Transmission Spectrum Of Long-period Grating Research Articles

Towards the design of a wideband reflective long period grating distributed sensor

In this paper, we computationally investigate the effects of metal coating length and coating coverage on the reflected spectrum of a long period grating (LPG) over a broad bandwidth. Simulation results indicate that coating the tail end of the fiber between the LPG and the end facet of the fiber provides a reflected spectrum that mimics the LPG transmission spectrum shape over a 400 nm bandwidth. Based on single LPG simulation results, we present the design of a distributed LPG structure containing a multiple number (n) of LPGs in reflection mode for the first time. Simulation results for n = 1, 2, and 3 are presented here to demonstrate the concept of a distributed reflective LPG design. It is expected that such a sensor will open a new window for distributed sensing using reflective LPGs.

Water-Induced Fused Silica Glass Surface Alterations Monitored Using Long-Period Fiber Gratings

Long-period gratings (LPGs) induced in optical fibers show exceptional refractive index (RI) sensitivity and thus they are often applied for label-free biosensing. However, during measurements at changing environmental conditions, the surface of the fiber made of fused silica is subjected to stresses that can cause alterations in its structure. The problem is particularly important when long-term or biosensing measurements are considered and when sensor surface may be exposed to subsequent drying and immersion in aqueous solutions. The aim of this paper was to investigate the influence of sequential drying in air and soaking in water of the fused silica fiber cladding surface on the optical response of LPGs. We demonstrated that transmission spectrum of LPG measured in water before and after drying in air differed significantly and further changed with subsequent drying steps indicating increase of the local RI. All these changes correspond to the alterations in the cladding surface, i.e., its fracturing and pores rearrangement, caused by the stress induced during drying and then immersing in water, as well as possible corrosion and formation of nanosized objects on the cladding surface. The effect can be additionally influenced by washing the samples in organic solvents. We confirmed that measurements done in a flow-cell system, where the sensor was kept wet during the sensing experiment may eliminate the water-induced fiber surface alteration effect and thus minimize the amount of false results. Described findings are highly important for biosensing applications of any optical-fiber-based devices.

Improved diamond‐like carbon coating deposition uniformity on cylindrical sample by its suspension in RF PECVD chamber

The paper discusses the uniformity of diamond‐like carbon (DLC) thin overlays deposited with radio frequency plasma‐enhanced chemical vapor deposition (RF PECVD) method on optical fibers with induced long‐period gratings (LPGs). When refractive index around the LPG increases, a spectral response of the LPG strongly depends on properties of the overlay. Using LPG suspended in the RF PECVD chamber, we have found that DLC thickness distribution around the fiber highly depend on a distance between the LPG and the electrode. More asymmetrical deposition, which takes place when the LPG is located closer to the electrode, induces appearance of a series of additional resonances in LPG transmission spectrum. It has been found that for the fiber suspended 7.2 mm over the electrode the coating has the highest uniformity. Higher uniformity obtained by fiber suspension in the chamber has been compared with the effect resulting from fiber rotation between two consecutive processes. It is shown that more symmetrical deposition is obtained for the suspended samples than for the rotated ones.

Optical fiber long period grating sensor with a polyelectrolyte alternate thin film for gas sensing of amine odors

The formation of poly(acrylic acid) (PAA) and poly(allyamine hydrochloride) (PAH) multilayers on an optical fiber long period grating (LPG) by the layer-by-layer (LbL) assembly technique for highly sensitive ammonia gas detection is reported. Observation of the LPG's transmission spectrum indicated that the refractive index (RI) of the PAH/PAA alternate layer film was changed on exposure to ammonia, possibly via a change in the structure of the polyelectrolyte multilayers. PAA could act as a receptor for binding of amine compounds including ammonia, which would induce changes in the coating properties such as optical thickness (OT), film thickness/density and electrostatic interaction, thus influencing the transmission spectrum of the LPG. The ammonia binding is based on the acid–base interaction to free carboxylic acid groups of PAA. An LPG of period 100μm with a 7-cycle PAH/PAA coating exposed to ammonia exhibited a limit of detection of 10.7ppm. Film morphology and thickness changes due to the binding of ammonia gas, explaining the sensing mechanism, were confirmed through atomic force microscopic (AFM) measurements.

Refractive index biosensor based on microstructured optical fiber long-period gratings: a theoretical analysis

A kind of label-free refractive index biosensor based on long-period gratings (LPGs) inscribed in microstructured optical fibers (MOFs) is theoretically studied. The finite element method is used to visualize the mode field distribution along the grapefruit MOF. The transmission spectrum of LPG in the air or liquid is simulated by the coupled-mode theory. An immobilized biorecognition molecules layer on the inner walls of the holes in the MOF provides the ability to capture analyte molecules selectively. Numerical results show that an ultrahigh refractive index sensitivity of approximately 397.3  nm/RIU over the refractive index range of 1.4–1.5 is achieved. Moreover, the sensitivity of the grapefruit MOF can be improved by reducing the periods of gratings. In addition, the shift of the resonant wavelength is linear to the analyte thickness with good sensitivity about 0.6  nm/nm. It is expected to play a great role in theoretical guidance for further development of biosensors.

