Quantum Walk Comb in a Dual Waveguide Quantum Cascade Laser

Abstract

Ring quantum cascade lasers (QCLs) proved to be a versatile tool for generating tunable and stable frequency combs in the mid infrared range in the form of quantum walk combs. By homogeneously integrating a racetrack QCL with a passive waveguide, which lays on top of the active region plane and therefore can be designed to be fully independent from the laser geometry, we improve the light outcoupling from the ring by more than 2 orders of magnitude reaching a maximum output power of 120 mW. In addition, we show that it is possible to achieve quantum walk comb operation in the devices under analysis. Finally, we prove that we can change the light dispersion by tuning the parameters of the passive waveguide, with a direct impact on the behavior of the generated comb.

Figures (10)

• Figure 1: a) 3D schematic of the double waveguide laser. b) Picture of a double waveguide laser mounted on a a standard PCB, and respective top-view showing the electrical contact and the trace of the U-shaped passive waveguide. c-d) SEM pictures of a cross section of a double waveguide laser with different passive waveguide widths. e-f) Far field measurements of the emitted light at the facet from the passive waveguide, wider and narrower than the active region, respectively.
• Figure 1: Detailed processing steps of the dual waveguide laser.
• Figure 2: Laser voltage vs current density (dashed line) and laser power vs current density (solid line) for: a) one reference racetrack without passive waveguide, b) two racetracks with wider top waveguide then the active region, c) two racetracks with a narrower top waveguide than the active region.
• Figure 2: b) Light-Current density and Power-Current density of the wide waveguide device used to measure the spectra in Fig.\ref{['fig:spectra']}.b. of the main text. a) Comparison of Voltage-Current density curves between a reference racetrack (black) and a wide waveguide laser (red) processed on the same active material (EV3032). For the latter, the Power-Current density curve is also reported.
• Figure 3: a-b) Symmetric and Anti-Symmetric TM mode solution, respectively, for a coupled waveguide system. Each inset, represents the mode profile in the center of the structure and the relative refractive index profile. The red-shaded and purple-shaded region, correspond to the active region and passive waveguide position, respectively. c-d) Simulated Group Velocity Dispersion (GVD) and mode overlap with the active material ($\Gamma$), for the two uncoupled waveguides and for the cases of a wider, narrower and same size top waveguide compared to the active region width. Both symmetric (+) and antisymmetric (-) solutions are displayed. The dispersion and the overlap factor of the lasing modes are indicated by thicker lines. The shaded region corresponds to the gain bandwidth of the active material under consideration. For all curves the active region width is chosen to be 5 $\mu$m. The top waveguide width is 3 $\mu$m and 8.5 $\mu$m for the narrow and wide waveguide, respectively.