Wavelength switchable mode-locked fiber laser with a few-mode fiber filter

We have proposed and constructed a few-mode fiber(FMF)-based comb filter realized by dislocation splicing a fewmode long-period fiber grating(FM-LPFG) with a single-mode fiber(SMF). From an all-fiber laser with a FMF-based comb filter, the generation of switchable single-, dual-, triple-, and quadruple-wavelength continuous light has been achieved.Moreover, wavelength switchable mode-locked pulses have been obtained with the increased pump power. In the experiment, the output wavelength of the mode-locked fiber laser was changed from 1567.72 nm to 1571.04 nm, while the signal-to-noise(SNR) ratio was maintained above 61 d B. The switchable multiwavelength continuous wave(CW) and mode-locked all-fiber lasers have potentially important applications for fiber sensing, wavelength-division multiplexing(WDM) and signal processing.

Machine Learning-based Inverse Model for Few-Mode Fiber Designs

The medium for next-generation communication is considered as fiber for fast,secure communication and switching capability.Mode division and space division multiplexing provide an excellent switching capability with high data transmission rate.In this work,the authors have approached an inverse modeling technique using regression-based machine learning to design a weakly coupled few-mode fiber for facilitating mode division multiplexing.The technique is adapted to predict the accurate profile parameters for the proposed few-mode fiber to obtain the maximum number of modes.It is for a three-ring-core few-mode fiber for guiding five,ten,fifteen,and twenty modes.Three types of regression models namely ordinary least-square linear multi-output regression,k-nearest neighbors of multi-output regression,and ID3 algorithm-based decision trees for multi-output regression are used for predicting the multiple profile parameters.It is observed that the ID3-based decision tree for multioutput regression is the robust,highly-accurate machine learning model for fast modeling of FMFs.The proposed fiber claims to be an efficient candidate for the next-generation 5G and 6G backhaul networks using mode division multiplexing.

Fast mode decomposition for few-mode fiber based on lightweight neural network

In this paper,we present a fast mode decomposition method for few-mode fibers,utilizing a lightweight neural network called MobileNetV3-Light.This method can quickly and accurately predict the amplitude and phase information of different modes,enabling us to fully characterize the optical field without the need for expensive experimental equipment.We train the MobileNetV3-Light using simulated near-field optical field maps,and evaluate its performance using both simulated and reconstructed near-field optical field maps.To validate the effectiveness of this method,we conduct mode decomposition experiments on a few-mode fiber supporting six linear polarization(LP)modes(LP01,LP11e,LP11o,LP21e,LP21o,LP02).The results demonstrate a remarkable average correlation of 0.9995 between our simulated and reconstructed near-field lightfield maps.And the mode decomposition speed is about 6 ms per frame,indicating its powerful real-time processing capability.In addition,the proposed network model is compact,with a size of only 6.5 MB,making it well suited for deployment on portable mobile devices.

Research and analysis of Brillouin distributed sensing system based on quasi-single-mode few-mode fiber

A distributed fiber sensor was fabricated by splicing two single-mode fibers(SMFs)using the few-mode fiber(FMF)technique.A Brillouin optical time domain analysis(BOTDA)system was developed to measure the sensor’s temperature and bending performance.Two-mode and four-mode step FMFs were combined to splice the few-mode segment.The results indicate that the temperature response coefficients of the few-mode segment are only slightly higher than those of the connected single-mode segment,measuring at 1.13 MHz/℃and 1.12 MHz/℃,respectively.The minimum bending radius for the sensor is 0.9 cm,and the four-mode bending response curve is superior to that of the two-mode one,proving that 4-SI-FMF offers better bending sensitivity.

