
Ralph P TatamCranfield University · Department of Engineering Photonics
Ralph P Tatam
BSc (Hons) (Exon), PhD, DSc (Exon), CEng, FInstP, FSPIE
About
521
Publications
97,254
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
11,162
Citations
Introduction
Background and positions at www.linkedin.com
Additional affiliations
April 1989 - present
February 1985 - present
Publications
Publications (521)
A spectrometer built using an external cavity pulsed quantum cascade laser is described. The spectrometer has a tuning range from 10 – 13 µm (1,000 – 769 cm⁻¹) and is designed to target volatile organic compounds (VOCs) which often exhibit water-free molecular absorption within the region. The spectrometer utilizes a hollow silica waveguide gas cel...
Additive manufacturing (AM) or three-dimensional (3D) printing stands out for its remarkable ability to manufacture custom-designed preforms for specialty silica optical fibres (SOFs) featuring sophisticated structures and diverse material compositions. Here, a novel scheme for manufacturing preforms for highly birefringent multi-core silica optica...
Part quality monitoring and control in wire-based directed energy deposition additive manufacturing (w-DEDAM) processes has been garnering continuous interest from both the academic and industrial sectors. However, maintaining a consistent layer height and ensuring that the wall height aligns closely with the design, as depicted in computer-aided d...
This paper describes the steps taken to improve the measurement speed of a combined low-coherence and confocal refractive index measurement system. The instrument measures the refractive index and thickness of transparent plates using a fibre-based low-coherence interferometer with a line-scan spectrometer. The spectrometer allows on-line dispersio...
In-process optical monitoring has led to impressive enhancements in the quality control of metallic parts made by wire and
arc additive manufacturing (WAAM). Identification of material defects during a WAAM process gives an opportunity to
reduce post-process inspection, to pause the deposition to address the defect problem or terminate the process...
Range resolved interferometry (RRI) applied to the interrogation of an extrinsic Fabry-Perot based pressure sensor in laboratory and wind tunnel environments is presented. A simple, compact sensor head design was fabricated and subsequently characterised using RRI, which was shown to have a sensitivity of 1.627×10−3 rad/Pa with a noise standard dev...
An external cavity quantum cascade laser spectrometer combined with a hollow silica waveguide gas cell is described. Measurements of propane are made in the 10.2-11.6 µm region with a minimum detectable absorbance of 3.6×10 ⁻⁵ .
A multiplexed array of optical fibre based angle sensors for deployment on multiple joints of a robotic arm is demonstrated. Two optical fibres, each containing an array of fibre segment interferometers, are bonded either side of the neutral axis of a thin, flexible metal tape. Differential measurement of the phases of corresponding interferometers...
The strain sensing performance of an optical fibre Bragg grating (FBG), demodulated using a state-of-the-art commercial interrogator, is compared directly with that of a fibre segment interferometer (FSI) of near equivalent gauge length. It is shown that the FSI segment displayed a 5 fold improvement in noise, 1n ε /Hz for a bandwidth of 1 kHz, com...
Multi-core high birefringence silica optical fibre (Hi-Bi SOF) preforms have been fabricated by digital light processing (DLP) 3D printing and post-printing thermal processing. These fibre preforms of high solid loading of 0.8 are doped with GeO 2 , TiO 2 , Al 2 O 3 and B 2 O 3 in the cores, cladding, and stress applying parts (SAPs), respectively,...
Fibre-optic strain and shape sensing instrumentation were deployed on rotor blades during a full-speed helicopter ground run. Data were streamed wirelessly from rotor hub-mounted sensor interrogators, providing insights into the blade dynamics excited by pilot inputs to the controls and changes in rotation frequency. This paper explores the challen...
Optical fibre strain and shape measurement sensors were deployed on a 5-m long rotor blade during a full-speed (rotation rate 6.6 Hz) helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In Part 1 of a 2-part paper series, the strain sensing capabilities of the two optical fibre-based sensing t...
Optical fibre strain and shape measurement sensors were deployed on a rotor blade during a full-speed helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In part 2 of a 2-part paper series, two-dimensional direct fibre-optic shape sensing (DFOSS), using fibre segment interferometry-based inter...
