All-fiber linearly polarized laser oscillator by fiber coiling loss control

In this paper,we demonstrate an all-fiber linearly polarized fiber laser oscillator.The single polarization of the oscillator is achieved through the careful designing of the active fiber coiling.The relationship between fiber coiling diameter and polarization extinction ratio and optical efficiency is studied,whose results lead to an optimized system.The thermal management of the oscillator is also refined,which allows the oscillator to reach a maximum output power of44.1 W with an optical-to-optical efficiency of 57.9%.A high average polarization extinction ratio of 21.6 d B is achieved during a 2-hour stability test.The oscillator also owns a narrow 3-d B bandwidth of 0.1 nm,as well as near-diffraction-limit beam quality of M^2～ 1.14.

A kW Continuous-Wave Ytterbium-Doped All-Fiber Laser Oscillator with Domestic Fiber Components and Gain Fiber

We demonstrate a kW continuous-wave ytterbium-doped all-fiber laser oscillator with M1 domestic fiber compo- nents： a 7× I fused fiber bundle combiner, a fiber Bragg grating and a double-clad gain fiber. The oscillator operates at 1079.48nrn with 80.94% slope efficiency and shows no limit of temperature and nonlinear effects. These indicate that the passive fiber components and the gain fiber are all qualified for the high power environ- ment. No evidence of the signal power roll-over shows that this oscillator possesses the capacity to higher output with available pump power.

Immersed liquid cooling Nd:YAG slab laser oscillator

An immersed liquid cooling slab laser is demonstrated with deionized water as the coolant and a Nd:YAG slab as the gain medium.Using waveguides,a highly uniform pump beam distribution is achieved,and the flow velocity distribution is also optimized in the channels of the gain module(GM).At various flow velocities,the convective heat transfer coefficient(CHTC)is obtained.Experimentally,a maximum output power of 434 W is obtained with an optical–optical efficiency of 27.1%and a slope efficiency of 36.6%.To the best of our knowledge,it is the highest output power of an immersed liquid cooling laser oscillator with a single Nd:YAG slab.

4.05 kW monolithic fiber laser oscillator based on home-made large mode area fiber Bragg gratings

With the increasing output power of the monolithic fiber laser oscillators, the stimulated Raman scattering （SRS） effect becomes one of the main limitations of power scaling. Employing fiber with a larger mode area is an effective technique to mitigate the SRS, but, for the monolithic fiber laser oscillators, the difficulty of the inscription of the high-reflection fiber Bragg gratings （FBGs） increases with the fiber mode area. In this work, we demonstrated a high-power monolithic fiber laser oscillator based on the home-made large mode area FBGs and ytterbium-doped fiber （YDF） with 25 μm core diameters. A maximum output power of 4.05 kW is achieved at the central wavelength of -1080 ？nm with a total 915 nm pump power of -6.7 kW. At the operation of 4.05 kW, the intensity of the Raman Stokes light is -25 dB below the signal laser, and the beam quality （M2-factor） is -2.2. To the best of our knowledge, this is the first detailed report of the monolithic fiber laser oscillator with an output power beyond 4 kW.

Widely Tunable Middle Infrared Optical Parametric Oscillator Pumped by the Q-Switched Ho：GdVO_4 Laser

We demonstrate a middle infrared ZnGeP_2 optical parametric oscillator pumped by the Q-switched Ho：GdVO_4 laser. When the incident Ho pump power is 4.12 W, the maximum average output power of the ZGP-OPO laser is 2.05 W, corresponding to a slope efficiency of 74.6%. The ZGP-OPO laser produces 4.2 ns mid-infrared pulses at a pulse repetition rate of 5 kHz. In addition, we obtain 0.8 um of tunable range for the signal wave and 2.1 um of tunable range for the idler wave.

A 515-nm laser-pumped idler-resonant femtosecond BiB_(3)O_(6) optical parametric oscillator

We report on an idler-resonant femtosecond optical parametrical oscillator(OPO)based on BiB_(3)O_(6)(BiBO)crystal,synchronously pumped by a frequency-doubled,mode-locked Yb:KGW laser at 515 nm.The idler wavelengths of OPO can be tuned from 1100 nm to 1540 nm.At a repetition rate of 75.5 MHz,the OPO generates as much as 400 mW of idler power with 3.1 W of pump power,the corresponding pulse duration is 80 fs,which is 1.04 times of Fourier transform-limited(FTL)pulse duration at 1305 nm.In addition,the OPO exhibits excellent beam quality with M^(2)<1.8 at 1150 nm.To the best of our knowledge,this is the first idler-resonant femtosecond OPO pumped by 515 nm.

