Mode-group selective photonic lanterns for multiplexing multi-order orbital angular momentum modes.

The lack of research on photonic lanterns multiplexing multi-order orbital angular momentum (OAM) modes hinders the development of OAM space division multiplexing systems. In this paper, an annular multicore photonic lantern (AMCPL) for multiplexing several OAM mode groups is proposed and demonstrated. Comprehensive simulations are carried out to investigate the effect of the multicore arrangements on the crosstalk (XT) between different OAM mode groups. Further optimization provides an inverted multicore arrangement of the OAM AMCPL with balanced XT between high-order OAM mode groups with topological charges |l| = 2 to 5 for the first time, of which the highest XT between target mode groups does not exceed -27.20 dB at wavelengths from 1300 nm to 1600 nm, and mode conversion efficiencies of all target mode groups exceed 99.5%. Furthermore, a quantum interpretation is given to reveal the characteristics of the evolution of the supermodes along the taper of the OAM AMCPL, which has not been reported.

Broadband generation of multiple high-order OAM modes in ring-core fibers using multi-pitch chirped long-period fiber gratings.

A multi-order broadband mode converter in a ring-core fiber (RCF) using a multi-pitch chirped long-period fiber grating (LPFG), where multiple pitches were introduced in each chirp to further increase the bandwidth, is proposed and demonstrated. The grating parameters were optimized both theoretically and experimentally to achieve broadband mode generation of OAM ± 2 and OAM ± 3 modes by increasing the number of chirps and pitches. The mode conversion efficiency is higher than 90% with a broadband of 57 nm from 1456 nm to 1513 nm and 51 nm from 1573 nm to 1624 nm, corresponding to the second-order OAM mode and third-order OAM mode, respectively. Additionally, the insertion loss is less than 0.8 dB, and the purity is over 90%. The demonstrated mode converter has successfully achieved simultaneous generation of multi-order broadband OAM modes in a RCF for the first time, which has promising potential for application in OAM mode-division multiplexing systems.

Observation of the "invisible" pulsation of soliton molecules in a bidirectional ultrafast fiber laser.

A novel optical soliton dynamics phenomenon, called "invisible" pulsation, has gradually attracted extensive interest in recent years, which can only be identified effectively with the help of real-time spectroscopy technique, i.e., dispersive Fourier transformation (DFT). In this paper, based on a new bidirectional passively mode-locked fiber laser (MLFL), the "invisible" pulsation dynamics of soliton molecules (SMs) is systematically studied. It is indicated that the spectral center intensity, pulse peak power and relative phase of SMs are periodically changed during the "invisible" pulsation, while the temporal separation inside the SMs is constant. The degree of spectral distortion is positively correlated with the pulse peak power, which verifies that self-phase modulation (SPM) is the inducement of spectral distortion. Finally, the universality of the SMs "invisible" pulsation is further experimentally verified. We believe our work is not only conducive to the development of compact and reliable bidirectional ultrafast light sources, but also of great significance to enrich the study of nonlinear dynamics.

Ultra-low-loss 5-LP mode selective coupler based on fused biconical taper technique.

Trapped in the stringent adiabatic transmission condition of high-order modes, low-loss fused biconical taper mode selective coupler (FBT-MSC) has long been challenging to achieve. We identify the adiabatic predicament of high-order modes to stem from the rapid variation of the eigenmode field diameter, which is caused by the large core-cladding diameter difference of few-mode fiber (FMF). We demonstrate that introducing a positive-index inner cladding in FMF is an effective approach to address this predicament. The optimized FMF can be used as dedicated fiber for FBT-MSC fabrication, and exhibits good compatibility with the original fibers, which is critical for the wide adoption of MSC. As an example, we add inner cladding in a step-index FMF to achieve excellent adiabatic high-order mode characteristics. The optimized fiber is used to manufacture ultra-low-loss 5-LP MSC. The insertion losses of the fabricated LP01, LP11, LP21, LP02 and LP12 MSCs are 0.13 dB at 1541 nm, 0.02 dB at 1553 nm, 0.08 dB at 1538 nm, 0.20 dB at 1523 nm, and 0.15 dB at 1539 nm, respectively, with smoothly varying insertion loss across the wavelength domain. Additional loss is less than 0.20 dB from 1465.00 nm to 1639.31 nm, and the 90% conversion bandwidth exceeds 68.03 nm, 166.68 nm, 174.31 nm, 132.83 nm, and 84.17 nm, respectively. MSCs are manufactured using commercial equipment and a standardized process that takes just 15 minutes, making them a potential candidate for low-cost batch manufacturing in a space division multiplexing system.

