Mode converter based on the long-period fiber gratings written in the two-mode fiber

Rocking filter in a highly birefringent fiber for resonant coupling between different LP11 modes and generation of cylindrical vector beams

We present a rocking filter in a highly birefringent two-mode fiber that enables resonant coupling between different modes in the LP11 group. Our simulations and experimental results prove that such a filter allows for resonant coupling between orthogonally polarized LP11 modes of the same spatial structure, as well as between modes of the same polarizations and orthogonal spatial distributions. Furthermore, we demonstrate that such rocking filters can be used to generate pure TE01, TM01 and HE21 beams or their coherent superposition.

Design of a bidirectional TM01(TE01)-LP01 mode converter with a metasurface-on-fiber

Mode conversion is crucial for coupling a light source to a desired waveguide. While traditional mode converters such as fiber Bragg gratings and long-period fiber gratings exhibit high transmission and conversion efficiency, the mode conversion of two orthogonal polarizations remains challenging. Here, we present a bidirectional metasurface mode converter that can convert the transverse electric (TE)01 or transverse magnetic (TM)01 mode to the fundamental mode (LP01) with orthogonal polarization, and vice versa. The mode converter is located on a facet of a few-mode fiber and connected to a single mode fiber. Through simulations, we find that 99.9% of the TM01 or TE01 mode is converted into the x- or y-polarized LP01 mode, and that 99.96% of the x- or y-polarized LP01 mode is converted to the TM01 or TE01 mode. Furthermore, we expect a high transmission of over 84.5% for all mode conversions, up to 88.7% for TE01 to y-polarized LP01 conversion.

Sub-kHz high-order mode Brillouin random fiber laser based on long-period fiber grating and distributed Rayleigh scattering in a half-open linear cavity

We demonstrate a narrow-linewidth high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on a long-period fiber grating (LPFG) and distributed Rayleigh random feedback in a half-open linear cavity. The single-mode operation of the laser radiation with sub-kilohertz linewidth is achieved thanks to distributed Brillouin amplification and Rayleigh scattering along kilometer-long single mode fibers whilst a few mode fiber-based LPFGs enable the transverse mode conversion among a broadband wavelength range. Meanwhile, a dynamic fiber grating (DFG) is embedded and incorporated to manipulate and purify the random modes, which hence suppresses the frequency drift resulting from random mode hopping. Consequently, the random laser emission with either high-order scalar or vector modes can be generated with a high laser efficiency of 25.5% and an ultra-narrow 3-dB linewidth of 230 Hz. Furthermore, the dependence of the laser efficiency and frequency stability on the gain fiber length are also experimentally investigated. It is believed that our approach could provide a promising platform for a wide range of applications such as coherent optical communication, high-resolution imaging, highly sensitive sensing, etc.

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.

A Taper-in-Taper Structured Interferometric Optical Fiber Sensor for Cu2+ ion Detection

Copper ion is closely associated with the ecosystem and human health, and even a little excessive dose in drinking water may result in a range of health problems. However, it remains challenging to produce a highly sensitive, reliable, cost-effective and electromagnetic-interference interference-immune device to detect Cu2+ ion in drinking water. In this paper, a taper-in-taper fiber sensor was fabricated with high sensitivity by mode-mode interference and deposited polyelectrolyte layers for Cu2+ detection. We propose a new structure which forms a secondary taper in the middle of the single-mode fiber through two-arc discharge. Experimental results show that the newly developed fiber sensor possesses a sensitivity of 2741 nm/RIU in refractive index (RI), exhibits 3.7 times sensitivity enhancement when compared with traditional tapered fiber sensors. To apply this sensor in copper ions detection, the results present that when the concentration of Cu2+ is 0-0.1 mM, the sensitivity could reach 78.03 nm/mM. The taper-in-taper fiber sensor exhibits high sensitivity with good stability and mechanical strength which has great potential to be applied in the detection of low Cu2+ ions in some specific environments such as drinking water.

