2012: Vol 103 No 1

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Table of Contents (~180 kB)

Guest editorial by M. du Plessis (~40 kB)

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1. Optimisation of CMOS compatible microbolometer device performance
2. Characterisation of the electrical response of a novel dual element thermistor for low frequency applications
3. Wafer level packaging with wedge seal method
4. Spectral measurement and analysis of silicon CMOS light sources
5. CMOS avalanche electroluminescence applications – microdisplay and high speed data communication
6. Ab initio frequency measurement and characterisation of frequency doubled fibre laser utilised for precision oscillators
7. Methodology for in situ characterisation of a highly birefringent photonic crystal fibre for supercontinuum generation
8. Digital design of broadband long-period fibre gratings by an inverse scattering algorithm with flip-flop optimisation
9. A lensless, automated microscope for disease diagnostics
10. Optical and thermal applications in grapevine (vitis vinifera L.) research – an overview and some novel approaches
11. Growth and characterisation of InAs photodetectors for MWIR applications

Optimisation of CMOS compatible microbolometer device performance by W. Maclean, Prof. M. du Plessis and J. Schoeman
Abstract: Uncooled IR (infrared) microbolometer performance is greatly affected by the thermal properties associated with the structural layout of each design. Equations are derived in this article which make use of basic structural dimensions to predict the expected thermal conductance and thermal capacitance of a microbolometer device. These equations enable a microbolometer designer to determine the estimated thermal time constant of a design without performing complicated analytical calculations for each layer in the design. Calculation results shown indicate the effect structural changes have on the thermal time constant of microbolometer devices. These changes aid microbolometer designers in adjusting the layout of the device to change the thermal time constant to the desired value. Structural deviations that occur during manufacturing of microbolometers are calculated and the possible causes are discussed.
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Characterisation of the electrical response of a novel dual element thermistor for low frequency applications by J. Schoeman and M. du Plessis
Abstract: This work is aimed at characterising the DC electrical response of a temperature sensitive microbolometer device. The contribution lies with the choice and the structure of the device, a novel bolometer infrared sensing structure consisting of dual sensing elements that are thermally very closely coupled on a single membrane supporting structure. A mathematical model is presented to characterise the behaviour of the device resistance and conductance for a given biasing current. A modified experiment of a well published non-optical method exploiting the normally unwanted Joule heating of a device when biased with a large direct current is employed for the experimental verification and validation of the theoretical model. The measured results indicate that the proposed model approximates the measured results well. Although some deviation occurs, this is to be expected and discussed.
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Wafer level packaging with wedge seal method by C. Versteeg, J. vd Vyver and P. van Rooyen
Abstract: Wafer level packaging could reduce the cost of MEMS based sensors through simplified processing and inexpensive components. A novel vacuum tight seal referred to as the wedge seal method is proposed in this paper. The seal consists of a silicon wedge forced into a pliable material (typically a metal) that is attached to the component wafer. The wedge-seal addresses some of the requirements of micro-bolometer packaging in that it provides a vacuum tight seal at low temperatures with tolerance to surface finish and topography. The concept was evaluated with regards to manufacturability, material suitability and performance. A demonstrator model was manufactured with Complementary Metal–Oxide–Semiconductor (CMOS) compatible processing equipment and procedures. The effectively penetrated the metal base and showed promising leak rates.
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Spectral measurement and analysis of silicon CMOS light sources by A.W. Bogalecki, M. du Plessis, P.J. Venter and C. Janse van Rensburg
Abstract: The emission spectra of pn-junction and punch-through (PT) carrier injection silicon (Si) CMOS light sources were measured at various current densities and temperatures. In contrast to the narrow-band forward-biased junction spectrum, that peaks around 1.1 µm (1.1 eV), the reverse-bias spectrum was found to extend from about 350 nm (3.4 eV) to about 1.7 µm (0.7 eV) covering the UV, Vis and NIR regions. Since the photon energy decreases with increasing wavelength, the significant NIR radiation implies that the quantum conversion efficiency of Si avalanche light sources is appreciably higher than previously reported. Calculating the photon flux at the emission source within the Si against photon energy allowed the deduction and quantification of the physical light emission processes with respect to silicon’s electronic band structure. Intra-conduction-band (c-c) electron (e^-) transitions seem to be the dominant physical mechanism responsible for the wide avalanche spectrum.
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CMOS avalanche electroluminescence applications – microdisplay and high speed data communication by M.E. Goosen, M. du Plessis, P.J. Venter, A.W. Bogalecki, A.C. Alberts and P. Rademeyer
Abstract: All-CMOS silicon light sources, although not the choice semiconductor process for light generation, offer the possibility of large scale manufacturing, integration with digital and driver electronics as well as a wide operating temperature range. These advantages do however come at a cost of reduced efficiency, but offer significant cost advantages inherent when using a standard CMOS technology. This paper presents two applications of standard CMOS integrated light sources. A fully functional microdisplay utilising avalanche electroluminescence for visible light generation and implemented in a completely standard 0.35 µm CMOS technology is presented. The microdisplay has an operating temperature range of -50 to 125 °C, which cannot be achieved by competing microdisplay technologies. Utilising the same silicon light sources, a 10 Mb/s optical communication link is established operating at a BER of less than 10^-12. The data communication link presented in this paper constitutes the fastest all-silicon data communication link achieved thus far.
