A high-efficiency grating fabrication method was, for the first time, demonstrated to inscribe helical long period fiber gratings (H-LPFGs) in small numbers by means of twisting a standard single-mode fiber (SMF) during hydrogen-oxygen flame heating and then cutting the helical fiber into in series of sections. Each section of the helical fiber was a desired LPFG whose resonant wavelength, i.e. grating pitch, can be changed by adjusting the twist rate of the helical fiber. The H-LPFG inscribed in a standard SMF could be used to generate orbital angular momentum (OAM) modes, i.e. OAM+1 mode, with a purity of 91% and a conversion efficiency of 87% within a large wavelength range from more than the cutoff wavelength of a SMF, which is highly advantageous to all-fiber optical communications based on the OAM mode-division multiplexing technique.

Optical orbital angular momentum (OAM) has been recently implemented in holography technologies as an independent degree of freedom for boosting information capacity. However, the holography capacity and fidelity suffer from the limited space-bandwidth product (SBP) and the channel crosstalk, albeit the OAM mode set exploited as multiplexing channels is theoretically unbounded. Here, we propose the ultra-dense perfect OAM holography, in which the OAM modes are discriminated both radially and angularly. As such, the perfect OAM mode set constructs the two-dimensional spatial division multiplexed holography (conventional OAM holography is 1D). The extending degree of freedom enhances the holography capacity and fidelity. We have demonstrated an ultra-fine fractional OAM holography with the topological charge resolution of 0.01. A 20-digit OAM-encoded holography encryption has also been exhibited. It harnesses only five angular OAM topological charges ranging from -16 to +16. The SBP efficiency is about 20 times larger than the conventional phase-only OAM holography. This work paves the way to compact, high-security and high-capacity holography.

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.

The elastic scattering of ${}^8$B by a ${}^{\mathrm{nat}}$Pb target was measured at an incident energy of 170.3 MeV. Special care was taken with the limited intensity and broad profile of the secondary beam. The measured angular distribution of the differential cross section shows that the Coulomb-nuclear interference peak (CNIP) is not suppressed in this system, in contrast to what was observed in the scattering of neutron halo nuclei by heavy targets at energies around the Coulomb barrier. Analyses of the angular distribution were performed both in terms of the optical model using a single-folding-type potential and the continuum discretized coupled-channels (CDCC) method, which explicitly takes into account the breakup-channel couplings to the elastic scattering. The overall pattern of the differential cross section is well reproduced by the CDCC calculations. The calculations show that the effect of breakup-channel couplings on the elastic scattering is small in the present case.

We demonstrate the fabrication of single-mode helical Bragg grating waveguides (HBGWs) in a multimode coreless fiber by using a femtosecond laser direct writing technique. This approach provides a single-step method for creating Bragg grating waveguides. Specifically, the unique helical structure in such an HBGW serves as a depressed cladding waveguide and also generates strong Bragg resonance due to its periodicity. Effects of pulse energy, helical diameter, and helical pitch used for fabricating HBGWs were studied, and a single-mode HBGW with a narrow bandwidth of 0.43 nm and a Bragg wavelength of 1546.50 nm was achieved by using appropriate parameters, including a diameter of 10 μm and a helical pitch of 1.07 μm. The measured cross-sectional refractive index profile indicates that a depressed cladding waveguide has been created in this single-mode HBGW. Moreover, five single-mode HBGWs with various Bragg wavelengths were successfully fabricated by controlling the helical pitch, and this technique could be used for achieving a wavelength-division-multiplexed HBGW array. Then, the temperature and strain responses of the fabricated single-mode HBGW were tested, exhibiting a temperature sensitivity of 11.65 pm/°C and a strain sensitivity of 1.29 pm/με, respectively. In addition, the thermal stability of the single-mode HBGW was also studied by annealing at a high temperature of 700°C for 15 h. The degeneration of the single-mode waveguide into a multimode waveguide was observed during the isothermal annealing process, and the peak reflection and the Bragg wavelength of the fundamental mode exhibited a decrease of ∼ 7 dB and a “blue” shift of 0.36 nm. Hence, such a femtosecond laser directly written single-mode HBGW could be used in many applications, such as sapphire fiber sensors, photonic integrated circuits, and monolithic waveguide lasers.