Controllable all-fiber orbital angular momentum mode converter

Optical vortex lasers [Invited]

The manipulation of light fields has become pivotal in advancing various domains of optics, including classical and quantum communications, optical trapping, optical metrology and high-resolution imaging. Among various techniques for creating optical vortices, the optical vortex lasers, distinguished by directly emitting customized beams with phase or polarization singularities, have emerged as a burgeoning tool in modern optics. This paper summarizes the recent progress of optical vortex lasers including their primary types, designing methods and diverse applications. We begin by introducing the underlying principles of optical vortices and then explore numerous methods for designing optical vortex lasers, including bulk laser, fiber laser and on-chip laser. We also highlight the optical vortex laser towards higher dimensions for shaping structured beams with more complex spatial and topological patterns. Furthermore, we outline the wide applications of optical vortex lasers and address their challenges and potential future developments. This paper serves as a thorough overview for the physics, optics and engineering communities looking to harness potential of optical vortex lasers in cutting-edge applications.

Simultaneous generation of first- to fourth-order OAM modes based on a cascaded preset-twist long-period fiber grating

We propose and demonstrate the simulation and fabrication of an all-fiber orbital angular momentum (OAM) mode converter capable of generating first- to fourth-order modes simultaneously, which is realized by inscribing a cascaded preset-twist long-period fiber grating (CPT-LPFG) in a six-mode fiber utilizing a CO2 laser. A new segmented Runge-Kutta method is proposed to simulate the preset-twist long-period fiber gratings. By calculating the twist angle and relative coupling coefficient for each pitch and then solving the coupled mode equations utilizing the Runge-Kutta algorithm. The simulation illustrates that the preset-twist method significantly improves the coupling coefficient of higher-order modes, thereby reducing coupling difficulty. In the experiment, by twisting the fiber at an angle of 1080° and fabricating cascaded gratings with periods of 745 μm, 310 μm, 204 μm, and 146 μm, it is feasible to generate first- to fourth-order OAM modes simultaneously, at wavelengths of 1635 nm, 1548 nm, 1460 nm, and 1334 nm, respectively. The insertion loss is less than 1 dB, and the mode purity is over 90 %. To the best of our knowledge, this is the first time that first- to fourth-order OAM modes are simultaneously generated utilizing a single long-period fiber grating.

Reuleaux triangle core fiber with triple rotational symmetry

A Reuleaux triangle core fiber (RTF) with triple rotational symmetry is proposed and fabricated. Then the RTF is twisted to form the chiral fiber grating, which converts the core mode into a vortex mode containing 3rd-order orbital angular momentum (OAM). Based on the Fourier expansion of the core boundary, the straight-sided and arc-sided triangular core profiles were analyzed, revealing the mechanism of high-efficiency OAM3 generation. The experimental results show a 3rd-order vortex mode with a high conversion efficiency and purity, and the polarization-independent characteristics endowed by the core shape are also confirmed. The proposed RTF provides a new, to the best of our knowledge, way for higher-order vortex beam generation, which can be used in optical fiber communication systems with OAM multiplexing.

All-fiber function devices for twisted lights

Lights carrying orbital angular momentum (OAM), also called twisted lights, have been applied in fields of optical manipulation, imaging, quantum communication, and mode-division-multiplexing (MDM) optical communication systems. Traditional approaches for manipulating twisted lights carrying OAM in free space paths such as Q-plates, spiral phase plates (SPPs), and spatial light modulators (SLMs) that are usually affected by diffraction effect and imperfect alignment between different optical components, limiting the practical applications of twisted lights. Here we design, fabricated, and package all-fiber function devices for twisted light carrying OAM such as all-fiber broadband OAM generator, all-fiber OAM (de)multiplexer, all-fiber OAM & WDM coupler, and all-fiber OAM 1 × 2 coupler. Base on coupled mode theory and phase-matching condition, twisted light can be generated and detected by pre-tapered single mode fiber (SMF) fusing with multi-mode fiber (MMF). The results show that the proposed all-fiber function devices for twist light have large working broadband (at least C band), high purity (above 95%), and low insert loss (less than 3 dB). The proposed devices will open a reliable way for twisted light applied in optical fiber communications and optical interconnections.

Broadband linearly polarized mode converter based on over-coupled long-period fiber grating

We demonstrate the fabrication of over-coupled long-period fiber gratings (LPFGs) in the 1.55-µm and 2-µm wavebands enabling broadband linearly polarized LP11 mode conversion using a CO2 laser. The birefringence of the fiber is caused by on one side laser exposure and increases with the increase of refractive index modulation depth, which realizes the conversion of linearly polarized modes. The mode conversion bandwidth can be significantly increased by using the over-coupled LPFG. The 10-dB bandwidth of the LPFGs with |κ|L values of π/2, 3π/2, and 5π/2 are 33.04, 80.84, and 114.08 nm at 1.55 µm waveband, respectively. The maximum bandwidth of the over-coupled LPFG is 3.79 times higher than that of conventional LPFG. The operating wavelength of the mode converter can be extended to 2.0 µm wavebands and the maximum 10-dB bandwidth reaches 161.32 nm. The proposed broadband linearly polarized mode converters could have potential application in the fields of mode division multiplexing systems, fiber laser systems.

