L.B. Lok

University College London, Londinium, England, United Kingdom

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Publications (34)16.34 Total impact

  • Lai Bun Lok · Paul V. Brennan · Samiur Rahman ·

    IET Radar Sonar ? Navigation 06/2015; DOI:10.1049/iet-rsn.2014.0355 · 1.14 Impact Factor
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    P.V. Brennan · L.B. Lok · Keith Nicholls · Hugh Corr ·
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    ABSTRACT: Ice shelves fringe much of the Antarctic continent, and, despite being up to 2 km thick, are vulnerable to climate change. Owing to their role in helping to control the ice sheet contribution to sea level change there is great interest in measuring the rate at which they are melting into the ocean. This study describes the development and deployment of an icepenetrating phase-sensitive FMCW radar, sufficiently robust and with sufficiently low-power consumption to be run through the Antarctic winter as a standalone instrument, yet with the stability and mm-precision needed to detect the very slow changes in ice shelf thickness in this exceptionally demanding environment. A number of elegant processing techniques are described to achieve reliable, high-precision performance and results presented on field data obtained from the Larsen-C ice shelf, Antarctica.
    IET Radar Sonar ? Navigation 08/2014; 8(7):776-786. DOI:10.1049/iet-rsn.2013.0053 · 1.14 Impact Factor
  • Samiur Rahman · Paul V. Brennan · Lai Bun Lok ·
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    ABSTRACT: Antarctic ice shelves are the fringes of ice sheets, that stifle the ice flow into the ocean. Determining the presence of internal layers in these ice shelves is very important for accurate measurement of the ice shelf melt rate. This paper introduces a novel method to detect presence of an internal layer (if there is any) that is masked by high amplitude antenna direct coupling returns or near surface clutter. The method uses the shape of the basal layer return to match with the radar signal returns throughout the range profile. Instead of usual cross correlation weighting, this method uses ranking system to determine likelihood. Experimental results are also shown and described in the paper. The raw data has been acquired from the Foundation ice stream, George VI, and Larsen ice shelf in Antarctica. Phase sensitive FMCW radar developed at UCL and pRES radar system developed at British Antarctic Survey (BAS) have been used for the data acquisition. The results validate that the method can be used for detection of internal layers or any other hidden features inside ice shelf.
    2014 IEEE Radar Conference (RadarCon); 05/2014
  • P.V. Brennan · Lai Bun Lok · H.F.J. Corr · K.W. Nicholls ·
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    ABSTRACT: A ground-based phase-sensitive FMCW radar system, and associated signal processing algorithms, have been developed at UCL for millimetre-precision monitoring of Antarctic ice shelf thickness to derive basal melt rates [1]. To support the practical realisation of this radar, and to confirm that suitable performance is obtained, rigorous bench testing have been performed. These tests were conducted on the prototype radar system using coaxial cables in the laboratory at room temperature and to -15°C. Since 2011, the radar system has successfully acquired sounding data from various field locations in Antarctica. The phase-sensitive radio echo sounding (pRES) technique was originally demonstrated by scientists at the British Antarctic Survey for monitoring ice-shelf melt rates, requiring millimetre-precision range measurements [3]. pRES is a stepped-frequency radar implemented using a commercial, laboratory-grade, vector network analyzer. However, it has drawbacks such as high power consumption (1kW petrol generator needed), low portability, high noise figure, and therefore the measurement datasets are limited in length because the instrument cannot be left unattended. This presentation describes our custom-designed radar instrument, based on a novel phase-sensitive FMCW technique [1] and low-cost antenna arrays, which has been developed to enable year-round monitoring and imaging of ice-shelves.