In this work a range resolved interferometry (RRI) instrument for absolute distance measurements is integrated into a wire + arc additive manufacturing (WAAM) system to provide in-process monitoring of layer height, and prospects for volume and profile monitoring are discussed. Interferometry as a coherent optical technique offers a straightforward i...
Displacement measuring interferometers, commonly employed for traceable measurements at the nanoscale, suffer from non-linearities in the measured displacement that limit the achievable measurement uncertainty for microscopic displacements. Two closely related novel non-linearity correction methodologies are presented here that allow for the correct...
This paper compares two fibre optic sensing techniques for vibration characterisation: (a) optical fibre Bragg grating (FBG) strain gauges and (b) a novel direct fibre optic shape sensing (DFOSS) approach based on differential interferometric strain measurements between multiple fibres within the same fibre arrangement. Operational mode shapes and...
Non-linearities in interferometric displacement measurements commonly affect both homodyne and heterodyne optical interferometers. Unwanted back reflections (ghost reflections) or polarisation leakage introduce non-linearity terms at harmonics of the illuminating wavelength that cannot be fully corrected for with standard non-linearity correction t...
We describe the first use of range-resolved interferometric signal processing for measurement of spectral transmission. This was applied to gas sensing using tunable diode laser spectroscopy, allowing the simultaneous and independent measurement of methane concentrations in multiple gas cells. The system uses a single injection-current modulated di...
We present a novel multi-dimensional positional encoder approach that uses range-resolved interferometry (RRI) to simultaneously interrogate multiple interferometers differing in optical path length, using only a single photo detector and a single diode laser. Employing only stage-mounted non-polarising beamsplitters and reflections from three orth...
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 u...
Laser standoff Instrumentation has been developed to measure surface reflectivity of UV-aged bitumen at four specific mid-IR wavelengths. Preliminary measurements reveal heterogeneity across individual bitumen samples and an overall trend with bitumen age.
High sensitivity pressure measurements are reported using two intrinsic optical fibre methods: Fibre Bragg Gratings and Fibre Segment Interferometry. The optical fibres sensors are surface mounted onto a composite membrane and respective sensitivities of 79.2 µɛ/kPa and 65.8 µɛ/kPa have been achieved at resolutions of 0.79 µɛ and 0.04 µɛ.
The use of polarizing optics for both beam steering and phase measurement applications in displacement measuring interferometer designs is almost universal. Interferometer designs that employ polarizing optics in this manner are particularly sensitive to the effects of unwanted optical cavities that form within the optics due to polarization leakag...
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 ligh...
Chemical sensors based on optical absorption require accurate knowledge of the optical pathlength of the sample cell. Integrating spheres offer increased pathlengths compared to single pass cells combined with tolerance to misalignment, making them attractive for use in challenging environments subject to vibration. However, the equivalent optical...
We have investigated the release of gases and volatile organic compounds (VOCs) from a carbon fibre reinforced epoxy composite matrix used in aircraft structural components. Analysis was performed at several temperatures both up to and above the recommended operating temperature (121 °C) for the material, to a maximum of 250 °C. Gas chromatography-...
A novel suite of instrumentation for the characterisation of materials held inside an air-tight tube furnace operated up to 250 °C has been developed. Real-time detection of released gases (volatile organic compounds (VOCs), CO2, NO, NO2, SO2, CO and O2) was achieved combining commercial off-the-shelf (COTS) gas sensors and sorbent tubes for furthe...
A dual beam optical coherence tomography (OCT) instrument has been developed for flow measurement that offers advantages over microscope derived imaging techniques. It requires only a single optical access port, allows simultaneous imaging of the microfluidic channel, does not require fluorescent seed particles, and can provide a millimetre-deep de...
We present the development of an external cavity Bragg grating stabilized laser for tunable diode laser spectroscopy (TDLS). Our design uses a planar integrated silica-on-silicon platform incorporating a custom written Bragg grating as the wavelength-selective element of the laser cavity. We have developed a prototype singlemode laser at 1651 nm an...