Effect of core ellipticity and core-induced thermal stress on the conversion of LP11 modes to vector vortex modes in gradually twisted highly birefringent fibers

We study the effect of the core ellipticity and core-induced thermal stress on the conversion of LP11 modes to vortex modes in gradually twisted highly birefringent PANDA fibers using an improved perturbation-based modeling method. We show that these two technologically unavoidable factors have a significant impact on the conversion process, which manifests itself in shortening the conversion length, altering the assignment between the input LP₁₁ modes and output vortex modes, and modifying the vortex mode structure. In particular, we demonstrate that for certain fiber geometries, it is possible to obtain output vortex modes with parallel and antiparallel spins and orbital angular momenta. The simulation results obtained using the modified method are in good agreement with recently published experimental data. Furthermore, the proposed method provides reliable guidelines for choosing fiber parameters that ensure a short conversion length and the desired polarization structure of the output vortex modes.

Dynamic generation of photonic spatial quantum states with an all-fiber platform

Photonic spatial quantum states are a subject of great interest for applications in quantum communication. One important challenge has been how to dynamically generate these states using only fiber-optical components. Here we propose and experimentally demonstrate an all-fiber system that can dynamically switch between any general transverse spatial qubit state based on linearly polarized modes. Our platform is based on a fast optical switch based on a Sagnac interferometer combined with a photonic lantern and few-mode optical fibers. We show switching times between spatial modes on the order of 5 ns and demonstrate the applicability of our scheme for quantum technologies by demonstrating a measurement-device-independent (MDI) quantum random number generator based on our platform. We run the generator continuously over 15 hours, acquiring over 13.46 Gbits of random numbers, of which we ensure that at least 60.52% are private, following the MDI protocol. Our results show the use of photonic lanterns to dynamically create spatial modes using only fiber components, which due to their robustness and integration capabilities, have important consequences for photonic classical and quantum information processing.

Reconfigurable ultra-broadband mode converter based on a two-mode fiber with pressure-loaded phase-shifted long-period alloyed waveguide grating

We present a reconfigurable ultra-broadband mode converter, which consists of a two-mode fiber (TMF) and pressure-loaded phase-shifted long-period alloyed waveguide grating. We design and fabricate the long-period alloyed waveguide gratings (LPAWG) with SU-8, chromium, and titanium via the photo-lithography and electric beam evaporation technique. With the help of the pressure loaded or released from the LPAWG onto the TMF, the device can realize reconfigurable mode conversion between the LP₀₁ mode and the LP₁₁ mode in the TMF, which is weak sensitive to the state of polarization. The mode conversion efficiency larger than 10 dB can be achieved with operation wavelength range of about 105 nm, which ranges from 1501.9 nm to 1606.7 nm. The proposed device can be further used in the large bandwidth mode division multiplexing (MDM) transmission and optical fiber sensing system based on few-mode fibers.

Transmission of an optical vortex beam in antiresonant fibers generated in an all-fiber system

We report an experimental study on transmission of orbital angular momentum mode in antiresonant fibers generated with a dedicated all-fiber optical vortex phase mask. The vortex generator can convert Gaussian beam into vortex beams with topological charge l = 1. Generated vortex beam is directly butt-coupled into the antiresonant fiber and propagates over distance of 150 cm. The stability and sensitivity of the transmitted vortex beam on the external perturbations including bending, axial stress, and twisting is investigated. We demonstrate distortion-free vortex propagation for the axial stress force below 0.677 N, a bend radius greater than 10 cm.

Super-resolution optical microscopy using cylindrical vector beams

Super-resolution optical microscopy, which gives access to finer details of objects, is highly desired for fields of nanomaterial, nanobiology, nanophotonics, etc. Many efforts, including tip optimization and illumination optimization etc., have been made in both near-field and far-field super-resolution microscopy to achieve a spatial resolution beyond the diffraction limit. The development of vector light fields opens up a new avenue for super-resolution optical microscopy via special illumination modes. Cylindrical vector beam (CVB) has been verified to enable resolution improvement in tip-scanning imaging, nonlinear imaging, stimulated emission depletion (STED) microscopy, subtraction imaging, superoscillation imaging, etc. This paper reviews recent advances in CVB-based super-resolution imaging. We start with an introduction of the fundamentals and properties of CVB. Next, strategies for CVB based super-resolution imaging are discussed, which are mainly implemented by tight focusing, depletion effect, plasmonic nanofocusing, and polarization matching. Then, the roadmap of super-resolution imaging with CVB illumination in the past two decades is summarized. The typical CVB-based imaging techniques in fields of both near-field and far-field microscopy are introduced, including tip-scanning imaging, nonlinear imaging, STED, subtraction imaging, and superoscillation imaging. Finally, challenges and future directions of CVB-illuminated super-resolution imaging techniques are discussed.

Multi-channel higher-order OAM generation and switching based on a mode selective interferometer

A multi-channel orbital angular momentum (OAM) mode generation and switching scheme is proposed and demonstrated based on an in-fiber mode selective interferometer (MSI), which is formed in a four-mode fiber. The MSI consists of two strong modulated long-period fiber gratings (LPFGs), which realizes the mode selected coupling between a target mode pair. With the optimized structural parameters, the MSI can couple a launched LP₀₁ (or OAM₀) into a desired higher-order azimuthal mode (HAM, LPl₁ or OAM_±l, l≥1) at multiple wavelength channels and generate the HAM with high-purity. To verify this concept, we fabricate two LPFGs in a four-mode fiber with designed distance and hence realize a MSI which can generate the second-order HAM (OAM₂ or LP₂₁) at 17 wavelength channels. The mode conversion efficiency is more than 90% at 17 wavelengths and the corresponding mode purity is no less than 97%, respectively. In addition, we also demonstrate that the selected mode pair (OAM₀ and OAM₂) can be switched at multiple channels by changing the state of the MSI. This MSI can also be used as a wavelength band-rejection filter on different spatial modes and find potential applications in optical communications and sensing.