    IET Colloquium on Antennas, Wireless and Electromagnetics 2014; 01/2014
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    ABSTRACT: We present the design, fabrication, and measurement of a 2-way modified Wilkinson divider constructed in a coplanar geometry exhibiting ultra wideband isolation, transmission, and port matching in the millimeter-wave frequency range. The proposed divider replaces the lumped resistor in the conventional Wilkinson divider with two quarter-wave length transmission lines, a phase inverter, and two 2Z0 resistors. Except for the three ports that are coplanar waveguides (CPWs), the main body of the divider uses coplanar striplines (CPS). The phase inverter is realized using a simple airbridge-based crossover which is compatible with a modern monolithic microwave integrated circuit process. The divider has a ring-like configuration fabricated on a 620 µm thick semi-insulating GaAs wafer using electron beam lithography (EBL) technology. Three-dimensional (3D) full-wave electromagnetic simulations have been carried out to optimize the design and investigate the possible effect of fabrication tolerance on the performance of the crossover and the divider. Two dividers working at center frequencies of 25 and 80 GHz have been designed, fabricated, and tested. They all show consistent performance in terms of bandwidth, isolation, and port matching. Experimental and simulation results are in excellent agreement.
    International Journal of Microwave and Wireless Technologies 06/2013; 5(03). DOI:10.1017/S1759078713000421 · 0.35 Impact Factor
  • S. Rahman · P.V. Brennan · L. Lok ·
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    ABSTRACT: Ice shelves are the fringes of ice sheets, found in both Polar regions (Antarctica and Greenland). Thickness changing patterns of these ice shelves have direct consequence to sea level rising. This paper introduces an ice-penetrating phase-sensitive FMCW radar system that is capable of measuring the depth of ice shelves to mm precision within 2 km. The signal processing technique for achieving this high precision range is described along with trial results verifying the accuracy. The radar had also been operated in Larsen North ice shelf in Antarctica in 2011-12 Austral summer for cross-sectional imaging. The processed results of the data obtained from that expedition are illustrated in this paper. The data processing for cross-sectional imaging of Larsen North ice shelf has been done by using both phased array and synthetic aperture radar (SAR) processing methods.
    Radar Conference (RADAR), 2013 IEEE; 01/2013
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    ABSTRACT: In this letter, we present a coplanar waveguide (CPW)-based ring power combiner that exhibits less than 0.8 dB insertion loss, better than 15 dB port match and higher than 22 dB isolation loss over the frequency range from 50 to 100 GHz. Compared with the conventional two-way Wilkinson combiner, the proposed ring power combiner replaces the resistor between the two input ports with two quasi quarter-wave CPWs, a 180° CPW phase inverter, and two resistors that lead to frequency-insensitive port isolation and wideband port match. The power combiner is realized using an electron beam-based GaAs MMIC process along with simple electron beam airbridge technology. These results agree well with 3-D full-wave simulations.
    IEEE Microwave and Wireless Components Letters 11/2012; 22(11):580 - 582. DOI:10.1109/LMWC.2012.2224854 · 1.70 Impact Factor

