Third-order nonlinear wavelength conversion in chalcogenide glass waveguides towards mid-infrared photonics

The increasing demand in spectroscopy and sensing calls for infrared(mid-IR)light sources.Here,we theoretically investigate nonlinear wavelength conversion of Ge_(28)Sb_(12)Se_(60)chalcogenide glass waveguide in the mid-IR spectral regime.With waveguide dispersion engineering,we predict generation of over an octave wavelength(2.8μm-5.9μm)tuning range Raman soliton self-frequency shift,over 2.5 octaves wavelength cover range supercontinuum(1.2μm-8.0μm),as well as single soliton Kerr comb generated in suspended Ge_(28)Sb_(12)Se_(60)waveguide.Our findings evidenced that Ge_(28)Sb_(12)Se_(60)chalcogenide glass waveguides can simultaneously satisfy the generation of Raman soliton self-frequency shift,supercontinuum spectrum,and Kerr frequency comb generation through dispersion engineering towards mid-IR on chip.

Mid‐infrared all‐fiber gain‐switched pulsed laser at 3μm

Mid-infrared(MIR)fiber pulsed lasers are of tremendous application interest in eye-safe LIDAR,spectroscopy,chemical detection and medicine.So far,these MIR lasers largely required bulk optical elements,complex free-space light alignment and large footprint,precluding compact all-fiber structure.Here,we proposed and demonstrated an all-fiberized structured gain-switched Ho3+-doped ZBLAN fiber laser operating around 2.9μm.A home-made 1146 nm Raman fiber pulsed laser was utilized to pump highly concentrated single-cladding Ho3+-doped ZBLAN fiber with different lengths of 2 m or 0.25 m.A home-made MIR fiber mirror and a perpendicular-polished ZBLAN fiber end construct the all-fiberized MIR cavity.Stable gain-switched multiple states with a sub-pulse number tuned from 1 to 8 were observed.The effects of gain fiber length,pump power,pump repetition rate and output coupling ratio on performance of gain-switched pulses were further investigated in detail.The shortest pulse duration of 283 ns was attained with 10 kHz repetition rate.The pulsed laser,centered at 2.92μm,had a maximum average output power of 54.2 mW and a slope efficiency of 10.12%.It is,to the best of our knowledge,the first time to demonstrate a mid-infrared gain-switched Ho3+:ZBLAN fiber laser with compact all-fiber structure.

Visible-light all-fiber vortex lasers based on mode selective couplers

We demonstrate visible-light all-fiber vortex lasers by incorporating the home-made mode selective couplers (MSCs). The MSC at green or red wavebands is fabricated by specially designing and fusing a single-mode fiber (SMF) and a few-mode fiber (FMF). The MSCs inserted into visible fiber cavities act as power splitters and mode converters from the LP01 to LP11 mode at green and red wavelengths, respectively. The red-light all-fiber vortex laser is formed by a 10-cm Pr3+/Yb3+:ZBLAN fiber, a fiber Bragg grating, a fiber end-facet mirror and the MSC at 635 nm, which generates vortex beams with OAM±1 at 634.4 nm and an output power of 13 mW. The green-light all-fiber vortex laser consists of a 12-cm Ho3+:ZBLAN fiber, two fiber pigtail mirrors, and the MSC at 550 nm, which generates vortex beams with OAM±1 at 548.9 nm and an output power of 3 mW.

Wide-color-gamut, high-purity, and high-brightness thin film structural colors based on modified Fano resonant structure

Recently, the Fano resonance has played an increasingly important role in improving the color performance of structural colors. In this study, we further elucidate the asymmetric spectral shape generated by Fano resonance from a phase perspective and explore four distinct continuum state structures. By integrating the proposed cavity-like structure with a metal±dielectric±metal discrete state, multilayered thin-film structural colors with minimal background reflection, as low as 8%, were successfully achieved. The reflection peak of this structure exhibits a bandwidth of approximately50 nm and reaches up to 80%, indicating heightened saturation and color brightness. Moreover, by adjusting the thickness,we effortlessly obtained a broader color gamut compared to Adobe RGB (45.2%), covering 56.7% of the CIE color space. Even adjusting a single layer can achieve a color gamut of 47.1%. In experiments, by deliberately choosing low oxygendependent materials, excellent RGB colors with high brightness and in high consistency with simulation results were successfully achieved. Therefore, the scheme's simple process for structural color creation, along with its excellent color performance and the ability to effectively replicate simulation characteristics makes it highly valuable in fields like anticounterfeiting, decoration, display devices, and solar cell panels.

