Long period grating Research Papers (original) (raw)

2025, NeuroQuantology

In this paper, some properties of optical fiber have been studied numerically by changing the values of graded-index as non-integer numbers using the finite element method (FEM) in the COMSOL environment. The results showed that the small graded orders gives a strange properties, such as a great value at nonlinearity and a new forms of dispersion relation are obtained. The paper enables us to choose the graded order and wavelength to achieve the desired values of dispersion and nonlinearity.

2025, Diamond and Related Materials

The paper presents diamond-like carbon (DLC) film as a coating for optical fibres with long-period gratings (LPG). The coatings were successfully obtained using the Radio Frequency Plasma Chemical Vapour Deposition (RF PCVD) method. Thin DLC coatings significantly change the properties of the LPG-based sensing elements. The sensing systems built using those elements can be very sensitive to concentration changes of various chemical solutions due to changes in their refractive index. DLC-coated LPGs can be as much as 15 times more sensitive than uncoated LPGs to variations within a specified range of the refractive index of the surrounding medium. The influence of RF PCVD process parameters on sensitivity is discussed.

2025, Optics and Lasers in Engineering

A novel all-fibre cavity ring down spectroscopy technique is proposed where a tilt fiber Bragg grating (TFBG) or long period grating (LPG) in the cavity provides sensitivity to surrounding medium. Such configuration with a LPG as the representative was theoretically analyzed. Two spectral bands were identified employable for sensing of surrounding refractive index for a weak LPG while only one band existed for a strong LPG. A TFBG, with enhanced sensitivity compared to usual LPGs, was used in a ring down cavity of 1m constructed with 2 fiber Bragg gratings as the reflectors and the decay time changed from 220ns to 450ns when the TFBG was immersed into water from air.

2025, (CLEO). Conference on Lasers and Electro-Optics, 2005.

A comprehensive eigenmode analysis is performed of the guided modes supported by typical photonic crystal fiber. These modes exhibit unusual phase matching conditions requiring multiple grating periods for resonant coupling. All the signature features of the experimentally observed transmission spectra are explained by multiple-period resonances

2025, Photonic Crystal Fibers

A numerical model of the sensitivity of long period gratings fabricated by electric-arc in photonic crystal fibres to strain, temperature and refractive index is proposed and evaluated by comparison to the experimental results. It is shown to be superior to the commonly used semi-analytical method. The generalized modelling procedure is thoroughly explained in order to facilitate its application to a wide range of long period gratings in different types of fibres.

2025, Journal of the Optical Society of America B

The spectral properties of long-period gratings (LPGs) fabricated in photonic crystal fibers using femtosecond laser pulses by the point-by-point technique, without oil-immersion of the fiber, are investigated in detail. Postfabrication spectral monitoring at room temperature showed significant long-term instability of the gratings and stable spectra only after 600 h. The stabilized spectral properties of the gratings improved with increasing annealing temperature. The observed changes in resonant wavelength, optical strength, and grating birefringence were correlated to the laser inscription energy and were further used to study the mechanism of femtosecond inscription. Furthermore, the femtosecond-laser inscribed LPGs were compared to electric-arc fabricated LPGs. Comparison of experimental results with theoretical models of LPGs and laser propagation during inscription indicate that the major processes responsible for the index change are permanent compaction and thermally induced strain, the latter can be significantly changed through annealing.

2025, Optical and Quantum Electronics

A numerical model of a long period grating in photonic crystal fibre fabricated by an electric arc is proposed that allows for the spectral characterisation of the grating. In the combination with the suggested model of the photonic crystal and the experimentally recorded grating growth it is used to find the index change induced by the electric arc.

2025, Optical and Quantum Electronics

Multiple period resonances in long period gratings in photonic crystal fibres j . s . p e t r o v i c 1, * , v. m e z e n t s e v 1 , h . d o b b 1 , d . j . w e b b 1 , k . k a l l i 2 a n d i . b e n n i o n 1

2025, Journal of Lightwave Technology

Sensing properties of long-period gratings (LPGs) fabricated in photonic crystal fibers by an electric arc are explained and quantified by semianalytical and numerical models. In particular, the grating's insensitivity to temperature and simultaneous sensitivity to strain and refractive index are simulated. The modeling procedure is generalized so that it can be applied to a wide range of LPGs in various fibers.

