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

Full C- and L-band covered second-order OAM mode generator based on a thinned helical long-period fiber grating

A full C- and L-band covered second-order orbital-angular-momentum (OAM) mode generator has been proposed and experimentally demonstrated, which is realized by using a helical long-period fiber grating (HLPG) but inscribed in a thinned four-mode fiber. By optimizing the design of grating period and fiber diameter of the proposed HLPG, an ultra-broadband rejection filter with a depth of ∼23 dB, a bandwidth of ∼156 nm @-10 dB (ranging from 1522 nm to 1678 nm) and a bandwidth of ∼58 nm @-20 dB (ranging from 1574 nm to 1632 nm), has been successfully obtained as a typical sample. To the best of our knowledge, this is the first demonstration of such ultra-broadband second-order OAM mode generator by using only one fiber component, i.e., the thinned HLPG. In addition, the proposed generator is less polarization-dependent and less temperature-sensitive than those of the conventional HLPGs, which is believed to be considerably helpful to find potential applications of the device itself in wavelength division multiplexing (WDM) and OAM mode division multiplexing (MDM) optical fiber communication systems.

Tunable Bessel beam generator based on a 3D-printed helical axicon on a fiber tip

We demonstrate a tunable and fully enclosed fiber-based Bessel beam generator that has the potential for applications in a tough environment. This generator consists of a few-mode fiber (FMF), a short section of graded index fiber (GIF), and a 3D-printed helical axicon. The FMF provides tunable modes that carry an orbital angular momentum (OAM). The GIF was fused to the FMF to expand and collimate the generated modes. The helical axicon was 3D-printed on the GIF tip without any holes or gaps, which reshapes the OAM modes into Bessel modes and adds an additional helical phase structure to them, resulting in the generation of zeroth-order, first-order, and second-order Bessel beams. The fully enclosed structure provides high mechanical strength and optical stability, which enable the generator to be suitable for imaging or particle manipulation in a complex liquid or air environment.

On-demand flat-top wideband OAM mode converter based on a cladding-etched helical fiber grating

A new method enabling to provide an on-demand flat-top wideband orbital angular momentum (OAM) mode converter is proposed and experimentally demonstrated, which is based on utilization of a cladding-etched helical long-period fiber grating (CEHLPG). By appropriately selecting the grating period and precisely controlling the diameter of the CEHLPG in-situ, both the radial order and central wavelength of the flat-top band for the generated OAM mode can be flexibly tailored according to specific requirements. As typical examples, the first azimuthal order OAM modes with a flat-top bandwidth of 95 nm at -20 dB, a central operating wavelength of ∼1500 nm, and the radial-orders of 9, 8, 5, and 2, respectively, have been demonstrated consecutively. The proposed method provides an excellent flexibility and robustness in controlling both the radial order and the central wavelength of the resulting flat-top wideband OAM mode conversion, which may support a variety of practical optical vortex applications.

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.

Probe-type all-fiber tiny-displacement sensor based on orbital-angular-momentum interferometry

In this study, a probe-type all-fiber tiny-displacement sensor is proposed and experimentally demonstrated, which is realized by using an all-fiber orbital-angular-momentum (OAM) interferometer, where a probe is especially adopted and inserted into the testing arm of the OAM interferometer. The proposed device takes full advantages of the OAM interferometer and the probe-type fiber sensor, making it completely available to the tiny-displacement measurement. As a result, changes in displacement (ranging from 0 nm to 750 nm) with a real resolution of ∼8.81 nm have been successfully measured. To our knowledge, this is the first demonstration of an all-fiber probe-type OAM interferometer, which may find potential application to high-precision tiny displacement in a small confined space.

Recognition of the orbital-angular-momentum spectrum for hybrid modes existing in a few-mode fiber via a deep learning method

In this study, we theoretically and experimentally demonstrate that the convolutional neural network (CNN) in combination with the residual blocks and the regression methods can be used to precisely and quickly reconstruct the OAM spectrum of a hybrid OAM mode no matter how the consistent OAM modes have the same or different order indices in both the azimuthal and the radial direction. For cases of the simulation testing, the mean errors of all recognized parameters for hybrid OAM modes in a four-mode fiber (4MF) and a six-mode fiber (6MF) are smaller than 0.003 and 0.008, respectively. To the best of our knowledge, this is the first time that all the OAM modes, probably existing in the core of 4MFs or 6MFs, can be precisely and quickly recognized from intensity distribution of the hybrid OAM mode itself via the deep learning method.