Confocal scanning combined with low-coherence interferometry is used to provide remote refractive index and thickness measurements of transparent materials. The influence of lens aberrations in the confocal measurement is assessed through ray-trace modeling of the axial point-spread functions generated using optical configurations comprised of pair...
A novel dual beam Optical Coherence Tomography (OCT) instrument has been developed for high velocity flow measurement, principally in microfluidics applications. The scanned dual beam approach creates a pair of image-frames separated by a small spatiotemporal offset. Metre-per-second flow measurement is achieved by rapid re-imaging by the second be...
This paper investigates the application of feature tracking algorithms as an alternative data processing method for laser speckle instrumentation. The approach is capable of determining both the speckle pattern translation and rotation and can therefore be used to detect the in-plane rotation and translation of an object simultaneously. A performan...
The development of reliable, affordable and efficient sensors is a key step in providing tools for efficient monitoring of critical environmental parameters. This review focuses on the use of tapered optical fibres as an environmental sensing platform. Tapered fibres allow access to the evanescent wave of the propagating mode, which can be exploite...
A new method for the analysis and design of fiber Bragg gratings (FBGs) based on the theory of transmission lines has been developed and verified both theoretically and experimentally. The method is an extension of the coupled-mode theory and utilizes the equivalent transmission lines in order to simulate any type of grating, with an easy and direc...
Long period gratings (LPGs) as a sensing platform have the potential for multi-parameter measurement through the utilization of 2nd order coupling resonance bands. Although the current literature has produced LPGs with 2nd order resonance bands, the fabrication parameters required to generate these features have not been elucidated. Here, using UV...
This paper reports on the development of a new, non-contact, cost-effective and widely-applicable optical sensor for the measurement of the three degrees of translational freedom of a robotic end-effector. The workpiece positioning sensor (wPOS) tracks the relative position between the workpiece and end-effector in real-time, using laser speckle co...
A Long Period Grating based sensor is developed for the rapid detection of Campylobacter jejuni. The sensitivity of the sensor is enhanced by bacterial staining allowing the detection of concentrations as low as 102 CFU/mL.
A novel dual beam Optical Coherence Tomography (OCT) system is presented for microfluidics applications. The dual sensing-light-sheet approach presented here achieves high velocity flow measurement because the rapid re-imaging by the second sensing-light-sheet allows for particle tracking between each image-frame of the pair. We have demonstrated m...
The process of fabricating tapered long period gratings (TLPGs) using a CO2 laser is described. The objective of this work is to optimize the CO2 laser based fabrication of TLPGs so that their transmission spectra are comparable with those made using the arc-discharge technique, which typically exhibits transmission losses below 2 dB. The losses cu...
This paper describes a technique for measuring refractive index and thickness of transparent plates using a fibre-optic low-coherence interferometer. The interferometer is used to independently measure quantities related to the phase and group refractive indices, np and ng, of the material under investigation. Additionally, the dispersion of the ph...
We describe a simple method for detecting formaldehyde using low resolution non-dispersive UV absorption spectroscopy. A two channel sensor was developed, making use of a strong absorption peak at 339 nm and a neighbouring region of negligible absorption at 336 nm as a reference. Using a modulated UV LED as a light source and narrow laser-line filt...
A dual-beam Optical Coherence Tomography system has been developed, using a bespoke dual optical fibre, to simultaneously image microfluidic channel structures and measure high velocity flows (presently 250μm/s) from a single optical access point.
All-optical switching using an optical fibre long-period gating (LPG) modified with bacteriorhodopsin (bR) is demonstrated. The switching process is based on the photo-induced RI change of bR, which in turn changes the phase matching conditions of the mode coupling by the LPG, leading to modulation of the propagating light. The effect was studied w...
The diameter of long-period fiber gratings (LPFGs) fabricated in optical fibers with a low cutoff wavelength was be reduced by hydrofluoric acid etching, enhancing the sensitivity to refractive index by more than a factor of 3, to 2611 nm/refractive index unit in the range from 1.333 to 1.4278. The grating period selected for the LPFGs allowed acce...