2025

We report the development and demonstration of a Long-Period Grating (LPG) based optical fiber sensor for determining the adulteration of coconut oil by palm oil. The fundamental principle of detection is the sensitive dependence of the resonance peaks of LPG on the changes of the refractive index of the environmental medium around the cladding surface of the grating. Refractive index sensing with LPGs employs light coupling between core and cladding modes in the grating section. The transmittance spectra of a long period grating element immersed in different mixtures of coconut oil and palm oil were recorded. Results show that resonance wavelengths and transmission intensities varied as a function of the adulteration level of coconut oil. Detection limit of adulteration was found to be 2 % for coconut oil–palm oil binary mixture. Copyright © 2010 IFSA.

2025, 23rd International Conference on Optical Fibre Sensors

An optical fibre long period grating (LPG), modified with a coating of silica gold (SiO 2 :Au) core/shell nanoparticles (NPs) deposited using the layer-by-layer (LbL) method, was employed for the development of a bio-sensor. The SiO 2 :Au NPs were electrostatically assembled onto the LPG with the aid of a poly(hydrochloride ammonium) (PAH) polycation layer. The LPG sensor operates at the phase matching turning point to provide the highest sensitivity. The SiO 2 :Au NPs were modified with biotin, which was used as a ligand for streptavidin (SV) detection. The sensing mechanism is based on the measurement of the refractive index change induced by the binding of the SV to the biotin. The lowest detected concentration of SV was 19 nM using an LPG modified with a 3 layer (PAH/SiO 2 :Au) thin film.

2025, IEEE Sensors Journal

This article presents a portable optical fiber sensor (OFS) design for continuous liquid level measurement, fabricated using two pieces of bare polymer optical fibers (POFs). The proposed sensor design utilizes the twisted coupling technique, where two optical fibers are twisted and coupled, leading to power coupling from the illumination fiber (IF) to the coupled fiber. The casing is fabricated through a 3-D printing method. The 3D-printed casing is utilized to mount the sensor onto a smartphone, facilitating the connection of the fibers to both the flashlight and the camera. The flashlight serves as the light source, while the camera records the intensity variations. The liquid level sensing relies on the coupling loss, where the light is transmitted to the IF, and the twisted coupled fiber couples the power, exhibiting intensity variations with changing liquid levels. The proposed liquid level sensor can measure depths up to 120 mm with a sensitivity of 0.053 ± 0.002% per mm and an impressive resolution of 0.05 mm. The experimental findings indicate that the proposed sensor has excellent stability and reliability. The sensor's straightforward, comprehensive, and cost-effective design enables its application in chemical, petroleum, and many other industries.

2025

ESEM image showing bacterial attachment onto the surface of the sensor. References [1] Tam, C. C. & O’Brien, S. J., PLoS One, 11, 1, 2016. [2] Wong, R. Y. N. et al., Appl. Opt. 53, 21, 2014. [3] Romero, A. et al., Proc. IEEE Sensors, 1-3, 2017. [4] Masdor, N. et al., Chemosensors, 5, 16, 2017. Optical image of the surface of the sensor with stained C. jejuni. Gram-negative bacteria display a pinkish red tone. Introduction

2025, SPIE Proceedings

The use of an optical fibre long period grating (LPG) as a soil moisture sensor is reported. Characterization of the device in both clay and sandy soils revealed a sensitivity to moisture levels in the range 10-50%, and the results were compared with the output from a Theta probe, the standard soil moisture sensor, which measures the impedance of the soil.

2025, Sensors and Actuators B: Chemical

An optical fibre long period grating (LPG) modified with a thin film of ZIF-8, a zeolitic immidazol framework material and a subgroup of metal organic framework family, was employed for the detection of organic vapours. ZIF-8 films were deposited onto the surface of the LPG using an in-situ crystallization technique by mixing freshly prepared 12.5 mM zinc nitrate hexahydrate and 25 mM 2-metyl-imidazole solutions in methanol. The ZIF-8 film was characterized by scanning electron microscopy. The thickness and refractive index (RI) of films deposited after 1,2,3,5 and 10 growth cycles were determined by using an ellipsometer. The crystallinity of the films was confirmed by X-ray diffraction pattern. The LPG was designed to operate at the phase matching turning point to provide the highest sensitivity. The sensing mechanism is based on the measurement of the change in the RI of the film induced by the penetration of the chemical molecules into the ZIF-8 pores. The responses of LPGs modified with 2 and 5 growth cycles of ZIF-8 to exposure to methanol, ethanol, 2-propanol and acetone were characterised. The sensitivity of the measurements to humidity as an interfering parameter was also investigated.