Low-noise-figure and high-purity 10 vortex modes amplifier based on configurable pump modes

We have explored an orbital angular momentum (OAM) amplifier of 10 vortex modes under different-order OAM pump modes, i.e. OAM₀, OAM₁, and OAM₂. The all-fiber amplification system consists of an active few-mode erbium-doped fiber (FM-EDF), a mode selective pump (MSP), and a mode selective signal (MSS). These mode selective components are based on fused-taper mode selective couplers (MSC) under different wavelengths fabricated by a passive ring-core fiber (RCF). Under different-order mode pumps, the OAM amplifier experimentally exhibits mode gains (MGs) above 15 dB for 10 vortex modes with the mode purities only 89%, essentially in line with the simulation results. Especially when the signal-mode profiles are better matched to the pump-mode profiles, i.e. the OAM pumps with the same order as signals, the obtained MGs are all over 20.2 dB and the amplified OAM mode purity is up to 97%; the acquired noise figures (NFs) are <4.9 dB and even the minimum NF is 3.2 dB. The results reveal that the OAM amplifier shows low-NF and high-purity characteristics under configurable pump modes in C-band. The amplified high-order OAM mode could be promising for uses in the long-distance mode division multiplexing (MDM) and in mitigation of the upcoming capacity crunch in optical fiber communication.

Orbital Angular Momentum Mode Sensing Technology Based on Intensity Interrogation

A novel optical fiber sensing technology based on intensity distribution change in orbital angular momentum (OAM) mode is proposed and implemented herein. The technology utilizes a chiral long-period fiber grating (CLPFG) to directly excite the 1st-order OAM (OAM₁) mode. The intensity changes in the coherent superposition state between the fundamental mode and the OAM₁ mode at the non-resonant wavelength of the CLPFG is tracked in order to sense the external parameters applied to the grating area. Applying this technology to temperature measurement, the intensity distribution change has a good linear relationship with respect to temperature in the range of 30 °C to 100 °C. When the intensity was denoted by the number of pixels with a gray value of one after binarization of collected images, the sensitivity was 103 px/°C and the corresponding resolution was 0.0097 °C. Meanwhile, theoretical and experimental results show that the sensitivity and resolution can be further improved via changing the area of the collected image. Compared with sensing methods based on spiral interference pattern rotation in previous work, this sensing technology has the advantage of exquisite structure, easy realization, and good stability, thus making it a potential application in practices.

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%.

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.

Wavelength-Tunable, Ultra-Broadband, Biconical, Long-Period Fiber Grating Mode Converter Based on the Dual-Resonance Effect

We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO₂ laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing and tapering, the dispersion turning point of the fiber is adjusted and wavelength-tunable broadband mode conversion is achieved efficiently. Theoretical simulation shows that the mode conversion bandwidth can cover the O + E + S + C band. In the experiment, we fabricated adiabatic tapers with cladding diameters of 113 μm and 121 μm and wrote gratings on these tapers to achieve dual-resonance coupling, thus realizing mode conversion from LP₀₁ to LP₁₁, with a 15 dB bandwidth of 148.8 nm from 1229.0 nm to 1377.8 nm and of 168.5 nm from 1319.7 nm to 1488.2 nm, respectively. As far as we know, this is the first time that fusion taper technology has been used to adjust the window of the dual-resonant coupling of an optical fiber. This work broadens the scope of application of the dual-resonance effect and proposes a general method for widening the bandwidth of a fiber grating with tunable wavelength.

High-power cylindrical vector beams generated from an all-fiber linearly polarized laser by metasurface extracavity conversion

Five-hundred-watt cylindrical vector beams (CVBs) at 1030 nm with the 3 dB linewidth being less than 0.25 nm have been generated from a narrow linewidth all-fiber linearly polarized laser by metasurface extracavity conversion. At maximum output power, the transmission efficiency and polarization extinction ratio of radially polarized cylindrical vector beams (RP-CVBs) are beyond 98% and 95%, respectively. The average power is approximately an order higher than previously reported high-power narrow-linewidth CVBs generated from fiber lasers. The temperature rise of the metasurface is less than 10°C at 500 W output power, which means that the system can be further power-scaled in the near future. The high-power, high-purity, and high-efficiency RP-CVBs generated by the metasurface demonstrate potential application of a metasurface in high-power CVBs lasers.

