In this study, a novel and efficient method enabling fabrication of a multichannel helical long-period fiber grating (HLPG) with almost the same channel spacing and the same rejection depth has been proposed and demonstrated for the first time both theoretically and experimentally, which is realized based on utilization of the so-called phase-only sampling technique. Unlike the previous amplitude-type sampling method where either a strong rectangular-type or a Sinc-like index-modulation distribution is generally demanded, the proposed method requires the minimum as well as a uniform index-modulation to the designed multichannel gratings, which considerably facilitates the fabrication process and makes the multichannel HLPGs to be fabricated in even a conventional single-mode fiber (SMF) by using the CO₂ laser writing technique. As examples, a 3-channel and a 9-channel HLPGs have been successfully demonstrated numerically and experimentally. The experimental results agree with the designed ones, which represents the first realization of a multichannel HLPG, to the best of our knowledge. It is believed that the proposed multichannel HLPG could find potential applications to multi-wavelength orbit-angular-momentum (OAM) mode converter as well as the wavelength division multiplexing (WDM) sensors.

A miniature temperature-independent curvature sensor was proposed and prepared by using a phase-shifted long-period fiber grating (LPG) fabricated from deeply tapered conventional single-mode fibers (SMFs). As a typical example, a sensor made from a phase-shifted deeply tapered long-period grating (PS-DTLPG) with a length of 3.4 mm and a maximum curvature sensitivity of 12.05 nm/m ^-1 was successfully demonstrated. The proposed curvature sensor is of the temperature-independence and has features of high sensitivity, high reliability, miniature size and low cost, which could have potential applications in structural health monitoring, especially for the medical industry and shape reconstruction.

An ultra-wideband orbital-angular-momentum (OAM) mode generator has been proposed and demonstrated both theoretically and experimentally, which is based on a helical long period fiber grating (HLPG) but written in a graded-index few-mode fiber (GIFMF). Unlike all the HLPG-based broadband OAM generators reported to date, where the utilized HLPG (fabricated only in step-index fibers) all were operated at wavelengths very close to the dispersion-turning point (DTP), the GIFMF-based HLPG proposed in this study is, however, operated at wavelengths far from its DTP, which provides a new and robust method enabling to produce a ultra-wideband OAM mode generator with an ultra-high conversion-efficiency and yet with a great flexibility in controlling the central wavelength of itself. As a typical example, the first-order OAM mode generator with a central wavelength of ∼1510 nm, a bandwidth of ∼235 nm, and a conversion-efficiency larger than 99% (∼20.7 dB) has been successfully obtained. To the best of our knowledge, this is the first report for practical demonstration of the GIFMF-based HLPG.

In this study, an ultra-broadband orbital angular momentum (OAM) mode generator based on a phase-modulated helical long period fiber grating (HLPG) working at high-radial order cladding mode is firstly proposed and numerically demonstrated. The proposed phase-only modulation method can considerably facilitate the HLPG’s fabrication by eliminating the grating’s apodization, which is extremely difficult to realize in the screw-type grating. Meanwhile, the proposed method provides more flexibilities in the design of HLPGs. As a typical example, a first-order OAM (OAM-1) mode generator with a bandwidth of ~63 nm, a conversion efficiency of ~100%, and purity of ~ 91% has been numerically demonstrated. Moreover, an OAM-1 mode generator with a bandwidth of ~469 nm, a conversion efficiency of ~100%, and purity of ~ 91% has been successfully obtained, covering all bands of fiber communication, i.e., original-(O-), extended-(E-), short-(S-), conventional-(C-), long-(L-), and ultralong-(U-) wavelength bands, which, based on our knowledge, are the best results reported to date among those of the HLPG-based OAM mode generators. The proposed component may find potential applications in the OAM mode division multiplexing (MDM) system combined with the wavelength division multiplexing technique.

A ring-shaped structure based on polymer coated no-core fiber (NCF) is presented for simultaneous temperature and pressure measurements. The sensing structure is fabricated by splicing the partially coated NCF between single-mode fibers and bending it into a loop. The anti-resonance effect is formed in the NCF and the resonant dips have wavelength shift responses to the temperature and displacement variations. It should be noted that the increase of temperature leads to a decrease in the polymer coating refractive index and changes the displacement measurement sensitivity, thus, the compensation algorithm is required to resolve this cross-sensitivity problem. Combining the ring-shaped structure with the elastic diaphragm encapsulation can convert seawater pressure into structural deformation displacement to achieve pressure measurement. The experimental and simulation results show that this sensor offers a high temperature sensitivity of −5.098 nm/°C with the resolution of $2\times 10^{-4}$ °C and a pressure sensitivity of −2.368 nm/MPa with the resolution of $0.4\times 10^{-3}$ MPa. In addition, the sensing performances could be changed by adjusting the NCF size and the ring-shaped structure diameter, which enables this sensing structure to have a broader application potential for seawater parameter measurement.