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Journal of Physics D Applied Physics

Direct 825th harmonic mixing of a 1 GHz source with an HCN laser in a Josephson junction

T. G. Blaney, D. J. E. Knight

Published Sep 21, 1974• 37 citations
Publisher

Abstract

Signals near 1 GHz were multiplied in frequency by up to 825 times and mixed with the hydrogen cyanide laser frequency at 891 GHz in point-contact Josephson junctions. The 1 GHz signals were generated by direct multiplication from a 120 MHz quartz-crystal oscillator, and no microwave transfer oscillator was needed. The maximum IF output powers obtained in these and other harmonic mixing experiments (using the same laser) were found to agree quite well with a simple theory which predicts a limit proportional to the square of the frequency being multiplied in the Josephson junction. This provides a criterion for determining the lowest frequency at which to begin multiplication in a Josephson device.

DOI
10.1088/0022-3727/7/14/306

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Signals near 1 GHz were multiplied in frequency by up to 825 times and mixed with the hydrogen cyanide laser frequency at 891 GHz in point-contact Josephson junctions. The 1 GHz signals were generated by direct multiplication from a 120 MHz quartz-crystal oscillator, and no microwave transfer oscillator was needed. The maximum IF output powers obtained in these and other harmonic mixing experiments (using the same laser) were found to agree quite well with a simple theory which predicts a limit proportional to the square of the frequency being multiplied in the Josephson junction. This provides a criterion for determining the lowest frequency at which to begin multiplication in a Josephson device.
Experiments are described in which Josephson point contacts were used to generate and mix harmonics (up to the 96th) of microwave klystrons with the radiation from an HCN laser at 891 GHz. A qualitative analysis of mixing of two external radiations in a Josephson junction is given in order to understand the existence of mixing components independent of the Josephson oscillation frequency and linewidth. The spectral analysis of the HCN laser shows the importance of the microwave source spectral purity when frequency multiplication by large factors is considered.
Experiments are described in which simple and relatively reliable point-contact Josephson junctions were used to generate and mix harmonics (up to the 50th) of microwave klystron frequencies with emissions from a hydrogen cyanide laser operating at 891 GHz. Data are presented illustrating the variation of the intermediate frequency signal with DC bias current through the junction and incident radiation power levels, and the results compared, where possible, with a simple qualitative theory of Josephson mixing. Details are also given of experiments in which such a mixer was used to phase-lock a klystron to the laser, enabling the laser frequency to be accurately and continuously monitored.
Experiments were carried out to investigate the performance of a heterodyne detector at 891 GHz using a cw hydrogen cyanide laser as the local oscillator and Nb-Nb point contact Josephson junctions as mixing devices. Using a second off-tuned HCN laser as the signal, intermediate frequencies (IF's) up to a few MHz could be studied. Both pre-set and adjustable types of junction were used, and the latter, arranged so that radiation could be directly focussed onto them gave the best ultimate sensitivity. Dependence of the IF power on dc current bias and local oscillator power is described, and compared with simple theory. IF power varied linearly with signal power for signals less than 10-5 to 10-6 W. The mixer conversion efficiency was independent of IF. The sensitivity of the detector was a strong function of junction characteristics and ranged over several orders of magnitude in the junctions studied. Noise at the output was dominated by the room temperature preamplifier with a noise temperature of about 500 K. The smallest observed value for conversion loss of the junction was about 29 dB. This figure includes the signal input matching loss. The best sensitivity of the whole receiving system was equivalent to 1 × 10-17 W/Hz.
In an extension of earlier work, the response of niobium-niobium point-contact Josephson junctions to radiation from hydrogen cyanide and deuterium cyanide lasers has been studied. Constant voltage steps corresponding to the 805, 891 and 964 GHz lines from a HCN laser have been observed as well as steps corresponding to the frequency mixtures of all these lines. In addition, subharmonic and harmonics of `mixed-frequency' steps have also been observed. The first observation of Josephson step structure corresponding to the 1540 and 1578 GHz emissions of a DCN laser is reported.
The performance of Nb-Nb point-contact Josephson junction mixers was investigated experimentally with signals from a 891 GHz laser. The local oscillator (LO) was provided by the appropriate junction-generated harmonic of either a 74 or 99 GHz klystron. The intermediate frequency (IF) power varied linearly with signal power for signals less than 10 −5 to 10 −6 W. The optimum conversion loss (from 891 GHz to the IF) was not significantly different for the two klystron frequencies used: the lowest useful conversion loss for the whole mixing system including the signal input matching loss, was 42 (±3) dB. This is 10 dB greater than that observed with a 891 GHz laser LO and the same junction apparatus. Conversion loss of the mixer element alone was independent of IF with the latter varied up to 1·2 GHz. The noise at the output of the IF system was usually dominated by that of the transistor preamplifier with a noise temperature of about 500 K. Thus the input signal necessary to give a signal-to-noise ratio of one at the IF was equivalent to 10 −16 W Hz −1 of IF bandwidth. This mixing system, which can be significantly improved, forms the basis of a fully tunable heterodyne receiver for the submillimetre region.
Thin film niobium nitride Josephson tunnel junctions have been used as harmonic mixers in experiments to measure the frequency of various submillimetre wavelength laser lines. The Josephson junctions were used as harmonic mixers with an 18-40 GHz microwave synthesiser providing the local oscillator signal. Harmonics up to the 140th of ∼18 GHz, generated within the junction, mixed with optically pumped laser lines up to 2.5 THz to produce output signals around 20 MHz. Results of frequency measurements and data on the performance of the mixing system, including signal to noise ratios and frequency response, will be presented.