Design of turn-around-point long-period gratings in a photonic crystal fiber for refractometry of gases

A fiber-optic long-period grating (LPG) operating near the dispersion turning point in its phase matching curve, referred to as a turn-around-point (TAP) LPG, is known to be highly sensitive to external parameters. So far TAP LPGs have been realized in specially designed or post-processed conventional fibers, not yet in PCFs, which allow a great degree of freedom in engineering the fiber's dispersion properties through the control of the PCF structural parameters. We have developed the design optimization technique for TAP PCF-LPGs employing the finite element method for PCF modal analysis in a combination with the Nelder–Mead simplex method for minimizing the objective function based on target-specific PCF properties. Using this tool TAP PCF-LPGs can be designed for specified wavelength ranges and refractive indices of medium in the air holes. We present here the dispersion optimization of PCF-LPG for refractometry of gases. The resonant wavelength of an optimized PCF-LPG is subsequently highly sensitive to the refractive index of gaseous analytes, however, its large shifts could be detected with a substantially reduced resolution because the resonance dip in the TAP LPG transmission spectrum is broad. We have resolved the resolution problem by proposing an interferometric scheme based on twin TAP LPGs.

A highly‐birefringent fiber loop mirror temperature sensor demodulation based on a long‐period grating in photonic crystal fiber with differential processing

AbstractA highly‐birefringent fiber loop mirror temperature sensor demodulation based on a long‐period grating (LPG) in photonic crystal fiber (PCF) with differential processing was proposed.Utilizing the property temperature insensitive and the filtering function of the LPG in PCF, the intensities of two signals located respectively within the positive and negative linear region of the LPG's transmission spectrum can be monitored simultaneously and accurately. The noise of fiber sensors is eliminated effectively by the algorithm of two signals, since the two signals (also including the noise of fiber sensors) transmit through the same way to the monitor. Experimental results show that the value of the algorithm remains constant when the power of light source varies ±10% and the relative error between the measured temperature change and the true change is only 0.34%. Compared with the sensing without the algorithm, the accuracy increases from∼97.3 to ∼99.7%. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:176–179, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26460

Reconstruction of parameters of an optical fiber from the transmission spectrum of long-period gratings induced in this fiber

An optimization technique is developed to determine parameters of a fiber from the transmission spectrum of long-period gratings written in this fiber and to identify the type of the cladding modes exited by the gratings. The technique is based on fitting the experimental dependence of resonance wavelengths on grating period or cladding radius by theoretical calculation. The resolutions that can be achieved by this method are 10nm in the core radius, 100nm in the cladding radius, and 10−5 in the refractive index.

Theoretical analysis and numerical calculation for the transmission spectrum of long-period fiber gratings with a rectangular index modulation

In this paper, we present a theoretical method to analyze the spectral characteristics of long-period fiber gratings (LPGs) with a rectangular index modulation. Furthermore, we numerically calculate the transmission spectrum of a long-period grating written in Corning FlexcorTM 1060 fiber. The calculation results agree with the experimental results.

Evanescent wave long-period fiber bragg grating as an immobilized antibody biosensor.

An immunosensor using a long-period grating (LPG) was used for sensitive detection of antibody-antigen reactions. Goat anti-human IgG (antibody) was immobilized on the surface of the LPG, and detection of specific antibody-antigen binding was investigated. This sensor operates using total internal reflection where an evanescent field interacts with bound antibody immobilized over the grating region. The reaction between antibody and antigen altered the LPG transmission spectrum and was monitored in real time as a change in refractive index, thereby eliminating the need for labeling antigen molecules. Human IgG binding was observed to be concentration dependent over a range of 2-100 microg mL-1, and equilibrium bound antigen levels could be attained in approximately 5 min using an initial rate determination. Binding specificity was confirmed using human interleukin-2 and bovine serum albumin as controls, and nonspecific adsorption of proteins did not significantly interfere with detection of binding. Antigen detection in a heterogeneous protein mixture and in crude cell lysate from Escherichia coli was also confirmed. Moreover, regeneration of the LPG surface via diethylamine treatment resulted in approximately 80% removal of bound antigen. Subsequently, fibers reexposed to antigen retained greater than 85% of the initial signal after five consecutive regeneration cycles.

Effects of hydrogen molecule diffusion on LP 0m mode coupling of long-period gratings

We analyzed the effects of hydrogen molecule diffusion on LP 0 m mode coupling of long-period gratings with two assumptions. The fibers were treated in a H 2 chamber at a temperature 90°C and pressure around 107 atm for five days. And then we fabricated long-period gratings with these fibers after holding them at room temperature and one atmosphere pressure for 15 min to 55 h and measured coupling of LP 0 m mode. We used diffusion coefficient of hydrogen molecule, which considered the fiber as a perfect cylinder, and assumed that the core and the cladding have the same diffusion coefficient. Corning ‘Flexcor1060’ was used as a photosensitive fiber which has cutoff wavelength, 920 ± 50 nm, and the cladding/coating diameter, 125 ± 2.0 μm/250 ± 15 μm. Core diameter is 5.0 μm and numerical aperture is 0.14. The hydrogen molecule concentration is averaged in the region of the core and the cladding with time and LP 0 m mode coupling was simulated with coupled mode theory and two assumptions. We were able to predict the coupling peak wavelengths that change with time. These results were useful for designing the transmission spectrum of long-period gratings.