Xin Wang

This paper presents a novel optical Fabry-Pérot (FP) interferometric pressure sensor with an extensive measurement range, specifically designed for the high-pressure and high-salinity conditions of the ocean. Utilizing ultra-high-precision demodulation techniques, the sensor adeptly captures minute structural deformations, significantly reducing material fatigue and enhancing overall sensitivity. It demonstrates exceptional performance features, including superior repeatability, excellent linearity over a wide measurement spectrum, and significant scalability—attributes crucial for diverse pressure measurement applications. Optimized for fatigue resistance and stability over extended operational periods, the sensor is positioned as an ideal tool for deep-sea exploration and as a critical tool for multi-parameter optical sensing integration. Additionally, an improved simulation model was developed to include processing and analysis errors, enhancing the system’s predictive accuracy under varied environmental conditions. We describe the working principles of the sensor and present both theoretical analyses and standard experimental comparisons. The experimental results highlight the sensor’s accuracy, long-term stability, expansive measurement capacity, and strong environmental adaptability. Key performance indicators include a dual-range sensitivity of 0.125 pm/bar within the 0–400 bar range and 0.066 pm/bar from 400 to 1200 bar, a pressure resolution of 0.75 bar (0.063% F.S.), and an Allan variance of 1.12 × 10⁻¹¹ nm under a 1000 bar load. Based on the experimental results, an online calibration method for optical measurement chamber length based on Free Spectral Range (FSR) is proposed, addressing the identified measurement challenges and offering constructive insights to improve the adaptability of marine optical sensors to environmental conditions.

A novel seawater density sensor has been developed, offering high resolution and improved accuracy. The sensor employs a Michelson interferometer as the core sensing element, measuring density by directly incorporating seawater samples into the measurement path. To enhance measurement precision, standard seawater is used in the reference path to mitigate temperature and dispersion effects. The sensor system consists of a superluminescent diode and an interrogator, forming a compact, high-sampling-rate wavelength demodulation system that is particularly suitable for measuring seawater density with high spatiotemporal variability. However, the spectrum collected by the interrogator suffers from discontinuity. To address this issue, we designed a high precision multi wavelength demodulation algorithm. This algorithm not only analyzes the movement of each resonance peak with a resolution of 0.0008 nm within a 70nm detection range, but also enables continuous tracking of specific resonance peaks via a dynamic adjustment mechanism. It overcomes the measurement range constraints typically faced by interference-based sensors, which are limited to a single free spectral range. Experimental validations demonstrate the sensor’s capability, achieving a density resolution of 1.79×10 ^-5 kg/m ³ and an extended measurement range exceeding 35 kg/m ³ . The sensor’s average relative error in density measurement is 0.0144%, highlighting its effectiveness and potential for advanced oceanographic research.

A high-resolution seawater salinity sensor, based on a temperature self-compensating resonator, has been developed, which accommodates simultaneous temperature variations in both the reference and sensing arms. The impact of external temperature fluctuations on the optical components affecting the optical path difference is significantly reduced and a salinity sensitivity of 145 pm/PSU and a salinity resolution of 0.0003 PSU are achieved. The sensor utilizes a structure that converts optical rays to spatial light, achieving non-contact measurements which is potentially pivotal for various applications in oceanography and marine studies.

An ultra-broadband 3-dB coupler based on a polymer dual-hollow-core anti-resonant fiber (DHC-ARF) is designed to work in the E + S + C + L + U communication band. By incorporating two elliptical-like cores and modulating the air gap between the two cores, the wavelength and polarization dependence of the DHC-ARF-based coupler is reduced effectively. The feasibility of using a 1.46 cm long DHC-ARF as the ultra-broadband coupler for the operating bandwidth of 400 nm in the range between 1.33 µm and 1.73 µm is demonstrated theoretically. The coupling ratio of each polarized mode stabilizes at 50 ± 2% and the coupling ratio difference between the two polarized modes changes within ±0.6%. This DHC-ARF coupler which is made of a polymer can be fabricated by high-resolution 3D printing. Compared to a silica-based DHC-ARF coupler, the polymer-based DHC-ARF coupler is easier to manufacture and the total loss of the latter is only 0.041 ± 0.006 dB in the operating bandwidth. The polymer hollow-core fiber coupler boasting an ultra-broadband, short component length, and low loss is very promising in next-generation, high-speed, and large-capacity hollow-core fiber communication systems.

