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Optical fiber sensing is a promising detection method for spacecraft health monitoring, since optical fiber sensors are lightweight, small in size, easy to integrate and immune to electromagnetic interference. As a significant optical sensor, fiber Bragg gratings (FBG) are widely used for force sensing because of their axial strain characteristics. However, it is necessary to detect not only one-dimensional strain but also plane strain and its deformation in order to comprehensively evaluate the condition of the structure. Therefore, it is very important to analyze the reflection spectrum of FBG under lateral stress. When FBG are subjected to lateral stress, the refractive index of the waveguide in the x and y directions changes, resulting in a birefringence phenomenon. This result causes the reflection spectrum of FBG to split into two peaks. In this paper, a transverse stress detection method based on spectral split sensing for the fiber Bragg grating is proposed, intended for monitoring spacecraft–small particle collisions. The FBG local lateral stress detection system is designed and verified by experiments. The wavelength pressure correlation is established in the experiment by adjusting the number of weights to change the lateral pressure on the FBG. The loading range of FBG lateral pressure is 4.0–7.0 N, the step size is 0.5 N, and round-trip measurement is carried out four times. The wavelengths of the peak and split point of the FBG reflection spectrum are recorded. The experimental results show that FBG’s split point and right peak pressure sensitivities are 16.57 pm/N and 45.14 pm/N, respectively. The spectral splitting phenomenon can be applied in spacecraft structure health monitoring systems and has certain reference value for the simplification of sensor systems.

Abstract An optical fiber microdisplacement sensor based on symmetric Mach-Zehnder interferometer (MZI) with a seven-core fiber and two single-mode fiber balls is proposed. The rationality and manufacturing process of the MZI sensing structure are analyzed. The fabrication mechanism of the Mach-Zehnder sensor by CO2 laser is described in detail. Experimental results show that temperature sensitivities of the two dips are 98.65 pm/°C and 89.72 pm/°C, respectively. The microdisplacement sensitivities are 2017.71 pm/mm and 2457.92 pm/mm, respectively. The simultaneous measurement of temperature and microdisplacement is demonstrated based on the sensitive matrix. The proposed Mach-Zehnder interference sensor exhibits the advantages of compact structure, simple manufacturing process, and high reliability.

A novel technology for the simultaneous and independent measurement of dual parameters is proposed and experimented. By using a single fiber Bragg grating half-pasted by 1C-LV epoxy under different curing conditions, the sensor structure is designed such that the reflective single-peak spectrum splits into a twin-peak spectrum, which makes the FBG spectrum form a natural spectral peak splitting bias. A measurement limitation exists in the FBG sensor packaging at room temperature, which can be solved by the high-temperature cured packaging method. To verify the validity of the theory and methodology, the experimental system is used. In the range from -1000 to +1000 με and from 35 to 75°C, the Bragg wavelength change is relative linear to the strain and temperature. The temperature and strain variations can be independently and simultaneously measured using the split peak, and the deviations of the FBG sensor are ±1°C and ±5 με, respectively. This single FBG sensor can realize dual-parameter measurement, which is valuable for narrow-space health monitoring. [ABSTRACT FROM AUTHOR]

