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Soviet Journal of Quantum Electronics

Data transmission in the far infrared range

É. M. Belenov, Сергей Иванович Веденеев

Published Nov 30, 1979• 0 citations
Publisher

Abstract

An investigation is made of the possibility of using superconducting Josephson weak links (bridges or point contacts) for data transmission and reception in the frequency band ~1012 Hz in the far infrared region. It is shown that a signal current ~(10–2–10–3)Ic is induced in real superconducting point contacts (Ic is the critical superconducting current). Such a signal is considerably greater than the superconducting contact noise or spontaneous noise of the laser medium at the carrier frequency.

DOI
10.1070/qe1979v009n11abeh009786

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STIN•May 1, 1978•0 citations
An investigation is made of the possibility of using superconducting Josephson weak links (bridges or point contacts) for data transmission and reception in the frequency band ~1012 Hz in the far infrared region. It is shown that a signal current ~(10–2–10–3)Ic is induced in real superconducting point contacts (Ic is the critical superconducting current). Such a signal is considerably greater than the superconducting contact noise or spontaneous noise of the laser medium at the carrier frequency.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb–Nb point-contact junctions at 4.2 °K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1, 0.3, and 9.2 GHz. The point contact is in a nonresonant structure and fed by a constant current source: neither a strong rf coupling of the contact with a cavity nor a stabilizing shunt resistance was proved necessary. The direct observation of frequency conversion was performed with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1600 GHz. The experiments with intermediate frequencies fi of 0.1 and 0.3 GHz indicate that conventional frequency conversion does not occur but that the observed signals are related to noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. The results show that low-frequency current components are induced in the junction, which can give large spurious signals when small i.f. are used.
We have performed mixing experiments with CO2 lasers in the 9 to 11 μm wavelength band using superconducting point contacts. The experiments involve difference frequencies ranging from 19 MHz to 106 GHz. The mixing mechanism definitely appears to require the superconducting state since the beat signal amplitude decreases by as much as 60 dB as the bath temperature is raised through the superconducting transition temperature. At least three conceptually distinguishable mechanisms may be involved in the observed response. These include : (1) a conventional Josephson response, (2) a thermally modulated Josephson response, and (3) a single particle tunneling response. The usual signature of a Josephson interaction is one or more current steps in the I-V curve. Steps, the first of which should be at 67 mV, are not observed in these experiments. However, we do not feel that is a definitive test for the mixing mechanism between two external sources at such high frequencies. In fact, Werthamer's theory of tunnel junctions predicts a large response for these experiments, a response as large as that from two X-band sources, for example. It should be noted, however, that the theory assumes the junction is maintained at a fixed low temperature and whether or not that is the case depends directly on the current density at the contact. Very little is known about the current density at the present time. These experiments emphasize a fact that is usually overlooked in Josephson junction mixing experiments with an external local oscillator. If the contact or tunneling region is sufficiently small, it may be thermally modulated at difference frequencies extending into the microwave range. This effect becomes more likely with increasing applied frequencies, particularly in the infrared. Theoretical modeling for the purpose of calculating ultimate sensitivities of these devices requires a detailed consideration of thermal effects. We fully expect large mixing signals from Josephson junctions even at optical frequencies if thermal effects do not prove to be a limitation.
Point-contact-type Josephson junctions have been developed as a harmonic mixer to measure far-infrared (FIR) laser frequencies in a frequency chain up to the infrared region. The junctions were formed by mechanically pressing a niobium wire of 70 μm in diameter with a sharply etched tip against a niobium post of 2 mm in diameter. The voltage-current characteristics of the mixers were investigated under the irradiation of FIR lights from an optically pumped methyl-alcohol laser. The Shapiro steps for FIR lights up to 3.1 THz were observed. The mixing properties of the mixers were studied through harmonic mixing experiments of 50-GHz microwaves and the FIR lights up to the frequency of 3.1 THz. The conversion efficiency of the beat signals to the incident FIR power was found to vary as the — 4th power of the FIR frequency. From the mixing properties, the combination of the critical current Icof the junction and the product IcRnwhere Rnis the normal resistance of the junction, was found important to optimize the mixing performance.
The high-frequency behavior of niobium cat-whisker point contacts is studied using radiation from an optically pumped far-infrared (FIR) laser. The FIR molecular laser had a dielectric-waveguide cavity and was pumped by a CO2 laser. The use of capacitive-mesh output couplers is extended throughout the FIR spectrum, between 42 micrometers and 1.2 mm, and have found the optimum grid constants for several lines. At shorter wavelengths, the laser performance was improved by the use of a novel hybrid capacitive-mesh hole output coupler. The coupling of the FIR radiation to the point contact is found to depend in a simple manner on the resistance of the contact. The FIR laser radiation induces constant-voltage steps on the dc I-V curves of our junctions, allowing us to study the high-frequency ac Josephson effect.