Double Cladding Seven-core Photonic Crystal Fiber

A double cladding seven-core PCF was presented for high power supercontinuum generation. The calculated zero dispersion wavelength is located at 912 nm, which has a good agreement with the measurement. The attenuation is measured 6 dB/km at 1590 nm and lower than 14.5 dB/km at 1060 nm, the water-loss peak at 1380 nm is about 134 dB/km;Supercontinuum spanning over more than 1500 nm was generated when the designed seven-core PCF was pumped by a gain-switching Yetterbium-doped fiber laser. These results will be helpful in the future design of multicore photonic crystal fibers (MCPCF) with proper guidance properties for high power supercontinuum generation.

A high-quality-factor ultra-narrowband perfect metamaterial absorber based on monolayer molybdenum disulfide

In order to significantly improve the absorption efficiency of monolayer molybdenum disulfide(M-MoS_(2)), an ultranarrowband M-MoS_(2)metamaterial absorber was obtained through theoretical analysis and numerical calculation using the finite difference time domain method. The physical mechanism can be better analyzed through critical coupling and guided mode resonance. Its absorption rate at λ = 806.41 nm is as high as 99.8%, which is more than 12 times that of bare MMoS_(2). From the simulation results, adjusting the geometric parameters of the structure can control the resonant wavelength range of the M-MoS_(2). In addition, we also found that the maximum quality factor is 1256.8. The numerical result shows that the design provides new possibilities for ultra-narrowband M-MoS_(2) perfect absorbers in the near-infrared spectrum.The results of this work indicate that the designed structure has excellent prospects for application in wavelength-selective photoluminescence and photodetection.

Sb_2Te_3 mode-locked ultrafast fiber laser at 1.93μm

Ultrafast pulse generation was demonstrated in thulium doped fiber laser mode locked by magnetron sputtering de- posited Sb2Te3 with the modulation depth, non-saturable loss, and saturable intensity of 38%, 31.2%, and 3.3 MW/cm2, respectively. Stable soliton pulses emitting at 1930.07 nm were obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average output power of 130 mW, and signal-to-noise ratio （SNR） of 84 dB. To our knowledge, this is the first demonstration of Sb2Te3-based SA in fiber lasers at 2-p.m regime. Ultrafast pulse generation was demonstrated in thulium doped fiber laser mode locked by magnetron sputtering de- posited Sb2Te3 with the modulation depth, non-saturable loss, and saturable intensity of 38%, 31.2%, and 3.3 MW/cm2, respectively. Stable soliton pulses emitting at 1930.07 nm were obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average output power of 130 mW, and signal-to-noise ratio （SNR） of 84 dB. To our knowledge, this is the first demonstration of Sb2Te3-based SA in fiber lasers at 2-p.m regime.

Design of Seven-core Photonic Crystal Fiber with Flat In-phase Mode for Yb: Fiber Laser Pumping

We numerically investigate the seven-core photonic crystal fiber (PCF) with the zero dispersion wavelength designed in the range of 1000 - 1080 nm, particularly suitable for the ytterbium-doped fiber laser pumping. Also, the PCFs are well designed for obtaining a flat in-phase mode by carefully adjusting the diameter of inner layer six holes, and the corresponding empirical values of fiber structure are summarized and listed. The variations of inner six holes to the amplitude of in-phase mode are further investigated, and our results show that a better tolerance can be achieved in the fiber structures with lower filling ratio configuration.

Reconstructing coherent dynamics of bound states induced by strong attosecond XUV pulses

We propose a scheme that utilizes weak-field-induced quantum beats to investigate the electronic coherences of atoms driven by a strong attosecond extreme ultraviolet(XUV)pulse.The technique involves using a strong XUV pump pulse to excite and ionize atoms and a time-delayed weak short pulse to probe the photoelectron signal.Our theoretical analysis demonstrates that the information regarding the bound states,initiated by the strong pump pulse,can be precisely reconstructed from the weak-field-induced quantum beat spectrum.To examine this scheme,we apply it to the attosecond XUV laser-induced ionization of hydrogen atoms by solving a three-dimensional time-dependent Schr?dinger equation.This work provides an essential reference for reconstructing the ultrafast dynamics of bound states induced by strong XUV attosecond pulses.