An in-line optical fiber Mach–Zehnder interferometric (MZI) sensor is proposed and experimentally demonstrated for magnetic field measurement. The fabrication mechanism of the optical fiber microstructures by CO2 laser is described in detail. The sensor with symmetrical structure is fabricated by splicing a 7-mm length of seven-core fiber and two fiber peanuts, which is encapsulated with magnetic fluid in the capillary. The transmission spectrum of the MZI is modulated by the refractive index of the magnetic fluid, which is affected by the magnetic field. The experimental results indicate that the mode interference is sensitive to the magnetic field and different dips of the interference spectrum have different sensitivities. Hence, simultaneous measurement of the temperature and magnetic field can be achieved by simultaneously monitoring two different dips. The sensitivities of the magnetic field and temperature can reach up to 0.062 nm∕Gs and 0.081 nm∕°C, respectively. The proposed Mach–Zehnder interference sensor has a more compact structure, simple manufacturing process, and high reliability, which would find potential applications in weak magnetic sensing fields. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study is to propose a method to construct corneal biomechanical model which is the foundation for simulation of corneal microsurgery. Methods: Corneal material has two significant characteristics: hyperelastic and viscoelastic. Firstly, Mooney-Rivlin hyperelastic model of cornea obtained based on stored-energy function can be simplified as a linear equation with two unknown parameters. Then, modified Maxwell viscoelastic model of the cornea, whose analytical form is consistent with the generalized Prony-series model, is proposed from the perspective of material mechanics. Results: Parameters of the model are determined by the uniaxial tensile tests and the stress-relaxation tests. Corneal material properties are simulated to verify the hyper-viscoelastic model and measure the effectiveness of the model in the finite element simulation. On this basis, an in vivo model of the corneal is built. And the simulation of extrusion in vivo cornea shows that the force is roughly nonlinearly increasing with displacement, and it is consistent with the results obtained by extrusion experiment of in vivo cornea. Conlusions: This paper derives a corneal hyper-viscoelastic model to describe the material properties more accurately, and explains the mathematical method for determination of the model parameters. The model is an effective biomechanical model, which can be directly used for simulation of trephine and suture in keratoplasty. Although the corneal hyper-viscoelastic model is taken as the object of study, the method has certain adaptability in biomechanical research of ophthalmology. [ABSTRACT FROM AUTHOR]

Background. Artificial Neural Network (ANN), as a potential powerful classifier, was explored to assist psychiatric diagnosis of the Composite International Diagnostic Interview (CIDI). Method. Both Back-Propagation (BP) and Kohonen networks were developed to fit psychiatric diagnosis and programmed (using 60 cases) to classify neurosis, schizophrenia and normal people. The programmed networks were cross-tested using another 222 cases. All subjects were randomly selected from two mental hospitals in Beijing. Results. Compared to lCD-10 diagnosis by psychiatrists, the overall kappa of BP network was 0.94 and that of Kohonen was 0.88 (both P< 0.01). In classifying patients who were difficult to diagnose, the kappa of BP was 0.69 (P<0.01). ANN-assisted ClDl was compared with expert system assisted CIDI (kappa=0.72-0.76); ANN was more powerful than a traditional expert system. Conclusion. ANN might be used to improve psychiatric diagnosis. [ABSTRACT FROM AUTHOR]

A dual-wavelength erbium-doped fiber laser with bow-tie polarization maintaining regenerated fiber Bragg grating (PM-RFBG) as a sensor head is proposed. The PM-RFBG with enhanced thermal resistance is obtained through isothermal annealing treatment towards a saturated seed grating. The optical signal to noise ratio (SNR) is higher than 61 dB and the 3 dB line width is less than 18 pm at room temperature. The temperature characteristic of the laser sensor is investigated at temperatures ranging from 200 °C to 800 °C. The temperature sensitivities of the laser wavelength in fast (λf) and slow (λs) axes are 16.2 pm °C−1 and 15.1 pm °C−1, respectively. The simultaneous measurement of force and temperature could be realized at temperatures ranging from 200 °C to 400 °C, for the force sensitivities of λf and λs are 1.333 nm F−1 and 1.346 nm F−1, respectively. Due to the cross-sensitivities of temperature and force, the force sensitivity shows an approximately linear increase at temperatures between 400 °C and 600 °C. The dual-wavelength laser sensor is capable of operation at temperatures up to 800 °C due to the high optical SNR and narrow line width. [ABSTRACT FROM AUTHOR]

The characterization of a fiber Bragg grating in pure-silica-core and Ge-doped-core optical fiber is demonstrated under tensile strain at different temperatures. The peak wavelength of the fiber Bragg grating increases slowly under tensile strain of 1000 µε when the temperature exceeds a certain value. The wavelength response of the pure-silica-core fiber Bragg grating is less sensitive under the same high-temperature strain as compared with the Ge-doped-core one. The results show a little predominance for the pure-silica-core fiber Bragg grating in high-temperature strain sensing. The two kinds of fiber Bragg grating are capable of measuring temperatures of up to 800 °C, but it is recommended that strain measurements are conducted below 500 °C to ensure good stability and repeatability. [ABSTRACT FROM AUTHOR]