Abstract:Abstract-The spectral properties of grating-assisted directional couplers are studied using an improved coupled mode formulation. Key parameters for the design of these structures, such as the grating period, the coupling length, and other structural parameters, are calculated. The frequency response, the filter bandwidth, and the tuning range are analyzed. The technique is used to examine a specific case of InGaAsP-InP tunable filter, and the results are compared to a prior experiment.
“…More sophisticated coupler designs exhibiting appreciable dispersion have been studied in the past. Some examples are grating assisted couplers [45,46] and couplers implemented in asymmetric vertical structures [47], including Bragg Reflection waveguides [48]. These can provide more compact and efficient ways of achieving the necessary dispersion.…”
Integrated optics has brought unprecedented levels of stability and performance to quantum photonic circuits. However, integrated devices are not merely micron-scale equivalents of their bulk-optics counterparts. By exploiting the inherently dispersive characteristics of the integrated setting, such devices can play a remarkably more versatile role in quantum circuit architectures. We show this by examining the implications of linear dispersion in an ordinary directional coupler. Dispersion unlocks several novel capabilities for this device, including in-situ control over photon spectral and polarization entanglement, tunable photon time-ordering, and entanglement-sensitive two-photon coincidence generation. Also revealed is an ability to maintain perfect two-photon anti-coalescence while tuning the interference visibility, which has no equivalent in bulk-optics. The outcome of this work adds to a suite of state engineering and characterization tools that benefit from the advantages of integration. It also paves the way for re-evaluating the possibilities offered by dispersion in other on-chip devices.
“…More sophisticated coupler designs exhibiting appreciable dispersion have been studied in the past. Some examples are grating assisted couplers [45,46] and couplers implemented in asymmetric vertical structures [47], including Bragg Reflection waveguides [48]. These can provide more compact and efficient ways of achieving the necessary dispersion.…”
Integrated optics has brought unprecedented levels of stability and performance to quantum photonic circuits. However, integrated devices are not merely micron-scale equivalents of their bulk-optics counterparts. By exploiting the inherently dispersive characteristics of the integrated setting, such devices can play a remarkably more versatile role in quantum circuit architectures. We show this by examining the implications of linear dispersion in an ordinary directional coupler. Dispersion unlocks several novel capabilities for this device, including in-situ control over photon spectral and polarization entanglement, tunable photon time-ordering, and entanglement-sensitive two-photon coincidence generation. Also revealed is an ability to maintain perfect two-photon anti-coalescence while tuning the interference visibility, which has no equivalent in bulk-optics. The outcome of this work adds to a suite of state engineering and characterization tools that benefit from the advantages of integration. It also paves the way for re-evaluating the possibilities offered by dispersion in other on-chip devices.
“…Here, we are interested on the latter and propose a scheme where coupling strengths are constant but vary from waveguide to waveguide following an abstract symmetry plus periodical modulation of individual refractive indices. Our symmetry-based proposal complements grating assisted couplers [29][30][31][32][33], where dissimilar waveguides are coupled by resonant periodic variations in the effective refractive index. The coupling allows near complete power transfer between the waveguides.…”
We consider a coupled mode system where the effective propagation constants of localized modes are amenable to modulation. Starting from an unmodulated system where power transfer is heavily suppressed, we demonstrate that small, periodic modulation of the propagation constants enhance power transfer using a slowly varying envelope approximation for the field mode amplitudes. We calculate an approximate modulation frequency enabling complete transfer between otherwise negligibly coupled elements. The ability to control these modulations by electrical or thermal effects allows for reconfigurable multiport switching. We use an array of coupled silica waveguides and the thermo-optic effect to test our predictions in the telecom C-band. However, this requires a refractive index modulation with period of the order 10−3 m and yields total power transfer with a propagation distance of the order of 10−1 m, which might make it unattractive for integrated photonic applications. Nevertheless, our results are valid for devices described by an equivalent coupled mode matrix for space or time propagation; for example, arrays of microring or terahertz resonators, microwave cavities, radio frequency antennas, or RLC circuits.
The exchange of power between two identical coupled waveguides with embedded periodic structure along the z direction is studied in the framework of Floquet-Bloch theory. A one-period propagator is calculated and a spectrum of its eigenvalues (eigenphases) is studied. We show that due to special symmetry properties of the Floquet-Bloch operator, a strong enhancement or a total suppression of power exchange can be obtained. The power exchange control is realized by the interaction of lowest order waveguide modes with high-order modes. A numerical example is presented in which the beat length is shortened by more than five orders of magnitude.
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