In this work,a confined-doped fiber with the core/inner-cladding diameter of 40/250μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate ga...In this work,a confined-doped fiber with the core/inner-cladding diameter of 40/250μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique,and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability(TMI)mitigation.Notably,the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments.It is found that the TMI threshold could be significantly affected by the seed laser mode purity.The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis.Finally,a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83,which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier.This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.展开更多
The effect of transverse mode instability (TMI) is currently the main limitation for thefurther average-power scaling of fiber laser systems with diffraction-limited beamquality. In this work a main driving force for ...The effect of transverse mode instability (TMI) is currently the main limitation for thefurther average-power scaling of fiber laser systems with diffraction-limited beamquality. In this work a main driving force for TMI in fiber amplifiers is identified. Ourexperiments and simulations illustrate that the performance of fiber laser systems interms of their diffraction-limited output power can be significantly reduced whenthe pump or seed radiation exhibit intensity noise. This finding emphasizes the factthat the TMI threshold is not only determined by the active fiber but, rather, by thewhole system. In the experiment an artificially applied pump intensity-noise of 2.9%led to a reduction of the TMI threshold of 63%, whereas a similar seed intensitynoisedecreased it by just 13%. Thus, even though both noise sources have animpact on the TMI threshold, the pump intensity-noise can be considered as themain driver for TMI in saturated fiber amplifiers. Additionally, the work unveils thatthe physical origin of this behavior is linked to the noise transfer function insaturated fiber amplifiers. With the gained knowledge and the experimental andtheoretical results, it can be concluded that a suppression of pump-noise frequenciesbelow 20 kHz could strongly increase the TMI threshold in high-power fiber lasersystems.展开更多
Transverse mode instability(TMI),induced by nonlinear thermal-optical coupling,poses a primary challenge for the power scaling of fiber lasers.In the fiber oscillator,a sealed resonant cavity,TMI could become particul...Transverse mode instability(TMI),induced by nonlinear thermal-optical coupling,poses a primary challenge for the power scaling of fiber lasers.In the fiber oscillator,a sealed resonant cavity,TMI could become particularly complex due to the mode competition during the laser oscillation.While traditional theories of TMI predominantly address two-mode coupling,this paper explores the TMI phenomena in few-mode fiber oscillators utilizing a holistic approach that includes solving steady-state thermal-optic coupling equations.The simulation shows that there is a non-monotonic correlation between bending loss and the TMI threshold,which is contrary to the monotonic associations suggested by two-mode interaction theory.When one high-order mode experiences net gain,fluctuations of the TMI threshold would occur,leading to the amplification of a new mode within the uncoupled frequency region,thus affecting the gain saturation.By designing the linewidth of a low-reflection grating(LR),the modal power management in the uncoupled frequency domain can be achieved.An excessively broad LR linewidth exacerbates mode coupling within the shared frequency region,thus exacerbating TMI.To validate the theoretical simulation,we carefully fabricated LRs and optimized the fiber coiling to elevate the TMI threshold.Through careful optimization of LR linewidth and bending radii,we achieved a record-breaking laser output of 10.07kW using a monolithic fiber oscillator,with no observable evidence of TMI.Our work demonstrates that modal power redistribution in independent frequency domains offers a novel approach to mitigating TMI in high-power fiber lasers.Additionally,it provides new insights into mode decoupling strategies pertinent to fiber communications.展开更多
In order to balance the suppression of stimulated Raman scattering(SRS)and transverse mode instability(TMI)in high-power fiber lasers,in this Letter,a new type of spindle-shaped ytterbium-doped fiber(YDF)with asymmetr...In order to balance the suppression of stimulated Raman scattering(SRS)and transverse mode instability(TMI)in high-power fiber lasers,in this Letter,a new type of spindle-shaped ytterbium-doped fiber(YDF)with asymmetric longitude distribution was designed and produced,which had a small-sized input end,large-sized transmission section,and moderate output end,enabling a good fit with a seed laser and mitigating SRS as well as TMI effects.A counter-pumped fiber laser amplifier was established using this YDF,and two kinds of laser diodes(LDs)were adopted for increasing the TMI threshold.Finally,the maximum output power reached 6 kW,and the beam quality(M~2factors)indicated near-single-mode output.The SRS suppression ratio under 6 kW output power was 36 dB,and no dynamic TMI was observed,which revealed that further enhancement of output power was limited only by pump power.展开更多
