In this study, two fiber Bragg grating sensors are multiplexed using time division multiplexing. The fiber Bragg gratings are setup in a dual bus configuration and the sensor signals are detected using a transmit-reflect detection system. The results demonstrate that both the transmitted and reflected signals from both fiber Bragg gratings can be resolved when the sensors are separated by a sufficient distance.
In this paper, we report the response of Cobalt-60 gamma irradiation on Photonic Crystal Fibre Bragg gratings (PCFFBGs) and standard commercial FBGs (STD-FBGs). Optical measurements were performed to determine the shift of the Bragg wavelength as a function of accumulated dose and relaxation time. To simulate real time conditions of a radiation dosimeter, the FBGs are examined through three consecutive radiation stages followed by very limited recovery times. We were able to obtain a Bragg wavelength shift with both sets of FBGs. The PCF-FBGs response included strong recovery after each irradiation compared to the STD-FBGs. This makes the PCF-FBGs strong candidates as optical fibre FBG sensors in the area of radiation dosimetry.
In this study, we have simulated the effect of light sources with different spectral output functions on the generation of A-scans in optical coherence tomography using a fundamental physics-based interferometric model. Many different source function were examined, and compared to a standard Gaussian source. These sources included truncated Gaussians, multiple Gaussians, other non-Gaussian, Lorentzian, square and triangular sources. Only the pure Gaussian source produced A-scans without false artefacts such as satellite peaks, that could produce misinterpretation of real OCT images that may be used for patient diagnosis. A triangular source produces the next best response with small extraneous peaks, whereas all other sources have significant false artefacts present in their A-scans.
The axial resolution is a critical parameter in determining whether optical coherent tomography (OCT) can be used to resolve specific features in a sample image. Typically, measures of resolution have been attributed to the light source characteristics only, including the coherence length and the point spread function (PSF) width of the OCT light sources. The need to cost effectively visualize the generated PSF and OCT cross-correlated interferogram (A-scan) using many OCT light sources have led to the extrinsic evolution of the OCT simulation model presented. This research indicated that empirical resolution in vivo, as well as depending on the light source’s spectral characteristics, is also strongly dependent on the optical characteristics of the tissue, including surface reflection. This research showed that this reflection could be digitally removed from the A-scan of an epithelial model, enhancing the stratum depth resolution limit (SDRL) of the subsurface tissue. Specifically, the A-scan portion above the surface, the front surface interferogram, could be digitally subtracted, rather than deconvolved, from the subsurface part of each A-scan. This front surface interferogram subtraction resulted in considerably reduced empirical SDRLs being much closer to the superluminescent diodes’ resolution limits, compared to the untreated A-scan results.
Using a Low Coherence Interferometry (LCI) model, a comparison of broadband single-Gaussian and multi-Gaussian
light sources has been undertaken. For single-Gaussian sources, the axial resolution improved with source bandwidth,
confirming the coherence length relation that resolution for single Gaussian sources improves with increasing spectral
bandwidth. However, narrow bandwidth light sources resulted in interferograms with overlapping strata peaks and the
loss of individual strata information. For multiple-Gaussian sources with the same bandwidth, spectral side lobes
increased, reducing A-scan reliability to show accurate layer information without eliminating the side lobes. The
simulations show the conditions needed for resolution of strata information for broadband light sources using both single
and multiple Gaussian models. The potential to use the model to study LCI and OCT light sources, optical delays and
sample structures can better characterise these LCI and OCT elements. Forecasting misinformation in the interferogram,
may allow preliminary corrections. With improvements to the LCI-OCT model, more applications are envisaged.