2025, 23rd International Conference on Optical Fibre Sensors

2025, 19th International Conference on Optical Fibre Sensors, Pts 1 and 2

A chirped long period grating (LPG) is used as a directional flow sensor by monitoring the change in the transmission spectrum upon immersion in an oil of refractive index higher than that of the fibre cladding. The change in profile of an attenuation band is shown to depend on the direction of immersion, indicating the possibility of using such devices in applications such as resin flow sensing in the manufacture of fibre reinforced plastic composite structures.

2025

A novel approach to chemical application of long period grating (LPG) optical fibers was demonstrated, which were modified with a film nanoassembled by the alternate deposition of SiO 2 nanoparticles (SiO 2 NPs) and poly(diallyldimethyl ammonium chloride) (PDDA). Nanopores of the sensor film could be used for sensitive adsorption of chemical species in water, which induced the changes in the refractive index (RI) of the light propagating in the cladding mode of the optical fiber, with a concomitant effect on the transmission spectrum in the LPG region. The prepared fiber sensor was highly sensitive to the change in the RI of the surrounding medium and the response time was very fast within 10 s. In addition, chemical infusion into the film was tested using a porphyrin compound, tetrakis-(4-sulfophenyl)porphine (TSPP), which could be saturated within a few min. The lowest detectable concentration of the TSPP analyte was 10 µM. The TSPP infusion led to the development of well-pronounced dual resonance bands, indicating a large increase in the optical thickness of the film. The RI of the film was dramatically increased from 1.200 to ca. 1.540.

2025, Third European Workshop on Optical Fibre Sensors

The concept of nested fibre optic long period grating (LPG) based interferometers is introduced. A number of in-series, identical LPGs may be used to form a set of nested, multiplexed Mach-Zehnder interferometers that may demodulated and demultiplexed by virtue of a Fourier analysis of the optical spectrum. The concept is demonstrated by the use of three LPGs to form a nested set of interferometers.

2025, Sensors and Actuators B: Chemical

The chemical sensing capabilities of a long period grating (LPG) coated with a functional material are explored. The LPG period and coating thickness are chosen such that the mode transition region, characteristic of LPGs coated with nanostructured coatings, corresponds with the phase matching turning point, ensuring optimum sensitivity. The functional material is a calixarene, which is shown to exhibit sensitivity to the aromatic compounds toluene and benzene, while being relatively insensitive to the aliphatic hydrocarbon hexane. A nanoscale cavity in the calixarene molecule entraps the VOC molecule, thereby altering the refractive index of the coating and influencing the transmission spectrum of the LPG. A sensitivity to toluene of 231 ppmv has been achieved, based on a spectrometer resolution of 0.3nm. This is a weak molecular interaction thereby ensuring the sensor gives a reversible response, when the VOC is removed. The recovery time is of the order of 15 seconds.

2025, Optics Letters

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.

2025, Optics Letters

A liquid-level sensor based on the refractive-index sensitivity of long-period fiber-optic gratings is proposed and demonstrated. The form of the transmission spectrum of the long-period grating is dependent on the fraction of the length of the long-period grating that is surrounded by the liquid. The sensor shows a large linear range, with sensitivity of 4.8% change in transmission per millimeter, for a long-period grating with a length of 40 mm and a periodicity of 400 mm.

2025, Measurement Science and Technology

Surrounding a long period grating, fabricated in a standard optical fibre, with a material with a high thermo-optic coefficient enhances the temperature sensitivity of a long period grating. Temperature sensitivities as high as 19nm/°C are reported over a temperature range of 1.1°. The temperatures at which the LPG exhibits enhanced sensitivity may be tailored by a suitable choice of material.