Propagation of transverse photonic orbital angular momentum through few-mode fiber

Spatiotemporal optical vortex(STOV)pulses can carry transverse orbital angular momentum(OAM)that is perpendicular to the direction of pulse propagation.For a STOV pulse,its spatiotemporal profile can be significantly distorted due to unbalanced dispersive and diffractive phases.This may limit its use in many research applications,where a long interaction length and a tight confinement of the pulse are needed.The first demonstration of STOV pulse propagation through a few-mode optical fiber is presented.Both numerical and experimental analysis on the propagation of STOV pulse through a commercially available SMF-28 standard telecommunication fiber is performed.The spatiotemporal phase feature of the pulse can be well kept after the pulse propagates a few-meter length through the fiber even with bending.Further propagation of the pulse will result in a breakup of its spatiotemporal spiral phase structure due to an excessive amount of modal group delay dispersion.The stable and robust transmission of transverse photonic OAM through optical fiber may open new opportunities for transverse photonic OAM studies in telecommunications,OAM lasers,and nonlinear fiber-optical research.

Applications of weakly-coupled few-mode fibers[Invited]

Space-division multiplexing(SDM)has attracted significant attention in recent years because larger transmission capacity is enabled by more degrees of freedom(DOFs)in few-mode fibers(FMFs)compared with singlemode fibers(SMFs).To transmit independent information on spatial modes without or with minor digital signal processing(DSP),weakly-coupled FMFs are preferred in various applications.Several cases with different use of spatial DOFs in weakly-coupled FMFs are demonstrated in this work,including single-mode or mode-groupmultiplexed transmission,and spatial DOFs combined with time or frequency DOF to improve the system performance.

A novel three-ring-core few-mode fiber with large effective area and low nonlinear coefficient

A novel three-ring-core few-mode fiber with large effective area and low nonlinear coefficient is proposed in this paper. The fiber characteristics based on the full-vector finite element method(FEM) with perfect matched layer boundary conditions show that four supermodes with large effective area, low nonlinear coefficient and low differential mode group delay(DMGD) are achieved. With the increase of input wavelength, the effective areas of three-ring-core few-mode fiber are increased, and the nonlinear coefficients are decreased. The bending losses are increased with the increase of input wavelength, and are decreased with the increase of bending radius. Moreover, the proposed fiber performs a nonlinear coefficient and DMGD flattened profile at a large wavelength range.

Efficient silicon integrated four-mode edge coupler for few-mode fiber coupling

Here,we designed a broadband,low loss,compact,and fabrication-tolerant silicon-based four-mode edge coupler,composed of a 1×3 adiabatic mode-evolution counter-taper splitter and a triple-tip inverse taper.Based on mode conversion and power splitting,the proposed structure can simultaneously realize efficient mode coupling from TE_(0),TM_(0),TE_(1),and TM_(1) modes of multimode silicon waveguides to linearly polarized(LP),LP^(01,x),LP_(01,y),LP_(11a,x),and LP_(11a,y),modes in the few-mode fiber.To the best of our knowledge,we proposed the first scheme of four LP modes coupling,which is fully compatible with standard fabrication process.The 3D finite-difference time-domain simulation results show that the on-chip conversion losses of the four modes remain lower than 0.62 dB over the 200 nm wavelength range,and total coupling losses are 4.1 dB,5.1 dB,2.1 dB,and 2.9 dB for TE_(0)-to-LP_(01,x),TM_(0)-to-LP_(01,y),TE_(1)-to-LP_(11a,x),and TM_(1)-to-LP_(11a,y),respectively.Good fabrication tolerance and relaxed critical dimensions make the four-mode edge coupler compatible with standard fabrication process of commercial silicon photonic foundries.

Design and optimization of weakly-coupled few-mode fiber with low nonlinearity

By investigating the influence of the difference of refractive index between core and cladding （nco - ncl）, normalized frequency （V） and core radius （α） on both the intramodal and intermodal nonlinear coefficients （NCs） respectively, we design a novel weakly-coupled four-mode fiber with low nonlinearity. In general, under the premise of ensuring the low NCs between two non-degenerate modes, this design can reduce the intermodal NCs （〈 0.5 W-l·km^-1） between two degenerate modes and optimizes the parameters of differential group delays （DGDs） and chromatic dispersion. The optimized few mode fiber （FMF） is eligible for transmission and mode de-multiplexing in the receiver.