"Invisible" pulsation and period-doubling bifurcation of harmonic mode locking in a bidirectional fiber laser.

The harmonic mode-locking (HML) "invisible" pulsation (IP) is reported, here, in a bidirectional passively mode-locked fiber laser (BPMLFL). With the help of dispersive Fourier transform (DFT) technology, it is found that due to the alike nonlinear effects experienced by two pulse trains in HML, their evolution is consistent during the IP. Further, as the increase of pump power, period-doubling bifurcations (PDBs) can be observed based on the IP phenomenon in the HML regime, the PDB path experienced by the HML from steady to chaotic is statistically obtained. Finally, the IP and PDB in the bidirectional laser are reproduced and studied through numerical simulations. The effect of IP on the coherence of solitons is further analyzed. We believe our research results will provide new insights into the study of soliton dynamics in fiber lasers.

High-efficiency broadband third-order OAM mode converter based on a multi-period preset-twist long-period fiber grating.

All-fiber mode converters for generating orbital angular momentum (OAM) beams have many applications in optical communications, optical sensing and lasers. Currently, it is a great challenge to use a long-period fiber grating (LPFG) to broadband excite high-order OAM modes above the second-order. Here, we demonstrate a preset-twist LPFG fabrication method, which introduces asymmetry in the refractive index modulation area, for efficient generation of third-order modes. Through optimization, the generation of third-order OAM modes with 99.55% conversion efficiency, 0.81 dB insertion loss, and over 99% purity is achieved with only 40 pitch number. In addition, a multi-period preset-twist LPFG is proposed and demonstrated to achieve the excitation of broadband third-order mode with conversion efficiency of more than 99%, insertion loss of less than 1 dB, and mode purity of more than 90%. The 15 dB bandwidth (96.8% conversion efficiency) of the LPFG is 109 nm in the wavelength range from 1475 nm to 1584 nm, and the 20 dB bandwidth (99% conversion efficiency) of the LPFG is 92 nm from 1488 nm to 1580 nm. To the best of our knowledge, this is the first time to generate efficient and broadband third-order mode using a long-period fiber grating.

Observation and characterization of the high order modes in a six-mode fiber using an OFDR method.

High order modes in a six-mode fiber are separately observed and characterized using an Optical Frequency Domain Reflectometry (OFDR) method. Due to the difference in group refractive index between fundamental mode and the high order modes, Fresnel reflection peaks for each mode can be separated in beat frequency domain with their corresponding time delay. In the experiment, the fundamental mode and high order modes are excited in turn and observed at a 6.6 m six-mode fiber end, which agree with their beat frequency difference in theoretical simulation. The demonstration provides a flexible and feasible method for mode identification and characterization of all kinds of fibers.

Broadband dispersion compensating ring-core fiber for orbital angular momentum modes.