Modeling of the conversion of LP modes to vector vortex modes in gradually twisted highly birefringent fibers

We present a new method for the efficient modeling of the conversion of LP modes to vortex modes in gradually twisted highly birefringent fibers, employing the coupled-mode approach in helicoidal coordinates. The method is applicable to a class of highly birefringent fibers with cylindrical cores and stress-applying elements. We analyzed the effects of refractive index contrast, birefringence, and twist rate profile on the quality of the converted vortex beams, including the intensity and polarization distributions, as well as on the crosstalk between different eigenmodes at the output of the twisted fibers. The obtained results prove the possibility of a broadband quasi-adiabatic generation of vortex beams of high purity in gradually twisted highly birefringent fibers a few centimeters long and provide hints for optimization of the conversion process.

Singularities splitting phenomenon for the superposition of hybrid orders structured lights and the corresponding interference discrimination method

It is the basic characteristic of pure vortex light that there is a phase singularity at the origin. Such a singularity may be multiple degenerate, which determines the order of vortex light. Singularities splitting phenomenon means that singularities no longer concentrate at the origin but distribute around the space, usually occurring in impure vortex light. In this paper, we demonstrate the singularities splitting phenomenon and propose an analysis method, based on which one may rapidly estimate the modal components of impure vortex light. As two common singularity discrimination methods, the spiral and fork wire interference patterns are compared in distinguishing splitting singularities. The most widely used spiral interference pattern is revealed to be the worst form because of the low resolution. Instead, the fork wire interference pattern is with higher and easily adjusted resolution. 1‰ impurity is still able to be distinguished through fork wire interference patterns in the experiment.

Femtosecond laser inscribed helical long period fiber grating for exciting orbital angular momentum

A method employing femtosecond lasers to inscribe helical long period fiber grating (HLPFG) for exciting orbital angular momentum (OAM) of light is experimentally demonstrated. In this method, the refractive index modulation (RIM) of HLPFG is realized by three-dimensional translation of a fiber without rotation, indicating better stability, repeatability and flexibility. The coupling efficiency can be customized by varying the radius of the helical RIM, except laser energy. The characteristics of phase and polarization purity of the coupled modes in HLPFGs are studied. Results show that HLPFGs can directly excite OAM modes, the polarization state and helical phase of the mode can be adjusted independently, and the purity is the highest at resonant wavelength, over 91%.

Orbital angular momentum and beyond in free-space optical communications

Orbital angular momentum (OAM), which describes tailoring the spatial physical dimension of light waves into a helical phase structure, has given rise to many applications in optical manipulation, microscopy, imaging, metrology, sensing, quantum science, and optical communications. Light beams carrying OAM feature two distinct characteristics, i.e., inherent orthogonality and unbounded states in principle, which are suitable for capacity scaling of optical communications. In this paper, we give an overview of OAM and beyond in free-space optical communications. The fundamentals of OAM, concept of optical communications using OAM, OAM modulation (OAM modulation based on spatial light modulator, high-speed OAM modulation, spatial array modulation), OAM multiplexing (spectrally efficient, high capacity, long distance), OAM multicasting (adaptive multicasting, N-dimensional multicasting), OAM communications in turbulence (adaptive optics, digital signal processing, auto-alignment system), structured light communications beyond OAM (Bessel beams, Airy beams, vector beams), diverse and robust communications using OAM and beyond (multiple scenes, turbulence-resilient communications, intelligent communications) are comprehensively reviewed. The prospects and challenges of optical communications using OAM and beyond are also discussed at the end. In the future, there will be more opportunities in exploiting extensive advanced applications from OAM beams to more general structured light.

Spin-Orbit Mapping of Light

Spin-orbit photonics, involving the interaction between the spin angular momentum (SAM) and orbital angular momentum (OAM) of light, plays an important role in modern optics. Here, we present the spin-orbit mapping of light in a few-mode fiber that originates from the mode degeneracy lifting (TM_{01} and TE_{01}) property. We demonstrate two kinds of spin-orbit mapping phenomena, i.e., mapping from intrinsic SAM to OAM and mapping from polarization direction rotation to field pattern rotation. The demonstrated spin-orbit mapping shows high efficiency, large bandwidth, availability for short pulses, and scalability to high-order OAM states.

Excitation of high order orbital angular momentum modes in ultra-short chiral long period fiber gratings

A class of ultra-short chiral long period fiber gratings (CLPFGs) are prepared by writing a spiral curve on the surface of a six-mode fiber. The CLPFGs are applied to excite ±2^nd- and ±3^rd-order orbital angular momentum (OAM) modes. The coupling efficiency of the CLPFG in these modes can be as high as 99%, when the length is only 0.5cm. The polarization characteristic of the excited higher-order OAM modes in CLPFGs was theoretically analyzed and experimentally investigated. Results show that the obtained ±2^nd- and ±3^rd-order OAM modes are polarization independent, as expected.

