In-line fibre-optic laser doppler velocimeter using bragg grating interferometric filters as frequency to intensity transducers

Three dimensional complex flows particularly those of turbomachinery present challenges to current measurement technology in terms of restricted optical access, measurement accuracy for the on-axis velocity component, the need to resolve flow turbulence and measurement difficulty from close to surface or intra-channel measurements in rotating machinery. A novel non-intrusive in-line fibre-optic laser Doppler velocimeter is presented specifically for the measurement of the on-axis component of velocity. The measurement principle is based on a Doppler frequency to intensity transducer in the form of a fibre-optic Bragg grating based Fabry-Perot interferometric filter. The filters were fabricated at 514.5 nm but in principle any desired wavelength may be used thus permitting any laser wavelength source to be used. Filters with appropriate features were designed with the aid of the theoretical models based on the coupled mode theory and transfer matrix approach. The argon-ion laser emission wavelength was locked to a corresponding Doppler broadened absorption line of molecular iodine vapour while the Fabry-Perot interferometer phase was controlled in an independent feedback system using digital lock-in amplifiers. The optical frequency was stabilized to within 10 MHz for at least one hour while the phase was controlled to an equivalent of (within) ± 3 MHz in frequency. Both feedback loops utilized custom designed PID electronic circuit controllers. The bandwidth of the filter was tunable by up to 400 MHz, with a resolution of between 0.2 ms'1 and 1 ms"1, and a sensitivity range of between 0.5 [GHz]'1 and 1.7 [GHz]'1. In this technique the filter was tuned to the optical wavelength, rather than tuning the laser wavelength to match the filter. The finished instrument was applied to the measurement of the on-axis component of velocity, of a rotating disc, over an available range of up to ± 42 ms'1, limited only by the maximum velocity of the disc. The detection system was reconfigured for low velocity measurements at twice the sensitivity over a velocity range of ± 7 ms'1. This technique demonstrates a unique contribution to fluid dynamics for the measurement of the traditionally difficult in-line component of velocity.Ph

A simple method for fabricating phase-shifted fibre Bragg gratings with flexible choice of centre wavelength

A simple technique for fabricating phase-shifted fibre Bragg gratings (PSFBGs) without the use of a phase-shifted phase mask is presented. Two, 3-mm long, standard fibre Bragg gratings (FBGs) were inscribed sequentially in singlemode fibre at the same Bragg wavelength such that the FBGs physically overlapped by one grating period. This induces a spectral-hole in the middle of the reflection spectrum of a standard FBG, equivalent to a π-phase shifted FBG. The flexibility of the technique in writing PSFBGs at any choice of wavelength is demonstrated. The results show that PSFBG devices produced by this method are highly reproducible and the process is fas

A transverse loading technique to enhance the pressure measurement capability of fibre Bragg gratings

A fibre optic pressure sensor based on the application of a transverse load onto an epoxy-resin material embedded over a sub-section of a fibre Bragg grating (FBG) is presented. When a transverse load is applied to the epoxy material it deforms and transfers the load onto the optical fibre in the form of an axial strain that changes the period of the FBG over the embedded region. This introduces a phase shift between the two resulting identical gratings that lie either side of the loaded section, creating a spectral drop-out within the bandwidth of the FBG. A 2 mm section at the centre of FBGs of length 6 mm, recorded in both single mode and highly linearly birefringent optical fibres, was embedded in resin, and were subsequently subjected to a transverse load. The loading technique, while offering protection to the optical fibre from mechanical damage, enhances transverse load sensitivity, without introducing birefringence

Multiplexing curvature sensors using fibre segment interferometry for lateral vibration measurements

Dynamic fibre-optic curvature sensing is demonstrated by interrogating chains of fibre segments, separated by broadband Bragg grating reflectors, using range-resolved interferometry (RRI). Four fibre strings, containing four fibre segments each of gauge length 20 cm, are attached to the opposing sides of a support structure and the resulting differential strain measurements allow inference of lateral displacements of a cantilever test object. Dynamic tip displacement resolutions in the micrometre range at an interferometric bandwidth of 21 kHz demonstrate the suitability of this approach for highly sensitive and cost-effective fibre-optic directional vibration measurements of smart structures. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use onl

Interferometric fibre-optic curvature sensing for structural, directional vibration measurements

Dynamic fibre-optic curvature sensing using fibre segment interferometry is demonstrated using a cost-effective rangeresolved interferometry interrogation system. Differential strain measurements from four fibre strings, each containing four fibre segments of gauge length 20 cm, allow the inference of lateral vibrations as well as the direction of the vibration of a cantilever test object. Dynamic tip displacement resolutions in the micrometre range over a 21 kHz interferometric bandwidth demonstrate the suitability of this approach for highly sensitive fibre-optic directional vibration measurements, complementing existing laser vibrometry techniques by removing the need for side access to the structure under test