A well designed ring-core fiber can theoretically support numerous orbital angular momentum (OAM) modes with low crosstalk for space-division-multiplexing (SDM) data transmission, which is considered as a promising solution for overcoming the capacity crunch in optical communication network. However, the accumulated chromatic dispersion in OAM-fiber could limit the data speed and transmission distance of communication systems. A potential solution is to insert a dispersion compensation ring-core fiber with opposite-sign of dispersion in the transmission fiber along the fiber link. In this work, we propose a triple ring-core fiber with broadband negative dispersion. A highest negative dispersion of -24.47 ps/(nm·km) at 1550 nm and an average dispersion slope in the C band from -0.182 ps/(nm2·km) to 0.065 ps/(nm2·km) can be achieved to compensate multi-order dispersion. The effects of Ge-doping concentration fluctuation in the high-index ring core and fabrication errors on fiber geometric structures are also investigated. Furthermore, the effective mode area decreases as the widths of high-index rings increase due to the enhanced confinement ability. The designed triple ring-core fiber could offer potential for compensating OAM fiber links with positive dispersions.

19-ring-air-core fiber supporting thousands of OAM modes for spatial division multiplexing.

We propose and design a 19-ring-air-core fiber that can support about 3000 orbital angular momentum (OAM) modes (156 modes in each ring) with <-80 dB inter-ring cross talk across the entire C and L bands after 100-km fiber propagation. Moreover, the eigenmodes are all separated from their adjacent modes by effective index differences >2.67 × 10-4 and mode groups by > 1.90 × 10-2, which can guarantee the stable transmission of OAM modes. This designed fiber is a potential candidate for applications in spatial division multiplexing (SDM) of optical channels to improve the capacity of next-generation high-speed optical communication systems, especially in short-distance applications. In this Letter, we also show the relationship between supported OAM mode numbers, total cross talk, and effective refractive index of intra-ring modes during the optimization of fiber through numerical simulations. It can provide a related reference for the future design of multi-ring-core fibers.

Wideband, large mode field and single vector mode transmission in a 37-cell hollow-core photonic bandgap fiber.

Stable generation and propagation of ultrafast high-order mode beams has become an important research direction. A core diameter not more than 10 times the wavelength is regarded as the upper limit for single mode transmission. However, a high-power laser requires a core diameter 20 to 40 times the wavelength to achieve high-power and stable output, which exceeds the design limit of the traditional fiber. In this paper, a novel 37-cell hollow core photonic bandgap fiber (HC-PBF) that only supports pure TE01 mode over a bandwidth of 50 nm with the lowest loss of 0.127 dB/km is proposed. The HC-PBF has a core diameter of more than 40 µm. Single mode guidance is achieved by adjusting the lattice size in a particular of the cladding. The best single mode performance with a loss ratio as high as 150,000 between TE01 mode and other modes with minimum loss is obtained. The fiber also has low bend-loss and thus can be coiled to a small bend radius of 1 cm having 1.6 dB/km bend loss. The tunability of the single-mode window and the manufacturing feasibility of the proposed fiber are also discussed.

Single-resonator, stable dual-longitudinal-mode optofluidic microcavity laser based on a hollow-core microstructured optical fiber.

A single-resonator, stable dual-longitudinal-mode optofluidic microcavity laser based on a hollow-core microstructured optical fiber is proposed and experimentally demonstrated. The resonator and microfluidic channel are integrated in the hollow-core region of the fiber, inside which a hexagonal silica ring is used as the only resonator of the laser. Experimental results show that with mixing a small amount of Rhodamine B into a 1 mM Rhodamine 6G solution to form a dual-dye solution as a gain medium, the laser obtained by the method of lateral pumping can operate at dual longitudinal modes, with a threshold of 90 nJ/mm2. By adjusting the concentration of Rhodamine B, the lasing wavelength of the laser and the power ratio of the two wavelengths can be controlled. And because the laser emission is co-excited by different kinds of dye molecules, the mode competition is diminished, enabling the simultaneously efficient optical gain and therefore lasing at dual longitudinal modes stably with a maximum lasing intensity fluctuation of 3.2% within 30 minutes even if the dual longitudinal modes have the same linear polarization states. This work can open up promising opportunities for diverse applications in biosensing and medical diagnosis with high sensitivity and integrated photonics with compact structure.

