Friedemann Mohr

Application of kalman filters as a tool for phase and frequency demodulation of IQ signals

Frequency or phase modulated electrical signals arise in multiple applications in communication systems as well as in measurement setups such as interferometry / vibrometry, velocimetry etc. common demodulation techniques are often based on analog techniques (e.g. a PLL in the RF band) or - after mixing into the base band and generating IQ signals - on digital signal processing algorithms.

Bias error in fiber optic gyroscopes due to elasto-optic interactions in the sensor fiber

It has become a standard to use Shupes nonreciprocity formula for calculating the thermally induced bias error in fiber gyroscopes. This effect is initiated by thermal waves propagating through the material. However, it seems as if it has not been recognized yet that elastooptical interactions in the fiber can also lead to a nonreciprocity effect. In this case it is not required that a thermal wave propagate through the material, a simple temperature change -- even if homogeneous -- would have the same effect. In this contribution, we demonstrate the nature of the effect and, based upon the standard architecture of a sensor coil assembly, report on calculations (including FEM) elucidating the origination of elastic stress leading to a nonreciprocity.

Rigorous treatment of fiber-environmental interactions in fiber gyroscopes

Thermal and elastomechanical effects on fiber optic gyroscopes (FOGs) are a major source of error signals and have been dealt with mainly by Shupe and Mohr. It has become a standard to use Shupepsilas nonreciprocity formula for calculating the thermally induced bias error in fibre gyroscopes. This effect is initiated by thermal waves propagating through the material. However, the Shupe formula does not describe the effect of homogeneous, but transient heating of the fiber coil, which leads to an inhomogeneous deformation of the fibers and thus to nonreciprocity. Furthermore, the effect of inertial forces such as vibrations has been described first in the work of Mohr and Schadt (2004) and shall be extended in this contribution. Wepsilall describe a simulation model of the fiber coil using analytical and - where necessary - finite element methods. Finally, wepsilall verify our simulation model with measurement results from an FOG.

Aspects of Signal Processing in Laser Vibrometry and their Embedded-Realisation on FPGA with NI-LabVIEW

The most common approach in todays interferometric laser vibrometry is the so called heterodyne technique. This about 20 year-old method works with two optical beams with two different optical frequencies that are lead over separate ways and, after one of them has interacted with the vibrating target, are superimposed on optical detectors to produce interference signals [1] However, this concept requires expensive optical components such as a Bragg Cell or a Zeeman-Laser. In order to reduce the manufacturing cost of an interferometic laser vibrometer we persecute a strategy which reduces the optical complexity for the prize of higher signal processing demand. Based on the fact that today highly integrated signal processing hardware such as DSPs (Digital Signal Processors) FPGAs or ASICs solutions are much less expensive than in the past, this so called HWSHD (Homodyne With Synthetic Heterodyne Demodulation) strategy is therefore competitive to the classical usage of expensive optical components in the heterodyne concept. This paper will, after a brief introduction into the homodyne vibrometer concept, describe the idea and realisation aspects of the signal processing algorithm by usage of NI-LabVIEWs FPGA Module.

Observation of prestress loss in post-tensioned concrete with FBG and LVDT sensors

We report on long-term measurement results on post-tensioned concrete specimens. A total number of 8 specimens, each of dimensions 700×150×150mm3 and four each of two different concrete mix designs, are subjected to prestress by post-tensioning iron bars conducted in a central longitudinal hole. During their drying phase the specimens undergo a prestress loss from creep and shrinkage, resulting in a reduction of their longitudinal size. This change of dimension is observed by a number of sensors applied to the arrangement. As the main sensors for strain, FBG sensing fibers spot bonded to the specimen and FBG sensing fibers bonded on their entire length are used. FBG sensing fibers are applied to both sides of the specimen to allow observation of bending. The experimental arrangement further includes a conventional length measurement apparatus consisting of LVDT sensors (also applied to opposite sides of the specimens) so comparisons can be made between the novel technique based on FBGs and this conventional measurement approach. As each of the 3 types is applied to both opposing sides of the specimens bending can also be monitored. In addition, our data logger will also monitor both room and specimen temperatures. We are especially interested in the performance of the FBG sensors bonded on their entire length that are measuring the local strain in the concrete through direct elastomechanical interaction as we want to clarify the appropriateness of this novel technique for future SHM (structural health monitoring) installations in civil engineering. The readout of the FBG sensors which are all arranged in an optical sensor network is done by a tunable laser approach. Preparation of the set-up has been accomplished recently, and data are now being collected. The observation of the prestress loss process is planned to run over several months, and results will be presented at the conference.

