In this paper the authors present a practical solution to the problem of optical isolation. This solution is as simple as it is elegant, and is the type of idea that seems obvious in retrospect but could well be game-changing in the way that isolation is currently approached. The central concept is to employ a twinned set of Mach-Zehnder modulators, separated by a delay line. The modulators are fed with high-frequency square waves, which are synchronized with the length of the delay line so that the phase changes induced in the optical signal add up in one direction but cancel out in the other. This difference in phase leads to the preferred direction required by non-reciprocity, which is parlayed into isolation by mixing the signals at the output.
Using this set-up, the authors demonstrate a proof-of-principle isolation device using off-the-shelf components. They demonstrate an impressive 12.5dB of isolation over an extremely broad bandwidth (8.7 THz). The impact of these numbers is emphasised in the paper by the comparison with other state-of-the-art isolation schemes. The authors also examine the effect of the device on high-speed data transmission, via “eye –diagrams”, using an on-off key encoding of the data. As the authors note, the device would not be expected to work for phase-based modulation schemes such as DPSK because the scheme results in the flipping of the optical phase in the “open” direction; however the beauty of the results lead one to contemplate whether a simple adaptation could be concocted. As a final flourish, the authors demonstrate that this scheme can also be used as an optical circulator.
Despite the fact that this demonstration uses distinct optical components, the authors argue convincingly that there is nothing to prevent the integration of this method in CMOS-compatible platforms using present-day technology. This argument is supported by a useful discussion on the likely performance of such a device.
This paper is a beautiful example of how an elegant idea can be treated both clearly and comprehensively. The discussion of the work is both informative and interesting, pointing out the impact of the work as well as placing the research in the broader context of non-reciprocal systems. Finally, the Appendix contains a simple model for the operation of the device, written at a level that makes it a useful educational introduction. Best of all, this paper stimulates thought and discussion on the practical implementation of non-reciprocal devices, as well as on their importance and potential uses in the field of photonics.
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