Intrinsic, multiplexable sensors for electric field strength using structural slow light in phase-shifted fibre Bragg gratings

In this paper we demonstrate through simulation the potential for phase-shifted fibre Bragg gratings incorporating structural slow light to enable intrinsic reflection-mode point sensors for electric field or voltage. It is shown that lo-bi FBGs incorporating multiple phase shifts yield large enhancements in group index (group delay) at resonance, thus amplifying and localizing time-dependent non-reciprocal effects. A relative, multiplexable measurement of electric field by comparison of the phase unbalance between linear modes on and off resonance is proposed, yielding static resolutions of 24 V and 18 mV respectively in unpoled (dc Kerr effect) and poled (Pockels effect) fibres

High-speed, solid state, interferometric interrogator and multiplexer for fiber Bragg grating sensors

We report on the design and prototyping of a robust high-speed interferometric multiplexer and interrogator for fiber Bragg grating sensors. The scheme is based on the combination of active WDM channel switching and passive, instantaneous interferometry, allowing the resolution of virgin interferometric interrogators to be retained at MHz multiplexing rates. In this article the system design and operation are described, and a prototype scheme is characterized for three sensors and a multiplexing rate of 4 kHz, demonstrating a noise floor of 10 nε/√Hz and no cross-sensitivity. It is proposed that the system will be applicable to demanding monitoring applications requiring high speed and high resolution measurements across the sensor array

Temperature characterisation of an optically-interrogated Rogowski coil

An investigation into the performance of an optically-interrogated Rogowski coil over temperatures up to 80 °C is presented. Preliminary thermal tests reveal that the sensor response is temperature-dependent and the measurement errors are increased at higher temperatures. Compensation of temperature effects is, therefore, necessary in order for the sensors to meet the requirements of protection and metering classes specified by relevant IEC standards over the considered operating temperature range. This can, however, only be achieved when the utilised sensor interrogator is adapted to ensure sufficient resolution and accuracy from a generally low-output transducer. As such, the design of a new multiplexing, interferometric interrogation platform is also proposed in this paper

Interferometric time division FBG interrogator and multiplexer with static, dynamic, and absolute wavelength measurement capabilities

We report on the design and preliminary testing of an interferometric interrogator capable of large-scale time-division multiplexing of FBG sensors. The scheme employs a passive algorithm for phase demodulation, allowing changes in FBG sensor reflected wavelengths to be calculated instantaneously upon arrival, and incorporates a technique for identification of initial absolute sensor wavelengths in order to overcome the measurement ambiguity associated with interferometric schemes. The proposed system will allow for high-speed interrogation of large-scale FBG sensor arrays with interferometric resolution and the capability for dynamic, static, and absolute FBG wavelength measurement

High-speed interferometric FBG interrogator with dynamic and absolute wavelength measurement capability

A passive, interferometric wavelength demodulation technique has been extended to measure the absolute wavelengths of a multiplexed array of fiber Bragg grating sensors. The scheme retains its original strain resolution of 10 nε/√{Hz}. A proof-of-concept interrogation system was able to determine the absolute wavelength of Bragg peaks to within 20 pm (17 με). Static and dynamic Bragg grating strains were accurately demodulated in both absolute and relative wavelength measurement modes. This demonstration indicates that interferometric techniques are able to provide absolute, static and dynamic measurements of strain within a single platform

All-optical differential current detection technique for unit protection applications

In this paper we demonstrate a novel, all-optical differential current protection scheme. By monitoring the optical power reflected from two matched hybrid fiber Bragg grating current sensors and using a simple optoelectronic threshold detector, an immediate response to an increase in differential current is achieved. A preliminary laboratory embodiment is constructed in order to characterize the performance of the scheme. The proposed technique does not require a complex sensor interrogation scheme, usually characterized by a limited sampling frequency, and thus will be capable of facilitating inexpensive and fast-acting differential protection over long distances

Solid-state interferometric interrogator and multiplexer for high-speed dynamic and absolute FBG wavelength measurement