Transmission and Generation of Orbital ANGULAR Momentum Modes in Optical Fibers

The orbital angular momentum (OAM) of light provides a new degree of freedom for carrying information. The stable propagation and generation of OAM modes are necessary for the fields of OAM-based optical communications and microscopies. In this review, we focus on discussing the novel fibers that are suitable for stable OAM mode transmission and conversion. The fundamental theory of fiber modes is introduced first. Then, recent progress on a multitude of fiber designs that can stably guide or generate OAM modes is reviewed. Currently, the mode crosstalk is regarded as the main issue that damages OAM mode stability. Therefore, the coupled-mode theory and coupled-power power theory are introduced to analyze OAM modes crosstalk. Finally, the challenges and prospects of the applications of OAM fibers are discussed.

Excitation of high-quality orbital angular momentum vortex beams in an adiabatically helical-twisted single-mode fiber

A novel method to control the parameters of a chiral fiber grating structure is proposed. Mode couplings are controlled in real time during the twisting fabrication process. This chiral grating structure can satisfy the phase-matching condition for generating high-quality orbital angular momentum (OAM) beams, with an order mode of conversion efficiency over 99.9%. Both theoretical analysis and experimental results of this OAM mode conversion have been investigated, with good agreement. The results demonstrate a dual-OAM beam converter with a charge of ±1 for the right- and left-handed CLPGs, respectively. The high-quality OAM beam generated in this twisted single-mode fiber process may find excellent applications in optical communications.

High-order orbital angular momentum generation in a helically twisted pig-nose-shaped core microstructured optical fibers

We propose a helically twisted pig-nose-shaped core microstructured optical fiber (HPC-MOF) for orbital angular momentum (OAM) mode generation. It comprises seven air-hole rings hexagonally arranged with two air holes and one air-hole ring replaced, forming two cores in a line 3 µm from the fiber center and one ring-shaped core. The fiber is helically twisted along the rotation axis. In this fiber, supermodes in inner dual-core can be coupled to high-order modes in outer ring-core, yielding OAM ring-shaped modes at different certain wavelengths, and various OAM modes at different twist rates were investigated in this paper. We demonstrate the distinct coupling differences of symmetric and antisymmetric supermodes in inner dual-core when the supermode coupled to ring-core mode. A modal matching rule is presented to characterize the coupling differences, which is suitable for describing supermode coupling characteristics in HPC-MOFs. Compared to conventional methods, these properties indicate that the fiber can generate higher-order OAM modes and more easily integrate into all-fiber optical communication systems, with potential in OAM generators, light-controlling devices, and integrated optics applications.

Simultaneous generation of the second- and third-order OAM modes by using a high-order helical long-period fiber grating

An all-fiber orbital angular momentum (OAM) mode generator enabling simultaneous generation of the second- and the third-order OAM modes with conversion efficiencies larger than 95% has been proposed and experimentally demonstrated, which is realized by using a high-order helical long-period fiber grating (HLPG) written in a thinned four-mode fiber. This is the first time, to the best of our knowledge, that two such OAM modes have been simultaneously obtained at wavelengths ranging from 1450 to 1620 nm by using only one fiber component, i.e., the HLPG. The proposed method provides a new way to simultaneously generate different orders of the OAM modes, which would further expand the OAM's applications to the fields of the optical tweezers, microscopy, and fiber communication, etc.

Recent progress in all-fiber ultrafast high-order mode lasers

Ultrafast high-order mode (HOM) lasers are a relatively new class of ultrafast optics. They play a significant role in the fieldsof scientific research and industrial applications due to the high peak power and unique properties of spatial intensity and polarization distribution. Generation of ultrafast HOM beams in all-fiber systems has become an important research direction. In this paper, all-fiber mode conversion techniques, pulsed HOM laser strategies, and few-mode/multi-mode fiber (FMF/MMF) lasers are reviewed. The main motivation of this review is to highlight recent advances in the field of all-fiber ultrafast HOM lasers, for example, generating different HOM pulses based on fiber mode converters and mode-locking in the FMF/MMF lasers. These results suggest that mode selective coupler can be used as a broad bandwidth mode converter with fast response and HOM can be directly oscillated in the FMF/MMF laser cavity with high stability. In addition, spatiotemporal mode-locking in the FMF/MMF is also involved. It is believed that the development of all-fiber ultrafast HOM lasers will continue to deepen, thus laying a good foundation for future applications.

High purity optical vortex generation in a fiber Bragg grating inscribed by a femtosecond laser

In this Letter, a method for orbital angular momentum (OAM) mode generation is proposed and experimentally demonstrated using a fiber Bragg grating (FBG) and off-axis incidence. The FBG fabricated by a femtosecond laser was used to couple the incidence beam into backward high-order modes. The generated modes were then reformed into ring-shaped OAM modes by adjusting the off-axis displacement of the input beam. The intensity distribution, phase vortex, and mode purity of the output light were experimentally investigated. Results indicates that the order of the generated OAM modes is dependent on the resonant wavelength of the FBG, and the sign of the OAM topological charge is determined by the displacement value of the off-axis incident light. In the experiment, ±1- and ±2-order OAM modes were achieved and confirmed, with purities as high as 90%, 91%, 89%, and 88%, respectively.