Tunable self-seeded multi-wavelength Brillouin-erbium fiber laser based on few-mode fiber filter

A tunable self-seeded multi-wavelength Brillouin-erbium fiber laser(BEFL) is proposed and demonstrated based on a few-mode fiber filter(FMFF) with varying temperature. The FMFF configuration is a section of uncoated few-mode fiber(FMF) sandwiched between two up-tapers. As the temperature varies from 25 °C to 125 °C, the transmission spectrum of FMFF moves towards the longer wavelength. The self-excited Brillouin pump is internally achieved by cascaded stimulated Brillouin scattering(SBS) in the single mode fiber(SMF). Then employing the FMFF temperature variation characteristics in the ring cavity fiber laser, the multi-wavelength of the output laser can be tuned, and the tunable range is about 8.0 nm. The generation of up to 15 Brillouin Stokes wavelengths with 16 d B optical signal-to-noise ratio(OSNR) is realized.

Influence of Internal Stresses in Few-Mode Fiber on the Thermal Characteristics of Regenerated Gratings

The pre-treatment of few-mode fibers (FMFs) has been successfully done with CO2 laser The wavelength difference, AA between the two resonant wavelengths in the few-mode fiber Bragg grating (FMFBG) varies with temperature increment during the annealing process. The results show that the treated fibers with lower stresses have lower thermal sensitivity in △λ than that of non-treated fiber. However, the treated fibers produce FMFBGs with better thermal durability and regeneration ratio. It is conceived that the presence of those stresses in the pristine fiber is responsible for the high thermal sensitivity in △λ. The thennal relaxation of stresses and structural rearrangement during the thennal annealing process are responsible for the degradation of the strength and resilience of the regenerated grating.

Multimode oscillation Q-switched erbium-doped fiber laser with a few-mode fiber cavity

We propose and demonstrate experimentally a multimode oscillation Q-switched erbium-doped fiber laser with a few-mode fiber cavity. The different coupling processes of reflection peaks of fiber Bragg gratings(FBGs) which act as cavity mirrors allow the different oscillating states. We obtain microsecond light pulse output states whose oscillation and output beams have different mode compositions, including high order mode. Such a structure enables the high order mode oscillation in the whole cavity with an all-fiber structure, which has potential applications in mode-division multiplexed systems and research of multimode nonlinear effect.

High-power,electronically controlled source of user-defined vortex and vector light beams based on a few-mode fiber amplifier

Optical angular momentum(OAM)-based structured light beams provide an additional degree of freedom for practical applications ranging from optical communication to laser-based material processing. Many techniques exist for generating such beams within laser sources and these primarily rely upon the use of specially designed optical components that limit laser power scaling and ready tunability of the topological charge and polarization of the output OAM beams. Here we show that some of these limitations can be overcome by employing a computer controlled reflective phase-only spatial light modulator to adaptively tailor the input(and subsequent output) beam wavefront and polarization in a few-mode fiber amplifier. In this way modal-coupling-induced beam distortion within the fiber amplifier can be mitigated and we are able to generate at will any desired supported spatial mode guided in the fiber, including conventional linearly polarized(LP) modes, scalar OAM modes, and cylindrical vector modes, at average powers >10 W and with a peak power of >11 kW. Our results pave the way to the realization of practical high-power structured laser sources with tunable chirality and polarization.

Mach-Zehnder Interferometer for High Temperature(1000℃)Sensing Based on a Few-Mode Fiber

A Mach-Zehnder interferometer(MZI)for high temperature(1000°C)sensing based on few mode fiber(FMF)was proposed and experimentally demonstrated.The sensor was fabricated by fusing a section of FMF between two single-mode fibers(SMFs).The structure was proven to be an excellent high temperature sensor with good stability,repeatability,and high temperature sensitivity(48.2 pm/C)after annealing process at a high temperature lasting some hours,and a wide working temperature range(from room temperature to 1000 C).In addition,the simple fabrication process and the low cost offered a great potential for sensing in high temperature environments.

