Long period fiber grating for biosensing: an improved design methodology to enhance add-layer sensitivity

We present our theoretical study on the design of LPFG sensor where its add-layer sensitivity is enhanced. addlayer sensitivity quantifies the sensitivity of the sensor to the changes taking place within few tens of nanometers around the receptor molecules. Two different methodologies: the use of dual overlay layer and tailoring of the intermodal separation between two cladding modes, have been used to enhance the add-layer sensitivity. Using coupled mode analysis we compute several examples to carry out a detailed comparative analysis between the results obtained, focusing on the cladding mode near mode transition.This work was supported by Council of Scientific Research (CSIR), India during the 12th Five Year Plan. Project Nos. ESC-0102 and ESC-0110. The author I. D. Villar thankfully acknowledge the support of the Spanish Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) (TEC2016-78047-R) and the Government of Navarre through the project with reference 2017/PI044

Sensitivity Analysis of Sidelobes of the Lowest Order Cladding Mode of Long Period Fiber Gratings at Turn Around Point

A new methodology to enhance the sensitivity of a long period fiber grating sensor (LPFG) at the Turn Around Point (TAP) is here presented. The LPFG sensor has been fabricated by etching the fiber up to 20.4 mu m, until the sidelobes of dispersed LP0,2 cladding mode appeared near TAP in aqueous medium. The dual peak sensitivity of the sidelobes was found to be 16,044 nm/SRIU (surrounding refractive index units) in the RI range from 1.333 to 1.3335

Long-Period Fiber Grating Probe: An Improved Design Suitable for Biosensing

Add-layer sensitivity of long-period fiber grating (LPFG) near mode transition (MT) has been studied in reflection configuration to realize a highly sensitive LPFG-based sensor probe suitable for biosensing. The dependency of the sensitivity on the separation between the resonant cladding modes considered during MT has also been investigated. The separation between two consecutive cladding modes was precisely increased or stretched to a desired value by reducing the cladding diameter. The probe was formed by cleaving the mode stretched LPFG (MSLPFG) through the grating region. The MSLPFG was designed to operate near MT by deposition of electrostatic self-assembled (ESA) polymeric overlay layers on the grating surface. We demonstrate that the polymeric overlay not only increases the sensitivity of the sensor but also eliminates undesirable multiple resonant bands that appear due to the introduction of arbitrary phase while cleaving the grating. Add-layer sensitivity of the MSLPFG sensor probe was obtained and found to be similar to 2.0 nmWL/nmTH with a significantly enhanced peak attenuation of similar to -30 dB around the MT region

Design and realization of a femtosecond-laser-inscribed fiber Bragg grating for accurate measurement of liquid level and liquid density

An exceptionally sensitive liquid level and density sensor system using Femtosecond-Laser-Inscribed Fiber Bragg Gratings is designed and demonstrated utilizing the basic Archimedes' Law of Buoyancy. The sensor works based on principles of Archimedes' law of buoyancy and basic strain sensitivity of Fiber Bragg Grating (FBG). A cylindrical mass of 22.5 cm long is suspended from one end of the FBG and whereas the other end of the fiber is fixed. The mass is partially immersed into the liquid under consideration to measure the liquid level as well as liquid density. Following Archimedes' law of buoyancy, the relative weight of the suspended mass gets reduced and the corresponding shift in Bragg wavelength is measured using an interrogator connected at the fixed end of the sensor lead fiber. The relative weight of the immersed mass also depends on the liquid density. Therefore, a change in liquid density also is reflected by the shift in Bragg wavelength. The system can be calibrated to measure the accurate liquid level and liquid density. A liquid (water) level measurement sensitivity of 5.47 to 5.60 pm/mm is achieved using the system under three times of consecutive experiments which is very close to the simulated value (5.9 pm/mm). A density measurement sensitivity of - 0.71 nm/(gm/cm3) is calculated in this system and experimentally achieved sensitivity is - 0.7 nm/(gm/cm3). The LOD (limit of detection) was also found to be 1.008 gm/cc for the density measurement and the average error during the measurement was 0.0021 nm/(gm/cm3).It is also shown that the density measurement sensitivity is highly dependent on the amount of immersed portion of the suspended mass and more amount of immersed mass is desirable for better sensitivity

Long Period Fiber Grating Near Turn Around Point: Suitable Design for Bio-Sensing

