Zourab Brodzeli

Sensors at Your Fibre Tips: A Novel Liquid Crystal- Based Photonic Transducer for Sensing Systems

Recent advances in both nano-fabrication and nano- materials open up rich new avenues in the optical sensing area. Using nano-grid electrodes, nanometer-scale photo-alignment layers and deformed helix ferro-electric liquid crystals with a sub-wavelength helix pitch of 150 nm, we have realised a fully integrated polarisation-based photonic transducer which exhibits fast response to an applied voltage/electric field, excellent linearity and allows multiplexing using standard, low cost telecom optical fibers.

A distributed-feedback fiber-laser-based optical fiber hydrophone system with very high sensitivity

This paper reports the development of a very compact and very-high sensitivity optical fibre hydrophone system using a distributed-feedback fibre laser with a cavity length of 10cm. A theoretical system design making use of a Mach-Zehnder interferometer, homodyne demodulation scheme and digital signal processing is described. At the time of writing, the system is only partially completed; therefore the content of this paper will focus on the distributed-feedback fibre laser sensor head. Results for noise spectrum below 100kHz are presented, as well as discussions on some key issues with designing such hydrophone systems. Although not the intention for the system, initial results also indicate the suitability of the DFBFL for intensity modulated sensing.

Reflective mode of deformed-helix ferroelectric liquid crystal cells for sensing applications

We present a detailed theoretical and experimental study of the reflectance response of a deformed-helix ferroelectric (DHF) liquid crystal (LC) cell to an applied voltage under cross-polarisers. Using a model based on the effective dielectric tensor approximation, we derive simple analytical formulas to design a LC cell with maximum modulation depth and optimal linearity of the electro-optical response intensity versus the electric field. Our experimental results show that the cell works at frequencies up to 10 kHz and exhibits excellent linearity, with a total harmonic distortion as low as −70 dB. These findings suggest that DHF-LCs can be exploited to develop simple and accurate optical sensors.

Fast Electro-optical Mode in Photo-aligned Reflective Deformed Helix Ferroelectric Liquid Crystal Cells

The electro-optical behavior of deformed helix ferroelectric liquid crystal in reflective mode is described in this paper. The electrically controlled reflectance has been measured at subkilohertz driving voltage frequency for different polarizations of the incident light and compared quite successfully with the simulation results.

Voltage Sensor with wide Frequency Range using Deformed Helix Ferroelectric Liquid Crystal

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Voltage sensor based on Deformed Helix Ferroelectric Liquid Crystal

In this paper we propose a new approach to fibre optic voltage sensors via voltage-controlled Liquid Crystals (LC), which would allow direct measurement of up to 400 kV/m electric fields at multiple points. In addition, a novel polarization independent fibre optic sensor configuration is presented that exhibits a linear electro-optic (EO) response to variations of the electrical field under test.

Electrical energy harvesting device for current/voltage fibre-based sensors

In the context of current/voltage sensing by electric field monitoring it can be of importance to amplify the effect of the field at the sensors head. We present such an amplification - or energy harvesting - device in the form of a simple antenna coupled with a liquid crystal-based sensor for the monitoring of high-power lines.

Distributed hydrophone array based on liquid crystal cell

We describe a fibre optic hydrophone array system that could be used for underwater acoustic surveillance applications e.g. military, counter terrorist and customs authorities in protecting ports and harbors, offshore production facilities or coastal approaches as well as various marine applications. In this paper we propose a new approach to underwater sonar systems using voltage-controlled Liquid Crystals (LC) and simple multiplexing method. The proposed method permits measurements of sound under water at multiple points along an optical fibre using low cost components (LC cells), standard single mode fibre, without complex interferometric measurement techniques, electronics or demodulation software.

Reflection from gold-coated deformed-helix ferroelectric liquid crystal cells: theory and experiment

Liquid crystal (LC) cells can be used in conjunction with optical fibres to develop cheap and efficient sensors, such as voltage sensors or hydrophones. In this paper we apply an effective tensor model to describe reflection from gold-coated deformed-helix ferroelectric liquid crystal (DHFLC) cells. We show that, depending on the polarisation of the incident light, it is possible to obtain a linear electro-optical response to the voltage applied to the cell. Theoretical results are compared with experimental results yielding accurate agreement.

State of charge of battery indicator based on fibre optic probe

A low-cost, high-accuracy fibre optic sensor probe to determine state of charge (SOC) of battery is demonstrated. Capability of extension to multipoint measurements of SOC of stock batteries is also discussed.