Power scaling based on traditional ytterbium-doped fibers(YDFs)is limited by optical nonlinear effects and transverse mode instability(TMI)in high-power fiber lasers.Here,we propose a novel long tapered fiber with a c...Power scaling based on traditional ytterbium-doped fibers(YDFs)is limited by optical nonlinear effects and transverse mode instability(TMI)in high-power fiber lasers.Here,we propose a novel long tapered fiber with a constant cladding and tapered core(CCTC)along its axis direction.The tapered-core region of the fiber is designed to enhance the stimulated Raman scattering(SRS)threshold and suppress higher-order mode resonance in the laser cavity.The CCTC YDF was fabricated successfully with a modified chemical vapor deposition(MCVD)method combined with solution doping technology,which has a cladding diameter of 400µm and a varying core with a diameter of~24μm at both ends and~31μm in the middle.To test the performance of the CCTC fiber during high-power operation,an all-fiber laser oscillator based on a CCTC YDF was investigated experimentally.As a result,a maximum output power of 3.42 kW was achieved with an optical-to-optical efficiency of 55.2%,although the TMI effect was observed at an output power of~3.12 kW.The measured beam quality(M^(2)factor)was~1.7,and no sign of the Raman component was observed in the spectrum.We believe that CCTC YDF has great potential to simultaneously mitigate the SRS and TMI effects,and further power scaling is promising by optimizing the structure of the YDF.展开更多
Unlike conventional continuous-wave lasers with wide spectra,the amplification of single-frequency lasers in optical fibers is much more difficult owing to the ultra-high power spectral density induced nonlinear stimu...Unlike conventional continuous-wave lasers with wide spectra,the amplification of single-frequency lasers in optical fibers is much more difficult owing to the ultra-high power spectral density induced nonlinear stimulated Brillouin scattering effect.Nevertheless,over the past two decades much effort has been devoted to improving the power scaling and performance of high-power single-frequency fiber amplifiers.These amplifiers are mostly driven by applications,such as high precision detection and metrology,and have benefited from the long coherence length,low noise,and excellent beam quality of this type of laser source.In this paper,we review the overall development of high-power single-frequency fiber amplifiers by focusing on its progress and challenges,specifically,the strategies for circumventing the stimulated Brillouin scattering and transverse mode instability effects that,at present,are the major limiting factors of the power scaling of the single-frequency fiber amplifiers.These factors are also thoroughly discussed in terms of free-space and all-fiber coupled architecture.In addition,we also examine the noise properties of single-frequency fiber amplifiers,along with corresponding noise reducing schemes.Finally,we briefly envision the future development of high-power single-frequency fiber amplifiers.展开更多
Achieving an all-fiber ultra-fast system with above kW average power and mJ pulse energy is extremely challenging.This paper demonstrated a picosecond monolithic master oscillator power amplifier system at a 25 MHz re...Achieving an all-fiber ultra-fast system with above kW average power and mJ pulse energy is extremely challenging.This paper demonstrated a picosecond monolithic master oscillator power amplifier system at a 25 MHz repetition frequency with an average power of approximately 1.2 kW,a pulse energy of approximately 48μJ and a peak power of approximately 0.45 MW.The nonlinear effects were suppressed by adopting a dispersion stretched seed pulse(with a narrow linewidth of 0.052 nm)and a multi-mode master amplifier with an extra-large mode area;then an ultimate narrow bandwidth of 1.32 nm and a moderately broadened pulse of approximately 107 ps were achieved.Meanwhile,the great spatio-temporal stability was verified experimentally,and no sign of transverse mode instability appeared even at the maximum output power.The system has shown great power and energy capability with a sacrificed beam propagation product of 5.28 mm·mrad.In addition,further scaling of the peak power and pulse energy can be achieved by employing a lower repetition and a conventional compressor.展开更多
基金supported by the National Natural Science Foundation of China(62035015)the Innovative Research Groups of Hunan Province(2019JJ10005)the Hunan Provincial Innovation Construct Project(2019RS3018).