Experimental investigation of point-by-point off-axis Bragg gratings inscribed by a femtosecond laser in few-mode fibers

Off-axis Bragg gratings with varied horizontal and vertical distances off the center in a step-index two-mode fiber were fabricated by 800 nm infrared-femtosecond laser pulses through a point-by-point technique. In this article, we experimentally investigate these gratings via measuring the transmitted power and the reflected intensity profiles under different input polarization, with multiple characteristics reported for the first time to the best of our knowledge. To highlight, we find that the birefringence induced to the LP₀₁ reaches its maximum magnitude at an intermediate offset, followed by the fast and slow axes switching at a further slightly increased offset. We also show that the peak reflectivity of the LP₁₁ exhibits strong polarization dependence, with the much stronger peak reflectivity constantly corresponding to the polarization perpendicular to the damage-point-to-center line, whereas the peak reflectivity of the LP₀₁ has almost no polarization dependence. Moreover, we report that the reflected mode patterns of the cross-coupling of the LP₀₁ and LP₁₁ are linked to the direction of linear polarization, through which one can selectively excite an arbitrarily oriented LP₁₁ by merely altering the polarization.

High-order mode conversion in a few-mode fiber via laser-inscribed long-period gratings at 1.55 µm and 2 µm wavebands

We demonstrate high-order mode conversion in a few-mode fiber (FMF) via CO₂ laser inscribed long-period fiber gratings (LPFGs) at both the 1.55 µm and 2 µm wavebands. At the 1.55 µm waveband, five high-order core modes (the LP₁₁, LP₂₁, LP₀₂, LP₃₁, and LP₁₂ modes) can be coupled from the LP₀₁ mode with high efficiency by the FMF-LPFGs. The orbital angular momentum beams with different topological charges (±1,±2,±3) are experimentally generated by adjusting the polarization controllers. At the 2 µm waveband, three high-order modes (the LP₁₁, LP₂₁, and LP₀₂ mode) can be coupled by the FMF-LPFGs with different grating periods. The proposed LPFG-based mode converters could have a potential prospects in mode-division multiplexing and multiwindow broadband optical communication applications.

Bandwidth optimization of cascaded long-period gratings for broadband mode conversion over 1.0-2.2 µm waveband

We investigated theoretically and experimentally the cascaded long-period fiber gratings (c-LPFGs) in a few-mode fiber (FMF) for the generation of LP₁₁ core mode in a broad wavelength range. The dependence of the transmission spectra of the c-LPFGs on the spacing between the gratings, and grating periods are studied in detail. The c-LPFGs experimentally generate LP₁₁ core mode in a 10-dB bandwidth of 193.6 nm in 1.55 µm waveband and 447.5 nm in 2 µm waveband, respectively. The first-order orbital angular momentum mode can be converted by the c-LPFGs with the same broadband wavelength range. The 10-dB bandwidth and corresponding wavelength range for mode conversion can be adjusted by changing the grating spacing and grating periods.

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.

Enhancing the sensitivity of plasmonic optical fiber sensors by analyzing the distribution of the optical modes intensity

Improving the sensitivity of plasmonic optical fiber sensors constitutes a major challenge as it could significantly enhance their sensing capabilities for the label-free detection of biomolecular interactions or chemical compounds. While many efforts focus on developing more sensitive structures, we present here how the sensitivity of a sensor can be significantly enhanced by improving the light analysis. Contrary to the common approach where the global intensity of the light coming from the core is averaged, our approach is based on the full analysis of the retro-reflected intensity distribution that evolves with the refractive index of the medium being analyzed. Thanks to this original and simple approach, the refractive index sensitivity of a plasmonic optical fiber sensor used in reflection mode was enhanced by a factor of 25 compared to the standard method. The reported approach opens exciting perspectives for improving the remote detection as well as for developing new sensing strategies.

Compact and broad wavelength range tunable orbital angular momentum mode generator based on cascaded helical photonic crystal fibers

A compact and broad wavelength range tunable orbital angular momentum (OAM) generator was experimentally demonstrated by cascading two helical photonic crystal fibers (HPCFs) with opposite helicity, i.e., clockwise-twisted + anticlockwise-twisted HPCF. Such an OAM generator exhibited a length of approximately 9 mm and generated a high-quality OAM mode with a wavelength range of 35 nm. Moreover, the wavelength range is expected to be tuned from 17.9-51.3 nm by applying mechanical torsion.

Orthogonal long-period fiber grating for directly exciting the orbital angular momentum

An orthogonal long-period fiber grating (OLPFG) is proposed and demonstrated for directly exciting the orbital angular momentum (OAM), without the need for other devices. This grating was produced using CO₂ laser exposure in the orthogonal direction. A helical phase was then optically induced in the OLPFG, with a chirality determined by the structure of the OLPFG. In this study, ±1-order OAM resonances were respectively observed in OLPFGs with a different orthogonal direction. The conversion efficiency of OAM mode in this process was 99%, and the purity was higher than 98%. In addition, incident light in any polarization state was observed to excite OAM with the same polarization.

All-fiber third-order orbital angular momentum mode generation employing an asymmetric long-period fiber grating

The third-order orbital angular momentum (OAM_±3) guided mode generation is demonstrated for the first time, to the best of our knowledge, by employing an asymmetric long-period fiber grating (AS-LPFG). The proposed AS-LPFG is modeled by coupled local-mode theory, which is extended to the coupling of core modes and is fabricated by multicycle scanning ablation with increasing power in a six-mode fiber. The experiments demonstrate that one fabricated AS-LPFG can convert the LP₀₁ mode to the third-azimuthal-order (3AO, LP₃₁ or OAM_±3) guided mode with efficiency of ∼99.8%. The model and the method presented, in principle, can be used to generate any other high-order modes.

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.