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

Wave-band-tunable optical fiber broadband orbital angular momentum mode converter based on dispersion turning point tuning technique

A wave-band-tunable optical fiber broadband orbital angular momentum (OAM) mode converter based on a helical long-period fiber grating (HLPFG) and dispersion turning point (DTP) tuning technique is demonstrated both theoretically and experimentally. The DTP tuning is achieved by thinning the optical fiber during the HLPFG inscription. As a proof of concept, the DTP wavelength of the LP_1,5 mode is successfully tuned from the original ∼2.4 µm to ∼2.0 µm and ∼1.7 µm. With the help of the HLPFG, broadband OAM mode conversion (LP_0,1→LP_1,5) is demonstrated near the 2.0 µm and 1.7 µm wave bands. This work addresses a longstanding problem that the broadband mode conversion is limited by the intrinsic DTP wavelength of the modes and provides a new, to the best of our knowledge, alternative for broadband OAM mode conversion at the desired wave bands.

High-order orbital angular momentum mode conversion based on a chiral long period fiber grating inscribed in a ring core fiber

A chiral long period fiber grating (CLPFG) was designed according to the phase-matching condition and conservation law of angular momentum, and was inscribed in a ring core fiber (RCF). This CLPFG was used to directly excite the ±2^nd- and ±3^rd-order orbital angular momentum (OAM) modes. The coupling efficiency of the OAM mode is up to 98.7% and the insertion loss is within 0.5 dB. The uniformity of the annular mode intensity distribution, polarization characteristics, and the mode purity of coupled OAM modes were investigated in detail. Results show that the coupled high-order OAM modes possess a relative uniform annular intensity distribution, its mode purity is up to 93.2%, and the helical phase modulation is independent on the polarization state of incident light. These results indicate that the RCF-based CLPFG is an ideal OAM mode converter for future high-capacity optical fiber communication systems.

Multichannel ±2 order orbital angular momentum mode converter based on an elliptical-core helical intermediate-period fiber grating

We propose and demonstrate a multichannel ±2 order orbital angular momentum (OAM) mode converter based on an elliptical-core helical intermediate-period fiber grating (E-HIPFG). By decreasing the grating pitch to ∼17.5 µm, ten wavelength channels are observed in the transmission spectrum of the E-HIPFG. Within the wavelength range of 1240-1650 nm, the ±2 order OAM modes are identified at each wavelength channel. The proposed E-HIPFG is ∼2.6 mm in length, which is more than one order of magnitude shorter than the conventional device, and thus may be more resistant to external disturbances, such as bending. Furthermore, the device exhibits an ultralow temperature drift of ∼5.84 pm/°C. Therefore, the proposed E-HIPFG can be a good candidate for a multichannel higher-order OAM mode converter.

Direct generation of orbital angular momentum in orthogonal fiber Bragg grating

We experimentally demonstrated an all-fiber reflective orbital angular momentum (OAM) generator based on orthogonal fiber Bragg grating (OFBG). The OFBG is formed by using a femtosecond laser to prepare two fiber Bragg gratings with a certain spacing in orthogonal planes. The ±1st- and ±2nd-order OAM modes were directly excited in this OFBG, and the chirality of the OAM modes depends on the relative positions of the two FBGs. The mode coupling properties and effects of center-to-center distance on OAM modes were investigated as well.

Synthesizing the complex orbital-angular-momentum spectrum of hybrid modes existed in a few-mode fiber

In this study, a simple and reliable method enabling to well synthesize the complex orbit-angular-momentum (OAM) spectrum of hybrid mode in a few-mode fiber is proposed and numerically demonstrated, which is realized by using the so-called inverse scattering method based on the genetic algorithm (GA), where the main Fourier components of a specially-selected ring in intensity distribution of the hybrid mode is used as the optimization objective. As a proof-of-concept example, power spectrum of a hybrid mode consisted of the first- and second-order OAM modes was successfully reconstructed with an accuracy higher than 0.99. This is the first time, to the best of our knowledge, that the complex OAM spectrum of a fiber hybrid mode consisted of more than two kinds of OAM modes is synthesized directly from the intensity distribution of the hybrid mode itself.

