Changle Wang

An Injection-Locked Single-Longitudinal-Mode Fiber Ring Laser With Cylindrical Vector Beam Emission

We demonstrate a fiber ring laser with narrow bandwidth single-longitudinal-mode cylindrical vector beam (CVB) output at C-band wavelength range for the first time to the best of our knowledge. A step index two-mode fiber Bragg grating is used as a transverse mode selector for CVB generation, while both the injection-locking technique and narrow bandwidth of the fiber Bragg grating lead to single-longitudinal-mode operation. The 3-dB bandwidth of the laser output is measured to be <0.01 nm near a single wavelength of 1551.4 nm with a signal-to-background ratio of >60 dB. Mode distribution and optical spectra of few-mode fibers with periodic modulated refractive index profile, namely the few-mode fiber Bragg gratings with bent radius, are investigated theoretically and experimentally, which provide a comprehensive exploration of CVBs generation.

Theoretical and experimental analysis of excessively tilted fiber gratings

We have theoretically and experimentally investigated the dual-peak feature of tilted fiber gratings with excessively tilted structure (named as Ex-TFGs). We have explained the dual-peak feature by solving eigenvalue equations for TM0m and TE0m of a circular waveguide, in which the TE (transverse electric) and TM (transverse magnetic) core modes are coupled into TE and TM cladding modes, respectively. Meanwhile, in the experiment, we have verified that one of the dual peaks at the shorter wavelength is due to the TM mode coupling whereas the other one at the longer wavelength arises from TE mode coupling when a linearly polarized light launched into the Ex-TFG. We have also investigated the peak separation of TE and TM cladding mode for different surrounding medium refractive indexes (SRI), revealed that the dual peaks separation is decreasing as increasing of SRI, which agrees very well with the theoretical analysis results.

Small-period long-period fiber grating with improved refractive index sensitivity and dual-parameter sensing ability

We UV inscribe and characterize a long-period fiber grating with a period of 25 μm. A series of polarization-dependent dual-peak pairs can be seen in the transmission spectrum, even though only the symmetrical refractive index modification is introduced. The fabricated grating exhibits a lower temperature sensitivity compared with standard long-period gratings and an enhanced refractive index sensitivity of ∼312.5  nm/RIU averaged from 1.315 to 1.395, which is more than four-fold higher than standard long-period gratings in this range. The full width at half-maximum of the fabricated grating is only about 0.6 nm, allowing for high-resolution sensing. Moreover, the grating period is so small that the attenuation dip corresponding to a high-order Bragg resonance can also be seen, which can act as a monitor of the unwanted perturbation to realize dual-parameter sensing.

Graphene-induced unique polarization tuning properties of excessively tilted fiber grating

By exploiting the polarization-sensitive coupling effect of graphene with the optical mode, we investigate the polarization modulation properties of a hybrid waveguide of graphene-integrated excessively tilted fiber grating (Ex-TFG). The theoretical analysis and experimental results demonstrate that the real and imaginary parts of complex refractive index of few-layer graphene exhibit different effects on transverse electric (TE) and transverse magnetic (TM) cladding modes of the Ex-TFG, enabling stronger absorption in the TE mode and more wavelength shift in the TM mode. Furthermore, the surrounding refractive index can modulate the complex optical constant of graphene and then the polarization properties of the hybrid waveguide, such as resonant wavelength and peak intensity. Therefore, the unique polarization tuning property induced by the integration of the graphene layer with Ex-TFG may endow potential applications in all-in-one fiber modulators, fiber lasers, and biochemical sensors.

In-Line Mach-Zehnder Interferometer With D-Shaped Fiber Grating for Temperature-Discriminated Directional Curvature Measurement

A high-sensitivity curvature sensing configuration is implemented by using a fiber Mach-Zehnder interferometer (MZI) with D-shaped fiber Bragg grating (FBG). A segment of D-shaped fiber is fusion spliced into a single mode fiber at both sides, and then a short FBG is inscribed in the D-shaped fiber. The fiber device yields a significant spectrum sensitivity as high as 87.7 nm/m−1 to the ultralow curvature range from 0 to 0.3 m−1 , and can distinguish the orientation of curvature experienced by the fiber as the attenuation dip producing either a blue or red wavelength shift, by virtue of the asymmetry of D-shaped fiber cladding. In addition, by tracking both resonant wavelengths of the MZI and embedded FBG, the temperature and curvature can be measured simultaneously.

