There is an urgent demand for high sensitivity and high stability online CO2 monitoring in greenhouse gas management and industrial applications. To address this demand, this paper proposes and experimentally demonstrates a Fabry–Perot (F-P) interferometric CO2 sensor. The sensor is based on an optical fiber microcavity formed by two ceramic ferrules inside a ceramic sleeve, with side openings that allow gas exchange. The microcavity is fully filled with a polyethyleneimine/polyethylene glycol (PEI/PEG) composite medium. Conventional external-cavity and end-face-coated configurations suffer from reflection amplitude mismatch and beam divergence, which degrade the interference signal quality. In contrast, the proposed structure provides symmetric refractive-index interfaces and a weak waveguiding effect that suppresses coupling losses. This improves the extinction ratio and signal-to-noise ratio of the interference spectrum. Experimental results show that the sensor exhibits a linear response in the CO2 concentration range of 200-1000 ppm, with a sensitivity of 7.44 pm/ppm. The response time is approximately 1 min, and the recovery time is about 6 min. The sensor also shows selectivity toward CO2 over other gases. We also evaluated the effect of ambient temperature on the sensor response. A temperature cross-sensitivity compensation model is established to reduce measurement errors caused by ambient temperature fluctuations, thereby improving the reliability of CO2 concentration estimation under varying temperature conditions. Overall, the fully filled microcavity design provides a structurally simple solution and offers a promising approach for high-resolution CO2 monitoring applications.
Optical biosensors require high sensitivity, stability and measurement accuracy for trace-level biomolecule detection. Tacrolimus (TAC) is a first-line immunosuppressant with a narrow therapeutic window (5–20 ng/mL), and its blood concentration varies greatly among patients, making real-time and accurate monitoring essential for clinical medication safety after organ transplantation. In this work, a fiber-optic biosensing system integrated with a laser ring cavity is proposed for sensitive, rapid and accurate detection of TAC. A nested dual-tapered multimode fiber-multi-core fiber-multimode fiber (MMF-MCF-MMF) probe was fabricated to strengthen the evanescent field, boost light-matter interaction and improve sensor sensitivity. The nested dual-taper structure was analyzed using RSoft software, and the optimized probe was verified by experimental characterization. An erbium-doped fiber ring laser with a full width at half-maximum (FWHM) of 0.116 nm acts as the narrow-linewidth near-infrared excitation source. ZnCdS quantum dots are immobilized on the probe surface to improve sensing responses, while Apt122 aptamers functionalized on the fiber enable specific molecular recognition of small-molecule TAC. The developed biosensor achieved a sensitivity of 0.447 nm/lg(ng/mL) and a limit of detection (LoD) of 193.8 pg/mL, about 15.85-fold lower than that of the conventional amplified-spontaneous-emission-based sensing system. Selectivity, stability and pH-response tests further confirmed the reliable quantitative detection performance of the sensor. This sensor provides reliable technical support for real-time and precise monitoring of blood TAC concentrations after organ transplantation.
Optical fiber integrated photodetectors are attracted considerable attention for next generation optical communication, environmental monitoring, and remote sensing applications particularly in harsh environments where conventional electronic devices exhibit limited reliability. However, achieving efficient light to electrical energy conversion within the confined geometry of an optical fiber remains a significant challenge. In this work, a high performance H-shaped side polished optical fiber photodetector was developed by partially removing the fiber cladding exposing the fiber cladding to enhance the evanescent field interaction. The exposed region was functionalized with a hybrid graphene oxide (GO) nanolayer and a gold nanoparticle (Au) film to exploit the surface plasmon resonance (SPR) phenomenon. The GO layer provides broadband optical absorption while the Au layer enhances electron photon coupling through localized surface plasmon resonance. Experimental results demonstrated a peak responsivity and quantum efficiency of 2.48 and 558,93 A/W, respectively, at 550 nm across the visible wavelength range of 380–750 nm. The proposed hybrid architecture enhances light absorption, accelerates the photo response, and improves photoelectric conversion efficiency, demonstrating its potential as a robust platform for high performance optical fiber photodetectors in environmental sensing and biomedical monitoring applications.
This study demonstrates the generation of stable, high-pulse-energy noise-like pulses (NLPs) from a dumbbell-shaped Tm-Ho co-doped fiber laser, wherein that a 50:50 nonlinear optical loop mirror (NOLM) and a 30:70 NOLM serve as the reflectance mirrors. The performance optimization of NLPs is systematically investigated by tailoring the length of single-mode fiber (SMF) integrated within the 30:70 NOLM and adjusting the output coupling ratio. Experimental results confirm that targeted manipulation of nonlinear phase shift and output coupling efficiency is crucial for achieving high pulse energy output. In the optimized configuration, which incorporates a 40 m SMF segment and a 50 % output coupling ratio, stable NLP operation was realized with a maximum pulse energy of up to 165 nJ, accompanied by a broad 3 dB bandwidth of 57.67 nm centered at 1975 nm. The laser exhibits excellent stability, as validated by a signal-to-noise ratio (SNR) of 62 dB. These findings highlight that this dumbbell-shaped cavity provides a reliable and efficient platform for high-performance NLPs generation in the 2 μm spectral region, holding significant potential for applications in mid-infrared spectroscopy, remote sensing, and nonlinear optics.
Magnetic field measurement plays an important role in many fields such as resource exploration, national defense security, and disaster monitoring. In order to solve the problem of low sensitivity of the current optical fiber magnetic field sensor, a Fiber Bragg Grating (FBG) magnetic field sensor based on V-hinge structure was proposed. The V-shaped hinge structure integrates the design of symmetrical thin plates and a driving plate into the FBG magnetic field sensor. This design allows for the amplification of the microstrain of the Terfenol-D rod, thereby enhancing the sensitivity of the sensor. Firstly, the sensor model was established and the theoretical analysis was carried out. Secondly, the sensor model was imported into COMSOL for simulation analysis of the sensor. Finally, the actual sensor was developed and tested in the zero magnetic space. The experimental results show that when the bias magnetic field is 35mT, the highest sensitivity of the sensor reaches 164.46 pm/mT, the magnetic field resolution is 6.08 µT, and the sensor directivity basically conforms to the cosine-squared law. The research results provide reference for the development of high-sensitivity fiber optic magnetic field sensors.