High-power mid-infrared femtosecond master oscillator power amplifier Er:ZBLAN fiber laser system

High-power femtosecond mid-infrared(MIR)lasers are of vast importance to both fundamental research and applications.We report a high-power femtosecond master oscillator power amplifier laser system consisting of a singlemode Er:ZBLAN fiber mode-locked oscillator and pre-amplifier followed by a large-mode-area Er:ZBLAN fiber main amplifier.The main amplifier is actively cooled and bidirectionally pumped at 976 nm,generating a slope efficiency of 26.9%.Pulses of 8.12 W,148 fs at 2.8μm with a repetition rate of 69.65 MHz are achieved.To the best of our knowledge,this is the highest average power ever achieved from a femtosecond MIR laser source.Such a compact ultrafast laser system is promising for a wide range of applications,such as medical surgery and material processing.

Memristive switching in two-dimensional BiSe crystals

In spite of the explosive rise of research on memristive switching,more improvements in tunability,versatility,and hetero-integration are required through the discovery and application of novel materials.Herein,we report resistance switching in nano-thick two-dimensional(2D)crystals of bismuth selenium(BiSe).The BiSe devices exhibit nonvolatile bipolar resistance switching,volatile switching,and electrical bistable behavior in different conditions.The different memristive behavior of BiSe devices may be related to the concentration of Bi ions in this Bi-rich structure,which directly affects the capability of filaments forming.Furthermore,the external mechanical strain is applied in modulation of multi-layer BiSe devices.The memristive BiSe devices show a large on/off ratio of~10^(4)and retention time of~104 s.The discovery of memristive switching behavior in multi-layer BiSe is attributed to the forming of Bi filaments.The resistance switching behavior in multi-layer BiSe demonstrates the potential application in the flexible memories and functional integrated devices.

Scaling all-fiber mid-infrared supercontinuum up to 10 W-level based on thermal-spliced silica fiber and ZBLAN fiber

We demonstrate an integrated all-fiber mid-infrared(mid-IR)supercontinuum(SC)source generated by a 1.95μm master oscillator power amplifier system and a single-mode ZBLAN(ZrF_4–BaF_2–LaF_3–AlF_3–NaF)fiber.The maximum average output power is 10.67 W with spectral bandwidth covering from～1.9 to 4.1μm.The single-mode ZBLAN fiber and silica fiber are thermal-spliced to enhance the robustness and practicability of the system.It is,to the best of our knowledge,the first high-power integrated compacted all-fiber mid-IR SC source based on thermal-spliced silica fiber and ZBLAN fiber.

High energy soliton pulse generation by a magnetron-sputtering-deposition-grown MoTe_2 saturable absorber

The pulse energy in the ultrafast soliton fiber laser oscillators is usually limited by the well-known wave-breaking phenomenon owing to the absence era desirable real saturable absorber （SA） with high power tolerance and large modulation depth. Here, we report a type of microfiber-based MoTe2 SA fabricated by the magnetron-sputtering deposition （MSD） method. High-energy wave-breaking free soliton pulses were generated with pulse duration/pulse energy/average output power of 229 fs/2.14 nJ/57 mW in the 1.5 μm regime and 1.3 ps/13.8 nJ/ 212 mW in the 2 μm regime, respectively. To our knowledge, the generated soliton pulses at 1.5μm had the shortest pulse duration and the highest output power among the reported erbium-doped fiber lasers mode locked by transition metal dichalcogenides. Moreover, this was the first demonstration of a MoTe2-based SA in fiber lasers in the 2 ltm regime, and the pulse energy/output power are the highest in the reported thulium-doped fiber lasers mode locked by two-dlmensional materials. Our results suggest that a microfiber-based MoTe2 SA could be used as an excellent photonic device for ultrafast pulse generation, and the MSD technique opens a promising route to produce a high-performance SA with high power tolerance and large modulation depth, which are beneficial for high-energy wave-breaking free pulse generation.

Efficient improvement of 2.7 μm luminescence of Er^(3+):oxyfluoride glass containing gallium by Yb^(3+) ions codoping

Mid-infrared laser materials with low phonon energy have significant applications. However, the development of available glass systems for high-power laser gain medium have posed a great challenge.Therefore, we investigated the 2.7 μm spectroscopic properties of Er^(3+)/Yb^(3+) -codoped oxyfluoride glass containing gallium, which were prepared by typically melting and quenching methods. The 2.7 μm luminescence properties of the Er^(3+)/Yb^(3+)-codoped oxyfluoride glass containing gallium were recorded by a 980 nm laser diode. The Judde Ofelt parameters, decay curves, emission cross section, energy transfer efficiency and quantum efficiency were calculated. The maximum emission cross section of YbFGa-0.5 is 1.63 × 10^(-20) cm^2 by 980 nm excitation. The energy transfer efficiency is calculated to be77.8% for the YbFGa-0.5 glass around 2700 nm. The quantum efficiency at 1530 nm is 65.6%. The result reveals that the best doping concentration ratio of Er^(3+):Yb^(3+) ions is 1:0.5, and suggests an effective energy transfer from Yb^(3+) to Er^(3+) ions.