2025, Journal of Lightwave Technology

The CO 2 laser based fabrication of tapered long period gratings (TLPGs) is optimized so that their transmission spectra are comparable with those made using the arc-discharge technique, which typically exhibits transmission losses below 2 dB. The reduction in transmission loss was achieved by optimizing the micro-taper geometry and the duty-cycle of the device. A 6-period TLPG of period 378 µm exhibited a pass-band transmission loss of 0.6 dB, resonance band extinction values of 3 dB and had a physical length of 2.27 mm. The average refractive index sensitivity of a 6 period TPLG was measured and found to be 372 nm/RI.

2025, IEEE Sensors Journal

The use of chirped long period gratings (CLPGs) for monitoring the flow, the direction of the flow and the subsequent cure of an epoxy resin is presented. The asymmetric properties of the CLPG and its sensitivity to refractive index changes were exploited to facilitate the measurement of the direction of the flow. The performances of a continuously chirped LPG (CCLPG) and a step chirped LPG (SCLPG) were compared, with the CCLPG showing improved spatial resolution. The CCLPG and SCLPG were also used to monitor changes in the refractive index of the resin during cure, with the results showing close agreement with measurements undertaken simultaneously using a fibre optic Fresnel refractometer.

2025, Measurement Science and Technology

The thermal response of the attenuation bands of an optical fibre long period grating was monitored over a temperature range 4.2K to 280K. A linear dependence of the central wavelength of the band, of gradient 0.2 nm/K was observed over... more

2025, Sensors and Actuators B: Chemical

Long-period gratings (LPGs) inscribed in a novel fiber, in which the smoothed refractive index of the cladding increases approximately quadratically with radius (IGI fiber), are investigated for temperature sensing. Fabrication of the fiber and LPGs is briefly described and results of a theoretical analysis are given. The analysis yields fields and effective indices of the LP 01 core mode and LP 0i cladding modes of the IGI fiber. Wavelength-dependent effective indices, computed for 11 lowest-order cladding modes and an external index of 1.33, are incorporated into a diagram of the phase-matching condition covering also coupling through higher harmonics of the grating. Temperature sensitivities of LPGs submerged in water and in a liquid with a refractive index that decreases with temperature and is close to that of the pure-silica fiber cladding are determined. Average sensitivities of 0.60 and 0.56 nm/ • C are obtained for the LPGs with periods of 1000 and 500 m in water. For the same LPGs in the above liquid, average sensitivities of 5.40 (3.95) and 3.76 (3.11) nm/ • C are obtained in a 10 • C (20 • C) temperature interval with a lower limit of sensing at about 50 • C. A significant improvement in sensitivity is observed after slightly bending an LPG in the liquid.

2025

In this work we studied the behavior of an in fiber Mach-Zehnder interferometer for salinity measurements. The salt species used are KCl, NaCl e NaCOOH (organic salt) within the concentration range from 3 g.L -1 to 150 g.L -1 . The results are compared with traditional salinity measurement methods (conductivity and refractive index) and show that the device can distinguish and identify the concentration of each species saline.

2025, Measurement Science and Technology

In this work, a comparative study of the behaviour of an in-fibre Mach-Zehnder interferometer for salinity measurement in a water solution is presented. The fibre transducer is composed of two nearly identical long period gratings forming an inseries 7.38 cm long device written in the same fibre optic. Two inorganic and one organic salts (NaCl, KCl, NaCOOH) were characterized within the concentration range from 0 to 150 g L -1 . For the long period grating interferometer, the average obtained sensitivities were -6.61, -5.58 and -3.83 pm/(g L -1 ) for the above salts, respectively, or equivalently -40.8, -46.5 and -39.1 nm RIU -1 . Salinity measured by means of fibre refractometry is compared with measurements obtained using an Abbe refractometer as well as via electrical conductivity. For the long period grating refractometer, the best resolutions attained were 1.30, 1.54 and 2.03 g of salt per litre for NaCl, KCl and NaCOOH, respectively, about two times better than the resolutions obtained by the Abbe refractometer. An average thermal sensitivity of 53 pm °C-1 was measured for the grating transducer immersed in water, indicating the need for the thermal correction of the sensor. Resolutions for the same ionic constituent in different salts are also analysed.