文摘In this work,a confined-doped fiber with the core/inner-cladding diameter of 40/250μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique,and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability(TMI)mitigation.Notably,the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments.It is found that the TMI threshold could be significantly affected by the seed laser mode purity.The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis.Finally,a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83,which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier.This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.
基金Deutsche Forschungsgemeinschaft(DFG,German Research Foundation)-416342637,416342891,GRK 2101(259607349)Fraunhofer Gesellschaft–Fraunhofer Cluster of Excellence“Advanced Photon Sources”.
文摘The effect of transverse mode instability (TMI) is currently the main limitation for thefurther average-power scaling of fiber laser systems with diffraction-limited beamquality. In this work a main driving force for TMI in fiber amplifiers is identified. Ourexperiments and simulations illustrate that the performance of fiber laser systems interms of their diffraction-limited output power can be significantly reduced whenthe pump or seed radiation exhibit intensity noise. This finding emphasizes the factthat the TMI threshold is not only determined by the active fiber but, rather, by thewhole system. In the experiment an artificially applied pump intensity-noise of 2.9%led to a reduction of the TMI threshold of 63%, whereas a similar seed intensitynoisedecreased it by just 13%. Thus, even though both noise sources have animpact on the TMI threshold, the pump intensity-noise can be considered as themain driver for TMI in saturated fiber amplifiers. Additionally, the work unveils thatthe physical origin of this behavior is linked to the noise transfer function insaturated fiber amplifiers. With the gained knowledge and the experimental andtheoretical results, it can be concluded that a suppression of pump-noise frequenciesbelow 20 kHz could strongly increase the TMI threshold in high-power fiber lasersystems.
基金support from the National Natural Science Foundation of China(NSFC)(62405373,11974427)the Science and Technology Innovation Program of Hunan Province(2021RC4027).
文摘Transverse mode instability(TMI),induced by nonlinear thermal-optical coupling,poses a primary challenge for the power scaling of fiber lasers.In the fiber oscillator,a sealed resonant cavity,TMI could become particularly complex due to the mode competition during the laser oscillation.While traditional theories of TMI predominantly address two-mode coupling,this paper explores the TMI phenomena in few-mode fiber oscillators utilizing a holistic approach that includes solving steady-state thermal-optic coupling equations.The simulation shows that there is a non-monotonic correlation between bending loss and the TMI threshold,which is contrary to the monotonic associations suggested by two-mode interaction theory.When one high-order mode experiences net gain,fluctuations of the TMI threshold would occur,leading to the amplification of a new mode within the uncoupled frequency region,thus affecting the gain saturation.By designing the linewidth of a low-reflection grating(LR),the modal power management in the uncoupled frequency domain can be achieved.An excessively broad LR linewidth exacerbates mode coupling within the shared frequency region,thus exacerbating TMI.To validate the theoretical simulation,we carefully fabricated LRs and optimized the fiber coiling to elevate the TMI threshold.Through careful optimization of LR linewidth and bending radii,we achieved a record-breaking laser output of 10.07kW using a monolithic fiber oscillator,with no observable evidence of TMI.Our work demonstrates that modal power redistribution in independent frequency domains offers a novel approach to mitigating TMI in high-power fiber lasers.Additionally,it provides new insights into mode decoupling strategies pertinent to fiber communications.
基金supported by the Hunan Outstanding Youth Fund(No.2023JJ10057)the Changsha Excellent Innovative Youth Training Program(Nos.kq2206006 and kq2206002)the National Natural Science Foundation of China(No.62005315)。
文摘In order to balance the suppression of stimulated Raman scattering(SRS)and transverse mode instability(TMI)in high-power fiber lasers,in this Letter,a new type of spindle-shaped ytterbium-doped fiber(YDF)with asymmetric longitude distribution was designed and produced,which had a small-sized input end,large-sized transmission section,and moderate output end,enabling a good fit with a seed laser and mitigating SRS as well as TMI effects.A counter-pumped fiber laser amplifier was established using this YDF,and two kinds of laser diodes(LDs)were adopted for increasing the TMI threshold.Finally,the maximum output power reached 6 kW,and the beam quality(M~2factors)indicated near-single-mode output.The SRS suppression ratio under 6 kW output power was 36 dB,and no dynamic TMI was observed,which revealed that further enhancement of output power was limited only by pump power.
