A flexible and stable optical fiber sensor based on the Sagnac loop is proposed and experimentally demonstrated for the measurement of salinity and temperature, simultaneously. The sensing unit consists of a tapered polarization maintaining fiber (tPMF) and a fiber Bragg grating (FBG) connected in series in the Sagnac loop. The temperature response comes from the high birefringence of PMF inside Sagnac loop, and the salinity response is enabled by the high-order modes excited in the tapered area of PMF. Besides, the FBG is for temperature compensation. We have succeeded in implementing dual-parameter measurements with the sensitivities of 0.356 nm/‰ for salinity and 0.616 nm/°C for temperature, respectively. The designed sensor has the potential for long-term monitoring of real ocean states.

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.

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.

We demonstrate a new concept for an all-fiber inclinometer based on a tapered fiber Bragg grating (tFBG) in a fiber ring laser (FRL) with the capability of measuring the tilt angle and temperature simultaneously. The sensor performance is analyzed theoretically and investigated experimentally. The dependence of tilt angle on the spectral response in variable temperature conditions was measured. Two inclinometers with different lengths have been fabricated and characterized in FRL. The sensitivity is 0.583 dB/° and 0.849 dB/°, respectively, in the range of 0° to 90°. Thanks to the FRL system, narrow 3-dB bandwidth (<0.1 nm) and high optical signal-to-noise ratio (~60 dB) are achieved. The tFBG in the FRL system can be used for working as a temperature insensitive inclinometer. The results suggested that the proposed inclinometer has the advantages of compact size and convenient manufacture, enhancing its potential for application prospect.

An optical fiber ring laser (FRL) cavity-based sensitive temperature and salinity sensor is proposed and experimentally demonstrated. The sensor consists of a Sagnac loop with a waist of 15 µm and a total length of 30 cm made of tapered polarization-maintaining fiber (PMF). Sagnac loop dual parameter sensing was theoretically modeled and presented. The salinity sensitivity of 0.173 nm/‰ was made possible by the efficient interaction between the tapered PMF cladding mode and the external refractive index. In addition, temperature sensitivity of 0.306 nm/°C was achieved through ultrahigh birefringence of PMF. Apart from that, the previous sensing system used a broadband light source (BBS) as the input light, resulting in a wide bandwidth and a poor signal-to-noise ratio (SNR). The Sagnac loop integrated into the FRL system can achieve a high SNR of approximately 50 dB and a narrow bandwidth of 0.15 nm while serving as the filter and sensor head. Additionally, the developed sensor has the advantages of simple design, low cost, and easy fabrication. It can also extend sensing distance indefinitely within a given range, which is anticipated to have positive effects on the testing of marine environments in laboratories.

The measurement of salinity and temperature is vital for ocean observation. However, due to the cross-sensitivity phenomenon of fiber sensors, traditional fiber-based demodulation algorithms are difficult to measure the dual-parameter accurately and simultaneously. In this research, a cascaded structure of a tilted fiber Bragg grating (TFBG) and a Sagnac interferometer (SI) is employed to monitor salinity and temperature simultaneously. The transfer learning based ResNet network (TFRes) is proposed to demodulate two variables at the same time. A total of 1680 spectral samples were collected for training and testing when the sensor was immersed in sodium chloride solution with different salinities ranging from 0% to 25% with 5% intervals and temperatures ranging from 20°C to 32°C with 3°C intervals. The SI exhibits a temperature sensitivity of -1.458 nm/°C, accompanied by a mean absolute error (MAE) of 0.28 °C. The sensitivity of the TFBG core mode to temperature is 0.009 nm/°C, and the sensitivity of one of the cladding modes to salinity is 0.004 nm/%. After training through the TFRes, we successfully achieved precise demodulation of salinity and temperature variations by analyzing the composite reflection spectra of SI and TFBG. The MAE amounted to 0.07304 °C for temperature and 0.07285 % for salinity, outperforming traditional analyzer demodulation methods by a factor of four. The monitoring experiment of dual parameter simultaneous changes is conducted, with a temperature error of 0.18 °C and a salinity error of 0.1%. The designed sensing system is poised to play a significant role in marine physical quantity monitoring.