Spectro-temporal unitary transformations for coherent modulation: design trade-offs and practical considerations

TL;DR

This paper evaluates spectro-temporal unitary transforms for coherent optical modulation, highlighting their lossless nature and analyzing design trade-offs to achieve high SDRs suitable for advanced optical communications.

Contribution

It introduces a detailed analysis of performance limits and design trade-offs for spectro-temporal transforms, demonstrating their potential for high SDR in optical systems.

Findings

High SDRs (>30 dB) are achievable with few stages and reasonable parameters.

Performance is influenced by the number of stages, dispersion, modulator bandwidth, and signal power.

Penalties from errors and limited DAC resolution are also addressed.

Abstract

This paper analyzes the performance of spectro-temporal unitary transforms for coherent optical modulation. Unlike conventional IQ modulation, such transforms are based on a cascade of phase modulators and dispersive elements, so are theoretically lossless and not limited by the bandwidth of the constituent modulators. We analyse the performance limits and design trade-offs of this scheme: estimating how the number of stages, amount of dispersion, modulator bandwidth, symbol block length and electrical signal power impacts the achievable signal-to-distortion ratio (SDR). Importantly, we show that high (>30 dB) SDRs suitable for modern >200 GBd class coherent optical communications are achievable with a low (<6) number of stages and reasonable parameters for driver power, modulator bandwidth and on-chip dispersion. Finally we address the SDR penalties associated with potential phase, amplitude, or dispersion errors, and limited DAC resolution.