We have experimentally demonstrated a 16-wavelength high-power DFB laser array with 200 GHz (1.6 nm) channel spacing based on the asymmetric equivalent π phase shift. Good single-longitudinal-mode (SLM) operations are obtained by introducing asymmetric equivalent π phase shifts. The effect of random phase on the high-reflective (HR) coating facet also is weakened by introducing asymmetric equivalent π phase shifts which are implemented at the 1/5 laser cavity close to the facet with HR coating. The average channel spacing is 1.62 nm, which deviated 0.02 nm from our design under the same injection current (300 mA) of each laser. The output power of 16 channels is above 100 mW at the bias current of 400 mA and the average slope efficiency is 0.41 W/A at 25 °C. Good single-longitudinal-mode are obtained for all the 16 channels with side mode suppression ratios of above 50 dB. Besides, the relative intensity noise at an injection current of 200 mA is below -157 dB/Hz.

Simultaneous measurement of pressure and temperature in seawater is realized based on the polydimethylsiloxane (PDMS) sealed microfiber Mach-Zehnder interferometer (MZI). Benefitting from the high thermal-optic coefficient and large elasticity of PDMS, high sensitivity and robust sensing structure can be obtained. In theoretical modeling, significant enhanced pressure response in fiber is observed clearly, and pressure and temperature sensitivities are calculated theoretically. In experiment, PDMS sealed MZI is fabricated by a two-step sealing method. Using the sealed MZI, sensitivity of 13.31 nm/MPa for pressure sensing and 7.41 nm/C for temperature sensing are demonstrated, respectively, which are about one order higher than the bare fiber MZIs (without PDMS sealing). To verify the accuracy of the sensor, several tests under arbitrarily pressure and temperature are performed with average errors of 4.38% and 1.44%, respectively. In addition, effects of the encapsulation on avoiding cross-sensitivity with salinity, PDMS thickness on pressure sensitivity, response time, repeatability, time stability, and polarization of the sensor are also evaluated by experiment. Sensors demonstrated here show advantages of low cost, simple fabrication, robust and compact structure, high-pressure resistance, high sensitivity, good repeatability and long-term stability.

Wavelength interrogation has been widely used for fiber-optic sensors based on Fabry-Perot interferometers (FPIs) because of its high measurement resolution. However, phase ambiguity often limits the measurement range to be less than one free-spectral range (FSR) of the FPI. Here, we investigate a simple but effective method that can eliminate the phase ambiguity of a silicon FPI temperature sensor using a fiber Bragg grating (FBG) fabricated on the lead-in fiber close to the silicon FPI. The reflection spectrum of the sensor shows sinusoidal fringes from the FPI and a single peak from the FBG. Assuming the FBG and the silicon FPI have similar temperatures, the FBG peak can be used merely as a marker to identify an FPI fringe peak without ambiguity even when it shifts by more than an FSR. Although the temperature resolution of the FBG is lower than that of the silicon FPI, the measurement range of the silicon FPI can be greatly improved without sacrificing the measurement resolution. Using a temperature sensor based on a 75-μm-thick silicon FPI, we demonstrated temperature measurement over a range of -60 to 140 °C (corresponding to a spectral shift of about five FSRs) with a resolution of 0.003 °C. The measurement range with unambiguous peak recognition is expected to be larger than demonstrated in the experiment and only limited by the spectral bandwidth of the sensor system and the capability of materials used for the sensor construction.