Low-repetition-rate, all-polarization-maintaining Yb-doped fiber laser mode-locked by a semiconductor saturable absorber

A low-repetition-rate, all-polarization-maintaining（PM）-fiber sub-nanosecond oscillator is presented, which is simple and low-cost, composed of standard components. The ring cavity is elongated by 114-m-long standard PM fiber, and passively mode-locked by a fiber pigtailed semiconductor saturable absorber. Linearly polarized pulses with 1.66 MHz repetition rate and 22 dB polarization extinction ratio are generated at a wavelength of 1030 nm, which is determined by an intracavity filter. In addition, to demonstrate that the oscillator is a good seed for high energy pulse generation, an all-fiber master oscillator power amplifier is built and amplified pulses with energy about 2 μJ are obtained.

Simultaneously Suppressing Low-Frequency and Relaxation Oscillation Intensity Noise in a DBR Single-Frequency Phosphate Fiber Laser

Effective multiple optoelectronic feedback circuits for simultaneously suppressing low-frequency and relaxation oscillation intensity noise in a single-frequency phosphate fiber laser are demonstrated. The forward transfer function, which relates the laser output intensity to the pump modulations, is measured and analyzed. A custom two-path feedback system operating at different frequency bands is designed to adjust the pump current directly. The relative intensity noise is decreased by 20dB from 0.2 to 5kHz and over lOdB from 5 to lOkHz. The relaxation oscillation peak is suppressed by 22dB. In addition, a long term （24h） laser instability of less than 0.05% is achieved.

Laser Oscillating of Nickel Alloyed Welds on Press‑Hardened Steel

Laser oscillating welding was employed to fabricate Al-Si coated press-hardened steel(PHS)to improve the element homogeneity in the fusion zone.Laser oscillating welding was employed with various oscillation amplitudes(0 mm,0.5 mm and 1.3 mm)in this present.Ni foil of 0.06 mm thickness was used as an interlayer between two tailored PHS welded.The weld morphology,elemental profile,microstructure and tensile strength of welded joints were studied.The results showed that full penetration weld without any weld defects were achieved for any oscillation amplitudes,and weld width increased with increasing oscillation amplitudes.With the oscillation amplitudes increased,Ni and Al had an uneven elemental profile due to strong stirring force,but the Ni and Al content in the weld was decreased and Ni had a sharp descent compared to Al element.Only fewδ-ferrite was presented in fusion line with the oscillation amplitudes increased to 1.3 mm.The oscillation amplitudes did not have an effect on the tensile properties,which was similar to that of base metal.But if keeping increasing the oscillation amplitudes or reducing the thickness of Ni interlayer,it has a potential risk to form more and moreδferrite such that deteriorate the mechnical properties of welded joints.

The MING proposal at SHINE:megahertz cavity enhanced X‑ray generation

The cavity-based X-ray free-electron laser(XFEL)has promise in producing fully coherent pulses with a bandwidth of a few meV and very stable intensity,whereas the currently existing self-amplified spontaneous emission(SASE)XFEL is capable of generating ultra-short pulses with chaotic spectra.In general,a cavity-based XFEL can provide a spectral brightness three orders of magnitude higher than that of the SASE mode,thereby opening a new door for cutting-edge scientific research.With the development of superconducting MHz repetition-rate XFEL facilities such as FLASH,European-XFEL,LCLS-II,and SHINE,practical cavity-based XFEL operations are becoming increasingly achievable.In this study,megahertz cavity enhanced X-ray generation(MING)is proposed based on China’s first hard XFEL facility-SHINE,which we refer to as MING@SHINE.

Highly efficient H-β Fraunhofer line optical parametric oscillator pumped by a single-frequency 355 nm laser

A highly efficient laser system output at the H-13 Fraunhofer line of 486.1 nm has been demonstrated. A high pulse energy single-frequency hybrid 1064 nm master oscillator power amplifier was frequency-tripled to achieve 355 nm laser pulses, which acted as the pmnp source of the beta barium borate nanosecond pulse optical para- metric oscillator. With pump energy of 190 mJ, the laser system generated a maximum output of 62 mJ blue laser pulses at 486.1 nm, corresponding to conversion efficiency of 32.6%. The laser spectrum width was measured to be around 0.1 ran, being in conformity with the spectrum width of the solar Fraunhofer line.