Design, fabrication, and characterization of a highly nonlinear few-mode fiber

We present the design, fabrication, and characterization of a highly nonlinear few-mode fiber(HNL-FMF) with an intermodal nonlinear coefficient of 2.8 W · km-1, which to the best of our knowledge is the highest reported to date. The graded-index circular core fiber supports two mode groups(MGs) with six eigenmodes and is highly doped with germanium. This breaks the mode degeneracy within the higher-order MG, leading to different group velocities among corresponding eigenmodes. Thus, the HNL-FMF can support multiple intermodal four-wave mixing processes between the two MGs at the same time. In a proof-of-concept experiment, we demonstrate simultaneous intermodal wavelength conversions among three eigenmodes of the HNL-FMF over the C band.

Modeling of Few-Mode Multi-Core Optical Fiber Channe Based on Non-Uniform Mode Field Distribution

In this paper, the influencing factors that affect few-mode and multi core optical fiber channel are analyzed in a comprehensive way. The theoretical modeling and computer simulation of the information channel are carried out and then the modeling scheme of few-mode multicore optical fiber channel based on non-uniform mode field distribution is put forward. The proposed modeling scheme can not only exponentially increases the system capacity through fewmode multi-core optical fiber channel, but has better transmission performance compared to the channel of the same type to the uniform channel revealing from the simulation results.

High-Power Raman Soliton Generation at 1.7 μm in All-Fiber Polarization-Maintaining Erbium-Doped Amplifier

An all-fiber polarization maintaining high-power laser system operating at 1.7 μm based on the Ramaninduced soliton self-frequency shifting effect is demonstrated. The entirely fiberized system is built by erbiumdoped oscillator and two-stage amplifiers with polarization maintaining commercial silica fibers and devices, which can provide robust and stable soliton generation. High-power soliton laser with the average power of 0.28 W,the repetition rate of 42.7 MHz, and pulse duration of 515 fs is generated directly from the main amplifier.Our experiment provides a feasible method for high-power all-fiber polarization maintaining femtosecond laser generation working at 1.7 μm.

Optical Nonlinearity of Violet Phosphorus and Applications in Fiber Lasers

A D-shaped fiber is coated with a new two-dimensional nanomaterial,violet phosphorus(VP),to create a saturable absorber(SA)with a modulation depth of 3.68%.Subsequently,the SA is inserted into a fiber laser,enabling successful generation of dark solitons and bright–dark soliton pairs through adjustment of the polarization state within the cavity.Through further study,mode-locked pulses are achieved,proving the existence of polarization-locked vector solitons.The results indicate that VP can be used as a polarization-independent SA.

Time Transfer in a 1839-km Telecommunication Fiber Link Demonstrating a Picosecond-Scale Stability

An implementation of high-precision time transfer over a 1839-km field fiber loop back link between two provincial capitals of China,Xi’an and Taiyuan,is reported.Time transfer stabilities of 6.5 ps at averaging time of 1 s and 4.6 ps at 40000 s were achieved.The uncertainty for the time transfer system was evaluated,showing a budget of 56.2 ps.These results stand for a significant milestone in achieving high-precision time transfer over a field fiber link spanning thousands of kilometers,signifying a record-breaking achievement for the real-field time transfer in both stability and distance,which paves the way for constructing the nationwide high-precision time service via fiber network.

On the generation of high-quality Nyquist pulses in mode-locked fiber lasers

Nyquist pulses have wide applications in many areas,from electronics to optics.Mode-locked lasers are ideal platforms to generate such pulses.However,how to generate high-quality Nyquist pulses in mode-locked lasers remains elusive.We address this problem by managing different physical effects in mode-locked fiber lasers through extensive numerical simulations.We find that net dispersion,linear loss,gain and filter shaping can affect the quality of Nyquist pulses significantly.We also demonstrate that Nyquist pulses experience similariton shaping due to the nonlinear attractor effect in the gain medium.Our work may contribute to the design of Nyquist pulse sources and enrich the understanding of pulse shaping dynamics in mode-locked lasers.