  • EuMC 2012; 10/2012
  • C Li · L B Lok · A Khalid · V Papageorgiou · J Grant · D.R.S. Cumming ·
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    ABSTRACT: We present the design, fabrication and measurement of an in-phase equal power ring divider exhibiting ultra wideband isolation, transmission and port matching. The design uses coplanar techniques to achieve a compact and uniplanar design. The device has been fabricated using a single metal level GaAs monolithic microwave integrated circuit (MMIC) process with electron beam airbridge technology. The device has a dimension of 0.6 mm×0.6 mm including test pads. The measured in-band insertion loss is 1.3 dB and the port match is better than 18 dB over the frequency range from 44 GHz to 110 GHz. The output port-to-port isolation is better than 20 dB over the entire measured frequency range from 10 MHz to 110 GHz. Full-wave simulation results show this performance up to 118 GHz.
    European Microwave Conference; 10/2012
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    Chong Li · Lai Bun Lok · Ata Khalid · Iain G Thayne · David R S Cumming ·
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    ABSTRACT: A one-port load-pull measurement has been carried out in order to investigate the effect of loading on the RF power performance of a planar Gunn diode operating in the transit-time mode at 102 GHz. A W-band manual E-H tuner was applied between a waveguide mixer and a wafer probe to vary the load impedance on the Gunn diode. It has been found that more than 25 dB variation of output power was obtained by systematically adjusting the tuner. By de-embedding the S-parameters of the probe, E-H tuner and mixer, the relationship between RF power and load impedance for the planar Gunn diode was derived. This method is extremely useful for assisting the design of matching networks to improve power output of one-port oscillator devices.
    IEEE Microwave and Wireless Components Letters 10/2011; 21(10):556-558. DOI:10.1109/LMWC.2011.2163496 · 1.70 Impact Factor
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    Chong Li · Lai Bun Lok · Ata Khalid · David R. S. Cumming ·
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    ABSTRACT: In this letter, the design and measurement of a new ring power divider exhibiting wideband high isolation is presented. Coplanar techniques are used to achieve a compact and truly uniplanar design. The design is demonstrated by a prototype, operating at K- band, that has been monolithically fabricated using a GaAs MMIC process with airbridge technology. The measured insertion loss is 0.6 dB at the center design frequency of 25 GHz. The output port isolation is better than 20 dB across a wide bandwidth from 10MHz to 50 GHz. The output amplitude and phase balance is within ±0:5 dB and ±2±° respectively, in the bandwidth from 10MHz to 43 GHz.
    Progress In Electromagnetics Research Letters 06/2011; 25:1-10. DOI:10.2528/PIERL11041408
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    J Grant · Y Ma · S Saha · L B Lok · A Khalid · D R S Cumming ·
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    ABSTRACT: We present the simulation, implementation, and measurement of a polarization insensitive resonant metamaterial absorber in the terahertz region. The device consists of a metal/dielectric-spacer/metal structure allowing us to maximize absorption by varying the dielectric material and thickness and, hence, the effective electrical permittivity and magnetic permeability. Experimental absorption of 77% and 65% at 2.12 THz (in the operating frequency range of terahertz quantum cascade lasers) is observed for a spacer of polyimide or silicon dioxide respectively. These metamaterials are promising candidates as absorbing elements for thermally based terahertz imaging.
    Optics Letters 04/2011; 36(8):1524-6. DOI:10.1364/OL.36.001524 · 3.29 Impact Factor
  • Chong Li · Lai Bun Lok · Ata Khalid · David R.S. Cumming ·
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    ABSTRACT: In this paper, a circular power combiner/divider using coplanar strip-lines operating at the centre frequency of 30 GHz is demonstrated. The combiner/divider has a coplanar strip-line (CPS) crossover phase inverter and it shows good isolation (< −22 dB) up to 55 GHz. The combiner/divider is suitable for combining planar Gunn oscillators that operate in a wide frequency range.
    Passive RF and Microwave Components Seminar, 2nd Annual; 03/2011
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    ABSTRACT: An ultra-low-power monolithic amplifier using 50-nm gate-length GaAs metamorphic high-electron-mobility transistor (MHEMT) has been designed and fabricated by a coplanar waveguide monolithic microwave integrated circuit process. A double (5-doped In<sub>0.52</sub>Al<sub>0.48</sub>As/In<sub>0.53</sub>Ga<sub>0.47</sub>As MHEMT technology with optimal doping profiles was used to achieve both ultra-low dc power consumption and good dynamic-range performance. The single-stage amplifier operates in the 24-GHz band and shows typical gain of 7.2 dB, ±0.5 dB bandwidth of 1.2 GHz, return losses better than 9 dB, and input IP<sub>3</sub> (IIP<sub>3</sub>) of +3 dBm while consuming only 0.9 mW of dc power. These experimental results demonstrate the outstanding potential of MHEMT technology for ultra-low-power applications such as wireless sensor networks.
    IEEE Electron Device Letters 12/2010; 31(11-31):1230 - 1232. DOI:10.1109/LED.2010.2070484 · 2.75 Impact Factor