Oscillatory self-organization dynamics between soliton molecules induced by gain fluctuation.

In passively mode-locked fiber lasers (PMLFLs), the dissipative solitons (DSs) can self-organize to form complex structures through delicate interactions. However, it is still elusive to control these soliton structures by external influences. We here find that at a certain critical power, the location between two soliton molecules can be controlled by a slow modulated pump power. After applying the pump power with periodic fluctuation, two soliton molecules oscillate from the state of soliton molecular complex to stable distribution with maximum inter-molecular separation. During this process, the internal structure of each soliton molecule keeps steady. The slow gain depletion and recovery mechanism which plays a dominant role affects the motion of soliton molecules. These results could further expand the molecular analogy of spectroscopy and stimulate the development of optical information storage and processing.

Non-zero dispersion-shifted ring fiber for the orbital angular momentum mode.

As the dimension of orbital angular momentum (OAM) is orthogonal to the other degrees of freedom for photon, such as wavelength, it can be utilized to further increase data capacity in the wavelength division multiplexing (WDM) systems. However, the non-zero dispersion-shifted fiber (NZDSF) for the OAM mode has not yet been investigated or even proposed. In this work, we propose and design a ring fiber with low chromatic dispersion for the HE2,1 mode, which can serve as NZDSF for its corresponding OAM1,1 mode. A low dispersion of 3.3 ps/(nm·km) at 1550 nm and <2.9 ps/(nm·km) dispersion variation from 1530 to 1565 nm for the OAM1,1 mode is achieved in simulation, which satisfies the standard of the ITU-T G.655.C. The designed fiber with ring width from 1.5 µm to 3.5 µm can support the OAM1,1 mode within the C-band, and a large effective area of about 646 µm2 is obtained. We also note that the fiber with larger inner radius and ring width are more tolerant to the perturbations, such as fiber ellipticity and bending. In the fiber-based optical communication systems, the designed ring fiber could be used as a candidate for supporting OAM modes with low dispersion and reduced nonlinear effects.

Optical characterization of Ge11.5As24S64.5 glass for an on-chip supercontinuum.

An on-chip supercontinuum (SC) source spanning from 900 nm to 2000 nm has been experimentally presented and analyzed based on a Ge11.5As24S64.5 (GeAsS) planar waveguide at telecommunication wavelength. The nonlinear response parameter (γ) of the GeAsS waveguide is estimated to be ∼12/W/m at the pump wavelength using resonant grating waveguide (RGW) nonlinear refractive index (n2=2×10-18m2/W), which is measured by the z-scan technique. The dispersion of the waveguide is carefully engineered based on the refractive index of the GeAsS film where the film structure is confirmed by a Raman spectrum exhibiting consistency with the corresponding glass. The results suggest that the GeAsS glass is expected to be an ideal platform for on-chip devices.

Mode transmission analysis method for photonic lantern based on FEM and local coupled mode theory.

In this paper, a novel full vector numerical simulation method based on the finite element method (FEM) and local coupled mode theory (LCMT) for analyzing the mode transmission characteristics of photonic lantern (PL) with arbitrary input mode field is proposed. Compared with the traditional numerical simulation methods for PL, our method can greatly reduce the computational complexity and ensure high precision. Taking a three-core PL as an example, we verify the validity of our method. The advantages and properties of our method are also discussed in detail and found instructive for optimization design of PL. Through specifically optimizing the geometric parameters of the PL according to the properties, mode selectivity of LP01 and LP11 can be respectively improved up to 44.5 dB and 54.7 dB with more than 95% coupling efficiency.

Orbital angular momentum generation in a dual-ring fiber based on the phase-shifted coupling mechanism and the interference of supermodes.