In this study, a fibre Bragg grating (FBG) was embedded beneath three common flooring materials acting as a pressure
switch for in-ground intrusion detection. This is achieved using an intensiometric detection system, where a laser diode
and FBG were optically mismatched so that there was a static dc offset from the transmitted and reflected optical power
signals. As pressure was applied, in the form of a footstep, a strain induced wavelength shift occurred that could then be
detected by converting the wavelength shift into an intensity change. The change in intensity caused a significant change
in the DC offset which behaved as on optical switch. This switch could easily be configured to trigger an alarm if
required. The intention is to use the FBG sensor as an in-ground intrusion detection pressure switch to detect an intruder
walking within range of the sensor. This type of intrusion detection system can be applied to both external (in soil, etc)
and internal (within the foundations or flooring of the home) security systems. The results show that a person's footstep
can clearly be detected through solid wood flooring, laminate flooring, and ceramic floor tiles.
The use of Optical Coherence Tomography (OCT) in early cancer detection is still under development. While the
specificity and precision of the technique has improved, the development of affordable, portable OCT configurations is
important for increased clinical access by general practitioners. To this end, a proposed microphotonic time domain (TD)
OCT system is being developed, based on a liquid crystal array and a microphotonic stepped mirror structure. In order to
characterize the practicality of this system and its performance compared to other optical delay line (ODL) and OCT
configurations, a previously demonstrated analytical simulation model has now been extended to retrieve from the
interferogram, depth profiles and reflectivities for better strata OCT definition. Based on a Michelson interferometer
configuration, the model allows user definition of the broadband light source, the sample's characteristics and the ODL
configuration. User defined sample characteristics include the number, thickness and reflectivities of layers. The purpose
of the forwards model was to compare the conventional moving ODL reference arms with their quasi-stationary and
stationary alternatives. The primary goal of the current investigation is to determine the efficacy of the backward fitting
model (BFM) that uses a genetic algorithm to iteratively optimize solutions for the layer thickness and layer reflectivities
for a given simulated interferogram. The genetic algorithm does retrieve the depth and reflectance of the layers identified
in the interferogram, improving in precision and accuracy with each generation. The BFM can deconvolve
interferograms produced using different types of ODL, with the prospect of improving the proposed discrete-step quasistationary
optical delay line functionality.
The current development of UV-Blue sensitive photo-detectors has lead to investigations with the polywell-stacked
gradient poly-homojunction (StaG) configuration. Backwall illumination is of interest due to increased fill factor and
pixel wavelength band tailoring. The StaG architecture has benefits. However, for benefit to backwall illumination the
space charge region needs to be depleted to the backwall, possibly dispensing with the need for the StaG multi-layer.
This research is an initial investigation of the benefit to crosstalk and sensitivity of the deep single well in high
resolution, 5 μm pitch, photodiode arrays. The results indicate that geometries that are of fabricatable morphology can
benefit backwall illumination especially in the U/V-blue wavelength spectrum.
In this study, we compare the practical implementation of both silicon and germanium Photovoltaic Power Converters
(PPCs). Simulations have previously shown that silicon PPCs can produce up to 43% optical to electrical power
conversion and germanium PPCs can produce conversion efficiencies as high as 22% when illuminated by 980nm light.
Moreover, germanium can produce conversion efficiencies of up to 36% when illuminated by 1550nm light. Here, we
compare these results to real power conversion efficiencies of off-the-shelf silicon and germanium photodiodes,
producing 9.9% and 8.0% conversion efficiencies, respectively for 980nm. Furthermore, we show germanium produces
conversion efficiencies up to 14.6% under illumination of 1550nm light. A discussion of the limitations is made. The
results show there is a peak efficiency point corresponding to a specific input optical power. We also show that the
power over fibre signal can be successfully combined with communications signals, using wavelength division
multiplexing, and that the multiplexed signals can be separated without significant loss of signal, or power conversion
efficiency. In addition, we investigate the affects of free space problems, such as divergence and misalignment, in both
the lateral and longitudinal directions. As expected, optical alignment plays a significant role in producing maximum
power conversion.
