Proposal and efficiency analysis of cascaded adiabatic frequency conversion in coupled microrings

Adiabatic frequency conversion (AFC) in microring resonators is a promising alternative for fully integrable and tunable frequency shifting of optical signals. Nonetheless, the magnitude of the frequency shift via simple AFC in a single ring is fundamentally limited by the material platform. To overcome this limitation, we propose and analyze a scheme to induce cascaded AFC (CAFC) along a chain of coupled, yet initially detuned rings, without requiring unloading the optical signal into a bus waveguide between successive modulations. For concreteness, we examine thoroughly the simplest nontrivial case of a chain of two rings and briefly discuss the generalization to a chain of an arbitrary number of rings. We analyze the temporal dynamics of this CAFC process using temporal coupled mode theory. We examine the transformation of the input into the frequency-shifted output as a rank-one linear operator in the vector space of finite-energy pulses. In this way we show that the energy efficiency of CAFC depends on the input pulse shape through a Schwarz inequality, just as in single-ring AFC. We propose a numerical scheme to maximize the CAFC efficiency with respect to the process's timescales and discuss the physics involved. We show that the resulting CAFC efficiency converges in a polynomial manner to a maximum as the process becomes progressively idealized. Furthermore, we show that this maximum efficiency is identical to that for single-ring AFC, e.g., 0.7951 for a symmetric, single-lobe input pulse. Thus, we show that CAFC can become more efficient than multiple instances of single-ring AFC. We explain the polynomial convergence of the CAFC efficiency as a consequence of its real analyticity as a function of the process's timescales under our scheme. We leverage this polynomial convergence to model the CAFC efficiency as a simple polynomial in few normalized timescales. We then utilize this polynomial model to predict optimal values for the remaining free parameters and the scaling of the CAFC with the interring detuning.