Polarization dependent cladding modes coupling and spectral analyses of excessively tilted fiber grating

Trace detection of cadmium (II) ions based on an excessively tilted fiber grating

Cadmium (Cd2+) ion is one of the most crucial industrial pollutants that cause serious harm to the human body. We proposed and experimentally demonstrated a highly sensitive Cd2+ sensor based on hydrogel coated excessively tilted fiber grating. The hydrogel with the functional monomer of the allyl thiourea can specifically bind to Cd2+, and hence forming a complex. The grating excites high order cladding modes, and ensures a sufficient interaction between the light and hydrogel binding to Cd2+, providing highly sensitive monitoring. The results show that the sensor can detect 0-160 pM Cd2+ in aqueous solution. The maximum sensitivity is 10600 nm/µM, and the minimum detection concentration is 20 pM (about 0.004 ppb), which is much less than that of the international standard (3 ppb). The proposed sensor exhibits high sensitivity, ultra-low detection limit, specificity, and a compact structure, offering potential as a tool for Cd2+ detection in aqueous solution.

Mode-locked fiber laser based on a small-period long-period fiber grating inscribed by femtosecond laser

We demonstrate stable mode-locked pulses in an erbium-doped fiber laser (EDFL) using a femtosecond laser-inscribed small-period long-period grating (SP-LPG). The SP-LPG has a period of 25 µm and a length of 2.5 mm. The polarization dependent loss (PDL) of the SP-LPG reaches 20 dB at the wavelength of 1556 nm and 25 dB at the wavelength of 1607 nm, which is sufficient to trigger the mode-locking mechanism. In addition, a mode-locked fiber laser (MLFL) based on the SP-LPG has been demonstrated to generate 1.58-ps pulses at 1577 nm with a bandwidth of 4 nm and a repetition rate of 1.54 MHz. The signal-to-noise ratio (SNR) of 50 dB shows the high stability of this system. This work indicates various potential applications of the SP-LPG in ultra-fast laser technologies due to its simple fabrication, compact structure, and high damage threshold.

Femtosecond laser-inscribed off-axis high-order mode long-period grating for independent sensing of curvature and temperature

We propose and demonstrate a novel curvature and temperature sensor based on an off-axis small-period long-period fiber grating (SP-LPG) which is inscribed in a single mode fiber by a femtosecond laser in one step. The total length of the SP-LPG is only 2.1 mm. The period of the SP-LPG is 30 µm, which is smaller than that of conventional long period fiber gratings. Essentially, the SP-LPG is a high-order mode long period fiber grating. Due to the off-axis structure, the SP-LPG can be used for two-dimensional vector bending sensing. The curvature can be demodulated by the intensity variation of the dips in the transmission spectrum. When the incident light is polarized, the instantaneous curvature sensitivity of the SP-LPG can exceed 20 dB/m^-1. Meanwhile, a series of Bragg resonant peaks can be observed in the reflection spectrum, which can be used to monitor the fluctuation of temperature. The transmission dip is insensitive to temperature and the reflection peak is insensitive to curvature, which allows the SP-LPG to measure curvature and temperature independently. The characteristics of high curvature sensitivity, two-dimensional bending direction identification, real-time temperature measurement, and compact structure make the device expected to be applied in the field of structural health monitoring and intelligent robotics.

Cladding mode characteristics simulation of an excessively tilted fiber grating coated with gold nanoshells

The cladding mode characteristics simulation of an excessively tilted fiber grating (ExTFG) coated with gold nanoshells was conducted in this study. First, the effective refractive indices of the core and cladding mode before coating were obtained by solving the eigenvalue equation of the three-layer waveguide structure, and the coupling characteristics were briefly analyzed. Then HE_1,m and EH_1,m modes were selected as the research objects, and the spectral characteristics of ExTFG coated with gold nanoshells were simulated by the finite element method. The simulated refractive index sensitivity of HE_1,29 and EH_1,29 modes is 160.16 and 185.03 nm/RIU, respectively. Compared with the non-localized surface plasmon resonance (LSPR) effect, it increased by 10.76 nm/RIU (7.2%) and 19.53 nm/RIU (11.8%), respectively. Thus, the LSPR effect was verified to be beneficial to improve the refractive index sensitivity of ExTFG.

Fabrication and Characterization of Line-by-Line Inscribed Tilted Fiber Bragg Gratings Using Femtosecond Laser

In this paper, we studied the basic characteristics of tilted fiber Bragg gratings (TFBGs), inscribed line-by-line. Experimental results showed that if the TFBGs were located within different planes parallel to the fiber axis, the spectra performed differently. For 2°TFBG, if it was located near the central plane, the Bragg resonance was stronger than ghost mode resonance, and the order reversed if it was located near the boundary between core and cladding. As the tilted angle increased, the range of cladding mode resonance increased. When the tilted angle was larger than 12°, the birefringence effect was observed. Based on the birefringence phenomenon, torsion characteristics were experimentally studied; the sensitivity was about 0.025 dB/degree in the linear variation range. The harmonic order of TFBGs also affected the transmission spectrum. Leaky mode resonance was observed in the 8th order TFBG, and torsion (or polarization) influenced the spectrum of the 8th order TFBG. Our research represented the theory of line-by-line inscribed TFBGs and provided an inscription guidance for TFBGs.