Multiple orbital angular momentum mode switching at multi-wavelength in few-mode fibers

Ultra-broadband flat-top circular polarizer based on chiral fiber gratings near the dispersion turning point

Based on the dual-resonance principle around the dispersion turning point, a scheme of chiral long-period fiber gratings (CLPGs) formed by twisting a high-birefringence (Hi-Bi) fiber is herein proposed to realise ultra-broadband flat-top circular polarizers. The coupling bandwidth is approximately seven times larger than that of traditional CLPGs. In addition, by introducing chirp characteristics in these CLPGs, an ultra-broadband flat-top circular polarizer with ∼200 nm@3 dB was conveniently achieved. Subsequently, by optimising the chirped CLPGs, a circular polarizer with a bandwidth extinction ratio of approximately 30 dB and a high level of ∼100 nm at 1 dB was realised. It was shown that the mode-controlling performances of the CLPGs can be remarkably improved, which has significant applications in light-field regulation. Finally, for the first time, it was proved that the CLPG cannot generate a vortex beam.

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.

Phase-Inserted Fiber Gratings and Their Applications to Optical Filtering, Optical Signal Processing, and Optical Sensing: Review

Phase-inserted fiber gratings (PI-FGs) refer to those gratings where there exist a number of the phase-shifts (spatial spacing) among different sections (or local periods) of the gratings themselves. All the PI-FGs developed to date can mainly be divided into three categories: phase-shifted gratings, phase-only sampled gratings, and phase-modulated gratings, of which the utilized gratings could be either the Bragg ones (FBGs) or the long-period ones (LPGs). As results of the proposed the PI-FGs where the numbers, quantities, and positions of the inserted phases along the fiber direction are optimally selected, PI-FGs have already been designed and used as various complex filters such as the ultra-narrow filters, the triangular (edge) filters, the high channel-count filters, and the flat-top band-pass/band-stop filters, which, however, are extremely difficult or even impossible to be realized by using the ordinary fiber gratings. In this paper, we have briefly but fully reviewed the past and recent advances on PI-FGs, in which the principles and design methods, the corresponding fabrication techniques, and applications of the different PI-FGs to the fields of optical filtering, optical signal processing, and optical sensing, etc., have been highlighted.

Thermally Tunable Orbital Angular Momentum Mode Generator Based on Dual-Core Photonic Crystal Fibers

The combination of mode division multiplexing (MDM) based on orbital angular momentum (OAM) modes with wavelength division multiplexing (WDM) has attracted considerable attention due to its ability to increase optical transmission capacity. However, the switching of the multi-wavelength and multi-order OAM mode in an all-fiber structure has always been a challenge. As a solution, a thermally tunable dual-core photonic crystal fiber (DC-PCF) is proposed to achieve multi-order and multi-wavelength switching of the OAM mode. The results show that the OAM mode with topological charge m = ±1 can be excited with the linear polarization fundamental mode (LPFM) and circular polarization fundamental mode (CPFM). In addition, the device can effectively excite a high-purity ±1st order OAM mode with wavelengths ranging from 1520 to 1575 nm by thermal tuning. The purity of the mode is in excess of 99%, and the energy conversion efficiency (ECE) is above 95%. The proposed design is expected to be applied in all-fiber communication systems combined with MDM and WDM.

Security strategy of parallel bit interleaved FBMC/OQAM based on four-dimensional chaos

A parallel bit-interleaved filter-bank multicarrier/offset quadrature amplitude modulation (FBMC/OQAM) security strategy based on four-dimensional chaos is proposed in this paper. After the QAM constellation point distribution is disturbed, the modulated FBMC bits and symbols are interleaved and encrypted to realize the improvement of the FBMC/OQAM system physical layer security performance. The chaotic sequence generated by the four-dimensional hyperchaotic system is optimized and calculated to control the disturbance process, which enhances the performance of the system against illegal malicious attacks. The parallel encryption scheme proposed in this scheme increases the encryption efficiency by 1.43 times; can provide a keyspace of 10⁹⁰ size, which effectively resists brute force attacks; and improves the physical layer security of the system. The proposed FBMC/OQAM parallel bit interleaved encryption scheme using a 5 km weakly coupled four-mode fiber achieves a 3×10 Gb/s multiple-input multiple-output-free transmission. The experimental results show that this scheme can effectively improve the security performance of the system, and combines the few-mode multiplexing technology with advanced modulation. It is a candidate for the future large-capacity and high-security optical transmission system.

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.