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.

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.

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.

Low short-wavelength loss fiber Bragg gratings inscribed in a small-core fiber by femtosecond laser point-by-point technology.

A femtosecond-laser-induced fiber Bragg grating (FBG) usually has a higher insertion loss at the shorter wavelength than at the reflection wavelength, i.e., so-called short-wavelength loss. High-quality FBGs are inscribed in different types of small-core single-mode fibers (SMFs) by the use of femtosecond laser point-by-point technology in order to investigate the effect of the fiber core diameter on the grating inscription efficiency and on the short-wavelength loss. A lower laser pulse energy is required to achieve the same grating reflectivity in a smaller-core fiber than in a large-core fiber. The short-wavelength loss of the small-core FBG is lower than that of the large-core FBG with the same reflectivity. Furthermore, a series of FBGs with a low short-wavelength loss are inscribed in a small-core SMF along the fiber axis to achieve so-called series-integrated FBGs (SI-FBGs). Finally, the effect of the input light direction on the reflection peak of the SI-FBGs is investigated to reduce the influence of the grating short-wavelength loss in the sensing and communication applications.

Transverse-load, strain, temperature, and torsion sensors based on a helical photonic crystal fiber.

In this Letter, we demonstrate a fabrication method of a helical photonic crystal fiber (HPCF) and an inflated HPCF (IHPCF) by use of an inflation-assisted hydrogen-oxygen flame heating technique. The transverse load, strain, temperature, and mechanical torsion properties of the HPCF and IHPCF were investigated experimentally to develop high-sensitivity sensors. The experimental results show that the transverse-load sensitivity could be greatly enhanced by means of enlarging the size of the air holes in the IHPCF; that is, the transverse-load sensitivity, i.e., 15.50 nm/(N·mm-1), of the IHPCF is two times higher than the transverse-load sensitivity of the HPCF, i.e., 4.45 nm/(N·mm-1). Moreover, both the HPCF and IHPCF exhibit high strain, temperature, and torsion sensitivities. Hence, such an HPCF/IHPCF could have great potential in sensing applications.

Temperature and liquid refractive index sensor using P-D fiber structure-based Sagnac loop.

A cost-efficient P-D fiber structure-based Sagnac loop sensor is proposed and experimentally demonstrated for measuring temperature and liquid refractive index (RI). The P-D structure is fabricated by fusion splicing a section of polarization-maintaining fiber (PMF) to a piece of multimode D-shaped optical fiber (MMDF). Then the P-D structure is built into a Sagnac loop using a 3dB coupler. The temperature and RI characteristics of the sensor are investigated experimentally. The results show that two resonant dips have different spectral responses of temperature and RI, which indicate that the sensor can realize simultaneous temperature and RI measurement. The high sensitivities of -1.804nm/°C and -131.49nm/RIU are achieved. The obtained resolutions of temperature and RI of the proposed sensor can reach 0.01°C and 2.46 × 10-4RIU, respectively. The proposed sensor has the potential application in biological and chemical fields.