High Efficiency Quantum Walk Comb via Double Waveguide Integration

Recently, Heckelmann et al. [1] showed that quantum cascade laser (QCL) ring cavities can support frequency comb formation with a bandwidth which is fully controllable by the use of an external RF signal, i.e. quantum walk combs (QWC). The ring geometry limits the output power, since the extracted light can only come from bending losses thus not guaranteeing a regular far-field. To this end, we exploit a coupled waveguides technique [2], to allow a better light out-coupling. At the same time, the light in the cavity will experience a modified dispersion, which will impact the stability of the comb. Compared to a coupling with an active element [3], the use of a fully passive waveguide is more power efficient and allows to define completely arbitrary geometries, opening the possibility of defining passive optical elements on the same chip as the active element, with interesting outcomes for interferometry, sensing or spectroscopy. In this first proof of concept, for the passive waveguide, we follow a simple U geometry (Fig. 1.a) made of silicon doped InGaAs, and completely buried in InP (Fig.1.b). In Fig.1.c we report a light-current-voltage (LIV) plot for a reference racetrack without an extractor element. The output power reaches a maximum of 430μW, with a blurred far-field (Fig.1.d). The results is directly compared with two racetracks which have a U-shaped top waveguide design. Typical values span from tens of mW up to 1 IOmW, proving that the light extraction is highly efficient and that the out-coupling is enhanced by two orders of magnitude, with a greatly improved far-field (Fig.1.d). The shape is the one of a higher order waveguide mode, due to the mode mismatch between the coupled waveguides section and the one of the uncoupled passive waveguide, also responsible for the power oscillation in the LI curve. We also measure a frequency spectral map for the reference racetrack (Fig.1.e) comparable with what already showed in [1]. The same measurement is performed on a double waveguide device and reported in Fig.1.f. In this case, we can reproduce a QWC, even if the tuning shape is not as clean as the reference one, due to the effect of third order dispersion. In conclusion, we proved it is possible to produce a QWC with an out -coupling efficiency increased by two orders of magnitude. At the same time the dispersion of the light in the cavity is significantly modified. Further work will try to improve the out-coupling mechanism, by a more accurate mode matching between the coupled cavity modes and the uncoupled passive waveguide mode, and improve the comb bandwidth by a more careful choice of the third order dispersion. The novelty of this work pushes forward the ability of designing active and passive components on the same chip, with possible outreaches in the spectroscopy and sensing fields.