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.

https://iopscience.iop.org/article/10.1088/0957-0233/14/5/201/pdf

Optical fibre long-period grating sensors: characteristics and application

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 μm.

Fiber-optic liquid-level sensor using a long-period grating

A long-period fiber-grating sensor with a high strain sensitivity of −7.6 pm/μe and a low temperature sensitivity of 3.91 pm/°C is fabricated by use of focused CO2 laser beam to carve periodic grooves on a large- mode-area photonic crystal fiber. Such a strain sensor can effectively reduce the cross-sensitivity between strain and temperature, and the temperature-induced strain error obtained is only 0.5 μe/°C without using temperature compensation.

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Highly sensitive long-period fiber-grating strain sensor with low temperature sensitivity.

Knowledge of integrity of in-service structures can greatly enhance their safety and reliability and lower structural maintenance cost. Current practices limit the extent of real-time knowledge that can be obtained from structures during inspection, are labor-intensive and thereby increase life-cycle costs. Utilization of distributed sensors integrated with the structure is a viable and cost-effective means of monitoring the structure and reducing inspection costs. Acellent Technologies is developing a novel system for actively and passively interrogating the health of a structure through an integrated network of sensors and actuators. Acellent's system comprises of SMART Layers, SMART Suitcase and diagnostic software. The patented SMART Layer is a thin dielectric film with an embedded network of distributed piezoelectric actuators/sensors that can be surface-mounted on metallic structures or embedded inside composite structures. The SMART Suitcase is a portable diagnostic unit designed with multiple sensor/actuator channels to interface with the SMART Layer, generate diagnostic signals from actuators and record measurements from the embedded sensors. With appropriate diagnostic software, Acellent's system can be used for monitoring structural condition and for detecting damage while the structures are in service. This paper enumerates on the SMART Layer and SMART Suitcase and their applicability to composite and metal structures.

SMART Layer and SMART Suitcase for structural health monitoring applications

https://iopscience.iop.org/article/10.1088/2040-8986/aac68d/pdf

Biomedical application of optical fibre sensors

We demonstrate that temperature- and strain-insensitive long-period gratings can be fabricated in conventional optical fibers. The former is employed to measure strain with resolution of 20 µ? under thermal fluctuations in the surroundings, while the latter is used to detect temperature variations as small as 0.8°C in the presence of axial strain.

Temperature-insensitive and strain-insensitive long-period grating sensors for smart structures

Life extension programs for military metallic aircraft are becoming increasingly important as defense budgets shrink and world economies realign themselves to an uncertain future. For existing military weapon systems, metallic corrosion damage costs an estimated $8 billion per year. One approach to reducing this cost is to develop a reliable method to detect and monitor corrosion in hidden metallic structure with the use of corrosion sensors which would give an early indication of corrosion without significant disassembly. This paper describes the current status of the development, analysis, and testing of a fiber optic corrosion sensor developed jointly by Boeing and Virginia Tech Fiber & Electro-Optics Research Center and sponsored by USAF Wright Laboratory, Materials Directorate, contract #F33615-93-C-5368. In the sensor which is being developed under this contract, the normal cladding is removed in the sensor region, and replaced with aluminum alloy and allowed to corrode on coupons representative of C/KC-135 body structure in an ASTM B117 salt spray chamber. In this approach, the optical signal out of the sensor is designed to increase as corrosion takes place. These test results to determine the correlation between sensor output and structural degradation due to corrosion are discussed.

Novel NDE fiber optic corrosion sensor

Life extension programs for military metallic aircraft are becoming increasingly important as defense budgets shrink and world economies realign themselves to an uncertain future. For existing military weapon systems, metallic corrosion damage costs as estimated $DOL8 billion per year. One approach to reducing this cost is to develop a reliable method to detect and monitor corrosion in hidden metallic structure with the use of corrosion sensors which would give an early indication of corrosion without significant disassembly, thereby reducing maintenance costs. This presentation describes the development, analysis, and testing of a fiber optic corrosion sensor developed jointly with the Virginia Polytechnic Fiber and Electro-Optics Research Center and sponsored by Wright Laboratory Materials Directorate. In the sensor which was researched, the normal cladding is removed in the sensor region, and replaced with aluminum alloy and allowed to corrode on coupons representative of C/KC-135 body structure in an ASTM B117 salt spray chamber and a Boeing developed Crevice Corrosion Cell. In this approach, the optical signal output of the sensor was originally designed to increase as corrosion takes place, however interaction with the corrosion byproducts yielded different results than anticipated. These test results to determine a correlation between the sensor output and the structural degradation due to corrosion are discussed.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Aluminum alloy clad fiber optic corrosion sensor

Long-period gratings have recently been demonstrated as highly versatile strain, temperature and refractive index sensors that can be implemented with simple demodulation schemes. The cross-sensitivity of long-period gratings to ambient temperature variations can be a limiting factor in applications that require their use as strain or refractive index sensors. The thermal-induced spectral shift in the grating transmission spectrum can be attributed to the material and waveguide changes in the grating characteristics. By designing special optical fiber refractive index profiles, these two effects can be counter- balanced to produce gratings that have an order of magnitude smaller temperature wavelength shift than conventional long- period gratings. We present experimental results for strain and refractive index sensors using these temperature insensitive long-period gratings. Such transducers are shown to possess strain- and refractive index-induced wavelength shifts that are comparable to those of standard gratings. The cross-sensitivity to temperature is determined at different magnitudes of strain and ambient index to determine the effectiveness of these gratings in practical applications.

Temperature-insensitive long-period gratings for strain and refractive index sensing

An important part of traffic management and intelligent transportation systems is the availability of cost effective, reliable sensors for vehicle classification and traffic flow monitoring. Fiber optic sensors have numerous advantages over conventional sensors and are thus prime candidates for use in these traffic systems. In this paper, an optical fiber vehicle sensor is described for axle detection. Results from field tests are presented. The performance of the fiber optic vehicle sensor is evaluated and methods of vehicle weight-in-motion are also discussed.

Noel Zabaronick

Papers

Temperature-insensitive and strain-insensitive long-period grating sensors for smart structures

Novel NDE fiber optic corrosion sensor

Aluminum alloy clad fiber optic corrosion sensor

Temperature-insensitive long-period gratings for strain and refractive index sensing

Performance of fiber optic vehicle sensors for highway axle detection