2025, IEEE Transactions on Nuclear Science

In-fiber Bragg and long-period gratings as well as Mach-Zehnder interferometers based on germanium-and nitrogendoped silica fibers have been investigated under y-rays. The majority of the experimental results suggest that both types of... more

2025, Journal of the Indian Institute of Science

Optical fibers are now taken for granted as signal transmission media for telecommunication and the Internet. Optical fibers also provide a versatile platform for sensing a host of physical and chemical effects, and parameters of interest. Though optical techniques for measurements were well known for a long time, utility of optical fibers for sensing and measurements has been recognised and widely appreciated only in the last two to three decades. It has indeed become a subject of intense activity over recent times due to a variety of attractive features, notably their immunity to electromagnetic interference, enabling distributed sensing, and remote collection of sensed data for post processing owing to their compatibility to optical fiber telemetry. Fiber optics offers a versatile platform for realizing a multitude of sensors for a variety of applications in areas as broad as civilian, defence, biomedical, and so on. Functional principles of some of these sensors could be extremely simple, while some sensors could be ultra-sensitive that could yield extremely small magnitudes of certain measurands. This article is an attempt to review the basic principles that underpin this area of research and expose the readers to few such fiber optical sensors by describing their applications.* *The article is to a large extent adapted from a book chapter "Optical Waveguide Sensors" that I wrote for the upcoming book "Handbook of Optical Sensors" ed by José Luís Santos

2025, Optics Letters

A novel Mach-Zehnder interferometer based on a fiber multimode interference structure combined with a long-period fiber grating (LPG) is proposed. The multimode interference is achieved through the use of a MMF section spliced between two single-mode fibers, with a length adjusted to couple a fraction of light into the cladding modes. A LPG placed after the MMF couples light back into the fiber core, completing the Mach-Zehnder interferometer. This novel configuration was demonstrated as a bending sensor.

2025, SPIE Proceedings

In this work an all-optical hot-wire flowmeter based on a silver coated fiber combining a long period grating and a fiber Bragg grating (FBG) structure is proposed. Light from a pump laser at 1480 nm propagating down the fiber is coupled by the long period grating into the fiber cladding and is absorbed by the silver coating deposited on the fiber surface over the Bragg grating structure. This absorption acts like a hot wire raising the fiber temperature locally, which is effectively detected by the FBG resonance shift. The temperature increase depends on the flow speed of the surrounding air, which has the effect of cooling the fiber. It is demonstrated that the Bragg wavelength shift can be related to the flow speed. A flow speed resolution of 0:08 m=s is achieved using this new configuration.

2025, Talanta

The analysis of the quality of food oils is of paramount importance, because the degradation of oils can lead to formation of harmful substances to the human organism. With the increase of the degradation of oils an increase of its refractive index occurs. The objective of this work is to develop and to characterize optical fiber refractometers sensitive to variations of refractive index of food oil samples. The optical fiber refractometers thanks to the intrinsic characteristics make them suitable for monitoring the quality of frying oils. They possess the advantages to require small volumes of sample for analysis, do not contaminate the sample, and supply the response in real time. In this work a long period grating (LPG) as refractometer is used because of their sensitivity to refractive index of the external media: degraded and not degraded frying oil samples. The oil samples had been characterized by the analysis of total polar components. The refractive index of oil is above 1.47, this region the LPG does not show enough sensitivity, a nanolayer of an organic material was coated onto the fiber. Using the Langmuir-Blodgett technique the response of LPG is modified according to the refractive index and thickness of the film. The deposition of the film modifies the rates effective modes of cladding, thus improving the response of the changes in the refractive index of the external media higher than that the refractive index of the cladding (n = 1.457).

2025, Sensors and Actuators B: Chemical

In this work is studied the response of optical fiber long period grating (LPG) to changes of the refractive index of the external media relatively to variations of wavelength and in transmission. The response of the LPG to refractive index greater and lesser than to cladding is investigated. A nanolayer was deposited onto the fiber to increase the sensitivity of the LPG to refractive index of the external media higher than cladding. The film modifies the rates of effective modes of cladding, thus improving the response of the changes in the refractive index of the external media higher than that in the refractive index of the cladding (n cl ≈ 1.457). The Langmuir-Blodgett technique was used for the deposition of the nanolayer.