Mode converter with C+L band coverage based on the femtosecond laser inscribed long period fiber grating

The mode converter (MC) is one of the key components for mode division multiplexing (MDM). Here, we experimentally demonstrate an all-fiber long period grating (LPG) based MC inscribed in the few-mode fiber (FMF) with the line-by-line femtosecond laser irradiation technique. Experimental characterization results agree well with the theoretical calculations, and a clear mode evolution from the LP₀₁ mode to the LP₁₁ mode is observed with the tuning of the operation wavelength. An average mode conversion efficiency of more than 90% and an average insertion loss of less than 5 dB, together with a polarization-dependent loss of less than 3 dB, are achieved over the C+L band with a good repeatability. The proposed MC based on the LPG inscribed into the FMF has the advantages of mode scalability, compact size, and wideband operation, which is desired for the wavelength division multiplexing (WDM) and MDM hybrid transmission.

Cascaded stimulated Brillouin scattering erbium-doped fiber laser generating orbital angular momentum beams at tunable wavelengths

We experimentally demonstrate a method to obtain central wavelength tunable orbital angular momentum beams with switchable topological charges (+1 or -1) in a stimulated Brillouin scattering erbium-doped fiber laser. Multiwavelength operation is achieved through cascaded stimulated Brillouin scattering in a single-mode fiber with a length of 6 km initiated by an external Brillouin pump. High-efficiency mode conversion between the fundamental mode and the orbital angular momentum modes is realized through a broadband two-mode long-period fiber grating. High-purity orbital angular momentum beams with up to 10 stable wavelength channels with a tuning range of 35 nm are achieved, which is the highest number of operating wavelengths and tuning range in an all-fiber laser for orbital angular momentum beam emission to the best of our knowledge. Both the operational central wavelength and number of operating wavelengths can be tuned by adjusting the primary pump power and the center wavelength of the tunable bandpass filter in conjunction with changing the Brillouin pump wavelength.

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.

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.

Gas sensing with mode-phase-difference photothermal spectroscopy assisted by a long period grating in a dual-mode negative-curvature hollow-core optical fiber

We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed CO₂ laser, which enables pump propagation in the fundamental LP₀₁ mode to achieve maximum photothermal phase modulation while exciting both the LP₀₁ and LP₁₁ modes at the probe wavelength to form a dual-mode interferometer for detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise equivalent concentration of ∼2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.

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.

Few-mode fiber-embedded long-period fiber grating for simultaneous measurement of refractive index and temperature

A few-mode fiber (FMF)-embedded long-period fiber grating is proposed as a sensor for simultaneous measurement of refractive index and temperature. Periodically embedding the FMFs induces the local refractive index modulation to achieve a compact sensor size and obtains a low insertion loss. The simulated results show that the two resonance dips have opposite waveguide dispersion coefficients. Therefore, they show different refractive indices and temperature sensitivities in the experiment. At the same time, the spectral characteristics of double-resonance dips provides a condition for simultaneous measurement of two parameters. By monitoring wavelength shift of the two dips, the simultaneous measurement of refractive index and temperature is easily realized.

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 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.

Vortex soliton molecule in a fiber laser

We report the generation of vortex soliton molecules (VSMs) in a passively mode-locked fiber laser based on a mode selective coupler (MSC). ±1-order VSMs with variable numbers of molecules are observed. By adjusting the polarization state of the light in the cavity, we further demonstrate the process in which one VSM splits to multiple. During this process, the number of the solitons inside the VSM also varies and their separation gradually increases while the spectral modulation being unobservable, and vice versa. The obtained results have potential applications in fields of optical communications, especially in information coding.

