The authors overview recent progress of fiber loop mirror based sensors in China Jiliang University. With the unique characteristics of fiber loop mirror made by various inserted fibers, fiber loop mirror based sensors have the potential to be exploited in a variety of wide applications.

The aim of this project was to develop non-contact fiber optic based displacement sensors to operate in the harsh environment of a “light gas gun” (LGG), which can “fire” small particles at velocities ranging from 1km/s.8.4km/s. The LGG is used extensively for research in aerospace to analyze the effects of high speed impacts on materials. Ideally the measurement should be made close to the center of the impact to minimize corruption of the data from edge effects and survive the impact. We chose to develop a non-contact “pseudo” confocal intensity sensor, which demonstrated resolution comparable with conventional polyvinylidene fluoride (PVDF) sensors combined withhigh survivability and low cost. A second sensor was developed based on “fiber Bragg gratings” (FBG) to enable a more detailed analysis of the effects of the impact, although requiring contact with the target the low weight and very small contact area of the FBG had minimal effect on the dynamics of the target. The FBG was mounted either on the surface of the target or tangentially between a fixed location. The output signals from the FBG were interrogated in time by a new method. Measurements were made on carbon fiber composite plates in the LGG and on low velocity impact tests. The particle momentum for the low velocity impact tests was chosen to be similar to that of the particles used in the LGG.

Scanning the absorption spectral line of water vapor through wavelength around 1368.597nm is successfully used to measure the value of micro-moisture content. The synchronous superposition average of original signal algorithm based on labview is innovated and applied to detecting weak spectrum absorption signal instead of low pass filter. Two data processing methods are used to get the concentration of water vapor in ppm: one is a general formula method which has newly deduced a general formula to calculate the concentration of gas with temperature and beam intensity ratio when the pressure is equal to or greater than 1atm; the other is engineering calibration method which is proved to have high resolution and accuracy with the fitted curve of beam intensity ratio and concentration in ppm when the temperature changes form 258K to 305K and the pressure ranges from 1atm to 5 atm.

A simple nanostrain direct current (DC) measurement system based on a chirped Bragg grating Fabry-Perot (FP) structure is presented. The FP cavity, formed between the chirped fiber Bragg grating (CFBG) and the fiber end face, presents an aperiodic behavior due to the CFBG. A laser located in the fringe pattern slope is used to interrogate the sensing head. The optical power parameter is analyzed when strain is applied, for long and short period fringe pattern wavelengths, and sensitivities of –2.87 μW/με and –5.48 μW/με are respectively obtained. This configuration presents a resolution of 70nε.

Inorganic silica-titania thin films with thicknesses 150 nm–200 nm are deposited on high purity and polished silicon wafer and silica glass substrates by sol-gel dipping process and are patterned by capillary force lithography technique. Subsequently grating structure is embossed in green stage. The patterned gel films are subjected to stepwise heat treatment to 500 ℃ and above in pure oxygen atmosphere in order to achieve major conversion of mixed-gel to oxide optical films which are characterized by Ellipsometry, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) to optimize the fabrication parameters and to get perfectly matched film. Removal of organics and formation of perfectly inorganic silica-titania network at optimized heat treatment in controlled environment are ensured by FTIR spectral study. The difference in refractive indices between the substrate and coated film as calculated theoretically matches exactly with the developed waveguides for operating wavelength (632.8 nm) and the measured optical properties show the planar waveguide behavior of the films.

The authors present a new design of high resolution and wide dynamic range photonic crystal pressure sensor. This sensor is based on two-dimensional photonic crystal with square array of silicon rods surrounded by air. The sensor consists of a photonic crystal waveguide which is coupled to a photonic crystal nanocavity. The waveguide is configured by removing one row of Si rods and nanocavity is formed by modifying the radius of one Si rod. The sensor is designed for 1300nm–1400nm wavelengths. Simulation results show that resonant wavelength of nanocavity is linearly shifted to larger wavelengths by increasing the pressure. The designed sensor has a linear behavior between 0.1GPa to 10GPa of applied pressure and 8nm/GPa of pressure sensitivity.

The authors review their recent advances in the development of optical fiber Bragg grating (FBG) sensor technologies. After a brief review of the fiber grating sensors, several newly developed FBG sensors are described. With the continuous development of fiber materials, microstructures and post-processing technologies, FBG sensors are still creative after the first demonstration of permanent gratings thirty years ago.

The combination of fiber optics with nano-structure technologies and sensitive thin films offers great potential for the realization of novel sensor concepts. Miniatured optical fiber sensors with thin films as sensitive elements could open new fields for optical fiber sensor applications. Thin films work as sensitive elements and transducer to get response and feedback from environments, in which optical fibers are employed to work as signal carrier. This article presents some research work conducted at the National Engineering Laboratory for Optical Fiber Sensing Technologies in recent years. Concrete examples are: Pd/WO3 co-sputtered coating as sensing material for optical hydrogen sensors shows robust mechanical stability and meanwhile good sensing performance; TbDyFe magnetostrictive coating directly deposited on fiber Bragg grating (FBG) demonstrates its possibility of miniature optical magnetic field/current sensors, and 40-pm shift of the FBG wavelength happens at a magnetic field order of 50mT.

An overview of recent researches of surface plasmon resonance (SPR) sensing technology in Laboratory of Science and Technology of Micro-Nano Optics (LMNO), University of Science and Technology of China, is presented. Some novel SPR sensors, such as sensors based on metallic grating, metal-insulator-metal (MIM) nanoring and optical fiber, are designed or fabricated and tested. The sensor based on localized surface plasmon resonance (LSPR) of metallic nanoparticles is also be summarized. Because of the coupling of propagating surface plasmons and localized surface plasmons, the localized electromagnetic field is extremely enhanced, which is applied to surface-enhanced Raman scattering (SERS) and fluorenscence enhancement. Future prospects of SPR and/or LSPR sensing developments and applications are also discussed.

We proposed two schemes of generating and localizing dynamic gratings in optical fibers: one is based on the gain saturation in erbium-doped fiber; the other is based on Brillouin scattering in the fiber. By using these dynamic gratings, fully distributed strain/temperature sensors have been demonstrated. In this presentation, we review the principles, basic schemes, and experimental demonstrations of the novel dynamic grating techniques.

The authors review the recent advances in fabricating long-period gratings (LPGs) in photonic crystal fibers (PCFs). The novel light-guiding properties of the PCFs allow the demonstration of novel sensors and devices based on such LPGs. The sensitivity of these PCF LPGs to temperature, strain and refractive index is discussed and compared with LPGs made on conventional single-mode fibers. In-fiber devices such as tunable band rejection filters, Mach-Zehnder interferometers are discussed.