XPM- and XGM-Based Optical RAM Memories: Frequency and Time Domain Theoretical Analysis

TLDR

The study presents a comprehensive frequency‑ and time‑domain analysis framework to understand how design parameters affect optical RAM cell performance. The authors analyze two SOA‑based optical RAM architectures—one using cross‑gain modulation (XGM) switches and the other employing cross‑phase modulation (XPM) in a Mach‑Zehnder interferometer—by deriving their frequency responses via first‑order perturbation theory and validating speed and signal quality with a transfer‑matrix SOA simulation. The analysis shows that the free spectral range depends on the coupling length.

Abstract

We demonstrate a frequency and time domain analysis for optical random access memory (RAM) cells that rely on semiconductor optical amplifier (SOA)-based switches but employ different switching mechanisms. The first RAM cell utilizes SOA cross gain modulation (XGM) switches both for the access gate as well as latching mechanism, whereas the second RAM cell configuration utilizes SOA-Mach-Zehnder interferometer cross phase modulation (XPM) switches. The frequency domain analysis exploits first-order perturbation theory approximations towards deriving the RAM cell frequency response, which is shown to exhibit in both RAM cell layouts a comb like resonant behavior. The free spectral range is dictated by the coupling length between the coupled switches that form the latching element, whereas the finesse depends on the temporal response of the switching mechanism employed. The qualitative speed and signal quality results obtained in the frequency domain are confirmed by a respective time-domain analysis carried out for both RAM cell layouts, using an experimentally validated time-domain SOA simulation model that relies on the transfer matrix method. Performance analysis in the time domain reveals in addition important quantitative RAM output signal measures like the extinction ratio and its dependence on the coupling length and the operational speed, as well as the input power dynamic range for successful RAM operation. Our holistic frequency- and time-domain analysis framework provides an in-depth understanding of performance-critical design parameters and their relationship to expected RAM cell performance characteristics. This is then utilized for a one-by-one system level comparison between the two RAM cell layouts in terms of readout extinction ratio, maximum speed, footprint, and power consumption concluding that the SOA-XGM-based RAM cell offers certain advantages when operational speeds not higher than 10 Gb/s are targeted, and the SOA-XPM-based RAM cell setup dominating when higher RAM serial speeds even up to 40 GHz are targeted.