Optical Fiber Long-period Grating Research Articles

Superlens-Assisted laser nanostructuring of Long Period optical fiber Gratings (LPGs) for enhanced refractive index sensing

We introduce an innovative approach to enhance refractive index sensing utilizing Long Period Optical Fiber Gratings (LPGs) processed through microsphere-assisted superlens laser nanostructuring technology. This method involves the self-assembly of a silica microsphere monolayer on the outer surface of LPGs, followed by pulsed laser irradiation to create nanoholes (with a diameter of 300–500 nm) on the surface, for forming the nanohole-structured LPGs (NS-LPGs). The sensing capabilities of NS-LPGs, featuring two different nanohole densities (5 % and 7.9 %), were experimentally evaluated in sucrose and glycerin solutions. The results revealed a notable improvement in sensitivity, with increases of 16.08 % and 19.57 %, respectively, compared to conventional LPGs. This suggests that higher nanohole density contributes to greater enhancement in refractive index sensing. Furthermore, the permanent nanohole structures showed improved durability and lifespan in various environments compared to surface-coating-based LPGs. Further enhancements can be achieved by refining the nanostructuring density and controlling the dimensions of the nanoholes.

Simultaneous Measurement of Thermal Gradient and Average Temperature in Concrete Blocks Using Nonuniform Long-Period Optical Fiber Gratings

A recent increase in structural integrity requirements has lead to a higher demand for accurate heat flux and temperature readings of building material substrates. In the present work, simultaneous two parameter measurements of the average temperature and the temperature difference in concrete blocks are reported. To this end long period gratings (LPGs) were commissioned in a boron doped photosensitive optical fiber using a pulsed CO₂ laser. The average temperature and the temperature differences were shown to be a linear combination of the spectral shifts of the center wavelength and the changes in the variations in transmitted power <i>I</i>_c minimums. The sensitivity and cross-sensitivity coefficients of the sensitivity matrix are also reported.

Refractive Index Sensors Based on Long-Period Grating in a Negative Curvature Hollow-Core Fiber.

Long-period optical fiber gratings (LPGs) are one of the widely used concepts for the sensing of refractive index (RI) changes. Negative curvature hollow-core fibers (NCHCFs), with their relatively large internal diameters that are easy to fill with liquids, appear as a very interesting medium to combine with the idea of LPGs and use for RI sensing. However, to date, there has been no investigation of the RI sensing capabilities of the NCHCF-based LPGs. The results presented in the paper do not only address this matter, but also compare the RI sensitivities of the NCHCFs alone and the gratings. By modeling two revolver-type fibers, with their internal diameters reflecting the results of the possible LPG-inscription process, the authors show that the fibers’ transmission windows shift in response to the RI change, resulting in changes in RI sensitivities as high as −4411 nm/RIU. On the contrary, the shift in the transmission dip of the NCHCF-based LPGs corresponds to a sensitivity of −658 nm/RIU. A general confirmation of these results was ensured by comparing the analytical formulas describing the sensitivities of the NCHCFs and the NCHCF-based LPGs.

Discretized superimposed optical fiber long-period gratings.

A novel technique, to the best of our knowledge, for the inscription of superimposed long-period gratings with arbitrary grating pitches is proposed and experimentally validated. The technique is based on the discretization of an ideal continuous sinusoidal refractive index (RI) pattern with a step function. The RI variation is induced by means of the irradiation of a photosensitive fiber with a 248 nm UV laser beam. The nonlinear relation between the induced RI change and the UV fluence was experimentally derived. Two superimposed long-period grating (LPGs) with different grating pitches have been realized with the discretization technique; the transmission spectrum was compared with that of two superimposed LPGs obtained with the traditional square wave RI modulation. The validity of the proposed technique was demonstrated by the better spectral characteristics of the discretized superimposed LPGs.