Mechanically-Induced Long-Period Fiber Gratings Using Laminated Plates

This work presents a formation method of mechanically-induced long-period fiber gratings using laminated plates. The mechanically-induced long-period fiber grating is temporarily inscribed by compressing the optical fiber between a flat plate and the proposed laminated plate. In turn, the new laminated plate consists of a parallel assembling of single-edged utility blades. We present the experimental characterization of mechanically-induced long-period fiber gratings while employing three laminated plates with a period of 480 ± 20 µm and low duty cycles. These mechanically-induced long-period fiber gratings display a leading rejection band (>15 dB) with a couple of shallow rejection bands (<2 dB) in the range of 1100–1700 nm. This spectral behavior is due to the new mechanical fabrication process that is based on laminated plates that we have proposed, which consists of piling multiple blades with trapezoidal edges that are polished with different levels to obtain different duty-cycles. With the proposed method, we can obtain values of duty-cycles around 10%, much lower than those obtained using traditional methods. Additionally, with this new method, the required mechanical pressure to form the grating is remarkably reduced, which minimizes the probability of the optical fiber failure in the mechanically-induced long-period fiber gratings (MI-LPFGs). Moreover, the proposed mechanically-induced long-period fiber gratings with a single rejection band open the feasibility to implement coarse wavelength division multiplexing systems that are based on long-period fiber gratings.

Wavelength-switchable all-fiber laser-emitting radially polarized beams

We present two kinds of mode-selective-coupler-based ultrafast radially polarized lasers delivering switchable wavelengths and pulsewidths. One is a linear-cavity fiber laser mode locked in the 1.5 µm region, which produces not only wavelength-agile radially polarized pulses in the spatial domain, but also switchable femtosecond and picosecond pulses in the temporal domain. The other one is a nonlinear-polarization-rotation-technique-assisted passively mode-locked fiber ring laser in the 1.0 µm region, presenting an ideal broadband spectral switching with picosecond radially polarized pulses output. The presented fiber lasers offer a type of compact laser source, enabling a flexible option for radially polarized beams in spectral and temporal domains.

Tailoring the magnetic field induced by the first higher order mode of an optical fiber

In this paper, according to the inverse Faraday effect (IFE), the amplitude, phase, polarization and field distribution of the first higher order mode of an optical fiber are tailored carefully, and a magnetic field with arbitrary orientation is generated in the focal region. Compared with traditional strategies to generate a magnetic field with arbitrary orientation, where the configurations are complicated and the components employed for the system are costly, the first higher order mode of a fiber, which has two lobes with opposite instantaneous electric fields, draws more attention for generating a magnetic field with arbitrary orientation. We believe that such an arbitrary orientation state of magnetic field can be applied in the field of confocal and magnetic resonance microscopy and spin dynamics, especially for the use of optical magnetic recording, where laser pulses are used to trigger the magnetization switching.

Ultra-broadband fiber mode converter based on apodized phase-shifted long-period gratings

We have demonstrated an ultra-broadband fiber mode converter based on CO₂-laser inscribed length apodized phase-shifted long-period gratings (LPGs) with a three-section linear length apodization profile, where a π-phase shift was introduced between two adjacent grating sections. The grating parameters were optimized theoretically to achieve the broadband mode conversion between the LP₀₁ mode and LP₁₁ mode. The demonstrated device provides mode conversion efficiency higher than 90% over an ultra- broad bandwidth of ∼182nm. The insertion loss of the LPGs is negligible. Orbital angular momentum modes with left- and right-handed circular polarization can be generated from the demonstrated ultra-broadband mode converter successfully.

Realization of linear-mapping between polarization Poincaré sphere and orbital Poincaré sphere based on stress birefringence in the few-mode fiber

Based on the spatial profiles and polarization states evolution process of the first-order modes resulted from stress-induced birefringence in the few-mode fiber (FMF), we analyze the mapping relationship between the input polarization states represented on polarization PS and the output spatial profiles represented on the orbital PS of the FMF with respect to the magnitude and orientation of birefringence. When the input mode lobe orientation and the phase differences between the four eigenmodes of FMF induced by the stress birefringence satisfy a given condition, the mapping relationship between the input polarization PS and the output orbital PS is linear. Thus, the arbitrary points on the orbit PS can be generated at the output of stressed FMF by controlling the polarization state of the input modes. Then we experimentally verify that, an electrical single-mode polarization controller, a mode converter for converting fundamental mode to higher-order mode, a polarization controller mounting a coil of two-mode fiber and a polarizer can be employed to generate arbitrary first-order spatial modes on the orbital PS by controlling the input single-mode polarization states. The positions on the orbital PS of the generated first-order modes, which are obtained by calculating the three normalized Stokes parameters of output modes, agree well with the simulation ones. The correlation coefficients between the theoretical mode profiles and the experimental ones are higher than 80%. Since the spatial profile evolutions depend on the variations of the input polarization states, a potential advantage of this method is high-speed switching among desired first-order modes by using the commercial devices switching the state of polarization.

All-fiber second-order orbital angular momentum generator based on a single-helix helical fiber grating

An all-fiber orbital angular momentum (OAM) generator enabling direct turning of the fundamental mode (${{\rm HE}_{11}}$HE₁₁) to the second OAM modes (${ l} = {\pm 2}$l=±2) with an efficiency of $\sim90\% $∼90% has been proposed and experimentally demonstrated, which is realized based on utilization of a second-order helical fiber grating written in a few-mode fiber. This is the first time, to the best of our knowledge, that an all-fiber second-order OAM has been achieved with using only one component, i.e., the helical long-period fiber grating. The proposed method opens a new way to efficiently generate an all-fiber higher-order OAM using a conventional multimode fiber.