Burst-mode pulse generation in passively mode-locked all-fiber green/orange lasers at 543 nm and 602 nm

We report on the experimental realization of,to the best of our knowledge,the first green and orange passively mode-locked all-fiber lasers.Stable mode-locking in the burst-pulse status is achieved at the wavelengths of 543.3 nm and 602.5 nm.The figure-9 cavity comprises the fiber end-facet mirror,gain fiber(Ho^(3+):ZBLAN fiber or Pr^(3+)/Yb^(3+):ZBLAN fiber),and fiber loop mirror(FLM).The FLM with long 460 HP fiber is not only used as an output mirror,but also acts as a nonlinear optical loop mirror for initiating visible-wavelength mode-locking.The green/orange mode-locked fiber lasers with the fundamental repetition rates of 3.779/5.662 MHz produce long bursts containing ultrashort pulses with the 0.85/0.76 GHz intra-burst repetition rates,respectively.The ultrashort intra-burst pulses stem from the dissipative four-wave-mixing effect in the highly nonlinear FLM as well as the intracavity Fabry–Perot filtering.Long bursts of 22.2/11.6 ns with ultrashort pulses of 87/62 ps are obtained in our experiment.The visible-wavelength passively mode-locked lasers in an all-fiber configuration and burst-mode would represent an important step towards miniaturized ultrafast fiber lasers and may contribute to the applications in ablation-cooling micromachining,biomedicine imaging,and scientific research.

Direct generation of 3.17 mJ green pulses in a cavity-dumped Ho^(3+)-doped fiber laser at 543 nm

High-energy pulsed lasers in the green spectral region are of tremendous interest for applications in space laser ranging,underwater detection,precise processing,and scientific research.Semiconductor pulsed lasers currently are difficult to access to the so-called“green gap,”and high-energy green pulsed lasers still heavily rely on the nonlinear frequency conversion of near-IR lasers,precluding compact and low-cost green laser systems.Here,we address this challenge by demonstrating,for the first time to the best of our knowledge,millijoule-level green pulses generated directly from a fiber laser.The green pulsed fiber laser consists of a 450 nm pump laser diode,a Ho^(3+)-doped ZBLAN fiber,and a cavity-dumping module based on a visible wavelength acousto-optic modulator.Stable pulse operation in the cavity-dumping regime at 543 nm is observed with a tunable repetition rate in a large range of 100 Hz–3 MHz and a pulse duration of 72–116 ns.The maximum pulse energy of 3.17 mJ at 100 Hz is successfully achieved,which is three orders of magnitude higher than those of the rare-earth-doped fiber green lasers previously reported.This work provides a model for compact,high-efficiency,and high-energy visible fiber pulsed lasers.

Temperature-sensing scheme based on a passively mode-locked fiber laser via beat frequency demodulation

In this paper,we propose a temperature-sensing scheme utilizing a passively mode-locked fiber laser combined with the beat frequency demodulation system.The erbium-doped fiber is used in the laser ring cavity to provide the gain and different lengths of single-mode fibers inserted into the fiber ring cavity operate as the sensing element.Different temperature sensitivities have been acquired in the experiment by monitoring the beat frequency signals at different frequencies.The experimental results indicate that the beat frequency shift has a good linear response to the temperature change.The sensitivity of the proposed sensor is about-44 kHz/℃ when the monitored beat frequency signal is about 10 GHz and the ratio of the sensing fiber to the overall length of the laser cavity is 10 m/17.5 m,while the signal-to-noise ratio(SNR)of the monitored signal is approximately 30 dB.The proposed temperature-sensing scheme enjoys attractive features such as tailorable high sensitivity,good reliability,high SNR,and low cost,and is considered to have great potential in practical sensing applications.

