Low-Complexity Direct-Detection Polarimetric Receiver for Mode Vector Modulation

Mode vector modulation (MVM) is a new type of multidimensional spatial modulation format for short-haul optical communications systems using multimode or multicore fibers or free space modes. It offers higher sensitivity than conventional modulation formats, such as pulse amplitude modulation (PAM), typically used in short distances. One of the attractive features of MVM is that it requires a direct-detection polarimetric receiver and, therefore, avoids the use of expensive coherent intradyne receivers like those used in long-haul communications. Nevertheless, the complexity of MVM direct-detection polarimetric receivers scales quadratically with the number of spatial and polarization degrees of freedom <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N$</tex>. This constitutes a major obstacle for the eventual adoption of this type of modulation in short links, since upfront cost is the main consideration for judging the viability of various technological solutions. In this paper, for the first time, we propose and study a low-complexity direct-detection MVM receiver architecture that is based on asymmetric optical hybrids and requires only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3 N-2$</tex> photodiodes. Through analysis and numerical simulations, we compare its performance to alternative architectures in different noise regimes and for different values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N$</tex>. We show that our proposed design is superior to conventional polarimetric receivers for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N=2$</tex> in terms of performance, as well as cost-effectiveness. For <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N&gt;2$</tex>, unamplified transmission is unlikely and, therefore, it is anticipated that amplifier noise will be dominant. In this regime, the performance of the proposed receiver is equal to the quadratic hardware complexity receivers while its cost is significantly lower.