Highly sensitive detection of low-concentration sodium chloride solutions based on polymeric nanofilms coated long period fiber grating.

Long period fiber gratings (LPFGs) have special advantages in the detection of salt concentrations due to small volume, corrosion resistance and immunity to electromagnetic interference. However, it is very difficult to distinguish low-concentration salt solutions with usual LPFGs owing to the poor sensitivity. In this paper, the detection capability of the LPFG to low-concentration salt solutions was significantly improved by assembling salt-containing poly (diallyldimethylammonium chloride) (PDDA) and salt-containing poly (sodium-p-styrenesulfonate) (PSS). Experimental results showed that, the responsive wavelength range of the LPFG was remarkably broadened in low-concentration salt solutions after assembling nanofilms. The suitable detection range of the PDDA/PSS films coated LPFG for salt concentrations was 0-3%. In such a range, the average refractive index sensitivity and the average salinity sensitivity of the LPFG was as high as 29545.9 nm/RIU and 52.2 nm/% respectively. Compared with the LPFG without nanofilms, the discrimination ability of the PDDA/PSS films coated LPFG to 0-3% salt solutions increased by 568 times. The analysis demonstrated that PDDA and salt in the assembly solutions played a pivotal role in the above effects. The proposed sensor has extensive application prospects in the monitoring of salt concentration in many fields such as seawater, food processing, fermentation process, etc.

Extremely sensitive multi-order mode refractive index sensor using TiO2 nanograss film and weakly bounded waveguide modes.

An extremely sensitive multi-order mode refractive index (RI) sensor was fabricated by coupling titanium dioxide nanograss film coated FTO conductive glass with Kretschmann prism. Both calculation and experimental studies were carried out. Theoretical analysis by employing resonant waveguide modes indicated that the maximum sensitivity could be achieved when the mode worked at the weakly-bounded condition. The experimental results showed that for p-polarized and s-polarized light, the sensor exhibited a maximum RI sensitivity of 2938.21 nm/RI unit (RIU) and 1484.39 nm/RIU in the 1st order mode, respectively. Its maximum figure of merit was as high as 77.77. The proposed sensor is promising to be applied in environmental monitoring, immune analysis, nucleic acid test, etc.

Highly sensitive refractive index sensor based on a TiO2 nanowire array.

We propose a novel, highly sensitive refractive index (RI) sensor by means of combining the Kretschmann prism with a TiO2 nanowire array and do not use a metallic layer in the Kretschmann configuration. Its RI sensing performance was investigated through measuring different concentrations of sodium chloride solution. Experimental results showed that, with increasing RI of liquid, the resonant wavelength in the reflectance spectrum redshifted gradually in the visible light range. There was a very good linear relationship between resonant wavelength and RI in the range of 1.3330 to 1.3546. More importantly, in contrast to the surface plasmon resonance (SPR) sensor, the interferometric sensors showed higher sensitivity to the external RI. In the case of the transverse magnetic mode, the RI sensitivity is up to 320,700.93 a.u./RIU (refractive index unit) by expression of light intensity, which is 9.55 times that of the SPR sensor. As for the transverse electric mode, it achieves 4371.76 nm/RIU by expression of the resonant wavelength, which is increased by a factor of 1.4 in comparison with the SPR sensor. Moreover, the experimental results have favorable repeatability. A TiO2 nanowire array sensor has also other advantages, such as easy manufacturing, low cost, and in situ determination, etc. To our knowledge, this fact is reported for the first time. It has great potential applications in the field of biological and chemical sensing.

An Ultrasensitive Long-Period Fiber Grating-Based Refractive Index Sensor with Long Wavelengths.

The response of a novel long-period fiber grating (LPFG) with a period of 180 µm to a surrounding refractive index (RI) was investigated. The results displayed that, with the increase in RI of the surrounding media of cladding glass in the grating region, the resonant peak located at 1336.4 nm in the transmission spectrum gradually shifts towards a shorter wavelength, while the resonant peak located at 1618 nm gradually shifted towards a longer wavelength. Moreover, the resonant peak at 1618 nm is much more sensitive to the surrounding RI than that of the one at 1336.4 nm. Compared with the conventional LPFG and other types of wavelength-interrogated RI sensors, such as ring resonators, surface plasmon resonance sensors, and Fabry-Perot interferometric sensors, this novel LPFG possesses a higher sensitivity, which achieved 10,792.45 nm/RIU (RI unit) over a RI range of 1.4436-1.4489.

[Spectral characteristics of refractive index based on nanocoated optical fiber F-P sensor].

An optical fiber Fabry-Perot (F-P) interferometer end surface was modified using layer-by-layer assembly and chemical covalent cross linking method, and the refractive index (RI) response characteristics of coated optical fiber F-P sensor were experimentally studied. Poly diallyldimethylammonium chloride (PDDA) and sodium polystyrene sulfonate (PSS) were chosen as nano-film materials. With the numbers of layers increasing, the reflection spectral contrast of optical fiber F-P sensor presents from high to low, then to high regularity. And the reflection spectral contrast has good temperature stability. The reflection spectra of the optical F-P sensor coated with 20 bilayers for a series of concentration of sucrose and inorganic solution were measured. Experimental results show that the inflection point extends from 1.457 to 1.462 3, and the reflection spectral contrast sensitivity to low RI material and high RI material is 24.53 and 3.60 dB x RI(-1), respectively, with good linearity. The results demonstrate that the functional coated optical F-P sensor provides a new method for biology and chemical material test.

TiO2 nanoparticle thin film-coated optical fiber Fabry-Perot sensor.

In this paper, a novel TiO(2) nanoparticle thin film coated optical fiber Fabry-Perot (F-P) sensor had been developed for refractive index (RI) sensing by monitoring the shifts of the fringe contrast in the reflectance spectra. Using in situ liquid phase deposition approach, the TiO(2) nanoparticle thin film could be formed on the fiber surface in a controlled fashion. The optical properties of as-prepared F-P sensors were investigated both theoretically and experimentally. The results indicated that the RI sensitivity of F-P sensors could be effectively improved after the deposition of nanoparticle thin-films. It was about 69.38 dB/RIU, which was 2.6 times higher than that of uncoated one. The linear RI measurement range was also extended from 1.333~1.457 to 1.333~1.8423. More importantly, its optical properties exhibited the unique temperature-independent performance. Therefore, owing to these special optical properties, the TiO(2) nanoparticle thin film coated F-P sensors have great potentials in medical diagnostics, food quality testing, environmental monitoring, biohazard detection and homeland security, even at elevated temperature.