Multi-channel mode converters based on in-line fiber modal interferometer

A modal interferometer was proposed to realize multi-channel mode conversion in two mode fiber. The near-filed pattern confirmed the LP01 mode was converted into LP11 mode at the destructive wavelengths. The mode conversion was realized at 20-channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss lower than 0.6 dB.

Potential bending sensor based on small-period long-period gratings

Small-period long-period gratings (SP-LPGs) have been fabricated by a UV laser of 244 nm in hydrogen-loaded single mode fibers (SMFs). The period of the SP-LPG is determined to be 25 μm by a custom-designed amplitude mask, which is placed close to the SMF. The fabricated SP-LPGs were annealed at 80°C for 48 h to stabilize the property. The transmission spectrum measured with non-polarized light clearly shows a series resonances of dual-peak pairs (DPPs). Fig. 1(a) bottom trace depicts the typical transmission spectrum showing two pairs of dual-peaks at1474.7 nm and 1481.0 nm as one pair and 1575.6 nm and 1582.1 nm as the other pair, which agrees well with the simulation in ref [1], in which only cladding modes with the lowest azimuthal order are taken into account. Coupling to high azimuthal order cladding modes should be zero in a circularly symmetrical refractive index (RI) perturbation for the SP-LPGs [2], thus we see from Fig. 1(a) very weak DPP appears around the 1525nm.

Polarization modulation based on graphene-integrated Ex-TFG in thin-cladding fibre

Light modulation by means of polarization management is crucial to avoid signal fading and errors in coherent optic communications [1]. In recent years, graphene has exhibited a great potential in light modulation due to its unique interaction with the electromagnetic field, and attracted increasing attention for applications in photonics and optoelectronics. [2-4]. To provide sufficient interaction length, an optical fibre with enhanced evanescent field generated by etched or tilted fibre grating structures [5, 6] is a promising approach. In this paper, we report an integration of multi-layer graphene onto an excessively tilted fibre grating (Ex-TFG, the tilted angle of ∼82°) inscribed in thin-cladding fibre, and reveal its special polarization tuning characteristics. In the hybrid waveguide of graphene-integrated thin-cladding Ex-TFG, with strong and distinctive core-to-cladding mode coupling property, the grating can provide not only an accessible evanescent field for the enhancement of the light-graphene interaction, but also two sets of polarization-dependent cladding modes. As depicted in the upper part of Fig. 1(a), we can clearly see the two waveguide attenuation peaks corresponding to the TE and TM polarization status from the bare Ex-TFG. After the graphene coating, from the lower part of Fig. 1(a), we can observe that the larger change of coupling intensity in the TE mode and more pronounced wavelength shift in the TM mode, which attests to the polarization-dependent coupling and interaction with graphene.

Fiber specklegram sensor based on the twist-induced effect in tilted two-mode fiber Bragg gratings

We demonstrate a fiber specklegram sensor based on the twist-induced effect in tilted two-mode fiber Bragg gratings (TM-FBGs). The measured LP<inf>21</inf> mode specklegram rotates 4.73 (radm⁻¹) with twist in the tilted TM-FBGs.

Temperature-calibrated high-precision refractometer using a tilted fiber Bragg grating

We present a refractometer with main- and vernier-scale to measure the refractive index (RI) of liquids with high precision by using the fine spectrum structure of a tilted fiber Bragg grating (TFBG). The absolute RI values are determined by the accurate wavelength of cut-off mode resonances. The main- and vernier-scale are calibrated by measuring large groups of fine spectra at different cut-off mode resonances in a small RI range, and the use of vernier-scale certainly reduces the RI measurement uncertainty resulted from the discrete cladding mode resonances. The performance of the TFBG-based vernier refractometer is experimentally verified by exploring the temperature dependence of RI of anhydrous ethanol in a near infrared region, showing an enhanced accuracy to the order of 10-4, high repeatability and temperature self-calibration capability.