THz Hilbert-Transform Spectrum Analyzer Based on High- $T_{c}$ Josephson Junction in Stirling Cryocooler

A demonstrator of a terahertz (THz) Hilbert-transform spectrum analyzer with subsecond scanning times and accuracy of the order of 10^-3 has been developed. The spectrum analyzer is based on a high-T_c Josephson junction in a compact Stirling cryocooler. Operation of the spectrum analyzer has been critically considered in the THz range and sources of errors have been studied. The voltage and frequency scales of the analyzer have been calibrated with accuracy of 7 ·10^-4 by Shapiro steps, induced on the I(V) curve of the junction by intensive monochromatic radiation. Dynamic errors of the analyzer have been minimized with scanning rates up to 4 THz/s. An instrumental function of the spectrum analyzer is of a Lorentz type with a spectral width of 1.5 GHz and free from any harmonic and subharmonic contributions with accuracy of around 10^-3. Rapid and detailed characterization of the THz sources, based on frequency multiplication of microwave radiation, has been demonstrated with the developed spectrum analyzer. The developed spectrum analyzer will be effective in new demanding THz applications, where a combination of high speed and accuracy is required.