High-power Yb-doped Fiber Laser Research Articles

High-temperature-insensitivity high-power Yb-doped fiber laser directly in-band pumping by 1018 nm laser diodes.

In this Letter, we demonstrate a high-power ytterbium-doped fiber laser (YDFL) based on a directly in-band pumping scheme (DIPS) which employs 1018 nm laser diodes (LDs) as pump sources. The wavelength of the LDs is designed theoretically, considering the distribution of Yb3+ absorption cross section (σa) as well as quantum defect (QD). The flat distribution of σa around 1018 nm ensures excellent temperature insensitivity and flexibility for the YDFL. Besides, lower QD and more compact structure promise higher optical-to-optical (O-O) and electrical-to-optical (E-O) efficiencies. Based on the experimental setup, as the cooling temperature of the 1018 nm LDs ranges from 6 to 23°C, an output power of 2 kW level is achieved that varies by only 2.01% without adjusting the operating current of the LDs subjectively. The output power is then scaled up to 5 kW level. Furthermore, there is a great potential to achieve higher output power and E-O efficiency in YDFLs based on the DIPS.

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

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

Thermal modeling of high-power Yb-doped fiber lasers with irradiated active fibers.

With both radiation effects and thermal effects taken into consideration, a multiphysics thermal model concerning high-power Yb-doped fiber lasers operated with post-irradiated active fibers is established. Radiation-related parameters, including propagation losses, refractive indexes and lifetime, are considered. And, with the temperature profile of the active fiber, temperature-dependent parameters, including absorption and emission cross-sections, refractive indexes and lifetime, are updated every loop to simulate the output parameters. Simulation results show that radiation induces great changes to the thermal profiles of the active fiber. And severe performance degradation of high-power Yb-doped fiber lasers are recorded, featuring a remarkable drop in output power and an even steeper decline in the transverse mode instability threshold, which is a predominant limitation at high radiation doses. With a deposited radiation of 100 Gy, an output decline of about 50% and a mode instability threshold drop over 85% are observed. And it's shown that, with the exploited active fiber, it's hardly possible for the investigated fiber laser to generate stable single-mode output at kilowatt levels with accumulated radiation doses beyond 50 Gy. At low radiation doses within 20 Gy, to maintain safe and stable single-mode operation of the laser system, longer active fibers with lower absorption coefficients are preferred despite a small rollover of the output power.

Theoretical and experimental study on the thermally dependent transient response of the high power continuous wave Yb-doped fiber laser.

In this paper, transient thermal effects of the Yb-doped fiber and the laser diode (LD) in high power Yb-doped fiber lasers (YDFLs) are studied theoretically to analyze the transient response of the fiber laser. Based on the transient heat conduction equation, we simulated the temperature variation of the YDF and LD. It is found that the transient response of the laser is mainly affected by the thermally induced wavelength shifting of LDs. We modified the rate equations of the fiber laser according to the temperature variation of the LD. To improve the transient response speed of the fiber laser, we present three methods: (a) raising the cooling temperature, (b) increasing the length of gain fiber, and (c) using the YDF with high doping. Finally, experiments were carried out to verify the theory, and the results are in good agreement.

High power narrow linewidth LP11 mode fiber laser using mode-selective FBGs

We demonstrated a high power Yb-doped fiber laser capable of delivering LP11 mode laser output. The oscillation of LP11 mode was realized by incorporating a pair of few-mode fiber Bragg gratings (FBGs), whose LP11 mode reflection peaks are matched with each other. The temperature-sensitivity of the FBGs was used to optimize the matching degree of the LP11 mode reflection peaks. Based on this configuration, over 50 W of LP11 mode output power with a linewidth of around 70 pm was obtained, which, to the best of our knowledge, is the highest power obtained in LP11 mode all-fiber lasers. Moreover, this laser possesses a certain stability; the standard deviation of whose output power is ~0.07% over a period of 300 s stability test. This work could provide a stable and high power LP11 transverse mode laser source for a wide range of applications.

High-Power and High-Order Random Raman Fiber Lasers

High-order random Raman fiber lasers are investigated for generating high-power widely wavelength-tunable light sources. By cascaded random Raman lasing and adjusting pump laser wavelength, continuous wavelength tuning from 1 to 1.9 μm is reported. For power scaling, a high-power Yb-doped fiber laser at 1064 nm with improved temporal stability is developed as the pump source. The ninth-order cascaded random Raman fiber laser with a spectral purity of 86.6% is achieved at 1806 nm. Up to 100.1-W inband power is obtained with an optical efficiency of 38.4% from 1064 nm and 27.2% from 915 nm. The results prove that cascaded random Raman fiber laser is a versatile technology to generate high-power fiber laser at wavelength outside the rare earth emission bands.