Towards visible-wavelength passively mode-locked lasers in all-fibre format

Mode-locked fibre lasers(MLFLs)are fundamental building blocks of many photonic systems used in industrial,scientific and biomedical applications.To date,1–2μm MLFLs have been well developed;however,passively modelocked fibre lasers in the visible region(380–760 nm)have never been reported.Here,we address this challenge by demonstrating an all-fibre visible-wavelength passively mode-locked picosecond laser at 635 nm.The 635 nm mode-locked laser with an all-fibre figure-eight cavity uses a Pr/Yb codoped ZBLAN fibre as the visible gain medium and a nonlinear amplifying loop mirror as the mode-locking element.First,we theoretically predict and analyse the formation and evolution of 635 nm mode-locked pulses in the dissipative soliton resonance(DSR)regime by solving the Ginzburg-Landau equation.Then,we experimentally demonstrate the stable generation of 635 nm DSR mode-locked pulses with a pulse duration as short as~96 ps,a radio-frequency signal-to-noise ratio of 67 dB and a narrow spectral bandwidth of <0.1 nm.The experimental results are in excellent agreement with our numerical simulations.In addition,we also observe 635 nm noise-like pulse operation with a wide(>1 nm)and modulated optical spectrum.This work represents an important step towards miniaturized ultrafast fibre lasers in the visible spectral region.

Direct generation of an ultrafast vortex beam in a CVD-graphene-based passively mode-locked Pr:LiYF4 visible laser

We report on the direct generation of passively mode-locked vortex lasers in the visible spectral region, for the first time to the best of our knowledge, using a Pr :Li YF4(Pr:YLF) crystal as the gain medium. A stable mode-locked TEM00 mode has been achieved with a maximum average output power of 75 m W using a graphene saturable absorber mirror. The mode-locked pulse width is measured to be as short as about 73.4 ps at a repetition rate of about 140 MHz, and the laser wavelength is at about 721 nm with spectral width of about 0.5 nm. By slightly misaligning the laser resonator, a first-order Laguerre-Gaussian mode(LG0,1) has also been obtained with output power reduced to about 22 m W. The achieved LG0,1 mode has been verified via a home made improved Fizeau interferometer. This work provides a simple and universal method for direct generation of an ultrafast vortex laser,which can be readily extended to other spectral regions by using different laser gain mediums.

Nonlinear optical absorption of few-layer molybdenum diselenide （MoSe2） for passively mode-locked soliton fiber laser [Invited]

In this paper,both nonlinear saturable absorption and two-photon absorption（TPA） of few-layer molybdenum diselenide（MoSe2） were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge.Few-layer MoSe2 nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer,Raman spectroscopy,and atomic force microscopy.The obtained fewlayer MoSe2 dispersion is further composited with a polymer material for convenient fabrication of MoSe2 thin films.Then,we investigated the nonlinear optical（NLO） absorption property of the few-layer MoSe2 film using a balanced twin-detector measurement technique.Both the saturable absorption and TPA effects of the few-layer MoSe2 film were found by increasing the input optical intensity.The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of 3.5%,corresponding to the relative modulation depth of 18%.The TPA effect occurred when the input optical intensity exceeds 260 MW∕cm2.Furthermore,we experimentally exploit the saturable absorption of few-layer MoSe2 film to mode lock an all-fiber erbium-doped fiber laser.Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps.It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity.Our results indicate that layered MoSe2,as another two-dimensional nanomaterial,can provide excellent NLO properties（e.g.,saturable absorption and TPA） for potential applications in ultrashort pulse generation and optical limiting.

Self-Q-switched and wavelength-tunable tungsten disulfide-based passively Q-switched Er:Y2O3 ceramic lasers

We report on diode-pumped Er:Y_2O_3 ceramic lasers at about 2.7 μm in the tunable continuous-wave, selfQ-switching and tungsten disulfide(WS2)-based passively Q-switching regimes. For stable self-Q-switched operation, the maximum output power reaches 106.6 m W under an absorbed power of 2.71 W. The shortest pulse width is measured to be about 1.39 μs at a repetition rate of 26.7 kHz at maximum output. Using a spin-coated WS2 as a saturable absorber, a passively Q-switched Er:Y_2O_3 ceramic laser is also realized with a maximum average output power of 233.5 m W(for the first time, to the best of our knowledge). The shortest pulse width decreases to 0.72 μs at a corresponding repetition rate of 29.4 kHz, which leads to a pulse energy of 7.92 μJ and a peak power of 11.0 W. By inserting an undoped YAG thin plate as a Fabry–Perot etalon, for the passive Q switching, wavelength tunings are also demonstrated at around 2710, 2717, 2727, and 2740 nm.