Most automated industrial processes use Distributed Control Systems (DCSs) or Programmable Logic Controllers
(PLCs) for automated control. PLCs tend to be more common as they have much of the functionality of DCSs, although
they are generally cheaper to install and maintain. PLCs in conjunction with a human machine interface form the basis of
Supervisory Control And Data Acquisition (SCADA) systems, combined with communication infrastructure and Remote
Terminal Units (RTUs). RTU's basically convert different sensor measurands in to digital data that is sent back to the
PLC or supervisory system. Optical fibre sensors are becoming more common in industrial processes because of their
many advantageous properties. Being small, lightweight, highly sensitive, and immune to electromagnetic interference,
means they are an ideal solution for a variety of diverse sensing applications. Here, we have developed a PLC Optical
Fibre Sensor Interface Module (OFSIM), in which an optical fibre is connected directly to the OFSIM located next to the
PLC. The embedded fibre Bragg grating sensors, are highly sensitive and can detect a number of different measurands
such as temperature, pressure and strain without the need for a power supply.
In conventional time-domain Optical Coherence Tomography (OCT), a moving mirror is used as a reference optical
delay line. This motion can result in instrument degradation, and in some situations it is preferable to have no moving
parts. Stationary optical delay lines using a variety of methods have been proposed. Of particular interest, due to its low
cost, is the use of a micro-photonic stationary optical delay line, made up of an addressable Stepped Mirror Structure
(SMS) using a liquid crystal optical switch. Here the individual steps of the SMS can be selected by the liquid crystal
array. For use in OCT, the discrete nature of the SMS needs to be overcome by having the step height less than the
coherence length of the low coherent light source. Typical coherence lengths in current OCT systems are on the order of
10μm. Hence, micrometer size steps require the use of a relevant fabrication method. In this paper, we compare SMSs
fabricated using wet and dry etching methods. Specifically, Reactive Ion Etching (RIE) using CF4/O2 and chemical bath
etching, using a solution of HF, HNO3 and Acetic acid. Three inch diameter silicon wafers, 400μm thick, were etched by
both methods. The RIE was used to produce a SMS with five 5μm high steps each step approximately 1 cm wide. The
wet etching produced an SMS with three 15μm steps approximately 2 cm wide. The overall structures of the SMSs were
compared using optical profilometry. The RIE step quality was far superior to the wet etch method due to the ability to
control the anisotropy of the RIE method.
In this paper, we demonstrate the use of wireless acoustic communications through the human body, in-vivo. The
acoustic communications signals are intended to be used for fixed in-vivo biomedical devices. In-vivo biomedical devices
include, for example, pacemakers, but more importantly, neural implants. The use of acoustic communications for neural
implants represents a significant improvement as wired and wireless RF communications cannot be utilised. The acoustic
communications channel comprises of a piezoelectric transducer as the transmitter, a section of the human body as the
transmission medium, and a second piezoelectric transducer as the receiver. In this initial work, a forearm was used as
the transmission medium.
Communicating acoustically through the human body was successfully achieved. We present results showing the
performance of the acoustic communications channel. The frequency response, transfer function and transient response
(at resonance) of the communications channel were measured. Due to the frequency response of the communications
channel, phase shift keying was chosen as the digital modulation method. Sample communications signals are included.
For comparison, amplitude shift keying results are also shown. The results suggest that a data rate of over 10kbps could
be achieved with the configuration used.
Acoustic sensors are used in Structural Health Monitoring (SHM) for the detection of impacts and strain. However,
secondary damage may result from the initial damage. This secondary damage, such as delamination or cracking, may
not be detectable by the SHM system. This is a significant problem for passive sensing systems, such as those based on
fibre optics, where signals cannot be actively generated to interrogate the structure. The integration of NDE by robotic
agents into a SHM sensor network enables the detection and monitoring of a wider variety of damage. Communicating
via acoustic transmissions represents a wireless communication method for robotic agents to communicate to the SHM
system without the addition of extra hardware, as piezoelectric transducers are commonly used in NDE. The effect of
Carbon Fibre Composites (CFC) on the ability to use acoustic transmission needs to be determined.