Pump Wavelength Dependence of Photodarkening in Yb-Doped Fibers

Photodarkening in Yb-doped fibers is known to be dependent of Yb population inversion. In addition to this dependence, we report that pump wavelength selection plays a significant role in photodarkening dynamics. It is found that 976 nm pump source can lead to more severe photodarkening effects than 916 nm pumping at the same inversion levels. This observation is consistent in Yb-doped fibers with various compositions. Our work reveals pump wavelength is another determinant factor of photodarkening besides the inversion level and emphasizes the importance of pump wavelength selection in high power Yb-doped fiber lasers and amplifiers.

掺镱光纤光暗化研究进展——溶胶凝胶法抑制光暗化

高功率掺镱光纤激光器的光暗化现象导致激光器功率降低，阻碍了激光器功率的进一步提升，因此研究掺镱光纤光暗化现象对促进掺镱光纤激光器的发展具有重要意义。本文首先从光暗化的表征、光暗化对激光器的影响及其机理，光暗化的影响因素及抑制手段方面对光暗化进行了综述，阐述了光暗化的研究进展；然后简单介绍了采用溶胶凝胶法工艺制备掺镱光纤的研究进展，并提出了采用溶胶凝胶法制备的掺镱光纤可能具有抑制光暗化效果的猜想。 Photodarkening of high power Yb-doped fiber lasers leads to the decrease of laser power, which limits the further development of high power laser. Therefore, it is significantly important to study the photodarkening behavior of Yb-doped fiber lasers. Firstly, the research progress of photodarkening in Yb-doped fiber is reviewed in this paper, including characterization, impact factors, mechanism, and suppression. Then based on the research progress of Sol-Gel method, it is proposed that the Yb-doped fiber prepared by Sol-Gel method may inhibit photodarkening.

Diode-Pumped 1178-nm High-Power Yb-Doped Fiber Laser Operating at 125 C

We demonstrate a high-power Yb-doped fiber laser operating at 1178 nm based on commercial silica double-cladding fiber and laser diode (LD) pumping regime at 976 nm. It is experimentally proved that heating the active fiber is an efficient way to suppress amplified spontaneous emission (ASE) and to increase the parasitic lasing threshold in long-wavelength Yb-doped fiber lasers. By adopting output coupling fiber Bragg grating (OC FBG) with high reflectivity and heating the Yb-doped fiber to 125 $^{\circ}\text{C}$ , a record output power of 31 W is obtained, and the corresponding slope efficiency is $\sim$ 44% with respect to the pump power. To the best of our knowledge, it is the highest output power of Yb-doped fiber lasers, which adopt commercial double-cladding active fiber and operate in the range beyond 1160 nm. Furthermore, it is first experimentally proved that the parasitic lasing threshold is related to the power of the backward ASE and the length of the active fiber.

Spectral broadening in high-power Yb-doped fiber lasers employing narrow-linewidth multilongitudinal-mode oscillators.

High-power continuous wave fiber lasers with narrow linewidths are required for spectral beam combining systems. A 2.9kW Yb-doped fiber laser with an output linewidth as narrow as 0.31nm employing a narrow-linewidth multilongitudinal-mode oscillator is experimentally demonstrated. The numerical simulations reveal that the spectral broadening of the bandwidth grows almost linearly with injected mode number, which provides a potential way to further suppress the nonlinear spectral broadening in high-power fiber lasers.

A fast algorithm to predict spectral broadening in CW bidirectionally pumped high-power Yb-doped fiber lasers

A detailed, fast-converging iterative numerical method has been developed to model continuous-wave bidirectionally pumped Yb-doped fiber lasers with output powers of several hundred watts. The analysis shows nonlinearity-induced broadening of the lasing spectrum, which also modifies power efficiency. Cavity dynamics is described by combining the effects of Kerr nonlinearities with power evolution equations and rate equations. The fast method to find steady-state solutions for cavity setups is based on setting the temporal phase evolution as a stochastic process with proper spectral filtering. Spectral properties of bidirectionally pumped lasers are calculated within few minutes using a commercial desktop computer, and very good agreement with experimental measurements is obtained for up to 922 W total pump and 708 W output power.

Short-Term and Long-Term Stability in Ytterbium-Doped High-Power Fiber Lasers and Amplifiers

In this paper we discuss recent progress in exploring short-term and long-term stability of high power Yb-doped fiber lasers and amplifiers. Long-term stability is associated with photodarkening effects that can significantly reduce operational lifetime of a high-power laser system. Short-term stability is associated with so-called transverse modal instabilities that degrade output beam quality at average powers above a certain threshold. In this paper we review ongoing studies that provide experimental characterization, and explore physical causes and mitigation strategies of the different physical phenomena involved. Those studies are critical for achieving stable operation of high power fiber lasers.

YDFL Operating in 1150–1200-nm Spectral Domain

A family of high-power Yb-doped fiber lasers operated in the range of 1150–1180 nm with output powers of up to 35 W and optical efficiencies up to 60% is realized. Operation at 1200 nm is also demonstrated. Amplified spontaneous emission increase with output power increase is analyzed in frames of the inhomogeneous broadening concept.

