Heraclitean Conflagration?

This work considers whether the doctrine of a general conflagration first emerged in Heraclitus’s thought and whether this doctrine is compatible with the rest of his philosophical theory, for which we have evidence in the surviving fragments

Advanced fibre optic sensors developed using femtosecond laser inscription

Currently optical fibres are widely used for telecommunications and sensing applications due to their immunity to electromagnetic fields, and their light weight and small design. In particular for sensing purposes, different types of specialised fibres have been development either with special designs, such as microstructure patterns or using different kinds of materials. In this way, the sensitivity of the fibres can be enhanced or become unresponsive to specific parameters. However, not all the speciality fibres are photosensitive and efficiently processed with conventional UV lasers to manufacture sensing structures; as a result, most of the ‘alternative’ material or designs fibres are not fully exploited.On the other hand, the femtosecond lasers modify materials using a different principle. The material photosensitivity is not required to induce refractive index changes in the fibres, indeed minimising linear absorption is desirable and the greater the transparency of the material the better for femtosecond laser processing. In this case it is multi-photon absorption that occurs at the laser focus that is important for laser-induced material modifications. In this thesis, we present the development and manufacturing of advanced fibre optic sensors using a femtosecond laser operating at 517 nm. A wide range of different optical fibres processed allows for the fabrication of not only fibre Bragg grating (FBG) sensors but also chirped gratings, Fabry-Perot cavities, and Mach-Zehnder interferometers (MZIs).I summarise several examples that a unique to femtosecond laser inscription. The precise and accurate control of the inscription parameters during the fabrication of sensors in a multi-mode optical fibre can lead to control of the coupling between a Bragg grating and the higher order modes of a low loss polymer optical fibre (POF). A result is that we are able to inscribe single-peak FBG sensors as an array in the multi-mode optical fibre. We demonstrate this by inscribing FBGs in a low loss POF, in which it is not possible to inscribe gratings using conventional UV-laser sources, and hence demonstrate applications using polymer fibres over several tens of meters, which exceed the operating length of POF-based sensors by three orders of magnitude at an operating wavelength of 1550 nm.Andreas Othonos, Stavros PissadakisComplete

Recent trends and advances of fibre Bragg grating sensors in CYTOP polymer optical fibres

Polymer optical fibre (POF) sensors have been the focus a many research studies for approximately twenty years, due to their unique mechanical characteristics that offer the potential for new and exciting applications; they are however, plagued with high loss. As we discuss in this paper efficient polymer sensing is possible using perfluorinated polymer fibres. We present a brief review of the latest achievements regarding the Bragg grating inscription in multimode gradient index CYTOP POFs, presenting their characterisation for different measurands and a number of applications that have been examined during the last five years

All-in-Fiber Cladding Interferometric and Bragg Grating Components Made via Plane-by-Plane Femtosecond Laser Inscription

We introduce a method of inscribing in-fiber devices using a femtosecond laser that is applicable to crucial components, such as cladding waveguides (CWGs), cladding Mach-Zehnder interferometers (MZIs), embedded waveguide Bragg gratings (WBGs), and waveguide Fabry-Perot cavities using the same key femtosecond laser parameters, via an 'inscribe and step,' plane-by-plane approach, applied as necessary on two orthogonal axes. This leads to femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fiber Bragg gratings; the unique sensing characteristics of the filters are maintained and provide complementary measure and information. This ensures a single inscription process, offering reliability and repetition in component manufacture, as the basic conditions to inscribe the here-demonstrated elements are common. We characterize CWG-WBG devices for axial strain, bend, and response to refractive index. The MZI-WBG is exposed to temperature and humidity excursions, confirming that the unique sensor responses are maintained for this compact, compound sensor. The MZI exhibits response to external refractive index, a large, negative wavelength response with temperature and high sensitivity to humidity, whereas the MZI-located WBG displays a similar sensitivity to conventional core-based Bragg grating sensors to temperature and no response to relative humidity. We consider that this research is an important step in the development of compact, smart optical fiber sensors

Carbon Cantilever Beam Health Inspection Using a Polymer Fiber Bragg Grating Array

We demonstrate a quasi-distributed sensor for cantilever health inspection measurements using a fiber Bragg grating (FBG) array inscribed in a polymer optical fiber. The FBGs were characterized and calibrated for axial strain, temperature, and relative humidity prior to their mounting on a carbon cantilever beam, the tail rotor of a helicopter. By using the zero-crossing demodulation algorithm, we recovered the time-dependent, wavelength response from each Bragg grating sensor and the vibration response of the beam was extracted. We used the response of the beam to study how the addition of masses at different positions on the beam influences the vibrational behavior and mimics the location of 'damage' through the time-dependent results. We show that health inspection measurements are feasible with polymer-based fiber Bragg gratings, offering accurate and rapid detection of damage points on a structural beam

Improvements on the cross-correlation algorithm used for tracking fractional Bragg grating wavelength shifts in multimode fibres

In this paper we report on the deployment of Finite Impulse Response (FIR) filters as an extension to the common cross-correlation algorithm (CCA), for accurate peak tracking of multimode and single mode fibre Bragg gratings when dealing with wavelength shifts below the sampling resolution of the demodulator; the well-known peak-locking effect. The performance of the common CCA under these conditions was studied using simulations for single and multiple peak grating spectra and was compared with the performance of proposed, improved CCA method. We studied the cases where fibre Bragg gratings are wavelength-shifted with triangular and sinusoidal manner and how the proposed method behaves under these conditions. We experimentally evaluated our proposed method by utilising strain measurements using a fibre Bragg grating inscribed in a multimode gradient index CYTOP polymer fibre. The method and algorithm showed significant improvement concerning the peak-locking effect, while maintaining the response speed

Femtosecond laser inscribed Bragg grating arrays in long lengths of polymer optical fibres; A route to practical sensing with POF

The first practical demonstration of readily useable fibre Bragg gratings in long lengths (>20 m) of polymer optical fibres (POFs) is reported; this is an increase in fibre length that exceeds two orders of magnitude for use in the C-band. A low loss perfluorinated fibre and a femtosecond- laser, direct-write method to inscribe gratings at two different wavelengths are used. Gratings are fabricated to operate at ∼800 nm, where the optical loss is minimal, and in the C-band, commonly used with traditional optical fibres for the majority of telecommunication applications. The development of POF Bragg gratings inscribed in long fibre lengths is the first important step for the integration of practical sensing devices and could promote the use of multiplexed POF sensors. The sensors were tested and characterised and demonstrate their compatibility with a commercial Bragg grating demodulator

Low loss polymer fiber Bragg gratings sensors for effective optical sensing of strain and temperature

We present a strain and temperature polymer fiber sensor with exceptional optical properties based on femtosecond inscribed fiber Bragg gratings (FBG). The gratings were interrogated with a detection setup optimized for multimode gratings. The sensors' strain and temperature response were characterized and showed promising results for realistic sensing applications

Dynamic damage detection of a cantilever carbon beam using a FBG array inscribed in polymer gradient index multimode CYTOP fibre

In this paper we report on the use of a multiple-FBG array inscribed in gradient index multimode CYTOP POF using an efficient femtosecond laser inscription method. The FBG array was mounted across the cantilever as a quasi-distributed sensor and the time-depended wavelength information of each particular FBGpoint sensor was recovered using a commercial spectrometer, designed to operate at 1550nm. We adjusted the degree of damage detection by using different weights suspended in the middle of the flexible beam