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The rf impedance of a superconducting weak link with negligible capacitance is calculated for a variety of operating conditions. It is shown that the impedance is always real between harmonic steps of the I-V characteristic. The real part of the impedance is negative under certain conditions of bias for small rf signals, thus indicating the region of self-oscillation of the weak link.

We derive the frequency spectrum of the light output from Mach-Zehnder-type waveguide modulators and demonstrate the excellent agreement of the experimental results with theory. From the measurement of the frequency spectrum the rf modulation characteristics can be deduced. This method is advantageous as it permits the evaluation of the modulator performance up to frequencies above 1 GHz without requiring high optical power, thus eliminating the risk of optical damage in the waveguides. The tested modulator shows a 3-dB cutoff frequency of 1.1 GHz; the figure of merit is Q≈130 μW/MHz.

Coherent optical transmission systems require that the polarization of the received signal and the local oscillator are matched. Endless polarization control, polarization diversity, and data-induced polarization switching are the most promising solutions for this problem. However, in spite of a host of publications, no clear favorite has emerged, maybe because few laboratories have implemented more than one method and no direct comparison was ever made. In this paper we directly compare these three methods, and active, data-synchronous polarization switching, for the first time to our knowledge. We will see that endless polarization control is potentially the most powerful candidate, however, the choice of polarization control devices remains questionable. Polarization diversity is as versatile as polarization control and is potentially the fastest method; however, it yields lower receiver sensitivity. Endless control or a well-designed diversity receiver should be used for coherent trunk systems. Data-induced polarization switching is restricted to FSK systems. It promises a loss span similar to that of diversity but is far simpler which makes .it recommendable for FSK distribution systems.

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