In information processing and communication technologies, the data rates of electronic devices have been constantly increasing in recent years. In order to keep pace with this development, national metrology institutes, such as PTB, have to introduce new techniques for the traceable measurement of the transfer characteristics and intrinsic rise times of high-speed electronic devices. Routinely, sinusoidal voltage signals at discrete frequencies are used for this purpose. Variation of the discrete frequency yields the transfer characteristics in frequency space. Such measurements are feasible up to frequencies of approximately 100 GHz.

Optoelectronic measurement techniques allow one to extend the frequency range to the sub-THz and THz region. Femtosecond optics forms the basis of the methods that are utilized by the Terahertz Optics group. Instead of sinusoidal voltage signals, ultrashort voltage pulses are used a test signals. The shape of these voltage pulses is determined with time-resolved optical sampling techniques before and after the pulses have passed the device under test (DUT). The transfer characteristics of the DUT can be calculated from the distortion that the pulses have experienced during propagation through the DUT. Very often the DUT can be described as a linear system. Its impulse response can then be obtained from the deconvolution of the output pulse with the input pulse.
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Example of an optoelectronic measurement technique: A semiconductor circuit and laser radiation (red-coloured) are used for generation and detection of ultrashort voltage pulses. On the left-hand side, the head of a 50 GHz sampling oscilloscope is seen, whose rise time is calibrated.

The temporal shape of ultrashort voltage pulses can be determined with all-optical sampling methods using femtosecond laser pulses. Frequently, the electro-optic sampling method is employed. An electro-optic crystal (e. g. LiTaO3) is brought in close vicinity to the electronic circuit in which the voltage pulses propagate. The electric field of the pulses changes the refractive index of the electro-optic crystal and, in turn, the phase of the probe laser pulses that are focussed in the crystal. The phase change is a direct measure of the voltage. The shape of the voltage pulses is determined varying the time delay between the probe laser pulse and the laser pulse that excites the voltage transients. Such measurements can be performed with a time resolution of 300 fs. A mode locked Titanium:sapphire laser generates the laser pulses.

Schematic electro-optic sampling set-up. The blue curve represents a voltage pulse whose temporal shape is to be determined.

Unfortunately, the external electro-optic crystal introduces a discontinuity in the transmission line on which the voltage pulse propagates. This discontinuity distorts the voltage pulse. As a consequence the shape of the measured voltage pulse deviates from the shape of the true, undisturbed voltage pulse. In the past, the Terahertz-optics group has optimized electro-optic sampling with external sampling tips. Using these optimized techniques it is possible to significantly reduce the distortion which the external sampling tip imprints on the measured voltage pulse.

The Terahertz-Optics group additionally investigates and develops alternative optical sampling techniques for the time-resolved measurement of ultrafast electric fields. Recently, a method was presented that is based on spectral integration over electric field-induced Franz-Keldysh oscillations in the absorption spectrum of a semiconductor.

For further information please contact Mark Bieler.