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Ab initio frequency measurement and characterisation of frequency doubled fibre laser utilised for precision oscillators by J.P. Burger, C. Matthee and R. Kritzinger
Abstract: An ab initio measurement of a free running, frequency doubled erbium fibre laser is made with an optical frequency comb in concert with an internally calibrated wavemeter. The measurement is validated via a Monte Carlo uncertainty analysis. The operation and characteristics of a new high performance optical metrology source for an optical frequency standard is also verified in the process.
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Methodology for in situ characterisation of a highly birefringent photonic crystal fibre for supercontinuum generation by J.P. Burger, A. Ben Salem, R. Cherif and M. Zghal
Abstract: A novel methodology for precisely determining the eigenaxes and effective twist of a solid-core polarisation maintaining fibre with a slightly elliptical effective core in an experimental setup with an ultrashort pulse laser is presented. This geometrical identification relies on experimental modal analysis and by utilising an incoherent ultrashort optical pulse fragment measurement and modelling procedure, and is a prerequisite for precise measurements and characterisation of this class of fibres. This orienting method is applied in the study of low threshold and temporally and polarisation stable supercontinuum generation in ~2m length of photonic crystal fibre, utilizing a modelocked Ti:Sapphire laser.
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Digital design of broadband long-period fibre gratings by an inverse scattering algorithm with flip-flop optimisation by R. Kritzinger, J. Burger, J. Meyer and P.L. Swart
Abstract: A discrete inverse scattering method, known as layer-peeling, is used to synthesise a LPFG (long-period fibre grating) from a desired complex spectrum by a direct solution of the coupled-mode equations, while simultaneously determining the physical properties of the layered structure. The physical properties of the grating structure are determined in a recursive layer-by-layer manner by using causality arguments to design LPFGs exhibiting a flat-top spectral profile. The results obtained from the layer-peeling method are optimised using the flip-flop method to ease the fabrication process. We found that the layer-peeling method has the highest performance and executes in the least amount of time. A discussion of possible applications where optimised broadband LPFGs could be utilised in the field of telecommunications and sensing demonstrates the importance of the results.
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A lensless, automated microscope for disease diagnostics by S. Hugo, T. Naidoo, H. Swart, S. Potgieter, P. van Rooyen and K. Land
Abstract: Optical microscopy is widely accepted as one of the gold standards in disease diagnosis. However, factors such as cost and the need for a trained eye limit the prevalence of such equipment, particularly in resource-limited areas such as rural clinics. Lensless microscopy, which is based on principles of digital holography, has illustrated the possibility of using simple and cheap optical components combined with software algorithms to implement microscope platforms. We present a digital in-line holographic microscope (DIHM) platform to be used with image processing and classification algorithms to provide a low cost, portable and automated microscope. Initial results show that the images obtained using the DIHM platform are similar to those obtained using a conventional bright field microscope. Applications of this work are targeted towards the implementation of a full blood count, which could provide resource-limited areas with improved healthcare facilities and diagnosis times.
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Optical and thermal applications in grapevine (vitis vinifera L.) research – an overview and some novel approaches by A.E. Strever, D. Bezuidenhout, R. Zorer, T. Moffat and J.J. Hunter
Abstract: In this article, some optical and thermal applications in grapevine research are reviewed and methods to quantify the light and temperature regime around a grape bunch are discussed. This includes temperature measurement techniques (thermocouples and thermal imaging) as well as methods to quantify light quantity (hemispherical photography) as well as light quality (spectroradiometric applications) around a grape bunch. Available methods for real-time quantifi cation of grapevine canopy size and density for application in variable rate technology sprayers are discussed, and a novel and simple approach of using opto-electronic sensors for quantifi cation of grapevine canopy thickness and density is presented. Some scientifi c as well as practical applications of these individual techniques are discussed, along with their potential integration to improve knowledge of the grape bunch and canopy interaction with the environment.
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Growth and characterisation of InAs photodetectors for MWIR applications by M.C. Wagener, V. Wagener and J.R. Botha
Abstract: This paper reports on the development of InAs photodiodes by the Department of Physics at Nelson Mandela Metropolitan University. The device structures have been grown by metal-organic vapour phase epitaxy and processed using conventional photolithography techniques. Due to the narrow band gap of these materials, the detectivity of the devices are often limited by the junction leakage currents associated with avalanche multiplication and trap assisted tunnelling and generation effects, as well as surface related conduction channels. The various contributions to the leakage current and photo-response have been analysed, and correlated to the material and electrical characteristics of the device structures grown.
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