Long period fiber grating is a kind of transmission type optical fiber grating. Due to the advantages such as low insertion loss, wide bandwidth, low-level reflection, high sensitivity, low cost and ease of compactness, LPFGs have been widely applied in optical fiber sensing and optical fiber communication. The Mode coupling of LPFG is the coupling between the fiber core mode and the cladding mode in the same transmission direction. If the ordinary LPFG is combined with bitaper or taper, we can effectively change the original LPFG's transmission spectrum to obtain the composite LPFG, which can stimulate new resonant peaks in the original wavelength-dependent transmission loss of the grating basis, thus applying to the dual-parameter simultaneously measuring field. We report a novel all-fiber narrow-bandwidth intermodal Mach– Zehnder interferometer (MZI) based on a long-period fiber grating (LPFG) combined with a fiber bitaper. The LPFG is written by high-frequency CO<sub>2</sub> laser pulses, and the bitaper is connected in series with the LPFG, forming the Mach– Zehnder interferometer (MZI). Experimental results indicate that the MZI has good temperature sensitivity, The temperature sensitivity of the two loss peaks are 55.35pm/°C and 48.18pm/°C respectively. The strain sensitivity of the two loss peaks are 3.35pm/με and -4.925pm/με respectively. By using the different temperature and strain response characteristics of the loss peaks, the temperature and strain measurement can be realized simultaneously. the proposed device has good repeatability and stability, which would be a promising candidate for precise dual-parameter sensing application.
We successfully fabricate and demonstrate a type of in-line M-Z interferometer by using single mode fiber and suspended dual-core hollow fiber. We fuse one section of dual-core hollow fiber (about 5-10cm length used as interferometer arms) with two part of single mode fiber by arc discharging on their cross sections. After this new M-Z interferometer’s fabrication and related experiments, we improved it by absorbing drops of alcohol in its dual-core hollow body which can do much of contributions to its sensitivity. With bunches of temperature experiments conducted, we find this new interferometer has a good temperature sensitivity highly as 1.751nm/°C (from 20 to 100 °C) and interferometer with alcohol inside has a better temperature sensitivity highly as 3.015nm/ °C (from 20 to 60 °C) respectively.
Long period fiber gratings (LPFGs), which can couple the core mode to the forward propagating cladding modes of a fiber and have the advantage of small additional loss, no backward reflection, small size, which is widely used in optical fiber sensors and optical communication systems. LPFG has different fabricating methods, in order to write gratings on the twin-core at the same time effectively, we specially choose electric heating fused taper system to fabricate asymmetric dual-core long period fiber grating, because this kind of method can guarantee the similarity of gratings on the twin cores and obtain good geometric parameters of LPFG, such as cycle, cone waist. Then we use bending test platform to conduct bending test for each of the core of twin-core asymmetric long period fiber grating. Experiments show that: the sensitivity of asymmetrical twin-core long period fiber grating’s central core under bending is -5.47nm·m, while the sensitivity of asymmetric twin-core long period fiber grating partial core changed with the relative position of screw micrometer. The sensitivity at 0°, 30°, 90° direction is -4.22nm·m, -9.84nm·m, -11.44nm·m respectively. The experiment results strongly demonstrate the properties of rim sensing of asymmetrical twin-core fiber gratings which provides the possibility of simultaneously measuring the bending magnitude and direction and solving the problem of cross sensing when multi-parameter measuring. In other words, we can detect temperature and bend at the same time by this sensor. As our knowledge, it is the first time simultaneously measuring bend and temperature using this structure of fiber sensors.
Helical core optical fiber is used to achieve high power fiber laser output and polarization state control, and it has become a hot issue of current research due to its unique characteristics of chiral medium. The key difficult point is critical concentric alignment between the actual rotational center and the optical fiber core in helical core fiber manufacturing process. This paper proposed a new alignment method based on the coordinates of centroid. That is, injecting light to the main and sub fiber core which is fixed on the rotating clamping apparatus, acquiring two coordinates of the energy center in output field spot image with the help of the microscopic imaging system by using the squared weighted centroid method. X-axis is defined as the two fiber core connection direction, which perpendicular direction is Y-axis, and the origin is the center of main fiber core, so the correspondence between the main fiber core XY coordinate system and the rotating clamping apparatus X<sub>0</sub>Y<sub>0</sub> coordinate system is available. Main fiber core rotation center coordinates X'Y' can be obtained by fitting, and the X<sub>0</sub>Y<sub>0</sub> adjustment of clamping apparatus can be also acquired by coordinate transformation. Experimentally measured eccentricity error can be controlled around 8μm, which achieved concentricity precise adjustment. The experimental device has advantages of small size, stable performance, high positioning accuracy, and this method overcomes the key technical difficulties of the helical core optical fiber preparation.
In this paper, we carry out a periodically taper experiment on standard single-mode optical fiber by electro-thermal fused taper system, and fabricate a long period fiber grating (LPFG). The resonance amplitude of the LPFG is -23dB. Then we have an experimental study about axial strain characteristics of the LPFG. In the axial stretching process of the grating, the period of the grating become larger, the diameter become smaller, and because of the photo-elastic effect, the refractive index of the core and the optical cladding change. Experimental results show that the resonant wavelength peak position of the LPFG had a good linear relationship with the axial strain. With the increase of axial strain, the resonance wavelength shift to short and the sensitivity is about -1.40pm/μepsilon.
Long period fiber grating transmission is the core guided mode coupling the transmission of the cladding mode, when the ambient temperature, strain, refractive index, bending changed, the resonance peaks of the long-period fiber grating are very sensitive to changes. Long period fiber grating as sensing elements has a strong advantage and a wide range of applications. In this paper, we use a method of electro-thermal melting tapering to draw some of the columns cone on the fiber to form a periodic structure, and then prepare a long period fiber grating. On this basis, we use super-continuum light source and the spectrometer with a range of 600-1700nm wavelength, and build a period fiber grating bending experiments systems.