2025, Optics Communications

A long-period grating written in the SMF-28 fibre was heat treated at 1000 °C for 15 days. The spectrum of the grating shifted to longer wavelengths and the resonances depth decreased as a result of structural relaxation. The background loss increased considerably for times longer than 200 h, and this loss is caused by devitrification of the fibre.

2025, Fiber and Integrated Optics

In the field of aqueous environment studies, long-period fiber gratings are very attractive for the real-time monitoring of physical parameters, such as temperature and salinity. However, due to the fiber jacket removal, these are fragile devices when applied in real conditions, where mechanical loads and contamination with algae and other organic materials must be taken into account. This work describes a refractive index sensing head that has been developed and characterized for in situ measurement of water salinity and resistance to hard conditions.

2025, Applied Optics

We present results on the characterization of the response of a dual resonance observed in the spectrum of a single long-period grating arc-induced in a B/Ge co-doped fiber to different physical parameters. The dual resonance is formed by two overlapping resonances corresponding to coupling of the core mode to symmetric and antisymmetric cladding modes. The behavior of the resonances is studied when the grating is subject to strain, bending, torsion, temperature, or external refractive-index changes. The strain, bending, and torsion sensitivities of the two resonances differ, whereas the temperature sensitivities are almost the same. The sensitivities to variation in external refractive index are the same for the two resonances when the long-period grating is straight and differ when the fiber with the grating is curved.

2025, Measurement Science and Technology

In this work a modal interferometer based on arc-induced long-period gratings (LPGs) in a Mach-Zehnder configuration is evaluated as a sensing structure for environmental refractive index measurement. To interrogate this sensing device, coherence addressing and pseudo-heterodyne processing were used. The influence of geometric effects such as stretching, bending and twisting the interferometer on the sensitivity to refractive index changes was studied. It is shown that due to the antisymmetric nature of cladding modes in arc-induced LPGs, it is possible to tune the system sensitivity to external refractive index by simple mechanical action. The experimental results show that it is possible to tune the sensitivity to external refractive index by more than 50% by control of the curvature in the Mach-Zehnder interferometer.

2025, Microwave and Optical Technology Letters

A photonic compact eight-channel wavelength filter has been proposed and fabricated by incorporating serially connected ring resonators based on a silicon rib waveguide. Eight different rings are subsequently addressed by a common input bus and individually outcoupled by output buses. A vertical grating coupler is attached to the input and output buses for efficient light coupling. Each of the launched optical signals is routed to the drop port of the corresponding channel, whose center wavelength is dependent on the size of the ring. The device was designed by using a finite-difference time-domain method and fabricated by exploiting a standard complementary metal-oxidesemiconductor silicon-on-insulator technology. The silicon rings, serially placed with a gap of 50 lm, have radii varying from 6.0 to 6.07 lm, leading to a channel wavelength ranging from 1546.3 to 1557.6 nm. As regards, the overall performance of the fabricated device, the quality factor is 13,600,thechannelspacing13,600, the channel spacing 13,600,thechannelspacing1.6 nm, the propagation loss 3dB,andthechannelcrosstalk3 dB, and the channel crosstalk 3dB,andthechannelcrosstalk27 dB. Finally, a group of optical signals modulated by a data signal at 3.2 Gbps were appropriately routed to yield a decent eye diagram. V

2025, Electronics

This paper reviews fiber Bragg grating sensing technology with respect to its use in mainstream industrial process applications. A review of the various types of sensors that have been developed for industries such as power generation, water treatment and services, mining, and the oil and gas sector has been performed. A market overview is reported as well as a discussion of some of the factors limiting their penetration into these markets. Furthermore, the author's make recommendations for future work that would potentially provide significant opportunity for the advancement of fiber Bragg grating sensor networks in these mainstream industries.