We present a solid-state FBG array interrogator and multiplexer capable of determining absolute FBG wavelengths and of providing high-speed, high-resolution static and dynamic measurements. Using a described procedure, deployable on multiplexing passive-interferometric schemes, the system is able to determine initial sensor wavelengths and thereafter track wavelength changes with interferometric resolution. The scheme allows high-resolution interrogation of FBG sensor arrays to be applied to many industrial applications, where previously the lack of combined absolute and quasi-static wavelength measurement precluded the use of interferometric techniques. Using a preliminary laboratory embodiment, we demonstrate a wavelength determination accuracy of <0.3 nm and a measurement resolution of 10 fm/√Hz, and propose pathways to improved performance and miniaturisation

Controlled fabrication of tunable delay using compound phase shifted resonators

Fine tuned, narrowband group delay (“slow light”) is obtained using a compound phase shifted grating and superposing resonances. Both simulation and experiments are reported

FBG-based fibre-optic current sensors for power systems protection : laboratory evaluation

Conventional differential current unit protection schemes rely on a pair of electronic protection relays that measure current phasors separately at the boundaries of the protected zone. The scheme requires a separate, often optical, communications channel for the sharing of measurement information to enable the timely identification of and reaction to internal faults. The high voltage environment that the transducers must operate in poses a number of engineering problems stemming from the need for electrical isolation and requirement for transformation of high primary system current magnitudes. Additionally, when either the number of relays or distance between relays is increased, timing problems can arise due to the limited bandwidth, speed and changeable latencies of the communication channels and the increased computation requirements. Fibre-optical sensor systems are maturing as a technology and offer a number of advantages over conventional electronic sensor regimes, including the possession of inherent electrical isolation, chemical inertness, immunity to electromagnetic interference, and their small size and serial multiplexing capability. Fibre sensor systems are therefore experiencing increased uptake in industries that operate in harsh environments, such as oil and gas, or where specific requirements such as large step-out distances or resistance to radiation prohibit the use of electronic sensors. The Advanced Sensors Team within the Institute for Energy and Environment has developed fibre-optic point sensors for voltage and electrical current, based on fibre Bragg grating (FBG) technology, that have been applied successfully to power systems diagnostics. With the photonic systems capability to interrogate up to 100 km from source at kHz sample rates with up to 30 sensors in series, it is possible and highly desirable to adapt this technology for use in power systems protection, where immediate applications in unit and distance protection are clear. In this paper, the application of the FBG-based hybrid current sensor system to power systems protection is presented for the first time. Experimental tests of the response of an optical unit protection system to a range of internal and external fault scenarios are also reported. Secondary current inputs to the system are modelled using ATP and injected into the prototype test system via an APTS3 (Advanced Protection Testing System) unit. Fibre sensors, separated optically by 24 km of fibre, provide all measurement information via a single interrogation system situated at one end of the protected zone. Experimental results confirm high performance of the optical unit protection both in terms of sensitivity to internal faults and stability under external fault conditions. Therefore, the systems ability to overcome problems experienced in electronic relaying systems using conventional current sensing technologies is demonstrated. No separate communications channel is required in this configuration, with fault algorithms being deployed only at one location that need not be close to the protected zone. The fibre-optic current sensor systems capacity for long-distance interrogation and high sensor count qualify it for further applications in more complex protection schemes, or over larger distances, where a single fibre could form the basis of highly novel distributed protection schemes. This potential will also be discussed in detail in the paper

Nanoscale resolution interrogation scheme for simultaneous static and dynamic fiber Bragg grating strain sensing

A combined interrogation and signal processing technique which facilitates high-speed simultaneous static and dynamic strain demodulation of multiplexed fiber Bragg grating sensors is described. The scheme integrates passive, interferometric wavelength-demodulation and fast optical switching between wavelength division multiplexer channels with signal extraction via a software lock-in amplifier and fast Fourier transform. Static and dynamic strain measurements with noise floors of 1 nanostrain and 10 nanostrain/sqrt(Hz), between 5 mHz and 2 kHz were obtained. An inverse analysis applied to a cantilever beam set up was used to characterise and verify strain measurements using finite element modeling. By providing distributed measurements of both ultahigh-resolution static and dynamic strain, the proposed scheme will facilitate advanced structural health monitoring