Based on the phased-shifted interference between supermodes, a novel method that can directly convert LP01 mode to orbital angular momentum (OAM) mode in a dual-ring microstructure optical fiber is proposed. In this fiber, the resonance between even and odd HE11 modes in inner ring and higher order mode in outer ring will form two pairs of supermodes, and the intensities and phases of the complete superposition mode fields for the involved supermodes created by the resonance at different wavelengths and propagating lengths are investigated and exhibited in this paper. We demonstrate that OAM mode can be generated from π/2-phase-shifted linear combinations of supermodes, and the phase difference of the even and odd higher order eigenmodes can accumulate to π/2 during the coupling process, which is defined as "phase-shifted" conversion. We build a complete theoretical model and systematically analyze the phase-shifted coupling mechanism, and the design principle and optimization method of this fiber are also illustrated in detail. The proposed microstructure fiber is compact, and the OAM mode conversion method is simple and flexible, which could provide a new approach to generate OAM states.

Real-time observation of multi-soliton asynchronous pulsations in an L-band dissipative soliton fiber laser.

Based on the experimental platform of an L-band normal-dispersion mode-locked fiber laser, we report the first observation, to our knowledge, of three novel types of multi-soliton asynchronous pulsation phenomena by virtue of the dispersive Fourier transform technique. The experimental results provide new insights into the complex multi-soliton dynamics under unstable mode-locking conditions. It is confirmed that more than one pulsating solution can coexist in a multi-pulse situation and that each soliton may evolve periodically in different ways. This implies that subsequent experimental and theoretical studies on multi-soliton need to take the differences among pulses into account and retrieve more degrees of freedom.

Real-time observation of dissociation dynamics within a pulsating soliton molecule.

Real-time detection of the ultrafast dynamics in complex nonlinear optical systems provides novel insights into pulse interactions and dynamic patterns, especially for soliton molecules. Herein, the concept of soliton molecule is extended to the pulsating regime, revealing the dynamical diversity of soliton molecule and the universality of pulsating behavior. By virtue of the dispersive Fourier transform (DFT) technique, we present the first experimental observation of the dissociation dynamics within a pulsating soliton molecule generated in an L-band normal-dispersion mode-locked fiber laser. The results provide valuable references for resolving the interactions in complex dissipative systems.

Transient behaviors of pure soliton pulsations and soliton explosion in an L-band normal-dispersion mode-locked fiber laser.

As one of the most striking localized structures in dissipative systems, pulsating soliton has been widely studied in theory but rarely observed in experiments. Here, three typical types of soliton pulsations are experimentally demonstrated in an L-band normal-dispersion mode-locked fiber laser via the dispersive Fourier transform (DFT) technique. According to the distinctive features, they are classified as single-periodic pulsating soliton, double-periodic pulsating soliton and soliton explosion. These pulsations have common features such as energy oscillation, bandwidth breathing and temporal shift. However, the pulse is repeated every two oscillations for double-periodic pulsating soliton. When it comes to soliton explosion, because of the intermittent overdriven nonlinear effect induced by the extreme energy oscillation, the spectrum cracks into pieces at a periodic manner. To the best of our knowledge, it is the first time that both pure soliton pulsations and soliton explosion are observed experimentally in the same fiber laser. The results will enhance a more comprehensive understanding for the soliton pulsating phenomena.

Microcavity-coupled fiber Bragg grating with tunable reflection spectra and speed of light.

After a fiber Bragg grating (FBG) is fabricated, the reflection spectrum of the FBG is generally not tunable without mechanical deformation or temperature adjustment. Here we present a microcavity-coupled FBG with both a tunable reflection lineshape and dispersion using electromagnetically induced transparency. The Fano interference of light in the FBG and the microcavity allows for dramatic modification of the reflection spectrum. The phase of the reflected spectrum is continuously tunable between 0 and 2π to produce various Fano lineshapes. The dispersion of the output light is adjustable from normal dispersion to abnormal dispersion, consequently providing an adjustable speed of light. Additionally, it allows the FBG to switch from a notch filter to a bandpass filter at the resonant wavelength, which is not possible in a conventional uniform FBG.