3,063 research outputs found
The frequency-dependent directivity of a planar Fabry-Perot polymer film ultrasound sensor
No full textA model of the frequency-dependent directivity of a planar, optically-addressed, Fabry-Perot (FP), polymer film ultrasound sensor is described and validated against experimental directivity measurements made over a frequency range of 1 to 15 MHz and angles from normal incidence to 80 degrees. The model may be used, for example, as a predictive tool to improve sensor design, or to provide a noise-free response function that could be deconvolved from sound-field measurements in order to improve accuracy in high-frequency metrology and imaging applications. The specific question of whether effective element sizes as small as the optical-diffraction limit can be achieved was investigated. For a polymer film sensor with a FP cavity of thickness d, the minimum effective element radius was found to be about 0.9d, and that an illumination spot radius of less than d/4 is required to achieve it
Characterization of a novel three-section tunable slotted fabry-perot laser
No full textA novel three-section tunable slotted-Fabry-Perot laser has been examined. Characterizations of the linewidth, SMSR and RIN indicate that this type of laser may be a suitable source for dynamic networks employing advanced optical modulation formats
Rouard's method as a modelling tool for the sensing characteristics of complex fibre Fabry-Perot interferometers formed between chirped fibre Bragg gratings
No full textPhase and spectral response characteristics of fibre Fabry-Perot interferometers
(FFPI), formed between linearly chirped fibre Bragg gratings (FBG), were
theoretically modelled using Rouard's method. The method models accurately the
phase shift through a complex grating structure which the transfer matrix model
is unable to achieve. The FBG FFPI cavities were modelled with variable free
spectral range that was tailored further to provide strain insensitivity and
strain enhancement by making the chirp parameter and cavity length satisfy the
desired conditions at a given wavelength of illumination
Micro-machining Techniques for the Fabrication of Fibre Fabry-Perot Sensors
No full textFabry-Perot optical fibre sensors have been used extensively for measuring a variety of
parameters such as strain, temperature, pressure and vibration. Conventional extrinsic fibre Fabry-Perot sensors are associated with problems such as calibration of the gauge length of each individual sensor, their relatively large size compared to the diameter of optical fibre and a manual manufacturing method that leads to poor reproducibility.
Therefore, new designs and fabrication techniques for producing fibre Fabry-Perot
sensors are required to address the problems of extrinsic fibre Fabry-Perot sensors.
This thesis investigates hydrofluoric acid etching and F2-laser micro-machining of optical fibres to produce intrinsic Fabry-Perot cavities. Chemical etching of single mode fused silica fibres produced cavities across the core of the fibres due to preferential etching of the doped-region. Scanning electron microscope, interferometric surface profiler and
CCD spectrometer studies showed that the optical quality of the etched cavities was
adequate to produce Fabry-Perot interference. Controlled fusion splicing of etched fibres produced intrinsic Fabry-Perot cavities. These sensors were surface-mounted on composite coupons and their response to applied strain was studied using low coherence interferometry. These sensors showed linear and repeatable response with the strain measured by the electrical resistance strain gauges.
To carry out F2-laser micro-machining of fused silica and sapphire substrates, a micro-machining station was designed and constructed. This involved the design of illumination optics for 157 nm laser beam delivery, the design and construction of beam delivery chamber, target alignment and monitoring systems. Ablation of fused silica and sapphire disks was carried out to determine ablation parameters suitable for micro-machining high aspect ratio microstructures that have adequate optical quality to produce Fabry-Perot
interference. Cavities were micro-machined through the diameter of SMF 28 and SM 800
fibres at different energy densities. CCD interrogation of these intrinsic fibre cavities ablated at an energy density of 25 x 10 4 Jm -2 produced Fabry-Perot interference fringes.
The feasibility of micro-machining high aspect ratio cavities at the cleaved end-face of the fused silica fibres and through the diameter of sapphire fibres was demonstrated. A technique based on in-situ laser-induced fluorescence monitoring was developed to determine the alignment of optical fibres and ablation depth during ablation through the fibre diameter. Ablation of cavities through the diameter of fibre Bragg gratings showed that the heat-generated inside the cavity during ablation had no effect on the peak reflection and the integrity of core and cladding of the fibre. Finally, a pH-sensor, a chemical sensor based on multiple cavities ablated in multimode fibres and a feasible design for pressure sensor fabrication based on ablated cavity in a single mode fibre were demonstrated
Thermospheric winds and temperatures above Mawson, Antarctica, observed with an all-sky imaging, Fabry-Perot spectrometer
No full textA new all-sky imaging Fabry-Perot spectrometer has been installed at Mawson station
(67°36' S, 62°52' E), Antarctica. This instrument is capable of recording
independent spectra from many tens of locations across the sky simultaneously. Useful operation
began in March 2007, with spectra recorded on a total of 186 nights. Initial analysis has focused
on the large-scale daily and average behavior of winds and temperatures derived from observations
of the 630.0 nm airglow line of atomic oxygen, originating from a broad layer centered around
240 km altitude, in the ionospheric F-region.