Recent Progress in Fabrications and Applications of Heating-Induced Long Period Fiber Gratings

This paper presents a review of our work concerning the recent progress in fabrications and applications of heating-induced long period fiber gratings (LPFGs). Firstly, three kinds of heating fabrication techniques based on CO₂ laser, hydrogen–oxygen flame and arc discharge are demonstrated to fabricate LPFGs, i.e., standard LPFGs (SLPFGs) and helical LPFGs (HLPFGs), in different types of optical fibers such as conventional fibers, photonic crystal fibers, and photonic bandgap fibers. Secondly, the all-fiber orbital angular momentum (OAM) mode converters based on heating-induced SLPFGs and HLPFGs in different types of fibers are studied to increase the transmission capacity. Finally, the heating-induced SLPFGs and HLPFGs are investigated to develop various LPFG-based strain, pressure, torsion and biochemical sensors.

Green/red pulsed vortex-beam oscillations in all-fiber lasers with visible-resonance gold nanorods

Optical vortex beams are of tremendous interest in diverse applications for optical tweezers, high-resolution imaging, quantum information and optical communications. So far, these vortex laser sources largely rely on extra-cavity mode conversion by bulk optical elements (e.g. spatial light modulators, phase plates, etc.), resulting in a relatively poor purity, low conversion efficiency, non-compact structure and expensive package. Vortex beams generated directly from cavity-mode lasers is naturally an ideal solution, but almost all of them are not extended into the important visible spectral region. Here, we address the challenge through demonstrating, for the first time, visible-wavelength all-fiber pulsed vortex lasers. By using the fiber offset splicing technique and all-fiber visible resonators, 543.6 nm (green) and 634.7 nm (red) vortex beams are generated directly from Er3+: ZBLAN and Pr3+/Yb3+: ZBLAN fiber lasers with topological charges of ±1 and ±2, respectively. In particular, by exploiting an excellent visible-wavelength saturable absorber, visible-resonance-controlled gold nanorods, we further realize stable short-pulse operation of the 543.6 nm/634.7 nm vortex beams in the miniaturized visible fiber lasers. The green/red vortex laser pulses are ∼500 ns in duration, have a 40-400 kHz tunable repetition rate, and a >45 dB RF signal-to-noise ratio. This work may pave a path towards compact visible-wavelength pulsed vortex lasers for specific applications in STED microscopy and visible-light communications.

A Review of Tunable Orbital Angular Momentum Modes in Fiber: Principle and Generation

Orbital angular momentum (OAM) beams, a new fundamental degree of freedom, have excited a great diversity of interest due to a variety of emerging applications. The scalability of OAM has always been a topic of discussion because it plays an important role in many applications, such as expanding to large capacity and adjusting the trapped particle rotation speed. Thus, the generation of arbitrary tunable OAM mode has been paid increasing attention. In this paper, the basic concepts of classical OAM modes are introduced firstly. Then, the tunable OAM modes are categorized into three types according to the orbital angular momentums and polarization states of mode carrying. In order to understand the OAM evolution of a mode intuitively, three kinds of Poincaré spheres (PSs) are introduced to represent the three kinds of tunable OAM modes. Numerous methods generating tunable OAM modes can be roughly divided into two types: spatial and fiber-based generation methods. The principles of fiber-based generation methods are interpreted by introducing two mode bases (linearly-polarized modes and vector modes) of the fiber. Finally, the strengths and weaknesses of each generation method are pointed out and the key challenges for tunable OAM modes are discussed.

Mode-Selective Photonic Lanterns for Orbital Angular Momentum Mode Division Multiplexing

We analyze the mode evolution in mode-selective photonic lanterns with respect to taper lengths, affected by possible mode phase differences varying along the taper. As a result, we design a three-mode orbital angular momentum (OAM) mode-selective photonic lantern by optimizing the taper length with mode crosstalk below −24 dB, which employs only one single mode fiber port to selectively generate one OAM mode.

Ultra-compact broadband polarization diversity orbital angular momentum generator with 3.6 × 3.6 μm2 footprint

Orbital angular momentum (OAM), one fundamental property of light, has been of great interest over the past decades. An ideal OAM generator, fully compatible with existing physical dimensions (wavelength and polarization) of light, would offer the distinct features of broadband, polarization diversity, and ultra-compact footprint. Here, we propose, design, fabricate, and demonstrate an ultra-compact chip-scale broadband polarization diversity OAM generator on a silicon platform with a 3.6 × 3.6 μm2 footprint. The silicon OAM chip is formed by introducing a subwavelength surface structure (superposed holographic fork gratings) on top of a silicon waveguide, coupling the in-plane waveguide mode to the out-plane free-space OAM mode. We demonstrate in theory and experiment the broadband generation of polarization diversity OAM modes (x-/y-polarized OAM+1/OAM-1) from 1500 to 1630 nm with high purity and efficiency. The demonstrations of an ultra-compact broadband polarization diversity OAM generator may open up new perspectives for OAM-assisted N-dimensional optical multiplexing communications/interconnects and high-dimensional quantum communication systems.