Torsion bidirectional sensor based on tilted-arc long-period fiber grating

Fiber torsion sensor has been researched for many years due to its various structure and sensitive response. In order to distinguish the torsion direction, fiber sensor still faces some difficult problems, including complex fabricating condition, special fiber structure and limited sensitivity. In this paper, a novel long-period fiber grating (LPFG) formed by tilted-arc grids is designed and fabricated in the normal simple-mode fiber, showing small size and high sensitivity. The asymmetrical tilted-arc grid structure can induce considerable chirality into the tilted-arc LPFG to enable it to distinguish torsion direction, which doesn't need any equipment to rotate or twist the fiber in the fabrication process. Theoretical analysis indicates that the structure can respond opposite wavelength shifting to the opposite torsion directions, and the torsion sensitivity is related to both the radius and tilted angle of grid. A series of tilted-arc LPFGs are fabricated with CO₂-laser scanning and tested in torsion experiment, all of whom can distinguish bidirectional torsion. The maximum sensitivity value can reach 0.514 nm/(rad·m^-1), which is higher than many normal tilted LPFGs and twisted fiber structures. The novel LPFG has the potential to be applied in directional torsion field due to its direction-distinguishing ability, high sensitivity and simple fabrication.

Tunable Mode Converter Device Based on Photonic Crystal Fiber with a Thermo-Responsive Liquid Crystal Core

A compact tunable mode converter device based on the thermo-optically characteristics of liquid crystals (LCs) is proposed and numerically analyzed herein. The proposed mode converter consists of an asymmetric dual-core photonic crystal fiber (PCF) with a highly thermo-responsive LC core. The verification of the proposed mode converter was ensured through an accurate PCF analysis based on the vector finite element method. With an appropriate choice of the design parameters associated with the LC core, phase matching at a single wavelength is available in the important O-band wavelength region. The simulation results showed that high conversion efficiencies between LP01 and LP11 mode are readily achieved over a broad wavelength range from 1278 nm to 1317 nm. Likewise, the tunable capability of the proposed mode converter was evaluated when it was submitted to thermal changes; thus, we evidence the strong thermo-responsive dependence of the operating wavelength, mode conversion efficiency and full-width at the half maximum (FWHM) bandwidth. Finally, the fabrication tolerances of the devices were also investigated. Therefore, the thermo-responsive characteristics of this novel PCF mode converter can be of fundamental importance in the future space division multiplexing technology.

Femtosecond laser micro-machining enabled all-fiber mode selective converter

We demonstrate a compact all-fiber mode selective converter enabled by femtosecond laser micro-machining on the few-mode fiber (FMF) facet. By introducing a micro-structure into the FMF core, we can achieve a π spatial phase difference to the fundamental mode of light to be converted. Theoretical optimization reveals that various high-order modes, including LP₁₁, LP₀₂, and LP₂₁ modes, can be selectively converted by various micro-structures on the FMF facet, with a mode extinction ratio of more than 25 dB and mode coupling efficiency of better than 45% over the C-band. Finally, a proof-of-concept experiment is conducted by inscribing a micro-slot on the two-mode fiber facet for the LP₀₁ to LP₁₁ mode conversion. A micro-slot with a width of 6.7 µm and a depth of 5.4 µm is fabricated under the optimal femtosecond laser parameters. A LP₀₁ to LP₁₁ mode conversion with an average insertion loss of 2.7 dB is realized over the C-band, together with a mode intensity profile correlation efficient of more than 65%. Efficient higher-order mode conversion is feasible with a precise femtosecond laser micro-machining.

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.

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.

Ring-core few-mode fiber for tunable true time delay line operation

We propose, for the first time to our knowledge, tunable true time delay line operation for radiofrequency signals on a few-mode fiber link. In particular, the custom design of a 7-LP-mode ring-core few-mode fiber together with a set of 5 broadband long period gratings inscribed at the proper positions along the fiber allows 4-sample true time delay line tunability over a 20-nm optical wavelength range. We study the performance of the designed true time delay line in the context of reconfigurable microwave photonics signal processing by theoretically evaluating microwave signal filtering and optical beamforming networks for phased array antennas.

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.

Sampled true time delay line operation by inscription of long period gratings in few-mode fibers

We propose and experimentally demonstrate distributed microwave photonics signal processing over a few-mode fiber link by implementing 4-sample true time delay line operation. The inscription of a set of long period gratings at specific locations along the few-mode fiber allows the excitation of the higher-order modes while adjusting the individual sample group delays and amplitudes that are required for sampled true time delay line behavior. Since solely the injection of the fundamental mode at the few-mode fiber input is required, we render this signal processing system independent of any preceding fiber link that may be required in addition to distribute the signal. We experimentally validate the performance of the implemented true time delay line when applied to radiofrequency signal filtering.