基金the National Natural Science Foundation of China(Nos.61735007 and 61705266).
文摘Power scaling based on traditional ytterbium-doped fibers(YDFs)is limited by optical nonlinear effects and transverse mode instability(TMI)in high-power fiber lasers.Here,we propose a novel long tapered fiber with a constant cladding and tapered core(CCTC)along its axis direction.The tapered-core region of the fiber is designed to enhance the stimulated Raman scattering(SRS)threshold and suppress higher-order mode resonance in the laser cavity.The CCTC YDF was fabricated successfully with a modified chemical vapor deposition(MCVD)method combined with solution doping technology,which has a cladding diameter of 400µm and a varying core with a diameter of~24μm at both ends and~31μm in the middle.To test the performance of the CCTC fiber during high-power operation,an all-fiber laser oscillator based on a CCTC YDF was investigated experimentally.As a result,a maximum output power of 3.42 kW was achieved with an optical-to-optical efficiency of 55.2%,although the TMI effect was observed at an output power of~3.12 kW.The measured beam quality(M^(2)factor)was~1.7,and no sign of the Raman component was observed in the spectrum.We believe that CCTC YDF has great potential to simultaneously mitigate the SRS and TMI effects,and further power scaling is promising by optimizing the structure of the YDF.
基金supported by the National Key R&D Program of China(No.2020YFC2200401)the National Natural Science Foundation of China(Nos.62005316 and 62035015)。
文摘Unlike conventional continuous-wave lasers with wide spectra,the amplification of single-frequency lasers in optical fibers is much more difficult owing to the ultra-high power spectral density induced nonlinear stimulated Brillouin scattering effect.Nevertheless,over the past two decades much effort has been devoted to improving the power scaling and performance of high-power single-frequency fiber amplifiers.These amplifiers are mostly driven by applications,such as high precision detection and metrology,and have benefited from the long coherence length,low noise,and excellent beam quality of this type of laser source.In this paper,we review the overall development of high-power single-frequency fiber amplifiers by focusing on its progress and challenges,specifically,the strategies for circumventing the stimulated Brillouin scattering and transverse mode instability effects that,at present,are the major limiting factors of the power scaling of the single-frequency fiber amplifiers.These factors are also thoroughly discussed in terms of free-space and all-fiber coupled architecture.In addition,we also examine the noise properties of single-frequency fiber amplifiers,along with corresponding noise reducing schemes.Finally,we briefly envision the future development of high-power single-frequency fiber amplifiers.
基金This work was financially supported by the CAS Project for Young Scientists in Basic Research(No.YSBR-065)the National Natural Science Foundation of China(Nos.62225507,62175230,and U2033211)the Scientific Instrument Developing Project of the Chinese Academy of Sciences(No.YJKYYQ20200001)。
文摘Achieving an all-fiber ultra-fast system with above kW average power and mJ pulse energy is extremely challenging.This paper demonstrated a picosecond monolithic master oscillator power amplifier system at a 25 MHz repetition frequency with an average power of approximately 1.2 kW,a pulse energy of approximately 48μJ and a peak power of approximately 0.45 MW.The nonlinear effects were suppressed by adopting a dispersion stretched seed pulse(with a narrow linewidth of 0.052 nm)and a multi-mode master amplifier with an extra-large mode area;then an ultimate narrow bandwidth of 1.32 nm and a moderately broadened pulse of approximately 107 ps were achieved.Meanwhile,the great spatio-temporal stability was verified experimentally,and no sign of transverse mode instability appeared even at the maximum output power.The system has shown great power and energy capability with a sacrificed beam propagation product of 5.28 mm·mrad.In addition,further scaling of the peak power and pulse energy can be achieved by employing a lower repetition and a conventional compressor.
