Fangcheng Shen

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.

Fabrication of surface nanoscale axial photonics structures with a femtosecond laser

Surface nanoscale axial photonics (SNAP) structures are fabricated with a femtosecond laser for the first time, to the best of our knowledge. The inscriptions introduced by the laser pressurize the fiber and cause its nanoscale effective radius variation. We demonstrate the subangstrom precise fabrication of individual and coupled SNAP microresonators having the effective radius variation of several nanometers. Our results pave the way to a novel ultraprecise SNAP fabrication technology based on the femtosecond laser inscription.

Compact eccentric long period grating with improved sensitivity in low refractive index region

We demonstrate a compact eccentric long period grating with enhanced sensitivity in low refractive index region. With a period designed at 15 µm for coupling light to high order cladding modes, the grating is more sensitive to surrounding refractive index in low refractive index region. The intrinsically low coupling coefficients for those high order cladding modes are significantly improved with the eccentric localized inscription induced by the femtosecond laser. The fabricated grating is compact with a length of 4.05 mm, and exhibits an average sensitivity of ~505 nm/RIU in low refractive index region (1.3328-1.3544). The proposed principle can also work in other refractive index region with a proper choice of the resonant cladding modes.

Switchable dual-wavelength erbium-doped fibre laser utilizing two-channel fibre Bragg grating fabricated by femtosecond laser

We propose and demonstrate a switchable dual-wavelength erbium-doped fibre ring laser. Competition between the lasing wavelengths in erbium-doped fibre laser at room temperature is suppressed by incorporating a two-channel fibre Bragg grating (TC-FBG), which consists of two highly localized sub-gratings fabricated by femtosecond laser in single mode fibre. Wavelengths and polarization states of the lasing lines are selected by the TC-FBG. Laser output can be switched between single- and dual-wavelength operations by simply adjusting the polarization controller. Stable dual-wavelength output is verified at room temperature with a power fluctuation less than 0.27 dB, and wavelength fluctuation less than 0.004 nm.

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.

Small period long period grating with enhanced sensitivity in low refractive index region

Long period grating with a grating period of 15 μm is fabricated with a femtosecond laser. Such a small period grating enables coupling of fundamental core mode with high order cladding modes that have effective refractive index close to water, and exhibits an enhanced sensitivity in low refractive index region.

Sensitive refractive index sensor based on an assembly-free fiber multi-mode interferometer fabricated by femtosecond laser

We propose and demonstrate a highly sensitive refractive index (RI) sensor based on a novel fiber-optic multi-mode interferometer (MMI), which is formed with a femtosecond-laser-induced in-core negative refractive index modified line in a standard single mode fiber. The proposed MMI structure is directly written with femtosecond laser in one step, which removes the splicing process needed in conventional MMI fabrication and also significantly improves the robustness. This device exhibits a high sensitivity to surrounding refractive index, with a maximum sensitivity up to 10675.9 nm/RIU at the RI range of 1.4484-1.4513. The distinct advantages of high sensitivity, compact, robust and assembly-free all-fiber structure make it attractive for real physical, chemical and biological sensing.

Microhole-based long period fiber grating fabricated by femtosecond laser-induced breakdown in distilled water

Long period fiber grating constructed with periodic microholes, which can enhance the interaction of surrounding environment with the core-propagating light, is fabricated in single mode fiber with femtosecond laser-induced breakdown in distilled water.