Janusz Rzepka

Zeeman laser for straightness measurements

In the paper we present the prototype of a straightness measuring device that uses a frequency stabilized Zeeman He-Ne laser. The He-Ne laser line is split by Zeeman effect into two circularly polarized laser beams. The frequency of the radiations differ of 1,2 MHz. The surface stabilized ferroelectric liquid crystal cell is used to stabilize the laser frequency. As the result of the laser frequency stabilization the power of both radiation is equal. The circular polarizations of two laser beams are converted into two linear polarizations perpendicular to each other. The two laser beams pass close to the measured axis. Along the axis the analyzing probe is moved. The analyzing probe changes the ratio of the power of the horizontal to the vertical polarization. This ratio is analyzed by the receiver composed of the ferroelectric liquid crystal switcher, the polarizer and the detector. The straightness of 2 m long optical bench was measured with this techniques. The resolution of 0,1 μm and the accuracy of 0.5 μm were obtained. The accuracy of presented technique is not so good as in the methods using laser interferometer but is comparable with methods using PSD, quadrant detectors or CCD at the same time offering bigger resolution.

Frequency stabilization of Zeeman He-Ne laser using a surface-stabilized ferroelectric liquid crystal

The Zeeman He-Ne laser frequency stabilization system using a surface stabilized ferroelectric liquid crystal cell for polarization switching in the frequency stabilization loop is presented. Our experiments showed that two circularly polarized laser beams with on opposite sense of circularity can be switched up to 3.7 kHz rate. No additional optical components (the quarterwave and halfwave plates) were needed in switching setup. During three hours of operation the long time stability of the laser frequency of 3 multiplied by 10-8 was achieved. The laser stabilization system is suitable for the two frequency interferometer setups.

Polarization switching detection method using a ferroelectric liquid crystal for dichroic atomic vapor laser lock frequency stabilization techniques

We present a concept of the polarization switching detection method implemented for frequency-stabilized lasers, called the polarization switching dichroic atomic vapor laser lock (PSDAVLL) technique. It is a combination of the well-known dichroic atomic vapor laser lock method for laser frequency stabilization with a synchronous detection system based on the surface-stabilized ferroelectric liquid crystal (SSFLC).The SSFLC is a polarization switch and quarter wave-plate component. This technique provides a 9.6 dB better dynamic range ratio (DNR) than the well-known two-photodiode detection configuration known as the balanced polarimeter. This paper describes the proposed method used practically in the VCSEL laser frequency stabilization system. The applied PSDAVLL method has allowed us to obtain a frequency stability of 2.7×10⁻⁹ and a reproducibility of 1.2×10⁻⁸, with a DNR of detected signals of around 81 dB. It has been shown that PSDAVLL might be successfully used as a method for spectra-stable laser sources.

A universal laser interferometer for high linearity measurements

In the paper we present the idea and measurement results of a modified laser interferometer with improved measurement linearity and measurement resolution. Presented solutions allow for linearity increase below 1nm mark with measurement resolution even of single picometers (depending on the noise parameters of the laser source and detection electronics) in the basic heterodyne configuration of the optical elements.

Two-dimensional sinusoidal signal quality improvement by combined software and hardware means

This article describes a two-dimensional, quadrature, sinusoidal type signal correction. Such signals are widely popular e.g. in motor control, laser interferometry and sensors applications. It is proved that in some situations it is necessary to improve the quality of the signals in order to obtain the required parameters. We show that the optimum solution is a combination of hardware and software correction mechanisms. We report the signal improvement of a factor of more than a hundred. The proposed unique construction is also characterized by a very fast dynamic response in the range of single microseconds. This feature makes the circuit suitable for wide range of applications.

The use of discharge electric field variations to CO2 laser stabilization

Abstract A method is described for detection of CO 2 laser power variations which requires no optical detector. It is based on electric field changes near the laser tube induced by the laser power fluctuations within the optical cavity. This phenomenon called by us “antenna effect” is used for laser stabilization. A simple and inexpensive technique for CO 2 laser power measurements, resonator adjustment, and for observation of the laser signature is described.

Intermodal beat frequency stability in a polarization stabilized HeNe laser with an optical switch

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

In-site defectoscopy of granite blocks with a laser vibrometer

Presented work is an attempt to develop a method for finding cracks and flaws in granite blocks using data obtained by vibrometry measurements. There is presented a unique resonance based technique. The biggest advantage of introduced method is simplification of measurements in outdoor conditions. There is also presented a mathematical model for material structure evaluation. The model uses s-wave a p-waves velocities to fit defects into blocks response. Those velocities are also obtained by the presented technique. Measurements were performed by laser vibrometer with 100 pm resolution, 50 kHz vibration bandwidth and dynamic range exceeding 200dB.

Frequency stabilization of a single mode 780nm VCSEL lasers with the use of a rubidium cell

Techniques of frequency stabilization of the single mode 780 nm VCSEL laser with the use of an external 85Rb rubidium cell are presented. There is presented a comparison of different active and passive stabilization methods. The best method, using Zeeman splitting phenomenon and an optical switch with SSFLC cell, allowed to achieve stability level of 3ldr10-8 which responds to frequency variations of only 10 MHz. The stabilized VCSEL laser could be used as the source of radiation in an optical communication and in a laser interferometry. The same technique with different cells (for example cesium cell) could be used for stabilization of another wavelength of laser radiation used in optical networks.

Portable digitally controlled iodine stabilized He-Ne laser with very long lock-up time

A digitally controlled iodine stabilized laser with an increased stabilization (lock-up) time and reduced dependence on the changes of external conditions is described. Its construction is very compact as it consists of only two units: a laser and a power supply and it does not require any external devices for operation, neither a computer nor an oscilloscope. The laser is constructed in such a way that it is possible to transport it from one place to another without necessity of realigning resonator mirrors. The digital control allows full supervision of the device operation and/or unattended operation of the laser. Such laser is suitable for all applications requiring ultimate frequency stability and simple use.