Direct generation of watt-level yellow Dy^(3+)-doped fiber laser

Yellow lasers(∼565–590 nm)are of tremendous interest in biomedicine,astronomy,spectroscopy,and display technology.So far,yellow lasers still have relied heavily on nonlinear frequency conversion of near-infrared lasers,precluding compact and low-cost yellow laser systems.Here,we address the challenge through demonstrating,for the first time,to the best of our knowledge,watt-level high-power yellow laser generation directly from a compact fiber laser.The yellow fiber laser simply consists of a Dy^(3+)-doped ZBLAN fiber as gain medium,a fiber end-facet mirror with high reflectivity at yellow and a 450-nm diode laser as the pump source.We comprehensively investigated the dependence of the yellow laser performance on the output coupler reflectivity and the gain fiber length and demonstrated that the yellow fiber laser with an output coupler reflectivity of 4% and a gain fiber length of∼1.8 m yields a maximum efficiency of 33.6%.A maximum output power of 1.12 W at 575 nm was achieved at a pump power of 4.20 W.This work demonstrated the power scaling of yellow Dy^(3+)-doped ZBLAN fiber lasers,showing their promise for applications in ophthalmology,astronomical exploration,and high-resolution spectroscopy.

3.6 W compact all-fiber Pr3+-doped green laser at 521 nm

Green semiconductor lasers are still undeveloped,so high-power green lasers have heavily relied on nonlinear frequency conversion of near-infrared lasers,precluding compact and low-cost green laser systems.Here,we report the first Watt-level all-fiber CW Pr3t-doped laser operating directly in the green spectral region,addressing the aforementioned difficulties.The compact all-fiber laser consists of a double-clad Pr3t-doped fluoride fiber,two homemade fiber dichroic mirrors at visible wavelengths,and a 443-nm fiber-pigtailed pump source.Benefitting from>10 MW∕cm2 high damage intensity of our designed fiber dielectric mirror,the green laser can stably deliver 3.62-W of continuous-wave power at∼521 nm with a slope efficiency of 20.9%.To the best of our knowledge,this is the largest output power directly from green fiber lasers,which is one order higher than previously reported.Moreover,these green all-fiber laser designs are optimized by using experiments and numerical simulations.Numerical results are in excellent agreement with our experimental results and show that the optimal gain fiber length,output mirror reflectivity,and doping level should be considered to obtain higher power and efficiency.This work may pave a path toward compact high-power green all-fiber lasers for applications in biomedicine,laser display,underwater detection,and spectroscopy.

Chip-scale broadband spectroscopic chemical sensing using an integrated supercontinuum source in a chalcogenide glass waveguide

On-chip spectroscopic sensors have attracted increasing attention for portable and field-deployable chemical detection applications. So far, these sensors largely rely on benchtop tunable lasers for spectroscopic interrogation. Large footprint and mechanical fragility of the sources, however, preclude compact sensing system integration. In this paper, we address the challenge through demonstrating, for the first time to our knowledge, a supercontinuum source integrated on-chip spectroscopic sensor, where we leverage nonlinear Ge_(22)Sb_(18)Se_(60) chalcogenide glass waveguides as a unified platform for both broadband supercontinuum generation and chemical detection. A home-built, palm-sized femtosecond laser centering at 1560 nm wavelength was used as the pumping source. Sensing capability of the system was validated through quantifying the optical absorption of chloroform solutions at 1695 nm. This work represents an important step towards realizing a miniaturized spectroscopic sensing system based on photonic chips.

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.

2080 nm long-wavelength, high-power dissipative soliton resonance in a dumbbell-shaped thulium-doped fiber laser

We demonstrate a 2080 nm long-wavelength mode-locked thulium(Tm)-doped fiber laser operating in the dissipative soliton resonance(DSR) regime. The compact all-fiber dumbbell-shaped laser is simply constructed by a 50/50 fiber loop mirror(FLM), a 10/90 FLM, and a piece of large-gain Tm-doped double-clad fiber pumped by a 793 nm laser diode. The 10/90 FLM is not only used as an output mirror, but also acts as a periodical saturable absorber for initiating DSR mode locking. The stable DSR pulses are generated at the center wavelength as long as 2080.4 nm, and the pulse duration can be tunable from 780 to 3240 ps as the pump power is increased. The maximum average output power is 1.27 W, corresponding to a pulse energy of 290 nJ and a nearly constant peak power of 93 W. This is, to the best of our knowledge, the longest wavelength for DSR operation in a mode-locked fiber laser.