The 1993 Horizontal Wind Model (HWM93), NRLMSISE-00 atmospheric model, and the Coupled
Thermosphere/Ionosphere Plasmasphere (CTIP) model were used for comparison. During the
geomagnetically quiet period studied, observed winds and temperatures were generally well modelled,
although temperatures were consistently higher than NRLMSISE-00 predicted, by up to 100 K. CTIP
temperatures better matched our data, particularly later in the night, but predicted zonal winds
which were offset from those observed by 70–180 ms−1 westward. During periods of increased
activity both winds and temperatures showed much greater variability over time-scales of less than
an hour. For the active night presented here, a period of 45 min saw wind speeds decrease by
around 180 ms−1, and temperatures increase by approximately 100 K. Active-period winds were
poorly modelled by HWM93 and CTIP, although observed median temperatures were in better agreement
with NRLMSISE-00 during such periods.
Average behavior was found to be generally consistent with previous studies of thermospheric winds
above Mawson. The collected data set was representative of quiet geomagnetic and solar conditions.
Geographic eastward winds in the afternoon/evening generally continued until around local
midnight, when winds turned equatorward. Geographic meridional and zonal winds in the afternoon
were approximately 50 ms−1 weaker than expected from HWM93, as was the transition to
equatorward flow around midnight. There was also a negligible geographic zonal component to the
post-midnight wind where HWM93 predicted strong westward flow. Average temperatures between 19:00
and 04:00 local solar time were around 60 K higher than predicted by NRLMSISE-00
Axially symmetrical Fabry-Perot oscillator with multiple devices inserted in dielectric substrate
No full textWe investigate an axially symmetrical Fabry-Perot oscillator with active devices inserted in a dielectric substrate for power combining of many more devices in the microwave and millimeter wave frequency range. Empirically in this oscillator, efficient power combining can be done when it oscillates approximately at the frequency which corresponds to the wavelength equal to twice the spacing between the devices. The wavelength in the dielectric is shorter than in free space, so we tried to insert the devices in the dielectric substrate in order to increase the number of devices. By measuring the oscillation frequency of the oscillator with sixteen devices at X-band, we confirmed that the spacing between devices was about a half wavelength in the dielectric. We achieved almost perfect power combining of sixteen device
Planar micromachined glass cantilevers utilising integrated Bragg Fabry-Perot cavities
No full textHere we demonstrate a glass cantilever based on a unique micromachining and etching approach, combined with UV written Bragg gratings. We shall also discuss the increase in sensitivity by using two Bragg gratings to form Fabry-Pérot cavity. Cantilevers are in ultra sensitive force sensors used in applications such as Atomic Force Microscopy, mass sensing and acoustic transducers
Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging
No full textA 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 /spl times/ 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-/spl mu/m-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-/spl mu/m Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing
Analytical method to find the optimal parameters for gas detectors based on correlation spectroscopy using a Fabry-Perot interferometer
No full textSeveral designs of infrared sensors use a Fabry-Perot Interferometer (FPI) to modulate the incident light. In this work we analyse the particular case where the FPI fringes are matched with very well defined ro-vibrational absorption lines of a target molecule such as CO. In this kind of sensor modulation is induced by scanning the FPI cavity length over one half of the reference wavelength. Here we present an analytical method based on the Fourier transform which simplifies the procedure to determine the sensor response. Furthermore, this method provides a simple solution to finding the optimal FPI cavity length and mirror reflectivity. It is shown that FPI mirrors with surprisingly low reflectivity (<50%) are generally the optimum choice for target gases at atmospheric pressure. Finally experimental measurements and simulation results are presented
An analytical method to find the optimal parameters for gas detectors based on correlation spectroscopy using a Fabry-Perot interferometer
No full textSeveral designs of infrared absorption based gas detector use a Fabry-Perot Interferometer (FPI) to modulate the incident light. In these systems, generally the FPI's fringes are matched with very well defined rotational absorption lines of a target molecule such as CO2, CO, N2O, CH4, etc. In order to obtain modulation the cavity length of the FPI is scanned over one half of the reference wavelength. In this work, we present a simple analytical method based on the Fourier Transform that describes the performance of these systems. Using this method the optimal reflectivity and optical spacing of the FPI can be determined. Furthermore, the modulated signal generated by the system as a function of the cavity length scan can be calculated by applying the inverse Fourier Transform. Finally, this method describes the underlying reasons why for some filters the background amplitude is severe, and gives guidance on the choice of optimised filters. Our method evaluates the optimal FPI parameters and the modulated signal much faster than the direct numerical computation which is used currently. Simulation results for different molecules in combination with diverse filters shapes are presented, with a comparison to directly computed results
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