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.
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.
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.
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.
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.
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.
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.
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