Analog digital conversion for terahertz data
The THz Photonics research group at the Braunschweig University has developed what it claims to be an entirely new approach to scanning and converting analog signals: All known samplers multiply the analog timing signal with sequences of very short pulses. In electronic samplers, the signal is multiplied with the gate function, a square wave signal generated by the respective circuit. In optical samplers, a direct multiplication of the signal and the short laser pulses takes place in a non-linear crystal or a non-linear fiber.
According to the rules of the Fourier transformation, a multiplication in the time domain corresponds to a convolution in the frequency domain. Thus, the new approach from the Braunschweig group is based on a convolution of the signal spectrum in a frequency comb. Though the underlying maths appear relatively complex, the practical implementation of the idea is rather simple, explained professor Thomas Schneider from the THz Photonics group. In contrast to electronic ADCs, the method at hand allows to achieve very high data rates at little effort. In addition, the technique can be integrated onto silicon photonic device with very little space requirements. All parameters can be controlled from the outside by electric signals.
The THz Photonics group is currently working to integrate their approach on silicon-on-insulator (SOI) semiconductors.
Traditionally analog-digital conversion is mostly a domain of electronic circuits. This is sufficient as long as values are converted at a relatively low data rate (or analog frequency). However, in the glass fiber networks of the internet, the data rate is continually rising. The electronic switches and converters used in this environment are already at their limit today. The circuits in use consume very much energy which leads to massive generation of heat. The removal of this heat requires additional energy. A further increase of the data rate would therefore require very significant efforts. Optical samplers, in contrast have the advantage to offer extremely high bandwidths in the terahertz range and therefore can process data rates in the dimension of terabit per second. For comparison: One terabit equals the amount of data required to stream more than 65.000 videos in UHD resolution. An additional benefit lies in the fact that the optical signal does not need to be converted first into an electrical one which is the case with electronic A/D converters. However, the optical converters require a source capable of providing very short pulses at a fixed, extremely stable repetition rate. Such mode locked lasers are relatively large and cannot be changed easily. In particular, mode locked lasers cannot be integrated on a chip. The method devised by the THz Photonics group therefore will offer a benefit once it is successfully implemented.
Further information (in German) www.tu-braunschweig.de/ihf
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