Mode-locked fiber laser of 3.5 μm using a single-walled carbon nanotube saturable absorber mirror

We report on a mid-infrared fiber laser that uses a single-walled carbon nanotube saturable absorber mirror to realize the mode-locking operation.The laser generates 3.5 μm ultra-short pulses from an erbium-doped fluoride fiber by utilizing a dual-wavelength pumping scheme.Stable mode-locking is achieved at the 3.5 μm band with a repetition rate of 25.2 MHz.The maximum average power acquired from the laser in the mode-locking regime is 25 mW.The experimental results indicate that the carbon nanotube is an effective saturable absorber for mode-locking in the mid-infrared spectral region.

High peak power femtosecond cylindrical vector beams generation in a chirped-pulse amplification laser system

Cylindrical vector beams(CVBs),with non-uniform state of polarizations,have become an indispensable tool in many areas of science and technology.However,little research has explored high power CVBs at the femtosecond regime.In this paper,we report on tne generation of high quality CVBs with high peak power and femtosecond pulse duration in a fiber chirped-pulse amplification laser system.The radially(azimuthally)polarized vector beam has been obtained with a pulse duration of 4A0 fs[430 fs]and a maximum average output power of 20.36 W[20.12 W].The maximum output pulse energy is〜20μJ at a repetition rate of 1 MHz,corresponding to a high peak power of-46 MW.The comparison between simulated intensity profiles and measured experimental results suggests that the generated CVBs have a remarkable intensity distribution.The proposed configuration of our laser system provides a promising solution for high quality CVBs generation with the characteristics of high peak power,ultrashort pulse duration,and high mode purity.

Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation.In ultrafast fiber lasers,enormous progress has been made toward controlling the interactions of many longitudinal modes,which results in temporally mode-locked pulses.Recently,optical vortex beams have been extensively investigated due to their quantized orbital angular momentum,spatially donut-like intensity,and spiral phase front.In this paper,we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser.The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser.The vortex mode-switching dynamics have four stages,including quiet-down,relaxation oscillation,quasi mode-locking,and energy recovery prior to the stable mode-locking of another vortex mode.The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method.The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses,which plays an essential role in the mode-switching dynamic process.The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

High-power, ultra-broadband supercontinuum source based upon 1/1.5 μm dual-band pumping

We experimentally demonstrate an all-fiber supercontinuum source that covers the spectral region ranging from visible to mid-infrared. The ultra-broadband supercontinuum is realized by pumping a cascaded photonic crystal fiber and a highly nonlinear fiber with a 1/1.5 μm dual-band pump source. A maximum output power of 9.01 W is achieved using the system,which is the highest power ever achieved from a supercontinuum source spanning from the visible to mid-infrared.

Generation of single solitons tunable from 3 to3.8μm in cascaded Er^(3+)-doped and Dy^(3+)-doped fluoride fiber amplifiers

High-power tunable femtosecond mid-infrared(MIR)pulses are of great interest for many scientific and industrial applications.Here we demonstrate a compact fluoride-fiber-based system that generates single solitons tunable from 3 to 3.8μm.The system is composed of an Er:ZBLAN fiber oscillator and amplifier followed by a fusion-spliced Dy:ZBLAN fiber amplifier.The Er:ZBLAN fiber amplifier acts as a power booster as well as a frequency shifter to generate Raman solitons up to 3μm.The Dy:ZBLAN fiber amplifier transfers the energy from the residual 2.8μm radiation into the Raman solitons using an in-band pumping scheme,and further extends the wavelength up to 3.8μm.Common residual pump radiation and secondary solitons accompanying the soliton self-frequency shift(SSFS)are recycled to amplify Raman solitons,consequently displaying a higher output power and pulse energy,a wider shifting range,and an excellent spectral purity.Stable 252 fs pulses at3.8μm with a record average power of 1.6 W and a pulse energy of 23 n J are generated.This work provides an effective way to develop high-power widely tunable ultrafast single-soliton MIR laser sources,and this method can facilitate the design of other SSFS-based laser systems for single-soliton generation.

