Tonglei Cheng

A fiber FI/FO device based on a micro-lens array (MLA) is proposed in which the compact fiber bundle method is combined with 3D printing, enabling low-loss and high-robustness fiber interconnection. Single-mode fibers (SMFs) are etched by a chemical process and assembled into a fiber array matched to a multi-core fiber (MCF), while a polymer MLA is directly printed on the MCF end facet using a Nanoscribe 3D printing system. The MLA reduces the alignment sensitivity of the fiber array and improves the tolerance of the conventional fiber bundle method, thereby effectively reducing the coupling loss. A seven-core MLA FI/FO device is fabricated as a proof of concept. Benefiting from the MLA’s effective beam control capability, the experimental results show that the insertion loss is significantly reduced under different alignment conditions. Under poor alignment conditions, the insertion loss of the device without lenses is about 1.62 dB and is reduced to 0.97 dB after introducing the MLA, corresponding to an improvement of 0.65 dB and demonstrating strong robustness against misalignment. Under good alignment conditions, the insertion loss is further reduced from about 0.77 dB to 0.48 dB, indicating both high tolerance and low-loss potential of the MLA-based FI/FO device. Further measurements show that the average inter-channel crosstalk remains below −60 dB over the entire C + L band, confirming the low-crosstalk performance of the proposed MLA FI/FO device.

A PDMS-optical fiber Fabry-Pérot interferometric (FPI) temperature sensor based on the Vernier effect is proposed. The sensing probe is formed by cascading two single-mode fibers (SMFs) with a PDMS cavity, while the reference probe consists of two SMFs spliced to a hollow-core fiber (HCF). The two cavities have lengths of 146.8 and 218.4 μm, respectively. The structure is compact and easy to fabricate. The air-filled cavity provides a highly stable reference interference signal, while the PDMS cavity, which is sensitive to temperature, undergoes significant cavity-length variation under temperature changes. Due to the slight mismatch between the free spectral ranges (FSRs) of the two cavities, a distinct Vernier envelope is generated in the reflection spectrum, effectively amplifying the temperature-induced spectral shift and thus greatly improving the sensing sensitivity. Experimental results show that the parallel FPI structure achieves a temperature sensitivity of approximately 21.54 nm/℃ in the range of 25-30 ℃, with a linearity better than 99.975%, which is significantly higher than that of conventional PDMS-based FPI sensors. The sensor also exhibits good temperature reversibility, demonstrating great potential for applications in industrial, medical, environmental, and aerospace fields.

We demonstrate a submillimeter-length single-helix chiral grating embedded in a high-numerical-aperture single-mode fiber (HNA-SMF) for efficient generation of third-order orbital angular momentum (OAM), specifically the OAM_{±3, 12} mode. This design facilitates enhanced coupling of higher azimuthal modes due to significant perturbations arising from both the geometric effect of the thin-core offset under high-NA conditions and the elasto-optic effect induced by intense helical stress with a small twist pitch. As a result, we achieve an unprecedented device length of 0.88 mm with a resonance dip of −28 dB, corresponding to a high conversion efficiency of 99.8%—the shortest among all fiber grating-based OAM generators reported to date, to the best of our knowledge. It also exhibits excellent robustness against external disturbances, demonstrating low responsivities to torsion, strain, and temperature at 12 pm/(rad/m), 0.5 pm/με, and 24 pm/°C, respectively. These features make it promising for applications in OAM-based particle sensing and manipulation, particularly for chiral compounds.

We designed and experimentally demonstrated an innovative temperature sensor based on Mach–Zehnder interference, which was formed by splicing a piece of no-core fiber (NCF) between two conventional single-mode fibers (SMFs) with a core offset. The intentional misalignment formed a gap that can be filled with ethanol, and temperature sensing was realized using the mode interference between the two distinct optical paths passing through the NCF cladding and the ethanol solution. Although the NCF length barely influenced the sensor sensitivity, it produced a major impact on the free spectral range (FSR) value. As a result, simulations were conducted and an NCF length of 620.7 µm was prepared. The sensor achieved a remarkable temperature sensitivity of 5.3574 nm/°C within the 30–36 °C range. Furthermore, the sensor boasted multiple advantages, including compact structure, high repeatability, and exceptional stability, rendering it a highly effective and versatile tool for precise thermal measurement in cell culture.

Two innovative methods, the different core couple (D-C) method and the tapered corrosion (T-C) method, are proposed to improve traditional fiber bundle corrosion techniques for fabricating fan-in/fan-out (FI/FO) devices. The D-C method utilizes fibers with varying core diameters to facilitate light transmission from smaller to larger diameters, enhancing the fault tolerance of a fiber core structure on the end-face of FI/FO devices and minimizing optical signal leakage at the multiplexer joint, thereby reducing insertion loss (IL). Taper-cut and fusion-splicing techniques ensure the compatibility of the device with standard optical communication systems. The T-C method presents the idea of tapered corrosion fibers applied to FI/FO device fabrication, significantly enhancing production efficiency through a simple, repeatable, and cost-effective process. The above two methods were used to fabricate the corresponding FI/FO device for four-core single-mode fibers (4C-MCF), which are employed for signal multiplexing and demultiplexing. The device has a low average IL value of less than 0.5 dB, with cross talk lower than −60 dB, meeting the requirements for fiber-optic communication. This work provides two techniques for the fabrication of FI/FO device in multi-core fiber (MCF) communication systems, which can be expected to promote the innovation and development of space division multiplexing (SDM) technology.