Xiaofu Li

Analysis of the Spectrum Distortions of Weak Fiber Bragg Gratings Fabricated In-Line on a Draw Tower by the Phase Mask Technique

Identical weak optical fiber Bragg grating (FBG) arrays can be fabricated in-line on a draw tower using the phase mask technique. On the FBG arrays, some grating spectrum distortions, for example, asymmetric side lobes or deformed side lobes, low extinction ratio of the main reflection peak and comb filter-type peaks, can be observed. We collected different distorted reflection spectra of the in-line-fabricated weak FBGs and compared them with the simulated spectra of weak FBGs. By comparing the experimental and theoretical weak FBG spectra, it is found that the different kinds of spectrum distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp. This analysis is helpful to improve the FBG array fabricating process by phase mask in-line on a draw tower. And we got good FBG arrays which have good central wavelength consistency, good reflectivity consistency and minor spectral distortion.

Layer-by-layer assembled highly reflective optical coatings

Polyelectrolyte/colloidal TiO2 composite films have higher refractive index than glass, while polyelectrolyte/colloidal SiO2 composite films have lower refractive index. By using a layer-by-layer assembling technique, polyelectrolyte/TiO2 nanoparticle-λ/4-layers and polyelectrolyte/SiO2 nanoparticle-λ/4-layers were alternatively deposited onto optical fibers and microscope glass slides. In this way, λ/4-high-reflection optical coatings were fabricated.

Theoretical calculations of crosstalk and time delay in identical FBG array in PM fiber

The spectral shadowing, multiple reflections and polarization crosstalk in an identical FBG array in PM fiber were theoretically studied. For a PM fiber FBG array (PMF-FBG array) with FBG peak reflectivity of -30 dB, grating spacing of 2 m and total fiber length of 2000 m, the reflection power of the PMF-FBG changed from -30.0 dB to -36.9 dB at the end of the array because of the spectral shadowing and the multiple reflections. Equations for the signal time delay of the PMF-FBG array were given. Higher birefringence in the PM fiber will result in a longer time delay.

Numerical analysis of a novel refractive index and temperature sensor based on a kagomé hollow-core photonic crystal fiber

A kagomé-lattice hollow-core photonic crystal fiber was fabricated, and based on it, a refractive index and temperature surface plasmon resonance sensor is proposed. The sensor consists of a layer of gold nanoparticles over the inner surface of the fiber core. Verified by theoretical calculations, the sensor possesses refractive index sensitivity of 1495 nm/RIU with resolution of 1.34×10−5 and temperature sensitivity up to 731.7 pm/°C. This kagomé fiber based sensor shows potentials for practical refractive index and temperature sensing.

Theoretical and Experimental Study on Crosstalk Caused by Multiple Reflections in Identical Weak Fiber Bragg Grating Array

Large scale distributed sensor networks based on fiber Bragg gratings (FBGs) can find a wide range of uses. However, when identical FBGs are connected in series, the crosstalk will occur because of multiple reflections among the FBGs. In this article, theoretical simulations were done on the crosstalk in an array with FBG peak reflectivity of -35dB. An array consisting of identical weak FBGs with peak reflectivities ranging from -33dB to -37dB and a spatial resolution of 2.5m was prepared in-line on a fiber draw tower. The testing results of the FBG array are in close agreement with the theoretical simulations and ghost gratings can be observed on the reflection spectrum of the FBG array.

Analysis of the spectrum distortions of optical fiber Bragg gratings fabricated in-line on a draw tower by the phase mask technique

Optical fiber Bragg grating (FBG) arrays can be fabricated in-line on a draw tower using the phase mask technique. On the FBG arrays, some grating spectrum distortions, for example, asymmetric side lobes or deformed side lobes, low extinction ratio of the main reflection peak and comb filter-type peaks, can be observed. We collected different distorted reflectance spectra of the in-line fabricated FBGs and compared them with the simulated spectra of FBGs. By comparing the experimental and theoretical FBG spectra, it is found that the different kinds of spectral distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp.

Producing regenerated gratings in hydrogen-loaded single mode fiber by heat treatment

Fiber Bragg grating (FBG) is a passive optical fiber component with the refractive index modulated along the fiber length and has been widely applied in fiber sensing systems. High-temperature stable fiber gratings are promising for uses at high temperature and receive extensive attentions. In this paper, FBGs were inscribed in hydrogen loaded standard single mode fibers with 248 nm excimer laser, and regenerated gratings were obtained through heat treatment. The center wavelength of the regenerated FBG has a good linearity with temperature, and the reflectivity of the regenerated FBG can almost keep unchanged at 800 °C.

Experimental Study on High Temperature Characteristic of Sn-Doped Fiber Bragg Grating

Fiber Bragg gratings (FBGs) were fabricated in Sn-doped single mode quartz fiber by UV exposure through phase mask. Linearity and repeatability of the FBGs were studied at temperature from 25 °C to 600 °C. The results of the study showed that linearity of the FBGs by high temperature heat treatment was better than that of sensor without standing heat treatment, and its linear correlation coefficient at least reached 0.997. Every time heating curve and cooling curve almost overlapped. Heating curves almost overlapped at different times. Cooling curves also were similar to heating curves at different times. Meanwhile, high temperature stability of the FBGs was further researched at 500 °C. It was showed that the reflectivity of the FBGs almost was keeping unchanging in decline to a certain value which depended on the preparation process of the FBGs after being heated 5 h at 500 °C. Therefore, the Sn-doped FBGs have excellent temperature sensing properties. It is possible to make low cost FBG sensors by using Sn-doped single mode quartz fiber in order to measure high temperature.

Reflection-enhancing coatings from layer-by-layer self-assembled polyelectrolyte/colloidal TiO2 multilayers

Colloidal TiO2 was prepared by hydrolyzing tetra-n-butyl titauate. Composite multilayer films of Poly(sodium 4-styrensulfonate), PSS, and colloidal TiO2 particles were electrostatically self-assembled onto optic fibers and microscope glass slides. As the PSS/TiO2 film was deposited onto the end face of a glass fiber, the reflected optic intensity periodically oscillated as the bilayer number of the film increased. After a 24-bilayer film was coated onto the both sides of a glass slide, the transmittance at 850 nm decreased more than 20%, which means that the film could serve the function as a reflection-enhancing coating. Data of X-ray diffraction and TEM electron diffraction analysis show that the main crystalline phase of the colloidal TiO2 particles is brookite and that the PSS/TiO2 films are polycrystalline films. Scratching experiments indicate that the composite films are of relatively high hardness.