Laser-based measurements of complex high-frequency signals

 

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P. Struszewski

Laser-based measurements of complex high-frequency signals

ISBN: 978-3-95606-592-7   |   Erscheinungsjahr: 2021    |    Auflage: 1
Seitenzahl: 188   |    Einband: Broschur    |    Gewicht: 564 g
Lieferzeit: 2-3 Tage
21,50 €
Inkl. 7% MwSt., zzgl. Versandkosten bei Auslandsbestellungen

The constantly growing demands in the telecommunications industry require electrical signals with large bandwidths that extend into the THz range. This poses considerable challenges for metrology since conventional purely electrical measurement methods can only cover this extended frequency range with large technical effort. As a consequence, optical time-domain sampling techniques based on femtosecond lasers are increasingly used. With these sampling techniques it is possible to detect ultrashort voltage signals with temporal width of a few ps, corresponding to a bandwidth of more than 500 GHz, by utilizing the linear electro-optic effect. The main challenge of these techniques is to identify all systematic effects caused by the electro-optic interaction during the detection process and by electrical propagation and reflections. Established optical approaches rely on electrical high-frequency measurements to identify these systematic influences which, however, lead to frequency limitations determined by the electrical instrumentation. Therefore, in this thesis new purely optical measurement techniques are developed to characterize the influence of the electro-optic interaction and electrical propagation and reflections on electro-optically measured signals. These findings can be used to traceably retrieve the original undistorted waveform of a complex high-frequency signal. In the first part of the thesis, a novel purely optical approach, based on electro-optic measurements at different positions on a planar waveguide, is studied to determine the electrical reflection and propagation properties of a transmission line. A systematic investigation enabled a deeper understanding of the physical principles of this novel approach and provided accurate reflection characteristics of a transmission line in a frequency range between 5 GHz and 500 GHz. A further improvement of the reflection measurement is achieved by implementing an alternative sampling method based on two asynchronous laser systems. This method requires no moving components and enables a significantly improved frequency resolution of the electro-optic detection from 500 MHz to 76 MHz. A key element of the alternative sampling method is the synchronization of the two laser systems. By implementing a new digital stabilization system, the temporal jitter of the measurement could be halved to below 46 fs while simultaneously enabling an unstabilized signal source. This significantly increases the range of applications for the electro-optic detection. In the second part, the physical properties of the electro-optic interaction are analyzed by describing this interaction within a linear response theory using an electro-optic transfer function. This investigation includes both a numerical simulation for the theoretical prediction of the transfer function and a new experimental measurement scheme relying on an analytical model for the transfer function. The comparison between numerical and experimental results reveals a good agreement and thus validates the physical model and the experimentally found transfer function. Using this function, the original waveform of voltage pulses on a waveguide can be found from the electro-optically measured signal resulting in a reduction of the pulse width from 2.21 ps to 1.94 ps. A precise knowledge of the electro-optic transfer function is also essential to provide traceable voltage values of the original high-frequency signals. These quantitative measurements are performed using a low-frequency signal with an exactly known voltage amplitude as a reference standard. However, in this thesis it was shown that the electro-optic transfer function in the frequency region of the reference signal is not constant as expected but exhibits a pronounced frequency dependence. This causes a scaling of the specified voltage values that is difficult to determine and thus makes traceability more difficult. By means of a systematic investigation, the value of this scaling factor could be narrowed down to an interval between 0.8 and 1.0. Finally, the performance of the newly-developed optical methods is demonstrated by the characterization of two different high-speed photodetectors. For a 100 GHz photodetector, the impulse response is determined where all systematic effects caused by mismatches of the transmission line and the electro-optic interaction are eliminated. This results in a response function with frequency components up to 190 GHz. These high-frequency components are essential to specify the original waveform in the time domain. A balanced high-speed photodetector is characterized by measuring the common-mode rejection. The results obtained are compared in an international study with the measurements of two different conventional methods. The comparison indicates a good agreement between the three measurement methods but also demonstrates that the characterization is very sensitive to tiny differences in the experimental alignment.

PTB E-118