Review on recent progress on Yb-doped fiber laser in a variety of oscillation spectral ranges

We present review on recent progress on high power Yb-doped fiber laser in a variety of oscillation spectral ranges. According to Kurkov (Laser Phys. Lett. 2006), the oscillation spectral range of YDFLs can be divided into three bands: convenient (C-band), short (S-band), and long (L-band). For the C-band YDFL, tandem-pumped high power radiation by using 1018 nm fiber laser or 1030 nm thin-disk laser is reviewed. For the S-band YDFL, three mainly technical solutions, i.e., special designed fiber, the employment of photonic crystal fiber (PCF), and tandem pumping using 930 nm Nd-doped fiber laser are presented. For the L-band laser, active heating or self-heating the active fiber can help the long-wavelength oscillation. Fiber Raman laser or amplifier can also boost the power of L-band YDFL up. Some discussions and prospective predictions are proposed at the end of this paper.

主振荡功率放大结构窄线宽全光纤激光器334 W高功率输出

A narrow-linewidth high power Yb-doped fiber laser in all-fiber format is set up. The seed laser has an output power of more than 40 mW with linewidth narrower than 100 MHz. The fiber laser employs three-stage master oscillator power amplifier(MOPA) configuration. It outputs 334 W narrow-linewidth laser when pumped with a total power of 405 W in the main amplifier. The optical-to-optical conversion efficiency is 82.4%. Presently, the output power is limited by the pump power. It can be predicted that the laser power can be further scaled with increased available pump power.

Effective area limit of large-mode-area solid-core photonic bandgap fibers for fiber laser applications

We investigate the bending characteristics of solid-core photonic bandgap fibers (SC-PBGFs) aiming to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for high-power Yb-doped fiber lasers and amplifiers. Through detailed numerical simulations based on the finite element method (FEM), we evaluate the impacts of the order of photonic bandgap on the bending performance and point out the limits of core size enlargement in the SC-PBGFs with a 1-cell core structure due to the increment of bending loss. In addition, under practical constraints, we find that the SC-PBGFs having a 7-cell core can achieve sufficient differential bending loss between the fundamental mode and the higher-order modes and a much larger effective area limit as compared with previously-reported index-guiding LMA fibers.

Wide Dynamic Range Optical Power Control Based on Cascaded Fused Tapered Couplers for High-Power Yb-Doped Fiber Lasers

Wide dynamic-range optical power control was achieved in a novel all-fiber device for a high-power Yb-doped fiber laser input using a serially cascaded fused tapered coupler pair. Torsional stress was independently applied over the coupling zones, which resulted in phase changes by photoelastic effects and subsequent linear shifts in transmission spectra. Dynamic range of 46 dB and low insertion loss below 1 dB were achieved for 1060-nm fiber laser with the incident fiber coupled power of 3.4 W. The detailed fabrication process of the device, characterization, and theoretical background are discussed.

Design of all-solid leakage channel fibers with large mode area and low bending loss

We investigate a novel design for all-solid large mode area (LMA) leakage channel fibers (LCFs) for high-power Yb-doped fiber lasers and amplifiers, based on a single down-doped-silica rod ring surrounding a seven-cell pure-silica core, aiming for effectively single-mode behavior and low bending loss characteristics. Through detailed numerical simulations based on the finite element method (FEM), we find that the proposed all-solid LMA-LCFs, having a seven-cell core and two different sizes of down-doped rods, can achieve sufficient differential mode loss and much lower bending loss, as compared with a previously-reported LCF with a one-cell core and six large down-doped-silica rods.

GaP waveguide emitters for high power broadband THz generation pumped by Yb-doped fiber lasers

We demonstrate the generation of broadband THz pulses by optical rectification in GaP waveguides pumped by high power Yb-doped fiber amplifiers. The dispersion of the GaP emitter can be controlled via the geometry of the waveguide; the peak frequency of the emitted THz radiation is tuned by varying the waveguide cross-section. Most importantly, the use of a waveguide for the THz emission increases the coherent buildup length of the THz pulses and offers scalability to higher power; this was investigated by pumping a GaP waveguide emitter with a high power Yb-doped fiber laser system. A 25-MHz-repetition-rate pulse train of THz radiation with 120 muW average power was achieved using 14 W optical power, which represents the highest average power for a broadband THz source pumped by fiber lasers to date.

Kinetic modeling of Q-switched high-power ytterbium-doped fiber lasers.

We present a kinetic model for Q-switched, cladding-pumped, high-power Yb-doped fiber lasers that are based on the rate equation, in a difference equation form, of ion population and propagation equations for both pumping and signal light. The effects of fiber-laser parameters, such as doping, length, pump power, and repetition rate on pulse characteristics are analyzed. This model is used to analyze the performance of Q-switched multicore fiber lasers and to show the output pulses with enhanced characteristics.