Rapid changes in the agricultural sector in the past two decades have given rise to several new technologies and superior products including genetically modified crops; the identification of which still requires robustness and rapidity. In this work, we report the use of continuous wave terahertz (CW THz) spectroscopy as a means to identify biomechanical changes at the tissue level based on systemic dehydration. We have also identified the factors affecting this progression and propose a biomechanical model towards genetic discrimination in plants. Our results indicate that within the same family, factors such as cell size and age, tissue composition, hydration retention capacity and water percentage to cell volume ratio affect the systemic dehydration in the plant and thereby show unique biomechanical profiles.
Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant
potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique
utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will
discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the
confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin
diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of
discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented
using a transgenic mouse model.
We propose a compact, self-aligned, low-cost, and versatile infrared diffuse-reflectance laser imaging system using a laser feedback interferometry technique with possible applications in in vivo biological tissue imaging and skin cancer detection. We examine the proposed technique experimentally using a three-layer agar skin phantom. A cylindrical region with a scattering rate lower than that of the surrounding normal tissue was used as a model for a non-melanoma skin tumour. The same structure was implemented in a Monte Carlo computational model. The experimental results agree well with the Monte Carlo simulations validating the theoretical basis of the technique. Results prove the applicability of the proposed technique for biological tissue imaging, with the capability of depth sectioning and a penetration depth of well over 1.2 mm into the skin phantom.
In this paper, we introduce the self-mixing phenomenon in terahertz quantum cascade lasers (THz QCLs) and present
recent advancements in the development of coherent THz imaging and sensing systems that exploit the self-mixing effect.
We describe an imaging method which utilises the interferometric nature of optical feedback in a THz QCL to employ it
as a homodyning transceiver. This results in a highly sensitive and compact scheme. Due to the inherently low penetration
depth of THz radiation in hydrated biological tissue, imaging of superficial skin is an ideal application for this technique.
We present results for imaging of excised skin tissue, showing high-contrast between different tissue types and pathologies.
In this work, the emission efficiency and spectral shift with respect to viewing angle were optimized by optimizing the
design of the multi-layer top mirror of a microcavity OLED device. We first established criteria for the emission side
mirror in order to optimize light intensity and spectral shift with viewing angle. Then we designed mirror using metallic
and dielectric layers based on the target defined. The electroluminescence emission spectra of a microcavity OLED
consisting of widely used organic materials, N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPB) as a hole
transport layer and tris (8-hydroxyquinoline) (Alq3) as emitting and electron transporting layer was then calculated.
Silver was used as the anode and back reflection mirror for the microcavity OLED. The simulation was performed for
both the conventional LiF/Al cathode/top mirror and the optimized 5-layered top mirror. Our results indicate that by
following the design procedure outlined, we simultaneously optimize the device for better light intensity and spectral
shift with viewing angle.
We investigate the combined effect of the diffraction-caused crosstalk noise (DCCN) and the stray-light crosstalk noise
(SLCN) on the performance of FSOI system. A numerical simulator was employed in this study to investigate OI
channel design. We determine that there exists an optimal focal length, which maximises the signal-to-noise ratio (SNR)
by minimising the combined effects of DCCN and SLCN. For the fundamental mode, the optimal focal length is
approximately 750 &mgr;m for both LG01 and LG10 modes, the optimal focal length occurs between f = 650 &mgr;m and f =
700 &mgr;m, depending on the interconnection distance and array pitch.
We report on detailed simulations of the emission from microcavity OLEDs consisting of widely used organic materials, N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq3) as emitting and electron transporting layer. The thick silver film was considered as a top mirror, while silver or copper films on quartz substrate were considered as bottom mirrors. The electroluminescence emission spectra, electric field distribution inside the device, carrier density and recombination rate were calculated as a function of the position of the emission layer, i.e. interface between NPB and Alq3. In order to achieve optimum emission from a microcavity OLED, it is necessary to align the position of the recombination region with the antinode of the standing wave inside the cavity. Once the optimum structure has been determined, the microcavity OLED devices were fabricated and characterized. The
experimental results have been compared to the simulations and the influence of the emission region width and position on the performance of microcavity OLEDs was discussed.
We investigate the effect of transmitter and receiver array configurations on the stray-light and diffraction-caused crosstalk in free-space optical interconnects. The optical system simulation software (Code V) is used to simulate both the stray-light and diffraction-caused crosstalk. Experimentally measured, spectrally-resolved, near-field images of VCSEL higher order modes were used as extended sources in our simulation model. Our results show that by changing the square lattice geometry to a hexagonal configuration, we obtain the reduction in the stray-light crosstalk of up to 9 dB
and an overall signal-to-noise ratio improvement of 3 dB.
The conventional self-mixing sensing systems employ a detection scheme utilizing the photocurrent from an integrated photodiode. This work reports on an alternative way of implementing a Vertical-Cavity Surface-Emitting Laser (VCSEL) based self-mixing sensor using the laser junction voltage as the source of the self-mixing signal. We show that the same information can be obtained with only minor changes to the extraction circuitry leading to potential cost saving
with reductions in component costs and complexity. The theoretical linkage between voltage and photocurrent within the self-mixing model is presented. Experiments using both photo current and voltage detection were carried out and the results obtained show good agreement with the theory. Similar error trends for both detection regimes were observed.
