Kilowatt-level supercontinuum generation in a single-stage random fiber laser with a half-open cavity

The random distributed-feedback fiber laser(RFL) is a new approach to obtain a high-power stable supercontinuum(SC) source.To consider both structure simplification and high-power SC output,an innovative structure achieving a kilowatt-level SC output in a single-stage RFL with a half-open cavity is demonstrated in this paper.It consists of a fiber oscillator,a piece of long passive fiber and a broadband coupler,among which the broadband coupler acting as a feedback device is crucial in SC generation.When the system has no feedback,the backward output power is up to298 W under the pump power of 1185 W.When the feedback is introduced before the pump laser,the backward power loss can be reduced and the pump can be fully utilized,which could promote forward output power and conversion efficiency significantly.Under the maximum pump power of 1847 W,a 1300 W SC with spectrum ranging from 887 to1920 nm and SC conversion efficiency of 66% is obtained.To the best of our knowledge,it is the simplest structure used for high-power SC generation,and both the generated SC output power and the conversion efficiency are highest in the scheme of the half-opened RFL output SC.

High-energy and high-peak-power GHz burst-mode all-fiber laser with a uniform envelope and tunable intra-burst pulses

We report a Yb-doped all-fiber laser system generating burst-mode pulses with high energy and high peak power at a GHz intra-burst repetition rate.To acquire the uniform burst envelope,a double-pre-compensation structure with an arbitrary waveform laser diode driver and an acoustic optical modulator is utilized for the first time.The synchronous pumping is utilized for the system to reduce the burst repetition rate to 100 Hz and suppress the amplified spontaneous emission effect.By adjusting the gain of every stage,uniform envelopes with different output energies can be easily obtained.The intra-burst repetition rate can be tuned from 0.5 to 10 GHz actively modulated by an electro-optic modulator.Optimized by timing control of eight channels of analog signal and amplified by seven stages of Yb-doped fiber amplifier,the pulse energy achieves 13.3 mJ at 0.5 ns intra-burst pulse duration,and the maximum peak power reaches approximately3.6 MW at 48 ps intra-burst pulse duration.To the best of our knowledge,for reported burst-mode all-fiber lasers,this is a record for output energy and peak power with nanosecond-level burst duration,and the widest tuning range of the intra-burst repetition rate.In particular,this flexibly tunable burst-mode laser system can be directly applied to generate high-power frequency-tunable microwaves.

2.1 μm,high-energy dissipative soliton resonance from a holmium-doped fiber laser system

We propose a 2.1μm high-energy dissipative soliton resonant(DSR)fiber laser system based on a mode-locked seed laser and dual-stage amplifiers.In the seed laser,the nonlinear amplifying loop mirror technique is employed to realize mode-locking.The utilization of an in-band pump scheme and long gain fiber enables effectively exciting 2.1μm pulses.A section of ultra-high numerical aperture fiber(UHNAF)with normal dispersion and high nonlinearity and an output coupler with a large coupling ratio are used to achieve a high-energy DSR system.By optimizing the UHNAF length to55 m,a 2103.7 nm,88.1 nJ DSR laser with a 3-dB spectral bandwidth of 0.48 nm and a pulse width of 17.1 ns is obtained under a proper intracavity polarization state and pump power.The output power and conversion efficiency are 0.233W and 4.57%,respectively,both an order of magnitude higher than those of previously reported holmium-doped DSR seed lasers.Thanks to the high output power and nanosecond pulse width of the seed laser,the average power of the DSR laser is linearly scaled up to 50.4 W via a dual-stage master oscillator power amplifier system.The 3-dB spectral bandwidth broadens slightly to 0.52 nm,and no distortion occurs in the amplified pulse waveform.The corresponding pulse energy reaches 19.1μJ,which is the highest pulse energy in a holmium-doped mode-locked fiber laser system to the best of our knowledge.Such a 2.1μm,high-energy DSR laser with relatively wide pulse width has prospective applications in mid-infrared nonlinear frequency conversion.

Record power and efficient mid-infrared supercontinuum generation in germania fiber with high stability

We report the demonstration of a mid-infrared(MIR)supercontinuum(SC)laser delivering a record-breaking average output power of more than 40 W with a long-wavelength edge up to 3.5μm.The all-fiberized configuration was composed of a thulium-doped fiber amplifier system emitting a broadband spectrum covering 1.9–2.6μm with pulse repetition rate of 3 MHz,and a short piece of germania fiber.A 41.9 W MIR SC with a whole spectrum of 1.9–3.5μm was generated in a piece of 0.2-m-long germania fiber,with a power conversion efficiency of 71.4%.For an even shorter germania fiber(0.1 m),an SC with even higher output power of 44.9 W(corresponding to a conversion efficiency of 76.5%)was obtained,but the energy conversion toward the long-wavelength region was slightly limited.A continuous operation for 1 hour with output power of 32.6 W showed outstanding power stability(root mean square 0.17%)of the obtained SC laser.To the best of the authors’knowledge,for the first time,this work demonstrates the feasibility of germania fiber on generating a 40-W level MIR SC with high efficiency and excellent power stability,paving the way to real applications requiring high power and high reliability of MIR SC lasers.

Au nanowires with high aspect ratio and atomic shell of Pt-Ru alloy for enhanced methanol oxidation reaction

The methanol oxidation reaction(MOR)is the limiting half-reaction in direct methanol fuel cell(DMFC).Although Pt is the most active single-metal electrocatalyst for MOR,it is hampered by high cost and CO poisoning.Constructing a Pt or Ru monolayer on a second metal substrate by means of galvanic replacement of underpotentially deposited(UPD)Cu monolayer has been shown as an efficient catalyst design strategy for the electrocatalysis of MOR because of the presumed 100%utilization of atoms and resistance to CO poisoning.Herein,we prepared one-dimensional surface-alloyed electrocatalyst from predominantly(111)faceted Au nanowires with high aspect ratio as the substrate of under-potential deposition.The electrocatalyst comprises a core of the Au nanowire and a shell of catalytically active Pt coated by Ru.Coverage-dependent electro-catalytic activity and stability is demonstrated on the Pt/Ru submonolayers on Au wires for MOR.Among all these catalysts,Au@Pt_(ML)@Ru_(ML)exhibits the best electrocatalytic activity and poisoning tolerance to CO.This presents a viable method for the rational catalyst design for achieving high noble-metal utilization efficiency and high catalytic performance.