Bacteriophage Adhesin-Coated Long-Period Grating-Based Sensor: Bacteria Detection Specificity

In this paper, we study a label-free detection specificity of an optical fiber long-period grating (LPG) biosensor working near the dispersion turning point of higher order cladding modes. The LPG sensor is functionalized with bacteriophage adhesin and tested with specific and nonspecific bacteria dry weights. It is shown that that such a biosensor is able to selectively bind to specific bacterial strains. In the experiments bacteria dry weights of E. coli B were used as a positive test and E. coli K12 and Salmonella enterica as negative tests. The resonance wavelength shift induced by E. coli B reaches over 90 nm, while for E. coli K12 and Salmonella enterica approximately 40 and 20 nm, respectively. Additionally, whole-cell-ELISA tests were made to confirm LPG-based biosensor outcomes, and numerical simulations were conducted to estimate biolayer impact on sensor's performance.

Reusable Bacteriophage Adhesin-Coated Long-Period Grating Sensor for Bacterial Lipopolysaccharide Recognition

In this study, we report an application of the reusable optical fiber long-period grating (LPG) biosensor working near the dispersion turning point of higher order cladding modes. The presented device is capable of bacterial lipopolysaccharide (LPS) recognition. We show that the LPG sensor biofunctionalized with bacteriophage adhesin can be regenerated and successfully reused for very specific LPS detection. Thus, we show three series of label-free binding effect for specific to the adhesin E . coli B LPS and nonspecific H. alvei 1189 LPS (as a reference test) on a single LPG-based sensor. The resonance wavelength shift induced by the specific tests is significantly higher than that induced by the reference test. The LPG-based sensing structure allows for monitoring of the binding phenomenon in real time and with good accuracy.

Detection of methicillin-resistant staphylococci by biosensor assay consisting of nanoscale films on optical fiber long-period gratings.

Methicillin-resistance among Staphylococcus species is a major health problem in hospitals, communities, and animals. There is a need for culture-free diagnostic assays that can be carried out rapidly, and maintain a high degree of sensitivity and specificity. To address this need an ionic self-assembled multilayer (ISAM) film was deposited on the surface of a long-period grating (LPG) optical fiber by immersion alternately in poly-allylamine hydrochloride and in poly-1-[p-(3'-carboxy-4'-hydroxyphenylazo) benzenesulfonamido]-1,2-ethandiyl (PCBS), resulting in terminal carboxyl groups on the LPG-ISAM. The terminal carboxyl groups were covalently conjugated to monoclonal antibodies (MAb) specific to penicillin-binding-protein 2a of methicillin resistant (MR) staphylococci. After exposure of the LPG-ISAM to 10(2) colony forming units (CFU)/ml of MR S. aureus (MRSA) for 50 min., light transmission was reduced by 19.7%. In contrast, after exposure to 10(6) CFU/ml of methicillin-sensitive S. aureus (MSSA) attenuation of light transmission was less than 1.8%. Exposure of the LPG-ISAM to extracts of liver, lungs, or spleen from mice infected with MRSA attenuated light transmission by 11.7-73.5%. In contrast, exposure of the biosensor to extracts from MSSA-infected mice resulted in 5.6% or less attenuation of light transmission. When the sensor was tested with 36 strains of MR staphylococci, 15 strains of methicillin-sensitive staphylococci, 10 strains of heterologous genera (all at 10(4) CFU/ml), or tissue samples from mice infected with MRSA, there was complete agreement between MR and non-MR bacteria determined by antibiotic susceptibility testing and the biosensor assay when the cutoff value for attenuation of light transmission was 6.3%. Thus, the biosensor described has the potential to detect MR staphylococci in clinical samples with a high degree of sensitivity and specificity.

