Laser Measurements Simplify Electronics Testing

Femtosecond lasers may provide a cost-effective alternative to conventional vector network analyzers for high-frequency measurements involved in electronics testing.

Researchers at Physikalisch-Technische Bundesanstalt (PTB) carried out such measurements using near-infrared laser emitting 100-fs pulses.

The laser beam was divided into a pump beam and a probe beam. The pump beam excited a photoconductive switch integrated in a planar waveguide. This excitation lead to voltage pulses approximately 2 ps long propagating on the planar waveguide. The probe beam was used to detect the electric field of the voltage pulses, employing the Pockels effect of the substrate on which the planar waveguide is fabricated. The shape of the voltage pulse could be accurately measured by changing the time delay between pump and probe beam through the use of a delay line.

Part of the experimental setup for laser-based vector network analysis. A cluster of planar waveguides on a semiconductor are contacted at either end using two microwave probes. A laser beam is focused from the front onto the waveguide and generates in a photoconductive gap ultrashort voltage pulses. The voltage pulses are detected by a second laser beam focused from the back through the substrate onto the planar waveguide. Courtesy of Physikalisch-Technische Bundesanstalt.

Such measurements are typically carried out with vector network analyzers (VNAs). These are among the most precise high-frequency measurement devices available today, usable up to frequencies of 1 THz (1012 Hz). However, VNAs are very expensive and require multiple frequency extenders in order to cover a wide frequency range.

The measurement principle of VNAs relies on the detection of power waves at discrete frequencies. Variation of the frequency allows frequency-resolved measurements. The measurement results are typically specified in terms of scattering parameters. In order to characterize a high-frequency device accurately, forward and backward propagating signals have to be separated, which is accomplished using directional couplers.

Using the laser technique, the PTB researchers achieved the separation of forward and backward propagating signals by detecting voltage pulses at different positions on the planar waveguide. This was possible even in the case of temporally overlapping forward- and backward-propagating signals.

With the new optoelectronic time-domain measurement method, the researchers demonstrated scattering parameter measurements on planar waveguides up to 500 GHz with a 500 MHz frequency spacing.

In addition to high-frequency measurements, the method could be used to characterize high-frequency coaxial devices and to help realize a precise voltage pulse standard.

The results were published in IEEE Transactions on Microwave Theory and Techniques (doi: 10.1109/TMTT.2015.2481426).