We present results for the detection of Acoustic Emissions and Transmissions (AET) in a CFC laminate. The optical
fibre AET detector was a Fibre Bragg Grating (FBG). Two FBG AET sensors were compared, one coupled to the surface
of the carbon fibre sheet, and one embedded within the lay-up. Results compare the transfer function, frequency
response, and transient response of the sensors. The embedded FBG receiver was also used to detect an actively
generated acoustic transmission. A piezoelectric receiver was also used for comparison. The embedded FBG was found
to give significantly better performance in all of the parameters considered for the surface coupled FBG.
In this paper the switching of a proposed Stationary Optical Delay Line (SODL) is demonstrated. This is intended for
proof of principle of the switching associated with such a SODL, to be applied to an Optical Coherence Tomography
(OCT) system.. The proposed SODL is made up of one dimensional beam expanding cylindrical lenses, a liquid crystal
transmissive Spatial Light Modulator (SLM), and a Stepped Mirrored Structure (SMS). The SLM is to be used as an
addressable optical switch. The SMS is an array of staggered mirrored steps, where the step height corresponds to half
the optical delay length. The required delay length from the SMS can then be selected with the SLM. In this work, beam
expanding optics and a nematic Liquid Crystal Display (LCD) are used to demonstrate the ability to select a spatial
region consisting of a row of 4 photodiodes substituting for the SMS. The principle of conventional sequential switching,
depth hoping, and multicasting of the four windows generated on the SLM are demonstrated. Rise and fall times were
260 and 150 ms, sufficient to prove the principle of switching at 1Hz. A maximum of 2 Hz could be achievable without
detriment to the contrast ratio. The contrast ratios between transparent and opaque LCD window states was 2.4 ± 0.2.
The contrast ratios between transparent and light-off states was 23 ± 4. Hence, the contrast ratios between opaque and
light-off states was 9 ± 1. These values were within the expected ranges for nematic LC SLMs.
Conventional time domain Optical Coherence Tomography (OCT) relies on a reference Optical Delay Line (ODL).
These reference ODLs require the physical movement of a mirror to scan a given depth range. This movement results in
instrument degradation. We propose a new optical fibre based time domain OCT system that makes use of a micro-photonic
structure as a stationary ODL. The proposed system uses an in-fibre interferometer, either a Michelson or a
Mach-Zhender. The reference ODL makes use of a collimator to expand the light from the optical fibre. This is them
expanded in one dimension via planar optics, that is, a cylindrical lens based telescope, using a concave and convex lens.
The expanded beam is them passed through a transmissive Spatial Light Modulator (SLM), specifically a liquid crystal
light valve used as an optical switch. Light is then reflected back through the system off the micro-photonic structure.
The micro-photonic structure is a one dimensional array of stagged mirror steps, called a Stepped Mirror Structure
(SMS). The system enables the selection of discrete optical delay lengths. The proposed ODL is capable of depth hoping
and multicasting. We discuss the fabrication of the SMS, which consists of eight steps, each approximately 150 μm high.
A change in notch frequency using an in-fibre Mach Zhender interferometer was used to gauge the average step height.
The results gave an average step height of 146 μm.
Conventional time domain Optical Coherence Tomography (OCT) relies on the detection of an interference pattern generated by the interference of backscattered light from the sample and a reference Optical Delay Line (ODL). By referencing the sample interference with the scan depth of the ODL, constructive interference indicates depth in the sample of a reflecting structure. Conventional ODLs used in time domain OCT require some physical movement of a mirror to scan a given depth range. This movement results in instrument degradation. Also in some situations it is necessary to have no moving parts. Stationary ODLs (SODLs) include dual Reflective Spatial Light Modulator (SLM) systems (Type I) and single Transmissive SLM with match-arrayed-waveguide systems (Type II).
In this paper, the method of fabrication and characterisation of a number of Stepped Mirrored Structures (SMS) is presented. These structures are intended for later use in proof-of-principle experiments that demonstrate Type II SODL: a six step, 2 mm step depth macro-SMS, an eight step 150 um deep micro-SMS with glue between steps, and a six step 150 um deep micro-SMS with no glue between steps.