Refractive Index and Strain Sensitivities of a Long Period Fiber Grating

A long period fiber grating （LPFG） fabricated upon the all-solid photonic bandgap fiber by CO2 laser irradiation was investigated, and its resonance wavelength was at 1335.76nm with a modulation depth of 15dB and a 3-dB bandwidth of 2.6nm. We studied its strain, temperature, and index sensor characteristics, the strain sensitivity of 0.992 pm/m was obtained by using linear fit, and the relationship between the refractive index and wavelength obeyed the distribution of quadratic function. Also, we demonstrated its temperature response was relatively insensitive （21.51 pm/~C）.

Recent development of saturable absorbers for ultrafast lasers [Invited]

As one of the greatest inventions in the 20 th century, ultrafast lasers have offered new opportunities in the areas of basic scientific research and industrial manufacturing. Optical modulators are of great importance in ultrafast lasers, which directly affect the output laser performances. Over the past decades, significant efforts have been made in the development of compact, controllable, repeatable, as well as integratable optical modulators(i.e., saturable absorbers). In this paper, we review the fundamentals of the most widely studied saturable absorbers, including semiconductor saturable absorber mirrors and low-dimensional nanomaterials. Then, different fabrication technologies for saturable absorbers and their ultrafast laser applications in a wide wavelength range are illustrated. Furthermore, challenges and perspectives for the future development of saturable absorbers are discussed and presented. The development of ultrafast lasers together with the continuous exploration of reliable saturable absorbers will open up new directions for the mass production of the nextgeneration optoelectronic devices.

Heterogeneous integrated phase modulator based on two-dimensional layered materials

Silicon nitride,with ultralow propagation loss and a wide transparency window,offers an exciting platform to explore integrated photonic devices for various emerging applications.It is appealing to combine the intrinsic optical properties of two-dimensional layered materials with high-quality optical waveguides and resonators to achieve functional devices in a single chip.Here we demonstrate a micro-ring resonator-based phase modulator integrated with few-layer MoS_(2).The ionic liquid is employed directly on the surface of MoS_(2) to form a capacitor configuration.The effective index of the composite MoS_(2)–SiN waveguide can be modulated via adjusting bias voltages to achieve different charged doping induced electro-refractive responses in MoS_(2) film.The maximum effective index modulation of the composite MoS_(2)–SiN waveguide can be achieved to 0.45×10^(−3).The phase tuning efficiency is measured to be 29.42 pm/V,corresponding to a V_(π)L of 0.69 V·cm.Since the micro-ring resonator is designed near the critical coupling regime,the coupling condition between the bus waveguide and micro-ring resonator can also be engineered from under-coupling to over-coupling regime during the charged doping process.That can be involved as a degree of freedom for the coupling tailoring.The ability to modulate the effective index with two-dimensional materials and the robust nature of the heterostructure integrated phase modulator could be useful for engineering reliable ultra-compact and low-power-consumption integrated photonic devices.

Temperature Insensitive Bending Sensor Based on In-Line Mach-Zehnder Interferometer

A simple and compact fiber bending sensor based on the Maeh-Zehnder interferometer was proposed. A photonic crystal fiber （PCF） with a length of 10mm was spliced by collapsing air holes with two conventional single mode fibers to consist of an all fiber bending sensor. The sensitivity of 0.53nm/m-1 was obtained at 1586nm for the curvature range from 0 to 8.514 m-1. The temperature sensitivity was very low. The measurement error due to the temperature effect was about 8.68x10-3 m-1/℃, and the temperature effect in the curvature measurement could be ignored. This device can avoid the cross sensitivity of the temperature in the curvature measurement.

Octave-spanning visible supercontinuum generation from an aluminum nitride single crystal pumped by a 355 nm nanosecond pulse

Supercontinuum generation（SC） of more than one octave spectrum spanning covering from 400 nm to 820 nm was achieved by pumping a piece of aluminum nitride（AIN） single crystal using a nanosecond 355 nm ultraviolet laser. The AlN with a thickness of ~0.8 mm was grown by an optimized physical vapor transport technique and polished with solidification technology. Compared to previously reported ones, the achieved visible SC exhibited the broadest spectrum spanning from bulk materials pumped by a nanosecond pulse laser. The visible supercontinuum in Al N presents new opportunities for bulk material-based white light SC and may find more potential applications beyond typical applications in integrated semiconductive photoelectronic devices.