Torsion-tunable OAM mode generator based on oxyhydrogen-flame fabricated helical long-period fiber grating

We demonstrate a class of all-fiber torsion-tunable orbital angular momentum (OAM) mode generators based on oxyhydrogen-flame fabricated helical long-period fiber gratings (HLPFGs). The 1-order and 3-order OAM modes are excited based on the HLPFGs inscribed in the single-mode fiber (SMF) and six-mode fiber (6MF), respectively. Theoretical analysis reveals that the twisting can result a resonant wavelength shift of the HLPFG, which means that the OAM modes can also be excited at various wavelength by simply applying a twist rate on the HLPFG. Experiments are carried out to characterize the torsional tunability of the OAM modes, and the results show that the 1-order and 3-order OAM modes can be excited at various wavelength of ∼1564 - 1585 nm and ∼1552 - 1574 nm, respectively, when the torsion angle varied from -360° to 360°, which is consistent with the theoretical analysis. Therefore, the HLPFG can be a candidate for all-fiber wavelength tunable OAM mode generator.

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.

Dynamic mode-switchable and wavelength-tunable Brillouin random fiber laser by a high-order mode pump

We experimentally investigate two schemes of Brillouin random fiber laser (RFL) by using high-order-mode (HOM) pump in a few-mode fiber (FMF). The core-mode conversion between LP₀₁ and LP₁₁ modes is obtained in the FMF by cascading long period fiber gratings (LPFG) working at the same wavelength region. Different transversal modes of stimulated Brillouin scattering (SBS) can be implemented based on broadband long period fiber gratings (LPFG) and acoustically induced fiber gratings (AIFG). The RFL base on two broadband LPFGs can obtain high purity LP₁₁ mode operating in the range of 1543 nm to 1565 nm. Moreover, the output mode can be dynamically switched between LP₀₁ mode, LP_11a mode and LP_11b mode by modulating frequency shift keying (FSK) signal of the AIFG. This work has potential application prospects in the fields of mode division multiplexing systems, speckle-free imaging, free-space optical communication, laser material processing.

Broadband flat-top second-order OAM mode converter based on a phase-modulated helical long-period fiber grating

In this study, a broadband flat-top second-order orbital angular momentum mode (OAM) converter is proposed and demonstrated using a phase-modulated second-order helical long-period fiber grating (HLPG). The proposed HLPG is designed to be inscribed in a thinned four-mode fiber and operated at wavelengths near the dispersion turning point (DTP). In contrast to most of the HLPG-based OAM mode generators reported to date, where the high-order OAM mode and flat-top broadband have rarely been achieved simultaneously, a second-order OAM(OAM-2) mode converter with a flat-top bandwidth of 113 nm @ -20 dB (ranging from 1530-1643 nm) and a depth fluctuation of less than 3 dB @-26 dB has been successfully demonstrated in this study, such flat-top bandwidth covers the entire C + L bands and represents the best result of the HLPGs reported to date. Tolerance analyses for the fabrication of the designed HLPG were also performed. It is believed that the proposed HLPG may find applications in all-fiber vortex lasers as well as the OAM mode division multiplex (MDM) system.

Broadband tunable orbital angular momentum mode converter based on a conventional single-mode all-fiber configuration

A broadband tunable orbital angular momentum (OAM) mode converter based on a helical long-period fiber grating (HLPFG) inscribed in a conventional single-mode fiber (SMF) is experimentally demonstrated. The proposed all-fiber OAM mode converter is based on the core-cladding mode dual resonance near the dispersion turning point (DTP). The converter can operate with a bandwidth of 303.9 nm @ -3 dB and 182.2 nm @ -10 dB, which is, as far as we know, the widest bandwidth for a conventional SMF. Furthermore, the bandwidth of the OAM mode can be dynamically tuned within a large dynamic range (>80 nm) by simply twisting the fiber clockwise (CW) or counterclockwise (CCW). The dynamic tunability of the bandwidth of the proposed OAM mode generator may find vital applications in large-capacity optical fiber communication systems.