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    ABSTRACT: An In<sub>0.23</sub>Ga<sub>0.77</sub>As-based planar Gunn diode operating in its fundamental transit-time mode of oscillation at 116 GHz with output power of -24 dBm is demonstrated. The diode has a pseudomorhpic HEMT-like structure grown on a semi-insulating GaAs substrate. The layer design was carried out using a two-dimensional drift-diffusion model. The realized devices show considerable potential as a source of millimeter-wave and even terahertz radiation.
    Infrared Millimeter and Terahertz Waves (IRMMW-THz), 2010 35th International Conference on; 10/2010
  • Chi-Jeon Hwang · Lai Bun Lok · Iain G. Thayne · Khaled Elgaid ·
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    ABSTRACT: This article presents the design and measurement of a microstrip band-pass filter (BPF) using modified branch-line coupler with asymmetric open-circuit stubs in the W-band frequency range. The fundamental topology consists of using the branch-line coupler as a filtering section by loading the coupled ports with suitable lengths of open-circuit stubs. Therefore, transmission responses with simultaneously good stopband and sharp cutoff slope are derived, while the passband bandwidth can be easily controlled by means of the design parameters of the filtering section. The proposed BPF structure has been designed and fabricated on GaAs substrate. The fabricated BPF shows a 3-dB bandwidth from 81 to 107 GHz with insertion loss <2 dB at center frequency, rejection of 51 dB, and return loss better than 18 dB. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1436–1439, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25185
    Microwave and Optical Technology Letters 06/2010; 52(6):1436 - 1439. DOI:10.1002/mop.25185 · 0.57 Impact Factor
  • Chi-Jeon Hwang · Lai Bun Lok · I.G. Thayne · K. Elgaid ·
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    ABSTRACT: This paper presents a wide bandpass filter (BPF) with low insertion loss and excellent harmonic out-of-band suppression over a very wideband. The filter consists of a standard ultra-wideband bandpass filters design approach combined with periodic defected ground structure (DGS), which is able to inhibit signal propagation over a wideband. To realize this filter, a wideband microstrip BPF with a bandwidth covering 3.7 GHz is implemented in an RO4350 substrate (¿<sub>r</sub> = 3.66, h = 0.762 mm). This bandpass filter shows a wide bandwidth with measured insertion loss better than 0.9dB and spurious suppression better than 24 dB.
    Microwave Conference, 2009. APMC 2009. Asia Pacific; 01/2010
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    ABSTRACT: In this paper a 10 GHz quasi-Hybrid/MMIC super-regenerative transceiver/antenna chip is presented. The circuit is the highest frequency super-regenerative transceiver presented in the literature and is amongst the lowest power - certainly the lowest power at X-band frequency. The chip is fabricated on GaAs substrate and uses a MMIC process for the passive components and an RFMD PHEMT chip device bonded into the circuit for the active components. The transceiver chip measures 10 × 10 mm and consumes 0.75 mW Tx and 0.9 mW Rx. When mounted into a pcb carrier substrate containing antenna, bias circuitry and low pass filtering the board measures 26 × 42 mm and operates over a range of 1 m.
    Microwave Conference, 2009. APMC 2009. Asia Pacific; 01/2010
  • L B Lok · C Li · A Khalid · N J Pilgrim · G M Dunn · D.R.S. Cumming ·
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    ABSTRACT: In this paper, we demonstrate the operation of a truly planar Gunn diode working as a self-oscillating mixer at millimeter-wave frequency. The Gunn diode was fabricated in a GaAs/Al0.23Ga0.77As layer structure. An initial proof-of-concept prototype yielded a measured conversion loss of around 20±2.5 dB between 30 GHz and 40 GHz.
    35th International Conference on Infrared, Millimeter and Terahertz Waves; 01/2010
  • L.B. Lok · C.-J. Hwang · I.G. Thayne · K. Elgaid ·
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    ABSTRACT: In this paper, we demonstrate the performance of a W-band (75–110GHz) parallel coupled-line bandpass filter implemented using a new in-house GaAs microstrip technology. The MMIC process at the University of Glasgow has recently been advanced to incorporate a new process module that includes wafer thinning, through-substrate via etching and backside processing capabilities. This enables the realization of microstrip and grounded coplanar waveguide monolithic circuits for microwave and millimetre-wave applications from a UK source [1]. To demonstrate an example of the high frequency performance of this new process module, a parallel coupled-line bandpass filter has been designed and realized on a 50µm thickness semi-insulating GaAs substrate.
    Passive RF and Microwave Components, IET Seminar on; 01/2010

Publication Stats

96 Citations
16.34 Total Impact Points


  • 2013-2014
    • University College London
      • Department of Electronic and Electrical Engineering
      Londinium, England, United Kingdom
  • 2008-2011
    • University of Glasgow
      • Division of Electronics and Electrical Engineering
      Glasgow, Scotland, United Kingdom
  • 2005-2008
    • University of Leeds
      • School of Electronic and Electrical Engineering (EEE)
      Leeds, England, United Kingdom