Self-coherent complex field reconstruction with in-phase and quadrature delay detection without a direct-detection branch

Abstract

Self-coherent detection with interferometric field reconstruction aims at retrieving the complex-valued optical field (amplitude and phase) by digitally processing delay interferometer (DI) measurements, in order to realize a differential direct detection receiver with capabilities akin to that of a fully coherent receiver with polarization multiplexing, albeit without requiring a local oscillator laser in the receiver. Here we introduce a novel digital recursive algorithm capable of accurately reconstructing the optical complex field (both amplitude and phase) solely from the quadrature DI outputs, eliminating the AM photo-detector branch. We analyze a key impairment namely the accumulation of errors and fluctuations in the reconstructed amplitude and phase due to ADC quantization noise, recirculating in the recursion. We introduce signal processing measures to effectively mitigate this noise impairment leading to a potentially practical self-coherent receiver, demonstrated in this paper for a single polarization. We also investigate the range of applicability of self-coherent detection concluding that it is most suitable to relatively low baud-rate systems such as passive optical networks, for which application the self-coherent receiver outperforms the coherent homodyne receiver due to its improved laser noise tolerance, obtained due to the removal of the optical local oscillator.

A self-coherent receiver for detection of PolMUX coherent signals

“…Any symbol from the transmitted constellation can be chosen for the symbols A x (8) and A y (8), which are used to start the field recovery algorithm. The preamble starting with the two zeros in the two polarizations serves as a uniquely identifiable symbol sequence which must not be part of the transmitted constellation.…”

“…Recently, self-coherent detection (SCD) using delay interferometers (DI) has been suggested as a substitute for coherent reception [2][3][4][5][6][7][8]. The concept extends self-homodyne differential detection of signals such as the differential phase-shift keying (DPSK) modulation format, where phase differences of zero or 180° are conveyed.…”

“…Especially the quantization noise of analog-to-digital converters (ADC) at the receiver is a degrading factor [8]. This OSNR penalty may be reduced with proper DSP algorithms [2,8,10].…”

“…Especially the quantization noise of analog-to-digital converters (ADC) at the receiver is a degrading factor [8]. This OSNR penalty may be reduced with proper DSP algorithms [2,8,10]. These algorithms, which were developed for single polarization, are not directly applicable to self-coherent receivers for PolMUX signals, since the unknown crosstalk between the two polarizations at the receiver strongly distorts the signal constellations.…”

“…Unfortunately, in SCD, post-detection noise accumulates in the field reconstruction recursion, leading to a random-walk process of the complex recovered signal [7,8] if no preventive measures are taken. Due to the independent noise random walks in the two polarizations, the PolDEMUX algorithms [18] commonly used in coherent detection system are not directly applicable for SCD system.…”