These SMS are characterized in terms of their fabrication, step alignment and step height increment precision. The degree of alignment of each step was verified using half of a bulk Michelson interferometer. Step height was gauged using a pair of vernier callipers measuring each individual step. A change in notch frequency using an in-fibre Mach-Zhender interferometer was used to gauge the average step height and the result compared to the vernier calliper results.
The best aligned SMS was the micro-SMS prepared by method B with no glue between steps. It demonstrated a 95% confidence interval variation of 1% in reflected intensity, with the least variation in intensity within steps. This SMS also had the least absolute variation in step height increment: less than 8 um. Though less variation would be ideal, for producing micro-SMS for proof of principle experiments for Type II stationary ODL, of the method compared, method B, with no glue between steps, produced more reproducible step height increments and step alignment.
Traditional Fibre Bragg Grating (FBG) sensing systems acquire data about the measurand via the spectral response of the
FBG. Edge filter methods are also used in the acquisition of data from FBGs. In edge filter systems, the spectral shift in
the FBG due to the measurand is converted into an optical power change. This optical power change can then be easily
measured using conventional optoelectronic devices. We demonstrate the use of a Transmit Reflect Detection System
(TRDS) for Fibre Bragg Grating (FBG) sensors. The TRDS is in essence a dual edge filter detection method. In
conventional edge filter detection schemes, the reflected portion of the incident spectrum is monitored to determine the
change in the measurand. In the TRDS, both the transmitted and reflected portions of the input spectrum, from a narrow
band light source, are utilised. The optical power of the transmitted and reflected signals are measured via two separate
photoreceivers, where each generates a single edge filter signal. As the spectral response of the FBG shifts due to the
measurand, the transmitted power will increase, and the reflected power will decrease, or vice versa. By differentially
amplifying the transmitted and reflected components, the overall signal is increased. This results in improved sensitivity
and efficiency of the photonic sensor. In this work, the FBG sensor and TRDS are used in the measuring and monitoring
of temperature, force and strain. As such, results are presented for the FBG TRDS for all of the measurands.
Wireless acoustic communications methods have been demonstrated. These include both electro-acoustic and acoustooptic
communications. The communications methods are intended for use by autonomous robotic agents in the Non-
Destructive Evaluation (NDE) of structures containing a distributed acoustic emission sensor network. The acoustic
emission sensors can be based on either piezoelectric or optical fibre sensors. The communications channel comprises of
a piezoelectric transducer as the transmitter, an aluminium panel as the transmission medium, and either a second
piezoelectric transducer or an optical fibre sensor as the receiver.
Distributed acoustic emission sensors are used in Structural Health Monitoring (SHM) for the detection of impacts
and/or strain, in real time. Secondary damage may result from the initial impact or strain. This damage may include
surface pitting, erosion, or cracking. These types of secondary damage may not be detectable, and hence may not be able
to be monitored by the SHM system; specifically in optical fibre based sensing systems. The integration of NDE by
robotic agents into a SHM sensor network enables the detection and monitoring of a wider variety of damage. Acoustic
communication represents a wireless communication method that does not require any additional hardware, as
piezoelectric transducers are commonly used in the NDE of materials.
Various modulation methods were investigated for the communications channel. These include Amplitude Shift Keying
(ASK), Frequency Shift Keying (FSK), and Phase Shift Keying (PSK). Successful communication was achieved using
both the piezoelectric and optical fibre receivers. The optical fibre sensor used was a Fibre Bragg Grating (FBG).
The response resolution of the stacked gradient homojunction vertical single junction photodiode can be improved
further by including a laterally stacked gradient homojunction in the form of inter-pixel nested ridges that extend from
each epilayer towards the frontwall of the photodiode (Fig 4). In this study, we have simulated the effect of inter-pixel
ridge height and width on the response resolution of a two dimensional CMOS compatible stacked gradient
homojunction photodiode array. The results demonstrate enhanced relative crosstalk suppression and slightly enhanced
maximum quantum efficiency compared to all photodiodes previously simulated by the authors, except for the double
junction photodiode which demonstrated better crosstalk suppression though being much reduced in sensitivity.