We report our experimental study on the add-layer sensitivity of LP0,11 cladding mode (CM) of Long Period Fiber Grating (LPFG) near turn around point (TAP). The CM response was tuned by controlled modification of the cladding diameter and deposition of overlay layers of polymer material. In this process an add-layer sensitivity of 2.308 nm(WL)/nm(TH) has been achieved. It is interesting to observe that add-layer sensitivity obtained in this process is close to the sensitivity of the same CM operated nearthe mode transition (MT). The advantage is that, the sensor designed as per the proposed methodology will not be plagued by the basic limitations of operating a CM near MT, i.e. loss of contrast of the resonant band and limited dynamic range. In our experiment, the dynamic range has found to be similar to 100 nm where the contrast remains greater than -10 dB throughout the range of measurement which is practically impossible to get for any CM operating at MT. The experimental result was supported by theoretical calculations. Also the SRI sensitivity of the sensor was found out to be 6400 nm/SRIU

Realization of Long Period Fiber Grating in Reflection Mode Operating Near Turn Around Point

A method of fabrication of a reflection-type long-period fiber grating (LPFG) operating near turn around point has been presented along with theoretical details. We have shown that removal of multiple resonant bands due to unwanted phase shift introduced after cleaving the grating at an arbitrary location can be accomplished by tailoring the phase matching curve of the cladding mode. This also allows us to enhance the sensitivity of the sensor. The grating can be cleaved at any arbitrary location within the grating structure. Therefore, the probe-type sensor head can be made considerably small. We have presented performance characteristics in detail. The sensitivity of the device has been found to be similar to 1300 nm/RIU and is superior to majority of the reflection-type LPFG sensors where mostly lower order cladding modes were considered

Investigations on the add-layer sensitivity near mode transition of a stretched mode long period fiber grating

The influence of the spectral separation between two consecutive resonant cladding modes of long period fiber grating (LPFG) on the add-layer sensitivity around mode transition (MT) has been investigated with a view to enumerating the dependency of the sensitivity on the separation between the resonant wavelengths of cladding modes. The separation between two consecutive resonant modes was increased or stretched by etching, and the specific mode of interest was brought to the MT region by the deposition of electrostatic self-assembled (ESA) layers of polymeric material on the sensor surface. The add-layer sensitivity of the mode-stretched LPFG (MSLPFG) operating around the linear part of the MT region was found to be similar to 2.59 nm(WL)/nm(TH). The result was found to be superior almost by 1.7 times as compared to a similar LPFG sensor at MT, but without mode stretching. Theoretical modelling has also been shown to corroborate the experimental results

Wideband Fiber Bragg Grating Accelerometer Suitable for Health Monitoring of Electrical Machines

A fiber Bragg grating (FBG) accelerometer suitable for health monitoring of electrical machines has been proposed. A polymer composite material is used to design the cantilever of the accelerometer. Thus it is ensured that there are no conducting parts in the accelerometer. Detailed finite element analysis of the cantilever is done to optimize the sensitivity and the resonant frequency. The sensitivity is significantly improved compared to the conventional FBG accelerometer having similar bandwidth of operation. Two different configurations, one having sensitivity 14.4 pm/g with resonant frequency 444 Hz, and the other having sensitivity 7.5 pm/g with resonant frequency 940 Hz have been designed and fabricated. The wide band optical accelerometers will enable monitoring of vibration modes of critical components of electrical machines and more importantly using commercially available standard FBG interrogators, instead of using any complex interrogation techniques. A good match between the simulation and experimental results was observed

Design of long period fiber grating with optimal sensitivity for detecting adhesion of nano-layer on the fiber surface

For detecting bio-molecular interaction using long period grating (LPG) we believe that a quantitative data concerning sensitivity for addition of layers on the surface and subsequently to optimize the same appears to be more usefull than defining LPG sensitivity for a surrounding refractive index change in bulk form. For the first time, to the best of our knowledge, we quantify the shift of resonant wavelength (Delta lambda(res)) of the mode of interest around the transition point as a function of unit bi-layer thickness (Delta d) of poly-electrolyte, deposited by ionic self assembly, and subsequently optimize the sensitivity Delta lambda(res)/Delta d. Experimental result show that a shift of similar to 12.5 nm/bi-layer is possible with optimum number of bi-layer deposition

Design of turn around point long period fiber grating sensor with Au-nanoparticle self monolayer

In this paper studies on the design and fabrication of a long period fiber grating (LPFG) with a self mono layer of gold nanoparticle (AuNP) has been presented. Refractive index (RI) sensitivity of a dispersed cladding mode (DCM) near turn around point (TAP) of its phase matching curve (PMC) has been investigated with and also without AuNP coated LPFG. The typical role played by the intermediate layer of AuNP on the effective index and thus on the sensitivity of the cladding mode to the surrounding RI has also been explored by carrying out coupled mode analysis of the requisite multilayer waveguide. Deposition of AuNP enhanced the sensitivity by more than a factor of 2. Measured sensitivity was found to be 3928 nm/refractive index unit (RIU) in the range of 1.3333-1.3428. (C) 2017 Published by Elsevier Ltd