Generation of LP11/LP21 modes with tunable mode lobe orientation controlled by polarization states

We propose and experimentally demonstrate a novel scheme to generate LP₁₁/LP₂₁ modes with tunable mode lobe orientation (MLO). Wherein, the MLOs have an excellent linear relationship with the linearly-polarized states of input fundamental modes. The proposed scheme is composed of a polarization controller (PC), a mode converter, a mode and polarization controller (PMC) which is twined with the few mode fiber (FMF) and a polarizer. Experimental results show that the deviations of MLOs between generated LP₁₁/LP₂₁ modes and simulated ones are less than 3.5 and 8 degrees over C band. Since polarization control up to nanosecond scale is available with GaAs or lithium based electro-optic modulator, the proposed scheme could enable nanosecond time scale MLO control, which would be immensely useful for optical trapping, fiber sensors and optical communications.

Generation of Orbital Angular Momentum Modes Using Fiber Systems

Orbital angular momentum (OAM) beams, characterized by the helical phase wavefront, have received significant interest in various areas of study. There are many methods to generate OAM beams, which can be roughly divided into two types: spatial methods and fiber methods. As a natural shaper of OAM beams, the fibers exhibit unique merits, namely, miniaturization and a low insertion loss. In this paper, we review the recent advances in fiber OAM mode generation systems, in both the interior and exterior of the beams. We introduce the basic concepts of fiber modes and the generation and detection theories of OAM modes. In addition, fiber systems based on different nuclear devices are introduced, including the long-period fiber grating, the mode-selective coupler, microstructural optical fiber, and the photonic lantern. Finally, the key challenges and prospects for fiber OAM mode systems are discussed.

Orbital angular momentum generation in two-mode fiber, based on the modal interference principle

In this Letter, we demonstrate, to the best of our knowledge, a novel method to generate an orbital angular momentum (OAM) based on the principle of the modal interference in a two-mode fiber. At the interference dips, the left- or right-handed circular polarized HE₁₁ modes can be ideally converted into the ±1-order OAM beam. To verify this concept, we employed the femtosecond laser micro-processing technology to write micro-waveguides in the two-mode fiber and hence realized the in-line modal interferometer. To enhance the mode conversion efficiency at the dips, we optimized the waveguide parameters both theoretically and experimentally. The interference spectrum and spiral/fork patterns confirm the OAM beam generation with an efficiency as high as 99%.

Twist-direction-dependent orbital angular momentum generator based on inflation-assisted helical photonic crystal fiber

A novel twist-direction-dependent high-order orbital angular momentum (OAM) generator based on an inflated helical photonic crystal fiber (IHPCF) was demonstrated by use of an inflation-assisted hydrogen-oxygen flame heating technique. Compared with the helical photonic crystal fiber, the IHPCF exhibits a perfect transmission dip without distinct splits, thus generating high-quality OAM_±6 modes. The helical phase of the generated OAM modes is dependent on the twist direction of the IHPCF and independent of the polarization state of the input light. In addition, the polarization state of the generated OAM modes is the same as that of the input light.

Dynamic mode-switchable optical vortex beams using acousto-optic mode converter

We propose a dynamic scheme to realize mode-switchable generation of LP_11a/b modes and ±1-order orbital angular momentum (OAM) modes simultaneously, which are induced by an acoustically induced fiber grating driven by radio frequency modulation. LP_11a/b mode degeneration in a few-mode fiber is induced by the geometric irregularity of optical fibers. A dual-wavelength resonance of mode coupling from LP₀₁ to LP_11a/b modes is found based on the combined effects of optical and acoustic birefringence. Within the configuration of continuous-wave intra-cavity laser output, we experimentally demonstrate a fast-switchable generation of LP_11a/b modes and optical vortex beams with ±1-order OAM at a switching speed up to 4.3 kHz. This approach has potential applications in mode-division multiplexing, particle manipulation, stimulated emission depletion microscopy, and quantum information science.

All-fiber orbital angular momentum mode multiplexer based on a mode-selective photonic lantern and a mode polarization controller

We demonstrate for the first time, to the best of our knowledge, an all-fiber orbital angular momentum (OAM) multiplexer that multiplexes both OAM modes of -l and +l up to the second order by using a mode-selective photonic lantern and a mode polarization controller. The experimentally obtained mode profiles are close to the theoretical results, and the mode purities are higher than 89% for all the OAM modes at 1550 nm. The losses for all mode generations are less than 3.8 dB in the C-band.

Interconnecting data based on vortex beams by adjusting the ellipticity of a ring-core fiber

In order to exchange data in a space-division multiplexing (SDM) system, a novel vortex-beam-based data interconnection concept, which is achieved by adjusting the ellipticity of a ring-core fiber, is proposed. A new ring-core fiber is also designed and fabricated for exchanging and propagating the data carried by first- or second-order vortex (orbital angular momentum) beams. The proposed scheme is not only analyzed and simulated in principle, but is also verified through experiments. The numerical results demonstrate that the vortex beams can be exchanged by appropriately adjusting the phase difference (with respect to the ellipticity of a ring-core fiber) between the even and odd vector modes. A new experimental platform is designed and established for the sake of investigating the feasibility of the proposed scheme. The experimental results are consistent with the results of the simulation, and demonstrate that the data carried by the first- or second-order vortex beams can be successfully switched with acceptable bit error rates (BERs) between the first-order vortex beams (L=1 or -1) or between the second-order vortex beams (L=2 or -2, left or right circular polarization), respectively. The measured BERs and constellation diagrams of 16-QAM are employed to evaluate the data exchange performance with respect to different cases (i.e., data exchange once or twice, and data exchange with or without crosstalk). The measured BERs and constellation diagrams also demonstrate that the performance degrades with increase in topological charge or crosstalk. The proposed scheme is flexible, simple, and reliable for data exchange in a SDM system.