Advances on Mode-Coupling Theories, Fabrication Techniques, and Applications of the Helical Long-Period Fiber Gratings: A Review

In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic helicity characteristics, which are especially suitable to control the loss, polarization, and orbit-angular-momentum (OAM) states of the light in optical fiber, HLPGs have recently attracted great research interest and have found various applications, such as the mode-converters, the torsion sensors, the band-rejection filters, wave plates, linear- and circular-light polarizers, and OAM mode generators, etc. It is believed that HLPGs and the HLPGs-based devices would find further applications to not only the fields of optical sensors and optical communication, but also other fields such as ultrahigh precision measurement, quantum optics, and biochemistry, etc.

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.

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.

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.

Eight-mode ring-core few-mode fiber using cross-arranged different-material-filling side holes

We propose an eight-spatial-mode ring-core few-mode fiber (RC-FMF), utilizing the cross-arranged different-material-filling side holes (CA DMFSH) for effective index difference improvement. Two GeO₂-doped-silica side holes and two air-filling side holes are arranged orthogonally around the ring core, which have directionally different effects on the refractive index and the mode field distribution in the RC-FMF. The results indicate that the effective index difference (Δn_eff) between adjacent spatial modes is larger than 1.96×10^-4, and the Δn_eff between adjacent non-degenerated modes can be above 1.01×10^-3 at the same time. Bend-resistant performance and low nonlinearity are achieved in the designed RC-FMF. Broadband performances ranging from 1510 to 1630 nm are also analyzed. The CA-DMFSH-assisted structure shows great potential for enlarging the effective index difference, and the proposed fiber targets applications in the short-reach space-division multiplexing optical networking while eliminating the complex multi-input/multi-output digital signal processing.

All-fiber bandwidth tunable ultra-broadband mode converters based on long-period fiber gratings and helical long-period gratings

We demonstrated the fabrication of bandwidth tunable ultra-broadband mode converters based on CO₂-laser inscribed long-period fiber gratings (LPFGs) and helical long-period gratings (HLPGs) in a two-mode fiber (TMF). The simulation and experimental results show that there is a dual-resonance coupling from LP₀₁ to LP₁₁ core mode at the dispersion turning point. The mode converters based on the TMF-LPFG and TMF-HLPG provide a 10-dB bandwidth of ∼300 nm and ∼297 nm, respectively, which covers O + E+S + C band. The 1^st order orbital angular momentum (OAM) mode based on TMF-LPFG was generated by adjusting the polarization controllers (PCs), while the 1^st order OAM mode can be generated directly by the TMF-HLPG. When the twist rate is varied from -36 rad/m ∼ 36 rad/m, the tunable range of the 10-dB bandwidth is ∼52 nm and ∼91 nm for the LPFG and HLPG mode converters, respectively. The ultra-broadband mode converter can be adopted as a bandwidth tunable mode converter, which can be applied in ultra-broadband mode-division-multiplexing transmission systems and optical fiber sensing systems based on few-mode fibers.

Demonstration of the mode-selection rules obeyed in a single-helix helical long-period fiber grating

A simple method enabling the validation of the mode-selection rules obeyed in a single-helix helical long-period fiber grating (SHLPG) has been demonstrated both theoretically and experimentally, which is realized by investigating and analyzing the circular dichroism (CD) and the polarization dependence loss (PDL) spectra of the utilized SHLPG. It is confirmed for the first time, to the best of our knowledge, that in terms of the SHLPG's helicity, the core mode ${{\rm HE}_{\textbf{11}}}$HE₁₁ with either the left circular polarization or the right circular polarization will be selectively coupled into the higher HE or the TE/TM mode, respectively. As a result, the SHLPG would exhibit an enhanced CD, as well as an enhanced PDL near the resonant wavelengths of the HE and TE/TM modes, respectively. The results presented in this Letter will be helpful for efficiently designing and producing SHLPG-based circular polarizers and orbital-angular-momentum generators.

Bend-induced long period grating in a helical core fiber

We report on a new type of long-period-grating generated in a helical core fiber by bending. The grating arises from bend-induced modulation of an equivalent refractive index in the helical core with a period equal to the helix pitch. We experimentally demonstrate that such grating induces multiple resonant couplings between the fundamental modes guided in the central core and the helical side-core. We have also shown that by varying a direction of bending, one can generate the phase shifted grating. The experimental results are supported by numerical simulations based on the coupled mode equations.