1.3 μm fiber grating in a thin-core fiber for LP01-LP11 mode converters and sensing ability

We demonstrate an all-fiber structure that can realize LP₀₁-LP₁₁ mode conversion and twist measurement. It is a thin-core fiber (TCF) grating at a wavelength of 1310 nm cascaded to a short segment of a TCF of a different core size. It is found that the different core size of the TCF between the fiber and the grating has an impact on the excitation of a higher-order mode and mode conversion efficiency. The fiber structure exhibits a good linear response to twisting, strain, and temperature. Depending on the associated mode, the mode intensity and the wavelength for exciting the peaks of the grating have different sensitivities to twisting angle, applied strain, and temperature. These properties can be exploited for simultaneous measurement.

Control of higher-order cladding mode excitation with tailored femtosecond-written long period fiber gratings

We report on the detailed investigation of the core to cladding mode coupling in femtosecond-written long period fiber gratings (LPFG). It is shown that the excitation of higher-order cladding modes with strong selectivity and high precision is possible. The coupling behavior of several gratings, as well as its dependence on the modified core cross-section, is determined theoretically and confirmed experimentally by its spectral response. The presented tool paves the way for a completely new class of tailored LPFGs for different fiber integrated devices.

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%.

Broadband LP01-LP02 mode converter for O-, E-, S-, C-, L-, and U-bands

This paper proposes a novel all-optical fiber mode converter for mode conversion from LP₀₁ to LP₀₂ and vice versa. The mode converter is formed by connecting a single-mode fiber, a taper-core multi-mode fiber, and a few-mode fiber (FMF) together. The taper fiber core is designed to convert LP₀₁ mode to LP₀₂. The few-mode fiber is used to cut off the modes that are higher than LP₀₂. It is shown that the proposed mode converter provides 20 dB extinction ratio and low insertion loss in a very broad optical bandwidth of 200 nm, from 1465 to 1665 nm. By further optimizing the FMF radius only for each of the O-, E-, S-, C-, U-, etc. bands, the mode converter can apply to any band of interest with higher performance. It is found that the performance of the mode converter has a large tolerance to structural parameters. The mode converter has the same performance in reciprocal operation, i.e., LP₀₂ to LP₀₁.

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.

Polarization-independent orbital angular momentum generator based on a chiral fiber grating

A polarization-independent orbital angular momentum (OAM) generator, based on a chiral fiber grating (CFG), is proposed and demonstrated for the first time, to the best of our knowledge. Helical phases were successfully excited in this device for the arbitrary polarization states of the input light, and the resulting OAM modes could be produced with a desired polarization state. The employed CFG was fabricated by twisting a fused few-mode fiber during hydrogen-oxygen flame heating. The polarization characteristics, helical phase, and purity of the coupled modes in the CFG were experimentally investigated for varying input light polarization states. The results showed that the coupled higher-order modes had the same polarization as the input fundamental mode. The chirality of the excited OAM was polarization-independent and determined solely by the helicity of the CFG. The purity of the OAM mode was higher than 93% with a slight fluctuation of 2% which occurred during measurement processing.

All-fiber high-order mode laser using a metal-clad transverse mode filter

We propose and demonstrate an all-fiber laser with LP₁₁ mode output. A transverse mode filter is designed and fabricated to suppress the fundamental mode and enable the fiber laser to oscillate in the second-order (LP₁₁) transverse mode. The mechanism is to introduce relatively low ohmic loss for the TE₀₁ mode and much higher ohmic losses for other modes through the loss of evanescent waves in the metal clad. The fiber laser operates at the center wavelength of 1053.9 nm with a narrow 3 dB linewidth of 0.019 nm. Four states of cylindrical vector mode with high modal purity are obtained through adjusting the intra-cavity polarization controller. This approach has great potentiality and scalability of realizing single high-order mode fiber laser, from which a wide range of applications could benefit.

