the small core diameter sections in two ends are 4 m to result in good beam quality. Finally, a near-single-mode QCW fiber laser oscillator with a beam quality of 1.38 (M2 factor) and peak power of 6.4 kW is achieved, accompanying with a high stimulated Raman scattering (SRS) suppression ratio of 20.3 dB. To the best of our knowledge, this is the first time to adopt a spindle-shaped YDF in a QCW fiber laser, and also the highest output power of near-single-mode QCW fiber lasers ever reported.
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