2025, SPIE Proceedings

We present data on the development a new type of optical fibre polariser and the characterisation of its wavelength properties. The device is fashioned using a two step process. Firstly, a standard UV long period grating (LPG) with a period of 330µm is inscribed into hydrogenated SMF-28, followed by femtosecond laser ablation of a groove parallel to the fibre axis. The UV inscribed LPGs have inherently low birefringence. However, the removal of the cladding layer parallel to the location of the LPG within the fibre core (as a result the ablation) modifies the cladding modes that couple with the LPG. Furthermore, the groove breaks the fibre symmetry introducing a non-uniform stress profile across the fibre cross section leading to significant birefringence. We show that increasing the depth of the groove increases the birefringence, and this behaviour coupled with the ability to control the wavelength location of the LPGs attenuations peaks results in a polariser able to operate at almost any wavelength and birefringence. The maximum birefringence reported here as polarisation mode splitting was approximately 39±0.1nm with a polarisation loss of 10dB.

2025, SPIE Proceedings

The distinct behaviour of femtosecond laser inscribed long period gratings, with a non-uniform index perturbation within the optical fibre core, has been studied experimentally. The non-uniform laser-induced perturbation results in light coupling from the core mode to a greater number of cladding modes than is the case with their UV laser inscribed counterparts, and this is made evident from the surrounding refractive index (SRI) grating response. Femtosecond inscribed long period gratings are shown to simultaneously couple to multiple sets of cladding modes. A 400µm LPG is shown to result in attenuation peaks that have both blue and red wavelength shifts over a 1250nm to 1700nm wavelength range. This gives rise to SRI sensitivities far greater than anything achievable by monitoring a single attenuation peak. The maximum sensitivity produced by monitoring a single attenuation peak was 1106nm/RIU, whereas monitoring opposing wavelength shifts resulted in a significantly improved sensitivity of 1680nm/RIU.

2025, Microstructured and Specialty Optical Fibres

Photonic crystal fibres (PCF) and more commonly microstructure fibres, remain interesting and novel fibre types and when suitably designed can prove to be "ideal" for sensing applications, as the different geometrical arrangement of the air holes alters their optical wave-guiding properties, whilst also providing tailored dispersion characteristics. This impacts the performance of grating structures, which offer wavelength encoded sensing information. We undertake a study on different air hole geometries and proceed with characterization of fibre Bragg and long period gratings, FBG and LPG, respectively that have been inscribed (using either a femtosecond or ultraviolet laser system) within different designs of microstructured fibre that are of interest for sensing applications.

2024, Applied Optics

We report the design criteria for the use of long period gratings (LPGs) as refractive-index sensors with output power at a single interrogating wavelength as the measurement parameter. The design gives maximum sensitivity in a given refractive-index range when the interrogating wavelength is fixed. Use of the design criteria is illustrated by the design of refractive-index sensors for specific application to refractive-index variation of a sugar solution with a concentration and detection of mole fraction of xylene in heptane (paraffin).

2024, Measurement Science and Technology

Recent research on fibre optic long-period gratings (LPGs) is reviewed with emphasis placed upon the characteristics of LPGs that make them attractive for applications in sensing strain, temperature, bend radius and external index of refraction. The prospect of the development of multi-parameter sensors, capable of simultaneously monitoring a number of these measurands will be discussed.

2024, Measurement Science and Technology

We propose a new scheme for the interrogation of long-period fiber gratings (LPGs) which traces the polarization trajectory on the Poincaré sphere. This method was applied to detect changes of the surrounding refractive index at a single wavelength. Sensitivities as high as 10 −6 r.i.u. can be achieved for a broader range of refractive indices. This method can also be used in combination with WDM schemes.

2024, Journal of Lightwave Technology

We present a Müller-Stokes analysis of the polarization properties of randomly birefingent long-period fiber gratings (LPGs). Using a previously derived Müller matrix, we perform simulations of the polarization behavior in photonic crystal fibersbased tapered long-period gratings with randomly varying birefringence axes. Computer simulations show good agreement with results from experiments with a broadband-amplified spontaneous emission source and a tunable laser in combination with a polarization analyzer.

2024, IEEE Transactions on Instrumentation and Measurement

The method and the required installations for fabricating tapered long-period fiber gratings can be simpler than that of standard fiber Bragg gratings, and the fabrication process is faster. To our knowledge, the tapered long-period fiber grating pressure sensor is presented here for the first time. In this paper, the fabrication method for tapered long-period fiber gratings, the sensing principle, the sensor structure, the measurement setup, and the preliminary results are presented and discussed. The pressure sensitivity of the sensor is as high as 5.1 pm/bar.