Wieslaw Jasiewicz

Shaping the spectral characteristics of fiber Bragg gratings written in optical fiber taper using phase mask method

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

Multiplexed FBG strain measurement system

The idea and, design and realization of fiber Bragg grating multiplexing system is given. Special Bragg gratings with very long and linear side slopes were practically realized. They were tuned for different wavelengths distanced 9nm in each measurement channel. The system was applied for strain control. Special spectrophotometer with linear photodiode array was made. For flexible distributed multiplexing of many sensor channels fiber-optic switches were used.

Wideband light source for fiber Bragg grating measurements

Wideband (L, C, S) light source for the third fiber transmission window was developed. It has replaced incandescent halogen lamp light source in grating monochromator to increase the input signal level during measurements. This replacement has resulted in measurement time decrease and dynamic range increase in narrowband FBG spectral measurements. The source is an offer for other fiber metrology equipment users.

Interferometric fiber Bragg gratings

The interferometric techniques of realization of fiber Bragg gratings are presented. The practically performed laboratory optical system is described. First results of Bragg gratings written by this method on hydrogenated standard telecom fibers are given.

Bragg gratings in optical fibers made by the phase mask method

A fiber Bragg grating is a very attractive passive device widely used in telecommunication networks, laser technologies, metrology and research laboratories. The Institute of Electronics Systems has recently opened the new laboratory for writing Bragg gratings directly on fiber cores. The application of a phase mask is a reasonably simple technique for material, fiber photosensitivity research and filter spectral characteristics control.

Fiber Bragg gratings: technology and measurement

At present fiber Bragg filters are widely used in telecommunication, laser technology and metrology. The new laboratory was organized in the Institute of Electronic Systems (ISE), Warsaw University of Technology for Bragg gratings writing directly on fiber cores. The first results obtained with the use of the phase mask technique are presented.

Fiber Bragg filter measurements

Narrowband fiber Bragg filters are commonly used in telecommunication, laser technology and metrology. Special techniques have to be used to achieve the sufficient resolution in the subnanometer range. The modernized standard monochromator system with the resolution of 0.16 nm is presented. Measurement results for some filter designs are presented.

Operating distance extender for photonic vibrometer

Photonic vibrometer is used for non-contact vibration measurement. Information about changes in distance between the vibrating target and the fiber bundle tip is expressed as a change in the intensity of back scattered light by the target. The optical extender accessory offers the advantage of considerably increased operating distance (from about 1mm to 40mm).

High-resolution computer-controlled monochromator system for fiber diffraction grating measurements

A simple modification of the typical wide-band monochromator based cut-back system for fiber transmission measurements is presented. To increase the spectral resolution for fiber Bragg gratings measurements the properly blazed monochromator grating was inserted, stepping motor resolution increased and mechanical chopper thoroughly redesigned to avoid signal phase fluctuations. As the result spectral resolution of 0.16nm and signal dynamic range of 30dB was achieved in the 1.5μm region.