Few-mode fiber true time delay lines for distributed radiofrequency signal processing

We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3-sample configurations with different time delay properties are implemented over the same 60-meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber position converts part of the LP₀₁ mode into the LP₀₂, permitting sample time delay engineering. Delay line performance is experimentally demonstrated when applied to radiofrequency signal filtering, example of fiber-distributed processing functionality exhibiting one order or magnitude gain in terms of compactness.

Transverse mode switchable all-fiber Brillouin laser

A transverse mode switchable all-fiber Brillouin laser architecture is proposed. This laser architecture consists mainly of an optical switch, a set of cascaded mode selection couplers (MSCs), and a few-mode fiber ring cavity. By switching the output signal of the optical switch, specific transverse modes generated by MSCs can be coupled into the ring cavity to excite the corresponding Brillouin scattering light. Based on the Brillouin nonlinear effect, the desired mode resonant amplification is guaranteed, and a transverse mode switchable laser beam is obtained. As a proof of principle, we have implemented an all-fiber Brillouin laser with switchable output of fundamental transverse (LP₀₁) mode and second-order transverse (LP₁₁) mode at the wavelength of 1550 nm. The slope efficiency and linewidth for LP₀₁ and LP₁₁ modes are 25.1% and 4.9 kHz, 20.9% and 4.96 kHz, respectively. Additionally, the orbital angular momentum laser beam with l=+1, -1, or 0 switchable output is also demonstrated from our all-fiber Brillouin laser system. Owing to the compact all-fiber architecture, this transverse mode switchable Brillouin fiber laser is reliable during long-term operation and thus promising for many practical applications.

All-fiber stable orbital angular momentum beam generation and propagation

An all-fiber scheme for stable orbital angular momentum (OAM) beam generation and propagation is proposed and demonstrated. This scheme is based on a self-designed and manufactured graded-index few mode fiber (GI-FMF) and compatible mode selection coupler (MSC). The MSC, which consists of a conventional single mode fiber (SMF) and the GI-FMF, can effectively couple the fundamental mode in the SMF to the desired OAM mode in the GI-FMF based on phase matching condition. Meanwhile, the GI-FMF breaks the degeneracy between the desired eigenmodes and neighboring vector modes, thereby allowing the preservation and propagation of the selectively excited OAM modes. As a proof-of-principle, we have implemented an all-fiber device operating in stable OAM modes with |l| = 1. The experimental results show stable propagation of OAM beams with the mode purity of ∼95% and bandwidth of 100 nm. This all fiber device could be useful for further development of wide bandwidth OAM mode division multiplexed application.

Orbital angular momentum transition of light using a cylindrical vector beam

We demonstrate that using a single cylindrical vector (CV) beam in two-mode fibers, the orbital angular momentum of light can be switched among -1, 0, and 1. The input CV beam can be a conventional radial and azimuthal polarization distribution or a generalized CV beam, and we first use and verify that a rocking-long period fiber grating generates the tunable generalized CV beam. Because of using a single CV beam as the light source, this approach not only provides an increased stability compared to the conventional superposed eigenmodes method, but also builds a bridge between the polarization singularity beams and the phase singularity beams.

All-fiber radially/azimuthally polarized lasers based on mode coupling of tapered fibers

We demonstrate a mode converter with an insertion loss of 0.36 dB based on mode coupling of tapered single-mode and two-mode fibers, and realize all-fiber flexible cylindrical vector lasers at 1550 nm. Attributing to the continuous distribution of a tangential electric field at taper boundaries, the laser is switchable between the radially and azimuthally polarized states by adjusting the input polarization. In the temporal domain, the operation is controllable among continuous-wave, Q-switched, and mode-locked statuses by changing the saturable absorber or pump strength. The duration of Q-switched radially/azimuthally polarized laser spans from 10.4/10.8 to 6/6.4 μs at the pump range of 38 to 58 mW, while that of the mode-locked pulse varies from 39.2/31.9 to 5.6/5.2 ps by controlling the laser bandwidth. The proposed laser combines the features of a cylindrical vector beam, a fiber laser, and an ultrafast pulse, providing a special and cost-effective source for practical applications.

