Junhui Sun

A new ultrafast interrogation is proposed based on the peanut shaped tapered fiber (PSTF) interference structure and time-stretching method. The single mode fiber (SMF) is tapered by tapering machine to achieve a minimum core diameter width of 10.86 μm. Thanks to the peanut structure as a beam expander and collector, multiple high order modes have been excited. The interference between core mode and cladding modes has a high response to changes in surrounding salinity, with a salinity sensitivity of 0.302 nm/‰ and a corresponding refractive index (RI) sensitivity of up to 1800.535 nm/RIU. The time-stretching method is put forward and experimentally demonstrated. Once the femtosecond light pulse is modulated by the PSTF interferometer, the pulse light carrying information is transmitted to a dispersion compensation fiber (DCF) to achieve wavelength to time mapping and the spectrum in the time domain can be directly collected on an oscilloscope (OSC). By monitoring the time shift of salinity modifications, a salinity sensitivity of -0.128 ns/‰ and a real-time acquisition speed of 50 MHz for the sensing system is realized. The proposed salinity monitoring system has the advantages of high sensitivity, fast response speed, low cost and high accuracy. It has significant potential for real-time monitoring and timely provision of salinity data, which can help improve water quality and achieve scientific aquaculture. The proposed interrogation technology has broad application prospects for any real-time monitoring based on fiber optic interference modulation.

Carcinoembryonic antigen (CEACAM5), as a broad-spectrum tumor biomarker, plays a crucial role in analyzing the therapeutic efficacy and progression of cancer. Herein, we propose a novel biosensor based on specklegrams of tapered multimode fiber (MMF) and two-dimensional convolutional neural networks (2D-CNNs) for the detection of CEACAM5. The microfiber is modified with CEA antibodies to specifically recognize antigens. The biosensor utilizes the interference effect of tapered MMF to generate highly sensitive specklegrams in response to different CEACAM5 concentrations. A zero mean normalized cross-correlation (ZNCC) function is explored to calculate the image matching degree of the specklegrams. Profiting from the extremely high detection limit of the speckle sensor, variations in the specklegrams of antibody concentrations from 1 to 1000 ng/mL are measured in the experiment. The surface sensitivity of the biosensor is 0.0012 (ng/mL)⁻¹ within a range of 1 to 50 ng/mL. Moreover, a 2D-CNN was introduced to solve the problem of nonlinear detection surface sensitivity variation in a large dynamic range, and in the search for image features to improve evaluation accuracy, achieving more accurate CEACAM5 monitoring, with a maximum detection error of 0.358%. The proposed fiber specklegram biosensing scheme is easy to implement and has great potential in analyzing the postoperative condition of patients.

Detection of trace tumor markers in blood/serum is essential for the early screening and prognosis of cancer diseases, which requires high sensitivity and specificity of the assays and biosensors. A variety of label-free optical fiber-based biosensors has been developed and yielded great opportunities for Point-of-Care Testing (POCT) of cancer biomarkers. The fiber biosensor, however, suffers from a compromise between the responsivity and stability of the sensing signal, which would deteriorate the sensing performance. In addition, the sophistication of sensor preparation hinders the reproduction and scale-up fabrication. To address these issues, in this study, a straightforward lasso-shaped fiber laser biosensor was proposed for the specific determination of carcinoembryonic antigen (CEA)-related cell adhesion molecules 5 (CEACAM5) protein in serum. Due to the ultra-narrow linewidth of the laser, a very small variation of lasing signal caused by biomolecular bonding can be clearly distinguished via high-resolution spectral analysis. The limit of detection (LOD) of the proposed biosensor could reach 9.6 ng/mL according to the buffer test. The sensing capability was further validated by a human serum-based cancer diagnosis trial, enabling great potential for clinical use. The high reproduction of fabrication allowed the mass production of the sensor and extended its utility to a broader biosensing field.