Online Tests of an Optical Fiber Long-Period Grating Subjected to Gamma Irradiation

This paper reports the outcomes of the tests that we conducted as online measurements for the evaluation of one optical fiber long-period grating produced by a fusion technique in a single-mode radiation-hardened optical fiber, and subjected to gamma irradiation. During the irradiation, the grating temperature was monitored. Before the irradiation, the temperature sensitivity of the grating was 27.7 $\hbox{pm}/^{\circ}\hbox{C}$ , while the value of this parameter postirradiation was found to be 29.3 $\hbox{pm}/^{\circ}\hbox{C}$ . The spectral characteristics of the grating were measured (i) in the laboratory with an ANDO AQ6317C optical spectrum analyzer and (ii) online, for the first time , with a LUNA OBR 4600 backscatter reflectometer, operating in the frequency acquisition mode. Such online measurement enables the study of recovery effects during the irradiation. The wavelength dip of the grating shifted under gamma irradiation, with 16 pm/kGy for the maximum total dose of 45 kGy. At room temperature, the recovery of the irradiation-induced shift of the wavelength dip was almost complete in about 120 h, at a rate of 6.7 pm/h. Postirradiation heating of the sensor produced the reversing of the recovery effect. The investigation indicated that, up to 45 kGy, the grating is more sensitive to radiation than other optical fiber sensors.

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&#x2009;&#x2009;nm/RIU over the refractive index range of 1.4&#x2013;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&#x2009;&#x2009;nm/nm. It is expected to play a great role in theoretical guidance for further development of biosensors.

Synthesis of Rejection Band Profile in Ultrasonically Induced Long-Period Optical Fiber Gratings

An ultrasonically induced long-period fiber grating functions as a tunable optical notch filter, since the pitch of refractive index modulation is changed according to the driving frequency of the ultrasonic vibration propagating along a fiber. In this study, two methods of synthesizing the band profile are investigated experimentally. The first method is to add two signals with different frequencies to excite the ultrasonic vibration, while the other is to switch two signals by frequency shift keying modulation. For both methods, the tunability of the rejection band profile is evaluated with respect to the difference in the two frequencies and the amplitude ratio of the two ultrasonic signals.

Tunable WDM filter with 0.8-nm channel spacing using a pair of long-period fiber gratings

A strain-tuned wavelength-division-multiplexing filter with 0.8-nm channel spacing has been demonstrated by using a pair of long-period optical fiber gratings. The response of a transmission spectrum to applied axial strain is studied theoretically and experimentally. A large operation bandwidth of over 20 nm and a linear tuning rate of -0.6 pm/μ/spl epsiv/ within a near 3-nm range have been achieved.

P2-21 Synthesis of the rejection band profile in ultrasonically induced optical fiber long-period grating(Short oral presentation for posters)

P2-21 Synthesis of the rejection band profile in ultrasonically induced optical fiber long-period grating(Short oral presentation for posters)

Optical fiber long-period gratings with Langmuir–Blodgett thin-film overlays

An overlay material was deposited by the Langmuir-Blodgett technique onto a single-mode optical fiber containing a long-period grating. The long-period grating exhibits characteristic attenuation bands in its transmission spectrum whose central wavelengths were observed to depend on the optical thickness of the overlay material, even for materials that have a refractive index higher than that of silica.

Long period gratings as solution concentration sensors

The on-line concentration measurement of solutions is of great interest in many industrial processes as a means of quality production control, in order to measure concentrations of harmful solutions or solutions that cannot be reached by the operator. An optical fiber long-period grating (LPG), tested for the refractometry of sodium chloride, calcium chloride and ethylene glycol aqueous solutions, is presented. Resolution values comparable to or even better than those offered by an Abbe refractometer, that is typically used in these types of measurements, are obtained.

Fabrication of long-period optical fiber gratings by use of ion implantation

We report the fabrication of long-period optical fiber gratings by use of a refractive-index increase induced by ion implantation. Helium ions were implanted in an optical fiber core through a metal mask that had a 170-microm -pitch grating with spacing of 60 microm . We obtained a wavelength-dependent effective transmission loss by use of the grating.