As inter-pixel nesting of ridges increases with increase in ridge height, the relative crosstalk reduces and the
maximum quantum efficiency is improved to a constant level above that of the conventional stacked gradient
homojunction photodiode. As the lateral gap between nested ridges increases and the ridges' widths reduce more
rapidly through the underlying epilayers, the relative crosstalk deceases while the maximum quantum efficiency remains
constant. Frontwall illumination is advantaged in reduced crosstalk due to the immediacy of illumination to the
depletion region and being far from the substrate. Backwall illumination is superior in sensitivity due to more carriers
being photogenerated outside the well and being focused into the depletion region by the two minority carrier mirrors.
Electro-Acoustic and Acousto-Optic communications channels have been investigated. The communications channels
are intended for use by robotic agents in the Non-Destructive Evaluation (NDE) of structures containing distributed
Acoustic Emission (AE) sensors. The AE sensors can be either piezoelectric or optical fibre sensors. The
communications channel comprises of a piezoelectric transducer as the transmitter, an aluminium panel as the
transmission medium, and either a second piezoelectric transducer or a fibre optics sensor as the receiver. The electroacoustic
communications channel uses the piezoelectric transducers as the transmitter and the receiver. The acousto-optic
communications channel uses a piezoelectric transducer as the transmitter, and a fibre optic sensor as the receiver.
Acoustic communications represents a wireless communications method that does not require any additional hardware,
as piezoelectric transducers are commonly used in the NDE of materials.
Phase Shift Keying (PSK) was used for the communications encoding. Successful communications was achieved
using both the piezoelectric and fibre optic receivers. The fibre optic sensor used was a Fibre Bragg Grating (FBG), and
the piezoelectric transducers were Lead Zirconate Titanate (PZT) piezoceramic disc transducers. The electro-acoustic
communications channel gave a data rate of 200kbps with a 1MHz square wave carrier. The acousto-optic
communications channel gave a data rate of 6.3568kbps with a 635.68kHz carrier wave.
The characterization of photodiode junction depth using laser beam induced current (LBIC) has long been ambiguous, due in part to the limited understanding behind the relevant physics governing this phenomena, and more importantly, the signal behavior for the various device geometries. In this work, the induced current behavior arising from the individual junction components that form the device for different geometric conditions is examined in detail. In particular, at low temperatures, the overall LBIC signal dependence to junction depth could be attributed to current crowding through the dominance of two competing current mechanisms which include a lateral current flow, Ilbic, and a transverse current flow, Iph. This study represents another step in the development towards a fully quantitative procedure for extracting junction depth and alternatively interpreting the current contributions arising from the individual junction components using LBIC.
In this paper, a numerical study was conducted on spreading of the current in a bottom emitting Vertical Cavity Surface Emitting Laser (VCSEL) with oxidation at the substrate. It was found that the current density profiles of etched VCSELs with small active diameters (< 125 μm) are similar to unetched VCSELs with a 500 μm active diameter. Larger active diameters of 150 μm to 225 μm also have higher density profiles than unetched VCSELs. The simulated current density profiles of large p-contact diameters are dependent on the oxide aperture diameter rather than the contact diameter. For smaller p-doped contact diameters, the density profiles are dependent on the contact diameter rather than the oxide aperture diameter. From current density profiles, higher output powers in the 980 nm wavelength regime are theoretically obtainable at lower threshold currents than previously reported. Maximum output powers of 489 mW, 690 mA and 787 mA at current thresholds of 102 mA, 271 mA and 442 mA were calculated for contact diameters of 50 μm, 100 μm and 150 μm, respectively, with a 50 μm oxide aperture. Depending on the geometric ratios of the simulated devices, required high output power VCSELs can be designed for specific applications.
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