We propose and demonstrate an all-fiber passively mode-locked laser with a figure-8 cavity, which generates pulsed cylindrical vector beam output based on a mode selective coupler (MSC). The MSC made of a two mode fiber and a standard single mode fiber is used as both the intracavity transverse mode converter and mode splitter with a low insertion loss of about 0.65 dB. The slope efficiency of the fiber laser is > 3%. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity which are measured to be > 94%. The laser operates at 1556.3 nm with a spectral bandwidth of 3.2 nm. The mode-locked pulses have duration of 17 ns and a repetition rate of 0.66 MHz.

We demonstrate label-free detection of low glucose concentration based on whispery gallery mode resonance in an optofluidic microcapillary (OFMC) biosensor. The wall surface of the OFMC is bio-chemically functionalized to detect cellular-level glucose concentration as low as 2.78 mM. The measured sensitivity is 0.966 pm/mM. The fabricated microcapillary has a thin wall thickness of 2 μm and a Q factor of 1.3 × 106, corresponding to a bulk refractive index sensitivity of 23.36 nm/RIU. The OFMC biosensor has advantages such as high resistance to environmental perturbation, small sample volume down to 90 nL and cost-effectiveness. Theoretical analysis shows the sensor sensitivity depends on the microcapillary wall thickness and liquid core refractive index.

Cylindrical vector beams (CVBs) with axial symmetry in both polarization and field intensity have attracted much attention because of their unique optical properties. Conventional methods to obtain CVBs including direct modulation of light beams in free space and high-order mode excitation by offset splicing single-mode fiber with few-mode fiber usually works at single wavelength with rather narrow bandwidth. Here, for the first time to the best of our knowledge, we demonstrate switchable dual-wavelength CVB generation from a passively mode-locked fiber laser using carbon nanotubes as saturable absorber for mode-locking and a home-made mode-selective coupler as both mode converter and birefringence filter. In experiments, the mode-locked fiber laser delivers CVB pulses of dual-wavelength (1532.5 nm and 1555.5 nm) and corresponding single wavelength with duration of hundreds of femtosecond, respectively. Moreover, the output polarization status is switchable between radially and azimuthally polarized states. The mode-locked CVBs with wavelength and polarization flexibility may have potential applications in mode-division multiplexing optical fiber communication, nanoparticle manipulation, material processing, nonlinear optics, and so on.

We propose and demonstrate, for the first time to the best of our knowledge, a passively mode-locked ytterbium-doped fiber laser generating cylindrical vector beams (CVBs) using a mode selective coupler (MSC) as the transverse mode converter and splitter, and a semiconductor saturable absorber mirror for mode-locking. According to the coupling mode theory and the phase matching principle, the MSC was fabricated by weakly fused technology, having a low loss of about 0.5 dB and achieving LP11 mode with a high purity of >96%. The CVB fiber laser operates at a center wavelength of 1042.3 nm, with a 3 dB spectral width of 1.5 nm. The repetition rate of the mode-locked laser pulses is 18.58 MHz. The radially and azimuthally polarized vector beams can be switched by adjusting the polarization controllers in the fiber ring cavity, with a high mode purity measured to be >95%. The mode-locked CVB ytterbium-doped fiber laser has potential applications in optical tweezers, optical imaging, etc.

To achieve high-sensitive refractive index (RI) sensing, an Au/Ti ₃ C ₂ T _x -MXene hybrid layered D-shaped photonic crystal fiber (PCF) based on surface plasmon resonance (SPR) sensor is proposed theoretically. The cladding of the D-shaped PCF has a hexagonal air-hole arrangement. In addition, the two air holes nearest to the metal layer in the first layer of cladding are reduced to enhance the plasma wave and provide an efficient leakage channel. The two air holes second nearest to the metal layer in the same layer are enlarged to limit the light scattering and guide more energy coupled with the surface plasmon polariton (SPP) mode. A thin layer of the Ti3C2Tx-MXene and Au film is applied on the surface of the D-shaped PCF to produce plasmon resonances across a wide range of wavelengths. By using the finite element method, the optical properties of the proposed PCF-SPR sensor are investigated. The results of the numerical analysis show that this sensor has a significant SPR effect, and the resonance loss peaks can be formed at different positions in the wavelength range of 800-2020 nm. The average RI sensitivity reaches 48120 nm/RIU and the maximum sensitivity is up to 64600 nm/RIU in the refractive index range of 1.390-1.415. The RI sensitivity of the Au/Ti ₃ C ₂ T _x -MXene layered sensor is 2.36 times higher than the sensor with only the Au layer. The proposed structure has its unique sensing properties, which can be effectively applied in bio-sensing fields.

We propose a concept of a high-order mode (HOM) pass filter based on the inverse-designed mode-routing, which enables an ultra-compact footprint and broad bandwidth. To validate the concept, we experimentally demonstrate two types of HOM pass filters using the direct-binary search topology optimization algorithm. In the first HOM pass filter, the mode-routing region is constructed using an inverse-designed adiabatic coupler, while the second filter utilizes a tapered asymmetric directional coupler. The subwavelength units based on the functional regions of both filters have an ultra-compact footprint of 4 µm × 800 nm. The experimental results indicate that the insertion losses of two HOM-pass filters are 3.13 and 1.94 dB, respectively, and their mode cross-talks are −15.8 and −27.36 dB at the center wavelength of 1550 nm. Both HOM pass filters exhibit high performance over a broad bandwidth of 130 nm.