LP01-LP11a mode converters based on long-period fiber gratings in a two-mode polarization-maintaining photonic crystal fiber

A long-period fiber grating (LPFG) mode converter based on a two-mode polarization-maintaining photonic crystal fiber (PM-PCF) is proposed and demonstrated. The mode converters realize conversions between the LP₀₁ modes and LP_11a modes with parallel polarization directions. Different from typical conventional mode converters, the PM-PCF-LPFG mode converters most notably can separate out two linearly polarized LP_11a modes at different wavelengths. The highest mode-conversion efficiency is more than 99%. In addition, the bandwidth of the mode converter is adjustable by changing the grating number of the LPFG.

High efficiency all-fiber cylindrical vector beam laser using a long-period fiber grating

We demonstrate a novel all-fiber laser generating cylindrical vector beams with high slope efficiency and low threshold by introducing a long-period fiber grating into the laser cavity. Highly efficient mode conversion is realized by an LPFG at ∼1548  nm. Mode selection and spectrum filtering are achieved in combination with a two-mode fiber Bragg grating (TM-FBG). The fiber laser operates at a single wavelength of 1547.95 nm with a 30 dB linewidth of less than 0.18 nm and a side-mode suppression ratio (SMSR) of more than 56 dB. The lasing threshold and slope efficiency of the laser are 24.5 mW and 35.41%, respectively. The output power is 72 mW with an absorbed pump power of 225 mW. The variation of slope efficiency with respect to the reflectivity of the TM-FBG is investigated. Through adjusting the intra-cavity polarization controller, high-purity radially and azimuthally polarized beams are both obtained.

All-fiber second-order optical vortex generation based on strong modulated long-period grating in a four-mode fiber

We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of CO₂-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (LP₀₁) to the high-order LP core mode (LP₂₁) with efficiency of 99.7% and then transform the LP₂₁ mode into a high-order OV mode (±2 order). This is the first time, to the best of our knowledge, that ±2-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.

All-fiber mode converter based on long-period fiber gratings written in few-mode fiber

We investigated an all-fiber mode converter based on long-period fiber gratings (LPFGs) written in the few-mode fiber. Mode conversion between the fundamental core mode and different higher-order core modes (LP₁₁, LP₂₁, and LP₀₂ modes) can be realized via a single LPFG with an efficiency of 99% at the resonance wavelength. Moreover, optimized mode conversion between the LP₀₁ and LP₂₁ modes can be realized by cascading two LPFGs with different grating pitches. The maximum conversion efficiency is estimated to be ∼99.5% at 1553 nm. The orbital angular momentum states with different topological charges (±1,±2) are demonstrated experimentally. The all-fiber LPFG mode converters could have promising applications in the mode-division multiplexing optical communications.

Multi-channel mode converter based on a modal interferometer in a two-mode fiber

In this Letter, we propose a multi-channel mode converter with the concept of a modal interferometer in a two-mode fiber (TMF). Two lateral stress points in a TMF function as in-line fiber mode couplers to construct the modal interferometer, and both transmission spectra and near-field patterns confirm that the LP₀₁ mode is successfully converted into an LP₁₁ mode at the multiple channels. The measured mode conversion efficiency almost completely follows the theoretical tendency. Finally, the mode conversion is realized at 20 channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss as low as 0.6 dB. Furthermore, the channel spacing can be freely tailored by adjusting the distance between two stress points.

Ultra-broadband mode converters based on length-apodized long-period waveguide gratings

We propose an ultra-broadband mode converter based on the structure of a length-apodized long-period grating, where π-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP₀₁-LP_11a and an LP₀₁-LP_11b mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP₀₁-LP_11a and LP₀₁-LP_11b mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general grating-based mode-coupling devices for a wide range of applications.

Characteristics of chiral long-period fiber gratings written in the twisted two-mode fiber by CO2 laser

We demonstrated the fabrication of a chiral long-period grating (CLPG) by twisting a two-mode fiber (TMF) when a CO₂ laser beam was sweeping along the fiber axis. The torsion, temperature, and surrounding refractive index characteristics of the fabricated TMF-CLPG were investigated experimentally. The fabricated TMF-CLPG has a high torsion sensitivity [0.7768 nm/(rad/m)], and can measure the twist rate and twist direction simultaneously. This kind of CLPG would have great potential applications in high-sensitivity optical sensors.