Visible Raman and Brillouin lasers from a microresonator/ZBLAN-fiber hybrid system

Raman and Brillouin lasers based on a high-quality(high-Q) whispering gallery mode microresonator(WGMR)are usually achieved by employing a tunable single-frequency laser as a pump source. Here, we experimentally demonstrate visible Raman and Brillouin lasers using a compact microresonator/ZrF4-BaF2-LaF3-AlF3-NaF(ZBLAN)-fiber hybrid system by incorporating a WGMR with a fiber-compatible distributed Bragg reflector/fiber Bragg grating to form a Fabry–Perot(F-P) fiber cavity and using a piece of Pr:ZBLAN fiber as gain medium.The high-Q silica-microsphere not only offers a Rayleigh-scattering-induced backreflection to form the ~635 nm red laser oscillation in the F-P fiber cavity, but also provides a nonlinear gain in the WGMR itself to generate either stimulated Raman scattering or stimulated Brillouin scattering. Up to six-order cascaded Raman lasers at0.65 μm, 0.67 μm, 0.69 μm, 0.71 μm, 0.73 μm, and 0.76 μm are achieved, respectively. Moreover, a Brillouin laser at 635.54 nm is clearly observed. This is, to the best of our knowledge, the first demonstration of visible microresonator-based lasers created by combining a Pr:ZBLAN fiber. This structure can effectively extend the laser wavelength in the WGMR to the visible waveband and may find potential applications in underwater communication, biomedical diagnosis, microwave generation, and spectroscopy.

212-kHz-linewidth,transform-limited pulses from a single-frequency Q-switched fiber laser based on a few-layer Bi_2Se_3 saturable absorber

Conventional Q-switched fiber lasers operating at multi-longitudinal-mode oscillation usually suffer from selfmode-locking-induced temporal instability, relatively strong noise, and low coherence. Here, we address the challenge through demonstrating, for the first time, to the best of our knowledge, a single-longitudinal-mode(SLM)Er-doped fiber(EDF) laser passively Q-switched by a few-layer Bi_2Se_3 saturable absorber(SA). The Bi_2Se_3 SA prepared by the liquid-phase exfoliation method shows a modulation depth of ～5% and saturation optical intensity of 1.8 MW∕cm^2. A section of 1-m unpumped EDF together with a 0.06-nm-bandwidth fiber Bragg grating is used as an ultra-narrow autotracking filter to realize SLM oscillation. Stable SLM Q-switching operation at 1.55 μm is successfully achieved with the spectral linewidth as narrow as 212 kHz and the pulse duration of2.54 μs, manifesting near-transform-limited pulses with a time-bandwidth product of 0.53. In particular, we found that the SLM Q-switching possesses the higher signal-to-noise ratios of 62 dB(optical) and 48 dB(radio frequency), exhibiting its advantages of low noise and high stability. Such an SLM Q-switched fiber laser could gain great interest for some applications in coherent detection, coherent optical communications, and high-sensitivity optical sensing.

New ultrashort pulsewidth measurement technology based on interference jitter and FPGA platform

Conventional ultrashort pulsewidth measurement technology is autocorrelation based on second-harmonic generation;however,nonlinear crystals and bulky components are required,which usually leads to the limited wavelength range and the difficult adjustment with free-space light alignment.Here,we proposed a compact all-fiber pulsewidth measurement technology based on the interference jitter(IJ)and field-programmable gate array(FPGA)platform,without requiring a nonlinear optical device(e.g.nonlinear crystal/detector).Such a technology shows a wide measurement waveband from 1 to 2.15μm at least,a pulsewidth range from femtoseconds to 100 ps,and a small relative error of 0.15%-3.8%.In particular,a minimum pulse energy of 219 fj is experimentally detected with an average-power-peak-power product of 1.065×10^(-6)W^(2).The IJ-FPGA technology may offer a new route for miniaturized,user-friendly,and broadband pulsewidth measurement.

Passively Q-switched green laser operation using CdTe/CdS quantum dots

The direct generation of passively Q-switched lasers at a green wavelength has rarely been investigated in the past.In this Letter,we demonstrate a passively Q-switched praseodymium-doped yttrium lithium fluoride green laser at 522 nm using CdTe/CdS quantum dots as a saturable absorber.A maximum average output power of 33.6 m W is achieved with the shortest pulse width of 840 ns.The corresponding pulse energy and peak power reached 0.18μJ and 0.21 W,respectively.To the best of our knowledge,this is the first demonstration in regard to a quantum dots saturable absorber operating in the green spectral region.