Micropolarizers in real time polariscope

This contribution deals with a polariscope, an instrument for determining internal strain in a transparent medium by exploiting its polarization transmission: When a sample of birefringent material is inserted in a polarized beam of light a fringe pattern is observed. The pattern is captured by a camera and processed to reveal internal stresses in the probe. However, in order to completely specify stresses, four patterns obtained under different optical conditions are used. These can be achieved either by using a rotating analyzer in the optical setup or by applying the so called “Multispec Imager” approach where the output beam is divided into several parallel beams which can then be directed to separate sections of a single camera or to several individual cameras. Disadvantages of the former solution is the requirement of a mechanically rotated component (necessitating a driving motor and longer measurement time whence this concept is far from being suitable for real-time use) and of the latter is its price and higher complexity of the optical layout. The use of micropolarizers in a digital polariscope removes all stated disadvantages, hence such a polariscope processes a single beam (no need of beam-splitting) and one fringe pattern (no need of a rotating analyzer).

Influence of non-cooperative target on interferometrical signal

In this paper we discuss problems in an interferometer which arise when the interferometer uses a conventional, so-called dasianon-cooperativepsila target rather than a mirror. We discuss how the interference signal is influenced by using non-cooperative target which also can depolarize the incident beam. Finally we show how the detrimental effects arising from the speckle field can be reduced when a suitable imaging optics is used.

Methods for Measuring the Polarization Properties of Retarders and Beamsplitters

We develop methods to determine the polarization properties of basic optical devices like retarders and beamsplitters using a Soleil-Babinet compensator. A mathematical description of the optical components is also given. The description is then applied on a coherent state, which represents laser radiation.

Spatial Filter Design Techniques with Applications in Optical Velocity Measurement

This investigation was stimulated by the search for a simple and easy-to-apply method for measuring the movement of a car safety belt under a crash. Conventional methods would consist of gluing an incremental barcode-like strip on the belt, observing it with a reflective optical sensor, and counting the periods occurring while the belt is pulled out. However, as the woven structure of the belt constitutes a quite well-defined periodic texture by itself, the question arises if it is possible to determine the velocity from observing this. The method would then require imaging the texture to a suitably designed transmitting reference mask and evaluating the periodical signal observed behind the mask while the belt is moving. Mathematically, the proposed procedure can be called an optical spatial filter (OSF) technique. Of course, when applying OSF technique for velocity measurements of periodic objects it is vital to adapt the spatial filter closely to the moving object. While objects with a random surface or texture are quite easy to deal with, those with a periodic surface need special consideration. This investigation is aimed to explain the necessary considerations and to present techniques for suitably designing optical spatial filters.

Optical Zoom System for Laser Welding Robot

In this research, we established the design rules for providing a laser welding robot with an adaptable imaging system (a zoom lens optics). Goal of this system was the ability to deliberately move the focus point - the welding point - of the optical system in longitudinal direction. Such a system would make a robot much more flexible than a fixed-focus system as it would lead to a virtual increase of the robots arm length. While conventionally increasing the arm length of a robot would result in an increase of its mass, a higher amount of power needed, and a deterioration of its dynamic properties, a variable-focus system would widely avoid these drawbacks. Finally, this would also lead to higher working speed of the system and, thus, to higher manufacturing throughput.