Negative-index fiber Bragg gratings (FBGs) were fabricated using 800 nm femtosecond laser overexposure and thermal regeneration. A positive-index type I-IR FBG was first inscribed in H2-free fiber with a uniform phase mask, and then a highly polarization dependent phase-shifted FBG (PSFBG) was created from the type I-IR FBG by overexposure. Subsequently, the PSFBG was annealed at 800 °C for 12 hours. A negative-index FBG was obtained with a reflectivity of 99.22%, an insertion loss of 0.08 dB, a blue-shift of 0.83 nm, and an operating temperature of up to 1000 °C.
We reported a Bragg grating inscribed in gold-coated fiber (FBG) by NIR femtosecond laser (fs) for space application. Gold coating can shield the FBG from ultraviolet radiation and oxygen atom erosion. Cryogenic test, high temperature test, and gamma irradiation test were carried out. The reflectivity of the H2-free FBG remained stable at ± 120 °C for 100 h or with 50.4 krad γ irradiation, and the central wavelength shifted within 5 pm and 1.6 pm respectively. Regeneration of the fs-FBG was observed in case the FBG was annealed at 800 °C for 5 h, and the remained 5% in reflectivity after 19 h. Such fs-FBGs inscribed in gold-coated fiber could be employed as high performance fiber sensors for space application.
We investigated experimentally liquid crystal (LC) filled photonic crystal fiber’s temperature responses at different temperature ranging from 30 to 80°C. Experimental evidences presented that the LC’s clearing point temperature was 58°C, which is consistent with the theoretical given value. The bandgap transmission was found to have opposite temperature responses lower and higher than the LC’s clearing point temperature owing to its phase transition property. A high bandgap tuning sensitivity of 105 nm/°C was achieved around LC’s clearing point temperature.
We demonstrated an ultrasensitive temperature sensor based on a unique fiber Fabry-Perot interferometer (FPI). The FPI was created by means of splicing a mercury-filled silica tube with a single-mode fiber (SMF). The FPI had an air cavity, which was formed by the end face of the SMF and that of the mercury column. Experimental results showed that the FPI had an ultrahigh temperature-sensitivity of up to -41 nm/°C, which was about one order of magnitude higher than those of the reported FPI-based fiber tip sensors. Such a FPI temperature sensor is expected to have potential applications for highly-sensitive ambient temperature sensing.
Fiber Bragg gratings (FBGs) in gold-coated SMF have been successfully inscribed with NIR femtosecond laser and a phase mask for high temperature sensing application. The spectrums of FBGs inscribed by femtosecond laser are broader and asymmetrical with flat-toped profile which degrades the accuracy of FBGs interrogation with common peak detection techniques. A smart interrogation algorithm based on pattern matching (PMSIA) is reported in this paper. In this algorithm, an adjustable fitting spectrum template was proposed which enables the ability to suit for various spectrum patterns was proposed. The results of simulation and experiment demonstrate the noise immunity and threshold reliability of PMSIA. Less than 7pm interrogation error PMSIA was obtained even if the spectrum changes greatly in the very large sensing temperature range (up to 700°C).
An improved arc discharge technique was demonstrated to inscribe high-quality LPFGs with a resonant attenuation of - 28 dB and an insertion loss of 0.2 dB by use of a commercial fusion splicer. Such a technique avoids the influence of the mass which is prerequisite for traditional technique. Moreover, no physical deformation was observed on the LPFG surface. Compared with more than 86 grating periods required by traditional arc discharge technique, only 27 grating periods were required to inscribe a compact LPFG by our improved arc discharge technique.
We demonstrated a high-sensitivity strain sensor based on an in-line Fabry-Perot interferometer with an air cavity whose was created by splicing together two sections of standard single mode fibers. The sensitivity of this strain sensor was enhanced to 6.02 pm/με by improving the cavity length of the Fabry-Perot interferometer by means of repeating arc discharges for reshaping the air cavity. Moreover, such a strain sensor has a very low temperature sensitivity of 1.06 pm/°C, which reduces the cross-sensitivity problem between tensile strain and temperature.