Multiplexing curvature sensors using fibre segment interferometry for lateral vibration measurements

Dynamic fibre-optic curvature sensing is demonstrated by interrogating chains of fibre segments, separated by broadband Bragg grating reflectors, using range-resolved interferometry (RRI). Four fibre strings, containing four fibre segments each of gauge length 20 cm, are attached to the opposing sides of a support structure and the resulting differential strain measurements allow inference of lateral displacements of a cantilever test object. Dynamic tip displacement resolutions in the micrometre range at an interferometric bandwidth of 21 kHz demonstrate the suitability of this approach for highly sensitive and cost-effective fibre-optic directional vibration measurements of smart structures. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use onl

All-electronic frequency stabilization of a DFB laser diode

A laser diode’s junction voltage is a sensitive measure of its temperature and can be used in a thermal control feedback loop. To compensate for the temperature dependence of the laser’s internal resistance, we have measured the dynamic resistance, ∂V/∂I, by modulating the injection current and measuring the demodulated voltage. The junction voltage was thus controlled while operating at fixed DC injection current. Over an external temperature range of 15°C to 35°C, this stabilised the centre frequency (wavelength) of a 1651 nm DFB laser diode with a residual mean frequency shift of 60 MHz (0.5pm), less than the uncertainty on the centre frequency of 80 MHz (0.7 pm). Under the same conditions, conventional thermistor control gave a systematic wavelength shift of −8.4 GHz (−76 pm), and control of the uncompensated forward voltage gave a shift of 9.9 GHz (90 pm)

Fabrication of fiber optic long period gratings operating at the phase matching turning point using an ultraviolet laser

It is known that optical fiber long period gratings (LPGs) exhibit their highest sensitivity to environmental perturbation when the period is such that the phase matching condition is satisfied at its turning point. The reproducible fabrication of LPGs with parameters satisfying this condition requires high resolution control over the properties of the grating. The performance of an LPG fabrication system based on the point-by-point UV exposure approach is analyzed in this paper, and the control of factors influencing reproducibility, including period, duty cycle, and the environment in which the device is fabricated, is explored

High sensitivity pressure measurement using optical fibre sensors mounted on a composite diaphragm

A pressure sensor specified for aerodynamic applications and based on optical fibre strain sensors mounted on a circular glass fibre reinforced polymer membrane is presented. The use of two fibre optic strain sensing technologies is explored, the novel intrinsic fibre segment interferometry (FSI) approach and fibre Bragg gratings (FBGs), with the use of FSI shown to offer a pressure resolution that is 15 times larger than that achieved using an FBG. A number of design and fabrication issues are considered, including the position of the fibres relative to the neutral axis of the membrane and the influence of the membrane support structure on the thermal and pressure sensitivities of the sensor, with particular regards to pressure and temperature discrimination

Production process monitoring and post-production strain measurement on a full-size carbon-fibre composite aircraft tail cone assembly using embedded optical fibre sensors

Multiplexed optical fibre sensors were embedded into a carbon-fibre-reinforced-preform during the industrial production of a full-sized, one-piece tail cone assembly for a regional jet aircraft. Optical fibre Fresnel sensors monitored both the infusion of the resin, via measurement of the refractive index-dependent attenuation in the reflected light signal, and the degree of cure of the resin, via measurement of the chemical cure reaction-dependent change in refractive index. The resin cure was also monitored by optical fibre Bragg gratings (FBGs) fabricated in high linearly birefringent optical fibre, which measured through-thickness strain development, while FBGs in standard single mode optical fibre measured longitudinal strain development. The magnitudes and profiles of the transverse and longitudinal strains developed during the curing process were consistent across different locations on the tail cone. Typical transverse and longitudinal strains, related to cure reaction-induced shrinkage, were −1500 ± 17 μepsilon and −500 ± 5 μepsilon, respectively. Post-production, the same embedded FBG sensors were used subsequently to monitor structural strains when the tail cone was subjected to vacuum pressure loading. The longitudinal strains measured using the embedded FBG sensors were generally in good agreement with the longitudinal strains measured by the surface-bonded resistance foil strain gauge (RFSG) sensors, both qualitatively and quantitatively. The in-plane transverse and circumferential strains, oriented collinearly, were measured by the embedded FBGs and appropriately oriented surface-bonded RFSG sensors, respectively, and were, qualitatively, in good agreemen