13.5 mJ polarized 2.09μm fiber-bulk holmium laser and its application to a mid-infrared ZnGeP2 optical parametric oscillator

A high pulse repetition frequency（PRF）, high energy Ho：YAG laser directly pumped by a Tm-doped fiber laser and its application to a mid-infrared ZnGeP_2（ZGP） optical parametric oscillator（OPO） is demonstrated.The maximum polarized 2.09 μm laser pulse energy is 13.46 mJ at a PRF of 1 k Hz. The corresponding peak power reaches 504 kW. In a double-resonant ZGP-OPO, a maximum mid-infrared laser pulse energy of 1.25 m J,corresponding to a peak power of 79 kW, is accomplished at a PRF of 3 kHz. The nonlinear conversion efficiency reaches 41.7%. The nonlinear slope efficiency reaches 53.3%.

Effect of cavity length detuning on the output characteristics for the middle infrared FEL oscillator of FELiChEM

FELiChEM is an infrared free electron laser（FEL） facility currently under construction, which consists of two oscillators generating middle-infrared and far-infrared laser covering the spectral range of 2.5–200 μm. In this paper, we numerically study the output characteristics of the middle-infrared oscillator with accurate cavity length detuning. Emphasis is put on the temporal structure of the micropulse and the corresponding spectral bandwidth.Taking the radiation wavelengths of 50 μm and 5 μm as examples, we show that the output pulse duration can be tuned in the range of 1–6 ps with corresponding bandwidth of 13%–0.2% by adjusting the cavity length detuning.In addition, a special discussion on the comb structure is presented, and it is indicated that the comb structure may arise in the output optical pulse when the normalized slippage length is much smaller than unity. This work has reference value for the operation of FELiChEM and other FEL oscillators.

Wire Oscillating Laser Additive Manufacturing of 2319 Aluminum Alloy: Optimization of Process Parameters, Microstructure, and Mechanical Properties

In this study,a wire oscillating laser additive manufacturing(O-WLAM)process was used to deposit 2319 aluminum alloy samples.The optimization of the deposition process parameters made it possible to obtain samples with smooth surfaces and extremely low porosities.The effects of the deposition parameters on the formability and evolution of the microstructure and mechanical properties before and after heat treatment were studied.The oscillating laser deposition of 2319 aluminum alloy,especially the circular oscillation mode,significantly reduced the porosity and improved the process stability and formability compared with non-oscillating laser deposition.There were clear boundaries between the deposition units in the deposition state,the interior of which was dominated by columnar crystals with many rod-and point-shaped precipitates.After the heat treatment,theθphase was significantly dissolved.The residual dot-and rod-shapedθ'phases were dispersedly distributed,exhibiting an obvious precipitation-hardening effect.The samples in the as-deposited state had a tensile strength of 245–265 MPa,an elongation of approximately 12.6%,and an 87 HV microhardness.After heat treatment at 530°C for 20 h and aging at 175°C for 18 h,the tensile strength,elongation,and microhardness reached 425–440 MPa,approximately 10%,and 153 HV,respectively.The performance improved significantly without significant anisotropy.Compared with the samples produced by wire arc additive manufacturing(WAAM),the tensile strength increased by approximately 10%,and the strength and microhardness were significantly improved.

Line-narrowed electro-optic periodically-poled-lithium-niobate Q-switched laser with intra-cavity optical parametric oscillation using a grazing-incidence grating

We report a line-narrowed electro-optic periodically-poled-lithium-niobate （PPLN） Q-switched laser with intra-cavity optical parametric oscillation using a grazing-incidence grating, producing 8-ns, 5-#J pulses at 10-kHz repetition rate when pumped with a 10-W diode laser at 808 rim. The output wavelength is centered at 1554.3 nm with a 0.03-nm spectral width. Wavelength tuning is achieved by rotating a mirror and changing the crystal temperature.

PTX-symmetric metasurfaces for sensing applications

In this paper,we introduce an ultra-sensitive optical sensing platform based on the parity-time-reciprocal scaling(PT^-symmetric non-Hermitian metasurfaces,which leverage exotic singularities,such as the exceptional point(EP)and the coherent perfect absorber-laser(CPAL)point,to significantly enhance the sensitivity and detectability of photonic sensors.We theoretically studied scattering properties and physical limitations of the PTX-symmetric metasurface sensing systems with an asymmetric,unbalanced gain-loss profile.The PTLY-symmetric metasurfaces can exhibit similar scattering properties as their Pr-symmetric counterparts at singular points,while achieving a higher sensitivity and a larger modulation depth,possible with the reciprocal-scaling factor(i.e.,X transformation).Specifically,with the optimal reciprocalscaling factor or near-zero phase offset,the proposed PTX-symmetric metasurface sensors operating around the EP or CPAL point may achieve an over 100 dB modulation depth,thus paving a promising route toward the detection of small-scale perturbations caused by,for example,molecular,gaseous,and biochemical surface adsorbates.