This work reports on simulation and experimental investigation into the charge transport and electroluminescence in a quantum well (QW) organic light emitting diode (OLED) consisting of a N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPB) as a hole transport layer, tris (8-hydroxyquinoline) aluminum (Alq3) as a potential barrier and electron transporting layer, and rubrene as potential well layer. Indium tin oxide was used as an anode, while LiF/Al was employed as a cathode. The carrier transport was simulated using one-dimensional time-independent drift-diffusion model. The influence of the well width, barrier width, and the number of QWs on the carrier distribution, recombination rate, and device performance was investigated. Finally, the device structures which yielded most promising simulation results were fabricated and characterized. The comparison between the experimental and theoretical results is discussed.
KEYWORDS: Optical coherence tomography, Signal to noise ratio, Sensors, Retina, Signal detection, Holography, Eye, Cornea, 3D image processing, 3D image reconstruction
We report a new approach in optical coherence tomography (OCT) termed full-field Fourier-domain OCT (3F-OCT). A three-dimensional image of a sample is obtained by digital reconstruction of a three-dimensional data cube, acquired using a Fourier holography recording system illuminated with a swept-source. This paper presents theoretical and experimental study of the signal-to-noise ratio of the full-field approach versus serial image acquisition approach, represented by 3F-OCT and "flying-spot" OCT systems, respectively.
Free-space optical interconnects (FSOIs) utilize arrays of vertical-cavity surface emitting lasers (VCSELs), microlenses, and photodetectors to effectively overcome the communication bottleneck caused by the poor performance of electrical interconnects. We derived a comprehensive FSOI link equation which can be used to determine the interconnect performance parameters, such as the receiver carrier-to-noise ratio. The link equation includes both optical and electrical noise components. The optical noise component is caused mainly by laser beam diffraction. We have simplified the modeling of optical noise by using the recently introduced Mode Expansion Method. The optical noise component strongly depends on the modal content of the incident VCSEL beam. The models used in the literature assume that the cross-sectional profile of the emitted laser beam resembles the fundamental Gaussian mode. Our link equation takes into account the modal structure of a multimode VCSEL beam. We have investigated the FSOI performance and we found that for each merit function there exists a single set of design parameters yielding the optimal performance. We have also found that the presence of higher-order modes negatively affects the performance. Our results show that FSOIs based on multimode VCSELs can be utilized in chip-level interconnects despite increased beam diffraction.
In this paper we investigate for the first time the effect of the crosstalk introduced due to laser beam imaging in a free-space optical interconnect (FSOI) system. Due to the overfill of the transmitter microlens array by the vertical cavity surface emitting laser (VCSEL) beam, one part of the signal is imaged by the adjacent microlens to another channel, possibly far from the intended one. Even though this causes increase in interchannel and intersymbol interference, to our knowledge this issue has been neglected so far. The numerical simulation has been performed using a combination of exact ray tracing and the beam propagation methods. The results show that some characteristics of stray-light crosstalk are similar to that of diffraction-caused crosstalk, where it is strongly dependent on the fill factor of the microlens, array pitch, and the channel density of the system. Despite the similarities, the stray-light crosstalk does not affect by an increase in the interconnection distance. As simulation models for optical crosstalk are numerically intensive, we propose here a crosstalk behavioral model as a useful tool for optimization and design of FSOIs. We show that this simple model compares favorably with the numerical simulation models.
In a self-mixing type laser range finder the current of the laser is modulated with a triangle wave to produce a range of optical frequencies. However, the electrical signal does not produce a perfect linear sweep in optical frequency due to thermal and other effects in the laser. This leads to errors in the accuracy and resolution of the range finder. In this paper, we describe and implement a method in software to systematically determine the optimal shape of the injected waveform needed to eliminate these thermally induced measurement errors. With this method we do not require the more complicated and expensive optical techniques used by other researchers to recover the optical frequency variations with regard to injection current. The averaging of a reasonable number of samples gave sub-millimeter accuracy when the optimal current shape was used. The uncertainty in the average measurements are improved by roughly six times compared to the conventional triangular modulation. The reshaping also results in the range finding system being less sensitive to changes in ambient temperature.
In this work we present detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq3) based OLEDs as a function of: the choice of cathode, the thickness of organic layers, and the position of the hole transport layer/Alq3 interface. The calculations fully take into account dispersion in glass substrate, indium tin oxide anode, and in the organic layers, as well as the dispersion in the metal cathode. Influence of the incoherent transparent substrate (1 mm glass substrate) is also fully accounted for. Four cathode structures have been considered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer, N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) was considered. As expected, emitted radiation is strongly dependent on the position of the emissive layer inside the cavity and its distance from the metal cathode. Although our optical model for an OLED does not explicitly include exciton quenching in vicinity of the metal cathode, designs placing emissive layer near the cathode are excluded to avoid unrealistic results. Guidelines for designing devices with optimum emission efficiency are presented. Finally, the optimized devices were fabricated and characterized and experimental and calculated emission spectra were compared.
In this work, we have calculated the emission wavelength dependence on the viewing angle for different combinations of metallic mirrors. The dispersion of the optical functions of ten different metals is fully taken into account using Lorentz oscillator model. The metals have been assigned to a function of top (cathode) or bottom (anode) mirror based on their work function. Refractive index dispersion of organic layers, N,N'-disphenyl-N,N'-bis(3-methylphenyl)-1,1'-disphenyl-4,4'-diamine (TPD) and tris (8-hydroxyquinoline) aluminum (emitting layer) is taken into acocutn via Cauchymodel. The change of the emission wavelength with angle has been calculated iteratively to fully take into account wavelength dependence of indices of refraction and phase change. Calculations have been performed for different hole transport materials and different thickness of the emitting layer.
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